Cell membranes are all referred to as plasma membranes and allow different conditions inside and
outside. The Cell-Surface membrane is the one between the cytoplasm and environment.
The bulk of the membrane is made of a phospholipid bilayer. The hydrophilic heads point out and
the hydrophobic ends are in the middle. This allows lipid soluble molecules to enter/ leave the cell
and prevents water/water soluble ion doing so. They also make the membrane flexible and self-
healing.
Proteins are found in two different ways in the bilayer. Firstly, in the surface which provides
mechanical support. If they are present in conjunction with glycolipids or lipids, they can act as cell
receptors for molecules (e.g. hormones). Secondly, proteins can go across the whole layer and
create channels. Carrier proteins are another kind of membrane transport mechanism. They bind to
a molecule and change shape to move the molecule. Proteins can also form cell surface receptors
and help cells adhere together.
Cholesterol is another molecule in the surface of the membrane. It is very hydrophobic and binds
with the heads of the phospholipids. This makes the tails pull together which limits lateral
movement and loss of water/ water soluble ions without making the membrane too rigid.
Glycolipids are carbohydrates bonded covalently with a lipid. The carbohydrate part extends directly
out of the bilayer. They act as a cell receptors for specific chemicals, recognition sites, maintain
stability of the membrane, and help cells attach together to form tissues.
Glycoproteins are carbohydrates bonded to proteins. A carbohydrate chain extends from an
extrinsic protein. They can act as cell surface receptors for hormones and neurotransmitters,
recognition sites, help cells attach together to form tissues. They help cells, such as lymphocytes,
recognise that they are cells from the same organism, preventing them from attacking the cell.
The fluid-mosaic model of the cell-surface membrane explains some of the properties of the
membrane. The fluid part refers to the fact that individual phospholipid molecules can move in
relation to each other. This means the membrane is flexible and always changing structure. The
mosaic part means that the components of the membrane are all varying in shape, size, and pattern.
Osmosis and movement across membranes
Water potential is the pressure created by water molecules. The lower (more negative) the water
potential is, the more osmosis will happen into it. The water potential of pure water is 0.
Diffusion is the net movement of molecules and ions from a region of high concentration to a region
of low concentration, until they reach a dynamic equilibrium (they are evenly distributed but
constantly moving). It is a passive process. There are 3 important conditions for a high rate of
diffusion. A steep concentration gradient, large surface area, and short diffusion pathway. Fick’s law
is an equation for the rate of diffusion.
SA × concentration gradient
Rate of diffusion α
diffusion pathway
The graph of rate of diffusion against concentration gradient is fairly linear up to a point then
plateaus. This is because all the carrier proteins available to move molecules across the membrane
are being used at once so the rate of diffusion can’t get any faster. At this point the proteins are
referred to as being saturated.
Only small, non-polar molecules (which are lipid soluble) can diffuse directly across the
phospholipid bilayer. Facilitated diffusion is also a passive process. It takes place in channel proteins
outside. The Cell-Surface membrane is the one between the cytoplasm and environment.
The bulk of the membrane is made of a phospholipid bilayer. The hydrophilic heads point out and
the hydrophobic ends are in the middle. This allows lipid soluble molecules to enter/ leave the cell
and prevents water/water soluble ion doing so. They also make the membrane flexible and self-
healing.
Proteins are found in two different ways in the bilayer. Firstly, in the surface which provides
mechanical support. If they are present in conjunction with glycolipids or lipids, they can act as cell
receptors for molecules (e.g. hormones). Secondly, proteins can go across the whole layer and
create channels. Carrier proteins are another kind of membrane transport mechanism. They bind to
a molecule and change shape to move the molecule. Proteins can also form cell surface receptors
and help cells adhere together.
Cholesterol is another molecule in the surface of the membrane. It is very hydrophobic and binds
with the heads of the phospholipids. This makes the tails pull together which limits lateral
movement and loss of water/ water soluble ions without making the membrane too rigid.
Glycolipids are carbohydrates bonded covalently with a lipid. The carbohydrate part extends directly
out of the bilayer. They act as a cell receptors for specific chemicals, recognition sites, maintain
stability of the membrane, and help cells attach together to form tissues.
Glycoproteins are carbohydrates bonded to proteins. A carbohydrate chain extends from an
extrinsic protein. They can act as cell surface receptors for hormones and neurotransmitters,
recognition sites, help cells attach together to form tissues. They help cells, such as lymphocytes,
recognise that they are cells from the same organism, preventing them from attacking the cell.
The fluid-mosaic model of the cell-surface membrane explains some of the properties of the
membrane. The fluid part refers to the fact that individual phospholipid molecules can move in
relation to each other. This means the membrane is flexible and always changing structure. The
mosaic part means that the components of the membrane are all varying in shape, size, and pattern.
Osmosis and movement across membranes
Water potential is the pressure created by water molecules. The lower (more negative) the water
potential is, the more osmosis will happen into it. The water potential of pure water is 0.
Diffusion is the net movement of molecules and ions from a region of high concentration to a region
of low concentration, until they reach a dynamic equilibrium (they are evenly distributed but
constantly moving). It is a passive process. There are 3 important conditions for a high rate of
diffusion. A steep concentration gradient, large surface area, and short diffusion pathway. Fick’s law
is an equation for the rate of diffusion.
SA × concentration gradient
Rate of diffusion α
diffusion pathway
The graph of rate of diffusion against concentration gradient is fairly linear up to a point then
plateaus. This is because all the carrier proteins available to move molecules across the membrane
are being used at once so the rate of diffusion can’t get any faster. At this point the proteins are
referred to as being saturated.
Only small, non-polar molecules (which are lipid soluble) can diffuse directly across the
phospholipid bilayer. Facilitated diffusion is also a passive process. It takes place in channel proteins
