1. The cell wall


Every cell is delimited from its environment by a

plasma membrane but many cells also posses a cell wall which lies out side the membrane.

A complex outer cell wall made of the carbohydrates

cellulose and peotin is a characteristics of most plant cells. It forms a permeable framework which strengthens and protects plant tissues. The many layers of the cell wall develop as the plant grows. first the extensible, cellulose primary wall is laid down

between the plasma membrane and the middle

lamella (an intercellular cement of pectin found in plant tissues). Then, as cell growth ceases, the

layered secondary wall is deposited on the inner

surface of the primary wall. The cellulose fibres of

each successive layer lie at different angles,

increasing the strength of the cell wall in a manner

analogous to the reinforcing effect of fibre layers

placed at different angles In the wall of a car tyre.

Lignin may be added to make a rigid secondary

wall when the greater strength of woody tissue is required.

Animal cells do not have true cell walls but


some are enveloped, wholly or partially, by wall-

like protective pelicles Such as the chitin pellicles

of insects and arthropods and the keratin layers of

some vertebrate cells.

The cells of tissues are often bound together by an intercellular matrix or layer which may serve several purposes. Plant cells are cemented together by the middle lamellae, and the matrix of

bone and cartilage tissues contributes largely to the supporting function of these tissues as a whole.2. The Plasma Membrane (Cell Membrane)

All cells are bounded by a lipoprotein plasma

membrane about 10nm thick. This flexible membrane surrounds the cell, controlling the entry

and exit of materials by means of its selective


The lipid constituents of the membrane (phospholipids, glycolipids and the sterol

cholesterol) play a structural role and their relative

amounts are very variable. The phospholipid

molecules are usually arranged in two layers sop

that the non-polar, water insoluble fatty acid tails

lie within the membrane and the polar, water-

soluble molecular heads make up the internal and

external surface layer. The protein molecules act

largely as enzymes and are responsible for specific functions of the membrane such as molecular

transport. Many different types of protein are arranged irregularly on or within the

membrane surtace. This two-layered lipid structure

peppered with varying amounts of protein is often

called a ‘unit’ membrane and it is characteristic of

membranes found in the cell.

The plasma membrane may form a simple cellular

envelope or it may be adapted in several ways.

Absorptive cells develop finger-like projections of the plasma membrane, microvilll, which greatly

increase their Surface area. The protective,

insulating myelin sheath of nerve fibres is formed

from the plasma membranes of Schwann cells

which extend to wrap spirally round the nerve

axon. The plasma membranes of neighbouring cells

in a tissue may develbp special areas of

attachment such as desmosome which involves

thickening of the adjacent membrane from which

intracellular filaments radiate.

Updated: July 24, 2020 — 11:53 pm

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