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SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY Respiration Learning objectives: Outline why plants, animals and micro-organisms need to respire, with reference to active transport and metabolic reactions; Describe, with the aid of diagrams, the structure of ATP; State that ATP provides the immediate source of energy for biological processes; Explain the importance of coenzymes in respiration, with reference to NAD and coenzyme A; State that glycolysis takes place in the cytoplasm; Outline the process of glycolysis beginning with the phosphorylation of glucose to hexose bisphosphate, splitting of hexose bisphosphate into two triose phosphate molecules and further oxidation to pyruvate, producing a small yield of ATP and reduced NAD; Key definitions: Compile a glossary by writing your own definitions for the following key terms related to the learning objectives above. Key term energy ATP metabolism anabolic catabolic photoautotrophs oxidation reduction coenzyme Definition SACKVILLE SCIENCE DEPARTMENT Key term A2 BIOLOGY Definition NAD glycolysis hexose hydrolysis triose hexose 1,6bisphosphate substrate-level phosphorylation Energy in cells and the role of ATP Respiration is the process whereby energy stored in complex organic molecules is used to make ATP. It occurs in living cells. Energy exists as potential (stored) energy and kinetic energy (movement). Moving molecules have kinetic energy that allows them to diffuse down a concentration gradient. Large organic molecules contain chemical potential energy. All living organisms need energy to drive their biological processes. All the reactions that take place within organisms are known collectively as metabolism. Metabolic reactions that build large molecules are described as anabolic and those that break large molecules into smaller ones are catabolic. Some of the energy from catabolic reactions is released in the form of heat. This is useful as metabolic reactions are controlled by enzymes, so organisms need to maintain a suitable temperature that allows enzyme action to proceed at a speed that will sustain life. Plants, some protoctists and some bacteria are photoautotrophs. They use sunlight energy in photosynthesis to make large, organic molecules that contain chemical potential energy, which they and consumers and decomposers can then use. Respiration releases the energy, which is used to phosphorylate (add inorganic phosphate to) ADP, making ATP. This phosphorylation also transfers energy to the ATP molecule. SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY The molecule ATP (adenosine triphosphate) is the universal energy carrier for the cell. ATP can release its energy quickly; only one chemical reaction (hydrolysis of the terminal phosphate) is required. This reaction is catalysed by the enzyme ATPase. Once ATP has released its energy, it becomes ADP (adenosine diphosphate), a low energy molecule that can be recharged by adding a phosphate. This requires energy, which is supplied by the controlled breakdown of respiratory substrates in cellular respiration. The most common respiratory substrate is glucose, but other molecules e.g. fats or proteins may also be used. In the presence of the enzyme ATPase, the ATP molecule loses a phosphate. The energy released from the loss of a phosphate (30.7kJ) is available for immediate work inside the cell i.e. powering chemical reactions. A free phosphate is released from the ATP (this may be reused later to regenerate ADP into ATP again). Respiration generates ATP by two methods: substrate-level phosphorylation where an enzyme transfers a phosphate group from a substrate to ADP; oxidative phosphorylation where glucose is oxidised in a series of redox reactions that provide the energy for the formation of ATP; SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY Cellular respiration Cellular respiration is the process by which organisms break down energy rich molecules e.g. glucose to release energy in a useable form (ATP). All living cells respire in order to exist, although the substrates they use may vary. Aerobic respiration requires oxygen. Forms of cellular respiration that do not require oxygen are said to be anaerobic. Some plants and animals can generate ATP anaerobically for short periods of time. Other organisms use only anaerobic respiration and live in oxygen-free environments. For these organisms, there is some other final electron acceptor other than oxygen e.g. nitrate or Fe2+. Respiration involves three metabolic stages, summarised below: The first two stages are the catabolic pathways that decompose glucose and other organic fuels. In the third stage, the electron transport chain accepts electrons from the first two stages and passes these from one electron acceptor to another. The energy released at each stepwise transfer is used to make ATP. The final electron acceptor in this process is molecular oxygen. 1. Glycolysis. This occurs in the cytoplasm and involves the breakdown of glucose into two molecules of pyruvate. 2. The Krebs cycle. This occurs in the mitochondrial matrix, and decomposes a derivative of pyruvate to carbon dioxide. 3. Electron transport and oxidative phosphorylation. This occurs in the inner membranes of the mitochondrion and accounts for almost 90% of the ATP generated by respiration. SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY Glycolysis Glycolysis is the first part of respiration that involves the breakdown of glucose in the cytoplasm. Glucose (a 6-carbon sugar) is broken into two molecules of pyruvate (also called pyruvic acid), a 3carbon acid. A total of 2 ATP and 2NADH + 2H+ are generated from this stage. No oxygen is required (the process is anaerobic). This pathway involves a sequence of ten reactions, each catalysed by a different enzyme. The coenzyme NAD is also involved. The pathway can be considered to involve four stages. SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY Stage 1: Phosphorylation Glucose is a hexose (6C) sugar and as its molecules are stable, they need to be activated before they can be split into two. One ATP molecule is hydrolysed and the phosphate group released is attached to the glucose molecule at carbon 6. Glucose 6-phosphate is changed to fructose 6-phosphate. Another ATP is hydrolysed and the phosphate group released is attached to fructose 6phosphate at carbon 1. This activated hexose sugar is now called fructose 1, 6-bisphosphate. The energy from the hydrolysed ATP molecules activates the hexose sugar and prevents it from being transported out of the cell. This activated, phosphorylated sugar is referred to as hexose 1, 6-bisphosphate. This stage has used two molecules of ATP for each molecule of glucose. Stage 2: Splitting of hexose 1, 6-bisphosphate Each molecule of hexose bisphosphate is split into two molecules of triose phosphate (3 carbon sugar molecules each with one phosphate group attached). Stage 3: Oxidation of triose phosphate Although this process is anaerobic, it involves oxidation. Two hydrogen atoms (with their electrons) are removed from each triose phosphate molecule (the substrate). This involves dehydrogenase enzymes. These are aided by the coenzyme NAD (nicotinamide adenine dinucleotide), which is a hydrogen acceptor. NAD combines with the hydrogen, becoming reduced NAD. At this stage of glycolysis, two molecules of NAD are reduced per molecule of glucose. Also at this stage, two molecules of ATP are formed. This is called substrate-level phosphorylation. Stage 4: Conversion of triose phosphate to pyruvate Four enzyme-catalysed reactions convert each triose phosphate molecule to a molecule of pyruvate. Pyruvate is also a 3-carbon compound. In the process another two molecules of ADP are phosphorylated (an inorganic phosphate group, P i is added) to two molecules of ATP (by substrate-level phosphorylation). SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY 1. Aerobic respiration may be summarised by the following equation: C6H12O6 + 6O2 → 6CO2 + 6H2O Although carbon dioxide and water are products of aerobic respiration, the equation is an over-simplification of the process. State and explain one way in which this equation is an over-simplification. ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ _____________________________________________________________ [2] 2. (a) Fig. 1.1 represents a molecule of ATP. (i) Name the parts of the ATP molecule labelled X, Y and Z. X ___________________________________________________________ Y ___________________________________________________________ Z ___________________________________________________________ [3] SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY (ii) With reference to Fig. 1.1, describe and explain the role of ATP in the cell. _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ __________________________________________________________ [3] 3. Organisms require energy in order to carry out essential metabolism. Organisms are able to release energy by carrying out both aerobic and anaerobic respiration. (a) Complete the table below to compare anaerobic respiration in mammals and yeast. (b) Suggest one benefit of anaerobic respiration to an organism. _____________________________________________________________ __________________________________________________________ [1] SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY 4. Adenosine triphosphate (ATP) is an important product of respiration. The ATP molecule is made up of five sub-units, as shown in Fig. 5.1. (i) In the space below, indicate how these sub-units are joined in a molecule of ATP. [2] (ii) Suggest the type of reaction that removes a phosphate group from an ATP molecule. _________________________________________________________ [1] 5. Glycolysis is the initial stage of cellular respiration. (a) State precisely where in the cell glycolysis occurs. __________________________________________________________ [1] SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY (b) Outline the process of glycolysis. In your answer, you should use appropriate technical terms, spelled correctly ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ _____________________________________________________________ [4] (c) Yeast cells can carry out anaerobic respiration. Fig. 5.1 outlines the process of anaerobic respiration in yeast. Identify the compounds W to Z. W ___________________________________________________________ X ___________________________________________________________ Y ___________________________________________________________ Z ___________________________________________________________ [4] SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY 6. (a) Fig. 2.1 represents the first stage of respiration. (i) Name the stage represented by Fig. 2.1. ________________________________________________________ [1] (ii) State precisely where in the cell this stage takes place. ________________________________________________________ [1] (iii) Identify the compounds D, E and F. D _____________________________________________________ E _____________________________________________________ F _____________________________________________________ [3] SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY (b) In anaerobic conditions, compound F does not proceed to the link reaction. Describe the fate of compound F during anaerobic respiration in an animal cell and explain the importance of this reaction. ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ ___________________________________________________________ [5] SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY The link reaction, Krebs cycle and electron transport chain Learning objectives: State that, during aerobic respiration in animals, pyruvate is actively transported into mitochondria; Explain with the aid of diagrams and electron micrographs, how the structure of mitochondria enables them to carry out their functions; State that the link reaction takes place in the mitochondrial matrix; Outline the link reaction, with reference to decarboxylation of pyruvate to acetate and the reduction of NAD; Explain that acetate is combined with coenzyme A to be carried to the next stage; State that the Krebs cycle takes place in the mitochondrial matrix; Outline the Krebs cycle, with reference to the formation of citrate from acetate and oxaloacetate and the reconversion of citrate to oxaloacetate (names of intermediate compounds are not required); Explain that during the Krebs cycle, decarboxylation and dehydrogenation occur, NAD and FAD are reduced and substrate level phosphorylation occurs; Outline the process of oxidative phosphorylation, with reference to the roles of electron carriers, oxygen and the mitochondrial cristae; Outline the process of chemiosmosis, with reference to the electron transport chain, proton gradients and ATP synthase; State that oxygen is the final electron acceptor in aerobic respiration; Evaluate the experimental evidence for the theory of chemiosmosis; Explain why the theoretical maximum yield of ATP per molecule of glucose is rarely, if ever, achieved in aerobic respiration; Explain why anaerobic respiration produces a much lower yield of ATP than aerobic respiration; Compare and contrast anaerobic respiration in mammals and yeast; Define the term respiratory substrate; Explain the difference in relative energy values of carbohydrate, lipid and protein respiratory substrates; Key definitions: Compile a glossary by writing your own definitions for the following key terms related to the learning objectives above. Key term link reaction pyruvate dehydrogenase Definition SACKVILLE SCIENCE DEPARTMENT Key term A2 BIOLOGY Definition pyruvate decarboxylase acetyl coenzyme A Krebs cycle substrate-level phosphorylation mitochondrial matrix cristae oxidative phosphorylation electron transport chain chemiosmosis anaerobic respiration respiratory substrate mole The link reaction Pyruvate produced during glycolysis is transported across the inner and outer mitochondrial membranes to the matrix. The pyruvate that enters the mitochondrion has carbon dioxide removed. Coenzyme A (CoA) picks up the remaining 2-carbon fragment of the pyruvate to form acetyl coenzyme A. The following equation summarises the link reaction: 2 pyruvate + 2NAD+ + 2CoA → 2CO2 + 2 reduced NAD + 2 acetyl CoA SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY Decarboxylation and dehydrogenation of pyruvate to acetate are enzyme-catalysed reactions: pyruvate dehydrogenase removes hydrogen atoms from pyruvate; pyruvate decarboxylase removes a carboxyl group, which eventually becomes carbon dioxide, from pyruvate; the coenzyme NAD accepts the hydrogen atoms; Coenzyme A (CoA) accepts acetate, to become acetyl coenzyme A. The function of CoA is to carry acetate to the Krebs cycle. Krebs cycle The Krebs cycle also takes place in the mitochondrial matrix. It is a series of enzyme-catalysed reactions that oxidise the acetyl group of acetyl CoA to two molecules of carbon dioxide. It also produces one molecule of ATP by substrate-level phosphorylation, and reduces three molecules of NAD and one molecule of FAD. NAD and FAD are hydrogen acceptors, transporting hydrogens to the electron transport chain. The acetyl group passes into a cyclic reaction and combines with a 4-carbon molecule to form a 6carbon molecule. The CoA is released for reuse. Successive steps in the cycle remove carbon as carbon dioxide. The acetate is offloaded from coenzyme A (which is then free to collect more acetate) and joins with a 4-carbon compound, called oxaloacetate, to form a 6-carbon compound, called citrate. Citrate is decarboxylated (one molecule of carbon dioxide removed) and dehydrogenated (a pair of hydrogen atoms removed) to form a 5-carbon compound. The pair of hydrogen atoms is accepted by a molecule of NAD, which becomes reduced. The 5-carbon compound is decarboxylated and dehydrogenated to form a 4-carbon compound and another molecule of reduced NAD. The 4-carbon compound is changed into another 4-carbon compound. During this reaction a molecule of ADP is phosphorylated to produce a molecule of ATP. This is substrate-level phosphorylation. The second 4-carbon compound is changed into another 4-carbon compound. A pair of hydrogen atoms is removed and accepted by the coenzyme FAD, which is reduced. The third 4-carbon compound is further dehydrogenated and regenerates oxaloacetate. Another molecule of NAD is reduced. SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY There is one turn of the cycle for each molecule of acetate, which was made from one molecule of pyruvate. Therefore there are two turns of the cycle for each molecule glucose. Electron transport chain The final stage of aerobic respiration involves electron carriers embedded in the inner mitochondrial membranes. These membranes are folded into cristae, increasing the surface area for electron carriers and ATP synthase enzymes. Hydrogen pairs are transferred to the electron transport chain. The hydrogens or electrons are passed from one carrier to the next, losing energy as they go. The energy released in this stepwise process is used to produce ATP. Oxygen is the final electron acceptor and is reduced to water. *FAD enters the electron transport chain at a lower energy level than NAD, and only 2ATP are generated per FAD.H2. SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY Reduced NAD and reduced FAD are reoxidised when they donate hydrogen atoms, which are split into protons and electrons, to the electron carriers. The first electron carrier to accept electrons from reduced NAD is a protein complex I, called NADH-coenzyme Q reductase (also known as NADH dehydrogenase). The protons (H+) go into solution in the matrix. The electrons are passed along a chain of electron carriers and then donated to molecular oxygen, the final electron acceptor. As electrons flow along the electron transport chain, energy is released and used, by coenzymes associated with some of the electron carriers (complexes I, III and IV), to pump the protons across to the intermembrane space. This builds up a proton gradient which is also a pH gradient and an electrochemical gradient. Thus, potential energy builds up in the intermembrane space. The hydrogen ions cannot diffuse through the lipid part of the inner membrane but can diffuse through ion channels in it. These channels are associated with the enzyme ATP synthase. This flow of hydrogen ions (protons) is chemiosmosis. Oxidative phosphorylation is the formation of ATP by the addition of inorganic phosphate to ADP in the presence of oxygen. As protons flow through an ATP synthase enzyme, they drive the rotation of part of the enzyme and join ADP and P i (inorganic phosphate) to form ATP. The electrons are passed from the last electron carrier in the chain to molecular oxygen, which is the final electron acceptor. Hydrogen ions also join so that oxygen is reduced to water. 4H+ + 4e- + O2 → 2H2O SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY Chemiosmosis Chemiosmosis is the process whereby the synthesis of ATP is coupled to electron transport and the movement of protons (H+ ions). Electron chain carriers are arranged over the inner membrane of the mitochondrion and oxidise NADH + H+ and FADH2. Energy from this process forces protons to move, against their concentration gradient, from the mitochondrial matrix into the space between the two membranes. Eventually the protons flow back into the matrix via ATP synthase molecules in the membrane. As the protons flow down their concentration gradient, energy is released and ATP is synthesised. Chemiosmotic theory also explains the generation of ATP in the light dependent phase of photosynthesis. SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY Evaluating the evidence for chemiosmosis By the early 1960s, research teams were extracting mitochondria from cells and examining them, using electron microscopes and special staining techniques. They could identify an outer and inner membrane with a space between them, and could see that the inner membrane was folded into cristae covered on the inner surface with many small (9nm in diameter) mushroom-shaped particles. In 1961, Peter Mitchell realised that the build-up of hydrogen ions on one side of a membrane would be a source of potential energy and that the movement of ions across the membrane, down an electrochemical gradient, could provide the energy needed to power the formation of ATP from ADP and Pi. he called this chemiosmosis theory. The inner mitochondrial membrane is therefore an energy-transducing membrane. He postulated that the energy released from the transfer of electrons along the electron transport chain was used to pump hydrogen ions from the matrix to the intermembrane space and that these protons then flowed through protein channels, attached to enzymes. The kinetic energy or the force of this flow, the proton motive force, drove the formation of ATP. At first his theory was greeted with great scepticism as it was radically different from the idea of a high-energy intermediate compound. However, by 1978 there was much evidence supporting the theory and Mitchell was awarded the Nobel Prize for chemistry. Since then scientists have established that the stalked particles are ATP synthase enzymes and have discovered how they function. It is also now known that some of the complexes in the electron transport chain have coenzymes that can use the energy released from electron transport to pump hydrogen ions across the membrane, into the intermembrane space, where a proton or electrochemical gradient builds up. SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY 1. Fig. 1.2 is an electron micrograph of a mitochondrion from an animal cell. (i) Name the structure labelled A. _______________________________________________________ [1] (ii) Name the specific process that is carried out by structure A in the mitochondrion. _______________________________________________________ [1] 2. Fig. 3.1 (on the next page) represents some of the reactions that take place in a leaf cell of a flowering plant. (a) Name the reaction pathways indicated by the letters W, X and Y. W ___________________________________________________________ X ___________________________________________________________ Y ___________________________________________________________ [3] SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY (b) Triose phosphate is a compound that is central to the metabolism of this cell. Explain how the three reaction pathways (W, X and Y) are able to work independently of each other in the same leaf cell. _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ __________________________________________________________ [3] SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY (c) Identify which of these three reaction pathways (W, X and Y) are associated with: photosynthesis _________________________________________________ aerobic respiration ___________________________________________ [2] (d) Fig. 3.1 shows that compounds from two of the three pathways are used in oxidative phosphorylation. State the products of oxidative phosphorylation. _____________________________________________________________ ___________________________________________________________ [2] (e) Explain the role of coenzymes in this leaf cell, with respect to the metabolic reactions outlined in Fig. 3.1. _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ __________________________________________________________ [3] 3. Fatigue is a symptom of some medical conditions. One feature of fatigue is extreme tiredness, due to a lack of energy. Medical conditions that have fatigue as a characteristic symptom include Type 2 diabetes, certain heart conditions, chronic fatigue syndrome (CFS) and emphysema. (a) Explain how emphysema could result in fatigue. _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ __________________________________________________________ [2] SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY (b) In Type 2 diabetes, the target cells do not respond correctly to the insulin produced when there is an increase in blood glucose concentration. Suggest why fatigue may occur in a person with Type 2 diabetes who is not taking medication. _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ __________________________________________________________ [2] (c) Certain heart conditions result in a weak and irregular heart beat. Suggest how a weak and irregular heart beat could result in fatigue. _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ __________________________________________________________ [2] (d) Chronic fatigue syndrome (CFS) is a condition in which symptoms vary from individual to individual. It is thought that a number of malfunctioning processes can contribute to this condition in an individual. CFS can affect every system in the body and is identified by symptoms that include fatigue, muscle weakness and aching muscles. (i) It has been suggested that, in the cells of people with CFS, pyruvate might not be transferred into mitochondria efficiently. Outline the consequences of an inefficient transfer of pyruvate into mitochondria and link this to the symptoms of CFS stated above. SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ________________________________________________________ [3] (ii) Some people with CFS overproduce T lymphocytes and associated cytokines. Despite this, the specific immune response is poor in these people, resulting in an increased susceptibility to infection. Suggest a reason for the poor specific immune response in people with CFS. ___________________________________________________________ ___________________________________________________________ ________________________________________________________ [1] 4. The formation of ATP is now widely accepted as being achieved by the process of chemiosmosis. Various pieces of evidence have been documented to support this theory. Three of these are described below: SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY Identify the piece of evidence above, 1, 2 or 3, that supports each of the following statements about the theory of chemiosmosis. Write ‘none’ if a statement is not supported by any of the pieces of evidence above. (i) Electron transfer occurs on the inner membrane of the mitochondrion. _____________ [1] (ii) (iii) Protons are actively pumped across the inner mitochondrial membrane into the intermembrane space. _____________ [1] Protons accumulate in the inter-membrane space. _____________ [1] SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY Measuring respiration In small animals or germinating seeds, the rate of cellular respiration can be measured using a simple respirometer: a sealed unit where the carbon dioxide produced by the respiring tissues is absorbed by soda lime and the volume of oxygen consumed is detected by fluid displacement in a manometer. Germinating seeds are also often used to calculate the respiratory quotient (RQ): the ratio of the amount of carbon dioxide produced during cellular respiration to the amount of oxygen consumed. RQ provides a useful indication of the respiratory substrate being used. The respiratory quotient (RQ) can be expressed simply as: RQ = CO2 produced O2 consumed When pure carbohydrate is oxidised in cellular respiration, the RQ is 1.0; more oxygen is required to oxidise fatty acids (RQ = 0.7). the RQ for protein is about 0.9. Organisms usually respire a mix of substrates, giving RQ values of between 0.8 and 0.9. Anaerobic pathways All organisms can metabolise glucose anaerobically (without oxygen) using glycolysis in the cytoplasm, but the energy yield from this process is low and few organisms can obtain sufficient energy for their needs in this way. In the absence of oxygen, glycolysis soon stops unless there is an alternative acceptor for the electrons produced from the glycolytic pathway. In yeasts and the root cells of higher plants this acceptor is ethanal, and the pathway is called alcoholic fermentation. In the skeletal muscle of mammals, the acceptor is pyruvate itself and the end product is lactic acid. In both cases, the duration of the fermentation is limited by the toxic effects of the organic compound produced. Although fermentation is often used synonymously with anaerobic respiration, they are not the same. Respiration always involves hydrogen ions passing down a chain of carriers to the terminal acceptor, and this does not occur in fermentation. In anaerobic respiration, the terminal H + acceptor is a molecule other than oxygen e.g. Fe2+ or nitrate. Alcoholic fermentation SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY In alcoholic fermentation, the H+ acceptor is ethanal (acetaldehyde) which is reduced to ethanol with the release of CO2. Yeasts respire aerobically when oxygen is available but can use alcoholic fermentation when it is not. At levels above 12-15%, the ethanol produced by alcoholic fermentation is toxic to the yeast cells and this limits their ability to use this pathway indefinitely. The root cells of plants also use fermentation as a pathway when oxygen is unavailable but the ethanol must be converted back to respiratory intermediates and respired aerobically. Lactic acid fermentation In the absence of oxygen, the skeletal muscle cells of mammals are able to continue using glycolysis for ATP production by reducing pyruvate to lactic acid (the H+ acceptor is pyruvate itself). This process is called lactic acid fermentation. Lactic acid is toxic and this pathway cannot continue indefinitely. The lactic acid must be removed from the muscle and transported to the liver, where it is converted back to respiratory intermediates and respired aerobically. SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY 1. One way of calculating the rate of respiration is to measure the volume of oxygen taken up over a period of time. A student carried out an experiment to investigate the effect of temperature on the rate of respiration in soaked (germinating) pea seeds and dry (dormant) pea seeds. A simple piece of apparatus called a respirometer was used, as shown in Fig. 4.1. The potassium hydroxide solution in this apparatus absorbs carbon dioxide. If the apparatus is kept at a constant temperature, any changes in the volume of air in the respirometer will be due to oxygen uptake. (a) State the stage or stages of aerobic respiration during which: (i) carbon dioxide is produced ___________________________________________________________ ________________________________________________________ [1] SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY (ii) oxygen is used ___________________________________________________________ ________________________________________________________ [1] (b) The student set up three respirometers, A, B and C, in water baths at two different temperatures. The respirometers were left for 10 minutes in order to equilibrate. The contents of each respirometer are shown in Table 4.1. At each temperature, respirometer C, which contained only glass beads, was a control. Respirometer B, at each temperature, also contained some glass beads. (i) Suggest why, at each temperature, respirometer B contained some glass beads. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ________________________________________________________ [2] SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY (ii) Suggest how the student determined the quantity of glass beads to place in respirometer B at each temperature. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ________________________________________________________ [2] (c) After the student had left each respirometer to equilibrate, a small volume of coloured fluid was introduced into each graduated tube. The respirometers were then left in the appropriate water baths for 20 minutes and maintained at the correct temperature. During this time, the coloured fluid in the graduated tube moved. The level of the coloured fluid in each respirometer was recorded at the start of the experiment and after 20 minutes. The results are summarised in table 4.2. SACKVILLE SCIENCE DEPARTMENT (i) A2 BIOLOGY Table 4.2 is incomplete. Calculate the missing value for the rate of oxygen uptake for soaked pea seeds (A) at 25°C. Show your working. Answer = _______________ cm3min-1 [2] (ii) Explain why there is an increased rate of respiration in soaked seeds at 25°C compared with soaked seeds at 15°C. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ________________________________________________________ [2] (iii) Suggest a reason for the difference in the rate of respiration between soaked and dry pea seeds. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ________________________________________________________ [2] SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY 2. The compound 2, 3, 5-triphenyl-tetrazolium chloride (TTC) is an electron carrier. TTC will diffuse into actively respiring cells and accept electrons from the electron transport chain. When TTC accepts electrons and becomes reduced, it changes from colourless to pink. The tissues in which this reaction takes place will be stained a pink colour. (a) State the precise location of the electron transport chain in the cell. __________________________________________________________ [1] (b) A student carried out an investigation into the respiratory activity of plant tissue. She used three groups of germinating bean seeds. These were first treated as shown in Table 3.1. The groups of seeds were then sliced longitudinally and placed cut surface down, in a shallow dish containing a small volume of TTC solution. The cut surfaces remained in contact with the solution for 10 minutes. The seeds were then removed from the dish. The excess TTC solution was wiped off and the cut surfaces of the seeds in each group were observed. The appearance of the seeds in each group is shown in Fig. 3.1. the shaded areas are the regions where the tissues have stained a pink colour. SACKVILLE SCIENCE DEPARTMENT (i) A2 BIOLOGY Describe the differences observed in the seeds in groups A, B and C. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ________________________________________________________ [1] (ii) Suggest reasons for the results observed in the seeds in group A. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ________________________________________________________ [2] (iii) Suggest reasons for the difference in the amount of staining observed in the groups B and C when compared to those in group A. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ________________________________________________________ [2] 3. In South-East Asia the main source of commercial sugar is the palm, Borassus flabellifer. Sap of this species has a high sugar content. Yeasts and bacteria, however, can contaminate the sap as it is collected and ferment the sugar, producing ethanol. This contamination makes it less suitable as a source of sugar. A study was carried out to investigate the effect of three treatments traditionally used to reduce fermentation during the collection of sap. The sap is treated in one of the following ways: with a weak alkaline solution (treatment A) with bark from the tree Vateria copallifera (treatment V) with bark from the tree Careya arborea (treatment C) the sap was collected from the palm trees over a 60-hour period. Samples of the collected sap were taken at 15 hour intervals. In each sample, the concentration of alcohol and the number of bacteria were recorded. The results are shown in Table 5.1 (see next page). (a) With reference to Table 5.1, describe the effect of the different treatments on the alcohol concentration of the treated samples compared with the control samples. _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ __________________________________________________________ [2] SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY (b) Suggest a reason for the difference in alcohol concentration at 60 hours between the two bark treatments V and C. _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ __________________________________________________________ [1] SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY (c) To be used as a source of commercial sugar, the sap needs to be as uncontaminated as possible. Suggest, with a reason, which of the treatments shown in Table 5.1 would be the best for use with sap so that it is suitable as a source of commercial sugar. _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ __________________________________________________________ [2] 4. Humans harvest a wide range of fruits and vegetables as food. Cellular respiration supplies energy and forms part of the natural ripening process in fruits and vegetables. This ripening process may continue after the fruits and vegetables are harvested, as the cells continue to respire. The rate of cellular respiration after harvesting affects the shelf-life of fruits and vegetables as it can lead to changes in food quality. After harvesting, some fruits and vegetables enter a dormant (inactive) state while others remain active during storage. Table 5.1 (on the next page) contains data that show the respiration rate of a selection of fruits and vegetables stored at different temperatures after harvesting. The respiration rate is measured by the rate of carbon dioxide produced. (i) Describe the pattern of respiration shown by cauliflower at increasing storage temperatures of 0°C to 20°C. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ________________________________________________________ [2] SACKVILLE SCIENCE DEPARTMENT (ii) A2 BIOLOGY Discuss what the data in Table 5.1 indicate about the best conditions for storage of fruits and vegetables. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ________________________________________________________ [2] (iii) Identify, with reasons, which fruit or vegetable listed in Table 5.1 is least likely to spoil during storage. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY ___________________________________________________________ ___________________________________________________________ ________________________________________________________ [3] (iv) Which fruit or vegetable listed in Table 5.1 is likely to be the most difficult to keep fresh during storage? Give a reason for your answer. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ________________________________________________________ [1] 5. Respiration can be aerobic or anaerobic. (a) Certain parasites live in the blood of mammals. Suggest why, even though blood carries oxygen, these parasites are adapted to respire anaerobically. _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ __________________________________________________________ [2] (b) The anaerobic respiration pathway in animal cells can be reversed but the anaerobic respiration pathway in yeast cells cannot be reversed. Explain why, using your knowledge of the differences between the two pathways. SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY In your answer, you should use appropriate technical terms, spelled correctly ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ _____________________________________________________________ [4] 6. Some animals conserve energy by entering a state of torpor (a short period of dormancy), in which they allow their body temperature to fall below normal for a number of hours. In an investigation into torpor in the Siberian hamster, Phodopus sungorus, the animal’s respiratory quotient (RQ) was measured before and during the period of torpor. The respiratory quotient is determined by the following equation: RQ = volume of carbon dioxide produced volume of oxygen consumed in same time SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY RQ values for different respiratory substrates have been determined and are shown in Table 1.1. Initially, the RQ value determined for the hamster was 0.95 but as the period of torpor progressed, its RQ value decreased to 0.75. What do these values suggest about the substrates being respired by the hamster during the period of the investigation? ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ _____________________________________________________________ [3] 7. The liver is an organ that is metabolically active, carrying out over 500 different functions. Some of its important functions include converting chemicals including toxins, into other compounds. Fig. 2.1 (on the next page) outlines some of the reaction pathways that take place in the liver cells. The underlined words represent toxic compounds. SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY (a) The lactate that enters pathway S is produced by cells, such as muscle cells, undergoing anaerobic respiration. Suggest why this lactate is converted into pyruvate by the hepatocytes (liver cells) rather than by the respiring cells in which it is produced. ______________________________________________________________ ______________________________________________________________ ___________________________________________________________ [1] Alcohol (ethanol) is oxidised in the liver by Pathway Q. if a person has a high alcohol intake, it will result in the production of excess reduced NAD. (b) Excess reduced NAD in the liver cells will influence some metabolic pathways by: inhibiting the conversion of lactate to pyruvate inhibiting fatty acid oxidation promoting fatty acid synthesis Using this information and the information in Fig. 2.1, suggest the consequences for liver metabolism if a person has a regular high alcohol intake. SACKVILLE SCIENCE DEPARTMENT A2 BIOLOGY _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ __________________________________________________________ [2] (c) State precisely where in the liver cell the excess reduced NAD can be reoxidised. __________________________________________________________ [2] 8. If oxygen is not present or is in short supply, respiration can take an anaerobic pathway after glycolysis. In plant cells, this pathway is the same as the one used in yeast cells. (i) Name the hydrogen acceptor in this pathway. _____________________________________________ (ii) Name the intermediate compound in this pathway. _____________________________________________ (iii) [1] Name the products of this pathway. _____________________________________________ (iv) [1] [1] Explain why this pathway is important for the plant cell. ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ________________________________________________________ [2]