Chemiosmosis & Coenzymes
Biology: Cambridge International A Level, topic 12.2.4
Chemiosmotic Theory
Chemiosmotic theory explains how oxidative phosphorylation and photophosphorylation work in relation to the production of ATP.
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Chemiosmosis
- Chemiosmosis is the movement of ions across a membrane down an electrochemical gradient.
- ATP synthesis is an example of chemiosmosis because protons move across a membrane down an electrochemical gradient.
- Chemiosmosis happens in photophosphorylation and oxidative phosphorylation.
ATP synthase
- ATP synthase is an enzyme found in the inner mitochondrial membrane of mitochondria, and in the thylakoid membrane of chloroplasts.
- Protons flow through ATP synthase, which releases energy needed to make ATP from ADP and P.
Proton gradients
- In mitochondria, protons are pumped from the matrix into the intermembrane space and in chloroplasts, protons are pumped from the stroma into the thylakoids.
- The energy needed to pump protons across the membrane comes from the electron transport chain.
- Pumping protons creates an electrochemical gradient. Protons move down the electrochemical gradient through ATP synthase, releasing energy.
Electron transport chain (ETC)
- The electron transport chain (ETC) is a series of electron carrier proteins in the thylakoid membrane and the inner mitochondrial membrane.
- As electrons move down the ETC, they lose energy, which is used to pump protons across the membrane.
- In respiration, electrons come from the oxidation of NADH and FADH2.
- In photosynthesis, electrons come from the photoionisation of chlorophyll.
Coenzymes in Respiration
NAD, FAD, and Coenzyme A are important coenzymes in respiration:
NAD
- NAD is a coenzyme that can be reduced to NADH.
- NADH is produced in glycolysis (2 molecules per glucose), the link reaction (1 molecule per pyruvate), and the Krebs cycle (3 molecules per acetate).
- NADH gets oxidised in oxidative phosphorylation, releasing electrons for the ETC and protons.
FAD
- FAD is a coenzyme that can be reduced to FADH2.
- FADH2 is produced in the Krebs cycle (1 molecule per acetate).
- FADH2 gets oxidised in oxidative phosphorylation, releasing electrons for the ETC and protons.
Coenzyme A
- Coenzyme A binds with acetate in the link reaction to form acetyl coenzyme A, which goes on to enter the Krebs cycle.
- The link reaction links together glycolysis and the Krebs cycle.
Test yourself on Chemiosmosis & Coenzymes
Which of the following statements about the link reaction are true?
How many molecules of reduced NAD are produced per turn of the Krebs cycle?
How many molecules of reduced NAD and reduced FAD are produced per molecule of glucose in respiration?