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INTRODUCTION From simple viruses and single-celled organisms to the most complex ecosystems, biology is the study of living things. The diversity of nature cannot be summed up in a short chapter in this handbook and so this chapter will explore just a few of the many interesting topics among the vast range of biological sciences – the very small (microbiology), the multicellular organism level (comparative anatomy of the respiratory and digestive systems) and the interactions among different organisms and their environments (ecology). MICROBIOLOGY Microbiology is the study of organisms that are so small that you can’t see them without a microscope. In this section, we will learn about bacteria, viruses and prions, which are all too small to see with the naked eye! Bacteria Did you know that the number of bacteria living in your mouth is more than the number of people who have ever lived? Bacteria have been around for a long, long time. In fact, the earliest fossils of bacteria are over 3.5 billion years old!! Having been around for so long, bacteria have had the opportunity to evolve into a wide variety of different types, adapting to a variety of different environments (including living inside your mouth!). Bacteria are single-celled organisms – unlike humans, who are made up of trillions of cells, each bacterium is just one single cell. Bacteria can be classified by their shape, as well as by something called Gram staining. 1 Table 1: Bacteria come in many different shapes! Bacteria Description Example Shape Cocci Sphere Figure 1: Staphylococcus epidermidis lives in human and other animal skin and does not commonly cause disease. Bacilli Rod Figure 2: Escherichia coli (often abbreviated as E. coli) lives in the lower intestines of birds and mammals. Some strains of E. coli can cause serious disease and even death (e.g., a strain called E. coli O157:H7) Spirilla Helix Figure 3: Campylobacter sp. can cause food-borne illnesses (Source: Agriculture & Agri-Food Canada) Vibrios Comma Shaped Figure 4: Vibrio vulnificus can cause infection after eating seafood and is closely related to Vibrio cholerae, the bacteria that causes cholera. 2 Gram staining is named for Hans Christian Gram who, in 1884, invented a technique that is still used today to identify bacteria. Using this special staining technique, bacteria can be classified as: Gram positive: cells that appear blue/purple when Gram stained; these cells possess a second cell wall that contains lipid Gram negative: cells that appear pink/red when Gram stained; these cells have a single cell membrane (a cell membrane is the structure that surrounds a cell, holding in all the contents in) that contains a lot of peptidoglycan (a large molecule that surrounds the cell membrane of the bacteria, giving structure and strength to the cell wall) Figure 5: Gram positive staining. The purple rods in this figure are Bacillus anthracis, which are Gram positive (the other cells in this photo are white blood cells). Bacillus anthracis is relatively common in some animals, and can cause anthrax, a serious disease, in humans. Diseases that can be spread from animals to humans are called zoonotic diseases. Figure 6: Gram negative staining. The red cocci shown here are Neisseriae sp., which are Gram negative. Most species within the Neisseriae family are nonpathogenic (i.e., they don’t cause disease) and reside in the upper respiratory tract. There are four different ways that bacteria can obtain nutrients and energy: 1. photoautotrophs: use light as an energy source and carbon dioxide as their carbon source to undergo photosynthesis 2. photoheterotrophs: use light as their source of energy and use organic compounds produced by other organisms as their source of carbon 3 3. chemoautotrophs: obtain energy through chemical reactions of inorganic (chemicals that aren’t carbon-based) substances (such as ammonium and hydrogen sulphide) and use that energy to fix carbon dioxide in a process similar to photosynthesis 4. chemoheterotrophs: obtain their energy and carbon from organic compounds (i.e., chemicals that are carbon-based) (just like you do when you eat food!) “Good” Bacteria and “Bad” Bacteria We often think of bacteria as being “bad” because many bacteria are pathogenic (i.e., they cause disease). Examples of pathogenic bacteria include: Clostridium tetani causes tetanus Salmonella typhi causes typhoid fever Mycobacterium tuberculosis causes tuberculosis Streptococcus pyogenes can cause strep throat Bacillus anthracis causes anthrax A number of bacteria including Vibrio vulnificus, Clostridium perfringens, Bacteroides fragilis can cause necrotizing fasciitis (more commoly known as flesh eating disease) Escherichia coli O157:H7 can cause very serious food-borne illness and has even resulted in death (Does anyone remember what happened in Walkerton, Ontario just a few years ago???) But did you know that many bacteria are very helpful? For example, we have harmless bacteria that live in our digestive tract and these bacteria help to prevent harmful microbes from growing there and making us sick. Bacteria are also used to make fermented foods, such as vinegar, sauerkraut and yogurt. Biotechnologists even use bacteria to clean up toxic spills and to produce therapeutic drugs like insulin. Viruses Viruses have a significant impact on humans. Diseases from the common cold to severe acute respiratory syndrome (SARS) and Acquired Immunodeficiency Syndrome (AIDS) are products of viral infection. Scientists have worked tirelessly for centuries to understand how these microorganisms work and to find better ways of combating them. 4 Viruses are very small, generally from 17 to 400 nanometres (a nanometre is 1/1000th of 1/1000th of a millimetre, or “very very tiny”) (define) in diameter, approximately 1000 times smaller than the diameter of a human hair, and are diverse in shape and complexity. Pictures of viruses resemble something out of science fiction, with polygonal (i.e., strange, many-sided shaped) heads and little jointed "legs" attached to tails, while others look like round popcorn. Figure 7: Viruses come in many shapes. Adenoviruses can cause diseases in humans and animals. Bacteriophage infect bacteria. Human immunodeficiency virus (HIV) causes a disease called Acquired Immunodeficiency Syndrome (AIDS) in humans. Figure 8: An electron micrograph of T4 bacteriophage. Viruses are composed of: Nucleic acid: a set of genetic material packaged in a protein shell, either DNA or RNA. Capsid: a structure that surrounds the DNA or RNA to protect it. Lipid membrane: a structure that surrounds the capsid (found only in some viruses, including influenza; these types of viruses are called enveloped viruses as opposed to naked viruses, which don’t have lipid membranes). 5 Figure 9: A drawing of the human immunodeficiency virus (HIV) showing its structure. Note how the capsid encases the nucleic acid, and the lipid membrane encases the capsid. Viral life cycle Believe it or not, viruses are not ‘alive’. They lack both metabolism and the ability to reproduce (i.e., produce more viruses) independently of another species. (Metabolism refers to all the chemical processes that occur in a living cell or organism that are needed for life). So, a virus must have a host cell (i.e., bacteria, plant or animal) in which to live and make more viruses. Regardless of the type of host cell, all viruses follow the same basic steps known as the lytic cycle (Figure 10): 1. The virus attaches to a host cell. Viruses have some type of protein on the outside of the capsid or envelope that "recognize" the proper host cells. 2. The virus injects its genetic material into the cell. 3. The viruses ‘hijack’ the cell’s replication machinery (i.e., the parts of the cell that allows it to make new cells) to produce multiple viral progeny. 4. The new viruses are assembled into the host 5. The viruses are released in the environment. 6 Figure 10: Lytic cycle of the viruses. (Adapted from http://science.howstuffworks.com/virus-human.htm). Once inside the host cell, some viruses do not reproduce right away. Instead, they combine their genetic material into the host cell's genetic material (i.e., so the cell carries it own DNA along with the virus’s genetic information). When the host cell reproduces, it also copies the viral genetic material along with its own (so each new cell made has the host DNA + the virus’s genetic material). This happens without any new viruses being produced until the cell receives a special signal (e.g., a signal from the environment) that triggers the production of viruses. The viral genetic instructions will then take over the host's machinery and make new viruses as described above. This cycle, called the lysogenic cycle, is shown in the figure below (Figure 11). Marine Viruses: Who said viruses were bad? Viruses are usually considered as bad. However, what we are only beginning to realize is that viruses may play an absolutely critical role in the world ecosystem, including the world ocean. Believe it or not, viruses are the most abundant group of organisms in the oceans! In every millilitre of seawater there 10-100 million viruses. Viruses from the world ocean are not a stable entity; they are sensitive to a variety of environmental stresses, such as ultraviolet radiation (like from the sun), which lead to their inactivation and destruction. They can only survive for a few hours to a few days. To maintain the abundance of the viral population, viruses must replicate and, in doing so, continually destroy many of their natural hosts, primarily bacteria and phytoplankton (plankton are tiny organisms that drift through the ocean; phytoplankton are plankton that use photosynthesis). Oceanographers believe that on average, one billion viruses per litre are produced and destroyed every few days! This results in the death of half of the ocean’s bacteria and phytoplankton every few days! 7 Figure 11: In the lysogenic cycle, the virus reproduces by first injecting its genetic material, indicated by the red line, into the host cell's genetic instructions (Adapted from http://science.howstuffworks.com/virus-human.htm) The presence of viruses in the oceans can also influence the genetic diversity and population sizes of the plankton community in many ways. Perhaps most importantly, they can have the effect of maintaining species diversity in the ocean environment by "killing the winner". What this means is that as the concentration of a particular planktonic host increases, the viruses prone to infecting this species will rapidly propagate throughout the host population, killing much of the 8 population. This prevents a single species from dominating and permits the coexistence of species. For example, blue, green, and red bacteria co-habit in the same environment. Blue bacteria however, have a greater ability to use nutrients than the others. Therefore, the concentration of the blue bacteria would increase until the point at which it dominates the environment. This could not only have the result of reducing the size of the other two bacterial species, but may actually result in their extinction. However, the presence of the virus helps to regulate the proportion of blue, red and green bacteria in the environment, and keeps the ‘loser’ in the race. Prions Short for proteinaceous infectious particle, prions are only made of protein (so, unlike bacteria and viruses, they do not contain any genetic (DNA or RNA) information). Prions are similar to normal proteins found in the host organism, but they are abnormally shaped and they can take the normal proteins in the host cell and cause them to take on the abnormal shape, just like the prion. As prions have only recently been discovered by scientists, it is still unclear just how prions cause these diseases or how someone cathc these diseases. Figure 12: How prions make new prions. 9 Prions cause a class of diseases called the transmissible spongiform encephalopathy diseases (TSEs). “Encephalopathy” means a diease of the brain and “spongiform” refers to the fact that the brain tissue ends up with large holes in it, resembling a sponge. : Examples of TSEs include: Figure 13: A tissue section taken from the brain of a cow affected with BSE. Note the vacuoles (small holes in the tissue which gives the tissue a spongelike appearance). (Photo by Dr. Al Jenny. Source: http://www.aphis.usda.gov/lpa/issues/ bse/bse_photogallery.html) scrapie (occurs in sheep) kuru (occurred in the Foré tribe in Papua New Guinea, who passed on the disease through canibalism) bovine spongiform encephalopathy (BSE or mad cow disease; occurs in cows) Creutzfeldt-Jakob disease (occurs in humans) variant Creutzfeldt-Jakob Disease (vCJD) (occurs in humans; is thought to be caused by consuming prions from cows with BSE There are no treatments avaialble for TSEs and they can be fatal. ANIMAL ANATOMY Multicellular organisms, as the name suggests, are made of more than one (and often many millions) cell. These cells are organised into tissues (a group of cells that perform a particular function in an organism; e.g., connective tissue, muscle tissue, bone, nervous tissue), those tissues are organized into organs (a group of tissues that perform a particular function) and those organs are organized into organ systems (a group of organs that work together to perform a particular function, e.g., cardiovascular system, reproductive system, skeletal system). There are many different organ systems in animals and we will be looking at two of them – the respiratory system and the digestive system. 10 The Respiratory System: A Comparative Approach All animals need oxygen from the environment to help produce the fuel for the different processes of living (e.g., running, learning, digesting). During the production of the body’s fuel, carbon dioxide is produced as a waste product as oxygen is used up. Therefore, animals must always obtain fresh oxygen and get rid of carbon dioxide for the processes to be maintained. This is the primary purpose of respiratory system: to release carbon dioxide and absorb oxygen. In this section we are going to take a comparative approach to looking at the respiratory system, which means that we are going to look at the strategies that different animals use to breathe in different environments. We are going to start by looking at the basic building blocks of the respiratory system for mammals. Respiratory system building blocks The mammalian respiratory system begins at the trachea, a stiff tube reinforced by cartilage rings, which splits into two bronchi (singular: bronchus). You can feel the cartilage rings in your trachea. Run your fingers along your throat. Can you feel the ridges? Those ridges are the cartilage rings! The bronchiole bronchus branches many times to form smaller tubes called bronchioles. These bronchioles trachea bronchi form a complex network of lungs millions of little tubes that alveoli lead to sacs called alveoli thoracic cavity (singular: alveolus). This complex network forms the ribs lungs, which are protected by diaphragm the rib cage in the thoracic cavity. A large dome-shaped muscle called the diaphragm Figure 14: Mammaliam respiratory organs. separates the thoracic cavity Adapted from Randall et al. (1997) from the abdominal cavity where the stomach and intestines are found. 11 The process by which air moves in and out of the lungs is called ventilation. The main muscle responsible for ventilation is the diaphragm (has your music teacher ever told you to breath from your diaphragm??). Other muscles that assist with ventilation are called the intercostal muscles and they are found between the ribs. When the diaphragm contracts, it changes its shape from “dome-like” to flat. The change in shape increases the space in the thoracic cavity that causes air to rush in via the trachea (through the mouth or nose) to fill the lungs. This phase of ventilation is inhalation. When the diaphragm is flat, it pushes all the organs in the abdomen down so that the stomach sticks out. You can try this yourself by taking a deep breath and imagining you are filling your lungs completely full. If you put your hands on your stomach, you can see how far it moves out and in during every breath. The second phase of ventilation is the relaxation of the diaphragm that causes it to return to its dome shape. The relaxation causes the muscle to move up and this movement plus contraction of the intercostal muscles causes the space in the thoracic cavity to get smaller so air is forced out of the lungs via the trachea (via the mouth or nose). This phase is called exhalation. This type of ventilation, where air leaves the lungs by the same way it entered, is called tidal ventilation (it moves in and out, like the tides) and is not used by all animals. Gas exchange Ventilation is the means by which oxygen is brought into the body from the environment and carbon dioxide is expelled. The blood is the primary method of transportation within the body, so in the lungs there are lots of tiny blood vessels, called capillaries, located in the alveoli. Both oxygen and carbon dioxide are gases that dissolve LUNGS into our blood and cells so that they can be transported around the body. Blood carries oxygen from lungs to cells Blood carries carbon dioxide from the cells to the lungs CELLS Figure 15: Path that oxygen and carbon dioxide follow in the body 1 First, let’s trace the path a molecule of oxygen takes from the environment. When an oxygen molecule is inhaled, it moves in through the mouth or nose, passes through the trachea, bronchi and bronchioles and into an alveolus. In an alveolus, the oxygen is dissolved into the fluid surface lining of the alveolus and diffuses1 into the blood in the Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. 12 capillary. In the lungs, there is a high concentration of oxygen, so the oxygen moves into the blood where there is a low concentration of oxygen. In the blood, a special molecule called haemoglobin, which carries the oxygen molecule until it reaches its destination, picks up the oxygen. When the oxygen molecules reaches the cell where it is going to be used, the haemoglobin molecule releases the oxygen molecules and it diffuses into the cell where there is a low concentration of oxygen. The cell uses the oxygen and produces carbon dioxide as a waste product. Let us now follow the carbon dioxide molecule as it leaves the body. Because there is a higher concentration of carbon dioxide in the cell compared to the blood, the carbon dioxide diffuses into the blood. The carbon dioxide dissolves into the blood and is carried to the lungs. The lungs have a low concentration of carbon dioxide compared to the blood, so carbon dioxide diffuses into the lungs. During exhalation, the carbon dioxide molecules leave the body by passing out of the alveolus, bronchioles, bronchi, and trachea to the environment via the nose or mouth. This process of absorbing oxygen and releasing carbon dioxide is called gas exchange. Gas exchange remains basically the same for all animals. Interesting air breathers This section has so far described the typical respiratory system of mammals (one group of animals). We are now going to look at the interesting differences between other animals that breathe air and mammals. Birds Mammals are the only animals that have muscular diaphragms which means that birds, reptiles and frog have to inflate their lungs using different methods. The lungs of birds are organized very differently. Look at the diagram very carefully. You can see that the lungs of the bird are surrounded by air air entering lung air leaving lung Front air sacs Lung to mouth Rear air sacs Figure 16: Bird respiratory organs. Blue arrows show fresh entering the lung and red arrows show air leaving. Adapted from Randall et al. (1997) 13 sacs, which collect the air as it moves through the respiratory system but do not exchange oxygen or carbon dioxide. The gas exchange takes place as the air is pushed through the lung. Birds do not use tidal ventilation like mammals, reptiles and frogs. Their ventilation is unidirectional (one direction) so that fresh air being inhaled never mixes with the old air being exhaled. During inhalation, intercostal muscles move the rib cage to make the rear air sacs larger, so air moves through the trachea to the rear air sacs. At the same time, air in the lung left from the previous breath moves into the front air sacs. During exhalation, rear air sacs are made smaller by the movement of the rib cage and the air in the rear air sacs moves into the lungs while the air in the front air sacs leaves the bird via the trachea. This system is designed so that the bird absorbs more oxygen out of the air than a mammal can because there is fresh air in the lung during both inhalation and exhalation. The air sacs are also advantageous because they make the bird less dense, which makes it easier to fly. Reptiles The respiratory system of reptiles is very similar to mammals except that reptiles do not have a diaphragm, therefore they evolved different ways of inflating their lungs. Different reptiles have different strategies. Lizards use their intercostal muscles to rotate the rib cage in a way that makes the thoracic cavity larger so that air moves into the lungs. Crocodiles have a large muscle that connects their pelvic (hip) bones to their liver. This muscle is called the diaphragmaticus and when it contracts, it pulls the liver toward their tail. This movement causes the Lung TA thoracic cavity to get bigger and so air moves into the lungs. When the diaphragmaticus relaxes and the OA intercostal muscles contract, the liver moves back into its place and pushes up on the lungs, forcing air out of the Figure 17: Arrangement of the transverse crocodiles. abdominis (TA) and oblique abdominis (OA), the ventilatory muscles in turtles. Adapted from Landberg et al. (2003). 14 Turtles are particularly challenged because their rib cage is fused to the shell, so instead they use limb movement to help inflate and deflate their lungs. They also have a large muscle called the transverse abdominis (TA) that wraps around the rear portion of the lungs. The TA contracts, resulting in exhalation by pushing up on the lungs and forcing the air out. The oblique abdominis (OA) muscles located at the back of the bottom shell cause inhalation when they contract because they flatten and make the space in the body cavity larger and air moves into the lungs. Reptile ventilation is different from mammals since the ventilation cycle starts with exhalation (exhale, inhale, hold breath, repeat) instead of inhalation (inhale, exhale, pause with lung empty, repeat). Frogs Frogs have two compartments where air moves in their respiratory system and the airflow between these two compartments is controlled by valves, which are flaps of tissues controlled by muscles to be opened or closed. The first set of valves is the nares, which are the paired openings on the nose of the frog. The nares lead to the buccal (or mouth) cavity. The buccal cavity and lungs are separated by another valve called the glottis. nares buccal cavity glottis lung Figure 18: The respiratory organs of frogs. Adapted from Randall et al. (1997). Ventilation in frogs occurs in this sequence: the lung is full and when the glottis opens, the air in the lungs moves into the buccal cavity and through the nares into the environment. This is exhalation (frogs also start with exhalation). To inhale, the glottis is closed and the nares are opened causing air to flow into the buccal cavity. Then the nares close and the glottis opens and the air moves into the lungs. The lungs are now full and the cycle starts again. Breathing water Since all animals have evolved from aquatic animals, all animals once breathed water. The organization of the respiratory system in fish is very different than for terrestrial animals, however all the important functions must be maintained. Study Table 2 to see which organs in fish perform the same functions as the organs in mammals. 15 Table 2: Comparison of respiratory organs between mammals and fish. Mammalian organ 1. Trachea Function Fish organ Passageway for air/water Mouth & pharynx 2. Lungs i) Bronchi – bronchioles ii) Alveoli Gas exchange organ Support for gas exchange surface Actual gas exchange site Gill arches Gill filaments Gill lamellae 3. Location of the gas exchange organ Operculum Inhalation and exhalation Muscles in mouth and operculum Thoracic cavity 4. Diaphragm & intercostal muscles A Operculum Gill arches B Mouth Pharynx Operculum Gill filaments View from top of head Figure 19: A. The location of the gill arches in the head of a fish. B. The fish respiratory system. The blue arrows show the flow of water. (Adapted from Randall et al., 1997). The fish gas exchange organs are the gills, therefore the fish must push water across the gill arches so that oxygen can be absorbed from the water by the gill lamellae. To create water flow over the gills, the mouth and pharynx expand so the space increases and water is sucked in. Muscles in the operculum, the bony covering over the gills behind the eye, move it in and out so water passes over the gill filaments and gill lamellae and out the operculum into the environment. Ventilation in fish is unidirectional. Fish that are very active (like sharks) use a type of ventilation called ram ventilation by swimming with their mouths open. The forward motion of their body forces water to move over the gill arches so that oxygen and carbon dioxide can be exchanged. 16 The Digestive System: A Comparative Approach All animals need nutrients from their environment; nutrients provide energy and material to build new cells and tissues, and vitamins and minerals that play many roles in body. Different types of animals have different ways of obtaining nutrients and in this section we are going to explore some of those ways! Flatworms posses a simple gastrovascular cavity (Figure 19), which has one opening through which both food enters and waste exits. Flatworms take food in through their mouth by contracting the muscles in the upper end of their gut to suck food in during feeding. Their gut is branched and this allows food to be digested and absorbed and then waste is ejected through the same opening as food was initially taken in. Figure 20: Diagram of a flatworm Tapeworms (Figure 20) don’t have a digestive system at all! Since tapeworms live inside the digestive tract of other animals, they just wait until the host animal digests the food and then they can absorb the nutrients directly. Figure 20: A tapeworm Many animals, including humans, have a tubular gut, with a mouth (for taking food in) at one end and an anus (for excreting wastes) at the other end. Different sections of the tubular gut play different roles in the digestive and absorptive process. Parts of the human digestive tract include: Buccal cavity (or mouth): the opening of the digestive tract where food enters; food is broken up here by the teeth Esophagus: a tube through which the food Figure 21: The human digestive tract. 17 passes from the mouth to the stomach. Muscular contractions (called peristalsis) push the food through the esophagus. Stomach: a muscular organ where food is stored and digested by churning (physical digestion) and by hydrochloric acid and the enzyme pepsin (chemical digestion) before it is passed along to the rest of the digestive tract Intestine: a long tube through which the food passes and where further digestion, as well as absorption of nutrients, occurs o The small intestine: it is called the “small” intestine because its diameter is smaller than that of the large intestine. The small intestine is actually quite long and contains three sections Duodenum: further digestion of food occurs here Jejunum: absorption of nutrients occurs here Ileum: also functions in absorption (especially the absorption of vitamin B12 and of bile salts) o The large intestine: has a larger diameter than the small intestine and consists of three parts: Caecum: a pouch at the beginning of the large intestine which is attached to the ileum; there are a number of bacteria present in the caecum which serve to breakdown fibre, which cannot be broken down by human enzymes Colon: absorption of water takes place here Rectum: stores fecal waste before excretion Anus: the opening at the end of the digestive tract through which fecal waste is excreted. Humans are known as monogastric animals because they only have one (“mono”) stomach. Ruminants Ruminants are animals that digest their food in two steps: 1. eating food and regurgitating (i.e., bringing the food back up from the gut into the mouth) it in a partially digested form (known as the cud) 2. chewing and re-swallowing the cud (this process is called rumination) Examples of ruminants include cows, sheep, goats, llamas, camels and giraffes. 18 The ruminant stomach is made up of four compartments: 1. rumen: the first and largest chamber of the stomach; contains microorganisms (like “good” bacteria) which digest cellulose (a fibre found in many plants which mammals do not have the necessary enzymes to digest on their own) 2. reticulum: the second chamber of the stomach; like the rumen, contains microorganisms which Figure 22: The stomach of a ruminant digest cellulose animal consists of four chambers: the 3. omasum: the next chamber of the rumen, the reticulum, the omasum and stomach; water is absorbed from the abomuasum. the partially digested food mixture here 4. abomasums: this compartment is analogous to the stomach of monogastric animals (i.e., it secretes hydrochloride acid and digestive enzymes) and is sometimes referred to as the glandular stomach. The ruminant stomach is quite large, taking up 3/4ths of the abdominal cavity! The way that ruminant digestion works is as follows: 1. food is mixed with saliva and enters the rumen 2. in both the rumen and the reticulum the food/saliva mixture separates into layers of solids and liquids 3. the solid layer clumps together to form the cud, which is regurgitated and chewed – this increases the surface area of the food so that the Figure 23: The digestive tract of a ruminant microorganisms can gain animal. access to more of the food in order to digest it once it is returned to the stomach 19 4. once the food has been sufficiently digested, it passes into the omasum, where the water is absorbed, resulting in a more concentrated mixture 5. the food passes into the abomasums, where digestion similar to that which occurs in monogastric animals takes place 6. the food then passes into the intestine where absorption occurs just as it does in monogastric animals Birds Unlike mammals, birds do not have teeth, so they cannot masticate (chew) their food. The physical breakdown of their food is accomplished by the beak and the gizzard (a muscular compartment that contains small stones; muscular contraction of the gizzard grinds the food together with the stones, resulting in the physical breakdown of the food). The gizzard (sometimes referred to as the muscular stomach) is found after the stomach (sometimes referred to as the glandular stomach or proventriculus) and the food is passed back and forth between the glandular stomach and the gizzard, resulting in a repeated cycle of physical and chemical digestion. Figure 24: The digestive tract of a bird. In addition to the stomach and gizzard, most (but not all) birds have a crop (depending on the species, the crop can be either a widening of the esophagus, or 1 or 2 esophageal pouches). The crop can store food before it enters the stomach. The small and large intestines of birds are similar to those of mammals. At the end of the digestive tract is the cloaca (a tubular structure that serves as a shared opening for the digestive, reproductive and urinary systems). 20 ECOLOGY Ecology is the study of interactions of organisms with their environment – including both the physical environment and other organisms in that environment. The interactions between two different organisms can result in benefit, harm or no effect on each of the organisms. Different types of interactions that organisms can have with one another are presented in Table 3. Table 3: Different types of ecological interactions Effect on Organism #2 Benefit Effect on Organism #1 Harm No Effect Benefit Harm No Effect Mutalism Predation or parasitism Commensalism Competition Amensalism Amensalism -- Predation or parasitism Commensalism In the situation where one organism benefits by harming another organism, we have a case of either a predator-prey relationship (e.g., when a lion eats a gazelle) or host-parasite relationship (e.g., when a tapeworm lives inside the digestive tract of a human, “stealing” the nutrients from the person). Predators are organisms that feed on other organisms, whereas parasites are a special type of predator that live on the inside (endoparasites) or the outside (ectoparasites) of the host’s body. A parasite that kills its host is called a parasitoid. When both of the organisms are mutually harmful to one another, we have a case of competition (e.g., two species of insects trying to live off of the same plants, but there are not enough of those plants to feed both species). When both of the organisms benefit from their relationship, it is called mutualism (e.g., when an insect pollinates a flower, the insect benefits by getting food from the flower and the flower benefits because its pollen is spread around, allowing the plant to reproduce). When one organism benefits and the other organism is unaffected, the interaction is called commensalism (e.g., plants that grow on trees causing neither harm nor 21 benefit to the tree. Plants that grown on other plants are called epiphytic plants) and when one organism is harmed and the other organism is unaffected, the interaction is called amensalism (e.g., the Black walnut tree secretes a chemical that often kills neighboring plants. The Black walnut tree does not gain anything by doing this). Ecosystems The term ecosystem refers to the organisms living within a given area + the physical environment in which they live and interact. Scientists who study ecosystems are interested in, among other things, the flow of energy through an ecosystem. To do so, they group organisms by their source of energy, with organisms that share a common source of energy being referred to as a trophic level (see Table 4). Table 4: Trophic levels Trophic Level Source of Energy Examples Primary producers Use photosynthesis to capture Plants, photosynthetic energy from the sun bacteria Herbivores Eating primary producers Cows, rabbits, deer, grasshoppers Primary carnivores Eating herbivores Spiders, wolves Secondary carnivores Eating carnivores Tuna fish, falcons, killer whales Omnivores Eating organisms from the Humans, crabs, robins, other trophic levels (primary bears producers, herbivores and/or carnivores) Detritivores Scavenge dead bodies and Vultures, fungi, worms, waste products of other many bacteria organisms A food chain is a set of linkages representing a linear series in which a primary producer is consumed by a herbivore, which is consumed by a carnivore, etc., as seen in Figure 25. 22 Figure 25: An example of a food chain Tertiary Consumers Secondary Consumers Primary Consumers Primary Producers In reality, food chains are usually interconnected into food webs as seen in Figure 261. Figure 26: An example of a food web (Source: U. S. Department of Agriculture . Adapted from http://soils.usda.gov/sqi/concepts/soil_biology/images/A-3.jpg) 23 References Farmer, C. G. and D. R. Carrier. 2000. Pelvic aspiration in the American alligator (Alligator mississippiensis). J. Exp. Biol. 203: 1679-1687. Landberg, T., Mailhot, J. D. and E. L. Brainerd. 2003. Lung ventilation during treadmill locomotion in a terrestrial turtle, Terrapene carolina. J. Exp. Biol. 206: 3391 – 3404. Randall, D., Burggren, W. and K. French. 1997. Eckert Animal Physiology. W. H. Freeman and Company: New York. Pp 727. Purves WK, Orians GH, Heller HC. LIFE: The Science of Biology, 4th ed. Sinauer Associates, Sunderland, Massachusetts, 1995. Websites http://science.howstuffworks.com/virus-human.htm http://www.path.ox.ac.uk/dg/vwork.html http://www.virology.net/Big_Virology/BVHomePage.html http://science-education.nih.gov/nihHTML/ose/snapshots/multimedia/ritn/prions/prions1.html http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion/index.html 24