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Biology

IB Biology Paper 2 Previous Questions & Marksheme [SL]

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An example of the document: Topic 1: Cell Biology Describe the characteristics of stem cells that make them potentially useful in medicine. [5] (stem cells) have/retain the capacity to divide; can be used to produce cell cultures/large number of identical cells; can be used to repair/replace damaged/lost cells/tissue; (stem cells) are undifferentiated / have not yet differentiated/specialized; can differentiate/specialize in different ways / are pluripotent/totipotent; can be used to form a variety of different tissues / form organs; used in medical research; used in treatment of (named) disease; Draw a labelled diagram to show the structure of membranes. [5] phospholipid bilayer – with head and tails; hydrophilic/phosphate/polar heads and hydrophobic/hydrocarbon/fatty acid/ non-polar tails labelled; integral protein – embedded in hydrophobic region of the phospholipid bilayer; channel protein – integral protein showing clear channel/pore; peripheral protein – on the surface (not embedded in hydrophobic region) can be attached to integral protein; glycoprotein – with carbohydrate attached on outside; cholesterol – shown embedded in bilayer; Explain the importance of surface area to volume ratio as a factor limiting cell size. [7] as volume of a cell increases, the ratio of its surface area to volume decreases; food/oxygen enters through the surface of cells; wastes leave through the surface of cells; the rate of substance crossing the membrane depends on surface area; more metabolic activity in a larger cell means more food and oxygen required; large volume means longer diffusion time; (large volume) means more wastes produced; excess heat generated will not be lost efficiently (with low surface area to volume ratio); eventually surface area can no longer serve the requirements of the cell; this critical ratio stimulates mitosis; (thus) the size of the cell is reduced and kept within size limits; Draw a labelled diagram of a prokaryotic cell. [5] a. cell wall – uniformly thick and drawn outside the plasma membrane; b. plasma membrane – a continuous single line; c. cytoplasm/cytosol; d. nucleoid/(naked) DNA – shown as a tangle of thread or irregular shape without a nuclear membrane; e. (70S) ribosomes – drawn as a small circle or dark dot; f. pili – hair like structures / flagellum – shown to be longer than any pili; g. plasmid – circular ring of DNA; h. capsule – drawn outside the cell wall; Award [1] for each structure clearly drawn and labelled which conforms to the italicized guidelines given above.

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