chapter 18 p3

Question 1

Small-scale and large-scale adaptations to low oxygen environments:

Answer 1

Many animals live in or around water and spend time underwater to hunt for food.
These animals are adapted in a variety of ways to survive periods of anaerobic respiration while they cannot breathe air.
Many bacteria also live in low oxygen environments.
There are many adaptations that have evolved in different organisms to overcome the problems of a temporary or permanent lack of oxygen:

Question 2

Bacterial adaptations:

Answer 2

Different groups of bacteria have evolved to use nitrate ions, sulphate ions, and carbon dioxide as final electron acceptors in anaerobic respiration.
This enables them to live in very low, or zero, oxygen environments.
Anaerobic bacteria present in the digestive systems of animals play an essential role in the breakdown of food and absorption of minerals.
Methanogens are a type of bacteria found in the digestive system of ruminants, such as cows.
They digest cellulose from grass cell walls into products that can be further digested, absorbed and used by the ruminants.
The final electron acceptor in the respiratory pathway of these bacteria is carbon dioxide, and methane and water are produced.
The methane builds up and eventually has to be released - it has been estimated that a cow produces around 500 L of methane per day.

Question 3

Mammalian adaptations:
Marine mammals that dive for long periods, such as seals and whales, have a range of different types of adaptations for surviving when they cannot take in more oxygen:

Answer 3

Biochemical adaptations

Physiological adaptations

Physical adaptations.

Question 4

Biochemical adaptations include

Answer 4

include greater concentrations of haemoglobin and myoglobin than land mammals, particularly in the muscles used in swimming.
This maximises their oxygen stores, delaying the onset of anaerobic metabolism.
Whales have a higher tolerance to lactic acid than human beings, so they can respire anaerobically much longer without suffering tissue damage.
They also have a greater tolerance of high carbon dioxide levels - they have very effective blood buffering systems that prevent a catastrophic rise in pH.

Question 5

Physiological adaptations

Answer 5

in many diving mammals include a modified circulatory system.
When they dive they show peripheral vasoconstriction, so blood is shunted to the brain, heart, and muscles.
The heart slows by up to 85% - this is known as bradycardia and reduces the energy demand of the heart muscle.
Whales also exchange 80-95% of the air in the lungs when they breathe - in humans, that figure is around 15%.
In some species dives can last up to two hours, so the adaptations are very effective.

Question 6

Physical adaptations

Answer 6

include streamlining to reduce drag due to friction from water while swimming, therefore reducing the energy demand during a dive.
The limbs of marine mammals are “fin-shaped”’ to maximise the efficient use of energy in propulsion

Question 7

Glucose is not the only organic molecule that is broken down to release energy for the synthesis of ATP. There are many other respiratory substrates:

Answer 7

Triglycerides are hydrolysed to fatty acids, which enter the Krebs cycle via acetyl CoA and glycerol.
Glycerol is first converted to pyruvate before undergoing oxidative decarboxylation, producing an acetyl group which is picked up by coenzyme A, forming acetyl CoA.
The fatty acids in a triglyceride molecule can lead to the formation of as many as 50 acetyl CoA molecules, resulting in the synthesis of up to 500 ATP molecules.
Gram for gram, lipids store and release about twice as much energy as carbohydrates.

Question 8

Alcohol contains more

Answer 8

energy than carbohydrates but less than lipids.
Proteins are roughly equivalent to carbohydrates.
Proteins first have to be hydrolysed to amino acids and then the amino acids have to be deaminated (removal of amine groups) before they enter the respiratory pathway, usually via pyruvate.
These steps require ATP, reducing the net production of ATP.

Question 9

Answer 9

Question 10

The respiratory quotient (RQ) of a substrate is calculated by

Answer 10

dividing the volume of carbon dioxide released by the volume of oxygen taken in during respiration of that particular substrate.
This is measured using a simple piece of apparatus called a respirometer

Question 11

It takes six oxygen molecules to

Answer 11

completely respire one molecule of glucose and this results in the production of six molecules of carbon dioxide (and six molecules of water).
This results in an RQ of 1.0.

Question 12

Lipids contain a greater proportion of

Answer 12

carbon-hydrogen bonds than carbohydrates which is why they produce so much more ATP in respiration.
Due to the greater number of carbon-hydrogen bonds, lipids require relatively more oxygen to break them down and release relatively less carbon dioxide.
This results in RQs of less than one for lipids.
The structure of amino acids leads to RQs somewhere between carbohydrates and lipids.

Question 13

The structure of amino acids leads to RQs somewhere between carbohydrates and lipids.

Answer 13

• carbohydrates = 1.0
• protein = 0.9
• lipids = 0.7

Question 14

So, by measuring the volume of oxygen taken in and carbon dioxide released, and calculating RQ…

Answer 14

the type of substrate being used for respiration at that point can be roughly determined.
During normal activity, the RQ is in the range of 0.8 to 0.9, showing that carbohydrates and lipids (and probably some proteins) are being use as respiratory substrates.
During anaerobic respiration, the RQ increases above 1.0, although this not easy to measure as the point at which anaerobic respiration begins is not easy to pinpoint.

Question 15

Low carbohydrate diets:

Answer 15

Many people choose low carbohydrate diets when they want to lose weight - and in particular to lose some body fat.
The diets can work - but the science suggests that you need to think carefully before cutting out the molecules that are most commonly used as fuel in your body.

Question 16

some of the facts about low carbohydrate diets for you to consider:
p1

Answer 16

Triglycerides are hydrolysed into fatty acids and glycerol.
The fatty acids are broken down in the mitochondria to give many two-carbon acetyl groups that combine with coenzyme A molecules and enter the Krebs cycle.

Triglycerides cannot act as the only respiratory substrate.
Carbohydrates are needed to keep the Krebs cycle going so that acetyl groups from the breakdown of fatty acids can be ‘fed in’.
If carbohydrates are in short supply the body will make them using a process called gluconeogenesis.
This process often uses glycerol, but it may also use pyruvate from glycolysis.

Oxaloacetate from the Krebs cycle can be used to make glucose when carbohydrate levels are low.
Reducing the number of oxaloacetate molecules in the Krebs cycle reduces the rate at which the acetyl groups produced during the breakdown of lipids can be fed into the cycle and produce ATP.

Question 17

some of the facts about low carbohydrate diets for you to consider:
p2

Answer 17

Oxaloacetate can be replaced by the conversion of pyruvate from carbohydrate breakdown in the mitochondria.
Pyruvate is also synthesised using glycerol from the breakdown of lipids.
However, the breakdown of a lipid molecule provides a relatively small quantity of glycerol and so a relatively small amount of pyruvate.
This means carbohydrates are still needed to ensure the continued respiration of fat.

Proteins can be hydrolysed into amino acids which are then deaminated in the liver. The remaining keto acids can be converted into glucose molecules.
Lean muscle is the protein of choice in this process, so a low carbohydrate diet can lead to the breakdown of muscle tissue.
The liver and kidneys also have to remove the nitrogenous waste.

If the level of acetyl CoA increases because it is not being taken into the Krebs cycle, the liver starts converting it into ketone bodies. Brain cells normally require glucose as an energy source.
They cannot use fatty acids as a respiratory substrate but they can use ketone bodies.

When the body is producing more ketone bodies than usual, it is said to be in ketosis.
This can lead to a dangerous condition known as ketoacidosis.
Ketoacidosis is the result of an accumulation of ketone bodies which cause the pH level of the blood to drop to dangerous, or even fatal, levels.
This condition is often seen in alcoholics, untreated diabetes, and during starvation.
It is often diagnosed by the fruity smell of propanone (acetone), a breakdown product of ketone bodies, on the breath of an affected person.

Question 18

Practical investigations into the factors affecting rate of respiration using respirometers: 1

Answer 18

A student carried out an experiment to investigate the effect of temperature on the rate of respiration in soaked (germinating) pea seeds and dry (dormant) pea seeds.
A respirometer was used, shown in Figure 2.
The potassium hydroxide solution in this apparatus absorbs carbon dioxide.
If the apparatus is kept at a constant temperature, any changes in the volume of air in the respirometer will be due to oxygen uptake.
The student set up three respirometers, A, B and C in water baths at two different temperatures.
The respirometers were left for 10 minutes to equilibrate.

Question 19

Practical investigations into the factors affecting rate of respiration using respirometers: 2

Answer 19

After the student had left each respirometer to equilibrate, a small volume of coloured fluid was introduced into each graduated tube.
The respirometers were then left in the appropriate water baths for 20 minutes and maintained at the correct temperature.
During this time, the coloured fluid in the graduated tube moved.
The level of the coloured fluid in each respirometer was recorded at the start of the experiment and after 20 minutes.