Passive transport

All molecules posses kinetic energy and move randomly in a gas or liquid. Net movement occurs from an area of high concentration to an area of low concentration, down a concentration gradient. Diffusion is a passive process since no additional energy is required.

Simple diffusion

Small and/or non-polar molecules, such as oxygen and steroid hormones, can fit in-between the hydrophobic fatty acid tails and pass directly through the phospholipid bilayer. Some small polar molecules, such as water and carbon dioxide can also pass through slowly.

Facilitated diffusion

Larger polar molecules and ions cannot diffuse across the phospholipid bilayer and require a specific transport protein (carrier or channel) to cross the membrane. Large molecules cannot diffuse across the phospholipid bilayer because they cannot pass between the phospholipids while polar molecules and ions cannot cross because they are repelled by the non-polar fatty acid tails.

Channel proteins provide a hydrophilic channel or pore through the hydrophobic interior of the phospholipid bilayer. Channel proteins are specific to the small molecules or ion being transported and may be open all the time or gated. Gated channels are open or closed depending on the presence of another molecule (ligand) or the potential difference (voltage) across the membrane.

Carrier proteins can bind a specific polar molecule e.g. glucose on one side of the membrane and then change shape (allostery) to release the molecule on the other side of the membrane.

In both types of facilitated diffusion, the rate of diffusion depends upon the number of transport proteins present in the membrane.

Active transport

Sometimes molecules need to be transported from low to high concentration, against their concentration gradient e.g. to accumulate molecules to a higher concentration inside an organelle or cell. Active transport requires a specific carrier protein (pump) and additional energy e.g. from the hydrolysis of ATP to change the shape of the carrier protein (allostery) after the molecule has bound so it can be released on the other side of the membrane.

Bulk transport

Bulk transport is the movement of large quantities of materials into or out of the cell. It involves the formation or fusion of vesicles with the plasma membrane and so relies on the fluid nature of the membrane. It is an active process requiring ATP (to form vesicles and move them around the cell).

There are two types of bulk transport: exocytosis and endocytosis.

Exocytosis is movement out of cells e.g. secretion of hormones or proteins.

Endocytosis is movement into cells. Phagocytosis is the uptake of solid particles e.g. a neutrophil engulfing a bacterium. Pinocytosis is the uptake of solutes in solution e.g. uptake of nutrients by egg cell.

Nutrient uptake in unicellular organisms using bulk transport:

1. Nutrient particles are taken up by endocytosis, as the plasma membrane surrounds the nutrient particles and fuses with itself, pinching off to form a phagocytic vesicle.

2. Lysosome containing hydrolytic enzymes fuses with phagocytic vesicle.

3. Lysosome releases digestive enzymes into vesicle; these breakdown the nutrient particles.

4. Soluble products of digestion are absorbed into the cytoplasm.

5. Insoluble, undigested material is removed from the cell by exocytosis as the vesicle fuses with the plasma membrane.