Question 1
Paper 1A style
Which structure is considered an organelle?
- Cell wall
- Cytoskeleton
- Ribosome
- Cytoplasm
Show mark scheme for question 1
Answer: C – ribosomes are organelles; the cell wall, cytoskeleton and cytoplasm are not [1]
Question 2
Paper 1B style
Liver tissue was homogenised in an ice-cold, isotonic, buffered solution and separated by differential centrifugation. Each fraction was tested for DNA and for the activity of two enzymes: succinate dehydrogenase, a Krebs cycle enzyme, and acid phosphatase, a hydrolytic enzyme found in lysosomes. The data are for practice. (a) Identify, with a reason, the fraction that contains most nuclei. [1] (b) Deduce which fraction contains most mitochondria. [1] (c) Suggest why some acid phosphatase activity is found in pellet 2. [1] (d) Explain why the homogenate was kept ice-cold, isotonic and buffered. [3]
| Fraction | Centrifugation | DNA / % | Succinate dehydrogenase activity / % | Acid phosphatase activity / % |
|---|---|---|---|---|
| Pellet 1 | 1 000 × g for 10 min | 95 | 4 | 3 |
| Pellet 2 | 10 000 × g for 20 min | 2 | 88 | 30 |
| Pellet 3 | 20 000 × g for 30 min | 1 | 6 | 55 |
| Final supernatant | None | 2 | 2 | 12 |
Show mark scheme for question 2
- (a) pellet 1, because it contains most of the DNA / 95 % of the DNA [1]
- (b) pellet 2, because it has the highest succinate dehydrogenase activity / 88 % [1]
- (c) some lysosomes are similar in size / density to mitochondria, so they sediment with them OWTTE [1]
- (d) cold reduces enzyme activity / stops hydrolytic enzymes digesting the organelles [1]
- (d) isotonic solution prevents water moving into or out of organelles by osmosis / stops organelles bursting or shrinking [1]
- (d) buffer keeps pH constant so proteins / enzymes are not denatured [1]
Question 3
Paper 2A style
Outline the advantage of separating the nucleus and cytoplasm into separate compartments.
Show mark scheme for question 3
- transcription (in the nucleus) is separated from translation (in the cytoplasm) [1]
- mRNA can be modified after transcription before it meets ribosomes [1]
- for example, introns removed [1]
- in prokaryotes mRNA may meet ribosomes immediately, so this is not possible [1]
- max 2
Question 4
Paper 2A style
Explain, using lysosomes and phagocytic vacuoles as examples, the advantages of compartmentalization in the cytoplasm of cells.
Show mark scheme for question 4
- enzymes and substrates / metabolites can be concentrated, so reactions are faster [1]
- incompatible processes are kept separate [1]
- lysosomes contain hydrolytic / digestive enzymes that would digest cell components if free in the cytoplasm [1]
- lysosome contents can be kept at acid pH, the optimum for these enzymes [1]
- phagocytic vacuole forms by endocytosis of a pathogen; lysosomes fuse with it so digestion happens inside a compartment [1]
- max 3