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Phospholipids are fat derivatives in which one fatty acid has been
replaced by a phosphate group and one of several nitrogen-containing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 ...view middle of the document...
membrane, as well as lipids, includes several proteins; the proteins
that are within the membrane are found buried or embedded into the
lipid bilayer. These proteins include enzymes, receptors and antigens.
There are four methods of transportation both in and out of a cell;
diffusion, facilitated diffusion, osmosis, and active transport.
Fick's law is used to measure the rate of diffusion:
Diffusion is the movement of atoms, molecules or ions from a region of
high concentration to a region of low concentration (down the
concentration gradient²). The energy for this to occur comes directly
from the particle itself; this is defined as passive (not requiring
energy). The reason for it not requiring energy from another substance
is the fact that the particle is in constant movement and is able to
use its kinetic energy for diffusion. Diffusion occurs in the cell
membrane where the membrane is fully permeable. In the case of the
plasma membrane, the membrane is permeable to non-polar substances
such as steroids and glycerol, as well as oxygen and carbon dioxide in
solution. Diffusion can also occur through pores in the membrane.
Water diffuses through these protein lined pores or can diffuse
through spaces between the phospholipid molecules. This situation is
only available to those molecules, however, that are a ‘regulation’
size; meaning that the particles must be small enough to have the
ability to travel through the small pores and thus diffuse.
In situations where a charged particle or large molecules are to be
transported across the membrane which could be soluble or insoluble in
lipids and is not able to diffuse through normal diffusion, molecules
are able to do so through another form of diffusion called facilitated
diffusion. This is where a large molecule is allowed to mover through
a protein- lined pore; the movement of these substances requires two
proteins: a channel protein and a carrier protein. The channel
proteins line a water-filled pore in the membrane so water-soluble
molecules can easily pass through. Different channels allow different
substances to pass through (the channels are selective). Some channels
are gated (they will only open when appropriately...