5.2

• 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 photosynthesis and is released into the atmosphere.
• The rate of photosynthesis is determined by carbon dioxide concentration, light intensity and 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 and it contains stroma which is the fluid surrounding the grana, stroma contains all the enzymes required for the light independent stage of photosynthesis.
• There are two stages of photosynthesis: the light-dependent reaction in which electrons are excited to a higher energy level by the energy trapped by chlorophyll molecules in the thylakoid membranes and the light-independent reaction also known as the Calvin cycle which uses ATP and reduced NADP to produce glucose.
• In the light-dependent reaction, electrons are passed down the electron transport chain from one electron carrier to the next, generating 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.
• The light-independent reaction, also known as the Calvin cycle, uses ATP and reduced NADP to produce glucose.
• In the light-independent reaction, RuBP is combined with carbon dioxide in a reaction called carbon fixation catalysed by the enzyme RUBISCO.
• RuBP is converted into two glycerate 3-phosphate (GP) molecules in the light-independent reaction.
• Reduced NADP and ATP are used to convert GP to triose phosphate (TP) in the light-independent reaction.
• 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 in the light-independent reaction.
• The remaining TP molecules are used to reform RuBP with the help of ATP in the light-independent reaction.
• Factors affecting photosynthesis include light intensity, CO2 concentration, and temperature.
• Aerobic respiration is the 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.
• Anaerobic respiration occurs in the absence of oxygen.
• Glycolysis is the first process of both aerobic and anaerobic respiration and it occurs in cytoplasm.
• In glycolysis, 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.
• 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.
• 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.
• The inner membrane is impermeable to H+.
• 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.
• The energy provided by the electrons to the electron carriers is used to move Hydrogen ions across the inner membrane into the intermembrane space.
• RQ = carbon dioxide produced / oxygen consumed.
• The ATP molecules produced by the Kreb’s cycle are done so by substrate level phosphorylation.
• Anaerobic Respiration occurs when the concentration of oxgen is low.
• Hydrogen atoms are produced from hydrogen ions and electrons.
• The hydrogen atoms are then combined with oxygen to produce water.
• In the process, NAD is reduced to NADH and CO2 is produced as pyruvate is decarboxylated.
• In order for some ATP to be produced anaerobic respiration allows glycolysis to continue.
• 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.
• ATP is produced on stalked particle using ATP synthase.
• 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 next step of aerobic reaction is the link reaction where each pyruvate is converted to acetyl which binds to coenzyme A.
• In mammals pyruvate is converted to lactate.
• 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.
• Oxidative phosphorylation is the process in which ATP is synthesised in the electron transport chain in mitochondria.
• Each glucose molecule guides the cycle to turn twice so per glucose we produce 4CO2, 4NADH, 2FADH and 2ATP molecules in the Krebs cycle.
• For instance, the number of hydrogens is greater in fatty acids than carbohydrates.