AQA GCSE Biology: Cell biology 6-mark questions

A sperm cell has a long tail (flagellum) which propels it towards the egg. It contains many mitochondria, which release energy by aerobic respiration to power the movement of the tail. Its streamlined shape reduces resistance as it moves through liquid. The nucleus contains the genetic material, which is delivered to the egg when the sperm fertilises it. A root hair cell has a long extension, giving it a large surface area so more water and mineral ions can enter from the soil. Its thin surface gives a short distance for these substances to pass through. Root hair cells extend between soil particles, increasing their contact with the soil solution. Water enters by osmosis, while mineral ions can be absorbed by active transport when their concentration is lower in the soil than in the cell. Many mitochondria provide the energy needed for this active transport.

A nucleus from one of the patient’s body cells could be inserted into an egg cell with its nucleus removed, producing an embryo that is genetically matched to the patient. Embryonic stem cells taken from this embryo are undifferentiated, so they can divide to produce more stem cells and differentiate into the specialised cell type that has been damaged. The new cells could then replace the damaged cells and restore some of the lost function. Because their genetic material matches the patient’s, the patient’s immune system is less likely to recognise them as foreign and reject them. However, there is a risk that a viral infection could be transferred during the transplant. There may also be ethical or religious objections to creating and using a human embryo.

When photosynthesis is taking place, the concentration of carbon dioxide is higher in the air outside the leaf than inside the palisade cells, because the cells use carbon dioxide. Therefore, carbon dioxide moves down its concentration gradient by diffusion, through the stomata into the leaf. It then diffuses through the air spaces between the cells and into the palisade cells, where it is used in photosynthesis. This use of carbon dioxide helps maintain the concentration gradient. The stomata provide openings with a large total area for carbon dioxide to enter, and the air spaces give it a short pathway between the stomata and the cells. Also, the leaf is thin, so the diffusion distance to the palisade cells is short, making diffusion rapid.