Biological Membranes and Fluidity

Study Notes

Membranes are sheets that protect hydrophobic regions of phospholipids from aqueous environments.
Membranes are impermeable to most water-soluble molecules.
Proteins can be inserted into the amphipathic bilayer to form an integral part of the membrane.
Proteins are connected by hydrophobic regions traversing the hydrophobic core of the bilayer and have hydrophilic regions protruding at the inside and outside faces of the membrane.
Proteins include enzymes, transport proteins, structural proteins, antigens, and receptors for various molecules.
The fluid mosaic model describes the lipid bilayer in which protein molecules are embedded.
The fluid mosaic model is the most acceptable model of the plasma membrane.
The cell membrane is an asymmetric structure with irregular distribution of proteins.

The fluidity of the membrane is highly dependent upon the lipid composition of the membrane and the presence of cholesterol.
Longer, more saturated fatty acid chains make the membrane less fluid.
The presence of unsaturated fatty acid chains, tends to increase the fluidity and decrease the compactness of the side chain packing.
Increasing the cholesterol/phospholipid ratio decreases fluidity.
Membranes are fluid, allowing movement of the hydrocarbon chains in the lipid bilayer.
High cholesterol to phospholipid ratios abolish fluidity.

Transport of small molecules can occur via passive diffusion or active transport.
Transport of large molecules can occur via endocytosis or exocytosis.
Passive diffusion occurs when molecules spontaneously move toward equilibrium.
Simple diffusion occurs when small molecules, like gases enter the cell down an electrochemical gradient across the membrane.
Passive transport is always down an electrochemical gradient, toward equilibrium.
Facilitated diffusion occurs when molecules diffuse down electrochemical gradients across membranes, with the aid of carrier proteins.
Many facilitated diffusion systems are stereospecific and require no metabolic energy, like simple diffusion.
"Ping-Pong" mechanisms occur in facilitated diffusion.
Active transport occurs when molecules are transported away from thermodynamic equilibrium.
Energy is required for active transport (ATP).
Energy is required to change the conformation or the nature of transporter proteins in active transport.
Membrane transport systems can be described functionally, based on the number of molecules moved and the direction of movement across the bilayer.
Na+- sugar
Na+-amino acid
Na+ in and Ca2 + out

Endocytosis involves an invagination of the membrane to engulf a substance.
Endocytosis requires energy (hydrolysis of ATP), calcium in extracellular fluids, and contractile elements in the cell (microfilaments).
Phagocytosis occurs in macrophages and granulocytes and involves the engulfment of large particles such as viruses, bacteria, cells, or debris.
Pinocytosis is a property of all cells and leads to the cellular uptake of fluid and its contents.
Exocytosis is the release of large molecules by most cells.
Exocytosis requires a signal, often a hormone and a change in calcium concentrations.
Molecules released by exocytosis include antigens, collagen, glycosaminoglycans, and hormones such as insulin.