4.13 End of Chapter Questions
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]
C;
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]
D;
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]
D;
4 Prepare a table to summarise briefly the major functions of phospholipids, cholesterol, glycolipids, glycoproteins and proteins in cell surface membranes. [15]
Suggested mark allocation:
phospholipids: [2]
cholesterol: [4]
glycolipids: [3]
glycoproteins: [3)
proteins: [3]
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.
cell wall, freely permeable, partially permeable, cell surface membrane, vacuole, tonoplast, cytoplasm, solute potential, pressure potential, water potential, turgid, osmosis, protoplast, equilibrium [14]
b 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]
Osmosis in plant cells
Unlike animal cells, plant cells are surrounded by cell walls, which are very strong and rigid. Imagine a plant cell being placed in pure water or a dilute solution. The water or solution has a higher water potential than the plant cell, and water therefore enters the
cell through its partially permeable cell surface membrane by osmosis. Just like in the animal cell, the volume of the cell increases, but in the plant cell the the cell wall pushes
back against the expanding protoplast (the living part of the cell inside the cell wall), and pressure starts to build up rapidly. This is the pressure potential, and it increases the water potential of the cell until the water potential inside the cell equals the water potential outside the cell, and equilibrium is reached. The cell wall is so inelastic that it takes very little water to enter the cell to achieve this. The cell wall prevents the cell from bursting, unlike the situation when an animal cell is placed in pure water or a dilute solution. When a plant cell is fully inflated with water it is described as fully turgid. For
plant cells, then, water potential is a combination of solute potential and pressure potential. This can be expressed in the following equation:
ψ = ψs + ψp
Figure 4.13c shows the situation where a plant cell is placed in a solution of lower water potential. An example of the latter would be a concentrated sucrose solution.
In such a solution, water will leave the cell by osmosis.
As it does so, the protoplast gradually shrinks until it is exerting no pressure at all on the cell wall. At this point the pressure potential is zero, so the water potential of the
cell is equal to its solute potential (see the equation above).
Both the solute molecules and the water molecules of the external solution can pass through the freely permeable cell wall, and so the external solution remains in contact
with the shrinking protoplast. As the protoplast continues to shrink, it begins to pull away from the cell wall. This process is called plasmolysis, and a cell in which it has happened is said to be plasmolysed. The point at which pressure potential has just reached zero and plasmolysis is about to occur is referred to as incipient plasmolysis. Eventually, as with
the animal cell, an equilibrium is reached when the water potential of the cell has decreased until it equals that of the external solution.
[Total: 29]
6 The diagram shows part of a membrane containing a channel protein. Part of the protein molecule is shaded.
a Identify the parts labelled A, B and C. [3]
A phosphate head (of phospholipid);
B fatty acid tail(s) (of phospholipid);
C phospholipid bilayer / membrane;
b For each of the following, state whether the component is hydrophilic or hydrophobic:
i A ii B iii darkly shaded part of protein iv lightly shaded part of protein. [2]
Award max. of 2 marks: 2 or 3 correct answers 1 mark, 4 correct answers 2 marks
i hydrophilic
ii hydrophobic
iii hydrophobic
iv hydrophilic;
c Explain how ions would move through the channel protein. [3]
ions move by diffusion;
channel has shape which is specific for particular ion;
channel is hydrophilic / water-filled / allowsmovement of polar substance;
ions move down concentration gradient;
d State two features that the channel proteins and carrier proteins of membranes have in common. [2]
both intrinsic proteins;
both have specific shape;
e State one structural diff erence between channel and carrier proteins. [1]
channel proteins have a fixed shape / carrier proteins have a variable shape;
f Calculate the magnification of the drawing. Show your working. [4]
width of C measured in mm;
mm converted to µm and µm converted to nm;
correct formula used magnification: M = I/A = width of C/7 accept mm, µm or nm;
correct answer in nm;
[Total: 15]
7 Copy the table below and place a tick or cross in each box as appropriate.
| Process | Uses energy in the form of ATP | Uses proteins | Specific | Controllable by cell |
| diffusion | ||||
| osmosis | ||||
| facilitated diffusion | ||||
| active transport | ||||
| endocytosis and exocytosis |
[20]
| Process | Uses energy | Uses proteins | Specific | Controllable by cell |
| diffusion | ✘ | ✘ | ✘ | ✘ |
| osmosis | ✘ | ✘ | ✔ | ✘ |
| facilitated diffusion | ✘ | ✔ | ✔ | ✘ |
| active transport | ✔ | ✔ | ✔ | ✔ |
| endocytosis and exocytosis | ✔ | ✘ | ✔ | ✔ |
NB: It could be argued that facilitated diffusion is controllable, because the number of channel proteins in the membrane can affect the rate.
8 Copy and complete the table below to compare cell walls with cell membranes.
| Feature | Cell wall | Cell membrane |
| is the thickness normally measured in nm or μm? | ||
| location | ||
| chemical composition | ||
| permeability | ||
| function | ||
| fluid or rigid |
[6]
Award 1 mark for each correct row
| Feature | Cell wall | Cell membrane |
| is the thickness normally measured in nm or µm? | µm | nm |
| location | surround some cells / not animal cells / only outside / surrounding cells | surround all cells / may be found inside cells |
| chemical composition accept any statements that serve to distinguish between cell wall and cell membranes. Examples are given. | contains cellulose in plants, peptidoglycans / murein in prokaryotes, (chitin in fungi) / contains a strengthening material / contains a polysaccharide (or polysaccharide like substance) AW | phospholipids, protein, (sometimes) cholesterol |
| permeability | freely permeable | partially permeable |
| function | mechanical strength | selective barrier AW |
| fluid or rigid | rigid | fluid |
9 A cell with a water potential of –300 kPa was placed in pure water at time zero. The rate of entry of water into the cell was measured as the change in water potential with time. The graph shows the results of this investigation.
Describe and explain the results obtained. [8]
description:
rate of entry of water is rapid at first but slows down gradually;
until rate is zero / no further entry of water or water enters until water potential of cell = water potential of pure water = 0 (= equilibrium);
exponential / not linear;
rate depends on / proportional to, difference in water potential between cell and external
solution; [max. 3]
explanation:
water (always) moves from a region of higher water potential to a region of lower water
potential;
(in this case) by osmosis;
through partially permeable cell surface membrane of cell;
as cell fills with water, cell / protoplast expands and pressure (potential) increases;
until water potential of cell = zero / water potential of pure water;
cell wall rigid / will not stretch (far), and prevents entry of more water; [max. 5]
cell is turgid;
10 The rate of movement of molecules or ions across a cell surface membrane is aff ected by the relative concentrations of the molecules or ions on either side of the membrane. The graphs below show the eff ect of concentration diff erence (the steepness of the concentration gradient) on three transport processes, namely diffusion, facilitated diff usion and active transport.
a With reference to the graphs, state what the three transport processes have in common. [1]
the greater the concentration difference, the greater the rate of transport;
b Explain the rates of transport observed when the concentration diff erence is zero. [3]
(net) diffusion and facilitated diffusion only occur if there is a concentration, difference / gradient, across the membrane
or
at equilibrium / if no concentration difference, there is no, net exchange / transport across membrane / rate of transport, is same in both directions; AW
active transport can occur even if no concentration difference;
because molecules / ions are being pumped; AW
c i Which one of the processes would stop if a respiratory inhibitor were added? [1]
active transport;
ii Explain your answer. [2]
active transport depends on a supply of ATP;
provided by respiration;
d Explain the difference between the graphs for diff usion and facilitated diffusion. [5]
graph for diffusion is linear / straight line (with no maximum rate);
purely physical process / not dependent on transport proteins / channel or carrier proteins;
graph for facilitated diffusion is a curve with a maximum rate; AW
facilitated diffusion depends on presence of, transport proteins / channel or carrier proteins;
as concentration increases, the receptor sites of these proteins become more and more saturated / the more saturated these become, the less the effect of increasing concentration;
rate reaches a maximum when all, transport / channel or carrier proteins, are working at full capacity / when all receptor sites are, full / saturated;
NB: This is similar to the effect of substrate concentration on rate of enzyme activity.
[Total: 12]
11 When a cell gains or loses water, its volume changes. The graphs show changes in the water potential (ψ),
pressure potential (ψp) and solute potential (ψs) of a plant cell as its volume changes as a result of gaining
or losing water. (Note that 80% relative cell volume means the cell or protoplast has shrunk to 80% of the
volume it was at 100% relative cell volume.)
a What is a protoplast? [1]
the living contents of a plant cell;
b i What is the pressure potential at 90%, 95% and 100% relative cell volume? [1]
at 90% = 22 kPa (accept 21 or 23 kPa), at 95% = 100 kPa, at 100% = 350 kPa;
ii Calculate the change in pressure potential between 90% and 95% relative cell volume and between 95%
and 100% relative cell volume. [2]
change 90–95 % = 78 kPa (accept 77 or 79 kPa);
change 95–100% = 250 kPa;
iii Explain why the pressure potential curve is not linear. [2]
as water enters the cell, the cell wall is stretched / protoplast pushes against cell wall;
cell wall is (relatively) rigid;
water cannot be compressed;
therefore pressure builds up more and more rapidly (for given volume of water) / small increase in amount of water has large effect on pressure; AW [max. 2]
(This could be compared with pumping up a bicycle tyre – pressure increases much more rapidly for a given amount of air towards the end due to the elastic limit of the tyre being reached.)
iv State the water potential when the cell reaches maximum turgidity. [1]
350 kPa;
The graph above shows that as the cell loses water, pressure potential falls and the relative cell volume
decreases (the cell shrinks).
c i What is the minimum value of the pressure potential? [1]
zero (kPa);
ii In a shrinking cell, what is the relative cell volume when the minimum value of the pressure potential is reached? [1]
86%;
iii What is the term used to describe the state of the cell at this point? [1]
incipient plasmolysis;
iv What happens to the values of water potential and solute potential at this point? [1]
water potential = solute potential;
v State the equation which links ψp, ψs and ψ. [1]
ψ = ψs + ψp;
vi Describe what is happening to the cell between the point identified in c ii and c iii above and 80% relative cell volume. [5]
the cell continues to lose water / protoplast continues to shrink;
protoplast pulls away from cell wall = plasmolysis;
shrinks until equilibrium is reached;
when water potential of cell = water potential of outside solution;
solute potential gets lower / more negative;
because cell contents becoming more concentrated;
d As the cell changes volume, the change in solute potential is much less than the change in pressure potential. Suggest an explanation for this. [3]
only a small amount of water is needed to bring about a large change in pressure;
because the cell wall is (relatively) rigid;
this is not enough to significantly change the concentration of the cell contents; AW
[Total: 20]
12 The diagram shows the concentration in mmol dm–3 of two diff erent ions inside a human red blood cell and in the plasma outside the cell.
a Explain why these concentrations could not have occurred as a result of diff usion. [1]
if it were diffusion, there would be (net) movement of ions from a region of higher concentration to a region of lower concentration until equilibrium is reached when concentration inside = concentration outside; AW [1]
R because concentrations different inside and outside
b Explain how these concentrations could have been achieved. [2]
active transport;
active transport involves pumping ions against a concentration gradient;
c If respiration of red blood cells is inhibited, the concentrations of potassium ions and sodium ions inside the cells gradually change until they come into equilibrium with the plasma. Explain this observation. [4]
if respiration is inhibited, no ATP is produced;
active transport uses ATP as energy source;
active transport stops;
diffusion continues;
ions move down concentration gradients by diffusion until equilibrium reached;
[Total: 7]