Respiration

Question 1

What is metabolism?

Answer 1

All the reactions of the organism.

Question 2

What is a metabolic pathway?

Answer 2

A sequence of enzyme controlled reactions in which the product from one reaction is the substrate for the next (eg respiration).

Question 3

What are the three types of phosphorylation?

Answer 3

1. Oxidative phosphorylation
2. Photophosphorylation
3. Substrate-level phosphorylation

Question 4

Describe oxidative phosphorylation and where it occurs

Answer 4

Occurs on the inner membranes of the mitochondrion in aerobic respiration. Energy comes from oxidation-reduction reactions and is released in the transfer of electrons along a chain of electron carrier molecules.

Question 5

Describe photophosphorylation and where it occurs

Answer 5

Occurs on the thylakoid membranes of the chloroplasts in the light-dependent stage of photosynthesis. The energy comes from light and is released in the transfer of electrons along a chain of electron carrier molecules.

Question 6

Describe substrate-level phosphorylation

Answer 6

Occurs when phosphate groups are transferred from donor molecules to ADP to make ATP in glycolysis or when enough energy is released from a reaction to bind ADP to an inorganic phosphate in the Krebs cycle.

Question 7

What are the three groups of organisms, classified by their respiration?

Answer 7

1. Obligate aerobes- use aerobic respiration
2. Facultative anaerobes- respire aerobically but can also respire without oxygen
3. Obligate anaerobes- bacteria and Archaea cannot grow in oxygen and only respire anaerobically.

Question 8

What are the four distinct stages of aerobic respiration, and where they occur?

Answer 8

1. Glycolysis- in cytoplasm, generates pyruvate, ATP and NADH2
2. Link reaction- in matrix, pyruvate converted to AcCoA, NADH2 and CO2
3. Krebs cycle- in matrix, generates CO2, NADH2 and FADH2
4. Electron transport chain- on cristae on inner membrane, generates ATP from ADP+Pi

Question 9

Why does glycolysis occur in the cytoplasm?

Answer 9

As glucose cannot pass through the mitochondrial membrane.

Question 10

What is dehydrogenation?

Answer 10

The removal of one or more hydrogen atoms from a molecule.

Question 11

Why is glucose initially phosphorylated in glycolysis?

Answer 11

*To make it more reactive so it requires less activation energy for the enzyme controlled reactions.
*To make it more polar so it is less likely to diffuse out of the cell.

Question 12

Fully describe the process of glycolysis

Answer 12

A glucose is phosphorylated by the addition of two phosphate groups, using two ATP, making hexose biphosphate. This is converted into two molecules of triose phosphate which are then dehydrogenated, oxidising them to pyruvate. The H atoms are transferred to NAD, making NADH2.

Question 13

How many ATP molecules are generated from one glucose molecule in glycolysis?

Answer 13

4ATP are generated but 2ATP are used to initially phosphorylate glucose, so net 2ATP made per glucose.

Question 14

What are the products of glycolysis?

Answer 14

Per glucose:
Two pyruvate
2ATP
2NADH2

Question 15

What is decarboxylation?

Answer 15

Removing a carboxyl group from a molecule, releasing CO2.

Question 16

Fully describe the link reaction

Answer 16

Pyruvate diffuses from the cytoplasm into the mitochondrial matrix and is dehydrogenated with NAD turning into NADH2. The pyruvate is also decarboxylated making acetate, which combines with coenzyme A to make acetyl coenzyme A which enters the Krebs cycle.

Question 17

What are the products of the link reaction?

Answer 17

Per glucose:
Two AcCoA
Two NADH2
Two CO2

Question 18

Fully describe the Krebs cycle

Answer 18

AcCoA enters the cycle by combining with a 4C acid to from a 6C compound, and CoA is regenerated. The 6C acid is dehydrogenated with NAD and decarboxylated to make CO2 and a 5C acid. The 5C acid is dehydrogenated with 2NAD and FAD and decarboxylated to make CO2 and to regenerate the 4C acid which can combine with AcCoA and repeat the cycle.

Question 19

How many ATP molecules are generated from one glucose molecule in the Krebs cycle?

Answer 19

Each turn produces one ATP by substrate-level phosphorylation but each glucose generates two Krebs cycles so two ATP.

Question 20

What are the products of the Krebs cycle?

Answer 20

Per glucose (two cycles):
Two ATP
Six NADH2
Two FADH2
Four CO2

Question 21

Fully describe the electron transport chain

Answer 21

Hydrogen atoms are carried into the electron transport chain using NAD and FAD. Each H2 on NADH2 synthesises three ATP using three proton pumps. Each H2 on FADH2 synthesises two ATP using two proton pumps as FAD feeds its hydrogen atoms directly to the second pump.

Question 22

Describe the movement of electrons through the electron transport chain

Answer 22

NADH2 donates the electrons of the hydrogen atoms to the first of a series of carriers. The electrons provide energy for the first proton pump and protons are pumped into the intermembrane space. The electrons pass along a chain of carriers, providing energy for each of the pumps in turn. At the end, they combine with protons and oxygen to form water.

Question 23

Describe the movement of protons through the electron transport chain

Answer 23

The inner membrane is impermeable to protons so they accumulate in the inter-membrane space. The concentration of protons in the space becomes higher than in the matrix so a concentration and charge gradient is maintained by the pumps. The protons diffuse back through ATP synthetase to make ATP and at the end they combine with electrons and oxygen to form water.

Question 24

What is the final electron acceptor in the electron transport chain?

Answer 24

Oxygen, as it removes protons and electrons to form water.

Question 25

What would happen to the electron transport chain without oxygen?

Answer 25

Protons and electrons would accumulate so the electron transport chain no longer functions. The proton gradient is not maintained so ATP synthetase cannot produce ATP and the cell dies.

Question 26

What are the products of the electron transport chain?

Answer 26

Per glucose: Receives ten NADH2 and generates 30 ATP Receives two FADH2 and generates 4 ATP

Question 27

What is the total number of ATP molecules generated per molecule of glucose respired?

Answer 27

38 ATP.

Question 28

Why might the full amount of ATP be made from one molecule of glucose?

Answer 28

*ATP is used to move pyruvate, NADH2 and FADH2 across the mitochondrial membrane. *The proton gradient may be compromised by proton leakage across the inner mitochondrial membrane rather than passing through ATP synthetase. *Molecules may also leak through membranes

Question 29

What is the only stage of aerobic respiration that can continue without oxygen?

Answer 29

Glycolysis.

Question 30

What must occur for glycolysis to continue without oxygen?

Answer 30

Pyruvate and hydrogen must be constantly removed and NAD must be regenerated. This is done by pyruvate accepting the H2 from NADH2.

Question 31

Describe anaerobic respiration in animals

Answer 31

Pyruvate is the H2 acceptor and is converted to lactate, which regenerates NAD. If oxygen becomes available, the lactate can be respired to CO2 and water, releasing the energy it contains.

Question 32

Describe anaerobic respiration in plants

Answer 32

Pyruvate is converted to CO2 and ethanal, a H2 acceptor by decarboxylase. Ethanal is reduced to ethanol and NAD is regenerated in alcoholic fermentation. This pathway is not reversible so even if oxygen is available, ethanol is not broken down and can rise to toxic concentrations.

Question 33

Why is the Krebs cycle called a 'metabolic hub'?

Answer 33

As the metabolic pathways of carbohydrates, proteins and lipids can feed into it, and in some situations, fats and proteins can be used as respiratory substrates.

Question 34

Why is acetyl coenzyme A the most significant molecule in respiration?

Answer 34

As it links the metabolism of the three types of macromolecule (carbohydrates, proteins and lipids).

Question 35

When is fat used as a respiratory substrate?

Answer 35

When carbohydrates levels in the body are low.

Question 36

Describe how fat is used as a respiratory substrate

Answer 36

Fat is hydrolysed into glycerol and fatty acids. Then the glycerol is phosphorylated with ATP to make glycerol-3-phosphate. This is then dehydrogenated with NAD and converted into triose phosphate which enters the glycolysis pathway.

Question 37

What happens to longer fatty acid chains when they are used as a respiratory substrate?

Answer 37

They are split into 2C fragments that enter the Krebs cycle as AcCoA. Longer fatty acids chains have: More carbons atoms, so more CO2 is produced More hydrogen atoms so more FADH2 and NADH2 are produced, making more ATP More hydrogen atoms, so more water is produced.

Question 38

When is protein used as a respiratory substrate?

Answer 38

Whenever dietary energy supplies are inadequate. In prolonged starvation, heart muscle and kidney tissue are the first the body breaks down to release protein.

Question 39

Describe how protein is used as a respiratory substrate

Answer 39

Protein is hydrolysed into its constituent amino acids, which are deaminated in the liver. The amino group is converted to urea and excreted. The residue is converted into AcCoA, pyruvate or other Krebs cycle intermediate, and oxidised.