Receptors

Figure 3 shows the wavelengths of light absorbed by three types of photoreceptor found in the human retina.

Use Figure 3 and your knowledge of colour vision to explain how an orange colour is seen at 600 nm

Which of the following statements is associated with high sensitivity to light in the retina?

Tick (✓) one box.

Variation in flower colour is considered one of the factors involved in the evolution of the following two species of monkeyflower.

• Erythranthe lewisii (Great Purple Monkeyflower), which has pink flowers, is mostly found at higher altitudes (1600 to 3000 metres) and attracts bumblebees.

• Erythranthe cardinalis (Scarlet Monkeyflower), which has red flowers, is mostly found at lower altitudes (up to 2000 metres) and attracts hummingbirds.

Bumblebees and hummingbirds are important in the pollination of flowers. Pollination involves the transfer of male gametes to female gametes.

Explain the different processes that may have been involved in the evolution of these two species of monkeyflower. Use the information provided to justify your answer.

Describe how stimulation of a Pacinian corpuscle produces a generator potential.

Scientists investigated the stimulation of a Pacinian corpuscle in the skin of a fingertip. The scientists applied two different pressures to the fingertip and recorded the changes in membrane potential of the Pacinian corpuscle’s sensory neurone.

Figure 1 shows the scientists’ results.

Use Figure 1 to describe what is meant by the all-or-nothing principle.

On Figure 1, from 0.6 ms to 4.0 ms, no new generator potential could be produced. What is this time period called?

Figure 4 shows a diagram of a Pacinian corpuscle.

Name the structures labelled P, Q and R shown in Figure 4.

Two students (A and B) investigated reaction time in response to touch.

• Student A sat with her eyes shut and her forearm resting on a worktop so that her hand was over the edge.

• Student B held a ruler vertically between student A’s thumb and first finger, with the ruler at 0 mm lightly touching student A ’s first finger.

• Student B released the ruler.

• As soon as student A felt the ruler fall, she closed her thumb and first finger to catch the ruler as shown in Figure 5.

• Student B measured the distance the ruler had fallen to the nearest mm

The test was repeated three more times using the same hand to catch the ruler. Table 3 shows student A ’s results.

The student was able to convert these distances into reaction times using Table 4.

Calculate the percentage uncertainty in the measurement of Trial 1 in Table 3.

Put a tick () in the correct box below.

In this investigation, it is not possible for a student to react in less than 45 ms

Suggest one explanation for the value recorded in Trial 3 in Table 3.

Student A estimated that the length of the nerve pathway involved was 175 cm

Use Table 3 and Table 4 to calculate the mean speed of nerve impulse transmission.

Do not use the value for Trial 3 in your calculation.

In response to touch, nerve impulses can be transmitted at speeds of 76.2 m s^–1

Suggest three reasons why, in this investigation, the estimated speed of student A ’s impulse transmission was less than 76.2 m s^–1

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.

Read the following passage.

Complete achromatopsia is a form of complete colour blindness. It is caused by having only rods and no functional cone cells. People with complete achromatopsia have difficulty in seeing detail. Complete achromatopsia is caused by an autosomal recessive allele and is usually very rare in populations with only one in 40 000 being affected. However on the Pacific 5 island of Pingelap ten percent of the population are affected.

One form of red-green colour blindness is caused by a sex-linked recessive allele which affects more men than women. People with this red-green colour blindness are unable to distinguish between red and green, and also between other colours. They have green-sensitive cones but the 10 photoreceptive pigment they contain does not function.

Scientists investigated the use of gene therapy to correct red-green colour blindness in monkeys. They injected viruses containing the gene for the green-sensitive pigment directly into the eyes of the monkeys. Although the monkeys maintained two years of colour vision, there is debate on whether 15 this form of gene therapy is worthwhile. No clinical trials of this procedure have been carried out on humans. Current research into the treatment of red-green colour blindness involves the use of induced pluripotent stem cells (iPS cells). The use of iPS cells could have advantages over the use of gene therapy. 20

Use the information in the passage and your own knowledge to answer the following questions.

People with complete achromatopsia have difficulty in seeing detail (lines 2–3).

Explain why.

Ten percent of the population on the Pacific island of Pingelap are affected by complete achromatopsia (lines 3–6).

Use the Hardy-Weinberg equation to calculate the percentage of this population who are heterozygous for this disorder. Show your working.

Red-green colour blindness affects more men than women (lines 7–8).

Explain why.

People with red-green colour blindness are unable to distinguish between red and green, and also between other colours (lines 8–10).

Explain why.

Current research into the treatment of red-green colour blindness involves the use of induced pluripotent stem cells (iPS cells) (lines 17–19).

Suggest how iPS cells could correct red-green colour blindness.

The use of iPS cells could have advantages over the use of gene therapy to correct red-green colour blindness (lines 19–20).

Using the information from the passage, suggest and explain reasons why.