Respiration (A-Level)

Respiration is the process by which organic molecules are broken down, releasing energy to synthesise ATP.

There are four stages:

1. Glycolysis
2. The link reaction
3. The Krebs cycle
4. The electron transport chain

Glycolysis

• 2 molecules of ATP are used to convert a molecule of glucose into 2 molecules of triose phosphate (TP).

glucose + 2 ATP → 2 triose phosphate + 2 ADP

• The 2 molecules of TP are then converted to 2 molecules of pyruvate

This process produces 4 molecules of ATP from 4 molecules of ADP

2 NAD coenzymes are reduced (into 2 reduced NAD coenzymes)

2 triose phosphate + 4 ADP + 2 NAD → 2 pyruvate + 4 ATP + 2 reduced NAD

The link reaction

• pyruvate + coenzyme A + NAD → acetylcoenzyme A + CO₂ + reduced NAD

The Krebs cycle

1. 4-carbon compound + acetylcoenzyme → 6-carbon compound

Oxidative Phosphorylation

Reduced NAD and Reduced FAD from other stages of respiration are passed to the electron transport chain on the inner membrane of mitochondria. The reduced NAD and FAD are then converted to NAD FAD and Hydrogen atoms. The Hydrogen atoms are split into electrons and H⁺ ions (a proton). The electrons released are used by the electron transport chain to release energy, this energy is used to synthesise ATD from ADP and P_i (inorganic Phosphate), the final electron acceptor is Oxygen, which combines with protons in the matrix to form water. Protons are then actively pumped into the intermembranal space in mitochondria using ATP from the electron transport chain, this lowers the pH (increases the conc. of H⁺) in the intermembranal space. Hence H⁺ ions diffuse into the matrix of mitochondria through ATP Synthase molecules, causing the synthesis of ATP.