The Plant Cell Wall
The presence of a cell wall above all other characteristics distinguishes plant cells from animal cells. It presence is the basis of many of the characteristics of plants are organisms. Cell walls are not merely outer, inactive covering of the plant cell itself but are critical in function such as:
a) Prevents enlargement of the plant cell.
b) Also play important roles in the absorption, transport and secretion of substances in plants.
Components of the Cell Wall
The most characteristic component of plant cell walls is cellulose, which largely determines their architecture. Cellulose is made of repeating molecules of glucose attached end to end in a
b(1-4) linkage. These long thin cellulose molecules are united into a "Microfibril". These microfibrils are arranged in a very regular, ordered arrangement and because of this the exhibit almost "crystalline" properties. These crystalline regions of the microfibrils are known as micelles. The microfibrils wind together to form fine threads which may coil around one another like a cable. Each "cable" is called a "Macrofibril".(Aside) (Cellulose molecules wound in this manner are as strong as an equivalent thickness of steel.)
The cellulose framework of the wall is interpenetrated by a cross-linked "matrix" of non-cellulose molecules.
Some of these are polysaccharides called:
a) Hemicellulloses
b) Pectic substances e.g. "Pectin".
Other substances found in some plant cell walls are:
Lignin - a very strong ridged molecule that is found in cells which have a strong supporting function.
Suberin Cutin Waxes- Fatty substances which are commonly found in the cell walls of cells on the outside surface of the plant, and act to water proof the outer surface of the plant to prevent evaporation and dehydration.
Animal cells lack a rigid cell wall, but most secrete and are embedded in a sticky layer of glycoproteins called the "Extracellular Matrix".
The Extracellular Matrix (ECM) is an elaborate covering outside animal cell membranes. The major ingredients of the are glycoproteins secreted by the cells. The most abundant glycoprotein in the ECM is collagen which forms strong fibers outside the cell.
This layer helps to hold cells together and can also have a protective and supportive function.The diagram below shows some of the components of this extracellular matrix.
Plant cells are attached by their cell walls but are not isolated from one another. There are small pores called "Plasmodesmata" which provide a conection between the cytoplasm of one cell and the cell adjacent to it.
Animal cells have several type of cell junctions.
Tight junctions: these bind cells togetherforming a leakproof sheet. Such a sheet of tissue lines the digestive tract and prevents the contents from leaking into the surrounding tissues.
Anchoring junctions: attaches adjacent cells to each other or to the extracellular matrix. Anchoring junctions rivet cells together with cytoskeletal fibers but still allow materials to pass along the spaces between cells.
Communicating junctions: are channels similar to the plasmodesmata in plant cells. They allow water and other small molecules to flow between neighboring cells.
