In cell respiration, as in photosynthesis (see Topic 8.2),

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1 Cell Respiration and Photosynthesis Cell respiration and photosynthesis 8.1 Cell respiration 8.2 Photosynthesis Cell respiration State that oxidation involves the loss of electrons from an element, whereas reduction involves a gain of electrons; and that oxidation frequently involves gaining oxygen or losing hydrogen, whereas reduction frequently involves losing oxygen or gaining hydrogen. In cell respiration, as in photosynthesis (see Topic 8.2), reactions often involve the enzyme controlled transfer of electrons. This type of reaction is called a redox reaction. In these reduction-oxidation reactions, one compound loses some electrons and the other compound gains them. Here is a useful mnemonic to help you remember this: OIL RIG Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons). The process of oxidation often involves gaining oxygen (hence its name) or losing hydrogen while reduction often involves losing oxygen or gaining hydrogen. A substance which has been reduced, now has the power to reduce other substances (and becomes oxidised in the process); e.g. NADH (respiration) and NADPH (photosynthesis). oxidation reduction electrons loss gain oxygen gain loss hydrogen loss gain If a molecule, atom or ion is oxidised, then it loses electrons. These electrons have to be accepted by another molecule, atom or ion which is reduced. Therefore, oxidation and reduction reactions always take place together, hence the name redox reactions Outline the process of glycolysis, including phosphorylation, lysis, oxidation and formation. Glycolysis takes place in the cytoplasm and produces two pyruvate molecules from every glucose according to the following reaction: Glucose + 2ADP + 2P i + 2NAD + 2Pyruvate NADH + 2H + + 2H 2 O Glycolysis is anaerobic and does not require oxygen and produces a small amount of and NADH and H +. One molecule of glucose contains sixatoms of carbon; one molecule of pyruvate contains three atoms of carbon. The structural formula for pyruvate is shown in Figure Biol Chapter 8 FINAL.indd /12/07 8:24:43 AM

2 Chapter 8 O C O - C CH 3 Figure 801 O Pyruvate Glycolysis uses glucose, a hexose sugar, with six carbon atoms to eventually produce two molecules of pyruvate, a triose, i.e. a monosaccharide with 3 carbon atoms. The first step of glycolysis involves phosphorylation. In this step, is used (invested) to add a phosphate group to glucose. It is followed by a second phosphorylation reaction, again using and producing hexose biphosphate. The hexose biphosphate still contains 6 carbon atoms. It is split in a lysis reaction, producing 2 triose phosphate molecules with 3 carbon atoms each. The next step is a combined oxidation phosphorylation reaction. The enzyme involved first oxidises the triose phosphate into a different triose phosphate (for those taking Chemistry HL: glyceraldehyde to glycerate or, in general, aldehyde to carboxylic acid). The oxidation has to occur because in the overall reaction, it was shown that glycolysis produces 2 NADH and 2H +. NAD + has been reduced to NADH, i.e. it has gained electrons. The electrons were supplied by a triose phosphate which is oxidised into a different triose phosphate. After the oxidation reaction, the enzyme will attach an inorganic phosphate from the cytoplasm to the triose phosphate to form a triose biphosphate. So this phosphorylation reaction does not involve. Finally, each triose biphosphate gives up one of its phosphate groups. This phosphate is taken up by ADP to form. This is repeated in the last step of glycolysis, again forming one, but now also producing pyruvate. The process of glycolysis is shown in Figure 802 and can be summarised by the following overall equation: Glucose + 2ADP + 2P i + 2NAD + 2Pyruvate NADH + 2H + + 2H 2 O glucose (6C) ADP ADP phosphorylation phosphorylation triose phosphate NAD + NADH+H + P triose biphosphate ADP ADP pyruvate hexose biphosphate lysis reduction of NAD + into NADH + H + oxidation of triose phosphorylation formation formation triose phosphate NAD + NADH+H + P triose biphosphate ADP ADP pyruvate Figure 802 Glycolysis Biol Chapter 8 FINAL.indd /12/07 8:24:44 AM

3 Cell Respiration and Photosynthesis Draw and label a diagram showing the structure of a mitochondrion as seen in electron micrographs. Mitochondria (singular: mitochondrion) are large organelles found in eukaryotic cells. They are surrounded by an outer membrane and an inner membrane. The inner membrane is folded and these folds, cristae (singular crista), project into the matrix of the mitochondrion. The matrix of the mitochondrion is a watery fluid which contains many molecules and enzymes. In the matrix of the mitochondrion, we also find ribosomes and DNA. The space between the outer and inner membrane of the mitochondrion is called the inter-membrane space. Figure 803 is an electron micrograph showing several mitochondria Explain aerobic respiration, including the link reaction, the Krebs cycle, the role of NADH + H +, the electron transport chain and the role of oxygen. When oxygen is NOT present in the cell, pyruvate will stay in the cytoplasm and be converted into lactate (in animals) or ethanol and carbon dioxide (in plants and yeasts) in the process of anaerobic respiration (see Topic 3.7). Glycolysis releases a small amount of energy but conversion of pyruvate to lactate or ethanol and carbon dioxide does not yield more. Therefore, anaerobic respiration releases only a small amount of the energy in glucose and is only used in the absence of oxygen, when aerobic respiration is not possible. If oxygen is present, a series of reactions take place which result in pyruvate being broken down to produce carbon dioxide and a relatively large amount of energy in the form of. The first of these reactions is called the link reaction because it forms the link between glycolysis (see Topic 3.7.3) and the Krebs cycle. Pyruvate, produced in the cytoplasm during glycolysis, is transported to the mitochondrial matrix according to the following equation: Pyruvate + CoA + NAD + Acetyl CoA + CO 2 + NADH + H + Figure 803 Mitochondria This process is known as decarboxylation of pyruvate because a molecule of carbon dioxide is removed from pyruvate. ribosome crista synthetase The Krebs cycle, (also known as the tricarboxylic citric acid cycle or TCA cycle), occurs in the matrix of the mitochondria and produces 2CO 2, 3NADH + 3H +, 1FADH 2 and 1 from 1 molecule of acetyl CoA. As the name suggests, it is a cyclic process. The logical place to start studying the Krebs cycle is at the point where acetyl CoA, a compound containing 2 carbon atoms and produced from pyruvate in the link reaction, enters the cycle. Figure 804 DNA outer membrane inner membrane matrix Structure of a mitochondrion intermembrane space Mitochondria are often drawn as a two-dimensional crosssection to show the internal structure. Refer to Figure 804. Acetyl CoA will combine with a four carbon compound, forming a six carbon compound. This six carbon compound will then be decarboxylated, producing a five carbon compound and carbon dioxide. The same sequence of reactions will happen again, producing a four carbon compound and another carbon dioxide. We are now back to the 4 carbon compound that originally reacted with acetyl CoA Biol Chapter 8 FINAL.indd /12/07 8:24:46 AM

4 Chapter 8 Figure 805 shows a simple diagram of the Krebs cycle. pyruvate C 3 LINK reaction C 4 acetyl CoA C 2 KREBS CYCLE C 6 CoA CO 2 energy stored in NADH + H + and in FADH 2 to produce more. The last step of this stage involves the use of oxygen. In the absence of oxygen, none of the reactions will occur. This will prevent the Krebs cycle from taking place and, instead, the process of anaerobic respiration will be carried out (see Topic 3.7). The last stage of aerobic respiration involves the electron transport chain and takes place on the cristae, the folds of the inner membrane of the mitochondrion. A series of protein complexes (electron carriers) are arranged in a specific order in the phospholipid bilayer of the inner membrane of the mitochondrion. These protein complexes pass electrons along, from one complex to the next. As the electrons move through the membrane, some hydrogen ions (protons) are pumped from the matrix into the intermembrane space. Finally the last member of the electron transport chain promotes the reduction of oxygen to form water. CO 2 C 5 Figure 805 The Krebs cycle One turn of the cycle will require the input of one acetyl CoA and produce one CoA molecule and two carbon dioxide molecules. However, the purpose of the Krebs cycle is to produce energy and so one turn of the cycle also produces 3 NADH + 3H +, 1 FADH 2 and 1. FADH 2 is like NADH + H + in that it has accepted hydrogen ions so it has become reduced. It now has reducing power - the ability to reduce another compound Explain oxidative phosphorylation in terms of chemiosmosis. In glycolysis, one glucose is split into two pyruvate, 2 and 2 NADH + 2H +. In the link reaction, one pyruvate is changed into one acetyl CoA, one carbon dioxide and 1 NADH + H +. In the Krebs cycle, one acetyl CoA is changed into 2 carbon dioxide, 3 NADH + 3H +, 1 and 1 FADH 2. This means that overall, one molecule of glucose has been changed into 6 molecules of carbon dioxide and, so far, no oxygen has been used and only a little has been produced. The final stage of aerobic respiration will use the The proton gradient, which is the result of the movement of hydrogen ions from the matrix into the intermembrane space, drives the production of (from ADP and P i ) by the enzyme, synthetase. This is the chemiosmotic theory, proposed by a British biochemist, Peter Mitchell in The chemiosmotic theory explains how the synthesis of is coupled to electron transport and proton movement. Initially, not many scientists accepted Mitchell s idea, but as more information became available, the chemiosmotic theory gained credibility and Peter Mitchell was awarded a Nobel Prize for Chemistry in The net result of this process is that 1 NADH + H + supplies enough energy to produce 3 from 3 ADP + 3 P i and 1 FADH 2 supplies enough energy to produce 2 from 2 ADP + 2 P i. During these reactions, NADH + H + and FADH 2 are returned to the ir oxidised forms NAD + and FAD. The mechanism of this series of reactions is that the energy from NADH + H + and FADH 2 is transferred to through a series of electron carriers. This series of electron carriers finally yields H + and electrons to oxygen (O 2 ) to form water (H 2 O). However if no oxygen is present, this reaction cannot take place. As a consequence, no NAD + or FAD is formed and hence the Krebs cycle cannot operate. This will cause acetyl CoA to accumulate and, as a result, it will no longer be produced from pyruvate. Glycolysis will continue to operate however, since, even without oxygen, it is possible to break down pyruvate and release some energy. This process is less efficient though, since the amount of energy produced is much lower in anaerobic than aerobic respiration Biol Chapter 8 FINAL.indd /12/07 8:24:47 AM

5 Cell Respiration and Photosynthesis Chemiosmotic coupling of electron transport chain and oxidative phosphorylation Electron transport chain nh + nh + NADH+H + 2e MATRIX NAD + 2H O 2 + H 2 O nh + ADP+P i 2e nh + synthetase nh + INTERMEMBRANE SPACE Figure 806 Chemiosmosis The chemiosmotic theory of Peter Mitchell It had already been obvious for some time that a link existed between the electrons being passed down the electron transport chain and the production of. Peter Mitchell discovered that during the passing of the high energy electrons down the electron transport chain, protons were being pumped across the inner mitochondrial membrane. There is a build up of H + ions in the intermembrane space. The concentration gradient (known as the proton motive force) will drive H + through the synthetase molecule which has chemiosmotic channels. As the H + ions go through the synthetase molecule, the potential energy they possess will be used to drive synthesis. Refer to Figure 806. The inter-membrane space has a higher concentration of H + ions (hence a lower ph) because of the electron transport chain. The inner membrane is folded into cristae to provide maximum space and surface area for the electron carriers and synthetase. It is impermeable to H + ions. Its structure is based on the fluid mosaic model with the electron carriers and the synthetase embedded among the phospholipid molecules. The synthetase molecules can be seen on the cristae. The inter-membrane space has a small volume so that the movement of even a limited number of hydrogen ions (protons) will greatly affect the concentration. The matrix contains the enzymes which enable the Krebs cycle to proceed Explain the relationship between the structure of the mitochondrion and its function. Keeping in mind all of the information presented in the previous sections, it is useful to return to the structure of the mitochondrion. Refer to Figures 803 and 804. The outer membrane is a regular membrane, separating the mitochondrion from the cytoplasm. Its structure is based on the fluid mosaic model. Glycolysis takes place in the cytoplasm. Pyruvate is transported to the matrix of the mitochondrion and decarboxylated to acetyl CoA which enters the Krebs cycle. The resulting NADH and H + and FADH 2 give their electrons to the electron carriers in the inner membrane. The electrons move through the membrane as they are passed from one electron carrier to another in a series of redox reactions. During this process, H + ions are pumped from the matrix into the intermembrane space, creating a potential difference. A concentration gradient drives the H + ions back to the matrix through the synthetase which uses the energy released to combine ADP and P i into, which is released into the matrix Biol Chapter 8 FINAL.indd /12/07 8:24:48 AM

6 Chapter Photosynthesis Draw and label a diagram showing the structure of a chloroplast as seen in electron micrographs. Photosynthesis occurs in the chloroplasts. Cells in the palisade layer often have a large number of chloroplasts because the main function of these cells is photosynthesis. Pictures of the chloroplast, taken with the electron microscope, allow its structure to be seen and studied in sufficient detail. See Figure 808 (a), (b) and (c), the approximate magnifications are shown. Figure 808 shows several electron micrographs showing the location and detail of a chloroplast. From similar EMs, a three-dimesional impression of the structure has been deduced. A diagram of the structure of a chloroplast is shown in Figure µm Figure 807 Light microscope view of chloroplasts Chloroplasts found in cells of green plants are 2-10 µm in diameter and ovoid in shape when found in higher plants (in green algae their shape varies). As you can see in Figure 807, chloroplasts can be seen with the light microscope. Figure 809 Diagram of a chloroplast granum stroma starch (b) nucleus chloroplast (a) (c) Figure 808 (a) A leaf cell (x400) (b) A chloroplast (x1200) (c) Granum and stroma (x5000) Biol Chapter 8 FINAL.indd /12/07 8:24:58 AM

7 Cell Respiration and Photosynthesis State that photosynthesis consists of light-dependent and light-independent reactions. As was shown in Topic 3.8, photosynthesis is NOT a simple one-step reaction. It consists of a series of reactions which can be grouped into a light-dependent stage and a light-independent stage. These two stages are shown in Figure 810. The light-dependent stage will only take place in the light. However, the light-independent stage can occur at any time, if the required materials are available. Outside the laboratory, these materials are provided ( and NADPH) come from the light-dependent stage. Details of the light-dependent stage The light-dependent stage takes place on the membrane of the grana, the stacks of thylakoid membrane in the chloroplast. There are two possible processes which will produce : non-cyclic photophosphorylation (Figure 811) cyclic photophosphorylation (Figure 812) Non-cyclic photophosphorylation X Y NADPH NADP + +H + O 2 LIGHT- DEPENDENT STAGE used in forming H 2O + light H + + electrons (+ NADPH) CO 2 Energy Level ADP+P i ADP+P i ADP + P i (NADP + ) Light Light PS I glucose + water LIGHT- INDEPENDENT STAGE H 2 O PS II 2H O 2 Figure 810 The two stages of photosynthesis Figure 811 Non-cyclic photophosphorylation in the light-dependent stage Some texts will still use the terms light stage and dark stage. These are incorrect since they imply that light is required for one stage and darkness for the other. This is not the case. Indeed light is needed for the light-dependent stage but the light-independent stage can take place in the presence or absence of light - which makes the name dark stage misleading Explain the light-dependent reactions. Outline In the light-dependent reaction, light energy is used (indirectly) to split water molecules into hydrogen ions, oxygen molecules and electrons. The oxygen is a waste product and will leave the chloroplast. The H + and electrons will be used to produce energy-rich and NADPH. In the light-independent stage, the and NADPH are used to combine 3 carbon dioxide molecules into 1 triose phosphate (TP) (C3) in the Calvin Cycle. Once two molecules of TP are produced, they are combined to form one molecule of glucose (C6). Non-cyclic photophosphorylation As can be seen in Figure 811, the light is absorbed by the pigments of photosystem II (PS II), which are mainly found in the grana of the chloroplast. Absorbing this light energy excites some electrons which, as a result, leave their normal position (circling the nucleus of the atom) and move away from the nucleus of the atom. This is called photoactivation of PS II. The electrons are taken up by an electron acceptor X, resulting in a chlorophyll a molecule with a positive charge. The electrons are then passed through a number of electron carriers in the membrane via oxidation-reduction reactions (see Topic 8.1) and will end up at photosystem I (PS I). This is a system of electron tranport. The presence of chlorophyll a + (Chl a + ) will induce the lysis of water so that oxygen, hydrogen ions and electrons are released. Chl a + is the strongest biological oxidant known. Since the lysis of water is the direct result of the photoactivation of PS II, the process is known as photolysis of water. The light is also absorbed by PS I which, like PS II, is found in the membranes of the grana. Again, the electrons absorb the light energy and move away from the nucleus Biol Chapter 8 FINAL.indd /12/07 8:24:59 AM

8 Chapter 8 This is called photoactivation of PS I. The electrons leave the chlorophyll a molecule and are taken up by electron acceptor Y. They are then passed on and taken up by NADP + which combines with an H + and is reduced to form NADPH. The Chl a + of PS I receives electrons from the electron carrier chain (ultimately from PS II) and becomes an uncharged Chl a molecule. GRANA THYLAK OID MEMBRANE H 2 O 2H O 2 LUMEN OF GRANUM light nh + STR OMA light 2e NADP + + H + NADPH Cyclic photophosphorylation In cyclic photophosphorylation (see Figure 812), the electrons from PS I go to electron acceptor Y, but instead of being used to produce NADPH, they go through the membrane via several electron carriers (electron transport) (redox reactions) and are returned to PS I. PS II is not involved. This process is cyclic, as its name suggests. It does not produce NADPH but it does produce. For this reason, cyclic photophosphorylation is a useful process, but as it does not produce NADPH, it is not able to drive the Calvin cycle and will not produce complex carbohydrates for long term energy storage. Energy Level X Cyclic photophosphorylation ADP + P i ADP + P i PS I Figure 812 Cyclic photophosphorylation in the light-dependent stage Explain photophosphorylation in terms of chemiosmosis. Y light Figure 813 ADP + P i Photophosphorylation Explain the light-independent reactions. The light-dependent stage uses light to produce the energy-rich compounds and NADPH and H + which are used to drive the Calvin cycle (see Figure 814) in the light-independent reactions. In the Calvin cycle, three molecules of carbon dioxide are combined to form the 3C compound triose phosphate (TP). TP will leave the Calvin cycle and be subsequently combined to larger, more complex, carbohydrates such as glucose and, eventually, starch. 3CO 2 (3 1C) RuBP carboxylase 3 ADP + 3 P i 3 Ribulosebiphosphate (RuBP) (3 5C) 3 6 Glycerate-3-phosphate (GP) (6 3C) Calvin Cycle 5 Triose phosphate (TP) (5 3C) 6 6 ADP + 6 P i 6 Glycerate-1,3-diphosphate (3 1C) 6 Triose phosphate (TP) (6 3C) 6 NADPH 6 NADP + 1 TP (1 3C) The electrons from photolysis of water are taken up by Chl a + in PS II. The following happens: Chl a + will be converted to Chl a oxygen is released as a waste product H + ions (protons) are pumped to the inside of the grana (the lumen). They accumulate there until the concentration gradient drives them through chemiosmotic proton channels in the synthetase, driving the phosphorylation reaction ADP + P i (see Topic 8.1). This is shown in Figure 813. Since the formation of is indirectly caused by light energy, the process is often described as photophosphorylation. 144 Figure 814 The Calvin cycle The Calvin cycle takes place in the stroma of the chloroplast. provides the energy and NADPH provides the reducing power needed for biosynthesis using carbon dioxide. RuBP is the carbon dioxide acceptor and (catalysed by RuBP carboxylase which is also known as RuBisCo) will take up CO 2, forming GP. GP will be reduced to TP but this conversion needs energy from and reducing power from NADPH. TP can be converted to glucose, sucrose, starch, fatty acids and amino acids and other products. Of course, RuBP is regenerated from TP to keep the cycle going. This process requires energy from Biol Chapter 8 FINAL.indd /12/07 8:25:00 AM

9 Cell Respiration and Photosynthesis TOK Is research science or art? For the series of experiments from which the mechanism of the Calvin cycle was determined, new procedures were devised and equipment designed. This is often the case when new knowledge is revealed. Melvin Calvin s team of scientists displayed amazing creativity and imagination in their investigation of the mechanism of photosynthesis. One device they designed and built was called the lollipop because of its shape. It was made from clear glass with light sources on both sides to illuminate the unicellular green algae, Chlorella, which was suspended in a watery solution. In this environment, substantial photosynthesis occurred and Chlorella could quickly and easily be removed from the lollipop and examined for the presence of chemicals involved in the stages of what is now known as the Calvin cycle. To some, the lollipop is just a machine. Perhaps it is viewed in this way because it was built for a specific purpose, to reveal knowledge. It can also be seen as the result of the creative minds of scientists. The development of new scientific protocols and devices have many parallels with the development of works of art. The light-independent reactions of the Calvin cycle take place in the stroma of the chloroplast. The concentration of the required enzymes, particularly RuBisCo, in the stroma of the chloroplast is much higher than would be possible in the cytoplasm. Also the concentration of magnesium ions (Mg 2+ ) in the stroma increases in light. Magnesium ions are needed for the proper functioning of Rubisco. Also, due to the fact that protons are pumped from the stroma into the lumen of the grana, the ph of the stroma slightly increases making it slightly alkaline (ph around 8). This facilitates the reactions of the Calvin cycle. Again, in the much larger volume of the cytoplasm, the effect of removing some protons would be much smaller Explain the relationship between the action spectrum and the absorption spectrum of photosynthetic pigments in green plants. For a person to see an object, the object needs to reflect light which then enters the person s eye. The colour of the object is the colour of the light that is reflected. All other colours are absorbed Explain the relationship between the structure of the chloroplast and its function. The light-dependent reactions of photosynthesis involve photoactivation, followed by a series of redox reactions during electron transport by electron carriers. The reactions need to take place in a certain order so the electron carriers are fixed in positions in the membrane of the grana thylakoid. Since the thylakoid membrane has a large surface area inside the chloroplast, many of lightdependent reactions can take place at the same time. Refer to Figures 808 and 809. Another process during the light-independent reactions is the movement of hydrogen ions (H + ) or protons. As the electrons move from the stroma through the membrane into the lumen of the grana, hydrogen ions are actively transported across the thylakoid membrane, into the lumen of the grana. Since the lumen of the granum is a small space, it has a small volume and even a limited change in the number of hydrogen ions will have a significant effect on the H + concentration. (The same situation occurs in the intermembrane space of mitochondria, refer Topic ) Chlorophyll is green. That means that it will absorb other colours better than the colour green, which will be reflected. An absorption spectrum can be produced from measurements of the percentage of the light of a certain colour that is absorbed. An absorption spectrum of chlorophyll a (one of the types of chlorophyll) is shown in Figure 815. It can be seen that the absorption of green light (500 nm) is nearly zero, indicating that this colour is reflected and will therefore enter a person s eye, creating the image of a green leaf. Absorption Figure 815 Wavelength (nm) absorption spectra β-carotene Chlorophyll b Wavelength (nm) Absorption spectra Chlorophyll a Biol Chapter 8 FINAL.indd /12/07 8:25:01 AM

10 Chapter 8 Chlorophyll needs to absorb light before it can use it in the light-dependent reactions. However, not all absorbed wavelengths (colours) of light are equally well used in photosynthesis. This is where an action spectrum is different from an absorption spectrum. An action spectrum will show how well the light of different wavelengths is used in photosynthesis. The amount of photosynthesis can be measured, for example, by the amount of oxygen produced (which is easiest to measure in water plants). In Figure 815, you can see that the different kinds of chlorophyll have different absorption spectra (you do not need to know the differences). You can also see that these different pigments (chlorophyll a, b, and carotenoid) work together in photosynthesis as can be seen from the absorbtion spectrum in Figure 815. The same applies for the concentration of carbon dioxide. Increasing the concentration of carbon dioxide will increase the rate of photosynthesis until a point when carbon dioxide concentration is no longer the limiting factor for photosynthesis and further increases do not affect the rate. See Figure 816 (b). However, the shape of the graph for temperature versus the rate of photosynthesis is different. When the temperature is low, it can be a limiting factor for the rate of photosynthesis. Increasing the temperature will increase the rate of photosynthesis until the optimum temperature is reached. A further increase in temperature will decrease the rate of photosynthesis because the enzymes will start to denature. See Figure 816 (c) which is only approximate Explain the concept of limiting factors in photosynthesis, with reference to light intensity, temperature and concentration of carbon dioxide. The factor the furthest away from its optimum value will limit the amount of photosynthesis. This is the limiting factor. If you improve this factor, the rate of photosynthesis will increase until another factor becomes the limiting factor. Limiting factors for photosynthesis are: light intensity temperature concentration of carbon dioxide. As can be seen in Figure 816, graph (a), light intensity can be a limiting factor in photosynthesis. If the intensity of the light is increased, the rate of photosynthesis will increase to a certain level, at which point further increases will not affect the rate of photosynthesis. At this point, light intensity is no longer a limiting factor in photosynthesis. TOK Does correlation mean causation? In experimental design, to be sure of the link between cause and effect, one must control all factors that could influence the outcome apart from one. Calvin s experiments were meticulously designed to achieve this outcome in an environment almost totally removed from the world in which most organisms live and interact. In the every day world, it is impossible to control all factors and draw definitive cause and effect conclusions. Epidemiology is an attempt by Scientists to study factors affecting health. It acknowledges at its foundation that humans cannot control all factors when we study health. Epidemiologists are quick to point out that they determine a correlation between a factor and a disease and that this is very different to proof that the factor is the cause of the disease. We can say that there is a correlation between longer life and vegetarianism as opposed to meateating. This does not prove that eating meat lowers life expectancy. What experiment would you have to perform to make such an assertion? Rate of photosynthesis Rate of photosynthesis Rate of photosynthesis (a) Light intensity (b) Carbon dioxide concentration (c) Temperature Figure 816 Limiting factors for photosynthesis Biol Chapter 8 FINAL.indd /12/07 8:25:02 AM

11 Cell Respiration and Photosynthesis Exercises 1 In a redox reaction: A both compounds lose electrons. B both compounds gain electrons. C electrons are not involved in the reaction. D one compound loses electrons and the other compound gains them: 2 Oxidation involves: A loss of electrons. B gain of electrons. C removal of oxygen. D removal of hydrogen. 3 If glycolysis is written as a one step equation, what are the reactants and the products? reactants products A glucose, oxygen carbon dioxide, water B glucose, ADP, Pi, NAD+ pyruvate,, NADH, H2O C glucose, pyruvate, ADP D glucose lactate Questions 4 and 5 refer to the schematic diagram of a mitochondrion. 6 A leaf is exposed to sunlight. Which colours are absorbed and which are reflected? A all colours are reflected B all colours are absorbed except red which is reflected C green is absorbed and all other colours are reflected D all colours are absorbed except green which is reflected 7 The Calvin cycle is a part of: A the light dependent stage of photosynthesis. B the light independent stage of photosynthesis. C fermentation. D respiration. 8 Which one of the following is not a limiting factor for photosynthesis? A light intensity. B temperature. C concentration of carbon dioxide. D concentration of oxygen. 9 (a) Where in the cell does glycolysis take place? (b) Where in the cell does the Krebs cycle take place? (c) Where in the cell is the electron transport chain found? X 10 Draw a diagram of the structure of a mitochondrion as seen with the electron microscope. 4 X is the: A intermembrane space B DNA C matrix D synthetase 5 Y is the: A crista B DNA C matrix D synthetase Y 11 (a) How does the structure of the site for the Krebs cycle relate to its function? (b) How does the structure of the site for the electron transport chain relate to its function? (c) What would happen to aerobic respiration if the outer membrane of the mitochondrion became permeable to protons (hydrogen ions)? 12 Draw a diagram of the structure of a chloroplast as seen with the electron microscope. 13 Outline how the light-independent reaction depends on the light dependent reaction Biol Chapter 8 FINAL.indd /12/07 8:25:03 AM

12 Chapter 8 14 (a) What are the functions of the and NADPH produced in non-cyclic photophosphorylation? (b) What would be the purpose of cyclic photophosphorylation? (c) What is the advantage of non-cyclic photophosphorylation over cyclic photophosphorylation for the plant? (d) What is the purpose of the Calvin cycle? 15 Explain how the structure of the chloroplast is suited to its function. 16 Compare and contrast the process of production in chloroplasts and mitochondria Biol Chapter 8 FINAL.indd /12/07 8:25:04 AM

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