#27 Summary of Cell membrane

1 The basic structure of a membrane is a 7 nm thick phospholipid bilayer with protein molecules
spanning the bilayer or within one or other layer. Phospholipids and some proteins move within the
layers. Hence the structure is described as a fluid mosaic – the scattered protein molecules resemble pieces of a mosaic.







2 Phospholipid bilayers are a barrier to most watersoluble substances because the interior of the
membrane is hydrophobic.

3 Cholesterol is needed for membrane fluidity and stability.

4 Some proteins are transport proteins, transporting molecules or ions across the membrane. They may be either channel proteins or carrier proteins. Channel proteins have a fixed shape; carrier proteins change shape.

5 Some proteins act as enzymes – for example, in the cell surface membranes of microvilli in the gut.

6 Glycolipids and glycoproteins form receptors – for example, for hormones or neurotransmitters.
They also form antigens, which are cell recognition markers.

7 The cell surface membrane controls exchange between the cell and its environment.

8 Some chemical reactions take place on membranes inside cell organelles, as in photosynthesis and
respiration.

9 Diffusion is the net movement of molecules or ions from a region of their higher concentration to one
of lower concentration. Oxygen, carbon dioxide and water cross membranes by diff usion through the
phospholipid bilayer.

10 Diffusion of ions and larger polar molecules through membranes is allowed by transport proteins. This process is called facilitated diff usion.

11 Water moves from regions of higher water potential to regions of lower water potential. When this takes place through a partially permeable membrane such as the cell surface membrane, this diff usion is called osmosis.

12 Pure water has a water potential (ψ) of zero. Adding solute reduces the water potential by an
amount known as the solute potential (ψs), which has a negative value. Adding pressure to a solution
increases the water potential by an amount known as the pressure potential (ψp), which has a positive
value.

The following equation is used:       ψ = ψp + pψs
                   Water Potential (ψ) = Pressure Potential (ψp) + Solute Potential (ψs)

13 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 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.

14 Some ions and molecules move across membranes by active transport, against the concentration gradient. This needs a carrier protein and ATP to provide energy.

15 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).


Multiple - choice Test 

1 The diagram shows a small part of a cell surface membrane.

Which regions are hydrophobic?

A 1 and 2 only
B 1 and 4 only
C 2 and 3 only
D 3 and 4 only

2 The diagram shows part of a cell surface membrane. In addition to the molecules shown, cholesterol is also present.

In which region is cholesterol found?

3 Which component of a cell surface membrane allows the movement of ions across the membrane?

A cholesterol
B fatty acids
C phospholipid
D protein

4 The molecules which make up biological membranes have different functions.

Which row matches molecules with their functions?

5 A substance moves into a cell through a protein pore. Which of the following describes this movement?

A an ion moving against its concentration gradient by active  transport
B glucose moving down its concentration gradient by diffusion
C oxygen moving against its concentration gradient by facilitated diffusion
D carbon dioxide moving against its concentration gradient by osmosis

6 Which of the following correctly describes the movement of water by osmosis?

A from a region of higher water potential to a region of lower water potential through a fully permeable membrane
B from a region of higher water potential to a region of lower water potential through a partially permeable membrane
C from a region of lower water potential to a region of higher water potential through a fully permeable membrane
D from a region of lower water potential to a region of higher water potential through a partially permeable membrane

7 Which of the processes that transport materials across a cell surface membrane require energy from the cell’s supply of ATP?

8 A protein is synthesised by a ribosome in an animal cell. What is the sequence of events leading to the secretion of the protein by exocytosis?

1 protein modified by the Golgi apparatus
2 protein enters the endoplasmic reticulum
3 secretory vesicles fuse with the cell surface membrane
4 secretory vesicles formed by the Golgi apparatus
5 vesicles bud off from the endoplasmic reticulum

A 1 → 2 → 5 → 4 → 3
B 2 → 5 → 1 → 4 → 3
C 3 → 4 → 1 → 5 → 2
D 5 → 4 → 3 → 2 → 1

9 Examples of the bulk transport of materials into or out of cells are listed.

1 white blood cell engulfing a bacterium
2 plant cell building up its cell wall
3 pancreas cell releasing digestive enzymes
4 ovum taking up liquid nutrients from a follicle cell

Which are examples of exocytosis?

A 1 and 2 only
B 1 and 4 only
C 2 and 3 only
D 3 and 4 only

10 A small piece of plant tissue was placed into each of three solutions, P, Q and R. After one hour, the tissues were examined using a light microscope and the appearance of the cells was recorded.

What conclusion can be made about the water potentials of the three solutions?

A The water potential of P is greater than that of R.
B The water potential of Q is greater than that of R.
C The water potential of R is greater than that of P.
D The water potential of Q is greater than that of P.

Answers for Multiple - choice Test 

1  D
2  C
3  D
4  C
5 A
6 B
7 A
8 B
9 C
10 A

End-of-chapter questions

1 Whatare the most abundant  molecules in the cell surface membranes of plant cells?
A cholesterol
B glycolipids
C phospholipids
D proteins

2 Whereare the carbohydrate portions of glycolipids and glycoproteins located in cell surface  membranes?
A the inside and outside surfaces of the membrane
B theinside surface  of the  membrane
C the interior of the membrane
D the outside surface of the membrane

3 The cells of the myelin sheath are wrapped in layers around nerve cell axons. Freeze-fractured     preparations of themyelin sheath cell surface membranes show very few particles. This indicates   that myelin membranes contain relativelfyew of which type of molecule?
A cholesterol
B glycolipids
C polysaccharides
D proteins

4 Prepare a table to summarise briefly the major functions of phospholipids, cholesterol,    glycolipids, glycoproteins and proteins in cell surface membranes.

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

b  Describe fully what will occur if a plant cell is placedin 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

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, Band C.
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.
c  Explain how ions would move through the channel protein.
d State two features that the channel proteins and carrier proteins of membranes have in common. e  State one structural difference between channel and carrier proteins.
f  Calculate the magnification of the drawing. Show  your  working.

7  Copy the table below and place a tick or cross in each box as appropriate.

8  Copy and complete the table below to compare cell walls with cell membranes.

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]

10 The rate of movement of molecules or ions across a cell surface membrane is affected by the  relative concentrations of themolecules or ions on either side of the membrane. The graphs below  show the  effect of concentration difference (the steepness of the concentration gradient) on three  transport  processes, namely diffusion, facilitated diffusion and activetransport.

a With reference to the graphs, state what the three transport processes have in common.[1]
b Explain the rates of transport  observed when the concentration difference is zero. [3]
c  i  Which one of the processes would stop if a respiratory inhibitor were added?[1]
    ii Explain your answer.[2]
d  Explain the difference between the graphs for diffusion and facilitated diffusion.[5]

 [Total: 12]

11 When a cell gains or loses water, its volume changes. The graphs overleaf show changes in the  water potential (Ψ), pressure potential (Ψ)  and  solute potential (Ψ) 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]

b  What is the pressure potential at 90%, 95% and 100% relative cell volume?     [1]

ii Calculate the change in pressure potential between 90% and 95% relative cell volume and between 95% and 100% relative cell volume,[2]

iii Explain  why the pressure potential curve is not linear.[2] 

iv State the water potential when the cell reaches maximum turgidity.[1]

The graph 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]

    ii In a shrinking cell, what is the relative cell volume when the minimum value of the pressure   potential is reached?                                                                              [1] 

   iii  What is the term used to describe the state of the cell at this  point?               [1]

   iv  What happens to the values of water potential  and solute potential at this point?   [1]

   v   State the equation which links Ψ p, Ψs and Ψ.[1]

  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]

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]

[Total: 20]

12 Thediagram shows the concentration in mmol dm^-3 of two different  ions inside a human red  blood cell and in the plasmaoutside the cell.

a  Explain why these concentrations could not have occurred as a result of diffusion.[1]

b  Explain how these concentrations could  have been achieved.[2]

c  If respiration of red blood cells is inhibited, the concentrations of potassium and sodium ions  inside the cells gradually change until they come into equilibrium with the plasma. Explain this  observation.[4]

[Total:  7]

End-of-chapter answers

1 C
2 D
3 D
4 Information for constructing this table can be found on pages 72–73.
5 a Information for answering this question can be found on page 77 and in the answer to SAQ 4.5.
  b Information for answering this question can be found on page 77 and in the answer to SAQ 4.5.
6 a A phosphate head (of phospholipid);
     B fatty acid tail(s) (of phospholipid);
     C phospholipid bilayer/membrane;
 b i hydrophilic
    ii hydrophobic
    iii hydrophobic
    iv hydrophilic
 c ions move by diff usion;
   channel has shape which is specifi c for particular ion;
   channel is hydrophilic/water-fi lled/allows movement of polar substance;
   ions move down concentration gradient;
d both intrinsic proteins;
   both have specifi c shape;
e channel proteins have a fi xed shape/carrier proteins have a variable shape;
f width of C measured in mm;
 mm converted to μm and μm converted to nm;
 correct formula used magnifi cation: M = I/A = width of C ÷ 7; accept mm, μm or nm;
 correct answer in nm;

N.B. It could be argued that facilitated diff usion is controllable, because the number of channel proteins in the membrane can aff ect the rate.

Exam-style questions

9 Description rate of entry of water is rapid at fi rst 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, diff erence 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 fi lls 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; cell is turgid; [max. 5]

 [Total: 8]

10 a the greater the concentration diff erence, the greater the rate of transport; [1]
    b (net) diff usion and facilitated diff usion only occur if there is a concentration, diff erence/gradient, across the membrane
or
at equilibrium/if no concentration diff erence,
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 diff erence;
 because molecules/ions are being pumped; AW  [3]
 c i active transport; [1]
    ii active transport depends on a supply of ATP;
        provided by respiration; [2]

d graph for diff usion is linear/straight line (with no maximum rate);
 purely physical process/not dependent on transport proteins/channel or carrier proteins;
 graph for facilitated diff usion is a curve with a maximum rate; AW
 facilitated diff usion 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 eff ect 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;
N.B. Th is is similar to the eff ect of substrate concentration on rate of enzyme activity. [max. 5]

 [Total: 12]

11 a the living contents of a plant cell; [1]
     b i at 90% = 22 kPa (accept 21 or 23 kPa), at 95% = 100 kPa, at 100% = 350 kPa; [1]
       ii change 90–95 % = 78 kPa (accept 77 or 79 kPa); change 95–100% = 250 kPa; [2]
       iii 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 eff ect 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 350 kPa; [1]
 c i zero (kPa); [1]
    ii 86%; [1]
    iii incipient plasmolysis; [1]
    iv water potential = solute potential; [1]
    v  ψ = ψs + ψp ; [1]
   vi 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; [max. 5]
 d 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 signifi cantly change the concentration of the cell contents; AW [3]

 [Total: 20]

12 a if it were diff usion, 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 diff erent inside and outside
 b active transport;
 active transport involves pumping ions against a concentration gradient; [2]
 c if respiration is inhibited, no ATP is produced;
 active transport uses ATP as energy source;
 active transport stops;
 diff usion continues;
 ions move down concentration gradients by diff usion until equilibrium reached; [max. 4]

 [Total: 7]