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Biology 30 Module 2: Lesson 5 Body Actions and Systems: Maintaining Life Body (Cell) Requirements Copyright: Ministry of Education, Saskatchewan May be reproduced for educational purposes Biology 30 1 Lesson 5 Biology 30 2 Lesson 5 Lesson 5 Body (Cell) Requirements Directions for completing the lesson: Text References for suggested reading: Read BSCS Biology 8th edition Pages 373-398 OR Read Nelson Biology Pages 122-135 Study the instructional portion of the lesson. Review the vocabulary list. Do Assignment 5. Biology 30 3 Lesson 5 Vocabulary alveoli autotrophs bronchioles bulk feeders catalysts chemical digestion chloroplasts cloaca closed digestive tube coelom diaphragm diaphragm diffusion digestion enzyme epiglottis esophagus extracellular digestion Biology 30 fluid feeders gastrovascular cavity glottis guard cells heterotrophs ingestion intracellular digestion minerals peristalsis phloem physical digestion stomata stomates vascular bundle villi, microvilli vitamins xylem 4 Lesson 5 Body (Cell) Requirements Introduction Observations of the different kinds of activities involving organisms – whether they are microscopic and unicellular or large and multicellular, would reveal that a great deal of time is spent in obtaining materials for bodies or cells. How materials are obtained and the manner in which they are processed by living things vary considerably; however, their essential purpose of maintaining life is the same for all. The action of photosynthesis is important in changing the energy of sunlight into the chemical energy of bonds in organic compounds. This process requires particular kinds of matter and also particular movements of gases. Once organic matter is formed, it can then be used in various ways by the photosynthetic organisms themselves or by heterotrophic organisms once they have acquired it. Some of the uses of organic matter can include: Direct energy sources for cell or body activities; Building materials for cells or bodies; or, formation of substances which either maintain the two previous actions or maintain suitable internal body conditions. Cellular respiration is an important process that is part of all of these actions. As with photosynthesis, certain kinds of gas movements are associated with cellular respiration as well. This lesson will show how matter or nutrients are obtained and processed by organisms in different kingdoms. It will also trace how gas exchanges, associated with these actions, are accomplished. Biology 30 5 Lesson 5 After completing this lesson, you should be able to: Biology 30 • name the basic purposes served by material taken into living bodies. • state how the nutritional requirements of autotrophs and heterotrophs differ. • list and indicate the importance of some nutrients important in sustaining life. • explain the purpose and value of digestion. • state the two general phases in the digestive process. • explain the general role of enzymes and how they function. • distinguish between intracellular and extracellular digestion. • describe some of the methods of obtaining and processing nutrients by organisms in various kingdoms. • distinguish between cellular respiration and respiration. • explain the need for carbon-dioxide or oxygen in plant or animal cells. • discuss the photosynthesis-respiration relationship in plants. • explain the significance of diffusion in respiration. • describe how gases move in and out of various plant cells or organs and the structures that may be involved with these movements. • indicate the major difference between plant and (multicellular) animal respiration. • distinguish between respiration and breathing. • trace the processes, along with possible specializations, related to respiration in “animal” kingdoms. 6 Lesson 5 Foods and Nutrients Every organism must continually take in matter if it is to sustain a living condition. Any such matter or material taken into cells or bodies to sustain life is given the term food or nutrient. Some references use the terms interchangeably. Others make a distinction by saying that food is all matter which goes into a cell or body while nutrients (or nutriments) make up that part of food which is actually used or is of value. This course will use the two terms, food and nutrient, interchangeably to refer to any substances taken into cells or bodies for purposes of work, growth or maintenance. Autotrophs – Heterotrophs The general definition given for food or nutrient means that autotrophs such as green plants or heterotrophs such as ourselves all require foods. Previous definitions of autotrophs may have stated that this general grouping could make its own food. In describing the general relationships of autotrophs and their requirements, a major difference will be emphasized as to the nature of plant requirements and those of animals. Autotrophic plants are plants that contain chlorophyll and can photosynthesize. They take in inorganic foods or nutrients and convert them into organic compounds by utilizing the energy of sunlight. There are certain varieties of bacteria that are autotrophic. Photosynthetic bacteria possess their own special type of chlorophyll in order to use the energy of sunlight. Chemosynthetic bacteria have enzymes capable of splitting some inorganic compounds (containing iron, sulphur and other matter) and obtaining energy from them. While autotrophs may obtain their initial energies from different sources, their foods or nutrients are still inorganic before being processed into organic compounds. Therefore, it would be better to say that autotrophs are organisms capable of converting inorganic matter into organic compounds. Biology 30 7 Lesson 5 Heterotrophic organisms are either directly or indirectly dependent on autotrophs. Their foods or nutrients are largely made up of pre-formed organic matter. These organic nutrients come from plant or animal sources. Heterotrophs may be classified differently depending on the ways in which they obtain their nutrients. Herbivores include those organisms whose main food sources are plant matter. Decomposers, saprophytes and scavengers rely on the bodies of plants and animals having died of natural causes or having being killed by other organisms. Carnivores and parasites either obtain their nutrients from living bodies or ones which they have killed. Processes Involving Foods Some of the values of matter and nutrients were mentioned as being energy sources and also sources for building and maintaining an organism's own cells or body. The processes of food breakdown to release energy (oxidation) and the formation of an organism's own cells and tissues (assimilation and synthesis) are really the final actions involving nutrients before they are used up or become part of the organism itself. Some important actions must occur before this. Getting nutrients into a cell (for a unicellular or plant organism) or into body cavities (of multicellular animals) is one of the first steps. For most plants, which are not mobile like animals, nutrients must come from the air or soil in contact with plant parts. Such plant parts often play passive roles themselves as the actions of diffusion or osmosis result in molecules moving through membranes and into cells. In some instances, nutrient molecules are actively transported through plant membranes against diffusion gradients. Such active transport requires some expenditure of plant energy. For mobile organisms, the actual capture of nutrients may require much more effort than is required to get those nutrients into cells or into body cavities. Many predators spend much time making unsuccessful stalks and rushes in capture attempts. The term ingestion is usually applied to the action of taking nutrients into a body cavity, like a stomach. Though such nutrients are within a body space, they are still outside of cells. Biology 30 8 Lesson 5 Enclosing nutrients within cells or within body cavities does not automatically make them available for use. Many are simply too large to be able to pass through a membrane and into the cytosol or the main part of a cell where they can be utilized. Even if some could pass through, they may still be in a form that can’t be used. Another action must occur. The process of digestion becomes very important at this point. Digestion involves actions that convert nutrients into smaller, soluble forms that could pass through membranes, be transported throughout plant or animal bodies and finally, be used by cells. Two phases of digestion can be involved: Phase One: Physical or Mechanical Foods are broken down into smaller amounts making them easier to ingest. Chewing or tearing with teeth and churning or grinding actions of stomachs or gizzards are some of the more common physical actions of digestion. Phase Two: Chemical In this phase, enzymes accomplish the final breakdown. Nutrients are usually converted to smaller molecules having different characteristics from their previous forms. Let’s review the role and actions of enzymes: Many chemical actions require higher temperatures to initiate them. Inside plant or animal bodies these higher temperatures are not possible. Instead, proteinbased enzymes are able to join or split molecules. The enzymes assume the role of catalysts in that they initiate and control rates of reactions but do not change the final nature of the products themselves. The enzymes are not used up either; after reactions are over, they become free to work on other molecules. Enzymes are specific, apparently operating on a "lock and key" principle. Each enzyme is constructed in such a way that only one particular molecule or set of molecules can fit onto it. This means that thousands of enzymes are required in digestive and metabolic processes. Many of the enzymes today are named by taking parts of the names of the substances upon which they act and adding "ase" to them. Thus an enzyme that acts upon a lipid (fat) is called lipase; one that acts upon sugar is sucrase. Closely associated with enzymes, especially in vertebrate animals, are hormones. These chemical secretions are produced by ductless glands and are released directly into circulatory systems. The importance of hormones to digestion relates to the ability that some of them have in stimulating the enzyme-secreting glands. Hormones are examined more closely in another section. Biology 30 9 Lesson 5 Even though plants, animals and other organisms differ widely in the kinds of nutrients they take in or in the actual manners of intake of these nutrients, some of the processes of digestion are very similar. Frequently, enzymes are similar and even the same in plants and animals. Some insectivorous plants have "digestive systems" where modified leaves are used to trap organisms (insects) and direct them into special cavities. Cells surrounding these cavities secrete enzymes which digest the bodies of the trapped victims. Once nutrients have been digested or converted into soluble forms, they may pass into and be utilized by adjacent cells. In larger, multicellular organisms, much of the digested matter must be transported in order that cells in all body areas satisfy their nutrient requirements. This transport is just as necessary and important in larger plants as it is in animals. Extracelluar – Intracellular Digestion Mechanical or chemical digestion of foods can occur in various places in organisms' bodies. Where the process does take place greatly depends on whether an organism is unicellular or multicellular and, if multicellular, how complex the body or digestive system is. Locations of digestion are generally classified as being either: Extracellular Extracelluar digestion is accomplished when enzymes leave the cells and carry out their actions outside of the cells. This type of digestion could occur completely outside an organism or in cavities or spaces within an organism. A cell or group of cells then absorbs the digested matter. Extracelluar digestion increases the possible food supplies for organisms, as they are no longer limited to very small foods. Large amounts of organic matter could be ingested and then partially digested in order to be able to enter individual cells. Some examples of organisms that show extracellular digestion are: bracket fungi rusts bread molds yeasts Biology 30 10 Lesson 5 Intracellular. Intracellular digestion takes place within the surrounding or boundary membranes of a cell. Food particles themselves may or may not be enclosed in smaller cavities or vacuoles enclosed by membranes. Some examples of organisms that show intracellular digestion are: Amoeba and Paramecium. Some Major Nutrients and Their Values Totally depriving any organism of all foods or nutrients will bring about its death when its body reserves are used up. For a human cut off from all food and water this may take about two weeks. Reducing or eliminating only certain kinds of foods may sustain lives for much longer periods of time. However, organisms lacking certain kinds or levels of nutrients could show poor growth, poor body functioning or actual deterioration in general body conditions. Such consequences as eventual death or poor body functioning indicate the general importance of nutrients to organisms. Nutrient values could be divided into more specific body uses: 1. Much of the food taken in by organisms is used as a source of energy. Energy is used for such actions as: Cell divisions Moving substances within bodies, Moving parts of bodies themselves (muscle contractions) Maintaining body temperatures. 2. Matter is also used for cell formation and cell and body maintenance. 3. Nutrients are re-organized to form an organism's own unique kinds of cells. Proteins and other substances are formed to keep the cells or body functioning properly. Some of the major organic nutrients taken in by organisms were examined earlier. Biology 30 11 Lesson 5 Carbohydrates, which include sugars and starches, are primarily fuel or energy sources. Glucose is the most immediate energy source in most cells. Excess amounts of this are converted to starches, which could be glycogen (animal "starch" in the liver). Another carbohydrate, cellulose, helps to form the cell walls of plants. Herbivorous animals (like cattle) can utilize the help of bacteria and protists in their digestive systems to break down cellulose. In other animals, including ourselves, cellulose is largely indigestible and passes out of the system. It is still valuable as roughage or bulk which helps matter to move along the digestive systems, partly by stimulating muscle contractions. Fats or vegetable oils (lipids) are also energy sources, providing about twice the energy levels supplied by comparable amounts of carbohydrates. In animals, excess amounts are frequently stored under the skin or in tissue spaces such as those around organs. Proteins are important for cell formation, growth and repair. They are also used in the production of enzymes that are so important in many metabolic activities. Excess amounts of protein or shortages of the other organic compounds will result in the breakdown of this organic nutrient. Vitamins are an organic group that has not been previously described. They are just as important. Vitamins are not like any of the other organic compounds since they are not broken down to provide energy. They are necessary for normal body growth and action. Their functions may not be particularly noticeable unless a shortage or deficiency exists. When this happens any number of conditions could develop. Two kinds of vitamins are: Fat soluble Water soluble Some vitamins, their sources and deficiency diseases are indicated in the following table. Biology 30 12 Lesson 5 Nutrients Some Functions Food Sources Deficiency Symptoms Fat Soluble Vitamins Vitamin A* • aids in the ability to see in dim light • keeps skin healthy • forms bones and teeth • needed for reproduction Dark green and yellow vegetables, yellow fruits, egg yolks, liver, milk and milk products, fortified margarine, fish liver oils Nightblindness; rough skin and mucous membranes; no bone growth; teeth cracked or decayed; dry eyes. Vitamin D* • builds strong bones and teeth Vitamin D fortified milks and margarines, fish or fish liver oils Rickets; bowed legs; soft bones prone to fracture; muscle spasms and twitching. Vitamin E* • protects vitamins A and C • formation of red blood cells, muscles and other tissues Vegetable oils, wheat germ, whole grain products Damage to blood cells, cell membranes. Vitamin K* • normal clotting of blood Dark green, leafy vegetables Hemorrhaging Water Soluble Vitamins Vitamin C* • maintains healthy bones, teeth and blood vessel walls • helps body use iron, calcium and folacin Citrus fruits and their juices, vitaminized apple juice, tomatoes, melons, berries, broccoli, brussel sprouts, cauliflower, cabbage, turnips, potatoes Scurvy; bleeding gums; dry and rough skin; loose teeth; degeneration of muscles. Thiamin (Vitamin B1) • releases energy from carbohydrates • normal functioning of nervous system Whole grain and enriched products, pork, organ meats, legumes (peas, beans, lentils), potatoes Berberi: muscular weakness; swelling of heart; leg cramps; mental confusion. Riboflavin (Vitamin B2) • releases energy from carbohydrates and fats • forms red blood cells • normal growth and development Milk, meat, fish, poultry, eggs, whole grain and enriched products Skin disorders - most noticeable around nose and lips; eyes more sensitive to and irritated by light. Niacin* (Vitamin B3) • releases energy from fat, protein and carbohydrates • helps to build body protein Meat, liver, poultry, fish, peanut butter, legumes, whole grain and enriched products Pellagra: skin disorders particularly where exposed to sun; diarrhea; irritability; mental confusion. Pyridoxine* (Vitamin B6) • helps the body use protein and fat • resists infection Meat, liver, fish, whole grain products, eggs, legumes, bananas Skin disorders; cracked mouth corners; dizziness; anemia; convulsions. Folacin* (Folic Acid) • forms genetic material • forms red blood cells Most fruits and vegetables, meat, fish, legumes, whole grain products Anemia; diarrhea. Cobalamin (Vitamin B12) • helps functioning of the nervous system • forms red blood cells Meat, liver, kidney, eggs, milk, cheese Anemia; degeneration of peripheral nerves. Biotin • forms fatty acids and protein • releases energy from carbohydrates Liver, kidney, milk, egg yolk, nuts, legumes Fatigue; depression; nausea; pains; appetite loss. Pantothenic Acid • releases energy from protein, fat and carbohydrate • aids absorption Liver, kidney, whole grain cereals, milk, mushrooms, dark green vegetables, citrus fruits, bananas, berries, cantaloupe, legumes Abdominal pain; vomiting; fatigue; sleep problems. Biology 30 13 Lesson 5 Most of the vitamins required by animals must come from their in their food sources. Only a few can be produced within bodies, such as vitamin D produced in the skin subjected to sunlight; or, certain bacteria producing vitamin K in the intestines of animals. Humans may also take in vitamins in the form of supplements. Animals can be given supplements as well. Excess amounts of water-soluble vitamins are usually excreted from animal bodies in the urine. Fat-soluble vitamins are stored in the liver and fatty tissue and are eliminated much more slowly. Because they are stored for long periods of time there is a greater risk of toxicity. Eating well-balanced meals is the best way to supply your body with vitamins. Consultations with dieticians or medical professionals would be wise courses of action before attempting supplemental programs. For both autotrophs and heterotrophs, inorganic substances can form varying amounts of the matter taken into bodies. Minerals, in the form of dissolved salts in soils or incorporated in other plant and animal tissues, can be used in diverse ways. Some, such as calcium and phosphorous, are important building components of animal body parts like bones and teeth. Other minerals are important in the proper functioning of muscles, nerves and plasma membranes. As with vitamins, the values of some minerals are not realized or appreciated until specific deficiency diseases develop. In humans, these can include: Anemia - deficiency of iron. Thyroid disorders - deficiency of iodine. Rickets - deficiency of calcium. Other abnormalities develop as a result of shortages in combinations of minerals or minerals and vitamins. In plants, mineral deficiencies are frequently noticed by poor growth performances of specific plant parts or entire plants. The following chart shows the mineral, its function, its food source, and the deficiency disease it can cause : Biology 30 14 Lesson 5 Nutrients Some Functions Food Sources Deficiency Symptoms Calcium • builds strong bones and teeth • helps blood clot • aids in muscle contraction Milk and milk products, canned salmon or sardines with bones, sunflower seeds, soybeans and tofu (soybean curd) Rickets; poor bone growth; osteoporosis; convulsion. Chromium • releases energy from carbohydrates Vegetables, whole grains, fruit, meat, cheese Reduction in ability to metabolize glucose. Copper* • part of many enzymes • aids in iron absorption Liver, shellfish, nuts, mushrooms, whole grain cereals Anemia; bone disorders. Fluoride* • forms strong bones and teeth Fluoridated water, fish, tea, wine Increased tooth decay. Iron* • forms hemoglobin, which supplies oxygen to cells • part of several enzymes and proteins • helps resist infection Kidney, liver, meat, legumes, whole grain or enriched products, eggs, dried fruits Anemia; shortness of breath; weakness. Magnesium* • involved in almost all reactions involving carbohydrate, protein and fat • conduction of nerve impulses to muscles • builds bones and teeth • helps regulate body temperature Vegetables, legumes, seafood, nuts, whole grain products, milk products Poor body growth; weakness; nerve and behavioral disturbances; muscle spasms. Selenium* • needed for growth • prevents breakdown of fats and other body chemicals Meat, dairy products, fruits and vegetables No known defects. Zinc* • part of over 40 enzymes which function to: maintain acid-base balance, digest protein, make genetic material Meat, seafood, whole grain cereals, legumes, eggs Growth failure; slow or incomplete sexual maturation; poor appetite; glucose intolerance. Potassium Acid-base balance; body water balance; nerve function Fruits, vegetables, legumes, cereals; meat; dairy products Muscle weakness; paralysis; body water retention; possible high blood pressure. Minerals * Large amounts of these nutrients have toxic effects in humans. Biology 30 15 Lesson 5 Water – A Food or Nutrient? Although many may find it hard to regard water as a food or nutrient, it easily fits into the general definition of being essential for the cells or bodies of organisms. Water makes up anywhere from sixty to over ninety percent of the composition of certain tissues or organisms. (The mass of our own bodies is between 60 and 70 percent water.) Functions of Water Water is a means of diluting waste substances (such as urea) and transporting them out of bodies. It is important as a dissolving agent or solvent, so that substances can be transported within bodies and through cell membranes. It functions as a lubricant (saliva, mucus...). Water acts as a temperature regulator (as perspiration evaporates from skin) and as a regulator of turgor in plant and animal cells. Other functions may not be as obvious, as water may have indirect involvements – as with the formation of blood, which is essential for oxygen transport, or as an activator of enzymes. Water can enter a body by: Absorbing it. Drinking it directly Entering bodies from the foods of a plant or animal. Some animals in desert regions manage to exist without ever drinking water. Dehydration synthesis and oxidation of certain foods meet their requirements. Biology 30 16 Lesson 5 How Organisms Obtain and Process Food Comparison of Food Ingestion and Processing Food The next section (pages 17-36) will look at how organisms ingest their food and how it is processed. The material will take a quick look at these processes in the fungus and plants and then move on to animals. The following chart shows the progression through the animals. It is important that you are able to understand: How food is ingested at each level. The key differences between each group. Use the process of digestion as it occurs in humans. Plants Inorganic nutrients required by plants could come from the atmosphere, soil or water. Gases such as carbon dioxide and oxygen in dissolved form pass through cell membranes or through openings such as stomates and lenticels. Dissolved minerals commonly diffuse into plant cells from the soil. Some minerals are actively transported through such membranes. Movement of water into plant cells or organs such as roots takes place by osmosis. Aquatic plants or even those grown hydroponically (in liquid nutrients rather than soils) derive much of their inorganic matter in dissolved form from the water. Insectivorous plants (such as Pitcher plants, Venus flytraps or Sundews) are unusual in their abilities to utilize the organic compounds in the bodies of small organisms such as insects. Quite often, such plants are able to function quite well on inorganic nutrients, just as most other plants do. Biology 30 17 Lesson 5 In other instances, the ability to digest organic matter is helpful to individuals, especially if they are growing in low-lying, boggy areas. The acid nature of the water and the soils in such areas makes them nitrogen deficient. Insectivorous plants are able to increase their supply of this valuable element by digesting trapped organisms. Nitrogen is an important building component of proteins and the highly protein bodies of insects are therefore able to yield this element to the plants. The process of photosynthesis, which takes place in chlorophyll bearing cells exposed to light, transforms inorganic nutrients into organic compounds. One of the principal endproducts, glucose (sugar), may be transported for use or storage in other cells. When stored, glucose is generally converted into longer, polysaccharide (starch) chains. If cells need to use some of this starch, it must be digested to return it to a smaller, more soluble sugar – for use within cells or for transporting elsewhere. Fungi and Heterotrophic Bacteria Whether these organisms are unicellular or multicellular, their nutrient requirements are satisfied by extracellular digestion. Enzymes produced within cells or hyphae are released into a surrounding substrate (possibly a suitable food supply) to begin breaking down the material. Digested matter can then be absorbed into the cells. The secretion and release of enzymes into substrates is typical of all fungi and most bacteria. Puffballs, mushrooms, bracket fungi, rusts, bread molds and yeasts are just some of the common fungal examples. Many of these are saprophytes in that they feed on dead bodies. The term "saprophyte" is the plant equivalent of the term "scavenger" applied to some animals. Other fungi such as the grain rusts and athlete's foot, or bacteria such as Salmonella, are parasitic. Their enzymes are released into the cells of living hosts, often resulting in harmful effects to the hosts as cell contents are digested or cell functions are disrupted. Some of the enzymes could even be poisonous in animals, resulting in muscle spasms or paralysis and nerve damage. Breaking down nonliving organic matter, such as plant or animal bodies or excretions from them, reduces the substances to simpler forms. The fungi and bacteria involved in such actions are really decomposers. A decomposing action may be considered harmful if it involves matter that is still useful to humans or to other organisms. The role of decomposers is very valuable in breaking down dead matter and returning it to the many cycles in the biosphere. Biology 30 18 Lesson 5 Protists (or Protozoans) These microscopic, unicellular organisms may fit into a number of different types of nutritional or feeding relationships. The different relationships include: Some are free-living, while "capturing" other small organisms or taking in bits of organic matter. Others are restricted to living in or on particular kinds of hosts, where they may be parasitic or mutualistic in their relationships. Protists such as Amoebae and Paramecia form vacuoles around food particles. An Amoeba flows around the food and encloses it in some membrane that breaks off from the external membrane. food vacuole food particle about to be enclosed In a Paramecium, hair-like cilia sweep food particles into an oral groove along one side of the organism. A food vacuole forms at the end of the groove and then breaks away to move about in the cytoplasm. In both Amoebae and Paramecia, digestive enzymes move into the vacuoles to break down the food and allow digested matter to be absorbed into the circulating cytoplasm. Even though membranes separate the vacuoles from the rest of the cytoplasm, the characteristic of being enclosed within the main unicellular organism makes the digestion intracellular. Undigested particles in an Amoeba remain enclosed in the vacuole and are simply expelled through the outer membrane at any point. Solid, undigested particles in a Paramecium are expelled at a point not far from the oral groove called the anal pore. Some protists, such as the flagellated Euglena, possess chlorophyll. This enables them to photosynthesize, in addition to capturing organic particles. Inorganic nutrients are absorbed through the external membrane at any point while organic matter may enter at localized points where vacuoles are formed. Biology 30 19 Lesson 5 Many other protists utilize extracellular digestion. Members of the sporozoan phylum are all parasitic. Living in the cells, bloodstreams or cavities of host organisms, they either release enzymes to break down material outside their own cells and then absorb it or they absorb already soluble nutrients. Many of these are disease-causing, such as the malaria-causing Plasmodium. Protists utilizing extracellular digestion may also be involved in mutualistic roles. There are large protist numbers in the digestive tracts of ruminant herbivores such as cattle or deer. These protists are able to release enzymes capable of breaking down cellulose, which their hosts cannot do. Both protists and hosts are then able to absorb the digested nutrients into their cells. Cnidarians By-passing sponges, which show many traits of colonial cells including intracellular digestion, we come to this multicellular animal group. Cnidarians are the first to show something like a body cavity. Cnidarians such as Hydras, jellyfish, sea fans, sea anemones that are found in corals, have a digestive cavity or pouch with one external opening. The characteristic of having only one opening leads to this cavity sometimes being called a "closed tube". Tentacles with stinging, paralysing cells capture other small organisms and push them through an opening or mouth and into a gastrovascular cavity. Some of the inner or endodermal cells release enzymes to begin digesting the food inside the cavity. This extracelluar digestion helps individual endodermal cells absorb food particles through their membranes, enclose them in food vacuoles and continue with intracellular digestion. Biology 30 20 Lesson 5 Worms Worm phyla show diversity in the manner of obtaining and digesting food – if it needs to be digested. Free-living flatworms have a closed tube with a branching posterior gut gastrovascular cavity. "Closed" means that there is only one opening to the cavity. A muscular pharynx draws in soft food particles that are pharynx frequently from decaying matter. Inside the branching cavity or "gut", anterior gut enzymes released from surrounding cells carry out some extracellular digestion. Nutrients are then absorbed and used by cells or enclosed in food vacuoles and subjected to further intracellular digestion. Parasitic flatworms tend to show a degeneration of systems since they appear to have taken backward steps in body developments. They have poorly developed nervous and muscular systems and generally lack digestive systems. Most could be considered as specialized fluid "feeders". Tapeworms, which may be found in many animals including ourselves, have heads with special hooks enabling them to fasten to intestinal walls. The remaining segments of the worms float free, absorbing the already digested matter from their hosts' intestines. These parasites have thick cuticles, or nonliving, outer layers of cells, to prevent the parasites from being digested by their own hosts. Biology 30 21 Lesson 5 The roundworm group shows two significant advances over previous groups. One advance is the appearance of a digestive tube with two openings – a mouth and an anus. This tube within a tube type of structure is more efficient as soft or fluid foods move slowly from one end to the other while being digested extracellularly. The efficiency of an open tube largely comes about with the varying degrees of specialization along its length. In roundworms, some specialization of this "open" tube includes a muscular section near the mouth called the pharynx, for generating suction, and then a thinner-walled intestine behind this. Both free-living and parasitic roundworms possess this type of system. The second important improvement that appears in roundworms is a small fluid-filled cavity or pseudocoelom between the digestive tube and the mesoderm. Circulation of fluids in this cavity around the digestive tube assists in food and oxygen circulation (and waste removal). The most advanced of the worm phyla are the annelids or segmented worms. Most are marine, although our familiar earthworm can serve as an example for the group. Body segmentation in any group of organisms is important as it enables certain areas of a body or certain systems within that body to become more specialized for particular functions. This can be seen in the earthworm's open tube digestive system with the development from mouth to anus of: a short, muscular pharynx for generating suction; an esophagus leading to a crop where the ingested matter is temporarily stored and moistened; a muscular gizzard for grinding the material; and, an intestine where the processes of extracellular digestion and absorption take place. Biology 30 22 Lesson 5 Annelids have also progressed to having a coelom or cavity within the mesoderm rather than between the endoderm and the mesoderm as in roundworms. The advantages to having a coelom are: A portion of the mesoderm has surrounded the digestive tube to become a layer of muscle independent from the rest of the body. The digestive tract has a muscle layer of its own to push food along independently of the outer body muscles. The fluid-filled coelomic cavity with its suspended digestive tube has even more space than a pseudocoelom. The intestine begins to twist and loop within this cavity, presenting a much greater area where digestion can occur. More nutrients and oxygen can circulate Wastes can be more efficiently removed. Mollusks Mollusks continue with an open (mouth-anus) tube and specialized areas along that tube. Some, like clams, are filter feeders. Cilia lining the gills sweep food particles entrapped in mucus towards a mouth at the edge of the gills. A snail or slug uses a rasping, tongue-like structure called a radula to scrap plant matter off surfaces and into its mouth. Biology 30 23 Lesson 5 Cephalopods, which include squids and octopuses, catch or move prey with their tentacles. Crustaceans, fish and shellfish (other mollusks) are brought to the mouth where beak-like teeth crush and tear the food so that it may be ingested. Arthropods The tube-like digestive tract, with some specialized areas along its length, continues in arthropods. Organisms in some classes (such as crayfish or grasshoppers) have hard tooth-like structures made of chitin in their stomachs or gizzards. Chitin is a hard, protein substance which is also used in the formation of the exoskeleton. These stomach or gizzard "teeth" assist in the mechanical digestion or breakdown of food substances. Digestive glands, which are also present in mollusks, are quite common in many members of a number of arthropod classes. It appears that foods move into these glands for the final stages of chemical digestion before nutrients are absorbed. Some arthropods have finger-like projections extending from areas near the stomach. Opinions vary on what these caeca actually do. Biology 30 24 Lesson 5 Caeca may increase the area where digestion may occur or they may be places where (mutualistic) bacteria are concentrated. Another structure in some arthropods is a pouch-like enlargement in the intestine just before the anal opening. This is called a rectum and it serves as a temporary storage area for solid wastes before they are expelled. Types of Feeders: Many of the arthropods are classified as bulk feeders where parts of, or whole, organisms are eaten. Mouth parts are adapted mainly for biting and chewing in this group. Other arthropods are fluid feeders, sucking out plant or animal liquids or soft tissues. Examples: Spiders inject enzymes into their prey and then suck the partly broken down, soft tissues into their digestive systems. Bees and mosquitoes have tubes for penetrating flowers or other plant parts and sucking out nectar or other plant juices. female mosquitoes will pierce and penetrate the skins of animals to remove blood as a necessary protein for egg development. Biology 30 25 Lesson 5 Vertebrates The varieties of food and the manners in which they are gathered or caught and then ingested by vertebrates seem almost endless. Sight, smell and sound are probably the most prominent senses which most organisms use to detect or locate their foods. Another variation of sound involving echo location is used by some organisms such as bats. Body temperatures or heat given off by some warm-blooded animals can help to guide some snakes (pit vipers) to their locations. Detection of vibrations in water or soil can be used to reveal the presence of prey for other predators. Locating Food After locating food, the major difficulty that may face herbivorous animals is that the food could be out of reach. In most instances, the herbivores simply move on searching for something more accessible. More determined or desperate animals will sometimes use physical skills or force that they normally wouldn't use – such as knocking or pushing down shrubs and small trees to get at leaves. Animals that prey upon other animals are usually faced with more challenges in obtaining food. Stealth and short bursts of speeds, as with cats; sustained chases by dogs or wolves; the swooping actions of hawks and owls; and, the "herding" of minnows into shallower, shorewaters by feeding walleye pike ("pickerel"), are some techniques more often rewarded by failure than success. Ingesting the Food Once food is within reach or captured, vertebrates demonstrate a variety of techniques in ingesting it. Long tongues may be used to reach plant nectars or insect prey and pull them into mouths; tongues can also wrap around and pull at grass or twigs. Many animals are bulk feeders in the sense of taking in food whole or in large chunks. Birds are good examples of this as they swallow food such as seeds, fish or animals whole or use their beaks to break or tear apart the food into smaller pieces. Amphibians and snakes also commonly swallow their prey whole. Snakes are especially well suited for this as many have lower jaws which can "unlock" from the top jaws. As well, their lower jaws are flexible enough to allow the two halves to move independently of each other. This enables snakes to slowly pull entire prey into their mouths and bodies. Biology 30 26 Lesson 5 Teeth are common to the digestive systems of many vertebrates, with the exception of the present day birds. The greatest variations appear in mammals, with teeth adapted for gnawing, chewing, crushing or tearing. Some organisms possess combinations of these as our own teeth illustrate, with biting and gnawing incisors at the front, two canines in each jaw and the crushing and grinding molars farther back. Although teeth shapes, sizes and numbers could be different between species, general structures are similar as shown by the illustration. molar premolars canine incisor premolars molars s In a few species, certain teeth experience constant growth. This is characteristic of some rodents such as the beaver and the Norway rat. Observations have indicated that a rat's incisors can grow approximately 15 cm in length in one year. Biology 30 27 Lesson 5 Variations In Digestive Systems Although the varieties of food and the methods of gathering or catching it are quite different, digestive systems and processes are fairly similar among all vertebrate classes. Some minor variations do occur and these seem to be adaptations to food or feeding techniques. A bird has a crop that enables it to hurriedly eat larger amounts of food in a short time and temporarily store and moisten it. A powerfully muscled stomach or gizzard grinds and crushes the food into smaller pieces. Some birds ingest gravel or small stones to assist in the grinding process. Birds, amphibians and reptiles have an enlargement in the digestive tract just before the anal opening called a cloaca. This chamber receives and temporarily stores both solid and liquid wastes and also eggs and sperm during reproductive periods. Stomach adaptations may include extra projections from the stomach, as with the caeca in some fish, or more stomach chambers, as in many ruminants. These provide more surface area along which digestion and absorption can occur. Ruminants (eg cows, giraffes, sheep, goats) can ingest larger amounts of grasses or twigs in a relatively short period of time. Later, they can regurgitate the food, in smaller amounts, for a more thorough chewing in a less hurried fashion. Biology 30 28 Lesson 5 The stomach chambers of some herbivores, particularly ruminants, are homes for many bacteria and protists. These microorganisms play an important role in releasing enzymes that can break down cellulose. Both host and microorganisms can then absorb and digest the nutrients. (Some ruminants have bacteria and protists which can digest only certain kinds of vegetable matter and this is why some animals, like deer or moose, can eat only certain kinds of plant matter.) Inside a Vertebrate Digestive System With vertebrate digestive systems being fairly similar, the human example will serve as a common representative for the actions happening to food once it enters the mouth. Biology 30 29 Lesson 5 What happens in the Mouth The tongue and teeth can begin mechanical digestion by breaking up the food into smaller pieces. Three pairs (in humans) of salivary glands and numerous mucous glands along the cheeks supply saliva and mucus to help moisten and break up the food. Saliva also has the enzyme amylase, which begins the chemical digestion of starches. After some chewing a swallowing action is initiated. The tongue moves backward toward the throat or pharynx region pushing food ahead of it to where it can be taken over by muscular contractions of the esophagus. Although we have some control of swallowing, there is little control of the muscular contraction, or peristalsis, once it begins. During the swallow, the trachea and larynx are pulled upwards against the epiglottis to close the tracheal opening and prevent food from going the wrong way (into the lung passages). Ever heard of food or water going down your Sunday throat? Your drink of water beat the epiglottis covering your trachea—so you cough to try to get the water out of your trachea! Peristalsis, a contraction of longitudinal and circular muscles, then pushes food along the esophagus and into the stomach. One of several valves along the digestive tract exists at the juncture of the esophagus and stomach. This thickened, circular muscle, called the lower esophageal sphincter, normally prevents partly digested food from moving back along the esophagus. Accidental relaxations or a disorder in the muscle could result in the acidic contents of the stomach being released into the esophagus, causing a burning sensation. This burning sensation is often referred to as heartburn. This sphincter and others in different locations serve more importantly in keeping foods in particular sections of the digestive tract for longer periods of time. This allows more efficient digestion and absorption, rather than pushing food too quickly through the system. Biology 30 30 Lesson 5 What happens in the Stomach? In the stomach both physical and chemical digestions occur. Physical Digestion Three muscle layers make up the stomach walls. Circular, longitudinal and oblique muscles cause rhythmic contractions along the stomach's length resulting in the mixing of food, mucus, acid and enzymes. Chemical Digestion Glands throughout the stomach wall secrete chemicals called gastric juices. These are: 1. Hydrochloric acid (HCl) – This acid helps the enzymes to function, dissolves minerals, kills bacteria and regulates the pyloric valve. It also stops the action of the starch and glycogen-digesting amylase that began in the mouth. Very little digestion of carbohydrates occurs in the stomach. 2. The enzymes: pepsin and gastric lipase. The enzyme pepsin begins the chemical breakdown of proteins into short-chain amino acids. Lipase acts upon fats and oils. 3. Mucus: Mucus, or mucin, is important as a lubricant for the food and also as a protector of the stomach wall against the actions of the acid and enzymes. Despite this, mucus itself is continually broken down along with cells lining the stomach. When the protection by the mucus is not complete and when the lining cells are not replaced fast enough, ulcers could result. Food may remain in the 1.5 to 2 liter capacity stomach for two to three hours. It eventually reaches a consistency and an acid level, that cause a pyloric sphincter between the stomach and the small intestine to open. A quantity of food, or chyme as it is called at this point, passes into the small intestine. Biology 30 31 Lesson 5 The Work of the Small Intestine The Influence of the Pancreas The acid nature of the chyme stimulates the secretion of two hormones by glands in the first part of the small intestine or duodenum. bile duct body of pancreas gall bladder head of pancreas junction of bile and pancreatic duct tail of pancrea s main pancreatic duct duodenum One of these hormones acts upon the adjacent pancreas causing it to produce pancreatic juice. Pancreatic juice makes its way from the pancreas through a duct that joins with a bile duct from the liver. The two ducts form one common duct that enters the duodenum a short distance below the pyloric sphincter or valve. Pancreatic juice is made up of: A number of enzymes - Pancreatic amylase continues the digestion of carbohydrates that began in the mouth and pancreatic lipase breaks down fats. A number of protein-digesting enzymes, including trypsin and chymotrypsin, are included in the pancreatic juice. A mixture of sodium bicarbonate - The carbonates convert the nature of the food from acid (pH 2-3) to alkaline (pH 9), enabling the pancreatic enzymes to function. As this happens, the stomach enzymes cease functioning. The pancreas is significant in another way besides secreting enzymes. Within the pancreas are also cells that release hormones. These hormones: insulin and glucagon, regulate the amount of sugar in the blood. The significance of maintaining certain sugar levels in the blood and the importance of these hormones will be looked at in another lesson. Biology 30 32 Lesson 5 The Influence of the Liver The other hormone produced in the duodenum affects the liver. This large organ continually produces bile that, in some organisms, is temporarily stored in a pouch-like cavity called a gall bladder. (Some animals such as horses, deer and rats do not have a gall bladder, but release bile continuously in small amounts.) Bile is made up of: Bile salts in the alkaline bile cause fats to emulsify or to break up into very fine droplets. This permits the enzyme lipase to continue with its digestion of the fats. Some of the fats may be emulsified so that they may be able to pass into the lymphatic circulatory system, where they recombine into larger molecules again. Bile also consists of bile pigments. These are largely made up of broken down red blood corpuscles and hemoglobin which the liver removes from the circulatory system. It is the brown color of these remains which results in the characteristic brown color of the solid wastes or feces which are eliminated from the large intestine. Apart from its involvement in digestion, the liver is also an important homeostatic organ. Blood collected from the small intestines makes its way by means of the hepatic portal vein into the liver. Before allowing the blood to continue into the general circulatory system, the liver carries out actions that affect the concentration levels of certain substances. Excess monosaccharides are converted to glucose and if glucose itself is above a certain concentration, it is changed to glycogen. (A lower amount of glucose will result in the opposite action.) Since amino acids cannot be stored, the liver breaks down excess amounts. The liver removes the nitrogen portion of the molecule and converts it to waste urea. The remaining portion can be oxidized to release energy. The liver also controls excess amounts of other chemicals, including vitamins and minerals. Drugs, including alcohol, are filtered through the liver and are neutralized or converted into other forms by special enzymes. Biology 30 33 Lesson 5 Absorption in the Small Intestine. In the small intestine, the work of the pancreatic enzymes and additional enzymes produced by intestinal glands, largely complete chemical digestion. What were once originally large molecules or macromolecules of carbohydrates, fats and proteins have been reduced to small molecules of glucose or monosaccharides, fatty acids and amino acids. In these small forms, such molecules are capable of being absorbed into the lymph and blood systems for transport to other body areas. Although some substances (such as alcohol and sugar) begin to be absorbed in the stomach, by far the greatest amount of absorption occurs through the walls of the small intestine. Examination of the inside of the small intestine of many animals would show the presence of many folds that increase the total absorptive area. The total absorptive area is increased by many finger-like projections called villi (sing. villus) as well as smaller projections called microvilli coming from the outer cell membrane that line the small intestine. Compared to an intestine that was completely smooth, the folds, villi and microvilli are estimated to increase the surface area by 500 to 700 times. A Cut Away Section of the Wall of the Small Intestine blood vessel microvilli capillary Individual Villus The small intestine would be up to 70 m long if villi did not exist! Biology 30 34 Lesson 5 Glucose and other monosaccharides, amino acids, vitamins and minerals pass through the epithelial cells of the villi and into the blood capillaries. While much of this movement is by diffusion, some does occur by active transport. In this last process, cells use up some energy in moving certain molecules towards a higher concentration in the body transport systems or cells. Droplets of (emulsified) fat and fatty acids, along with some fat-soluble vitamins such as A, D and K, make their way into the villi as well. However, rather than entering the (blood) capillaries, these larger molecules enter lymph ducts or lacteals which are more porous than the capillaries. The lymphatic system eventually joins the regular circulatory system in the shoulder areas where lymph enters veins. The Large Intestine The final stages in the digestive process occur in the large intestine. Relaxations of a sphincter at the junction of the small and large intestine permit chyme to leave the small intestine. This begins to occur about four to six hours after it had entered. In some herbivorous animals (horse, rabbit...) there is an enlargement of the intestine at this point called the caecum. Home to many bacteria, it may be an important area for the final breakdown of the hard-to-digest cellulose. caecum where small intestine joins large intestine appendix In humans, only a small, blind pocket exists of what may have been a larger caecum in early ancestors. It plays no major role now except perhaps in a negative way. Another vestigial structure, a finger-like appendix, is attached to the caecum and could become infected. This could require surgical removal. Small amounts of digestion and absorption of nutrients continue in the large intestine. The more important function of this part of the digestive tract appears to be reabsorption of water. A large amount of water has entered the system up to this point in saliva, mucus and as water. Reabsorption of it from the large intestine prevents rapid body dehydration. During the ten to fourteen hours in which chyme normally remains in the large intestine, it is converted from a watery consistency to the normally solid fecal matter. Biology 30 35 Disorders can shorten the reabsorption time, resulting in diarrhea or can increase the time, to bring about constipation. Lesson 5 The large intestine, unlike the stomach or the small intestine, has neither a strongly acid nor a strong alkaline environment. This, along with the still remaining nutrients in the chyme, makes it an ideal home for countless numbers of coliform bacteria. A member of this group, Escherichia coli, is a common inhabitant of humans and other warm-blooded animals. Its relationship could be considered as commensalism and even one of mutualism, as it could be involved in several useful functions for its host. Some of these may include: inhibiting the presence of harmful bacteria some breakdown of cellulose the production of K and B vitamins which could be absorbed by the host. Numbers and rates of reproduction of these bacteria are so high that ten to fifty percent of the dry fecal matter could consist of bacterial remains. Even if no food is eaten, solid fecal matter is still produced from dead body cells and dead bacteria. Solid wastes accumulate in the final portion of the large intestine. Two sphincters, one at the beginning and one at the end of a slightly enlarged portion called the rectum are involved in: The final elimination of the wastes. The last of these, the anal sphincter allows wastes to pass out of the body through the anal opening. Extracellular – Intracellular Digestion Much of the digestion that occurs in vertebrates is extracelluar. Digestive tracts, despite the different specialized "compartments" or areas among different species, are still really extensions of the "outside" into organisms' bodies. Nutrients begin to be incorporated into bodies when they are absorbed through cells lining the digestive tracts and into circulatory systems and finally into individual cells themselves. Some of the same enzymes secreted into the digestive tract, as well as others, continue the final nutrient breakdown inside the cells during intracellular digestion. Biology 30 36 Lesson 5 Comparison of Respiratory Systems The next section (pages 37-59) will look at how gas exchange occurs in organisms and the mechanisms the each organism uses for gas exchange to occur. The material will take a quick look at these processes in fungi and plants and then move on to animals. The following flow chart shows the progression from unicellular organisms to the more complex systems of vertebrates. It is important that you are able to understand: The progression in development of gas exchange and the development of new structures as the organisms get larger. The key differences between each group. How breathing occurs in humans. Gas Exchanges (Respiration) Photosynthesis and cellular respiration are both actions associated with particular movements of gases. Photosynthesis requires the carbon from carbon-dioxide gas to build an organic compound. In the process, oxygen is released. Cellular respiration is the breaking down of organic compounds into simpler forms to release energy in the form of ATP. This activity requires oxygen and releases carbon dioxide. The oxygen is needed to continually remove electrons during aerobic respiration. Both processes are distinct and unique in what they produce but there is a relationship that they both share. This relationship is gas exchange or respiration. Organisms must maintain a proper balance of required gases, depending on the dominant action(s) occurring in their cells or bodies. In some organisms, such as green plants, both photosynthesis and cellular respiration can be going on at the same time at different rates. Both shortages and excesses of certain gases can have damaging or lethal effects on cells and organisms. Biology 30 37 Lesson 5 Organisms must maintain a proper balance of required gases, depending on the dominant action(s) occurring in their cells or bodies. In some organisms, such as green plants, both photosynthesis and cellular respiration can be going on at the same time at different rates. Both shortages and excesses of certain gases can have damaging or lethal effects on cells and organisms. Diffusion Respiration, or the actual exchange of oxygen and carbon dioxide at the cellular or cell membrane level for all organisms, takes place by diffusion. Gas molecules, which are in constant motion and colliding with other molecules, move from areas of high concentration to areas of low concentration. Inside organisms, membranes form boundaries around cells and gases must be dissolved in order to pass through these membranes. Therefore, cell membranes must be moist for diffusion to occur. Should the membranes dry out, respiratory failure could occur. Respiration (gas exchange – oxygen moves in and carbon dioxide moves out) across cell membranes has other factors that affect the process. Diffusion and diffusion rates through membranes are affected by molecule concentrations on both sides of the membranes. Consequently, some of the actions of bringing or removing molecules near the membranes are almost as important as diffusion itself. The means by which organisms allow, regulate or are actively involved with gas movements vary greatly and are all part of the overall process of respiration. The next section will deal with respiration (gas exchange) and the importance of it, starting with the plants, then moving through the consumers and ending with gas exchange in humans. Biology 30 38 Lesson 5 Respiration in Plants and other Producers To look at some plants, plant parts or plant cells, one may find it difficult at first observation to see any evidence of either cellular respiration or of gas exchange which should accompany it. However, cells must continually release energy if they are to remain alive. Even seemingly dormant vegetables, potato tubers, fruits and seeds have enzyme systems which continue to function slowly. Actual evidences of cellular respiration may be noted from such observations as: Higher temperatures towards the middle of seed or fruit storage containers; Positive limewater tests by gases released from seeds Gradual weight losses over time. The fact that enzyme actions continue in some plants even after harvesting is evident by many tissues or seeds changing from being sweet and tender right after picking to less sweet and more "woody" as quickly as a day afterwards. Corn and peas, in particular, have their sugars quickly converted to starches. "New" potatoes also lose their sweetness and their tender, outer skins become thicker and more rigid over time. The rate of cellular respiration in plant cells can be affected by a number of external factors: Temperature - Lowering temperatures by refrigeration or having cool storage areas can decrease rates greatly. (Lowering the temperature around strawberries from 24C to a little above freezing for instance, can reduce their respiration rate by ten times.) Moisture - Surrounding humidity and internal moisture can also extend or shorten plant or seed life. Prairie grain producers are very aware of the difficulties of storing seeds above a certain moisture content. In many cereal seed varieties, once the moisture content is above 18%, the respiration rate doubles for every 1% increase in moisture above that. A high respiration rate could quickly lead to "heating" of grain and a loss in quality and germinating ability (or life) of the seeds. The amount of oxygen or carbon dioxide surrounding plant parts is another determining external factor. Internal conditions relating to injury or to age could also affect plant respiration and the need for gas exchange. Biology 30 39 Lesson 5 Respiration in Plant Organs Photosynthesis and Cellular Respiration In photosynthesis, carbon dioxide is taken in while oxygen is released. Cellular respiration reverses these movements by needing oxygen and releasing carbon dioxide. Cellular respiration in green plants could be going on at the same time as photosynthesis but they are occurring at different rates. (Check back to the conclusions formed from the experiments in assignment 4) Under conditions where there is light, the two processes can go on at the same time. Gases released by one process are raw materials for the other process. Daylight conditions generally result in photosynthetic rates being anywhere from five to ten times faster than the other process. At this time, a plant would be releasing oxygen to its surroundings. In darkness, photosynthesis slows and may even stop while cellular respiration goes on. The cells or plants would now take in oxygen while releasing carbon dioxide. At certain low light intensities (considered by some to be about 2% of full sunlight), the rates of photosynthesis and cellular respiration balance each other. At this time, gas requirements between the two processes balance each other and there may be no net movement of gases into or out of certain plant parts. Normally, over a longer period of time, green plants consume more carbon dioxide and release more oxygen, than the other way around. Biology 30 40 Lesson 5 Respiration in Plant Organs Leaves Leaves are common areas where the two processes of photosynthesis and cellular respiration could be going on at the same time. Most of the cells between the upper and lower epidermis are collectively called mesophyll. Some of the mesophyll consists of palisade mesophyll or palisade cells. The cells making up this layer are mainly photosynthetic in function. The remaining mesophyll is made up of spongy cells, which are much more loosely arranged. Intercellular (air) spaces are common among these cells. Both photosynthesis and cellular respiration occur in the spongy cells. The many air spaces within the spongy mesophyll allow gases to move among the cells and to diffuse relatively easily between the spaces and both palisade and spongy cells. The network of intercellular spaces can make up anywhere from fifteen to forty percent of the internal volume of particular leaves. Cross Section of a Leaf A major difference between plants and animals is that air occupies most of the intercellular spaces in plants while those of animals are fluid-filled. The surfaces of many plant leaves and other plant parts are often covered with close fitting epidermal cells and a waxy layer or cuticle. These are intended to reduce water loss. However, gas movements are also restricted. To overcome this problem, the branched air passages of many leaves are connected to special openings in the epidermis called stoma (also called stomates; plural stomata). These openings are regulated by the actions of bean-shaped guard cells which exist in pairs. Biology 30 41 Lesson 5 Guard Cells and Stomata Guard cells and stomata could be located in both the upper and lower epidermis of a horizontal leaf. The concentrations could vary according to plant species. Land plants frequently have more stomata on the undersides of leaves or in the lower epidermis. Vertical leaves of plants may have equal numbers on both sides. Some floating leaves of aquatic plants have their stomata on the upper surface where they are exposed to air. Biology 30 42 Lesson 5 The size of an opening or stoma changes during a 24 hour day. Most stomata are open during the day and closed at night. The actual opening and closing of a stomata is related to the structure of guard cells and turgor changes in them. Guard cells' epidermal walls next to the stoma are slightly thicker than the walls on the other sides. When water enters the guard cells and increases their turgor pressures, their outer walls are pushed outward more readily. This draws the inner walls outward as well, causing a stoma to open. When water leaves the guard cells and their turgor falls, the cell walls move inward reducing the stomatal opening or causing it to close almost completely. Open stomata during the day allow carbon-dioxide and oxygen to move in and out of a leaf more readily. This is when photosynthesis is commonly at its peak and movement of the gases (especially oxygen) is also greatest. Stomata could close in very hot, dry conditions when guard cells lose more water then usual. Stems Stomates are also present in herbaceous stems where a considerable amount of photosynthesis and cellular respiration takes place. Gases can diffuse in and out of such stems through these openings. Stems which increase in thickness or diameter year after year usually have an epidermis which is transformed into bark cells. Bark lacks stomates and is largely impermeable to movements of water and gases. However, in such woody stems the total number of living cells is no longer that large. Much of the "wood" of trees consists of non-living vascular tissue which does not require gas movements. A large number of the still-living cells are concentrated in a thin layer beneath the bark. Gas requirements for these cells are supplied by small, round areas of loosely-packed cells in the bark. These often-raised, corky areas create openings called lenticels through which gases can pass in and out of stems. Biology 30 43 Lenticels on a Twig Lesson 5 Roots The location of most roots beneath the soil surface or in shady, humid areas limits the action of their cells largely to cellular respiration. Oxygen is needed for cellular respiration to occur. Carbon dioxide is given off. Oxygen and carbon dioxide pass in and out of roots through their epidermal cells and root hairs. The gases move from cell to cell by simple diffusion. Biology 30 44 Lesson 5 Other Plant Parts Exchange of gases occurs in various other plant parts, some of which have been mentioned before. Flowers, fruits and seeds experience gas exchanges by diffusion through the moist membranes of epidermal cells or "skins". In these structures the main process is cellular respiration so that oxygen is taken in and carbon-dioxide must be released. However, there are usually no special openings or special adaptations for gas exchanges in these plant parts as there are in the other organs. Respiration In Other Producers There are significant numbers of non-vascular plants such as algae, mosses and ferns. As well, there are non-plant producers which include many single-celled or colonial organisms belonging to the Protist or Moneran kingdoms. All of these may carry out both photosynthesis and cellular respiration, which are dependent on different gas exchanges. Most of these organisms have no special structures associated with respiration. Respiration or gas exchanges are accomplished mainly by simple diffusion between atmosphere or soils and producer parts and then from cell to cell. Respiration in Consumers Organisms classed as consumers cannot make their own food. They lack the ability to carry out photosynthesis and must obtain their organic requirements from other organisms. Such individuals must still carry out cellular respiration in order to release the energy from the organic nutrients or to put those nutrients into forms that can be utilized for other purposes. The actual exchange of gases (or respiration) in consumers is still by diffusion. Molecules of oxygen and carbon-dioxide pass through cell membranes from areas of higher to lower concentration. This movement by diffusion is the same as that found in producers. For consumers which are unicellular, colonial or multicellular but small in size, diffusion is often the only process responsible for movements of gases. As consumers become larger in size, there is an increasing difficulty in getting sufficient movements of gases between the outer bodies and all inner cells and tissues. Consumers that maintain more active lives and therefore require higher energy levels further add to this problem. Increased rates of cellular respiration bring on greater oxygen demands while releasing larger amounts of waste carbon dioxide. Biology 30 45 Lesson 5 Increased body sizes and increased activities among many of the multicellular consumers lead to one of the more important differences in gas exchange when comparing consumers and producers – particularly animals and plants. Some multicellular animals, in addition to diffusion, are able to breathe. Multicellular animals are able to carry out mechanical or physical actions designed to move more air in and out of bodies faster. These breathing actions (and sometimes the term "respiration" is incorrectly applied to mean "breathing") involve: special muscles air passages tissues or organs such as gills or lungs. As well, an internal transport (circulatory) system in some of these animals is associated with breathing. While plants also have tissues designed to move substances, their transport systems are not as actively involved. Transport systems will be looked at and compared more fully in another lesson. The remainder of this lesson will look at the methods and developments related to respiration in various consumers. Fungi Fungi have the characteristic of being heterotrophic or nutritionally dependent on other organisms. The Fungus kingdom includes such diverse types as slime molds, bread and fruit molds, yeasts and mildews. Many of the larger fungal masses consist of thread-like hyphae that may or may not have cross walls. slime mold Whether fungi exist as single cells or as masses of mycelia, respiration takes place by diffusion. Maintaining moist membranes for proper diffusion is a necessary requirement and this is why most fungi prefer humid and dark or shady conditions. bread mold Biology 30 46 Lesson 5 Unicellular "Animals" Most unicellular animals are found either in aquatic or moist environments, which could include the body fluids of host organisms. Exchanges of dissolved gases occur by diffusion. Multicellular "Animals" Sponges, Cnidarians (Hydra) and Echinoderms (Starfish) Even though these particular organisms may consist of many cells and possibly a number of cell layers, their method of respiration is the same as that of unicellular animals. Individual cells are either in direct contact with the external environment or not many cells removed from it. This allows respiration by simple diffusion. Worms – Platyhelminthes, Nematoda and Annelida The three worm phyla receive oxygen and remove carbon dioxide by diffusion through an epidermis. For those normally found in water, the gases are dissolved in the liquid. Worms that occupy terrestrial environments must maintain a moist epidermis for gases to pass in or out. A dry skin would lead to respiratory failure and death. Biology 30 47 Lesson 5 Earthworms caught in warm, dry conditions could quickly have their skins dry out. They would not be able to receive oxygen nor would they be able to get rid of carbon dioxide and nitrogen wastes. To avoid too much drying, the worms try to remain in moist soil conditions. They also secrete some mucus onto the epidermal surface to help maintain moistness. On the other hand, if their burrows are completely submerged by rainwater, earthworms will come to the surface where diffusion of gases could be faster. G Parasitic worms use their hosts' body fluids to satisfy diffusion needs. Mollusks – Snails, Clams, Octopuses and Squids The soft-bodied mollusks could follow a basic three part body plan (which is shown by the top illustration in the following diagrams): A head region A muscular foot, which may remain as a single unit or divide into tentacles (squids and octopuses) A visceral hump which includes all the internal organs. Important to all mollusk classes is a special covering layer of cells over the visceral hump. This is called a mantle and it not only produces an exterior shell in some classes but by moving away from the visceral hump it creates a mantle cavity. Inside this mantle cavity water or air can circulate and diffusion can take place between the mantle and other special structures (gills). Biology 30 48 Lesson 5 Arthropods A. Crustaceans Larger crustaceans, such as the crayfish we have in Saskatchewan waters and the look-alike but larger ocean lobster, carry out respiration by means of gills. Gills are located just under a portion of the hardened exoskeleton called the carapace. Water can enter along the lower edges and pass upward and forward over the gills. Attachment to the legs allows some movement of the gills which increases gas exchanges between the water and their capillary-rich filaments. Water trapped under the carapace enables some of these organisms to continue functioning should they come out of the water for short periods of time. Smaller water crustaceans such as Daphnia, copepods and amphipods create water flows under their shells by leg and body movements. Their smaller body sizes enable them to satisfy respiration needs by diffusion between water and cells directly, without the help of gills. Land crustaceans—such as the sow bugs we may find in basements, around foundation walls or under piles of wood or lumber—are usually small and must remain in humid conditions. Openings along their flattened, ventral surfaces permit air to enter their bodies and pass over a series of special plate-like gills where diffusion occurs. Bruce Marlin A sow bug Biology 30 49 Lesson 5 B. Arachnids – Spiders and Scorpions Organisms adapted to a land or an air existence, as many of the arachnids are, show other developments which help in respiration. Circulatory systems, already evident in some previous groups, are very important in moving gases to and from all body parts and specific exchange points. It is at these exchange areas that transfers of gases between the circulatory systems and atmosphere occur most readily. Arachnids may show two developments different from the previous groups. 1. Along the ventral side of spiders is an opening to a chamber called a book lung. Many flat, folded membranes extend into this cavity (like the pages of a book). These membranes allow circulating blood to easily make gas exchanges. 2. Some species also have a pair of branched air tubes or tracheae connected to the external surface by openings called spiracles. These air tubes enable air to move farther into the bodies and closer to the circulatory systems and cells. Different arachnid species may have both a book lung and tracheae, just a book lung or just the tracheae. Biology 30 50 Lesson 5 C. Insects, Chilopods (Centipedes) and Diplopods (Millipedes) All three of these classes assist respiration with breathing movements which force air in and out of a series of interconnected tracheae or air tubes and air sacs. Rhythmic movements of thoracic and abdominal muscles in a grasshopper cause some of its front spiracles to open and allow air into the tracheal system. Further contractions move air through the system and, while closing the front spiracles, cause the spiracles located farther back along the midline of the abdomen to open and let air be expelled. The three classes have interconnected tracheae scattered quite extensively throughout the bodies. This arrangement appears to be a means of assisting the circulatory systems as much as possible. Enlargements of the tracheae into larger sacs at various points in the bodies permit more air to move through the system and to be exchanged. The system of tracheal tubes, air sacs and spiracles is present in aquatic insects also. Some move about on water surfaces, so they have no great difficulty in receiving the air they need. Some of the diving insects (and even some water spiders) can take air with them below the water surface as bubbles attached to their bodies (diving beetles) or as a thin layer of air completely around their bodies. Some may carry the air supply until it is used up while others temporarily store it in bubbles beneath various underwater structures. Biology 30 51 Lesson 5 Vertebrates A. The Fishes Almost all varieties of fish have blood vessels leading to a concentration in a gill area where they can be close to moving water. Water movement increases the rates of diffusion by bringing oxygenated water to the gill surfaces and removing water which has taken on carbon and nitrogen wastes. Water moves across the gills when a fish moves or when the fish uses a "gulping" action to suck water into its mouth and then force it out across the gills. Jawless fishes such as lampreys and cartilaginous fishes such as sharks, mantas and rays have anywhere from five to fourteen pairs of individual gill slits along the sides of the anterior ends. The greater number and variety of fresh water and marine bony fishes (Osteichthyes) have a number of gill arches, gill rakers and gill filaments on each side of the head. Unlike the two other classes of fish, bony fish have their gill structures grouped close together with only one gill slit and protective covering (operculum) on each side. Biology 30 52 Lesson 5 B. Amphibians Frogs, toads, salamanders and other amphibians may have up to three different structures or organs associated with respiration or gas exchange. 1. A thin, moist skin allows direct diffusion between the skin cells and capillaries and the surrounding air or water. Reduced respiration requirements for hibernating or estivating amphibians enable many of them to survive by means of diffusion through the skin alone, even while they remain completely submerged. 2. External or internal gills are common in the life cycles of many amphibians. In some varieties, gills remain throughout their life cycles. mud puppy 3. The third area where respiration takes place in some amphibians is in a pair of internal lungs connected by gullet and nostrils to the exterior. Thin-walled and lined with many blood vessels, the lungs also have many internal infoldings which increase the area through which diffusion occurs. Biology 30 53 Lesson 5 C. Reptiles Many of the reptilian groups have adapted to a land existence. To do so required a development to reduce or prevent the loss of water through the skin. This came about with the formation of a thick and largely impermeable skin covered with scales. This characteristic eliminated one of the means of respiration shown by groups before them – that of diffusion through the skin. After embryonic development and hatching, gills were also lacking. The only major difference between amphibian and reptilian lungs is that the lungs of the latter group have many more internal infoldings. These greatly increase the surface area across which diffusion can occur and makes up for the loss of gill or skin respiration. Minor adaptations may develop, as in some snakes, where one lung almost disappears while the other greatly enlarges or lengthens. D. Birds Birds expend a great deal of energy in sustaining certain body actions, particularly those involving flight. The large energy requirements lead to high rates of cellular respiration and demands for large volumes of air. The respiratory system of a bird consists of a cartilage-strengthened trachea leading to an enlarged structure called the syrinx ("voicebox"). From here the air tube splits into two smaller ones called bronchi. Biology 30 54 Lesson 5 Each bronchus makes its way into a lung. A unique feature about the birds' respiratory systems is the presence of numerous air sacs which extend out from the lungs and into various body cavities. Some of these even extend into some of the larger bones. Each bronchus extends through the lung and to the posterior air sac. Along the way, tubes branch off into the air sacs and the lung itself. Expansion of the thoracic cavity, especially on the upbeat of a bird's wings, draws air into and through the lungs to the air sacs. Some diffusion occurs as the air passes through the tubes in the lungs. Very little, if any, diffusion occurs in the air sacs. Muscle contractions then force much of the fresh air back from the air sacs and through many capillary-lined tubules in the lungs before final expulsion. The presence of the air sacs thus enables fresh air to pass through the lungs on both inhalation and exhalation. Air sacs are also of value since their presence reduces the weight of birds and forms storage areas for air when birds fly at oxygen-scarce, high altitudes. E. Mammals A number of major differences exist between the respiratory systems of birds and those of mammals. 1. The first involves the presence in mammals of a flat sheet of muscle which divides the body cavity into two. The upper or anterior thoracic cavity contains the heart and lungs. The cavity below or posterior to the muscle sheet is the abdominal cavity containing the liver, stomach, pancreas, intestines and organs of the urogenital system. The separating muscle sheet or diaphragm is important as part of the actual breathing process. Biology 30 55 Lesson 5 2. Missing from mammalian systems are the air sacs extending out from the lungs which are common to birds. 3. Another difference is that mammals have the bronchi dividing into smaller tubes which eventually end up in small, "deadend" air pockets called alveoli. In birds, the tubules are continuous in the lungs so that air could keep moving straight through while diffusion is occurring. The mammalian system is less efficient in that exhalation forces air that is "spent" or no longer fresh back the way it came in. How Breathing Happens The inhalation process of breathing in mammals mainly involves expansion of the thoracic cavity. Outward movement of the rib cage is caused by the contraction of muscles attached to the sides of the ribs and extending towards the back. As this is happening, the diaphragm muscle also contracts, drawing it downward or away from the lungs. Abdominal muscles may relax somewhat as the diaphragm pushes against the visceral organs. These actions result in an increase in the size of the thoracic cavity and reduced pressure immediately around the lungs, causing them to expand and to have air move in. Biology 30 56 Lesson 5 Lung expansion is also partly due to the presence of a double, pleural membrane. This membrane lines the inner surface of the thorax and the outer surface of the lungs. Moisture and mucus between the thoracic and lung membrane linings cause them to stick or to adhere to one another, although they can slide over each other. Therefore, when the thorax becomes larger or moves outward, it pulls the lung surface with it. Diseases, hard blows or other accidents to the thorax could cause a separation between the membranes resulting in a collapsed lung which can no longer function effectively. A collapsed lung is sometimes brought about deliberately in humans by medical means to give it a better opportunity to heal from some injury or disease. Gradual absorption of air from between the two membranes by the body will usually bring the outer surface of the lung and the inner thoracic surface into contact once more. In normal exhalation, relaxation of the outer rib muscles allows the thorax to move back (inward). Relaxation of the diaphragm allows it to resume its concave shape towards the thoracic cavity, also reducing its volume. The natural elasticity of the lungs then causes those organs to shrink, forcing out the air. An organism may consciously force more air out by contracting another set of rib muscles drawing the chest or rib area in even farther. At the same time, contraction of abdominal muscles pushes the viscera against the diaphragm moving it farther into the thoracic cavity. Biology 30 57 Lesson 5 Air enters and leaves a body through the nostrils or mouth. When it enters, it moves to the back of the throat area or pharynx. At this point there is an opening called the glottis which leads into a cartilage-strengthened trachea. In most mammals, a relatively prominent larynx or voicebox (containing vocal cords) is at the beginning of the trachea. A cartilage structure, the epiglottis, serves to cover the tracheal opening or glottis when an organism swallows. This prevents saliva or food from entering the respiratory system to cause choking, although there are occasions when the epiglottis may not close in time. Air moves along the trachea, which splits into bronchi leading to the right and left lungs. Inside the lungs, the bronchi split into smaller branches called bronchioles. The bronchi and larger bronchioles continue to be strengthened by horseshoe-shaped rings of cartilage. Bronchioles finally end in clusters of tiny, thin-walled sacs. It is through the walls of these alveoli and surrounding capillaries that diffusion of gases occurs (see diagram above). Exhalation simply reverses the pathway of air movement. In normal breathing, an average human adult inhales and exhales approximately 500 ml of air. Forced, deep breathing can increase this nine to ten times. (This is called vital capacity.) The total surface area of all the alveoli is considered to be about 70-90 square meters (750-975 square feet), which means that the area over which diffusion can occur is quite large. (This is the size of a small house). Biology 30 58 Lesson 5 The rate of breathing appears to be controlled by an area of the brain called the medulla oblongata. This area is sensitive to the amount of carbon dioxide carried in the blood. Blood leaving the capillaries surrounding the alveoli carries the same carbon dioxide concentration as the air in those alveoli. If the medulla senses a carbon dioxide concentration above a certain level in the blood, it sends impulses to the diaphragm and rib muscles to speed up breathing. A higher carbon dioxide concentration also causes the walls of the bronchioles to expand. This dilation of the air pathways allows a greater volume of air to move through them. As the carbon dioxide levels fall, the medulla's impulses and the rate of breathing slow down as well. In this way, the carbon dioxide levels in the blood and the medulla oblongata that senses them act as homeostatic devices to control breathing rates. At high altitudes where oxygen is scarce, the brain control could be fooled because the levels of carbon dioxide produced would be low as well. Breathing would not speed up even though the body is not receiving enough oxygen. A back-up control exists in receptors in the aorta and carotid arteries. These can detect lower oxygen levels in the blood causing them to send impulses to the medulla, which in turn speeds up breathing. Some Respiratory Disorders Before concluding mammalian respiration, brief mention will be made of some respiratory disorders that can affect humans. As mentioned earlier, the total diffusion area of human lungs could be up to 90 square meters. Any kind of situations or disorders that reduce this area could create a condition of oxygen starvation where not enough oxygen is diffusing into the blood system. Pneumonia infections lead to accumulations of lymph and mucus in alveoli and bronchioles that prevent air from reaching the walls of the alveoli. Emphysema is a condition that can develop slowly over a number of years. Emphysema is caused almost always by cigarette smoking. Air pollution may also play a role. An inherited form is rare. Emphysema is a Greek word meaning ‘inflation’. Emphysema is the over-inflammation and eventual destruction of the tiny air sacs, the alveoli in the lungs. The elasticity of the alveoli and walls of the airways is destroyed and air becomes trapped within the lungs. The transfer of oxygen from the air to the blood stream is affected. A person affected by Emphysema becomes very short of breath. Extra strain is placed on the heart. Biology 30 59 Lesson 5 Chronic bronchitis is also a condition that may develop from smoking. Healthy air passages are lined with (epithelial) cells possessing hair-like cilia. These cells secrete mucus that is swept along by the waving action of cilia. Mucus and cilia are designed to trap and then remove particles from the breathing passages. Tars from smoke can settle around the cilia and slow down their actions, leading to irritation of the epithelial cells. These cells swell and are even further irritated by accumulations of mucus. This can lead to extra coughing and clearing of the throat as one attempts to clear the mucus and any other irritants from the air passages. Smoking increases the incidence of cancer, where abnormal lung cells develop and interfere with diffusion and blood circulation. Such cancerous cells are initially hard to detect and are easily spread by the circulatory system from the lungs to other body areas. Summary All autotrophs and heterotrophs are constantly involved in the process of obtaining nutrients for their cells and bodies. These nutrients are the sources of matter for building and repairing cells. They are also the sources of energy for carrying out all the necessary life-sustaining activities. For most green plants, the nutrients are of an inorganic nature and are obtained in soluble form through cell membranes. Heterotrophic organisms also take in inorganic nutrients (water, dissolved minerals...) but a large amount is organic – from plant and animal bodies. Manners of actually obtaining nutrients vary greatly. Once nutrients, especially of an organic nature, are taken in or ingested, the processing and general outcomes are fairly similar among many species. The end result is to change them into forms which are soluble and readily usable. Synthesizing or breaking down organic compounds requires particular gases while releasing others. Respiration, or the exchange of gases at the cell membrane level, in all organisms occurs by the process of diffusion. Differences occur in the manners of trying to regulate concentrations of gases near membranes. As body sizes and complexities increase, especially among land animals, greater specializations appear in order to increase volumes and speeds of gas movements. Physical movement or "breathing" becomes apparent in larger animals and this is one of the major distinctions between animal and plant respiration. Along with breathing, animal orders also develop special structures or organs where gases can be rapidly exchanged with circulatory systems which transport those gases throughout the bodies. Gills, tracheae, book lungs and lungs make their appearances in various groups as adaptations to certain environments. The importance of respiration to organisms is often emphasized by conditions that slow or stop this action. Fish, crayfish or water plants soon die if left on land. Land plants and animals "drown" in water. Diseases or injuries which affect breathing passages or organs could impair the general health and normal actions of organisms involved and could also cause death. Biology 30 60 Lesson 5