Membranes And Their Functions Essay

1780 words - 7 pages

Membranes and Their Functions

Membranes form boundaries both around the cell (the plasma membrane)
and around distinct sub cellular compartments (e.g. nucleus,
mitochondria, lysosomes, etc.). They act as selectively permeable
barriers allowing the inside environment of the cell or the organelle
to differ from that outside. Membranes are involved in signaling
processes; they contain specified receptors for external stimuli and
are involved in both chemical and electrical signal generation. All
membranes contain two basic components: lipids (mainly phospholipids)
and proteins. Some membranes also contain carbohydrate. The
decomposition of lipid, protein, and carbohydrates vary from one
membrane to another.

The cell membrane's main function is to serve as a boundary between
the cell and its environment. It consists of a bimolecular
phospholipid layer with inwardly directed hydrophobic tails and a
variety of protein molecules with an irregular arrangement. Some
proteins occur on the surface of the phospholipid layer and are called
peripheral proteins. These normally act as antigens due to the
glycolipids that extend from the protein e.g. the human ABO blood
system. The glycolipids also help to make the membrane more stable.
The glycoproteins also allows recognition of substances such as
hormones or neurotransmitters as proteins have very specified shapes
and is thus ideal as receptor cells for chemical signaling. It also
means that they can recognize foreign antigens and attack them by the
immune system. Those that extend into the layer are called integral
proteins and usually act as enzymes e.g. microvilli contains digestive
enzymes in its membrane. The transmembrane protein that extends
completely across the layer provides support for the structure. These
proteins also assist the active transport of materials across the
membrane and could act as energy transducers or electron carriers.
Also contained in the membrane in between the hydrophobic tails are
steroids; cholesterol, which makes the membrane fluid at higher
temperatures but more fluid at lower temperatures so as to maintain
the structure of the membrane (more stable). It also acts as a plug,
reducing even further the escape or entry of polar molecules through
the membrane. The channel proteins in the membrane are involved in the
selective transport of polar molecules and ions across the membrane
e.g. water.

In short, phospholipids provide the basic structure of membranes and
restrict the entry and exit of polar molecules and ions. However, the
main function of the membrane is to obtain nutrients, excrete waste
substances, secrete useful substances, and generate the ionic
gradients essential for nervous and muscular activity and to maintain
a suitable pH and ionic concentration within the cell for enzyme

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