OCR (A) Biology A-level 5.2 Energy for Biological Processes PDF

OCR (A) Biology A-level Topic 5.2: Energy for biological processes Notes www.pmt.education  Photosynthesis Photosynthesis is a reaction in which light energy is used to split apart the strong bonds in water molecules in a pro...

OCR (A) Biology A-level Topic 5.2: Energy for biological processes Notes www.pmt.education  Photosynthesis Photosynthesis is a reaction in which light energy is used to split apart the strong bonds in water molecules in a process of photolysis in order to combine hydrogen with carbon dioxide to produce a fuel in the form of glucose. Oxygen is a waste product of this reaction and is released into the atmosphere. The rate of photosynthesis is determined by carbon dioxide concentration, light intensity and well as temperature. The chloroplast is the site of photosynthesis and it is adapted to photosynthesise in the following ways: It contains stacks of thylakoid membranes called grana which contain the photosynthetic pigments such as chlorophyll arranged as photosystems It contains stroma which is the fluid surrounding the grana, stroma contains all the enzymes required for the light independent stage of photosynthesis Figure 1 Biology TutorVista There are two stages of photosynthesis: Light-dependent reaction in which electrons are excited to a higher energy level by the energy trapped by chlorophyll molecules in the thylakoid membranes. Electrons are then passed down the electron transport chain from one electron carrier to the next and this process generates ATP from ADP and inorganic phosphate in a process called photophosphorylation. Reduced NADP is also generated in the light- dependent stage as the electrons are transferred to NADP (NADPH) along with a proton. Both ATP and reduced NADP are then used in the light-independent stage of photosynthesis. Light-independent reaction also known as the Calvin cycle is the final stage of photosynthesis which uses ATP (source of energy) and reduced NADP (reducing power) to produce glucose. Light independent reaction occurs as following: 1) RuBP is combined with carbon dioxide in a reaction called carbon fixation catalysed by the enzyme RUBISCO. 2) RuBP is converted into two glycerate 3-phosphate (GP) molecules 3) Reduced NADP and ATP are used to convert GP to triose phosphate (TP) 4) Some of TP molecules are used to make glucose which is then converted to essential organic compounds such as polysaccharides, lipids, amino acids and nucleic acids. 5) Remaining TP molecules are used to reform RuBP with the help of ATP. www.pmt.education  Factors affecting photosynthesis Photosynthesis is affected by many factors, the factor in lowest supply and therefore limiting the rate of the reaction is known as the limiting factor. Limiting factors include: Light intensity – if this is in short supply the light dependent reaction will slow therefore there will be lower amounts of ATP and NADPH created. This will then affect the Calvin cycle as these are needed to convert GP to TP. So the level of GP will rise and TP will fall which in turn causes RuBP levels to fall. CO2 concentration – if this is in short supply the light independent reaction will slow Temperature – if this is low Rubisco and other molecules will have lower levels of kinetic energy therefore the enzyme-controlled reactions are affected. Respiration Aerobic respiration is splitting of the respiratory substrate, to release carbon dioxide as a waste product and reuniting of hydrogen with atmospheric oxygen with the release of a large amount of energy whereas anaerobic respiration occurs in the absence of oxygen. Respiration is a multi-step process with each step controlled and catalysed by a specific intracellular enzyme. Glycolysis is the first process of both aerobic and anaerobic respiration and itoccurs in cytoplasm. In this process a molecule of glucose is phosphorylated to produce 2 molecules of pyruvate, 2 molecules of NADH and a net production of 2 molecules of ATP. The next step of aerobic reaction is the link reaction where each pyruvate is converted to acetyl which binds to coenzyme A. In the process NAD is reduced to NADH and CO2 is produced as Figure 2 IB guides pyruvate is decarboxylated. Co-enzyme A delivers Acetyl to the Krebs cycle where glucose is oxidised and carbon dioxide, ATP, reduced NAD and reduced FAD are produced. Each glucose molecule www.pmt.education causes the cycle to turn twice so per glucose we produce 4CO2, 4NADH, 2FADH and 2ATP molecules in the Krebs cycle. The ATP molecules produced by the Kreb’s cycle are done so by substrate level phosphorylation Oxidative phosphorylation Oxidative phosphorylation is the process in which ATP is synthesised in the electron transport chain in mitochondria. This process generates the majority of ATP in aerobic respiration and it occurs as following: Reduced coenzymes (NADH and FADH) carry hydrogen ions and electrons to the electron transport chain which occurs on the inner mitochondrial membrane Electrons are carried from one electron carrier to another in a series of redox reactions: the electron carrier which passes the electron on is oxidised whereas the electron carrier which receives it is reduced The energy provided by the electrons to the electron carriers is used to moveHydrogen ions across the inner membrane into the intermembrane space – as a result of that the concentration of the hydrogen ions in the intermembrane space is high The inner membrane is impermeable to H+ Hydrogen ions diffuse into the mitochondrial matrix down the electrochemical gradient through the ATPase enzyme. ATP is produced on stalked particle using ATP synthase Hydrogen atoms are produced from hydrogen ions and electrons. The hydrogen atoms are then combined with oxygen to produce water Oxgen acts as the final electron acceptor www.pmt.education There is a theoretical yield of 38 ATP molecules per glucose molecule but in real life this is rarely achieved due to the inner mitochondrial membrane being ‘leaky’ to H+ therefore not all H+ move through the ATPase. The pyruvate made during glycolysis in cytoplasm needs moving into the matrix by active transport and so this uses ATP. Respiratory substrates include carbohydrates, lipids and proteins which release varying amounts of energy, depending on the number of hydrogens in the structure which are oxidised to water. For instance, the number of hydrogens is greater in fatty acids than carbohydrates. The respiratory quotient (RQ) can be measured to determine which respiratory substrate is being used and to determine if the organism is undergoing anaerobic respiration. RQ = carbon dioxide produced / oxygen consumed Different respiratory substrates have different RQ values e.g. carbohydrates have a value of 1.0, lipids – 0.8 and proteins 0.9. Anaerobic Respiration This occurs when the concentration of oxgen is low. ATP production still needs to happen but this can’t be done by oxidative phosphorylation due to the lack of oxygen to act as the final electron acceptor. In order for some ATP to be produced anaerobic respiration allows glycolysis to continue. (Glycolysis has a net production of 2 ATP per glucose molecule). In mammals pyruvate is converted to lactate. Pyruvate acts as the hydrogen acceptor to enable NADH to be reoxidised to NAD which can then be used to continue the reactions in glycolysis. Lactate can then be converted back to pyruvate in the liver cells when the oxygen levels rise again. Yeast and plants use alcoholic fermentation to enable glycolysis to continue. In this process Pyruvate is decarboxylated to EthanAl which in turn is reduced to ethanOl reoxidising NAD in the process. So ethanAl is the hydrogen acceptor. The first step in this process produces CO2 and therefore this is an irreversible reaction. www.pmt.education