Respiration

Question 1

What are the stages of aerobic respiration?

Answer 1

Glycolysis: the splitting of 6-C glucose into two 3-C pyruvate molecules.
Link reaction: the 3-C pyruvate molecules form 2-C acetylcoenyme A.
Krebs Cycle: acetylcoenzyme A goes into a cycle of oxidation-reduction reactions to yield some ATP and a large quantity of reduced NAD and FAD.
Oxidative phosphorylation: electrons, reduced NAD and FAD synthesise ATP and produce water as a by-product.

Question 2

What is glycolysis?

Answer 2

The first stage of aerobic and anaerobic respiration.
It occurs in the cytoplasm.
It is the process by which a hexose sugar is split into two molecules of 3-carbon pyruvate.

Question 3

What is the phosphorylation of glucose to glucose phosphate (glycolysis 1)?

Answer 3

Glucose is made more reactive by adding two phosphate molecules.
The phosphate comes from the hydrolysis of two ATP molecules to ADP.
This provides the energy to activate glucose and lowers the activation energy for later enzyme-controlled reactions.

Question 4

What is the splitting of the phosphorylated glucose (glycolysis 2)?

Answer 4

Each glucose molecule is split into two 3-carbon molecules called triose phosphate.

Question 5

What is the oxidation of triose phosphate (glycolysis 3)?

Answer 5

Hydrogen is removed from each of the two triose phosphate molecules and transferred to a hydrogen carrier molecule NAD to form reduced NAD.

Question 6

What is the production of ATP (glycolysis 4)?

Answer 6

Enzyme controlled reactions convert each triose phosphate into another 3-carbon molecule pyruvate.
2 ATP molecules are regenerated from ADP in the process.

Question 7

What is the energy yield from glycolysis?

Answer 7

Two molecules of ATP.
Two molecules of reduced NAD.
Two molecules of pyruvate.

Question 8

How is glycolysis indirect evidence for evolution?

Answer 8

The enzymes for glycolysis are found in the cytoplasm of cells and so glycolysis does not need any organelle or membrane.
It doesn’t need oxygen, and the pyruvate produced in its absence can be converted to lactate or ethanol.
This is necessary to re-oxidise NAD.

Question 9

What is the Link reaction?

Answer 9

The pyruvate molecules are actively transported to the matrix of the mitochondria.
The 3-C pyruvate is oxidised to acetate. It loses a carbon dioxide molecule and two hydrogens - accepted by NAD to form reduced NAD.
The 2-C acetate combines with coenzyme A to produce acetylcoenzyme A.

Question 10

What is the Krebs cycle?

Answer 10

The 2-C acetylcoenzyme A combines with a 4-C molecule to form a 6-C molecule.
This loses carbon dioxide and hydrogen to give a 4-C molecule and a single molecule of ATP (produced as a result of substrate-level phosphorylation).
The 4-C molecule can now combine with a new acetylcoenzyme A to begin again.

Question 11

What is the yield from the link reaction and Krebs cycle?

Answer 11

Each molecule of pyruvate produce:
Reduced coenzymes NAD and FAD.
1 molecule of ATP
3 molecules of carbon dioxide.
2 pyruvates are produced for each original glucose, so these quanities are doubled.

Question 12

What is the significance of the Krebs cycle?

Answer 12

Breaks macromolecules (pyruvate) into smaller ones (carbon dioxide).
Produces hydrogen atoms that are carried by NAD to the electron transport chain and provide energy for oxidative phosphorylation.
Regenerates 4-C molecule that combines with acetylcoenzyme A.
Is a source of intermediate compounds used by cells to manufacture fatty acids, amino acids and chlorophyll.

Question 13

What is the importance of mitochondria?

Answer 13

The site of oxidative phosphorylation, the cristae are where the enzymes and other proteins are.
They occur in great numbers in metabollically active cells, which also have a more densely packed cristae to provide a greater surface area.

Question 14

What is the electron transfer chain (oxidative phosphorylation)?

Answer 14

Hyrdogen atoms combine with coenzymes NAD and FAD.
The reduced NAD and FAD donate electrons to the first molecule in the chain.
The electrons pass along the chain in oxidation-reduction reactions. The energy they release causes the active transport of protons across the inner mitochondrial membrane and into inter-membranal space.
The protons accumulate here before diffusing back through ATP synthase channels.
At the end of the chain, electrons combine with these protons and oxygen to form water.

Question 15

What is the importance of oxygen?

Answer 15

It acts as the final acceptor of the hydrogen atoms produced in glycolysis and the Krebs cycle.
Without this removing of hydrogen atoms, the protons and electrons would ‘back up’ and respiration would stop.

Question 16

Why is energy released in stages?

Answer 16

If lots removed in one step, most is released as heat and less is available for useful purposes.
So the electrons carried by NAD and FAD are passed along a series of electron transfer carrier molecules, each at a lower energy level.
Energy is therefore released gradually and more usefully.

Question 17

What is the respiration of lipids?

Answer 17

Lipids are first hydrolysed to glycerol and fatty acids.
Glycerol is then phosphorylated and converted to triose phosphate which then enters the glycolysis pathway.
The fatty acid is broken down into 2-C fragments which are converted to acetylcoenzyme A.
Lipids release double the energy of the same mass of carbohydrate.

Question 18

What is respiration of protein?

Answer 18

It is hydrolysed to amino acids, which have their amino group removed.
Where they enter the respiratory pathway depends on the number of carbon atoms they contain.
3-C are converted to pyruvate.
5-C are converted to intermediates in the Krebs cycle.

Question 19

Why does aerobic respiration need oxygen?

Answer 19

Without oxygen, neither the Krebs cycle or electron transfer chain can continue as soon all the NAD and FAD will be reduced.
So they can’t take up the protons from the Krebs cycle and so enzymes stop working.

Question 20

What is the production of ethanol in plants and some microorganisms?

Answer 20

The pyruvate formed at the end of glycolysis loses a molecule of carbon dioxide and accepts hydrogen from reduced NAD to produce ethanol.
pyruvate + reduced NAD –>ethanol + carbon dioxide + oxidised NAD

Question 21

What is the production of lactate in animals?

Answer 21

Each pyruvate molecule takes up two hydrogen atoms from the reduced NAD from glycolysis to form lactate.
pyruvate + reduced NAD –> lactate + oxidised NAD
At some point lactate is oxidised back to pyruvate once oxygen is available again.

Question 22

What are the problems with lactate?

Answer 22

It causes muscle cramp and fatigue if it is allowed to accumulate in muscle tissue.
As an acid, lactate causes pH changes which affect enzymes.