3.5.2 Respiration (A-level only)

Question 1

Define respiration.

Answer 1

= The process of releasing energy (in the form of ATP) from glucose.
—> energy used to power all the biological processes in the cell.

Question 2

Outline the process of Glycolysis.

Answer 2

NB => takes place in the cytoplasm; anaerobic process.
Stage 1 - Phosphorylation:
1. Glucose is phosphorylated using a phosphate from a molecule of ATP. This creates 1 molecule of glucose phosphate, using one molecule of ATP.
2. ATP is then used to add another phospate, forming hexose bisphosphate.
3. Hexose bisphosphate is then split into 2 molecules of TP.
Stage 2 - Oxidation:
4. TP is oxidised (loses H) to form 2 molecules of pyruvate.
5. NAD bonds with 2 removed H+ ions, forming 2 NADH.
6. 4 ATP are produced, but 2 were used in stage 1.
=> Net gain of 2 ATP.

Question 3

How can we ensure glycolysis continues even with a lack of oxygen?
What is the consequence of this?
Name the type of respiration where this occurs.

Answer 3

Pyruvate produced in glycolysis is converted into ethanol (in plants and yeast):
- Pyruvate -(-CO2)–> ethanal -(NADH->NAD)–> ethanol.
Or into lactate (in animals and some bacteria):
- Pyruvate -(NADH->NAD)–> lactate.
This production of ethanol or lactate regenerates oxidised NAD.
=> so a small amount of ATP can still be produced to keep some biological processes.
=> Aerobic respiration.

Question 4

Outline the link reaction.

Answer 4

1. Pyruvate is deoxycarbolated (1C removed from pyruvate in form of CO2)
2. Pyruvate is oxidised to form acetate and NAD is reduced to form NADH.
3. Acetate is combined with CoA to form acetyl coezyme A (acetylcoA).
   NB => NO ATP PRODUCED IN THE LINK REACTION.

Question 5

How many times must the link reaction occur for each glucose molecule? Why?

Answer 5

Link reaction occurs twice for every glucose molecule.
1. 2 molecules of pyruvate made for each glucose molecule entering glycolysis.
2. Link reaction and Krebs Cycle happen twice for every glucose molecule.
=> for each glucose molecule:
1. 2 molecules of acetylcoA go into Krebs.
2. 2 x CO2 molecules are released as a waste product of respiration.
3. 2 x NADH molecules formed and go to oxidative phosphorylation.

Question 6

Outline the Krebs Cycle.

Answer 6

= Series of oxidation-reduction reactions which take place of the mitochondria.
= Takes place once for every pyruvate, so goes around twice for every glucose molecule.
1. AcetylcoA from the link reaction combines with 4C oxaloacetate to form 6C citrate.
2. Coenzyme A goes back to the link reaction to be used again.
3. 6C citrate molecule converted to a 5C molecule. -CO2 / NAD—>NADH.
4. 5C molecule converted to 4C oxaloacetate molecule.
NAD—>NADH.
ADP + Pi—>ATP = produced by direct transfer of a phosphate group from an intermediate compound to ADP (substrate-level phosphorylation).
FAD—>reduced FAD.
NAD—>NADH.
=> Krebs therefore produces reduced coezymes and ATP.
Some products of the Krebs Cycle are used in oxidative phosphorylation.

Question 7

List the products of the Krebs Cycle and outline where they go.

Answer 7

1 x coenzymeA —> reused in next link reaction.
Oxaloacetate —> regenerated for use in next Krebs Cycle.
2 x CO2 —> released as waste product.
1 x ATP —> used for energy.
3 x NADH —> oxidative phosphorylation.
1 x reduced FAD —> oxidative phosphorylation.

Question 8

Outline the process of oxidative phosphorylation.

Answer 8

1. H atoms released from NADH and FADH2 (oxidation). The H atoms split into protons (H+) and electrons (e-).
2. Electrons move down ETC (made up of electron carriers) losing energy at each carrier.
3. This energy used by the electron carriers to actively transport H+ from the mitochondrial matrix into the intermembrane space.
4. [H+] now higher in the intermembrane space than in mitochondrial matrix —> forms an electrochemical gradient.
5. Protons then move down electrochemical gradient into mitochondrial matrix via ATP-synthase (embedded in the inner mitochondrial membrane).
   => movement drives ADP + Pi —> ATP.
   => process called chemiosmosis.
6. In the mitochondrial matrix at the end of the ETC, the protons, e- and O2 from blood combine to form water.
   => O2 said to be the final electron acceptor.

Question 9

Calculate how much ATP can be made from one glucose molecule.

Answer 9

Glycolysis 1 —> 2 ATP.
Glycolysis 2 —> 2 NADH —> 2x2.5 => 5 ATP.
Link Reaction x 2 —> 2NADH —> 2x2.5 => 5 ATP.
Krebs Cycle x 2 —> 2ATP.
—> 6 NADH —> 6x2.5 => 15 ATP.
—> 2 FADH2 —> 2x1.5 => 3 ATP.
=> 32 ATP.

Question 10

How do mitochondrial diseases affect ATP production?

Answer 10

1. Affect the functioning of mitochondria - can affect how proteins involved in oxidative phosphorylation or the Krebs Cycle function, reducing ATP production.
2. May cause anaerobic respiration to increase in order to compensate for some of the ATP shortage.
3. => Lots of lactate produced, which can cause muscle fatigue and weakness.
4. Some lactate will also diffuse into bloodstream, leading to high lactate concentrations in the blood.

Question 11

How can other respiratory substrates be used in aerobic respiration?

Answer 11

Some products from the breakdown of other molecules such as fatty acids from lipids, and amino acids from proteins can be converted into molecules that are able to enter the Krebs Cycle (usually converted to acetylcoA).