IB Biology [SL] Paper 2 Questions and Answers

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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.