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.