Describe cell membrane and its functions

The cell membrane, also known as the plasma membrane, is a double layer of lipids and proteins that surrounds a cell and separates the cytoplasm (the contents of the cell) from its surrounding environment. It is selectively permeable, which means that it only lets certain molecules enter and exit. It can also control the amount of some substances that go into or out of the cell.

The functions of cell membranes include:

1. Mechanical Structure
   1. Defines/encloses the Cell
   2. Re. Cytoskeleton
   3. Extracellular Matrix
   4. Protection
2. Selective Permeability
3. Active Transport
4. Bulk Transport:
   Exocytosis and Endocytosis
5. Markers & Signalling
   (for communication with other cells & the external environment)
6. Metabolic Activities

More about each of these functions of the cell membrane follows below:

1. Mechanical Structure

Defines/encloses the Cell
A cell membrane encloses and defines the cell. There are different ways to express this, e.g.

The cell membrane maintains the physical integrity of the cell. It's most obvious in the cases of animal cells (because they don't have cell walls) that the cell membrane holds the cell together by enclosing the cytoplasm and organelles within it.
The cell membrane forms a barrier between the inside of the cell and the environment outside the cell - enclosing cytoplasm and any organelles within the cell, and enabling different chemical environments to exist on each side of the cell membrane.
The cell membrane physically separates the intracellular components (e.g. organelles in eukaryotic cells) from the extracellular environment.
The 3 points listed above say much the same thing.

Re. Cytoskeleton
In many cases the cell membrane also helps to hold the cytoskeleton (which is within the cell) in place. This is is achieved by some proteins in the cell membrane attaching to some cytoskeletal fibres and helps to define and maintain the shape of the cell.

Extracellular Matrix
In many cases (but not all, e.g. not in the case of single celled-organisms) the cell membrane interacts with the cell membrane of adjacent cells e.g. to form plant and animal tissues.

Protection
The cell membrane protects the cell from some harmful chemicals in its external environmemt.
It also protects the cell from loss of useful biological macromolecules held within the cell by its plasma membrane.

2. Selective Permeability

The cell membranes that enclose cells (inside the cell wall in the cases of plant cells and prokaryotic cells) are selectively permeable. That is, the structure of these membranes is such that they allow certain particles, incl. e.g. molecules, - but not others - to pass through the membrane, hence into or out of the cell.

(This cell membrane function is one of several functions that facilitate the transport of materials needed for survival of the cell, others include "active transport", "exocytosis" and "endocytosis".)


3. Active Transport

Cell membranes, also known as "plasma membranes", can allow active transport of specific molecules across the cell membrane in either direction, i.e. either into or out of the cell.

That is - cell membranes can allow some particular molecules to move against a concentration gradient e.g. from a lower concentration outside the cell to a higher concentration inside the cell, or vice-versa. Active transport (movement against the concentration gradient) requires, that is it uses, energy.



See the pages about diffusion, osmosis and active transport for more information.


4. Bulk Transport: Exocytosis & Endocytosis

Exocytosis is the process by which a cell moves the contents of secretory vesicles out of the cell via the cell membrane.
Endocytosis is the opposite process by which the contents of secretory vesicles are moved into the cell via the cell membrane.

5. Markers & Signalling

Proteins called surface protein markers embedded in the cell membrane identify the cell, enabling nearby cells to communicate with each other.

Cell membranes often include receptor sites for interaction with specific biochemicals such as certain hormones, neurotransmitters and immune proteins. In this way the cell can recognize and process some signals received from the extracellular environment.


6. Metabolic Activities

Plasma membranes include as part of their structures certain proteins and enzymes that are involved in some of the metabolic processes of the cell.