6 exam-style questions with full mark schemes and model answers. Write your own answer and the AI examiner marks it against the mark scheme.
A single-celled amoeba can absorb all the oxygen it needs straight across its cell surface by diffusion. A large mammal such as a horse cannot rely on diffusion across its body surface alone and instead has lungs and a circulatory system.
Explain why small organisms can rely on diffusion across their surface, but large, active multicellular organisms need specialised exchange surfaces and transport systems. In your answer, refer to surface-area-to-volume ratio and diffusion distance. (6 marks)
Two cube-shaped blocks of agar jelly are used as models of cells.
(a) Calculate the surface-area-to-volume ratio of Block A. Show your working. (2 marks)
(b) Calculate the surface-area-to-volume ratio of Block B, then state which block models a cell that can exchange substances more efficiently by diffusion. (2 marks)
A student investigated osmosis using cylinders cut from a potato. Each cylinder was weighed, left in a different sucrose solution for 30 minutes, then dried and reweighed. The results for two cylinders are shown.
| Cylinder | Starting mass (g) | Final mass (g) |
|---|---|---|
| In distilled water | 5.0 | 5.6 |
| In concentrated sucrose | 4.0 | 3.4 |
(a) Calculate the percentage change in mass for the cylinder in distilled water. Show your working. (2 marks)
(b) Explain, in terms of water movement by osmosis, why the cylinder in concentrated sucrose lost mass. (1 mark)
Substances move into and out of cells by diffusion, osmosis and active transport.
(a) Root hair cells absorb mineral ions from the soil, even though the ions are at a higher concentration inside the cell than in the soil. Name the process used and state why it requires energy from respiration. (2 marks)
(b) State one way in which active transport differs from diffusion. (1 mark)
Stem cells are unspecialised cells that can divide to produce many cell types. They can be obtained from embryos or from adult bone marrow.
(a) State what is meant by the term stem cell. (1 mark)
(b) Give one potential benefit and one ethical or practical issue of using embryonic stem cells in medicine. (1 mark)
Plants transport water and dissolved mineral ions from the roots to the leaves through tubes called xylem. Water then evaporates from the leaves in a process called transpiration.
Name the type of plant tissue that, in contrast to xylem, transports dissolved sugars (made in the leaves) around the plant. (1 mark)