Wednesday, May 13, 2009

[Biology Form 4] Plasma Membrane

How do substances enter and leave a cell? They have to permeate the plasma membrane - a layer that separates the cell from its surrounding. To "permeate" means to "pass through".

The plasma membrane (also called the cell membrane or plasmalemma) is the biological membrane separating the interior of a cell from the outside environment.

It is a semipermeable lipid bilayer found in all cells. It contains a wide variety of biological molecules, primarily proteins and lipids, which are involved in a vast array of cellular processes such as cell adhesion, ion channel conductance and cell signaling.

The plasma membrane also serves as the attachment point for both the intracellular cytoskeleton and, if present, the extracellular cell wall.

The plasma membrane is about 7nm-8nm thick. Proteins and a double layer of phospholipid molecules are the main molecules that make up plasma membrane.

A phospholipid is an amphipathic molecule. It has a hydrophobic tail that is not attracted to water and a hydrophilic head that has an affinity for water.

The heads of the phospholipid molecules face an aquatic environment on the outside of the cell, while internally, the tails face the water-rich cytoplasm.

Since the plasma membrane is made up of lipids, only molecules that dissolve in fats/lipids and are non-polar in nature can pass through the phospholipid bi-layer freely. These molecules include steroid hormones eg: estrogen and testosterone and gases (like oxygen and carbon dioxide).

Other substances are allowed in selectively by the cell membrane. This means that the membrane "chooses" the molecules that can pass through it. To do this, the membrane has several types of proteins acting as passageways for the entry of specific molecules. Without these proteins, certain molecules cannot enter a cell at all.

For example, hydrophilic water-soluble polar molecules such as glucose and amino acids can only be taken into a cell if a special carrier protein for them is present in the membrane. These molecules avoid contact with the lipid bi-layer by passing through such transport proteins that span the membrane. Thus, glucose must bind to a carrier protein first before it can be transported across the membrane.

Some carrier proteins are called ion pumps because they hydrolyse ATP and engage in the active transport of ions such as potassium and sodium into and out of the cell.

Other special proteins also include pore proteins, which allow water and certain ions to enter the cell through them.

In short, the plasma membrane acts as an important barrier between the cell and its environment. It is very selective, allowing only certain molecules to pass through. This nature of the membrane is called "semi-permeable".

Illustration of an Eukaryotic cell membrane

Diagram of the arrangement of amphipathic lipid molecules
to form a lipid bilayer. The yellow polar head groups
separate the grey hydrophobic tails
from the aqueous cytosolic and
extracellular environments.

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