Cell walls and membranes The last part of this unit looks at cell walls and membranes. The diagram, on the next slide, lays out the external boundaries of bacteria, plant, and animal cell walls/membranes.

The cell membrane of an animal cell:

All cells need to maintain homeostasis Homeostasis is defined as “relatively constant physical and chemical conditions that organisms maintain” The Plasma membrane that exist in all cells – whether they are bacteria, animal, or plant cells – assist in maintaining homeostasis by: Selective Permeability – certain materials, like water, pass easily through the membrane but other materials do not. An example of this is the Na+ / K+ ion gates found in the membrane. Sodium and Potassium ions are only allowed in, and out, at certain times and through those specific protein gates.

Structure of cellular membranes Membranes are bi-layered lipid layers. As you recall, lipids have a hydrophilic end and a hydrophobic end. In cell membranes, the water loving hydrophilic ends face outward, and inward, with the hydrophobic ends sandwiched in the middle.

Cholesterol is also a component in membranes as it helps the hydrophobic tails from sticking together. Protein channels are also found in the membranes as these regulate the movement of materials in and out of the cells (ex. The Na+/K+ ion gateway is a protein channel).

Plant Cell Walls

Plant cell walls give plants form, structure, and support. a)Middle Lamella – This is found on the very outside of each cell and “ties” together each of the cells – giving support and allowing the movement of material from one cell to the next. b)Primary Cell Wall – This is composed of Pectin, Cellulose microfibers forming cross- linked Glycans. c)Cell membrane – found on the interior of plant cells

Bacterial Cell Walls Bacterial cell walls come in 2 different kinds: Gram Positive and Gram Negative. (Gram staining is used to divide bacteria into the 2 different groups and is a preliminary test. ) Gram Positive bacteria retain the purple color of the initial dye and Gram Negative bacteria are pink colored by the secondary dye used. (see the following picture)

Gram Positive and Gram Negative Staphylococcus aureus (+) and Escherichia coli (-)

The difference between Gram + and Gram – cell walls

-A thick layer of Peptidoglycans (the technical term for a sugar/amino acid polymer) holds the crystal violet dye used in the technique -the crystal violet dye is added first -iodine is added to fix the dye to the specimen. -The specimen is then rapidly “de-colourized” by washing the slide in acetone. This strips off the outer liposaccharide layer (if it exists). -A second “counter stain” of safranin is immediately added then washed off in distilled water. This pinkish dye colours the middle layer. Timing is very critical when doing gram stains, as the acetone can remove the royal violet dye if left on too long – resulting in a pink color after counter staining.