SL Biology
Unit 1: Cells
Topic: Transport across cell membranes
1. Membranes control the passage of substances into and out of cells
 Cell membranes surround cells and cell organelles; they are a barrier between the cell (or
organelle) and its environment.
 Cell membranes are partially permeable – some molecules are able to pass through and
others are not.
Cell membranes have a ‘fluid mosaic’ structure
 The structure of all membranes is basically the same; membranes are composed of lipids,
proteins and carbohydrates.
 A cell membrane, also called a plasma membrane, is said to be made of a phospholipid
bilayer.
 The cell membranes is composed mainly of phospholipid molecules (a glycerol molecules
joined to two fatty acids and a phosphate group).
 The fatty acid groups are hydrophobic (water-hating)
and so repel water; the phosphate group is hydrophilic
(water-loving) and so prefers to be in contact with water.
 If surrounded by water the phospholipids form a bilayer
– two layers with the phosphate groups pointing towards
the water and the fatty acids pointing away from the
water.
 The centre of the bilayer is hydrophobic so the membrane does not allow watersoluble substances to pass through it.
 The bilayer is ‘fluid’ because the phospholipids are constantly moving.
 Another component of plasma membranes are proteins embedded within the
phospholipids like a mosaic.
 The different proteins have many functions. Intrinsic proteins (carrier and channel
proteins) pass through both layers and are involved in transporting molecules through the
membrane. Extrinsic proteins exist only in one layer. On the outer surface of the
membrane, they act as receptors, detecting chemicals released from other cells (e.g. the
hormone insulin).
 Proteins may be associated with polysaccharide (carbohydrate) chains; these are called
glycoproteins. Lipids may also have a polysaccharide chain attached to them; they are
called glycolipids.
 Cholesterol is a type of lipid. Cholesterol is present (in varying quantities) in all
membranes, except bacterial cell membranes.
 Cholesterol molecules are found between the phospholipids where they bind to the
hydrophobic tails of the phospholipid, causing them to pack more closely together. This
restricts the movement of the phospholipids and increases the stability of the structure
so it is not too fluid.
 Cholesterol helps maintain the shape of animal cells which do not have a cell wall. This is
particularly important for cells that are not supported by other cells (e.g. red blood cells).
The function of the membrane
 The fluidity allows the membrane to self-seal and form different shapes (used in
phagocytosis).
 Any hydrophobic molecules that need to get in to the cell can simply diffuse straight
through the membrane.
 However, the hydrophobic interior of the membrane prevents the passage of any
hydrophilic, charged molecule through it. This means that hydrophilic molecules in the cell
such as enzymes, amino acids and glucose will not be lost out of the cell by diffusion. Any
hydrophilic molecules that need to get into the cell must travel through a specific intrinsic
protein.
 This means that the cell membrane can control what substance enters or leaves the cell.
2. The transport of substances across the cell membrane
A. Simple diffusion
 The movement of molecules or ions from a region of higher concentration to a region of
lower concentration (i.e. down a concentration gradient).
 Movement is in both directions, but overall the net movement is down the concentration
gradient until equilibrium is reached.
 Hydrophobic molecules and very small uncharged molecules (e.g. oxygen and carbon
dioxide) can diffuse straight through the membrane.
 The rate of diffusion will vary for different membranes; some are adapted for rapid
diffusion.
 The rate of diffusion is affected by:
 the surface area – the larger the surface area, the faster the rate of diffusion,
 the concentration gradient – the greater the difference between the concentrations
either side of the membrane (the steeper the gradient), the faster the rate of diffusion,
 the thickness of the exchange surface – the shorter the diffusion path, the faster the
rate of diffusion,
 the temperature – as the temperature rises and kinetic energy of molecules increases,
the faster the rate of diffusion.
B. Facilitated diffusion
 Hydrophilic molecules (e.g. ions and polar molecules) and larger molecules (e.g. glucose
or amino acids) would pass too slowly through the membrane by simple diffusion. They
diffuse through an intrinsic protein called a channel protein or a carrier protein. This
process is called facilitated diffusion.
 Charged particles pass through a channel protein which forms a pore
through the membrane. Different channel proteins are required for
different charged particles. The charged particles diffuse across the
membrane, down their concentration gradient.
 Large molecules are moved across the membrane using a carrier
protein. Different carrier proteins are required for the transport of
different large molecules. The protein carrier binds to the large molecule,
then the protein changes shape, releasing the large molecule on the
opposite side of the membrane.
 Facilitated diffusion only occurs down a concentration gradient and requires no energy
from ATP.
 The rate of facilitated diffusion depends on the number of channel proteins, the
concentration gradient and the temperature.
C. Osmosis
 The net diffusion of water molecules from a solution of high
water potential to a solution of lower water potential
through a partially permeable membrane.
 In the kidney, aquaporins are specialised channel proteins
which allow the facilitated diffusion of water molecules.
 Osmosis is a passive process; it does not require energy.
 Distilled water has the highest water potential of 0kPa
(kilopascals). Dissolving a solute in the water reduces the
water potential.
 A solution is described as being isotonic if its water potential is the same as another. There
will be no net movement of water molecules (i.e. no net osmosis).
 A solution is described as being hypertonic if it has a lower water potential (higher
concentration of solutes) than another; a solution is described as being hypotonic if it has
a higher water potential (lower concentration of solutes) than another.
 The rate of osmosis depends on the water potential gradient, the thickness of the
exchange surface, the surface area and the temperature.
D. Active transport
 Active transport also requires a specifically shaped intrinsic protein called
a carrier protein.
 Active transport moves molecules against a concentration gradient using
energy released from the hydrolysis of ATP produced from respiration.
 The molecule to be transported attaches to the carrier protein, and the carrier protein
changes shape, transporting the molecule across the membrane.
 The rate of active transport depends on the number and speed of individual carrier
proteins, the rate of respiration and therefore the availability of ATP, and temperature.