OCR A-Level Biology: Membranes, Cell Division and Cellular Organisation

The lining of the human small intestine is replaced roughly every three to five days. New cells are produced by mitosis in the crypts at the base of each villus and migrate up towards the tip, where worn cells are shed.

Describe and explain the main events of mitosis, and explain why mitosis is suited to replacing the cells of the intestinal lining. In your answer you should refer to the behaviour of chromosomes during the four phases and link the outcome of mitosis to the need for tissue replacement.

(6 marks)

A student investigated the water potential of parsnip tissue. They cut cylinders of parsnip of equal size, blotted and weighed each one, then left one cylinder in each of a range of sucrose solutions for 90 minutes. The cylinders were then blotted and reweighed. The results are shown below.

Sucrose concentration / mol dm⁻³	Initial mass / g	Final mass / g
0.00	4.00	4.36
0.20	4.00	4.12
0.40	4.00	3.88
0.60	4.00	3.60
0.80	4.00	3.40

(a) Calculate the percentage change in mass of the cylinder in the 0.60 mol dm⁻³ solution. Show your working. (2 marks)

(b) The student concluded that the water potential of the parsnip tissue was about −700 kPa, which corresponds to a sucrose concentration of roughly 0.30 mol dm⁻³. Using the data, explain how the cylinders behaved across the range of solutions, and how this conclusion was reached. (4 marks)

Equal-sized discs of beetroot were rinsed, blotted and placed individually into tubes of distilled water held at different temperatures for 30 minutes. The red pigment betalain leaks from beetroot cells when their membranes are damaged. After 30 minutes the absorbance of each solution was measured in a colorimeter (a higher absorbance means more pigment has leaked out). Mean results are shown below.

Temperature / °C	Mean absorbance / arbitrary units
10	0.08
25	0.12
40	0.21
55	0.74
70	0.96

Describe the trend shown by the data, and explain, in terms of the membrane, why the absorbance rises only slightly up to 40 °C but then increases sharply between 40 °C and 70 °C. (5 marks)

Three plant cells, P, Q and R, lie next to each other in a row in a leaf, so that water can pass between P and Q, and between Q and R. The solute potential ($\Psi_s$Ψs​) and pressure potential ($\Psi_p$Ψp​) of each cell are given below.

Cell	$\Psi_s$Ψs​ / kPa	$\Psi_p$Ψp​ / kPa
P	−1200	+500
Q	−1000	+200
R	−1400	+300

(a) Calculate the water potential ($\Psi$Ψ) of each cell. (2 marks)

(b) Using your values, state the direction of net water movement along the row of cells and explain your answer. (3 marks)

A patient has lost light-sensitive cells at the back of one eye. A research team plans to grow replacement cells in the laboratory and then transplant them. They can either use pluripotent embryonic stem cells taken from a donor embryo, or take a sample of the patient's own skin cells and reprogram them into induced pluripotent stem cells (iPSCs) before growing the replacement cells from them.

Suggest two advantages of using iPSCs made from the patient's own skin cells rather than the donor embryonic stem cells in this treatment, and explain each advantage. (4 marks)

A student examined a stained section of a garlic root tip under a microscope. In one field of view they counted 250 cells, of which 40 cells showed visible condensed chromosomes and were therefore in mitosis.

(a) Calculate the mitotic index of this field of view. (2 marks)

(b) Suggest one reason why the mitotic index is higher at a root tip than in a mature region further up the root. (1 mark)