6 Organisms respond to changes in their internal and external environments

Question 1

Give two safety precautions that should be followed when dissecting a heart. (1)

Answer 1

Use a sharp scalpel.

Safe disposal.

Question 2

Exercise causes an increase in heart rate. Describe the role of receptors and of the nervous system in this process. (4)

Answer 2

Chemoreceptors detect the rise in CO2, which sends impulses to the medulla. More impulses are sent to SAN, by the sympathetic nervous system which increases the heart rate.

Question 3

When the heart beats, both ventricles contract at the same time. Explain how this is coordinated in the heart after initiation of the heart beat by the SAN. (2)

Answer 3

Electrical activity passes only through the Bundle of His, so the wave of electrical activity passes over both the ventricles at the same time.

Question 4

Explain how the resting potential of -70mV is maintained in the sensory neurone when no pressure is applied. (2)

Answer 4

The membrane is more permeable to potassium ions and less permeable to sodium ions. This means that the sodium ions are actively transported out and the potassium ions are actively transported in.

Question 5

Suggest two advantages of simple responses. (2)

Answer 5

(Any 2 of these)

They are rapid responses.

They protect against damage to body tissues.

They do not have to be learnt.

They help escape from predators.

They enable homeostatic control.

Question 6

Explain how a resting potential is maintained across the axon membrane in a neurone. (3)

Answer 6

There is a higher concentration of potassium ions inside the neurone, which leaves a higher concentration of sodium outside the neurone. The membrane is more permeable to potassium ions. So sodium ions are actively transported out and the potassium ions are actively transported in.

Question 7

Explain why the speed of transmission of impulses is faster along a
myelinated axon than along a non-myelinated axon. (3)

Answer 7

Myelination provides electrical insulation. In a myelinated axon, saltatory conduction occurs, whereas in a non myelinated axon, depolarisation happens across the whole length of it.

Question 8

Describe the sequence of events involved in transmission across a cholinergic synapse. (5) Do not include details on the breakdown of acetylcholine in your answer.

Answer 8

Depolarisation of the presynaptic membrane causes calcium channels to open and calcium ions enter the synaptic knob. The calcium ions cause the synaptic vesicles to fuse with the presynaptic membrane which releases a neurotransmitter. The neurotransmitter diffuses across the synaptic cleft where it attaches to receptors on the post synaptic membrane. Sodium ions enter the postsynaptic neurone which leads to depolarisation.

Question 9

When a nerve impulse arrives at a synapse, it causes the release of
neurotransmitter from vesicles in the presynaptic knob.
Describe how. (3)

Answer 9

The nerve impulse causes the calcium channels to open, which lets calcium enter by facilitated diffusion. This causes the synaptic vesicles to fuse with the presynaptic membrane, releasing the neurotransmitter.

Question 10

In muscles, pyruvate is converted to lactate during prolonged exercise.
Explain why converting pyruvate to lactate allows the continued production
of ATP by anaerobic respiration. (2)

Answer 10

This conversion regenerates NAD so that glycolysis can continue.

Question 11

Explain the role of glycogen granules in skeletal muscle (2)

Answer 11

Glycogen granules act as a store of glucose which will then be used for respiration.

Question 12

During vigorous exercise, the pH of skeletal muscle tissue falls. This fall in
pH leads to a reduction in the ability of calcium ions to stimulate muscle
contraction.
Suggest how. (3)

Answer 12

The low pH changes the shape of the calcium receptors. This causes less calcium to bind to tropomyosin which then means that less tropomyosin moves away. Fewer binding sites on actin are revealed which causes fewer myosin heads to bind.

Question 13

Describe the roles of calcium ions and ATP in the contraction of a myofibril. (5)

Answer 13

Calcium ions diffuse into the myofibrils from the sarcoplasmic reticulum. The calcium ions cause movement of tropomyosin on actin which then causes the myosin heads to bend. The myosin heads attach to the binding sites on actin. Hydrolysis of ATP on the myosin heads cause the myosin heads to bend, which pulls the actin molecules. Attachment of a new ATP molecule to each myosin head causes the myosin heads to detach from the actin sites.

Question 14

ATP is an energy source used in many cell processes. Give two ways in
which ATP is a suitable energy source for cells to use. (2)

Answer 14

(Any 2 of these)
1. It releases a relatively small amount of energy
2. It releases energy instantly
3. It phosphorylates other compounds, making them more reactive
4. It can be rapidly resynthesized
5. It is not lost from cells.

Question 15

What is the role of ATP in myofibril contraction? (2)

Answer 15

ATP is used in a reaction to allow the binding of myosin to actin. It does this by providing energy to move the myosin heads.

Question 16

Neonatal diabetes is a disease that affects newly born children. The disease is caused by a change in the amino acid sequence of insulin.
This change prevents insulin binding to its receptor. Explain why this change prevents insulin binding to its receptor. (2)

Answer 16

The change in the amino acid sequence alters the tertiary structure. It is now not complementary to the receptor and cannot bind.

Question 17

Using your knowledge of the kidney, explain why glucose is found in the
urine of a person with untreated diabetes. (3)

Answer 17

There is a high concentration of glucose in the blood, meaning that not all of the glucose is reabsorbed at the proximal convoluted tubule. All of the carrier proteins are in use and cannot absorb all of the glucose.

Question 18

Describe the role of glucagon in gluconeogenesis.
Do not include in your answer details on the second messenger model of
glucagon action.

Answer 18

The glucagon attaches to receptors on the target cells and stimulates enzymes. These enzymes convert fatty acids into glucose.

Question 19

Each year, a few people with type I diabetes are given a pancreas transplant. Pancreas transplants are not used to treat people with type II
diabetes.
Give two reasons why pancreas transplants are not used for the treatment of type II diabetes. (2)

Answer 19

1. Type 2 diabetes usually produces insulin.
2. It is treated by changing diet and increasing exercise.

Question 20

Give two ways in which people with type 1 diabetes control their blood
glucose concentration. (2)

Answer 20

1. Treated with insulin injections.
2. Control of their diet and sugar intake.

Question 21

Describe how ultrafiltration occurs in a glomerulus. (3)

Answer 21

There is a high hydrostatic pressure, which causes substances such as (any 2 of - water/glucose/urea/ions) through the small pores in the capillary endothelium, and through the capillary basement membrane.

Question 22

More than 99% of biological molecules are reabsorbed from the filtrate in the proximal convoluted tubule.
Despite this, the concentration of fluid in this tubule remains constant.
Explain why. (1)

Answer 22

Water is also reabsorbed.

Question 23

Give the location of osmoreceptors in the body of a mammal. (1)

Answer 23

The hypothalamus.

Question 24

When a person is dehydrated, the cell volume of an osmoreceptor decreases.
Explain why. (2)

Answer 24

Water potential of the blood will decrease, so water moves from the osmoreceptor into the blood by osmosis.

Question 25

Stimulation of osmoreceptors can lead to secretion of the hormone ADH. Describe and explain how the secretion of ADH affects urine produced by
the kidneys. (4)

Answer 25

The secretion of ADH increases the permeability of the membrane to water, so more water is absorbed from the distal tubule. There is a smaller volume of urine so the urine becomes more concentrated.