Transport Across Plasma Membrane

Transport Across Plasma Membrane

The plasma membrane covers all living cells, enabling the cells’

contents to be held together and controls movement of substances into

and out of the cell.

Plasma membranes are made of phospholipids, proteins and

carbohydrates. The phospholipids are essentially made out of two fatty

acid chains and a phosphate-glycerol group. They are arranged in a

bilayer with the hydrophilic phosphate head facing outwards and the

hydrophobic fatty acid chains facing inwards and to each other in the

middle of the bilayer. This effectively provides a barrier to all but

the smallest molecules.

In the membrane are proteins that have a number of functions. Those

that span across the entire bilayer (from hydrophobic head to

hydrophobic head) act as channels and carriers to transport substances

across the membrane in active or facilitated diffusion. Those on the

surface act as receptors with specific binding sites where hormones

and other chemicals can bind. This binding in turn triggers other

events in the cell.

Finally, those that are on the inside surface maintains the cell’s

shape and motility, such as enzymes.

Glycoprotiens and glycolipds are found on the surface of the cell.

They form a cell coat (glycocalyx) and are important in cell

protection and recognition.

The bilayer is described as fluid since the lipids within it are able

to slide around and exchange places with each other freely. This

feature allows processes such as phagocytosis to occur.

There are five main ways in which transport across cells can occur:

Simple Diffusion;

The only substances that can diffuse directly th...

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...ules gets near the cell, the membrane forms a slight dip or pit.

When they are close enough, they are enclosed by a fold of the

membrane, which then pinches shut to form a closed vesicle. As the

vesicle moves into the cell, the molecules are digested and the

product molecules released. When the materials are small and often

liquid particles, endocytosis is known as pinocytosis. When the

materials are large and often solid particles, the process is known as

phagocytosis.

Exocytosis is the transport of materials out of a cell. It is the

exact reverse of endocytosis. The molecules to be transported have to

be enclosed in a membrane vesicle first, usually from the Golgi

apparatus and RER. It then moves to the membrane and fuses with it,

forcing the substances out.

These processes are controlled and require energy.