AQA A2 biology chapter 4

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4.1 Types of respiration

Aerobic respiration - Requires oxygen & produces carbon dioxide, water & lots of ATP

Anaerobic respiration (fermentation) - Happens without oxygen, produces lactate (in animals) or ethanol & CO2 (in plants) but only a little ATP in both cases 

Aerobic respiration has 4 stages:

1. Glycolysis - splits 6-Carbon glucose molecule into 2 pyruvate molecules (3-C)

2. Link reaction - converts pyruvate into CO2 & a 2-C molecule, acetylcoenzyme A

3. Krebs cycle - introduces acetylcoenzyme A into a cycle of oxidation-reduction reactions that yield some ATP & lots of electrons

4. Electron transport chain - uses the electrons produced in Krebs cycle to synthesise ATP. Water is produced as a by-product

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4.1 Glycolysis

There many enzyme-controlled reactions within glycolysis, textbook puts them into 4 stages:

1. Activation of glucose by phosphorylation - Glucose needs to be made more reactive. This is achieved using 2 P molecules (the P molecules came from hydrolysing ATP). This provides the energy to activate glucose. 

2. Splitting of the phosphorylated glucose - The glucose molecule is split into two 2-C molecules called TP (triose phosphate)

3. Oxidation of TP - Hydrogen removed from TP molecules & binds with NAD, reducing it 

4. The production of ATP - Enzyme- controlled reactions convert each TP into a 3-C molecule (pyruvate). In doing so, 2 ATP molecules are formed from ADP

4 ATP molecules produced for each glucose - 2 used at start - net gain 2 

2 reduced NAD produced 

2 pyruvate produced 

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4.2 Link Reaction

The pyruvate molecules produced in the cytoplasm during glycolysis are actively transported into the matrix of mitochpndria & undergoes a series of reactions during which the following changes occur:

  • Pyruvate oxidised by removing hydrogen - this Hydrogen is accepted by NAD to produce reduced NAD (which is later used to produce ATP)
  • The 2-C molecule (called an acetyl group) that is formed, combines with a molecule called coenzyme A (CoA). This forms a compound called Acetylcoenzyme A
  • A carbon dioxide molecule is formed from each pyruvate

The overall equation for the link reaction can be summarised as:

pyruvate + NAD + CoA -----> acetyl CoA + reduced NAD + CO2

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4.2 Krebs cycle

Krebs is a series of O-R reactions & occurs in the mitochondrial matrix. It can be summarised as follows:

1. Acetyl CoA (from link reaction) combines with a 4-C molecule, making a 6-C molecule

2. This 6-C molecule loses CO2 & H to give a 4-C molecule & 1 ATP molecule as a result of substrate-level phosphorylation

3. The 4-C molecule can now combine with a new Acetyl CoA to begin the cycle again

For each pyruvate, link reaction and Krebs produce:

  • Reduced coenzymes, (NAD & FAD) that can potentially produce ATP & are important products of Krebs
  • One ATP molecule
  • Three CO2 molecules 

These values must be doubled for a glucose molecule (2 pyruvates produced from 1 glucose)

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4.2 Significance of the Krebs cycle

The Krebs cycle performs an important role in the cells of organisms for four reasons:

1. It breaks down macromolecules into smaller ones (pyruvate broken down into carbon dioxide)

2. It produces hydrogen atoms that are carried by NAD to the electron transport chain for oxidative phosphorylation. This leads to the production of ATP that provides the cell with metabolic energy.

3. It regenerates the 4-Carbon molecule that combines with acetylcoenzyme A which would build up without it 

4. It is a source of intermediate compounds used by cells in making other substances (e.g. amino acids and chlorophyll)

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4.2 Coenzymes

Coenzymes are molecules needed by some enzymes in order for them to function. Coenzymes play a major role in photosynthesis and respiration where they carry hydrogen atoms from one molecule to another. 

Examples of coenzymes include:

  • NAD - which is important throughout respiration
  • FAD - which is important in the Krebs cycle
  • NADP - which is important in photosynthesis

In respiration, NAD is the most important carrier. It works with dehydrogenase enzymes that catalyse the removal of hydrogen ions from substrates and transfer them to other molecules such as the hydrogen carriers involved in oxidative phosphorylation.

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4.3 The electron transport chain

ATP is made using the ETC as follows:

  • H atoms produced during glycolysis & Krebs combine with NAD & FAD that are attached to cristae of mitochondria
  • Reduced NAD & FAD donate electrons from the H atom to the 1st molecule in the ETC
  • This releases protons from the H atoms & they are actively transported across the inner mitochondrial membrane
  • The electrons pass along a chain of electron carrier molecules in Oxidation-Reduction reactions. They lose energy in doing so, some combines ADP with P, the rest is lost as heat. 
  • The protons accumulate in the space between the two mitochondrial membranes before they diffuse back into the mitochondrial matrix through special protein channels
  • At the end of the chain, electrons combine with the protons & oxygen to form water. Oxygen is the final electron acceptor in the ETC. 

Without O2 removing H ions, protons & electrons would back up along the chain & respiration would stop. Cyanide kills as it's a non-competitive inhibitor of the enzyme that combines protons & electrons with O2. Enzyme stops functioning - protons & electrons accumulate, respiration stops. 

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4.4 Production of ethanol

Anaerobic respiration leading to production of ethanol occurs in organisms such as certain bacteria & fungi (e.g. yeast) as well as in some cells of higher plants (e.g. root cells when waterlogged).

The pyruvate formed at the end of glycolysis loses a CO2 molecule and accepts hydrogen from reduced NAD to produce ethanol. The equasion for this is:

pyruvate + reduced NAD ------> ethanol + carbon dioxide + NAD 

This form of anaerobic respiration in yeast has been used for thousands of years in the brewing industry. In brewing, ethanol is an important product.

Yeast is grown in anaerobic conditions in which it ferments natural carbohydrates in plant products, such as grapes (wine production) or barley seeds (beer production) into ethanol. 

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4.4 Production of lactate in animals

Anaerobic respiration that forms lactate occurs in animals as a way of overcoming temporary O2 shortage. It occurs most in muscles as a result of strenuous exercise, in these conditions O2 can be used faster than it can be supplied & an O2 debt occurs. 

Often muscles must keep working despite the O2 debt (e.g. if the organism is fleeing a predator). Without O2, glycolysis would stop as reduced NAD accumulates. If glycolysis is to continue & release energy, reduced NAD must be removed. To do this, each pyruvate takes up 2 H atoms from reduced NAD to form lactate in the equasion shown below:

pyruvate + reduced NAD ---------> lactate + NAD

Eventually, the lactate must be oxidised back to pyruvate, this can be further oxidised to release energy or converted into glycogen. This happens when O2 becomes available again. 

Lactate causes cramps & muscle fatigue if allowed to accumulate in muscles. Although muscle has a certain tolerance to lactate, it is important that it's removed by the blood & taken to the liver to be converted to glycogen. 

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4.4 Energy yields from respiration

Energy from cellular respiration is derived in 2 ways:

  • Substrate level phosphorylation - in Glycolysis & Krebs cycle. This is the direct linking of inorganic phosphate (Pi) to ADP to produce ATP.
  • Oxidative phosphorylation - in the electron transport chain. This is the indirect linking of Pi to ADP to produce ATP using the hydrogen atoms from glycolysis & Krebs cycle that are carried on NAD & FAD. Cells produce most of their ATP in this way

In anaerobic respiration pyruvate is converted into either lactate or ethanol. Therefore, it is not available for Krebs cycle. Because of this, neither the Krebs cycle or electron transport chain can occur in anaerobic respiration.

The only ATP that can be produced in anaerobic respiration is that formed by glycolysis. This amount is very small when compared to the much greater quantities produced by aerobic respiration. 

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