Cell walls can vary in composition based on what organism is being discussed. In the case of plant cells, cellulose is the primary component. Bacteria use peptidolgycan in their cell walls, and fungi use chitin.

The cell wall is composed of cellulose and encloses a plant cell in a rigid framework. Plant cells have both cell membranes and cell walls. Plant cell walls cannot be digested by humans due to the beta glycosidic bond in its structure. The cell membrane forms the outer boundary of a cell and isolates cell contents from the environment. The cell membrane regulates movement of materials into and out of the cell. The cell membrane is also referred to as the plasma membrane. The nuclear envelope regulates movement into and out of the nucleus. The cytoskeleton gives shape and support to cells and is typically involved in cell movement.

The most abundant molecule in bacterial cell walls is peptidoglycan. Recall that peptidoglycan contains repeating NAG-NAM units, which are made from glucose. In addition, peptidoglycan also has peptides attached to these oligosaccharides. These units form bonds with each other to create a strong and a robust cell wall. Peptidoglycan is synthesized in cytoplasm and on cell membrane before finally being transported to the cell wall.

Recall that phosphate head groups are found on phospholipids, which make up cell membranes (not cell walls).

Gram stain is a laboratory technique used to distinguish bacteria. Gram positive and Gram negative bacteria are distinguished based on their cell walls. Gram positive bacteria have a single thick cell wall (outside its cell membrane). Upon Gram staining, these bacteria will hold the Gram stain because of their thick cell wall. The Gram negative bacteria also have only one cell wall; however, it is a lot thinner and is sandwiched between two cell membranes. The thin nature of the cell wall makes it easier for the Gram stain to leak out of the bacterial cell.

Cell walls are robust structures found in most bacteria, fungi, and plants. Gram stain depends on the properties of the cell wall. Some bacteria don’t stain well with Gram stain (such as Mycobacterium that causes tuberculosis) because of unique cell walls (not lack thereof). Fungal and plant cell walls are also made up of polysaccharides. Fungal cell walls contain a special molecule called chitin whereas plant cell walls contain cellulose. Recall that plants obtain energy from sunlight via photosynthesis; therefore, photosynthetic organisms do contain cell walls.

One of the biggest function of a cell wall is to prevent cellular swelling. Recall that a cell placed in a hypoosmolar solution will swell due to the influx of water; however, this cellular swelling will not occur in a cell containing cell wall because the robust cell wall prevents contents of the cell from expanding, thereby preventing cellular lysis.

Cellulose is a polymer of glucose linked by beta (1-4) linkages. Humans lack the enzyme that catalyzes the cleavage of this bond; thus cellulose (dietary fiber) is indigestible and passes through the digestive system unchanged.