Cambridge International A Level Biology Revision Guide

Chapter 4: Cell membranes and transport
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In dilute solutions, animal cells burst as water moves
into the cytoplasm from the solution. In dilute
solutions, a plant cell does not burst, because the
cell wall provides resistance to prevent it expanding.
The pressure that builds up as water diffuses into a
plant cell by osmosis is the pressure potential. A plant
cell in this state is turgid. In concentrated solutions,
animal cells shrink, while in plant cells the protoplast
shrinks away from the cell wall in a process known as
plasmolysis.
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Some ions and molecules move across membranes
by active transport, against the concentration
gradient. This needs a carrier protein and ATP to
provide energy. Exocytosis and endocytosis involve
the formation of vacuoles to move larger quantities
of materials respectively out of, or into, cells by
bulk transport. There are two types of endocytosis,
namely phagocytosis (cell eating) and pinocytosis (cell
drinking).
End-of-chapter questions
1 What are the most abundant molecules in the cell surface membranes of plant cells?
A cholesterol
B glycolipids
C phospholipids
D proteins [1]
2 Where are the carbohydrate portions of glycolipids and glycoproteins located in cell surface membranes?
A the inside and outside surfaces of the membrane
B the inside surface of the membrane
C the interior of the membrane
D the outside surface of the membrane [1]
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3 The cells of the myelin sheath are wrapped in layers around nerve cell axons. Freeze-fractured preparations of
the myelin sheath cell surface membranes show very few particles. This indicates that myelin membranes contain
relatively few of which type of molecule?
A cholesterol
B glycolipids
C polysaccharides
D proteins [1]
4 Prepare a table to summarise briefly the major functions of phospholipids, cholesterol, glycolipids,
glycoproteins and proteins in cell surface membranes. [15]
5 a Describe fully what will occur if a plant cell is placed in a solution that has a higher water potential
than the cell. Use the following terms in your answer.
b
cell wall, freely permeable, partially permeable, cell surface membrane, vacuole, tonoplast, cytoplasm,
solute potential, pressure potential, water potential, turgid, osmosis, protoplast, equilibrium [14]
Describe fully what will occur if a plant cell is placed in a solution that has a lower water potential than
the cell. Use the following terms in your answer.
cell wall, freely permeable, partially permeable, cell surface membrane, vacuole, tonoplast, cytoplasm,
solute potential, pressure potential, water potential, incipient plasmolysis, plasmolysed, osmosis,
protoplast, equilibrium [15]
[Total: 29]