5.7.8: Energy values of different respiratory substrates

Question 1

Besides carbohydrates, which biological molecules can also provide respiratory substrates?

Answer 1

lipids and proteins

Question 2

Why is the monosaccharide glucose the chief respiratory substrate?

Answer 2

Some mammalian cells, for example brain cells and red blood cells can only use glucose for respiration.

Question 3

How do animals and some bacteria store carbohydrate?

Answer 3

Animals and some bacteria store carbohydrate as glycogen, which can be hydrolysed to glucose for respiration

Question 4

How do plant cells store carbohydrate?

Answer 4

Plant cells store carbohydrate as starch, and this can be hydrolysed to glucose for respiration.

Question 5

-How can disaccharides be used in respiration?

Answer 5

Disaccharides can be digested to monosaccharides for respiration.

Question 6

-How can monosaccharides such as fructose and galactose be used in respiration?

Answer 6

Monosaccharides such as fructose and galactose can be changed, by isomerase enzymes, to glucose for respiration.

Question 7

How can lipids be used as respiratory substrates?

Answer 7

Triglycerides are hydrolysed by lipase to glycerol and fatty acids. Glycerol can then be converted to triose phosphate and respired.

Question 8

Describe fatty acids

Answer 8

• Fatty acids are long-chain hydrocarbons with a carboxylic acid group.
• Hence, in each molecule, there are many carbon and hydrogen atoms and very few oxygen atoms.

Question 9

Why do fats produce more ATP than an equivalent mass carbohydrate?

Answer 9

-These molecules are a good source of protons for oxidative phosphorylation, and so fats produce more ATP than an equivalent mass carbohydrate.

Question 10

How do the products of fat digestion enter the Krebs cycle?

Step 1:

Answer 10

1. With the aid of some energy from the hydrolysis of one molecule of ATP to AMP, each fatty acid is combined with coenzyme A.

Question 11

How do the products of fat digestion enter the Krebs cycle?
Step 1: With the aid of some energy from the hydrolysis of one molecule of ATP to AMP, each fatty acid is combined with coenzyme A.

Step 2:

Answer 11

2. The fatty acid-CoA complex is transported into the mitochondrial matrix,where it is broken down into two-carbon acetyl groups, each attached to CoA.

Question 12

How do the products of fat digestion enter the Krebs cycle?
Step 2: The fatty acid-CoA complex is transported into the mitochondrial matrix,where it is broken down into two-carbon acetyl groups, each attached to CoA.

Step 3:

Answer 12

3. This beta-oxidation pathway generates reduced NAD and reduced FAD.

Question 13

How do the products of fat digestion enter the Krebs cycle?
Step 3: This beta-oxidation pathway generates reduced NAD and reduced FAD.

Step 4:

Answer 13

4. The acetyl groups are released from CoA and enter the Krebs cycle by combining with the four-carbon oxaloacetate.

Question 14

For every acetyl group oxidised in the Krebs cycle, how many molecules of NADH, FADH and ATP are made?

Answer 14

• 3xNADH
• 1xFADH
• 1xATP, by substrate level phosphorylation

Question 15

Where are excess amino acids, released after the digestion of proteins deaminated?

Answer 15

In the liver

Question 16

What does the deamination of amino acids involve?

Answer 16

The removal of the amino group and its subsequent conversion to urea that is removed via the kidney.

Question 17

What happens to the rest of the amino acid molecule after deamination?

Answer 17

-The rest of the amino acid molecule, keto acid, enters the respiratory pathway as puryvate, acetyl CoA or a Krebs cycle acid such as oaxaloacetate acid.

Question 18

What happens during fasting, starvation, prolonged exercise, when insufficient glucose or lipid are available for respiration?

Answer 18

• Protein from muscle can be hydrolysed to amino acids which are then respired.
• These amino acids may be converted to pyruvate or acetate and enter the Krebs cycle.

Question 19

When is most of the ATP produced during aerobic respiration made?

Answer 19

During oxidative phosphorylation

Question 20

Why do respiratory substrates with more hydrogen atoms generate more ATP per molecule of substrate?

Answer 20

-The greater the availability of protons for chemiosmosis, the more ATP can be produced.
Therefore, the more hydrogen atoms are in a molecule of respiratory substrate, the more ATP can be generated per molecule of substrate.

Question 21

Why is more oxygen needed for the respiration of substrates with a high proportion of protons?

Answer 21

-As the protons (hydrogen ions) ultimately combine with oxygen atoms to form water, the greater the proportion of hydrogen atoms in a molecule, the more oxygen will be needed for its respiration.

Question 22

What is the formula for calculating the respiratory quotient, RQ, for different respiratory substrates?

Answer 22

RQ= CO2 produced/O2 consumed

ratio, so, no units

Question 23

Respiratory substrate:
Glucose
C6H12O6+6O2–>6CO2+6H2O
calculate RQ

Answer 23

6/6 = 1

Question 24

Respiratory substrate:
Palmatic acid (fatty acid)
C15H31COOH+23O2–>16CO2+16H2O
calculate RQ

Answer 24

16/23 = 0.7

Question 25

Respiratory substrate:
Amino acid (Keto acid =CH3COOH)
CH3COOH+5O2–>4CO2+4H2O
calculate RQ

Answer 25

4/5 = 0.8

values of 0.8-0.9 for the various amino acids

Question 26

What does an RQ value greater than 1 indicate?

Answer 26

• Some aerobic respiration is taking place.

- More CO2 is being produced than oxygen is being consumed .