3.5.2 Respiration

This is the first stage of both aerobic and anaerobic respiration and doesn't require specialized organelles.

Answer: In the cytoplasm

Glycolysis is the first stage of both aerobic and anaerobic respiration and takes place in the cytoplasm of the cell.

Adding phosphate groups from ATP initially requires energy input (activation energy).

Answer: It makes the glucose more reactive and traps it inside the cell.

Phosphorylation of glucose to glucose phosphate (using ATP) makes the molecule more reactive (lowering activation energy for the split) and, because it is now charged, prevents it from diffusing out of the cell.

Four ATP are produced, but some are used up early in the pathway.

Answer: Two molecules of ATP

Glycolysis requires an initial input of $2$ ATP to phosphorylate glucose. It later produces $4$ ATP during the oxidation of triose phosphate to pyruvate. Therefore, the net yield is $2$ ATP.

Pyruvate ($3C$) becomes an acetyl group ($2C$) and hydrogen is lost to NAD.

Answer: Oxidation and decarboxylation

In the link reaction, pyruvate is oxidised (losing hydrogen to form reduced NAD) and decarboxylated (losing carbon as $CO_{2}$) to form acetate, which then combines with coenzyme A.

This involves the direct transfer of a phosphate group from a donor molecule to ADP.

Answer: Substrate-level phosphorylation

In the Krebs cycle, ATP is generated directly by a single reaction from ADP and Pi. This direct transfer of a phosphate group from a reacting molecule is known as substrate-level phosphorylation.

Without oxygen, electrons would back up in the chain, halting the entire process.

Answer: It acts as the terminal electron acceptor, combining with electrons and protons to form water.

Oxygen is the terminal electron acceptor in the electron transfer chain. It accepts electrons coming off the chain and protons from the matrix to form water ($1/2 O_{2} + 2e^{-} + 2H^{+} \rightarrow H_{2}O$).

Glycolysis requires a continuous supply of the 'empty' coenzyme to pick up hydrogen.

Answer: It oxidises reduced NAD back to NAD, allowing glycolysis to continue producing a small amount of ATP.

Anaerobic respiration converts pyruvate to lactate to regenerate oxidised NAD from reduced NAD. Without this, glycolysis would halt due to a lack of NAD, stopping all ATP production.

Lipids are composed of two different structural components, each entering at a different stage.

Answer: They are hydrolysed into glycerol, which enters glycolysis, and fatty acids, which are broken down into acetylcoenzyme A to enter the Krebs cycle.

Lipids can act as respiratory substrates. They are hydrolysed; the glycerol is phosphorylated and converted to triose phosphate (entering glycolysis), while the fatty acids are broken down into $2$-carbon fragments (acetyl CoA) and enter the Krebs cycle.