Biology A2 Chapter 12 - Respiration

Question 1

What is aerobic respiration?

Answer 1

Respiration that requires oxygen and produces carbon dioxide, water and much ATP

Question 2

What is anaerobic respiration?

Answer 2

Respiration that takes place in the absence of oxygen and produces lactate (in animals) or ethanol and carbon dioxide (in plants and fungi) but only a little ATP in both cases

Question 3

What are the 4 stages of respiration? Can you give a brief summary of each?

Answer 3

1. Glycolysis - the splitting of the 6-carbon glucose molecule into two 3-carbon pyruvate molecules
2. Link reaction - the 3-carbon pyruvate molecules enter a series of reactions which lead to the formation of acetylcoenzyme A, a 2-carbon molecule
3. Krebs cycle- the introduction of acetylcoenzyme A into a cycle of oxidation-reduction reactions that yield some ATP and a large quantity of reduced NAD and FAD
4. Oxidative phosphorylation - the use of electrons, associated with reduced NAD and FAD, released from the Krebs cycle to synthesise ATP with water produced as a by-product

Question 4

Where does glycolysis occur?

Answer 4

In the cytoplasm of all living cells

Question 5

What is the first stage of glycolysis and what happens during it?

Answer 5

Phosphorylation of glucose to glucose phosphate:
Before it can be split into two, glucose must be made more reactive by the addition of two phosphate molecules. These come from the hydrolysis of two ATP molecules to ADP. This provides the energy to activate glucose and lowers the activation energy for the enzyme controlled reactions that follow

Question 6

What is the second stage of glycolysis and what happens during it?

Answer 6

Splitting of the phosphorylated glucose:
Each glucose molecule is split into two 3-carbon molecules known as triose phosphate (each 3-carbon molecule has one phosphate attached)

Question 7

What is the third stage of glycolysis and what happens during it?

Answer 7

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

Question 8

What is the final (fourth) stage of glycolysis and what happens during it?

Answer 8

The production of ATP:
Enzyme controlled reactions convert each triose phosphate into another 3-carbon molecule called pyruvate. In the process, two molecules of ATP are regenerated from ADP

Question 9

What is the overall yield of one glucose molecule undergoing glycolysis?

Answer 9

• two molecules of ATP (four produced, but two used up in the initial phosphorylation pf glucose)
• two molecules of reduced NAD
• two molecules of pyruvate

Question 10

Does glycolysis require oxygen?

Answer 10

No - it takes place in the cytoplasm and so does not require oxygen, nor any organelles

Question 11

Where does the link reaction occur?

Answer 11

In the matrix of the mitochondria

Question 12

What happens during the link reaction?

Answer 12

• pyruvate is oxidised to acetate. In this reaction, the 3-carbon pyruvate loses a carbon dioxide molecule and two hydrogens. These are accepted by NAD to form reduced NAD which is later used to produce ATP
• The 2-carbon acetate combines with a molecule called coenzyme A to produce an enzyme called acetylcoenzyme A

Question 13

What is the equation for the link reaction?

Answer 13

pyruvate + NAD + CoA —> acetyl CoA + reduced NAD + carbon dioxide

Question 14

Where does the Krebs cycle occur?

Answer 14

The matrix of the mitochondria

Question 15

Briefly summarise the events that occur in the Krebs cycle (3 bullet points)

Answer 15

1. The 2-carbon acetylcoenzyme A from the link reaction combines with a 4-carbon molecule to produce a 6-carbon molecule
2. In a series of reactions this 6-carbon molecule loses carbon dioxide and hydrogen to give a 4-carbon molecule and a single molecule of ATP produced as a result of substrate-level phosphorylation
3. The 4-carbon molecule can now combine with a new molecule of acetylcoenzyme A to begin the cycle again

Question 16

What does one glucose molecule produce during the Krebs cycle?
(Pyruvate will be half of this as two pyruvates = one glucose molecule?

Answer 16

• 6 reduced NAD
• 2 reduced FAD
• 2 ATP
• 4 carbon dioxide

Question 17

What are coenzymes?

Answer 17

Coenzymes are NOT enzymes. They are molecules that some enzymes require in order to function.

Question 18

Give three examples of coenzymes and what they do

Answer 18

All of the following carry hydrogen atoms from one molecule to another:
- NAD, which is important throughout respiration
- FAD, which is important in the Krebs cycle
- NADP, which is important in photosynthesis

Question 19

What are the four reasons why the Krebs cycle plays an important role in the cells of organisms?

Answer 19

• breaks down macromolecules into smaller ones
• produces hydrogen atoms that are carried by NAD to the electron transfer chain and provide energy for oxidative phosphorylation, leading to the production of ATP
• regenerates the 4-carbon molecule that combines with acetylcoenzyme A, which would otherwise accumulate
• a source of intermediate compounds used by cells in the manufacture of other important substances, such as fatty acids, amino acids and chlorophyll

Question 20

What does the link reaction produce per glucose molecule?

Answer 20

• 2 acetyl CoA
• 2 reduced NAD
• 2 carbon dioxide

Question 21

Where does oxidative phosphorylation occur?

Answer 21

In the mitochondria - within the inner folded membrane (cristae) are the enzymes and other proteins involved in oxidative phosphorylation and hence ATP synthesis

Question 22

What does the synthesis of ATP by oxidative phosphorylation involve?

Answer 22

The transfer of electrons down a series of electron carrier molecules which together form the electron transfer chain

Question 23

How does the electron transfer chain work?

Answer 23

• The hydrogen atoms produced during glycolysis and the Krebs cycle combine with the coenzymes NAD and FAD
• The reduced NAD and FAD donate the electrons of the hydrogen atoms they are carrying to the first molecule in the electron transfer chain
• The electrons pass along a chain of electron transfer molecules in a series of oxidation-reduction reactions. As these flow along the chain, the energy they release causes the active transport of protons across the membrane into the inter-membranal space
• The protons accumulate here before diffusing back into the matrix through ATP synthase channels in the inner membrane
• At the end of the chain the electrons combine with these protons and oxygen to form water. Oxygen is therefore the last acceptor of electrons in the chain

Question 24

Why are the electrons carried by NAD and FAD and not completed in one explosive step?

Answer 24

In order for energy to be released a little at a time, so less is released as heat and more can be used for useful processes

Question 25

How are electron transfer molecules arranged?

Answer 25

In decreasing energy level so energy is released gradually and the electrons can move down an energy gradient

Question 26

What are lipids hydrolysed to?

Answer 26

Fatty acids and glycerol

Question 27

How does glycerol enter the Krebs cycle (from lipids)?

Answer 27

It is phosphorylated and converted to triose phosphate which enters the glycolysis pathway and subsequently the Krebs cycle

Question 28

How do fatty acids enter the Krebs cycle (from lipids)?

Answer 28

Broken down into 2-carbon fragments which are converted into acetylcoenzyme A and then enters the Krebs cycle

Question 29

How do proteins enter the Krebs cycle?

Answer 29

- Hydrolysed to its amino acids - Amino group is removed (deamination) before entering the respiratory pathway at different points depending on the number of carbon atoms they contain - 3-carbon compounds are converted to pyruvate - 4 and 5-carbon compounds are converted to intermediates in the Krebs cycle

Question 30

What is the equation of production of ethanol in plants and some microorganisms (anaerobic respiration)?

Answer 30

Pyruvate + reduced NAD —-> ethanol + carbon dioxide + oxidised NAD

Question 31

What does anaerobic respiration produce in animals?

Answer 31

Lactate

Question 32

Why is lactate produced?

Answer 32

It is a result of strenuous exercise and occurs when oxygen is used up more rapidly than it can be supplied, and therefore an oxygen debt is built up

Question 33

What changes in glycolysis when oxygen is in short supply?

Answer 33

NAD from glycolysis can build up and must be removed. To achieve this, each pyruvate molecule produced takes up two hydrogen atoms from the reduced NAD produced in glycolysis in order to form lactate: Pyruvate + reduced NAD —-> lactate + oxidised NAD

Question 34

What happens to lactate when oxygen becomes available again?

Answer 34

It is oxidised back to pyruvate to either be further oxidised to release energy, or converted to glycogen

Question 35

What does lactate cause?

Answer 35

Cramp and muscle fatigue (it is an acid also, so needs to be removed as causes pH changes which effect enzymes)

Question 36

How is energy from cellular respiration derived?

Answer 36

- substrate level phosphorylation in glycolysis and the Krebs cycle. This is the direct transfer of phosphate from a respiratory intermediate to ADP to produce ATP - oxidative phosphorylation in the electron transfer chain. This is the indirect linking of energy from phosphate to ADP to produce ATP involving energy from the hydrogen atoms that are carried n NAD and FAD. Cells produce most of their ATP this way

Question 37

What is pyruvate converted to in anaerobic respiration, and what does this prevent?

Answer 37

Ethanol or lactate. This means pyruvate is not available for the Krebs cycle, therefore stopping the Krebs cycle and the electron transfer chain - the only ATP that can be produced is formed by glycolysis