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Biology 30 Module 1 Lesson 4 Food and Energy: A Closer Look Copyright: Ministry of Education, Saskatchewan May be reproduced for educational purposes Biology 30 165 Lesson 4 Biology 30 166 Lesson 4 Lesson 4 Food and Energy: A Closer Look Directions for completing the lesson: Text References for suggested reading: Read BSCS Biology 8th edition Photosynthesis Chapter 19 Pages 86; 489-495 Cellular respiration Chapter 4 Pages 86; 377-384 OR Nelson Biology Photosynthesis Chapter 3 Pages 87-90 Cellular Respiration Pages 92-96 ATP Page 84 Study the instructional portion of the lesson. Review the vocabulary list. Do Assignment 4. Biology 30 167 Lesson 4 Vocabulary ADP aerobic respiration anabolism anaerobic respiration ATP autotrophs Calvin Cycle catabolism catalyst chemosynthesis chemotrophs chloroplast Biology 30 coenzyme dark reaction enzyme fermentation grana light dependent reaction light independent reaction metabolism NADP photolysis stroma Thylakoid membranes 168 Lesson 4 Food and Energy: A Closer Look Introduction Previous lessons presented information on matter and energy, including ways in which these could be related to each other. Atomic and molecular structures, different methods of bonding and the formation or breakup of organic compounds have been described in order to show some of the characteristics of substances and actions occurring within living cells. An important part was omitted at the time. That part relates to the actual "capture" and utilization of energy and matter by living organisms. Probably the two most important and complex biological processes, photosynthesis and cellular respiration, are part of this. Even though the term "biological" is used to show relationship to living things, the actions also include principles of Physics and Chemistry. Although many of the general actions and steps are understood, there are still some which scientists have yet to fully explain. In the descriptions of photosynthesis and respiration in this lesson, explanations will be presented as an attempt to bring about a general understanding rather than the full details of all the individual steps. To deprive any living organism of either matter or energy, or both, would eventually result in death after a period of time. Matter, whether inorganic or organic, is necessary for providing the building material of cells and tissues. It usually flows in continuous cycles between the living and nonliving with the help of decomposers. A continuous source of energy is needed to maintain cell and body actions. Unlike matter, energy cannot be recycled and a large part of it is eventually lost from most living systems as heat. Photosynthesis is the single most important link on this planet in making matter and energy available to all organisms. Green plants are able to convert light energy and inorganic substances into organic compounds which they can utilize themselves or pass on to other organisms through food chains. Cellular respiration occurs in both functioning plant and animal cells. Organic compounds are broken down into smaller units, with the energy trapped in the chemical bonds being released for various cell or body uses. Biology 30 169 Lesson 4 After completing this lesson, you should be able to: Biology 30 • describe the general nature and role of ATP in cells. • explain the roles of chlorophyll and light in photosynthesis. • indicate how electrons transform light energy into chemical energy. • describe the light phase or light reactions of photosynthesis. • describe the dark phase or dark reactions of photosynthesis. • list some of the organic compounds which are possible from the products of photosynthesis. • distinguish between photosynthesis and chemosynthesis. • distinguish between respiration and cellular respiration. • describe the first steps common to the different types of respiration. • describe the aerobic stage of cellular respiration. • describe two forms of fermentation: alcohol fermentation and lactic acid fermentation. • summarize the main differences between photosynthesis and cellular respiration. • apply different testing results to help explain some features relating to photosynthesis. • explain possible differences in gas movements in green plants over a 24 hour period. 170 Lesson 4 Energy Storage and Use – ATP The organic compound which is most available as an energy source for cellular use is sugar specifically glucose. Although starch is regarded as a fuel, it is a long chain and is an insoluble (doesn't dissolve) compound. It must be broken down into shorter sugar chains to be usable. Evan a glucose molecule would release too much energy for a specific cell action. The work of enzymes and life sustaining cell actions require energy in smaller "packets" - more suited to the action that can be released as needed. A real life example would be the inconvenience of using a ten-dollar bill to buy a few jelly beans instead of a loonie. Similarly, the large amount of energy in a sugar molecule is inconvenient for a cell to use. A large amount of the energy would be given off - "wasted" as heat. Organisms and cells solve the energy problem by having special compounds in which chemical (bond) energy is stored in smaller, easily available amounts that can be released as needed. Of these compounds the most common and probably best known is adenosine triphosphate or ATP. ATP may be likened to a number of different things in the manner in which it functions. In the case of money, it could be like a small change purse ready with its smaller handouts of coins; in terms of electrical energy, it could be like a battery for a light source or for an engine. In such situations, change purses and batteries cannot function indefinitely without being “re-charged” from time to time. ATP and similar energy carriers operate on somewhat the same principle. ATP can store energy when there is an excess of it and then release it as needed. Then it needs to be “recharged”. ATP is made up of: a nitrogen base (adenine), a five-carbon sugar (ribose), three phosphate groups. Identify each in the diagram. Biology 30 171 Lesson 4 The three phosphate groups are the main features to remember of this molecule. The last two phosphate groups are the important ones. The wavy lines joining each phosphate group to the one before it are high energy bonds. The breaking apart of the high energy bonds releases energy that is needed for cell activities such as transporting molecules through membranes, or movement. Breaking off of two phosphate groups leaves adenosine monophosphate or AMP. Usually just the last phosphate group is broken off, releasing energy, leaving the two-phosphate molecule adenosine diphosphate, or ADP. When there is a surplus of energy, it is used to bond a phosphate group to the ADP to form ATP once more. Within cells, there is a constant cycle involving these compounds and energy. This cycle can be represented by the following diagram. This process can be likened to batteries in a flashlight. When the energy is all used, the batteries can be taken out and recharged. They are then ready for the energy to be used again. The energy used in forming ATP comes from either A high energy electron of chlorophyll which has been acted on by light, or The chemical bond(s) of sugar being broken down during cellular respiration. Formation of ATP occurs in: chloroplasts mitochondria In the mitochondria, ATP molecules are also broken down to release energy if it is needed. Biology 30 172 Lesson 4 The energy compound, ATP is found in all living microorganisms, plants and animals. It provides the source of energy which is the most directly usable for the greatest number of cell activities. Photosynthesis All cellular activities involving the synthesis or breakdown of substances and energy movements could be included under the general term of metabolism. Those metabolic activities where energy is used to synthesize or build organic compounds are part of anabolism. When organic compounds are broken down to release stored energy for active uses, the actions are part of catabolism. Although anabolic and catabolic actions are common to both plants and animals, green plants can take anabolism to a degree not possible in other organisms. Green plants can use radiant (light) energy to change simple, inorganic matter, such as carbon-dioxide and water, into the beginnings of organic forms. These initial organic compounds then become the sources of energy, the "building blocks" for all other living tissue - not only in plants, but in all organisms found in food chains and webs. Since green plants can actually make their own organic compounds, they are independent or autotrophs. Organisms which rely on other organisms for their organic compounds are said to be dependent. They are called heterotrophs. Ultimately, even heterotrophs which are high-order consumers (such as carnivores) depend on green plants for their energy and other nutrient requirements. In this respect, the action of photosynthesis assumes great importance as a major life-sustaining process for all organisms. NOTE: Photosynthesis is a very complex process. It can be difficult to understand. Thinking of electrons in minute structures like chloroplasts is not something that is commonly done. So to give an overview, this lesson will begin by explaining the role of some of the participants of photosynthesis and then move into explaining what occurs during photosynthesis. Biology 30 173 Lesson 4 Chloroplasts and Their Role Photosynthesis occurs in plant organelles called chloroplasts. Chloroplasts capture light energy and produce food. They contain a green pigment, chlorophyll. There are five kinds of chlorophyll designated as chlorophyll a to e. The two kinds that are the most important to photosynthesis are chlorophyll a and b. They absorb most wavelengths of the light spectrum but not green light. Plants can only use energy from the absorbed light. Green light is reflected, thus the reason for plants being green in colour. In the fall, chlorophyll a is broken down, then the other pigments such as carotenes, beta-carotenes, and xanothophylls (yellows, oranges, and browns) show up. Chlorophyll acts in the same way as catalysts do in chemical reactions. They initiate and control rates of reactions but do not enter into the final products. The exposure of the chlorophylls and the pigments to the energy of light gives them extra energy. This begins the actions eventually leading to the formation of organic compounds. This will be seen as the steps of photosynthesis are laid out. A chloroplast is a complex structure. Electron microscope studies of chloroplasts have shown an outer membrane and internal specialized membranes called thylakoid membranes. The thylakoid membranes are arranged in stacks of disks that are altogether called grana. Chlorophyll and other pigments and enzymes are embedded in the thylakoid membranes. The gel-like fluid that surrounds the grana is called the stroma. It too, contains enzymes as well as DNA, RNA, and ribosomes. An illustration that will help to remember this arrangement of the chloroplast is to think of stacks of four quarters (thylakoid membranes), altogether each stack is called a dollar (grana). These stacks of quarters are in a bowl (outer membrane) filled with colourless gelatin that is not quite set yet. This gelatin represents the stroma. Biology 30 174 Adapted from Miguelsierra Lesson 4 The Role of Light in Photosynthesis The amount of light. The importance of light is best shown by testing for the presence of starch in leaves of plants that have been placed in varying light conditions. It is easier to test for starch than testing for glucose (the product of photosynthesis) because after glucose is formed, it very quickly combines into a larger molecule of starch. A test for starch is done by treating the leaf to remove all pigment, then applying iodine to the leaf. If starch is present, a blue-black colour will appear. This is a positive test. A plant left in dim to no light produces no starch. A plant left in good light conditions shows the most starch production. Another indication of the importance of light is often shown by plants, such as grasses, which may have been covered by boxes or other objects for a time. Uncovering these plants later has shown that many or all had died. The last type of situation just mentioned has also shown that light appears to be important in another way. Without knowing the exact manner in which it occurs, scientists have indicated that certain plants or plant parts must be exposed to light for a certain amount of time in order for chlorophyll to develop. Stems, leaves and some seeds kept in the dark will lose their color as the chlorophyll that they had breaks down and is not replaced; or, will not develop in the first place. The greenness of the exposed parts of carrot roots and potato tubers shows the opposite situation, with light inducing (green) chlorophyll formation. Light can be broken up into a visible spectrum which includes a range of colors that are red, orange, yellow, green, blue, indigo, violet. The different colors have different wavelengths and different energy levels as well. Infrareds and reds have the longest wavelengths, but the lowest energy levels. Violets and ultraviolets have short wavelengths, but large amounts of energy. It appears that plants absorb very little of the ultraviolet or infrared light rays. Some scientists also feel that part of the middle portion of the spectrum, consisting of green and adjacent bands, is either reflected or transmitted right through – accounting for the green colors. However, other scientists feel that greens are absorbed almost equally as well and that there are only very slight differences in absorption and reflection. If this is accurate, the ability to absorb much of the visible light spectrum means that most plants can really be grown under artificial light. The only requirement is a light intensity much higher than most buildings normally have. Despite the large amounts of natural light to which plants can be exposed, estimates indicate that approximately 50% of the light striking plants is either reflected or transmitted right through plant surfaces. Of the amount available to the plants, only 1 to 3% is converted to chemical energy. Biology 30 175 Lesson 4 Through studies carried out by a Belgian physician and scientist, Van Helmont, the conclusion was made that “plant tissues came from water”. Although not entirely right, Helmont’s conclusion led to further investigations as to the origin of matter making up plant bodies. These investigations and results, generally accepted as fairly accurate at the present time, do indicate that approximately 94% of plant matter is developed through photosynthesis while only 6% comes from the soil. It is also known that atmospheric CO2 is a major contributor to plant matter, along with water. These are considered to be the “raw materials” for photosynthesis. What happens in Photosynthesis? An overall chemical equation that sums up what happens in photosynthesis is: 6 CO2 carbon dioxide + + 12 H2O water chlorophyll Light C 6 H12O6 glucose + + 6 O2 oxygen + + 6 H2O water In words, this equation says: carbon dioxide and water combine in the presence of chlorophyll and light to produce glucose, oxygen and water. This, of course, is very simplified representation of what actually goes on in the process of photosynthesis but it is a good reminder for when you are studying. Note: You should know this equation. The overall process of photosynthesis: absorbs light energy (Occurs in the thylakoid membranes of the grana.) converts light energy (Also occurs in the thylakoid membranes.) stores chemical energy in sugars. (Occurs in the stroma.) The absorbing and converting of light energy and storing of chemical energy happens in two major processes in Photosynthesis. They are: 1. 2. The Light Dependent Reaction or Photolysis; and The Light Independent Reaction, also called The Calvin Cycle or the Dark Reaction Note: Yes, you should be aware and know all the previous names. Biology 30 176 Lesson 4 The Light Dependent Reactions (also called Photolysis) The ATP’s formed also carry energy to the dark reaction. There are two different types of molecule clusters scattered in the Thylakoid membranes of the chloroplasts. They are given the names Photosystem I and Photosystem II. Each photosystem has chlorophylls, pigments, and enzymes slightly different than the other. This results in each cluster absorbing light of different wavelengths. Note: The organic-making process in green plants begins in Photosystem II. Please read each step through carefully. Be certain to compare each step number back to the diagram so it is easier to follow and understand. You may have to go back and do this several times. Refer to the end of the lesson for overall Photosynthesis diagram. Biology 30 177 Lesson 4 Photosystem II 1. Non-cyclic electron flow (#1 in the diagram) 2. Light hits chlorphylls and pigments in photosystem II (see #1 in diagram). This causes the electrons to become "excited" and they move to higher energy levels. Absorbing light sets up a flow of electrons. The now high-energy electrons are taken through several electron carriers until they are accepted by NADPH+ (#6) which is a coenzyme (electron carrier). (NADP is nicontinamide adenine dinucleotide phosphate.) The high-energy electrons, with a negative charge reduce the positive charge so the molecule becomes NADPH. This energy rich molecule carries energy to the dark reaction. Light energy has now been changed, by means of energized electrons, into chemical bond energy (in NADPH). Cyclic flow - (#2 in diagram) Some “excited” electrons are picked up and moved along by several electron carriers. These electrons give up some of their energy to bond a phosphate group to ADP to form ATP. The electrons then return to the chlorophyll and pigments and are ready to begin the cycle again. This cycle flow occurs very quickly. ATP is an energy carrier that moves into the dark reaction. Photosystem I (#3 in the diagram) Electrons from other pigments and chlorophylls are energized when light hits them. These "excited" electrons e move through a series of electron carriers. As they do this they release energy to add a phosphate to ADP to form ATP. These electrons now move into photosystem II (#1 & 2) to keep the supply of electrons flowing. The ATP's that have been formed in both the photosystems carry energy into the dark reaction, also called the Light Independent Reaction. The Water Molecule is split (also called Photolysis) (#5 in diagram) The chlorophylls and pigments of photosystem I need a source of The meaning of photolyis is light electrons. Water is the source of electrons for the light reaction. splitting. Light A special group of enzymes remove electrons e from water. energy splits the H2O molecule. This splits the water molecule (#5). Oxygen is given off as a "waste" product of photosynthesis. The Hydrogen ions H from the molecule of water combine with NADP (#4) and give it a positive charge NADPH . NADPH now has greater attraction for the high energy electrons from photosystem II. The oxygen from the split water molecule is given off as a gas. Biology 30 178 Lesson 4 Review of the Light Dependent Reaction (Photolysis) 1. 2. 3. 4. 5. 6. 7. The Light Reaction must occur in the light. The raw materials that are used are water, light, chlorphyll The water molecule is split by the light energy. Enzymes remove electrons from the water H2 O molecule (#5) to supply the photosystems I & II with a continuous supply of electrons. The Hydrogens (H+) join with NADP to form NADPH+ and oxygen O2 is given off as a gas. Light shines on the chlorophylls/pigments in the thylakoid membranes of the grana. The excited electrons are accepted by the coenzyme NADPH+ to become NADPH (#6 in diagram) The energy rich compound NADPH moves through for use in the Dark reaction. Another set of "excited" electrons (#2 & #3) release their energy to add a phosphate to ADP to form ATP and then return to the chlorophyll to start over again. ATP carries energy for use in the Dark Reaction. Two very important energy carriers are produced: ATP and NADPH. They move into the Dark Reaction (Light Independent Reaction) for use in forming organic compounds. The Calvin Cycle - The Light-Independent Reactions (also known as The Dark Reaction) The Light-Independent Reaction, also known as the Calvin Cycle, does not require light and occurs in the stroma of the chloroplasts. Even though this part of Photosynthesis does not require light, it does need the ATP and NADPH that was formed in the Light Dependent Reaction. The term carbon fixation can also be applied to this phase as this is actually what is happening. Inorganic carbon from carbon dioxide is fixed into organic compounds. The enzyme, RuBP carboxylase, is very important as it promotes carbon fixation. Almost half of the protein in a chloroplast is part of this enzyme. It is the most abundant protein in all organic life. Enzymes are important in driving the whole process of photosynthesis (Psis). Biology 30 179 Lesson 4 Note: Look back at the overall Photosynthesis equation. So far we have talked about the H2 O , the chlorophyll and the light. Now the raw material, CO2 enters the reaction. This is the first time it has been mentioned. Steps of the Calvin Cycle Carbon Fixation With enzymes aiding the reaction, CO2 joins with ribulose biphosphate (RuBP), a 5-carbon sugar at #1. The 6-carbon sugar that is formed is unstable and splits into two molecules of 3carbon PGA (phosphoglycerate) (see #2 in diagram) Energy from Light Reaction Added (#4 and #5) The ATP (#4) and NADPH (#5) from the Light Reaction enter a series of reactions that convert PGA (#3) into PGAL (#6) (phosphoglyceraldehyde), a 3-carbon sugar Chemical energy has now successfully been built in and stored in an organic compound. Most of the PGAL, with the actions of other enzymes and additional ATP, is converted to RuBP to keep the cycle going. (#7) some PGAL leaves the cycle (to form glucose) Biology 30 180 Lesson 4 There are a number of actions that can occur with the remaining useable PGAL. PGAL is the first major nutrient/organic compound of photosynthesis Some could be used immediately for cell activities needing energy. PGAL can go straight to cellular respiration as a fuel. PGAL can be stored in stems and roots because more PGAL is produced than is needed by the cells right away. PGAL is quite reactive. Before the completion of photosynthesis much of the PGAL is changed into a less reactive but moveable form called glucose, a 6-carbon sugar, the second nutrient of photosynthesis. PGAL can be used to synthesize more complex sugars, starches (for storage), oils, amino acids, proteins, lipids and other compounds that are needed. After reviewing this description of photosynthesis (which has not included all the details) one can see that the original formula given earlier is very simplified. Changing kinetic light energy into potential chemical bond energy involves transferring energy by means of high energy electron movements. In the description of Cellular Respiration in the next section we will see the role of electrons once again. Conditions Affecting Photosynthesis The rate of photosynthesis can be affected by a number of conditions. Some of these are: kinds (wavelengths) and intensity of light the amounts of the "raw" materials of carbon-dioxide and water. External temperatures could increase or decrease rates; extremes of warmth or cold could stop activity completely. Kinds and rates of plant activities often seem to be "synchronized" with photoperiods. These are the actual lengths of light and darkness in certain time periods. The types of plants (or perhaps the kinds of chlorophylls they have) injuries or general plant health could have some effects as well. These factors can affect each other so the combined effect must be looked at. Any one factor not at its optimum can effect the overall rate of photosynthesis. One type of plant may not grow at the maximum rate in a cold climate with full sun because it needs higher temperatures. Biology 30 181 Lesson 4 Importance of Photosynthesis Although the relative importance of photosynthesis as an organic and energy link for all living organisms has been mentioned more than once, the process has many other effects. Wherever plant products or plant uses are involved, such as animal feeds, textiles, lumber, pulp, resins, medicines and many others, photosynthesis has to be considered as the starting point. Scientists speculate that the atmospheric conditions during the early development of this planet lacked oxygen or had very little. The current twenty percent of the atmosphere that is oxygen has resulted largely as a byproduct of photosynthesis. Another very significant contribution from the process and plant products has been the development of our fossil fuel reserves in the form of coals, oil and gases. There is a concern with the destruction of the world's forests. How is this concern related to Photosynthesis? Which part of the process would it affect the most? “Plants that photosynthesize are 'oxygen factories.” Photosynthesizing plants 'put out' O 2 and at the same time take in CO2 . Destroying rain forests would reduce the amount of O 2 put into the air and slows the rate at which CO2 is removed from the atmosphere. The increase of CO2 in the atmosphere seems to affect 'global warming.' Biology 30 182 Lesson 4 Chemosynthesis Earlier, organisms were mentioned as being divided on the basis of those which relied on other organisms for organic nutrients (heterotrophs) and those which could make their own organic compounds (autotrophs). Most autotrophs are green plants or photosynthetic organisms. There is another group of autotrophs generally classed as chemotrophs, which are capable of making food through chemotropism. Mainly bacteria, found near soil surfaces, lake bottoms and ocean floors, these organisms use special enzymes to break down inorganic chemical compounds such as sulfides or oxides. The energy released by these chemicals can then be used to synthesize organic compounds. In comparison to photosynthesis, however, chemosynthesis produces only extremely small amounts of organic matter. Respiration In referring to "respiration", one may form the idea of breathing gases or taking certain gases into the body (commonly oxygen) and expelling others (carbon-dioxide). On a cellular level, there is a slightly different meaning, even though movements of gases are quite important. Cellular respiration is the breakdown of organic compounds to release energy for cell activities. The energy that is produced is usually transferred to smaller units, such as ATP. Cellular Respiration occurs in living cells all the time. Some respiration occurs in the cytoplasm, but most occurs in the mitochondria, the “powerhouse of the cell.” In humans, cellular respiration is usually aerobic. This means it requires oxygen which is carried by the blood along with small food molecules. The oxygen and food particles pass through the cell membrane by the process of diffusion to be used by the cell. The most common food particle used as fuel for cellular respiration is glucose, C 6H12 O 6 , although starches, lipids and amino acids can also be used as energy sources. The overall reaction, shown below, simplifies a "complex series of chemical reactions." C6H12O6 (glucose) 6O2 enzymes (oxygen) 6CO2 (carbon dioxide) 6H2O (water) Note: You should know this equation. Biology 30 183 Lesson 4 It is this catabolic part of metabolism, where energy is released from organic compounds that will be considered as respiration throughout the rest of this lesson. Respiration can be a very complex topic. We will begin by giving a brief summary. Read the summary and the charted diagram below to gain an overview of the catabolic process. Respiration can be divided into three general steps: 1. Glycolysis takes place in the cytosol of the cytoplasm of the cell. Glucose molecules are split with a limited amount of energy released in the form of ATP. Intermediate products are formed and move into the mitochondria. 2. Krebs Citric Acid Cycle Occurs in the mitochondria. The remaining structure of the glucose molecule is dismantled. All carbons of the original glucose are released as carbon dioxide. Only a limited amount of ATP is released. 3. Electron Transport Hydrogen atoms and electrons removed from various compounds are moved along a series of carrier molecules. As this occurs significant amount of energy is released and stored in ATP. At the end of this stage the availability of oxygen plays a significant effect on the entire process. Now view the chart diagram which follows. Biology 30 184 Lesson 4 Respiration Overview: Again, this is a basic overview. We will now look at the entire process in a little more detail. The main object here is to know what goes in, what happens to it, and what comes out at the end of the process. The Stages of Respiration Glycolysis Occurs in the cytosol of the cytoplasm. Oxygen is not necessary for this stage. It is anaerobic. ATP (energy) must be used to start the reaction. Glucose is split into two, 3-Carbon molecules called PGAL which are rearranged to form two, 3-Carbon molecules of pyruvic acid. Biology 30 185 Lesson 4 The Hydrogen atoms that are given off are picked up by a carrier molecule NAD (nicotinamide adenine dinucleotide) to become NADH. The electrons and protons from hydrogen separate. The electrons are moved along a series of coenzymes (carriers). The energy that the electrons give off is used to add a phosphate to ADP to form ATP. There is a net gain of 2 ATP's in glycolysis, the first stage of cellular respiration. This is only about 5-7% of the total energy available in the glucose molecule, so the pyruvic acid molecules still contain most of the original energy. The end products of glycolysis are: 1. 2. 3. Two molecules of pyruvic acid (3-C) Net gain of 2 ATP Two NADH molecules The move into the mitochondria: The availability of some oxygen will allow for the removal of one carbon from a pyruvic acid (3-C) molecule. The carbon and oxygen combine to release CO2 . The remaining 2-carbon portion becomes acetic acid. oxygen present Each acetic acid is picked up by a coenzyme (a carrier molecule) called CoA (coenzyme A) to form acetyl CoA. The Coenzyme A delivers the acetic acid molecules to the Krebs Citric Acid Cycle in the mitochondria. Biology 30 186 Lesson 4 Krebs Citric Acid Cycle Occurs in the mitochondria. The 2-carbon acetic acid joins with a 4-carbon acid to form a 6-carbon acid called citric acid. From here citric acid goes through several reactions. Chemical bonds are broken and atoms are given off or are re-arranged. Electrons are also given off in the form of carbon atoms (#1). These carbon atoms are from the original glucose molecule. The carbon atoms combine with oxygen to produce carbon dioxide (CO2). (A person exhales this). The carbon structure of the original glucose is completely dismantled. Hydrogen atoms e join with carriers NAD to form NADH (#2) to go to the next stage, which is the Electron Transport System. Whenever bonds are broken, energy is given off so heat is released and ATP is formed (#3). The series of acids produced in the cycle eventually lead back to the formation of the original 4-carbon acid. Reproducing the original 4-carbon acid enables the cycle to continue. (a portion of Krebs cycle from above) Biology 30 Electron Transport 187 Lesson 4 Moving into the final stage of cellular respiration the hydrogen atoms (electrons and protons) are transported along a series of electron carriers where they lose their energy to adding a phosphate to ADP to form ATP. Oxygen is most important at the end of the electron transport stage where it picks up the hydrogen ions and the electrons to form water which is released. This action enables the electrons to keep moving through the entire system. The availability of oxygen to keep removing the carbons and hydrogens (electrons) enables this process to be known as aerobic respiration. From the beginning of glycolysis to the end of aerobic respiration, each glucose molecule will produce 38 ATP (34 of which are formed in the electron transport system). During the process, the sugar is broken down gradually as the bonds between its atoms are broken. The 38 ATP produced represent only 40 to 60% of the original energy that went into glucose. The rest of the energy is lost mostly as heat. This is still a fairly good recovery rate compared to machines, which often average 30 to 40% conversion to useful energy. What happens if these processes are stopped? (NAD) Cyanide, a fast acting poison, combines with the final carrier NAD in the electron transport system and blocks any ATP production. Without any ATP, cell processes are not able to continue. Unconsciousness soon follows. Carbohydrates, fats and proteins are all sources of "fuel" for the process of cellular respiration. Through digestion they are broken down to the point that they can enter the Krebs Citric Acid Cycle. Carbohydrates are broken down to glucose so they are ready to enter glycolysis. Fats, as you learnt in lesson 3, are made up of fatty acids and glycerol. Glycerol is broken down into a 3-carbon compound and enters glycolysis, whereas fatty acids are further broken into acetic acid and join with CoA to form acetyl CoA to enter the mitochondria and enter the Krebs cycle. Proteins, made up of amino acids, has to have the amino group removed then the resulting carbon molecules can enter at glycolysis or the Krebs cycle. Biology 30 188 Lesson 4 Anaerobic Respiration Not all cells or organisms are in conditions where sufficient amounts of oxygen are available all the time, or even some of the time. Such cells or organisms still need energy to maintain or carry out necessary life-sustaining activities. Respiration is still carried out, but under conditions of oxygen deficiencies it is called anaerobic respiration. A limited number of organisms, mainly some kinds of bacteria, carry out respiration where there may be no free atmospheric oxygen. Anaerobes are able to break down certain inorganic compounds such as nitrates (N03), carbon-dioxide and sulfates (S04) and use the oxygen from these sources. In soils, especially those that are wet and have little oxygen, denitrifying bacteria may have an effect on crop production as they reduce the amounts of nitrates available to plants. In water-logged soils, such as in some sloughs or marshes, some bacteria remove oxygen from sulfates and in the process, form hydrogen sulfide. This produces the swamp gas or "rotten egg" smell. Removal of oxygen from C02 leaves behind methane in other anaerobic respirations. Fermentation After glycolysis, if oxygen is in very short supply or is totally absent, a process called fermentation occurs. If the hydrogen cannot be removed by oxygen, the process of breaking down pyruvic acid (formed in glycolysis) stops. Fermentation does not produce or release any more energy beyond the glycolysis stage. The 2 ATP's produced in glycolysis are all that is produced so fermentation is a very inefficient process. The products formed still have a lot of energy tied up in them. There are two types of fermentation: lactic acid fermentation and alcoholic fermentation. Biology 30 189 Lesson 4 Alcohol Fermentation Lactic Acid Fermentation Yeasts and many bacteria carry out alcohol fermentation. In the absence of oxygen, carbon dioxide is released from pyruvic acid and hydrogen is transferred to the pyruvic acid to form ethyl alcohol. Lactic acid fermentation is more common to animals (especially in the muscles) and to some bacteria. In this type of fermentation, the single reaction of hydrogen transferring to pyruvic acid (see *) is enough to form lactic acid. Muscle exertion and the inability of the body to supply oxygen quickly enough results in lactic acid This type of fermentation has commercial value in baking and in the brewing and winemaking industry. (In the production of "sweet wines", fermentation is halted earlier so that the wine still contains a large amount of sugar or glucose. "Dry" wines are allowed to ferment longer to convert more sugar to alcohol). In baking, the yeast releases the CO2 into the dough. The oven heat kills the yeast and the bubbles break, leaving "holes" in the bread or cake. Thus, the cake or bread is fluffy and light to eat. Biology 30 190 buildup which leads to muscle fatigue and soreness. This process is sometimes known as building up an "oxygen debt". Rest allows oxygen and aerobic respiration to break down lactic acid. Again, an over-accumulation could be toxic to the organisms in which it is produced. Depriving the human brain of oxygen for longer than five minutes leads to permanent damage and possible death. Certain bacteria can also produce lactic acid as they break down animal or plant products. Sour milk, sauerkraut and silage are some examples where lactic acid is formed. The characteristic of lactic acid being toxic at certain levels and killing organisms has sometimes been applied to preserving certain substances or foods – such as silage or sauerkraut. Lesson 4 As mentioned shortly before, fermentation is not an efficient process in terms of yielding energy for the organisms carrying it out. Glycolysis is the only part which releases a limited amount of energy. Fermentation itself, where hydrogen combines with pyruvic acid to form various substances, merely allows glycolysis to go on a little longer. In the end, however, this could be fatal to organisms as substances such as alcohol and lactic acid can be toxic in higher concentrations. Uses of Energy If cells and organisms are to maintain living conditions, they must have a continuing supply of energy. Maintaining a living condition involves many diverse actions. The release of energy also releases heat in many organisms. In most plants and cold-blooded organisms, this is quickly lost to the external environments and has little value for the organisms. For warm-blooded organisms, maintaining higher body temperatures is necessary for proper body functioning and too great a drop could be fatal. Much of the energy released initially during respiration goes towards the formation of ATP. The energy can be temporarily stored this way and then when it is required, it is readily available in smaller, more convenient amounts. Released from ATP, a considerable portion is used to synthesize many kinds of cell parts and cell substances. These include the three general groups of organic compounds, as well as vitamins, pigments, organic acids and alkaloids. Energy is commonly associated with motion as well. While obvious by the muscular contractions of many animals, movements occur in plants as well. In both plants and animals, protoplasmic streaming, chromosome movements and active absorption are some of the less obvious motions. Summary The process of photosynthesis by green plants, in which inorganic matter is changed to organic (food), is an action upon which most life depends. The conversion of light energy into the chemical energy of PGAL, ATP and eventually other organic compounds, provides sources of energy which are released by the process of respiration in all living cells. The following table illustrates some of the major differences between photosynthesis and respiration. Biology 30 191 Lesson 4 Photosynthesis Respiration • occurs in the green cells of plants • occurs in the living cells of all organisms. • must begin in the presence of light. • takes place all the time, in both light and dark. • requires water and CO2. • requires organic matter and O2. • releases O2. • releases water and CO2. • radiant energy is converted to chemical energy. • chemical energy is changed into heat or transformed into smaller units of ATP. • organic matter is produced. • organic matter is broken down. Once light energy has been converted to chemical bond energy, different kinds of actions can take place with the chemical energy and chemical compounds formed. The following illustration shows some of the different pathways that could be associated with the anabolic and catabolic stages of metabolism. Pathways of Metabolism Biology 30 192 Lesson 4 Processes of Photosynthesis The Light Dependent Reactions (also called Photolysis) The energy carriers NADPH and ATP move into the Dark Reaction (#4, #5 in the next diagram) Steps of the Calvin Cycle (The Light-Independent/Dark Reaction) Biology 30 193 Lesson 4 Overall Diagram of Cellular Respiration Biology 30 194 Lesson 4