Cambridge International A Level Biology Revision Guide

Chapter 4: Cell membranes and transport
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 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 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 (p.d.) is created across
the membrane. The significance of this in nerve cells is
discussed in Chapter 15 (pages 333–334).
2K +
Figure 4.18 The Na + – K + pump.
outside +
Na + –K +
pump
inside −
3Na +
ATP ADP + P i
potential
difference
maintained
across
membrane
Bulk transport
So far we have been looking at ways in which individual
molecules or ions cross membranes. Mechanisms also
exist for the bulk transport of large quantities of materials
into cells (endocytosis) or out of cells (exocytosis). Large
molecules such as proteins or polysaccharides, parts of
cells or even whole cells may be transported across the
membrane. This requires energy, so it is a form of active
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.
■■
■■
Phagocytosis or ‘cell eating’ – this is the bulk uptake
of solid material. Cells specialising in this are called
phagocytes. The process is called phagocytosis and
the vacuoles phagocytic vacuoles. An example is
the engulfing of bacteria by certain white blood cells
(Figure 4.19).
Pinocytosis or ‘cell drinking’ – this is the bulk uptake
of liquid. The vacuoles (vesicles) formed are often
extremely small, in which case the process is called
micropinocytosis.
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In Figure 4.18, you can see that the pump has a receptor
site for ATP on its inner surface. It acts as an ATPase
enzyme in bringing about the hydrolysis of ATP to ADP
(adenosine diphosphate) and phosphate to release energy.
Active transport can therefore be defined as the
energy-consuming transport of molecules or ions across a
membrane against a concentration gradient (from a lower
to a higher concentration). The energy is provided by ATP
from cell respiration. Active transport can occur either
into or out of the cell.
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 (Chapter 7), and to load inorganic ions from the
soil into root hairs.
Figure 4.19 Stages in phagocytosis of a bacterium by a white
blood cell.
Exocytosis is the reverse of endocytosis and is the process
by which materials are removed from cells (Figure 4.20).
It happens, for example, in the secretion of digestive
enzymes from cells of the pancreas (Figure 4.21). 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.