BIOLOGI SEL

components and connections between cells help coordinate cellular activities

Having crisscrossed the interior of the cell to explore its interior

components, we complete our tour of the cell by returning

to the surface of this microscopic world, where there

are additional structures with important functions. The

plasma membrane is usually regarded as the boundary of the

living cell, but most cells synthesize and secrete materials

that are external to the plasma membrane. Although these

materials and the structures they form are outside the cell,

their study is central to cell biology because they are involved

in a great many cellular functions.

Cell Walls of Plants

The cell wall is an extracellular structure of plant cells that

distinguishes them from animal cells. The wall protects the

plant cell, maintains its shape, and prevents excessive uptake

of water. On the level of the whole plant, the strong walls of

specialized cells hold the plant up against the force of gravity.

Prokaryotes, fungi, and some protists also have cell walls, but

we will postpone discussion of them until Unit Five.

Plant cell walls are much thicker than the plasma membrane,

ranging from O.1llm to several micrometers. The exact

chemical composition ofthe wall varies from species to species

and even from one cell type to another in the same plant, but

the basic design of the wall is consistent. Microflbrils made of

the polysaccharide cellulose (see Figure 5.8) are synthesized by

an enzyme called cellulose synthase and secreted to the extra·

cellular space, where they become embedded in a matrix of

other polysaccharides and proteins. This combination of materials,

strong fibers in a "ground substance~ (matrix), is the

same basic architectural design found in steel-reinforced concrete

and in fiberglass.

Sumber : Buku Biologi Edisi 8