T11 + 12 - Photosynthesis + Respiration Flashcards
Describe the light-dependent reaction of photosynthesis
Absorption, photoionisation, and the electron transport chain
Light energy is absorbed by pigments.
Photoionisation occurs, which is when these electrons are excited and lost from the pigments (the pigments are oxidised).
Electrons are transferred to an electron carrier molecule (the electron carrier is reduced).
Electrons are passed along the electron transport chain, releasing energy as they go.
Photolysis of water
Light is used to split water into electrons, protons, and oxygen: 2H2O → 4H+ + 4e- + O2.
The electrons replace those lost from pigments during photoionisation.
The protons are used for ATP production and combine with electrons to reduce NADP.
Oxygen gas is released as a by-product.
Chemiosmosis generating ATP and reduced NADP
The energy lost by electrons along the electron transport chain is used to pump protons across the thylakoid membrane into the thylakoid space.
This produces a proton gradient, where protons are in a higher concentration in the thylakoid space.
The protons then diffuse through ATP synthase into the stroma.
This movement powers ATP synthase to produce ATP from ADP and an inorganic phosphate.
NADP takes up protons and electrons in the stroma and is reduced.
Reduced NADP is carried into the light-independent reaction.
Describe the light-independent stage of photosynthesis
Carbon fixation:
Carbon dioxide reacts with a 5-carbon (5C) compound ribulose bisphosphate (RuBP) to form an unstable 6C compound.
This 6C compound splits into two 3C glycerate-3-phosphate (GP) molecules.
This is catalysed by the enzyme ribulose bisphosphate carboxylase (rubisco).
Reduction of GP:
GP is reduced into triose phosphate (TP).
This uses energy from the hydrolysis of ATP.
This also requires protons and electrons from reduced NADP, which itself is oxidised to regenerate NADP.
The NADP that is re-formed returns to the light-dependent reaction to be reduced again.
Regeneration of RuBP:
Most TP is used to regenerate RuBP using ATP.
The rest of the TP can be used to make other organic molecules.
For each turn of the Calvin cycle, five carbons are used to regenerate RuBP (5C) and only one carbon, from CO2 (1C), is available to make new organic compounds. This means six full turns of the Calvin cycle are needed to make one molecule of glucose (6C).
Describe glycolysis
Phosphorylation - Two ATP molecules donate phosphate groups to glucose.
Lysis - The phosphorylated glucose molecule is split into two molecules of triose phosphate (TP).
Dehydrogenation - A hydrogen is removed from each TP molecule (they are oxidised) and used to form two molecules of reduced NAD.
Production of ATP - The TP molecules are converted into two pyruvate molecules, also producing four ATP molecules through substrate-linked phosphorylation.
After glycolysis, if oxygen is available, pyruvate moves through mitochondrial membranes by active transport.
Describe the link reaction
Active transport of pyruvate - Pyruvate from glycolysis is actively transported into the mitochondrial matrix by specific carrier proteins.
Decarboxylation - In the mitochondrial matrix, each pyruvate molecule is decarboxylated, losing one molecule of CO2.
Removal of CO2 - CO2 diffuses out of the mitochondria as a waste product.
Oxidation of pyruvate - Two hydrogen atoms are removed from pyruvate to form a two-carbon molecule (acetate).
Reduction of NAD - The hydrogen atoms are used to reduce the coenzyme NAD, forming reduced NAD (an electron carrier).
Formation of acetyl CoA - Acetate binds to coenzyme A, forming acetyl coenzyme A (acetyl CoA).
Describe the Krebs cycle
Acetyl CoA (which contains 2C acetate) merges with a 4C molecule to create a 6C molecule.
The 6C molecule is decarboxylated, releasing two molecules of carbon dioxide.
The 6C molecule is also dehydrogenated (oxidised), releasing hydrogens that reduce NAD and FAD.
For each acetyl CoA that enters the cycle, one ATP (or GTP in some organisms) is synthesised directly via substrate-level phosphorylation.
The 4C molecule is regenerated for the next turn of the cycle.
Describe oxidative phosphorylation
Reduced NAD and reduced FAD release hydrogen, transferring protons (H+) and electrons (e-) into the mitochondrial matrix.
High-energy electrons are passed to an electron carrier from reduced NAD and reduced FAD.
The electrons are passed along a series of electron carrier molecules in the electron transport chain embedded in the inner mitochondrial membrane, releasing energy as they are transferred.
The energy is used to actively transport protons across the inner mitochondrial membrane from the mitochondrial matrix into the intermembrane space.
The accumulation of protons in the intermembrane space sets up a steep electrochemical gradient of protons across the inner membrane.
Protons diffuse back into the mitochondrial matrix down their electrochemical gradient through ATP synthase.
This releases energy and catalyses the synthesis of ATP from ADP and inorganic phosphate (Pi).
Oxygen is the final electron acceptor, and combines with electrons and protons to form water, helping to maintain the proton gradient.
Chemiosmosis
In aerobic respiration, chemiosmosis is the diffusion of protons across the partially permeable inner mitochondrial membrane, down their electrochemical gradient through ATP synthase channels.
The movement of the protons releases energy that is used to synthesise ATP. It converts the energy from the electrochemical gradient into chemical energy stored in ATP molecules.
How can lipids be used as a respiratory substrate?
Lipids are hydrolysed into glycerol and fatty acids.
Glycerol is converted into triose phosphate and enters the glycolysis pathway.
Fatty acids are broken down into two-carbon fragments and converted into acetyl coenzyme A, which enters the Krebs cycle.
How can proteins be used as a respiratory substrate?
Proteins are hydrolysed into amino acids.
The amino group is removed from the amino acids (deamination).
Three-carbon compounds are converted into pyruvate, while four- and five-carbon compounds are converted into intermediates in the Krebs cycle.
The amino acids produced from hydrolysis of proteins are generally used only as a last resort for releasing energy because they have many other important, specialised functions.
Describe anaerobic respiration in yeasts and some plants
Alcohol fermentation:
Pyruvate loses a molecule of CO2 and accepts a hydrogen from reduced NAD.
This produces ethanol, and regenerates NAD.
In yeast, it can be used to produce ethanol for wine and beer production.
Describe anaerobic respiration in some animals and bacteria
Lactate fermentation:
Pyruvate accepts a hydrogen from reduced NAD.
This produces lactate, and regenerates NAD.
Lactate is removed by the blood and taken to the liver to be converted to glycogen, or oxidised to regenerate pyruvate when oxygen is available.
What happens if too much anaerobic respiration takes place?
If too much anaerobic respiration occurs in muscle tissue, the reduced quantity of ATP produced is insufficient to maintain vital processes for extended time periods. This means lactate accumulates, causing cramp and muscle fatigue, and it also reduces the pH affecting enzymes.
