Cambridge International A Level Biology Revision Guide

Chapter 4: Cell membranes and transport
Drug runners
Liposomes are artificially prepared membrane-bound
compartments (vesicles). They can be prepared by
breaking up biological membranes into pieces, some
of which re-seal themselves into balls resembling
empty cells, though much smaller on average. Like
intact cells, they are surrounded by a phospholipid
bilayer and the interior is usually aqueous. They were
first described in 1961. Since then they have been used
as artificial models of cells and, more importantly, for
medical applications. In particular, they have been
used to deliver drugs.
To do this, the liposome is made while in a solution
of the drug, so the drug is inside the liposome. The
liposome is then introduced into the body and
when it reaches a target cell, such as a cancer cell or
other diseased cell, it fuses with that cell’s surface
membrane, delivering the drug inside the cell. Precise
targeting can be achieved by inserting the correct
recognition molecule – for example an antigen or
antibody – into the liposome membrane. Other
targeting methods also exist.
A recent (2013) discovery illustrating the
usefulness of liposomes is that they provide a safe
way of delivering the powerful anti-cancer drug
staurosporine. Although this drug has been available
since 1977, it kills any cells, including healthy ones that
it comes into contact with because it interferes with
several cell signalling pathways. Disguising agents
have been added to the outer surfaces of liposomes
carrying the drug which hide the drug from the
immune system and allow it to target cancer cells only.
Liposomes have many other uses. For example,
they are used in the cosmetics industry to deliver skin
care products, such as aloe vera, collagen, elastin
and vitamins A and E, when rubbed on skin. Liposome
delivery of food supplements by mouth has also been
tried with some success – absorption rates can be
much higher than with traditional tablets.
Figure 4.1 Liposomes.
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In Chapter 1, you saw that all living cells are surrounded
by a very thin membrane, the cell surface membrane.
This controls the exchange of materials such as nutrients
and waste products between the cell and its environment.
Inside cells, regulation of transport across the membranes
of organelles is also vital. Membranes also have other
important functions. For example, they enable cells to
receive hormone messages. It is important to study the
structure of membranes if we are to understand how these
functions are achieved.
Phospholipids
An understanding of the structure of membranes depends
on an understanding of the structure of phospholipids
(page 38). From phospholipids, little bags can be formed
inside which chemicals can be isolated from the external
environment. These bags are the membrane-bound
compartments that we know as cells and organelles.
Figure 4.2a shows what happens if phospholipid
molecules are spread over the surface of water. They form
a single layer with their heads in the water, because these
are polar (hydrophilic), and their tails projecting out of
the water, because these are non-polar (hydrophobic). The
term ‘polar’ refers to the uneven distribution of charge
which occurs in some molecules. The significance of this is
explained on pages 35–36.
If the phospholipids are shaken up with water, they
can form stable ball-like structures in the water called
micelles (Figure 4.2b). Here all the hydrophilic heads face
outwards into the water, shielding the hydrophobic tails,
which point in towards each other. Alternatively, twolayered
structures, called bilayers, can form in sheets
(Figure 4.2c). It is now known that this phospholipid
bilayer is the basic structure of membranes (Figure 4.2d).