Transport across membranes

Net movement of particles

From an area of high concentration to an area of low concentration, down a concentration gradient.

How is diffusion a passive process

No additional energy is required.

Which types of substances can cross a membrane via simple diffusion

Small and/or non-polar molecules. Some small polar molecules, such as water and carbon dioxide can also pass through slowly.

Why can’t large molecules diffuse across the phospholipid bilayer

They cannot pass between the phospholipids

Why can’t polar molecules or ions diffuse across the phospholipid bilayer

They are repelled by the non-polar fatty acid tails.

How do large and/or polar molecules and ions diffuse across the membrane

Via a specific transport protein

What are the 2 types of transport proteins

Channel and carrier

What do channel proteins provide

A hydrophilic channel or pore through the hydrophobic interior of the phospholipid bilayer.

How do gated channel proteins function

They open or close depending on the presence of another molecule (ligand) or the potential difference (voltage) across the membrane.

How do carrier proteins function

They bind to a specific polar molecule on one side of the membrane and then change shape (allostery) to release the molecule on the other side of the membrane.

When does active transport occur

When molecules need to be transported from low to high concentration, against their concentration gradient

What does active transport require

A specific carrier protein (pump) and additional energy e.g. from the hydrolysis of ATP

What is the function of bulk transport

To move large quantities of materials into or out of the cell.

What does bulk transport rely on

The fluid nature of the membrane.

How is bulk transport an active process

Requires ATP for the formation of vesicles and to move them around the cell.

What are the 2 types of bulk transport

Exocytosis and endocytosis.

Exocytosis

Bulk transport of substances out of the cell e.g. secretion of hormones or proteins.

Endocytosis

Bulk transport of substances into the cell.

What are the 2 types of endocytosis

Phagocytosis and pinocytosis.

Phagocytosis

The uptake of solid particles e.g. a neutrophil engulfing a bacterium.

Pinocytosis

The uptake of solutes in solution e.g. uptake of nutrients by egg cell.