Skeletal muscle contraction

Figure 1 shows a drawing of a neuromuscular junction.

Name the parts labelled A to D.

Following the release of acetylcholine into a neuromuscular junction, a muscle contraction occurs.

Describe the sequence of events, following the release of acetylcholine, that leads to stimulation of this contraction.

Do not include in your answer the events following the release of calcium ions in the myofibril.

Inhibitory synapses cause hyperpolarisation in postsynaptic neurones.

Explain how this inhibits synaptic transmission.

Figure 2 shows part of a relaxed myofibril.

The myofibril represented in Figure 2 has 34 sarcomeres. The length of every sarcomere is 2 µm After contraction of this myofibril, the length of each sarcomere changed by 20%.

Use the information provided to calculate the length of the myofibril after contraction.

Give your answer in mm and in standard form.

Show your working.

Which statement correctly describes the changes in length in a sarcomere when a myofibril contracts?

Tick (✓) one box.

Fast muscle fibres have a higher concentration of glycogen than slow muscle fibres.

Explain how the difference in glycogen concentration is related to the different properties of these muscle fibres.

ATP is essential for muscle contraction (line 1).

Describe the roles of ATP in muscle contraction.

Calculate how long maximum exercise would last if ATP was not resynthesised (lines 3–4).

Describe and explain how taking creatine supplements (lines 5–6) and ‘carbohydrate loading’ (lines 6–7) can improve performance of different types of muscle fibres during different types of exercise.

Suggest and explain how GLUT4 and CD36 transport proteins (lines 10–11) are beneficial during exercise.

An increase in muscle activity causes an increase in heart rate (lines 12–14).

Describe and explain how.

Following a body injury, bone marrow stem cells move to the site of damage and undergo cell differentiation.

Figure 5 shows how this differentiation occurs.

Suggest how SCFR is destroyed by a lysosome.

After a heart attack, cardiomyocytes (cardiac muscle cells) die, and become infarcted tissue. Infarcted tissue cannot contract.

Stem cells in bone marrow cannot move to the infarcted tissue and differentiate into cardiomyocytes. Scientists used laboratory rats to investigate if bone marrow stem cell transplants could be used to repair infarcted tissue resulting from a heart attack.

They split the rats into three groups.

• Control group did not get a transplant of bone marrow stem cells.

• c-KIT+ group got a transplant of bone marrow stem cells with a functioning c-KIT gene.

• c-KIT– group got a transplant of bone marrow stem cells with no functioning c-KIT gene.

After 9 days, the scientists measured the mean ventricular blood pressure of each of the three groups.

Figure 6 shows their results.

The differences between the groups were all statistically significant.

Using all of the information, suggest explanations for the results for the Control group and the c-KIT– group shown in Figure 6.

Nine days after transplantation, the c-KIT+ group showed that 68% of infarcted tissue was made up of new cardiomyocytes. The control group had no new cardiomyocytes.

Assuming that mean ventricular blood pressure is directly proportional to the number of cardiomyocytes, calculate the percentage of infarcted tissue that was made up of new cardiomyocytes in the c-KIT– group.

All new cardiomyocytes produced:

• Connexin-43, a channel protein that allows electrical impulses to pass between cardiomyocytes

• GATA-4, a transcriptional factor that stimulates the expression of genes for actin and myosin.

Suggest how production of Connexin-43 and GATA-4 could give the result seen in the c-KIT+ group in Figure 6 (on page 13).

Do not include details of transcription or translation in your answer.

The action of endopeptidases and exopeptidases can increase the rate of protein digestion. Describe how.

As humans age, there is a decrease in body protein.

Give the name of one body protein that could have resulted in:

reduced muscle power

Scientists investigated the effect of two types of dietary protein on the ability of old men to produce body proteins.

Table 2 shows information about the two types of dietary protein investigated.

Figure 4 shows the percentage of protein absorbed that becomes body protein in old men following a meal of casein or whey.

A statistical test confirmed that the difference between the results shown in Figure 4 was significant.

Suggest which type of dietary protein would be better for old men to eat to cause a net gain of body proteins. Use the information provided to explain your answer.

The iris in the human eye is a muscular structure. The iris changes the size of the pupil.

Figure 1 shows the muscles in the iris.

Suggest and explain how the interaction between the muscles labelled in Figure 1 could cause the pupil to constrict (narrow).

The fovea of the eye of an eagle has a high density of cones. An eagle focuses the image of its prey onto the fovea.

Explain how the fovea enables an eagle to see its prey in detail.

Do not refer to colour vision in your answer.

The retina of the human eye has an area of approximately 1.094 × 10³ mm²

The circular fovea in a human eye has a diameter of 3 × 10³ µm

Calculate the area of the fovea as a percentage of the area of the retina.

The area of a circle is πr². Use π = 3.14 in your calculation.

Show your working.

The retina of an owl has a high density of rod cells.

Explain how this enables an owl to hunt its prey at night.

Do not refer to rhodopsin in your answer.

Figure 3 shows the banding pattern of a single sarcomere.

Explain the banding pattern shown in Figure 3.

Creatinine is produced in muscle tissues. Creatinine diffuses into the blood. The kidneys then excrete creatinine.

A calibration curve can be used to determine the concentration of creatinine in urine. One method of producing a calibration curve needs:

• creatinine solution of known concentration

• distilled water

• creatinine-detecting solution

a colorimeter.

Creatinine-detecting solution reacts with creatinine to produce an orange colour.

Use the information provided to describe how you could produce a calibration curve for creatinine.

Do not include details on the use of glassware in your answer.

Describe how you would determine the concentration of creatinine in a urine sample using your calibration curve.

Describe the advantage of the Bohr effect during intense exercise.

A cyclist completed a fitness test on an exercise bike. The intensity of the exercise was increased every 10 seconds. The test finished when he was unable to cycle any further. The partial pressure of oxygen (pO₂) and of carbon dioxide (pCO₂) in air breathed out was measured.

Figure 3 shows the results of the cyclist’s fitness test.

Ventilatory threshold (VT) is a measure of the point when anaerobic respiration increases because aerobic respiration alone can no longer maintain muscle contraction.

VT can be identified as the first point when there is an increase in pO₂ breathed out, without an equivalent increase in pCO₂ breathed out.

Use Figure 3 to determine the time after the exercise started when the cyclist reached VT.

Calculate the ratio of pO₂ to pCO₂ in breathed-out air at this time.

Show your working.

An increase in the intensity of exercise produces an increase in the volume of carbon dioxide produced.

However, Figure 3 shows that the pCO₂ in air breathed out did not show a large increase during the exercise.

Suggest one physiological change that would cause this result. Explain how the physiological change would allow for the removal of the increase in the volume of carbon dioxide produced.

Physiological change

When muscle fibres have very low concentrations of ATP, they may get ATP in the

following ways.

• AMPK (an enzyme) oxidises fatty acids.

• Phosphocreatine donates phosphate to ADP in anaerobic conditions.

Figure 4 shows how these chemicals work.

At more than 80% of maximum muscle effort, ATP can only be made for a limited time.

Use Figure 4 to suggest one reason why.

Tick () the correct box.

GW1516 is a performance-enhancing drug. GW1516 activates AMPK and develops slow muscle fibres at rest.

Use Figure 4 to justify why professional athletes are not allowed to take GW1516.

Do not include details of chemiosmotic theory in your answer.

EPO is another performance-enhancing drug. It can increase the haematocrit (the percentage of red blood cells in blood).

A heart attack is caused by a lack of glucose and oxygen being delivered to cardiac muscle via the coronary arteries. The overuse of EPO can increase the risk of a heart attack.

Suggest how.

The normal haematocrit for human males is 47(±5)%. For professional male cyclists, the maximum haematocrit allowed is 50%.

A student suggested that professional male cyclists should be allowed to use EPO until their haematocrit is 50%.

Give two reasons why this suggestion is not valid.

Section B

one question.

You are advised to spend no more than 45 minutes on this section.

Scientists investigated the effect of a decrease in pH on muscle contraction. The scientists did the investigation with four different preparations of isolated muscle tissue: A, B, C and D.

A - mouse muscle fibres at typical pH of mouse muscle tissue (control 1).

B - mouse muscle fibres at 0.5 pH units below typical pH.

C - rabbit muscle fibres at typical pH of rabbit muscle tissue (control 2).

D - rabbit muscle fibres at 0.5 pH units below typical pH.

They measured the force of muscle contraction of the muscle fibres at 12 °C, 22 °C and 32 °C

Figure 3 shows the results the scientists obtained for B and D compared with the appropriate control.

A student looked at the results and concluded that a decrease in pH does cause a decrease in the force of muscle contraction.

Use Figure 3 to evaluate this conclusion.

Another group of scientists suggested that a decrease in the force of muscle contraction is caused by an increase in the concentration of inorganic phosphate, Pi, in muscle tissues.

Their hypothesis is that an increase in the concentration of Pi prevents the release of calcium ions within muscle tissues.

Explain how a decrease in the concentration of calcium ions within muscle tissues could cause a decrease in the force of muscle contraction.

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.

Scientists investigated the effect of regular exercise on skeletal muscle fibres in mice. The scientists compared the muscle fibres of mice after six weeks of regular exercise (trained mice) with those of mice that had not exercised (control mice). The scientists stained the muscle fibres from both sets of mice to show succinic acid dehydrogenase activity. The darker the stain the greater the succinic acid dehydrogenase activity.

Figure 3 shows a typical set of results they obtained.

Succinic acid dehydrogenase is an enzyme used in the Krebs cycle.

Suggest one reason for the difference in the staining between the muscle fibres of the control mice and the trained mice.

The scientists then compared the length of time that the control mice and the trained mice could carry out prolonged exercise. The trained mice were able to exercise for a longer time period than control mice.

Explain why.

The scientists determined the mean diameter of muscle fibres in trained mice using an optical microscope to examine sections of muscle tissue. The circular area (πr of one field of view was 1.25 mm². The diameter of this area was equal to the diameter of 15 muscle fibres.

Using this information, calculate the mean diameter in µm (micrometres) of muscle fibres in this section of tissue.

The scientists also compared the diameter of samples of muscle fibres taken from young mice and adult mice.

Some of their results are shown in Figure 4.

Describe two differences between these samples of muscle fibres.

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

Describe how.

The presynaptic knob contains actin filaments and myosin molecules.

The myosin molecules can attach to mitochondria and move them towards the presynaptic membrane, as shown in Figure 1.

Use your knowledge of how myosin and actin interact to suggest how the myosin molecule moves the mitochondrion towards the presynaptic membrane.

Do not include the roles of calcium ions and tropomyosin in your answer.

This movement of mitochondria happens when nerve impulses arrive at the synapse.

Suggest and explain one advantage of the movement of mitochondria towards the presynaptic membrane when nerve impulses arrive at the synapse.