Cell Physiology
Movement of molecules across the membrane
Lipid-soluble / non-polar (non-charged) molecules can diffuse directly through the phospholipid bilayer of the
cell membrane.
Water, oxygen and carbon dioxide can move through the phospholipid layers as they are small molecules
Polar molecules (ions and water-soluble molecules) cannot move through the phospholipid layers due to the
hydrophobic nature of the centre of the phospholipid bilayer.
Therefore they must move through protein channels or by using special transport proteins
The protein channels act as hydrophilic pores/channels to allow water-soluble molecules through the
phospholipid bilayer.
Transport proteins are selective and only certain molecules or ions will be allowed through
Diffusion
There are two types of diffusion:
o Simple
o Facilitated
Diffusion is the net movement of a substance from where it is in a higher concentration to where it is in a lower
concentration.
Diffusion is a passive process – it does not require ATP energy as the movement occurs along the concentration
gradient.
Diffusion takes place due to the kinetic energy that the molecules in the liquid or gas have
Diffusion is affected by:
Temperature: Diffusion will usually occur faster at higher temperatures due to the kinetic energy that the
diffusing molecule will have
Concentration Gradient: The greater the concentration gradient across the membrane, the faster diffusion will
occur
Size of the molecule: Small molecules diffuse faster than larger molecules
The thickness of Exchange Surface: Biological membranes are usually very thin allowing for faster rates of
diffusion
Surface Area of the Membrane: The larger the surface area, the faster diffusion will occur. Many cells involved in
diffusion will have adaptations to increase the surface area for diffusion.
Increasing the surface area across which the
particles diffuse, or increasing the size of the
concentration gradient will increase the rate of
diffusion
, Increasing the distance over which diffusion takes place will decrease the rate
Simple Diffusion
This type of diffusion does not require the use of proteins. Lipid-soluble molecules, O 2 and CO2, can all pass through
the membrane directly. Water molecules can also slip through this way.
Facilitated Diffusion
Polar molecules like ions (positively or negatively charged) cannot pass directly through the phospholipid bilayer
as the fatty acid tails of the phospholipids are hydrophobic.
Facilitated diffusion allows polar, hydrophilic (water-soluble) molecules to travel across the membrane with the
‘help’ of proteins.
There are two types of protein that are involved in a facilitated diffusion:
Carrier proteins:
The molecule that is diffusing across the membrane attaches to the protein carrier. Once attached, the protein
undergoes a conformational change in shape and the molecule is released to the other side of the membrane.
Different proteins have different shapes due to their tertiary structure and this makes the carrier protein specific
to the molecule that it transports. Only certain molecules will fit the receptor / binding site on the protein
carrier.
Different carriers will be used to transport different shaped molecules.
Channel protein:
The channel is formed by proteins with a central pore that allows charged particles to pass through
This creates a hydrophilic channel through the phospholipid bilayer.
Some of these channels are permanently open but others are gated channels.
Gated channels will open or close allowing control over the movement through them.
Many water molecules pass through specialised protein channels (pores) in the membrane called aquaporins.
The rate of facilitated diffusion is also affected by the number of specific carrier or channel proteins, that are present
in a particular cell membrane. This enhances selectivity – a cell requiring more of a particular substance will have
more carriers that are specific to that substance. Facilitated diffusion is also a passive process.
Active Transport
Active transport is the movement of substances from a region of low concentration to a region of high concentration
To achieve active transport, two factors are needed:
Protein carriers – with a specific shape to fit the substance
Energy as ATP – this is supplied by aerobic respiration. In cells where a lot of active transport occurs there are
high numbers of mitochondria
Any factor that affects the rate of respiration affects the rate of active transport.
Increasing temperature increases kinetic energy, therefore, increases the rate of all chemical reactions including
aerobic respiration
Increasing O2 uptake means more aerobic respiration – therefore more ATP production
Some substances are known to inhibit respiration – cyanide – such poisons reduce the rate of active transport.
As with facilitated diffusion, the rate of active transport will also be affected by the relative number of specific
carriers in a particular membrane.
Movement of molecules across the membrane
Lipid-soluble / non-polar (non-charged) molecules can diffuse directly through the phospholipid bilayer of the
cell membrane.
Water, oxygen and carbon dioxide can move through the phospholipid layers as they are small molecules
Polar molecules (ions and water-soluble molecules) cannot move through the phospholipid layers due to the
hydrophobic nature of the centre of the phospholipid bilayer.
Therefore they must move through protein channels or by using special transport proteins
The protein channels act as hydrophilic pores/channels to allow water-soluble molecules through the
phospholipid bilayer.
Transport proteins are selective and only certain molecules or ions will be allowed through
Diffusion
There are two types of diffusion:
o Simple
o Facilitated
Diffusion is the net movement of a substance from where it is in a higher concentration to where it is in a lower
concentration.
Diffusion is a passive process – it does not require ATP energy as the movement occurs along the concentration
gradient.
Diffusion takes place due to the kinetic energy that the molecules in the liquid or gas have
Diffusion is affected by:
Temperature: Diffusion will usually occur faster at higher temperatures due to the kinetic energy that the
diffusing molecule will have
Concentration Gradient: The greater the concentration gradient across the membrane, the faster diffusion will
occur
Size of the molecule: Small molecules diffuse faster than larger molecules
The thickness of Exchange Surface: Biological membranes are usually very thin allowing for faster rates of
diffusion
Surface Area of the Membrane: The larger the surface area, the faster diffusion will occur. Many cells involved in
diffusion will have adaptations to increase the surface area for diffusion.
Increasing the surface area across which the
particles diffuse, or increasing the size of the
concentration gradient will increase the rate of
diffusion
, Increasing the distance over which diffusion takes place will decrease the rate
Simple Diffusion
This type of diffusion does not require the use of proteins. Lipid-soluble molecules, O 2 and CO2, can all pass through
the membrane directly. Water molecules can also slip through this way.
Facilitated Diffusion
Polar molecules like ions (positively or negatively charged) cannot pass directly through the phospholipid bilayer
as the fatty acid tails of the phospholipids are hydrophobic.
Facilitated diffusion allows polar, hydrophilic (water-soluble) molecules to travel across the membrane with the
‘help’ of proteins.
There are two types of protein that are involved in a facilitated diffusion:
Carrier proteins:
The molecule that is diffusing across the membrane attaches to the protein carrier. Once attached, the protein
undergoes a conformational change in shape and the molecule is released to the other side of the membrane.
Different proteins have different shapes due to their tertiary structure and this makes the carrier protein specific
to the molecule that it transports. Only certain molecules will fit the receptor / binding site on the protein
carrier.
Different carriers will be used to transport different shaped molecules.
Channel protein:
The channel is formed by proteins with a central pore that allows charged particles to pass through
This creates a hydrophilic channel through the phospholipid bilayer.
Some of these channels are permanently open but others are gated channels.
Gated channels will open or close allowing control over the movement through them.
Many water molecules pass through specialised protein channels (pores) in the membrane called aquaporins.
The rate of facilitated diffusion is also affected by the number of specific carrier or channel proteins, that are present
in a particular cell membrane. This enhances selectivity – a cell requiring more of a particular substance will have
more carriers that are specific to that substance. Facilitated diffusion is also a passive process.
Active Transport
Active transport is the movement of substances from a region of low concentration to a region of high concentration
To achieve active transport, two factors are needed:
Protein carriers – with a specific shape to fit the substance
Energy as ATP – this is supplied by aerobic respiration. In cells where a lot of active transport occurs there are
high numbers of mitochondria
Any factor that affects the rate of respiration affects the rate of active transport.
Increasing temperature increases kinetic energy, therefore, increases the rate of all chemical reactions including
aerobic respiration
Increasing O2 uptake means more aerobic respiration – therefore more ATP production
Some substances are known to inhibit respiration – cyanide – such poisons reduce the rate of active transport.
As with facilitated diffusion, the rate of active transport will also be affected by the relative number of specific
carriers in a particular membrane.
