Structure And Function

In general, prokaryotes are made up of a cell wall, a cell membrane, and cytoplasm that has ribosomes, DNA, and small molecules and ions. Recall that prokaryotes do not have membrane-bound organelles. Additional distinctive structures help some bacteria adapt to their own niches.

Cell Wall

Most members of the domains Bacteria and Archaea have a cell wall. The cell wall gives a cell its shape and protects the cell from toxic substances. The cell walls of many disease-causing bacteria have structures that add to the disease-causing abilities of the organisms. Bacterial cell walls are made of peptidoglycan, not cellulose, which makes up plant cell walls. Archaeal cell walls do not have peptidoglycan; instead, some of them contain pseudomurein, a compound made of unusual lipids and amino acids.

Cell Membrane and Cytoplasm

Just inside the cell wall, prokaryotes have a cell membrane, or plasma membrane. Both bacterial and archaeal cell membranes are lipid bilayers that have proteins. However, the lipids and proteins of archaeal cell walls differ from those of bacterial cell walls.

The cell membrane acts as a selective barrier by controlling the molecules that move into and out of the cell. In this way, the cell membrane helps keep the internal environment of the organism stable. The cell membrane is also the place where many important metabolic functions occur. For example, enzymes that catalyze the reactions of cellular respiration are found in the bilayer.

Photosynthetic bacteria have cell membranes whose internal foldings are called thylakoids (THIE-luh-KOYDZ). These foldings are much like the stacked membranes of plant chloroplasts. Like thylakoids in plant chloroplasts, bacterial thylakoids have photo-synthetic pigments that trap light energy.

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