5.2.2: Respiration

Question 1

Why do organisms need to respire?

Answer 1

● Produces ATP as energy currency for:
a) active transport against concentration gradients e.g.
to absorb nutrients from small intestine/soil.
b) metabolic reactions e.g. to form peptide bonds in
protein synthesis.
c) muscle contraction.
● Releases heat energy for thermoregulation.

Question 2

Describe the structure of a mitochondrion.

Answer 2

surrounded by double membrane.
folded inner membrane forms cristae: site
of electron transport chain.
fluid matrix: contains mitochondrial DNA,
respiratory enzymes, lipids, proteins

Question 3

Name the 4 main stages in aerobic respiration and

where they occur.

Answer 3

● Glycolysis: cytoplasm.
● Link reaction: mitochondrial matrix.
● Krebs cycle: mitochondrial matrix.
● Oxidative phosphorylation via electron transfer
chain: membrane of cristae

Question 4

Outline the stages of glycolysis.

Answer 4

1. Glucose is phosphorylated to hexose
   bisphosphate by 2x ATP.
2. Hexose bisphosphate splits into 2x triose
   phosphate (TP).
3. 2x TP is oxidised to 2x pyruvate.
   Net gain of 2x reduced NAD & 2x ATP per glucose.

Question 5

How does pyruvate from glycolysis enter the

mitochondria?

Answer 5

active transport

Question 6

What happens during the link reaction?

Answer 6

1. Oxidation of pyruvate to acetate.
   per pyruvate molecule: net gain of 1xCO2
   (decarboxylation) & 2H atoms (used to reduce
   1xNAD).
2. Acetate combines with coenzyme A (CoA) to
   form Acetylcoenzyme A

Question 7

Give a summary equation for the link reaction.

Answer 7

pyruvate + NAD + CoA
→
Acetyl CoA + reduced NAD + CO2

Question 8

What happens in the Krebs cycle?

Answer 8

series of redox reactions produces:
● ATP by substrate-level phosphorylation
● reduced coenzymes
● CO2 from decarboxylation
Begins when acetyl group from Acetyl CoA (2C)
reacts with oxaloacetate (4C). Cycle regenerates
oxaloacetate.

Question 9

What is the electron transfer chain (ETC)?

Answer 9

Series of carrier proteins embedded in membrane of the cristae of mitochondria.
Produces ATP through oxidative phosphorylation via chemiosmosis during aerobic respiration.

Question 10

What happens in the electron transfer chain (ETC)?

Answer 10

Electrons released from reduced NAD & FAD
undergo successive redox reactions.
The energy released is coupled to maintaining
proton gradient or released as heat.
Oxygen acts as final electron acceptor.

Question 11

How does chemiosmosis produce ATP during

aerobic respiration?

Answer 11

Some energy released from the ETC is coupled to active
transport of H+ ions (protons) from mitochondrial matrix
into intermembrane space.
H+ ions move down concentration gradient into
mitochondrial matrix via channel protein ATP synthase.
ATP synthase catalyses ADP + Pi → ATP

Question 12

State the role of oxygen in aerobic respiration.

Answer 12

Final electron acceptor in electron
transfer chain.
(produces water as a byproduct)

Question 13

Name the stages in respiration that produce ATP by

substrate-level phosphorylation.

Answer 13

● glycolysis (anaerobic)

● Krebs cycle (aerobic)

Question 14

What happens during anaerobic respiration in

animals?

Answer 14

only glycolysis continues reduced NAD + pyruvate
→
oxidised NAD (for further glycolysis) + lactate

Question 15

What happens during anaerobic respiration in some

microorganisms e.g. yeast and some plant cells?

Answer 15

Only glycolysis continues, so much less ATP is
produced compared to aerobic respiration.
Pyruvate is decarboxylated to form ethanal.
Ethanal is reduced to ethanol using reduced NAD to
produce oxidised NAD for further glycolysis

Question 16

What are the benefits of being able to respire

anaerobically?

Answer 16

ATP production for vital metabolic processes
continues.
Production of ethanol/ lactate converts reduced
NAD back into NAD so glycolysis can continue
= maximum yield of ATP in the conditions.

Question 17

Suggest how a student could investigate the effect of
a named variable on the rate of respiration of a
single-celled organism

Answer 17

1. Use respirometer (pressure changes in
   boiling tube cause a drop of coloured liquid
   to move).
2. Use a dye as the terminal electron acceptor
   for the ETC.

Question 18

What is the purpose of sodium hydroxide solution in
a respirometer set up to measure the rate of aerobic
respiration?

Answer 18

Absorbs CO2 so that there is a net decrease in pressure as O2 is consumed.

Question 19

How could a student calculate the rate of respiration

using a respirometer?

Answer 19

Volume of O2 produced or CO2 consumed/
time x mass of sample.
Volume = distance moved by coloured drop x (0.5 x capillary tube diameter)2 x π.

Question 20

Name 2 types of molecule that can be used as

alternative respiratory substrates

Answer 20

● (amino acids from) proteins

● (glycerol and fatty acids from) lipids

Question 21

What is the respiratory quotient (RQ)?

Answer 21

RQ = carbon dioxide produced / oxygen consumed
Can be used to determine:
● respiratory substrate being used (carbohydrates:
1.0, lipids: 0.8, proteins 0.9)
● if organism is undergoing anaerobic respiration
(anaerobic values are larger)

Question 22

Why do different respiratory substrates have

different relative energy values?

Answer 22

Depends on the number of hydrogens in the structure which are oxidised to water e.g. the number of hydrogens is greater in fatty acids than carbohydrates