Respiration in Plants

Respiration is the step-wise oxidation of food inside the cell that releases energy and traps it in ATP. Combustion is a rapid, uncontrolled burning of a substance, usually outside the cell, in which energy is released mostly as heat.

### Differences
- Respiration occurs inside living cells and is controlled by enzymes.
- Combustion occurs outside living cells and is not enzyme-controlled.
- In respiration, energy is released slowly in steps and stored in ATP.
- In combustion, energy is released suddenly and mainly as heat and light.
- Respiration occurs at normal biological temperatures; combustion usually needs ignition.
- Respiration may be aerobic or anaerobic; combustion requires oxygen and is a complete burning process.

So, respiration is a controlled biological process, while combustion is an uncontrolled chemical burning process.

Glycolysis and Krebs' cycle differ in location, starting material, products and nature of reactions.

### Glycolysis
- Occurs in the cytoplasm.
- It is the first stage of respiration.
- Glucose is partially oxidised into two molecules of pyruvic acid.
- It does not require oxygen directly.
- It forms a small amount of ATP and NADH + H⁺.

### Krebs' cycle
- Occurs in the mitochondrial matrix.
- It is the second major stage of aerobic respiration.
- Acetyl CoA enters the cycle and is completely oxidised.
- Carbon atoms are released as CO₂.
- It produces a large amount of NADH + H⁺, FADH₂ and ATP.

So, glycolysis is the breakdown of glucose to pyruvate in the cytoplasm, while Krebs' cycle is the complete oxidation of acetyl CoA in the mitochondria.

Aerobic respiration and fermentation differ mainly in oxygen requirement, extent of oxidation and ATP yield. ### Aerobic respiration
- Occurs in the presence of oxygen. - Glucose is completely oxidised to CO₂ and H₂O. - Takes place mainly in the mitochondria after glycolysis. - Produces a large amount of ATP. - End products are non-toxic. ### Fermentation
- Occurs under anaerobic conditions. - Glucose is incompletely oxidised. - Takes place in the cytoplasm. - Produces very little ATP; only the ATP from glycolysis is obtained. - End products may be ethanol and CO₂ or lactic acid.

Respiratory substrates are the organic compounds that undergo oxidation in respiration to release energy. During this process, their carbon bonds are broken and energy is trapped in ATP.

The most common respiratory substrate is carbohydrate, especially glucose.

The schematic representation of glycolysis is the step-wise conversion of glucose to pyruvic acid.

A compact scheme is:

Glucose → Glucose-6-phosphate → Fructose-6-phosphate → Fructose-1,6-bisphosphate → Dihydroxyacetone phosphate + PGAL → 1,3-bisphosphoglycerate → 3-phosphoglycerate → 2-phosphoglycerate → Phosphoenolpyruvate (PEP) → Pyruvic acid

This pathway occurs in the cytoplasm and involves 10 enzyme-controlled reactions.

The main steps in aerobic respiration are:

1. Glycolysis: Glucose is broken down into pyruvic acid in the cytoplasm.
2. Oxidative decarboxylation of pyruvate: Pyruvate enters the mitochondrial matrix and is converted into acetyl CoA with release of CO₂.
3. Krebs' cycle: Acetyl CoA is completely oxidised in the matrix of mitochondria, producing CO₂, NADH + H⁺, FADH₂ and ATP.
4. Electron transport system and oxidative phosphorylation: NADH and FADH₂ are oxidised on the inner mitochondrial membrane; electrons pass to O₂, forming H₂O and ATP.

### Where it takes place
- Glycolysis: cytoplasm
- Pyruvate oxidation and Krebs' cycle: mitochondrial matrix
- ETS and ATP synthesis: inner mitochondrial membrane

So, aerobic respiration is a coordinated process occurring in the cytoplasm and mitochondria.

The overall view of Krebs' cycle can be written schematically as:

Acetyl CoA + Oxaloacetic acid (OAA) → Citric acid → Isocitric acid → $-ketoglutaric acid → Succinyl-CoA → Succinic acid → Fumaric acid → Malic acid → Oxaloacetic acid (OAA)

During this cycle:
- CO₂ is released in decarboxylation steps.
- NAD⁺ is reduced to NADH + H⁺ three times.
- FAD⁺ is reduced to FADH₂ once.
- ATP/GTP is formed by substrate-level phosphorylation.

The cycle occurs in the mitochondrial matrix.

The electron transport system (ETS) is a chain of electron carriers located on the inner mitochondrial membrane. It transfers electrons from NADH + H⁺ and FADH₂ step by step to oxygen, the final electron acceptor.

### How it works
- NADH donates electrons to complex I.
- FADH₂ donates electrons through complex II.
- Electrons move through ubiquinone, cytochromes and other carriers up to complex IV.
- At the end, electrons combine with oxygen and hydrogen to form water.
- The energy released during electron transfer is used to pump protons and drive ATP synthase (complex V), producing ATP.

### Significance
- It is the main site of ATP formation in aerobic respiration.
- Oxygen acts as the final hydrogen/electron acceptor.
- The process of ATP formation coupled to electron transport is called oxidative phosphorylation.