A-Level生物备考：考点之呼吸作用（二）

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上次介绍了呼吸作用的基本知识生物体内能量用途，ATP的结构，概念区别，有氧呼吸无氧呼吸的几大步骤，那么，这次我们就来看看详细过程！

Glycolysis  糖酵

是有氧和无氧的共同步骤，也是个步骤

Link reaction

1. The 2 pyruvate molecules enter the mitochondrion

2. CO2 and hydrogen ions are removed from each pyruvate to create two 2-C molecules. The hydrogen is transferred to NAD.

3. The 2-C molecule is then combined with coenzyme A (CoA) to form the 2C compound, acetyl CoA. (CoA is a vitamin derivative which acts as a transporter of the 'acetate' psrt left from the pyruvate molecule)

Krebs Cycle

also known as citric acid cycle

 or tricarboxylic acid cycle

For each glucose molecule, there were 2 pyruvic acid molecules formed,

(and therefore 2acetylCoA molecules formed) so the whole cycle takes place twice for every glucose molecule respired.

1.Each acetylCoA (2C) combines with an oxaloacetic acid (4C) to make a 6C compound (citric acid).

2. In a series of steps, for each 6C compound,

2 CO2 molecules are released,

3 NAD molecules are reduced,

1 FAD molecule is reduced,

1 ATP molecule is made directly.

3. The 4C compound is regenerated (by the removal of the 2 Cs in 2 CO2 molecules) so that the cycle can begin again with more molecules of acetyl CoA.

So far we have in total, from one glucose molecule——

4. 6CO2s

5. 4 ATPs made directly

6. 10 reduced NADs

7. 2 reduced FADs

Electron transport chain

and oxidative phosphorylation

Each hydrogen atom is split into its constituent H+ (hydrogen ion) and electron. The electron is the part that actually gets passed down the chain from carrier to carrier. The H+, however, remains in the mitochondrial matrix.

The electron carriers are at successively lower energy levels hence, as the electron moves on from one carrier to the next some energy is released.

This energy is used to pump H+ from the matrix into the space between the inner and outer mitochondrial membrane. The H+ concentration therefore increases, forming a concentration gradient.

This means that the H+ ions have electrical potential energy. H+ then flows back down the gradient into the matrix through protein channels.

电子从高能级跃迁到低能级释放出能量，这部分能量用于制造大量ATP

ATP synthesis

Associated with each channel is an enzyme, ATP synthase. As the H+ ions flow through, their energy is used to make ATP.

This theory about how the ATP is actually made is called the chemiosmotic theory. Oxygen acts as the final electron acceptor in the chain, so the oxygen, electrons and hydrogen ions together form water.

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