The Transport of Substances Across the Plasma Membrane

The Transport of Substances Across the Plasma Membrane

The plasma membrane or, the cell surface membrane, is made almost

entirely of protein and lipid. The plasma membrane controls the

movement of substances into and out of a cell. It is partially

permeable so some substances cross more easily than others.

The lipids found in the membrane are known as phospholipids.

Phospholipids are fat derivatives in which one fatty acid has been

replaced by a phosphate group and one of several nitrogen-containing

molecules. The phospholipids’ structure is such that it appears to

have a ‘head’ attached to a ‘tail’. The head section of the lipid is

made of a glycerol group which is then attached to an ionised

phosphate group that is the tail. This gives the ‘head’ region of the

lipid to have hydrophilic properties (water loving), where the ‘tail’

section has hydrophobic properties (water hating). As a result of this

contrast between the two regions, when in a water environment, the

‘head’ region will face towards the water while the ‘tail’ region will

be pointing away from the water. In relating this to a plasma membrane

two lines of lipids are present in a membrane, they arrange themselves

such that the hydrophilic ends of each lipid on the first line (the

inside part of the membrane); the phosphate head is attracted to the

fluid within the cell or organelle. The hydrophobic tail is then

facing away from the fluid. The outer line of lipids consists of the

phosphate head being attracted to the water outside of the cell, with

the hydrophobic tail moving away from the water. The resultant of the

alignment of these two lines of lipids is a lipid bilay...

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...imilar to those involved in facilitated diffusion.

If very large molecules or groups of molecules need to enter or exit a

cell, they do so using vesicles. The material to be transported out of

the cell is surrounded by membrane. The vesicle will fuse with the

cell surface membrane and the contents leave. This is called

exocytosis. Materials entering the cell can do so when the plasma

membrane encloses and thus surrounds the material. The membrane seals

off to form a vesicle, which can then move into the cell. This is

endocytosis.

The transportation of substances through the plasma membrane is very

specific and the procedures that facilitate diffusion are what allow

the different types of d transportation to take place. The

transportation of substances is fundamental to life processes and

therefore to existence.