Membrane Fluidity: The plasma membrane is a fluid combination of phospholipids, cholesterol, and proteins. This “elbow room” helps to maintain fluidity in the membrane at temperatures at which membranes with saturated fatty acid tails in their phospholipids would “freeze” or solidify.

The plasma membrane is an extremely pliable structure composed of 2 layers of back-to-back phospholipids (a “bilayer”). Cholesterol is also present between the phospholipids, which contributes to the fluidity of the membrane. There are various proteins embedded within the membrane that have a variety of functions.

Phospholipids are the most abundant type of lipid found in the membrane. Phospholipids are made up of two layers, the outer and inner layers. The inside layer is made of hydrophobic fatty acid tails, while the outer layer is made up of hydrophilic polar heads that are pointed toward the water.

When cellular membranes form, phospholipids assemble into two layers because of these hydrophilic and hydrophobic properties. The phosphate heads in each layer face the aqueous or watery environment on either side, and the tails hide away from the water between the layers of heads, because they are hydrophobic.

Phospholipid bilayers are critical components of cell membranes. The lipid bilayer acts as a barrier to the passage of molecules and ions into and out of the cell. However, an important function of the cell membrane is to allow selective passage of certain substances into and out of cells.

pH also has a large impact on the function of proteins. The second part showed that temperature has a large effect on the efficiency of proteins and the permeability of a cell membrane. The last section proved that a decrease in pH also denatures proteins and limits the effect of the membrane.

Three factors affect the permeability of a cell membrane:

• heat.
• ethanol.
• pH.

Although ions and most polar molecules cannot diffuse across a lipid bilayer, many such molecules (such as glucose) are able to cross cell membranes. Such transport proteins determine the selective permeability of cell membranes and thus play a critical role in membrane function.

Selective permeability is a property of cellular membranes that only allows certain molecules to enter or exit the cell. This is important for the cell to maintain its internal order irrespective of the changes to the environment.

The term, selectively permeable, refers to the fact that the membrane allows some substances or molecules to enter the cells, and others to be prevented from entering. Selectively permeable membranes are important in maintaining homeostasis, fluid and electrolyte balance, and cellular health.

The membrane is selectively permeable because substances do not cross it indiscriminately. Some molecules, such as hydrocarbons and oxygen can cross the membrane. Many large molecules (such as glucose and other sugars) cannot. Water can pass through between the lipids.

Explanation: Glucose cannot move across a cell membrane via simple diffusion because it is simple large and is directly rejected by the hydrophobic tails. Instead it passes across via facilitated diffusion which involves molecules moving through the membrane by passing through channel proteins.

Semipermeable membrane is a type of biological or synthetic, polymeric membrane that will allow certain molecules or ions to pass through it by diffusion—or occasionally by more specialized processes of facilitated diffusion, passive transport or active transport.

The salt ions can not pass through the membrane. The net flow of solvent molecules through a semipermeable membrane from a pure solvent (in this cause deionized water) to a more concentrated solution is called osmosis.