AQA A-Level Biology: Cells, Membranes and Immunity

A student is studying actively dividing cells in the meristem of a plant root tip.

Describe and explain the events of the cell cycle and mitosis, and explain why mitosis produces two genetically identical daughter cells.

In your answer, refer to the events of interphase, the four main stages of mitosis, the roles of the spindle and centromere, and cytokinesis.

A student prepared a stained squash of an onion root tip and counted the cells in two regions of the same slide. The results are shown in the table.

Region	Number of cells in mitosis	Total number of cells counted
A	24	200
B	45	300

The mitotic index is calculated using:

$\text{mitotic index} = \frac{\text{number of cells in mitosis}}{\text{total number of cells counted}}$mitotic index=total number of cells countednumber of cells in mitosis​

(a) Calculate the mitotic index for region A and for region B. Give each value as a decimal to 2 decimal places and as a percentage. (3 marks)

(b) Region B is closer to the root-tip meristem. Using your results, explain what a higher mitotic index indicates about this region. (3 marks)

Beetroot tissue contains a red pigment in the cell vacuole. A student cut identical discs of beetroot, placed each in water at a different temperature for the same time, then measured the absorbance of the surrounding water (a higher absorbance means more pigment has leaked out). The results are shown in the table.

Temperature / °C	Absorbance / arbitrary units
20	0.10
30	0.20
40	0.34
50	0.55
60	0.80

(a) Describe the trend shown by the data. (1 mark)

(b) Explain this trend in terms of the structure of the cell-surface and vacuole membranes. (2 marks)

(c) Calculate the percentage increase in absorbance between 30 °C and 60 °C. Show your working. (2 marks)

A person is exposed to a new strain of a virus for the first time. They recover, and several months later they are exposed to the same strain again. On the second exposure they produce antibodies much faster and in much greater quantity, and do not become ill.

Explain the humoral immune response to the first exposure, and explain why the secondary response to the second exposure is faster and greater. (5 marks)

An ELISA test is used to find out whether a specific antigen (a protein produced by a particular pathogen) is present in a patient's blood sample, and roughly how much of it is present.

Explain how an ELISA test uses antibodies and an enzyme to detect this specific antigen and to indicate its amount. (4 marks)

Glucose is a large, polar molecule and cannot diffuse directly through the phospholipid bilayer of the cell-surface membrane.

Describe how glucose crosses the cell-surface membrane by facilitated diffusion. (3 marks)