Question 1
Paper 1A style
Which structure is considered an organelle?
- Cell wall
- Cytoskeleton
- Ribosome
- Cytoplasm
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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 |
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- (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.
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- 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.
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- 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
Question 5
Paper 1A style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanWhich feature of the mitochondrion allows a high concentration of protons to build up quickly during aerobic respiration?
- The large surface area of the cristae
- The small volume of the intermembrane space
- The enzymes of the Krebs cycle in the matrix
- The double membrane surrounding the mitochondrion
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Answer: B – protons are pumped into a small volume, so their concentration rises quickly [1]
Question 6
Paper 2A style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanExplain how the structure of the chloroplast is adapted for photosynthesis.
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- thylakoid membranes have a large surface area / are stacked in grana [1]
- for many photosystems / more light absorption [1]
- small volume of fluid inside the thylakoids [1]
- so a steep proton gradient builds quickly for ATP production [1]
- stroma compartmentalizes the enzymes and substrates of the Calvin cycle [1]
- max 3
Question 7
Paper 2A style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanOutline why the nuclear membrane needs pores, and why it breaks into vesicles during mitosis.
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- pores allow mRNA to leave the nucleus [1]
- pores allow proteins (such as enzymes or histones) to enter from the cytoplasm [1]
- membrane breaks into vesicles so spindle microtubules can reach / attach to / move chromosomes [1]
- vesicles later fuse to re-form nuclear membranes around each set of chromosomes [1]
- Award [1] max for pores and [1] max for vesicles
- max 2
Question 8
Paper 2B style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanExplain how a protein that is secreted from a cell is synthesized, processed and released.
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- protein synthesized by ribosomes bound to the rough ER [1]
- free ribosomes make proteins retained in the cell, so secreted proteins are made on the rER [1]
- polypeptide enters the lumen of the rER [1]
- protein transported from the rER to the Golgi apparatus in vesicles [1]
- vesicles fuse with the cis face of the Golgi apparatus [1]
- protein processed / modified in the Golgi apparatus, for example carbohydrate added [1]
- secretory vesicles bud from the trans face of the Golgi apparatus [1]
- protein folds / is modified in the rER lumen, for example carbohydrate added [1]
- vesicles move to the plasma membrane [1]
- vesicle fuses with the plasma membrane and the protein is released by exocytosis [1]
- fusion and budding are possible because membranes are fluid [1]
- Do not credit clathrin as coating the vesicles that carry the protein from the rER to the Golgi apparatus
- max 7