ANSWERS TO REVIEW QUESTIONS – CHAPTER 05
1.
What is meant by selective permeability? Explain how the phospholipid bilayer and
membrane-spanning proteins each contribute to movement across membranes. (pp. 103–
106)
The composition of both cells and their internal membrane-bound compartments is regulated by
properties of the membranes. These are selectively permeable, allowing the passage of some molecules
rather than others. In particular, the phospholipid bilayer of membranes is hydrophobic and
impermeable to most water-soluble molecules, especially ionised molecules. By contrast, lipid-soluble
substances pass freely through the bilayer. Some other substances are moved into or out of cells by
enclosing them in membrane-bound vesicles, or via the mechanism of membrane-spanning proteins.
These proteins facilitate the passage of some water-soluble substances through membranes either by
creating channels or acting as carriers. Lipid-insoluble particles may also be transported across
membranes through the action of membrane-spanning proteins.
2.
Define concentration gradient and electrical gradient. How is each involved in the movement
of solutes across membranes? (pp. 103–106)
The concentration gradient is the difference in concentration between connected solutes. Solute
molecules tend to disperse evenly throughout a solution by diffusion, so there is a net movement of
molecules from the strong concentration to the weaker one until concentrations are equal. The greater
the concentration gradient, the faster the rate of diffusion. Diffusion can also be accelerated by
increases in temperature. If several chemicals are in solution, they diffuse down their individual
concentration gradients irrespective of the concentrations of other substances.
Although many ions cannot cross the lipid bilayer, they can diffuse across membranes via ion-selective
channels. The energy driving diffusion of ions is related to both the concentration gradient and the
electrical gradient relating to the ions. The interaction of these two factors can mean that ions can
diffuse against a concentration gradient (from a low concentration to a higher one) if the electrical
component exceeds the concentration one. The ion is said to diffuse passively along an electrochemical
gradient. Overall, diffusion along electrochemical gradients is a passive process, requiring no
expenditure of energy.
3. By what processes do each of the following substances enter a white blood cell? (pp. 105–
111)
(a) O2—By diffusion—see page 82 of the text.
(b) H2O—By osmosis, whereby water moves across a selectively permeable membrane along its
own concentration gradient—see page 92 of the text.
(c) glucose—By protein-mediated transport that moves glucose across a membrane in the
direction of its concentration gradient, i.e. from high to low concentrations. Note that alternative
mechanisms exist in some cells (e.g. in the intestine and the kidney) that draw glucose against a
concentration gradient using solute-coupled exchangers. See also the answer to extension question 1
and the reading by Lienhard et al. (1992) listed in the print resources at the end of this chapter.
(d) bacterium—Bacteria are engulfed in a process of phagocytosis—Chapter 24. Phagocytosis
involves intake of particulate material, as distinct from pinocytosis, which involves intake of liquid.
4.
By means of diagrams, distinguish between the mechanisms of action of voltage-gated
channels, ligand-gated channels and mechanically gated channels. (pp. 104–105)
In a simple analogy, ion-selective channels can be thought of as pipes that run through membranes
permitting the passage of ions. Specific filters on the pipes regulate which substances can pass through.
In reality, these channels are complex proteins spanning the membrane and the central region forms the
channel through which particles diffuse. Most channels are ion-selective, although other ions may be
able to use them at much lower rates. Many also have ‘open’ and ‘closed’ states and allow movement
only when they are open. Opening can be initiated by voltage change across the membrane (voltage-
gated channels), by interaction between the channel and a particular chemical (ligand-gated channel) or
by mechanical force (mechanically gated channel). The mechanisms of action of these three types are
illustrated in Figures 5.5, 5.6, pages 104, 105 and Box 5.2.
5. Outline the characteristic features of active transport through carrier proteins. (pp. 105–
106)
Active transport occurs via carrier proteins that span the membrane and expend energy to move
substances across the membrane against their electrochemical gradient. It has four main features:
1.
2.
3.
4.
greater speed compared to passive diffusion mechanisms
transport proteins can be saturated when substrate concentration increases
specificity of transport proteins for particular substances
transport can be inhibited by substances similar to that normally transported, which compete for
the binding site and exclude or reduce the transport rate of the normal target substance.
6. List three key features that distinguish transport ATPases from co-transporters. (pp. 105–
106)
Transport ATPases gain energy from the hydrolysis of chemical bonds, most commonly hydrolysing
ATP to ADP and inorganic phosphate. The Na +–K+ pump (see extension question 2) is a good
example. ATP is not used directly in solute-coupled exchangers. Instead, molecules move together with
sodium ions or protons that are moving down a concentration gradient from a high concentration to a
low concentration. This system is based on two key components:
1.
2.
using ATP to establish a down gradient in sodium ions or protons that is greater than the up
gradient of the molecule to be moved
moving the molecule along special coupled channels that take the molecule, together with either a
sodium ion or a proton, across the membrane.
7.
What is water potential? In plant cells, what are its two components? Compare what
happens when animal cells and plant cells are placed in distilled water. (p. 108)
Water potential, written as  (psi), is the total energy level of water measured in megapascals (MPa), a
unit of pressure. It is the sum of pressure potential (p) and osmotic potential (). The pressure
potential arises when hydrostatic pressure is applied to water by suction or pressure. Suction lowers the
water pressure, while pressure increases it. Osmotic pressure changes in relation to the concentration of
dissolved solutes. Solutes increase the volume of a solution while keeping the water content constant,
effectively diluting the water. The solutes further interact with the water, reducing the ability of the
water to diffuse freely. Water potential is normally standardised at 0 MPa for pure water at atmospheric
pressure. In animal cells, water potential arises from osmotic potential alone, while in plant cells the
cell wall contributes pressure potential as well. Water potential has significant implications for a wide
range of plant functions related to water transport, which are discussed in detail in Chapter 18.
Water moves into or out of cells depending on the water potential inside the cell relative to that of the
external environment. Water movement by osmosis across the cell membrane changes the internal
solute concentration, thus changing the water potential. In an animal cell placed in distilled water,
water will enter the cell and ultimately burst the cell membrane because water potential cannot be
equalised across the cell membrane to halt the movement of water. However, in the case of a plant cell,
the rigid cell wall begins to apply pressure to the cell contents as water enters the cell by osmosis. At a
particular pressure, the water potential inside the cell will equal that on the outside and net movement
of water will cease.
8. What do the terms ‘hypo-osmotic’, ‘iso-osmotic’ and ‘hyperosmotic’ indicate? (p. 108)
Hypo-osmotic—A solution with a lower solute concentration than another solution.
Iso-osmotic—Solutions with the same solute concentration.
Hyperosmotic—A solution with a higher solute concentration than another solution.
9.
Why is vesicle-mediated transport necessary? How do pinocytosis, phagocytosis and
exocytosis differ from each other? (pp. 110–111)
Vesicle-mediated transport is necessary to transport very large molecules, such as extracellular
enzymes, across membranes.
Pinocytosis is invagination of the cell membrane to transport liquid into a cell.
Phagocytosis is invagination of the cell membrane to transport solid material into a cell.
Exocytosis is fusion of vesicles with the plasma membrane to expel material from a cell.
10. Show diagrammatically the sequence of events in receptor-mediated endocytosis. (pp. 110–
111)
Refer to Figure 5.11, page 110.