Biology A2 2 - Respiration

ATP

- Adenosine triphosphate
- Has three phosphate groups combined with the nucleotide base adenine and a ribose sugar
- The presence of the phosphate groups provide ATP with its energy-releasing properties

Purpose of ATP

- Immediate source of energy in a cell, a (very) short-term store
- ATP drives metabolism
- Glucose can be used to make ATP but it cannot release energy directly
- ATP releases energy when hydrolysed

ADP/ATP interaction

- ATP is synthesised from ADP (adenosine diphosphate), a molecule with two phosphate groups and inorganic phosphate

Phosphorylation

The addition of phosphate to a molecule

What happens when the terminal phosphate is removed from ATP?

- Energy is released
- The breakdown of ATP to ADP involves hydrolysis - the splitting of a molecule using water
- This reaction is catalysed by the enzyme ATPase

What makes ATP so suitable as an immediate energy store?

- The hydrolysis of an ATP molecule releases a relatively small amount of energy, allowing energy to be released in small, manageable steps during energy-requiring reactions
- The hydrolysis of ATP is a single reaction involving the breaking of one bond releasing immediate energy, providing the cell with fine control over its immediate energy budget
- As a small, soluble molecule, ATP can be transported around the cell easily. This enables it to be transported from mitochondria to any part of the cell

Using ATP in the cell

- ATP has a role in active transport and muscle contraction
- Anywhere 'work' is required, ATP is used
- Provides the energy for many metabolic processes including anabolic reactions
- ATP plays a role in the activation of molecules

4 stages of respiration

1. Glycolysis - splitting of glucose into two 3-carbon pryuvate molecules
2. Link reaction - conversion of the pyruvate into 2-carbon acetyl coenzyme A (acetyl CoA)
3. Krebs cycle - The feeding of acetyl CoA into a cycle of oxidation-reduction reactions
4. Electron transport chain - Use of electrons and hydrogens produced in the Krebs cycle to synthesise ATP

Where does glycolysis occur?

Cytoplasm

Initial stage of glycolysis

- The activation of glucose by phosphorylation
- This makes the glucose more reactive
- The 2 phosphates required come from the hydrolysis of 2 ATP molecules
- The phosphorylation of the glucose converts it into fructose bisphosphate

What happens in glycolysis following the phosphorylation of glucose? (Step 2)

- The 6C fructose bisphosphate splits into two 3-carbon molecules of triose phosphate
- The triose phosphate is oxidised through the loss of hydrogen atoms to form pyruvate
- The hydrogen atoms are collected by the hydrogen carrier molecule NAD which becomes reduced to form NADH
- The removal of hydrogen involves dehydrogenase enzymes in dehydrogenation

How much ATP does glycolysis provide?

- In converting each molecule of triose phosphate into a pyruvate molecule 2 ATP molecules are produced
- However as each glucose molecule splits to form 2 triose phosphate molecules this produces 2 ATP molecules for each of the triose phosphate molecules, a gain of 4 ATP
- This gives a net gain of 2 ATP for glycolysis as 2 were initially used to activate to the glucose

Glycolysis summary

- The initial stage of the cellular respiration of glucose that does not require oxygen and takes place in the cytoplasm
- The reduction of NAD between the triose phosphate and pyruvate stages to give two reduced NAD (NADH)
- Net gain of 2 ATP

The link reaction

- The pyruvate produced in glycolysis is transported into the matrix of a mitochondrion
- During the link reaction the pyruvate is converted to acetyl CoA like this:
- The pyruvate is decarboxylated with the removal of one molecule of CO2
- Dehydrogenation also takes place with the removal of hydrogen leading to the formation of NADH
- Following decarboxylation and dehydrogenation, the resulting 2-carbon acetate combines with CoA to form acetyl CoA

Where does the Krebs cycle occur?

mitochondrial matrix

Key stages in the Krebs cycle

- The 2-carbon acetyl CoA from the link reaction combines with the 4-carbon acid (oxaloacetate) to form a 6-carbon acid (citrate)
- Decarboxylation of the 6-carbon acid (citrate) results in the formation of the 5-carbon acid (oxoglutarate) with the loss of a molecule of CO2
- Decarboxylation of the 5-carbon acid (oxoglutarate) results in the formation of the 4-carbon acid oxaloacetate with the loss of a molecule of CO2 and the cycle continues
- The reactions in the cycle involve dehydrogenation and dehydrogenase enzymes. At 3 points in the cycle hydrogen is released that subsequently reduces NAD to form NADH. At one point hydrogen is picked up not by NAD but by FAD to form FADH^2
- 1 molecule of ATP is produced by the transfer of a phosphate group from an intermediate compound to ADP. ATP produced in this way is called substrate-level phosphorylation

Where does the electron transport chain happen?

Based in and on the inner mitochondrial membranes (cristae)

What molecules are used in the ETC?

The hydrogen atoms collected by NAD from the dehydrogenation in glycolysis, the link reaction and Krebs cycle are carried

What happens in the electron transport chain?

The energy in the hydrogen is converted into ATP

Revisiting the mitochondrion

- The more deeply infolded the cristae, and the more infoldings there are, the more extensive the ultrastructure that exists for ATP production in the mitochondrion

Downstreaming

- The NAD, FAD and other coenzymes and carriers in the ETC are highly organised and arranged in a sequence of decreasing potential energy
- Each carrier downstream has slightly stronger reducing power than the one before it
- Therefore the hydrogens are able to move along the chain with carriers being successively reduced and oxidised as hydrogen/electrons pass along the chain in a series of oxidation-reduction (redox) reactions

The carriers of the electron transport chain

- The NAD/FAD operate as hydrogen carriers
- The NAD and FAD function by bringing the hydrogen to the chain

Steps of electron transport chain

- Hydrogen passes along the carriers NAD, flavoprotein and coenzyme Q
- Following this, the hydrogen dissociates into electrons and the ETC subsequently acts as an electron carrier
- The electrons pass along the cytochromes in a series of redox reactions

Final hydrogen acceptor

- Oxygen
- At this stage oxygen is used in respiration
- The oxygen combines with hydrogen to form water, a waste product
- The final stage in the electron transport chain is catalysed by the enzyme cytochrome oxidase

What benefit does the downstreaming of carriers bring?

Energy becomes available as the redox reaction takes place. At certain points there is enough energy to produce ATP by oxidative phosphorylation

How much energy is produced in respiration?

- For each reduced NAD sufficient energy is released to produce 3 ATP molecules in the ETC
- Reduced FAD enters the chain further along than reduced NAD and there is only sufficient energy to produce 2 ATP

How many ATP are produced by substrate-level phosphorylation and by oxidative phosphorylation?

* Substrate-level phosphorylation: 4
* Oxidative phosphorylation: 34

Anaerobic respiration

- Glycolysis will only continue if its products are removed and not allowed to accumulate
- The pyruvate is converted to lactate in animals and ethanol in plants/yeast
- The reduced NAD formed during glycolysis must be oxidised again so that NAD will be available to take up further hydrogen atoms from glycolysis
- If this did not happen all the NAD would be reduced and glycolysis would stop as there would be no hydrogen acceptors available
- The mopping up of these hydrogen atoms is achieved by the hydrogen being used in the reactions between the pyruvate and lactate/ethanol

Role of glycolysis in anaerobic respiration

- Glycolysis is the only energy producing stage of respiration, with a net gain of 2 ATP; inefficient compared to aerobic respiration
- It is a fast process; taking place throughout the cytoplasm and substances do not have to diffuse in/out of the mitochondrion in addition to it being only a short part of the normal aerobic pathway

Anaerobic respiration in animals

- In animals anaerobic respiration can be advantageous
- In mammals, anaerobic respiration is most likely to take place in the skeletal muscles
- During strenuous exercise, the muscles will be respiring aerobically and anaerobically, and the anaerobic respiration provides extra energy
- This extra energy may be enough to make the crucial difference between escaping from a predator, etc

How is oxygen debt formed?

- The lactate produced by anaerobic respiration accumulates in the muscles and can cause muscle fatigue/cramp
- It is removed when sufficient oxygen becomes available again and anaerobic respiration is no longer necessary
- Lactate can be converted back to glucose or metabolised in other ways, processes that require O2
- As the body is dealing with the lactate produced because of an oxygen shortage earlier on, the extra oxygen used to metabolise lactate is called the oxygen debt
- The extra oxygen is also used to resynthesise depleted ATP

Anaerobic respiration in plants and fungi

- In plants/fungi the end product of anaerobic respiration is ethanol, not lactate
- Anaerobic respiration in plants and fungi produces carbon dioxide as a waste product
- The ethanol is not reconverted to pyruvate but is eliminated as a waste product

Value of anaerobic respiration in plants/fungi

- A significant part of most plants/fungi penetrates through soil or other substrates
- Oxygen levels can often be low in these environments and the ability to respire anaerobically allows production of ATP to be maintained
- The lower metabolic rate in plants/fungi compared to animals means lower ATP yield from anaerobic respiration is not as significant an issue