Energy in ATP
Respiration is a process where the organic molecules such as glucose, fatty acids, glycerol and amino acids are broken down by a series of enzymatic chemical reactions to produce energy in ATP form. It is called catabolism. Catabolism involves three stages which shown in Fig.1 :
- Hydrolysis
- Conversion of building blocks into acetyl-CoA
- Oxidation of acetyl-CoA
Overview of Metabolism
The main fuels is glucose. Glucose is broken down in four stages:
- Glycolysis
- Link reaction
- Krebs cycle
- Oxidative phosphorylation
Glycolysis
- Lysis/breakdown of glucose.
- Multiple enzymatic steps.
- Break 6C glucose to 2 molecules of 3C pyruvates.
- Produces 4 ATP
- Uses 2ATP
- Net gain: 2ATP
Glycolysis
- In aerobic metabolism (presence of oxygen)
- Pyruvate loses carbon dioxide.
- The remaining two carbon atoms become linked to coenzyme A as an acetyl group to form acetyl-CoA, which then enters the citric acid cycle.
- Called aerobic glycolysis.
2. In anaerobic metabolism (lack oxygen)
- Called anaerobic glycolysis.
- Occur in cytoplasm.
- In muscle, reduction of pyruvate to lactate by lactate dehydrogenase.
- NAD is recycle in glycolysis.
- Allow some ATP produced even the oxygen is not available.
- Lactate is carried through plasma to liver and convert to pyruvate to glucose (gluconeogenesis).
Link reaction
- Pyruvate passes by active transport from cytosol into mitochondrion matrix.
- It is decarboxylated, dehydrogenated and combine with coenzyme A (CoA) to form acetyl CoA.
Link reaction
Krebs cycle
- Also known as tricarboxylic acid cyle (TCA cyle), citric acid cycle.
- Closed pathway of enzyme controlled reactions.
- In a Krebs cycle, 3 NADH, 1 FADH2 and 1 ATP are produced.
Link reaction and Krebs cycle
| Energy producing reaction | No of ATP produced |
| 3 NADH | 9 ATP |
| FADH2 | 2ATP |
| GTP | 1ATP |
| Total | 12 ATP |
Oxidative phosphorylation & electron transport chain
- Oxidative phosphorylation and electron transport chain are the final stage of aerobic respiration.
- It is aerobic pathway and occur in mitochondrion.
- Enzyme : oxidoreductase.
- NADH and FADH2 are passed to electron transport chain.
- Hydrogen ions are removed and split to hydrogen ion (H+) and electron.
- Electron is then transferred to the first series of electron carrier (cytochrome). Hydrogen ion remains in matrix.
- Electron is transferred to oxygen, a hydrogen ion will be drawn and reduce oxygen to water.
- Electron transfer makes energy available which is used to convert ADP + Pi to ATP.
Besides glucose, lipid and protein are also broken down into smaller substances and provide energy. The catabolism pathway of lipid and protein are linked into Krebs cycle.
Catabolism of Organic molecules