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Functions of the Cell Membrane

Functions of the Cell Membrane

Both eukaryotic cells, including plant cells and animal cells, and prokaryotic cells, e.g. bacteria, are enclosed by a cell membrane.

A cell membrane (see the simple diagram on the right) is a thin structure that is also known as the plasma membrane.

The main functions of the cell membrane are:

  • to maintain the physical integrity of the cell - that is to mechanically enclose the contents of the cell, and also
  • to control the movement of particles e.g. ions or molecules, into and out of the cell.

Lists of functions of a cell membrane ( = plasma membrane) can include various numbers of functions because particles can move across the membrane in several different ways, including diffusion via ion channels, active transport via membrane pumps, endocytosis (into the cell) and exocytosis (out of the cell). Some lists count these different transport mechanisms as different functions while other lists count "transport of materials" as one function of the cell membrane.


Table of Functions of the Cell Membrane
:

Functions of the Cell Membrane

1.

Mechanical Structure (a) 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.

(b) 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.

(c) 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.

(c) 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 and 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
(for communication with other cells & the external environment)

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.

The above is not a complete list of functions of the cell membrane (= "functions of the plasma membrane"). The cell membrane is probably the most multifunctional cellular structure and has many functions that can be described in different levels of detail. This list is suitable for many introductory cell biology courses.


Video Demo of Cell Membrane Functions:

The following video-clip lasts just over 2 minutes and has no sound. Press the "Play" button for a short animation of the structures and functions of a general plasma membrane (cell membrane). In order of appearance the features shown are:

  • The hydrophilic head and hydrophobic tails of the phospholipid molecules that form the phospholipid bilayer of the cell membrane - forming its physical / mechanical structure.
  • The ion channels responsible for the selective permeability of the cell membrane.
  • Membrane pumps that enable active transport of specific molecules across the cell membrane
  • Carrier proteins and receptor proteins that transport specific substance(s) across the cell membrane and receive (chemical) signals from outside the cell that tell the cell to take a specific action, respectively.
  • Cholesterol molecules and carbohydrate chains that support the structure of the membrane and its attachment to other tissues, respectively.
  • Surface protein markers that identify the cell.
  • Animation of endocytosis / exocytosis
    (can't tell which because it's not clear if the vesicle is moving into or out of the cell - i.e. if the outside of the cell is above or below the cell membrane shown).


Remember
: The cell membrane (also called the "plasma membrane") is part of the cell's structure. It is not a (type of ) cell.

See also movement across membranes in biology (living organisms).

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