Biology / Cell membranes & Transport

Cell membranes & Transport 

Biology

The cytoplasm is surrounded externally by a cell surface membrane, also called the plasma membrane. It is a partilly permeable membrane which controls subtances entering or leaving the cell. Similar membranes may also surround large spaces or vacuoles within the cell. 

In a plant, there is a cell wall which encloses the whole cell. This cell wall is made of cellulose. It protects the cell from injury. 

Learning Outcomes:
You should be able to:

• a. Describe the structure of cell membranes
• b. Explain the functions of the molecules that make up cell membranes
• c. Explain how substances enter and leave cells
• d. Find the water potential of plant tissues through experiment

Q.1. What is phospholipids?

Ans: A substance whose molecules are made up of a glycerol molecule, two fatty acids and a phosphate group; a bilayer of phospholipids forms the basic structure of all cell membranes.

Fluid mosaic model:

In 1972, two scientists, Singer and Nicolson, used all the available evidence to put forward a hypothesis for membrane structure. They called their model the fluid mosaic model. It is describes as 'fluid' because both the phospholipids and the proteins can move about by diffusion. 

The fluid mosaic model describes the structure of the plasma membrane as a mosaic of components —including phospholipids, cholesterol, proteins, and carbohydrates—that gives the membrane a fluid character. 

The membrane is a double layer (bilayer) of phospholipid molecules. The individual phospholipid molecules move about by diffusion within their own monolayers. 

The phospholipid tails point inwards, facing each other and forming a non-polar hydrophobic interior. The phospholipid heads face the aqueous (water-containing) medium that surrounds the membranes.

Some of the phospholipid tails are saturated and some are unsaturated. The more unsaturated They are, the more fluid the membrane. This is because the unsaturated fatty acid tails are bent and therefore fit together more loosely. Fluidity is also affected by tail length: the longer the tail, the less fluid the membrane. As temperature decreases, membranes become less fluid, but some organisms which cannot regulate their own temperatures, such as bacteria and yeasts, respond by increasing the proportion of unsaturated fatty acids in their membranes. 

Intrinsic proteins may be found in the inner layer, the outer layer or, most commonly, spanning the whole membrane, in which case they are known as transmembrane proteins. In transmembrane proteins, the hydrophobic regions which cross the membrane are often made up of one or more ∝-helical chains.

Intrinsic proteins have hydrophobic and hydrophilic regions. They stay in the membrane because the hydrophobic regions, made from hydrophobic amino acids, are next to the hydrophobic fatty tails and are repelled by the watery environment on either side of the membrane. The hydrophilic regions, made from hydrophilic amino acids, are repelled by the hydrophobic interior of the membrane and therefore face into the aqueous environment inside or outside the cell, or line hydrophilic pores which pass through the membrane.

Most of the intrinsic protein molecules float like mobile icebergs in the phospholipid layers, although some are fixed like islands to structures inside or outside the cell and do not move about.

The second type of protein molecule is extrinsic protein. These are found on the inner or outer surface of the membrane. Many are bound to intrinsic proteins. Some are held in other ways- for example, by binding to molecules inside or outside the cell, or to the phospholipids.

Many proteins and lipids have short, branching carbohydrate chains attached to that side of the molecule which faces the outside of the membrane, thus forming glycoproteins and glycolipids, respectively.

The total thickness of the membrane is about 7 nm on average. Molecules of cholesterol are also found in the membrane.

The currently accepted basic model of membrane structure, proposed by Singer and Nicolson in 1972, in which protein molecules are free to move about in a fluid bilayer of phospholipid molecules.

Cell surface membrane:

            Function: 

A very thin membrane surrounding all cells, it is partially permeable and controls the exchange of materials between the cell and its environment. Some chemical reactions take place on membranes inside cell organelles, as in photosynthesis and respiration.

            Structure:

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. Phospholipid bilayers are a barrier to most water-soluble substances becasue the interior of the membrane is hydrophobic. Cholesterol is needed for membrane fluidity and stability.

 

Cholesterol:

A small, lipid-related molecule with a hydrophilic head and a hydrophobic tail which is an essential constituent of membranes, particularly in animal cells, conferring fluidity, flexibility and stability to the membrane.

Cholesterol is a relatively small molecule. Like phospholipids, cholesterol molecules have hydrophilic heads and hydrophobic tails, so they fit neatly between the phospholipid molecules with their heads at the membrane surface. Cell surface membranes in animal cells contain almost as much cholesterol as phospholipid. Cholesterol is much less common in plant cell membranes and absent from prokaryotes. In these organisms, compounds very similar to cholesterol serve the same function.

At low temperatures, cholesterol increases the fluidity of the membrane, preventing it from becoming too rigid. This is because it prevents close packing of the phospholipid tails. The increased fluidity means cells can survive colder temperatures. The interaction of the phospholipid tials with the cholesterol molecules also helps to stabilise cells at higher temperatures when the membrane could otherwise become too fluid. Cholesterol is also important for the mechanical stability of membranes, as without it membranes quickly break and molecules help to prevent ions or polar molecules from passing through the membrane. This is particularly important in the myelin sheath (made up of many layers of cell surface membrane) around nerve cells, where leakage of ions would slow down nerve impulses.

  

Antigen: 

A substance that is foreign to the body and stimulates an immune response.

Cell signalling: 

The molecular mechanisms by which cells detect and respond to external stimuli, including communication between cells. 

Cell signalling is an important, rapidly expanding area of research in modern biology, with wide applications. It is important because it helps to explain how living organisms control and coordinate their bodies. Basically, signalling is getting a message from one place to another.

All cells and organisms must be able to respond appropriately to their environments.  This is made possible by means of a complex range of signalling pathways that coordinate the activities of cells, even if they are large distances apart in the same body. The basic idea of a signalling pathway can be summarised in a simple diagram. 

A signalling pathway includes receiving a stimulus or signal, transmitting the message and making an appropriate response. Conversion of the original signal to a message that is then transmitted is called transduction. Transmitting the message involves crossing barriers such as cell surface membranes. Signalling molecules are usually very small for easy transport.

Distances travelled may be short, as with diffusion within one cell, or long, as with long-distance transport in blood or phloem. There are usually many components and different mechanisms along the route. Signalling includes both electrical and chemical events and their interactions with each other - for example, the events associated with the nervous and hormonal systems in animals. These events involve a wide range of molecules produced by cells within the body (e.g. hormones and neurotransmitters) as well as outside stimuli (e.g. light, drugs, pheromones and odours).

The cell surface membrane is a critical component of most signalling pathways because it is a barrier to the movement of molecules, controlling what moves between the external and internal environments of the cell. In a typical signalling pathway, molecules must cross or interact with cell surface membranes.

Signalling molecules are very diverse. If they are hydrophobic, such as the steroid hormones (e.g. oestrogen), they can diffuse directly across the cell surface membrane and bind to receptors in the cytoplasm or nucleus. 

The receptor is a specific shape that recognises the signal. The signal brings about a change in the shape of the receptor, and since this spans the membrane, the message is in effect passed to the inside of the cells. Changing the shape of the receptor allows it to interact with the next component of the pathway, so the message gets transmitted.

This next component is often a 'G protein', which acts as a switch to bring about the release of a 'second messenger, a small molecule which diffuses through the cell relaying the message. 

Many second messenger molecules can be made in response to one receptor molecule being stimulated. This represents an amplification of the original signal, a key feature of signalling. 

There are three other ways in which a receptor can alter the activity of a cell:

• 1. opening an ion channel, resulting in a change of membrane potential.
• 2. acting directly as a membrane-bound enzyme.
• 3. acting as an intracellular receptor when the initial signal passes straight through the cell surface membrane. For example, the oestrogen receptor is in the nucleus and directly controls gene expression when combined with oestrogen.

  

Channel protein: 

A membrane protein of fixed shape which has a water-filled pore through which selected hydrophilic ions or molecules can pass see facilitated diffusion.

 

Sodium-potassium pump:

A membrane protein that moves sodium ions out of a cell and potassium ions into it, using ATP.

The Sodium-potassium pump system moves sodium and potassium against large concentration gradients. It moves 2 potassium gradients. It moves 2 potassium into cells where potassium levels are high, pumps 3 sodium ions out of the cell and into the extracellular cell. It pumps the 3 sodium ions out of the cell.

 

 

 

Sodium-potassium (Na+ - K+) pumps are found in the cell surface membranes of all animal cells. In most cells, they run all the time, and it is estimated that on average they use 30% of a cell's energy (70% in the nerve cells).

The role of the Na+ - K+ pump is to pump three sodium ions out of the cell at the same time as allowing two potassium ions into at the same time as allowing two potassium ions into the cell for each ATP molecule used. 

The ions are both positively charged, so the net result is that the inside of the cell becomes more negative than the outside - a potential difference is created across the membrane. 

Diffusion: 

The net movement of molecules or ions from a region of higher concentration to a region of lower concentration down a gradient, as a result of the random movements of particles. Oxygen, carbon dioxide and water cross membranes by diffusion through the phospholipid bilayer. Diffusion of ions and larger polar molecules through membranes is allowed by transport proteins. This process is called facilited diffusion. 

Osmosis: 

The net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane, as a result of their random motion. Water moves from regions of higher water potential to regions of lower water potential. When water moves from regions of higher water potential to regions of lower water potential through a partially permeable membrane, such as the cell surface membrane, this diffusion is called osmosis.

Active transport:

The movement of molecules or ions through transport proteins across a cell membrane, against their concentration gradient, using energy from ATP.

Active transport is the process in which energy is used to move the particles of a substance against a concentration gradient from a region when they are in lower concentration to a region where they are in higher concentration. 

Active transport is involved in a number of processes occurring within an organism. This includes the absorption of 

* dissolved mineral salts by the root hairs

* glucose and amino acids by cells in the small intestine of humans. 

Q. Why active transport is important?

Ans: Active transport is important in reabsorption in the kidneys, where certain useful molecules and ions have to be reabsorbed into the blood after filtration into the kidney tubules. It is also involved in the absorption of some products of digestion from the gut. In plants, active transport is used to load sugar from the photosynthesising cells of leaves into the phloem tissue for transport around the plant, and to load inorganic ions from the soil into root hairs.

ATP: 

Adenosine triphosphate - the universal energy currency of cells.

 

Expansins: 

Proteins in the cell walls of plants that loosen the attachment of microfibrils of cellulose during elongation growth.

 

Water potential:

A measure of the tendency of water to move from one place to another; water moves from a solution with higher water potential to one with lower water potential; Water potential is decreased by the addition of solute and increased by the application of pressure; symbol is Ψ or Ψw.

Endocytosis: 

The bulk movement of liquids or solids (phagocytosis) into cell, by the infolding of the cell surface membrane to form vesicles containing the substance; endocytosis is an active process requiring ATP. 

Bulk Transport:

Endocytosis involves the engulfing of the material by the cell surface membrane to form a small sac, or 'endocytic vacuole'. It takes two forms.

1. Phagocytosis or 'cell eating'- this is the bulk uptake of solid material. Cells specialising in this are called phagocytosis and the vacuoles phagocytic vacuoles. An example is the engulfing of bacteria by certain white blood cells.

2. Pinocytosis or cell drinking' - this is the bulk uptake of liquid. The vacuoles formed are often extremely small, in which case the process is called micropinocytosis.

Exocytosis is the reverse of endocytosis and is the process by which materials are removed from cells. It happens, for example, in the secretion of digestive enzymes from cells of the pancreas. Secretory vesicles from the Golgi body carry the enzymes to the cell surface and release their contents. Plant cells use exocytosis to get their cell wall building materials to the outside of the cell surface membrane.

If the product being secreted is a protein, the Golgi body is often involved in chemically modifying the protein before it is secreted, as in the secretion of digestive enzymes by the pancreas.

 

Q.1. Why does the cell surface membrane not provide a barrier to the entry of hydrophobic molecules into the cell?

Ans: The plasma membrane is selectively permeable; hydrophobic molecules and small polar molecules can diffuse through the lipid layer, but ions and large polar molecules cannot. 

Q.2. Suggest three reasons why exchange between the cell and its environment is essential.

 Ans: A very thin membrane surrounding all cells, it is partially permeable and controls the exchange of materials between the cell and its environment. 

The movement of molecules or ions through transport proteins across a cell membrane, against their concentration gradient, using energy from ATP.

 The net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane, as a result of their random motion.

 

Q.3. The diagram shows three cubes.

                          📦    📦      📦

calculate the surface area, volume and surface area: volume ratio of each of the cubes.

            The surface area: volume ratio decreases as the size of any three-dimensional object                                                        increases.

Q.4. The fact that surface area: 

volume ratio decreases with increasing size is also true for whole organisms. Explain the relevance of this for transport systems within organisms.

Ans: Large molecules require more energy to get them moving than small ones do, so large molecules tend to diffuse more slowly than small molecules. Non-polar molecules, such as glycerol, alcohol and steroid hormones, diffuse much more easily through cell membranes than polar ones, because they are soluble in the non-polar phospholipid tails.  

Q.5. In figure 4.10b, imagine that the solutions in A and B are in equilibrium - that is, there is no net movement of water molecules. What can you say about the water potentials of the two solutions?

More water molecules moved from A to B than from B to A, so the net movement has been from A to B, raising the level of solution in B and lowering it in A. This means that A will end up with fewer water molecules so that the solution becomes more concentrated with solute. B will end up with more water molecules so that it becomes more dilute. We will also find that the volume of liquid in B will increase, because it now contains the same number of solute molecules, but more water molecules.

Q.6. In figure 4.12:

a. Which solution has the highest water potential?

b. Which solution has the lowest solute potential?

c. in which solution is the water potential of the red cell the same as that of the solution?

 

a) Figure a shows that if the water potential of the solution surrounding the cell is too high, the cell swells and bursts.

b) If it is too low, the cell shrinks (Figure c). This shows one reason why it is important to maintain a constant water potential inside the bodies of animals.

c) Figure b solution is the water potential of the red cell the same so that of the solution.

Q.7. Figures 4.14 and 4.15 shows a phenomenon called plasmolysis. Why can plasmolysis not take place in an animal cell?

Ans: 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 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 plasmolysed 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.

Q.8. Two neighbouring plant cells are shown in figure 4.16.

a. In which direction would there be net movement of water molecules?

b. Explain what is meant by net movement.

c. Explain your answer to a.

d. Explain what would happen if both cells were placed in

i. pure water

ii. a 1 mol dm ⁻ ³ sucrose solution with a water potential of -3510kPa.

 

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

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 hte 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 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

 

 

You can download:

https://www.cambridgeinternational.org/images/255120-cambridge-international-as-and-a-level-factsheet-english.pdf 

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

            Ans:  Phospholipids provide barriers in cellular membranes to protect the cell, and they make barriers for the organelles within those cells. Phospholipids work to provide pathways for various substances across membranes.

Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell membranes.

Glycolipids are found on the surface of all eukaryotic cell membranes, where they extend from the phospholipid bilayer into the extracellular environment. 

 Glycoproteins function in the structure, reproduction, immune system, hormones, and protection of cells and organisms. Glycoproteins are found on the surface of the lipid bilayer of cell membranes.

Proteins also play important roles in the membranes of organelles. For example, in the membranes of mitochondria and chloroplasts, they are involved in the processes of respiration and photosynthesis.

  

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, water potential, turgid, osmosis, protoplast, equilibrium.

 

b. Describe fully what will occur if a plant cells 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, 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, B, and C.

            Ans: a. 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.

 b i hydrophilic

    ii hydrophobic

    iii hydrophobic 

    iv hydrophilic 

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.

 c ions move by diffusion;

   channel has shape which is specific for particular ion;

   channel is hydrophilic/water-filled/allows movement of polar substance;

   ions move down concentration gradient

d both intrinsic proteins;

   both have specific 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 magnification: M = I/A = width of C ÷ 7; accept mm, μm or nm;

 correct answer in nm; 

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

 

       Ans:

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.

 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; 

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; cell is turgid;

 

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

 

a. With reference to the graphs, state what the three transport processes have in common.

b. Explain the rates of transport observed when the concentration difference is zero.

c. i. Which one of the processes would stop if a respiratory inhibitor were added?

ii. Explain your answer.

d. Explain the difference between the graphs for diffusion and facilitated diffusion.

Ans:  a) the greater the concentration difference, the greater the rate of transport; 

    b) (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. active transport; 

    ii. active transport depends on a supply of ATP;

        provided by respiration; 

d) 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;

N.B. This is similar to the effect of substrate concentration on rate of enzyme activity.

Q.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. 

a. What is a protoplast?

b. i. What is the pressure potential at 90%, 95% and 100% relative cell volume?

ii. Calculate the change in pressure potential between 90% and 95% relative cell volume and between 95% and 100% relative cell volume.

iii. Explain why the pressure potential curve is not linear.

iv. State the water potential when the cell reaches maximum turgidity.

The graph above shows that as the cell loses water, pressure potential falls and the relative cell volume decreases.

C. i. What is the minimum value of the pressure potential?

ii. in a shrinking cell, what is the relative cell volume when the minimum value of the pressure potential is reached?

iii. What is the term used to describe the state of the cell at this point?

iv. What happens to the values of water potential and solute potential at this point?

v. State the equation which links Ψp, Ψs and Ψ.

vi. Describe what is happening to the cell between the point identified in c ii and c iii above and 80% relative cell volume.

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.

                Ans: Protoplast: The living contents of a plant cell, including the cell surface                                         membrane, but excluding the cell wall.

     b. i. at 90% = 22 kPa (accept 21 or 23 kPa), at 95% = 100 kPa, at 100% = 350 kPa; 

       ii. change 90–95 % = 78 kPa (accept 77 or 79 kPa); change 95–100% = 250 kPa; 

       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 

 (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;

 

 c. i. zero (kPa); 

    ii .86%; 

    iii. incipient plasmolysis;

    iv. water potential = solute potential; 

    v.  ψ = ψs + ψp ;

 

   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;

 

 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 

Q. 12. The diagram shows the concentration in mmol dm-3 of two different 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 diffusion.

b. Explain how these concentrations could have been achived.

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.

Ans: a) 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 

 R because concentrations different inside and outside

 

 b) active transport;

 active transport involves pumping ions against a concentration gradient;

 

 c) 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;