Roles of membranes in cells- The plasma (cell-surface) membrane
The plasma membrane is a partially permeable barrier between the cell and its environment. It
keeps the contents of the cell separate from its environment, and limits what molecules can
enter and leave the cell. It acts as the site for certain chemical reactions and enables the cell to
communicate with other cells through the process of cell signalling.
Internal membranes (organelle membranes)
Other membranes in the cell separate the organelles from the cytoplasm. These
compartmentalise the cell, separating processes so that each process can occur in a
specialised area of the cell. For example, all the enzymes involved in one process can be kept
together and other processes do not interfere. Concentration gradients can be formed across
the membranes. The membranes may act as the sites of specific chemical reactions, such as
oxidative phosphorylation in aerobic respiration.
The fluid mosaic model of membrane structure
The fluid mosaic model describes the molecular arrangement of the membranes in a cell. A fluid
mosaic membrane consists of:
• a bilayer of phospholipid molecules
• cholesterol which regulates the fluidity of the membrane, making it
more stable
• glycolipids and glycoproteins that function in cell signalling or cell
attachment
• protein molecules that float in the phospholipid bilayer. Some
proteins are partially held on the surface of the membrane — these
are called extrinsic proteins. Others are embedded in the membrane
— these are called intrinsic proteins. Some proteins float freely in the bilayer whereas others
may be bound to other components in the membrane or to structures inside the cell
Phospholipids
Phospholipids form a barrier that limits movement of some substances into and out of the cell,
or into and out of the organelles, so the membrane is partially permeable. Small, fat-soluble
substances dissolve into the phospholipid bilayer and diffuse across the membrane.
Water-soluble molecules and ions cannot easily dissolve and cross the membrane. Small
molecules like water may diffuse across slowly, but most require special transport mechanisms.
Cholesterol
Cholesterol fits between the tails of the phospholipid molecules. It inhibits movement of the
phospholipids, reducing the fluidity of the membrane. It also holds the phospholipid tails
together, for mechanical stability. Cholesterol makes the membrane less permeable to water
and ions. It can also be converted into steroid hormones like testosterone, is used for
waterproofing the skin, for making vitamin D, and for making bile salts. Its structure is similar to
that of carbohydrates in that it contains C, H, and O, has hydroxyl groups, and has six
membered rings.
Glycolipids and glycoproteins
The carbohydrate group on the protein or lipid molecule always has a specific shape and is
used to recognise the cell — to identify it as ‘self’ or ‘foreign’. Antigens on cell surfaces are
usually glycolipids or glycoproteins.
Drugs and hormones can bind to these membrane-bound receptors. Medicines can be made to
fit the receptors on certain cells. For example, asthmatics take salbutamol, which fits the
receptors on smooth muscle in the airways to cause relaxation.
Cells communicate in an organism by cell signalling to coordinate the activities of the organism.
The shape of the glycoprotein or glycolipid may be complementary to the shape of a signalling
molecule in the body. Such complementary shapes can be used as binding sites to which the
signalling molecules (e.g. hormones and neurotransmitter molecules in a synapse) attach. If the
correct binding site is not present, the cell cannot respond to the signalling molecule. Binding
sites are also used for cell attachment — the cells of a tissue bind together to hold the tissue
together.
Proteins
Proteins have a variety of functions, such as enzymatic activity and cell signalling. However,
many functions involve moving substances across the membrane. For example, some proteins
may form:
• pores that allow the movement of molecules that cannot dissolve in the phospholipid bilayer
• carrier molecules that allow facilitated diffusion
• active pumps
Membrane structure and permeability
Temperature
Membranes are partially permeable, fluid and stable at normal body temperature. If temperature
increases, the molecules gain kinetic energy and move about more. This increases the
permeability of the membranes to certain molecules. Any molecules that diffuse through the
phospholipid bilayer will diffuse more quickly. This is because as the phospholipids move about,
they leave temporary gaps between them, providing space for small molecules to enter the
membrane.
If temperature increases further, the phospholipid bilayer may lose its mechanical stability (it
may melt) and the membrane becomes even more permeable. Eventually, the proteins in the
membrane will denature. This will further damage the structure of the membrane and it will
become completely permeable.
Solvents
Solvents such as alcohol dissolve fatty substances. As the concentration of alcohol increases,
the membrane is more likely to dissolve.
The movement of molecules across membranes
Passive transport
Passive transport is the movement of molecules that does not need metabolic energy in the
form of adenosine triphosphate (ATP). It uses energy in the form of the kinetic (movement)
energy. It only occurs when molecules move down a concentration gradient.
Because molecules move randomly, some may move
in the ‘wrong’ direction — so you should describe
passive transport as the net movement of molecules
down their concentration gradient. Passive transport
can occur in three forms:
• Diffusion- the net movement of molecules away from
a concentrated source. This may occur across a
membrane if the molecules are fat-soluble or if they
are small and can fit between the phospholipids in a
membrane.
• Facilitated diffusion- diffusion across a membrane
that is helped by a transport protein in the membrane.
The protein could be a pore protein (which may be gated) or it could be a carrier protein.
• Osmosis- the net movement of water molecules across a partially permeable membrane.
Water molecules move down their water potential gradient (i.e. from an area of higher water
potential to an area of lower water potential).
The rate of diffusion
Diffusion occurs without using metabolic energy. It relies on the kinetic energy of the molecules.
The rate of diffusion is affected by:
• Temperature — a higher temperature gives molecules more kinetic energy. At higher
temperatures the molecules move faster, so the rate of diffusion increases.
• Concentration gradient — more molecules on one side of a membrane (or less on the other)
increases the concentration gradient. This increases the rate of diffusion.
• Size of molecule — small molecules or ions can move more quickly than larger ones.
Therefore, they diffuse more quickly than larger ones.
• Thickness of membrane — a thick barrier creates a longer pathway for diffusion, so diffusion is
slowed down by a thick membrane.
• Surface area — diffusion across membranes occurs more rapidly if there is a greater surface
area.
Active transport
Active transport involves the movement of molecules using metabolic energy in the form of ATP.
It can move molecules against their concentration gradient and uses membrane-bound proteins
that change shape to move the molecules across the membrane.
Bulk transport
Bulk transport is the movement of molecules through a membrane by the action of vesicles.
Endocytosis is the formation of vesicles by the plasma membrane, which moves molecules into
the cell. Exocytosis is the fusion of vesicles with the plasma membrane, which moves molecules
out of the cell. Bulk transport uses metabolic energy.
Water potential and osmosis
Water potential
Pure water has a water potential of zero. As solutes (sugars or salts) are added to a solution,
the water potential gets lower. Therefore, a salt solution has a water potential below zero, i.e. a
negative potential.
Water molecules will move from a solution with a higher water potential to a solution with a
lower (more negative) water potential. Therefore, water molecules always move down their
water potential gradient.
Osmosis
A cell placed in water has a lower (more negative) water potential than the surrounding water.
There is a water potential gradient from high outside the cell to lower inside the cell. As a result,
water molecules enter the cell.
A cell placed in a strong salt solution has higher (less negative) water potential than the
surrounding solution. There is a water potential gradient from higher inside the cell to lower
outside the cell, so water molecules leave the cell.
