9.2. - 9.3. Respiration Flashcards
Glycolysis…
The preparatory phase, occuring in the cytosol, in which glucose is split into 2x pyruvate molecules.
This generates a small amount of energy and no CO2.
Involves ten enzyme-catalysed reactions:
- Reactions 1-5: energy-investing reactions that require an input of ATP.
- Reactions 6-10: energy-harvesting reactions that yield NADH and ATP.
Produces:
- 2 molecules of pyruvate (3C).
- 2 molecules of ATP (produces 4, we net 2).
- 2 molecules of NADH (reduced NAD).
Pyruvate is oxidisied into acetate in the link reaction…
Pyruvate is oxidised into acetate, releasing CO2.
More NADH is produced using hydrogens released.
Some energy is stored by combining acetate and coenzyme A to form acetyl CoA.
Acetyl CoA is the starting point for the citric acid cycle…
An eight-reaction cycle in a steady state (the concentration of intermediates don’t change).
The cycle continues when starting materials are available again (acetyl CoA and reoxidised electron carriers).
Inputs: acetyl CoA, water and electron carriers (NAD+, FAD, GDP).
Outputs: 2x CO2, 4x reduced electron carriers (NADH, FADH2 and GTP (which converts extra ADP to ATP)).
ATP is produced via substrate level phosphorylation.
The electron transport chain…
(Including oxidative phosphorylation)…
Electrons from NADH and FADH2 (originally from food molecules) pass through the respiratory chain of membrane-associated carriers.
The electron flow results in a proton gradient in the mitochondria.
Complex one: electrons are stripped out of NADH (forming NAD+). Electrons pass through this complex and are passed to ubiquinone.
Complex two: electrons are stripped out of FADH2 (forming FAD). Electrons pass through this complex and are passed to ubiquinone.
Complex three: ubiquinone passes electrons to cytochrome C in complex three.
Complex four: cytochrome c oxidase removes electrons from cytochrome C.
Electrons combine with oxygen to form water.
At all stages, hydrogen (protons) are being passed from the matrix to the intermembrane space.
The intermembrane space therefore has a positive charge and high proton concentration. Protons move down the gradient, through ATP synthase. For every 3 H+ that pass through F0, F1 moves round to produce 1 ATP (this is chemiosmosis).
Uncoupling ATP synthesis…
If a different H+ diffusion channel is inserted into the mitochondrial matrix, the energy is lost as heat. This can be deadly.
The uncoupling protein thermogenin occurs in human infants and hibernating animals. H+ is released as heat instead of being coupled to ATP synthesis.
Lactic acid fermentation…
Occurs in microorganisms and some muscle cells.
Pyruvate is the main electron acceptor.
Lactate is the product and can build up.
A lactate dehydrogenase enzyme carries out this reaction.
Alcohol fermentation…
Yeast and some plant cells carry this process out.
Requires two enzymes to metabolise pyruvate to ethanol.
Acetaldehyde is reduced by NADH + H+, producing NAD+ and glycolysis continues.
Harvesting energy from glucose…
Cellular respiration yields more energy than fermentation per glucose molecule.
Glycolysis plus fermentation = 2 ATP.
Glycolysis plus cellular respiration = 32 ATP.
In some cells, NADH must be shuttled using ATP, leading to a net gain of 30 ATP.
Regulation…
An interchange of molecules occurs between metabolic pathways, which are interrelated by shared substances and regulated by enzyme inhibitors.
Catabolic interconversions:
- Polysaccharides: hydrolysed to glucose, enters glycolysis or the citric acid cycle.
- Lipids: broken down to (I) glycerol –> DAP; (II) fatty acids –> acetyl CoA.
- Proteins: hydrolysed to amino acids, enters glycolysis or the citric acid cycle.
Anabolic interconversions:
- Mostly reversible.
- Gluconeogenesis: glucose formed from the citric acid cycle and glycolysis intermediates.
Catabolism and anabolism are integrated:
- Negative and positive feedback controls.
- Concentration of biochemical molecules remains constant (i.e. glucose concentration in the blood).
The main control point in glycolysis is phosphofructokinase (allosterically inhibited by ATP).
The main control point in the citric acid cycle is isocitrate dehydrogenase (inhibited by NADH + H+ and ATP).
If ATP levels are too high…
Accumulation of citrate diverts acetyl CoA to produce more fatty acid synthesis.
Fatty acids may be metabolised later to produce more acetyl CoA.
