Passive transport

active transport

Diffusion is the movement of particles from a high concentration to a low concentration.While the particles themselves need to have energy to move and will diffuse quicker if they have more energy, the cell does not need energy for diffusion of particles to occur through the plasma membrane.

What is facilitated diffusion:

Facilitated diffusion still relies on the principles of diffusion so substances can still only move down their concentration gradient from areas of high concentration to areas of low concentration. Facilitated diffusion is for substances that cannot get through the phospholipid bilayer e.g. because they are too big or because they are charged. They use specific proteins in the membrane to enable them to get through the membrane that they would not normally be able to do. The two types of proteins involved are channel proteins and carrier proteins. No metabolic energy from ATP is required for facilitated diffusion.

Protein channels:

Protein channels are for small charged molecules such as ions.

They are proteins with a pore through the centre through which the particle can pass without having to pass through the phospholipid bilayer itself.

The protein channels are specific to the particle passing though

it so potassium ions cannot pass through a sodium ion channel and sodium ions cannot pass through a potassium ion channel. As the channel just provides a passage through the membrane, substances can only travel down their concentration gradients by facilitated diffusion. Protein channels can open and close as needed.

Carrier proteins:

Carrier proteins tend to be used for larger polar molecules such as carbohydrates. They, too, are specific to the substance they are transporting across the membrane. The substance, e.g. glucose, binds with the carrier protein and this changes the shape of the protein causing it to open up on the other side so the substance can pass through. The open end is on the side of the membrane where the highest concentration is. No metabolic energy is required for this process and substances can only move down their concentration gradients.

Gated protein channels:

Gate closed , Receptor, Gate open, Neurotransmitter, Gate closed, Gate open

Ligand gated

These are also known as ionotropic receptors. They open in response to binding with a chemical receptor. A good example is when a neurotransmitter binds with receptor sites in the post synaptic membrane it causes the sodium ion channels to open and sodium ions diffuse rapidly down the concentration gradient into the post synaptic cell and trigger a new nerve impulse.

Voltage gated:

Voltage gated ion channels open in response to a change in membrane potential near to the channel protein. The change needed will depend on the specific ion the channel is for. An example is a voltage gated sodium ion channel which remains closed when the outside of the cell is more positive but will be triggered to open if a local depolarization occurs and the outside becomes more negative. This is how nerve impulses travel along a neuron.