Biological Membranes: Structure and Function

Biological Membranes: Structure and Function

• Biological membranes are barriers and control permeability.
• They organize cellular functions by localization.
• Membranes facilitate transport, signal detection, and cell-to-cell interactions.

Types of Membranes

• Plasma membranes enclose the cell.
• Nuclear membranes surround the nucleus.
• Mitochondrial inner and outer membranes compartmentalize the mitochondria.
• ER membranes form the endoplasmic reticulum.
• Myelin sheet membranes insulate nerve cell axons.

Chemical Composition

• Membranes consist of lipids, carbohydrates, and proteins.
• Lipids in membranes are amphipathic, having a polar head and a nonpolar tail.

Lipids - Fatty Acids

• Fatty acids are major components with a glycerol 3-phosphate head.
• They are 50% saturated and 50% unsaturated.
• Unsaturated fatty acids include oleic, linoleic, linolenic, and arachidonic acids.
• Unsaturated acids increase membrane fluidity.

Lipids - Glycerophospholipids examples

• Phosphatidylethanolamine, also known as cephalin.
• Phosphatidylcholine, also known as lecithin.

Lipids - Sphingolipids and Cholesterol

• Sphingolipids, like sphingomyelin, cerebroside, and ganglioside, are mainly in the brain.
• Cholesterol is present in animal cells and modulates membrane fluidity.

Proteins

• Integral proteins are embedded within the membrane.
• Peripheral proteins are located on the membrane surface.
• Transmembrane proteins act as receptors for hormones and neurotransmitters.

Carbohydrates

• The carbohydrate coat on the cell surface is called the glycocalyx.
• Glycosylation is the addition of sugar to proteins.
• Glycosylation occurs at the N-glycosidic end of N-acetyl glucosamine to asparagine.
• It can also occur on the O of N-acetyl glucosamine to serine and threonine.

Lipid Bilayer Formation and Fluidity

• Singer and Nicholson proposed the fluid mosaic model.
• Lipids, such as sphingolipids and glycerophospholipids, form the bilayer.
• Proteins and carbohydrates exhibit transitional diffusion.
• Transition temperature (Tm) is the point at which the hydrophobic side chains transition from ordered to disordered.
• Unsaturated fatty acids increase fluidity, while saturated fatty acids require higher Tm.
• Increased fluidity raises permeability and lateral mobility of integral proteins.

Other Membrane Characteristics

• Lipid rafts are dynamic areas rich in cholesterol and saturated fatty acids, often surrounding proteins.
• Caveolae are specialized lipid raft proteins involved in signal transduction, containing caveolin protein.

Red Blood Cell (RBC) Membranes

• RBCs contain specific proteins essential for their function.
• Glycophorin and band 3 protein are integral proteins that serve as RBC receptors, and facilitate the exchange of HCO3- and H2O in the lungs.
• Spectrin, ankyrin, and actin are peripheral proteins that function as cytoskeletal components.

Transport System and Ion Channels

• Membranes allow the entry of molecules necessary for cell function.
• Ion channels are transmembrane proteins with gates.
• Ligand-gated channels open when a specific molecule binds to the receptor; an example is the acetylcholine receptor.
• Voltage-gated channels open or close in response to changes in membrane potential.

Ionophores, Water Channels, and Gap Junctions

• Ionophores act as shuttles for ion movement.
  • Valinomycin is a mobile ion carrier.
  • Gramicidins are channel formers.
• Aquaporins are water channels found in RBCs and kidneys.
• Gap junctions, containing connexin protein, mediate and regulate molecule passage between cells.

Transport Mechanisms

• Passive simple diffusion does not require energy.
• Facilitated diffusion requires a carrier or transport protein.
• Active transport requires energy to move molecules against concentration gradients.

Simple Diffusion

• Simple diffusion necessitates no carrier or energy.
• Water, gases, and sugars pass through the membrane via this process.
• Factors influencing net diffusion include concentration gradient, electrical potential, temperature, and permeability coefficient.

Facilitated Diffusion

• Similar to passive diffusion, yet requires a carrier or transport protein, resulting in a faster rate of diffusion.

Active Transport: Types

• Uniport systems transport one type of molecule.
• Cotransport systems transport two types of molecules simultaneously and include:
  • Symport, where molecules move in the same direction.
  • Antiport, where molecules move in opposite directions.

Transport of Molecules: Exocytosis

• Exocytosis transports antigens, hormones, and molecules that become part of the extracellular matrix.

Transport of Molecules: Endocytosis

• Endocytosis requires energy, Calcium (Ca++) and contractile elements.

Transport of Molecules: Types of Endocytosis

• Phagocytosis occurs.
• Pinocytosis: including fluid-phase and receptor-mediated types, occurs.

Clinical Aspects of Membrane Function

• Aquaporin gene mutations can cause diabetes insipidus.
• Cystic fibrosis, characterized by chronic bacterial respiratory infections due to high chlorine levels in sweat, results from mutations in transmembrane protein of chlorine channels.

Additional Key Points

• Phospholipids arrange as a bilayer, with fatty acid tails being hydrophobic and the phosphate group head being hydrophilic.
• Membranes are a mosaic of proteins and other molecules in a fluid lipid bilayer.
• Diffusion moves molecules from high to low concentration, a "passive" process requiring no energy.
• Facilitated diffusion involves protein channels and is energy-independent.
• Active transport moves molecules against concentration gradients, requiring energy and protein "pumps."

Osmosis

• Osmosis is the diffusion of water.
• High concentration of water to low.
• Selective permiable membrane for only water.

Balance

• Cellur suvrival depends on balancing water.
• hyper, hypo, and isotonic all refer to levels of water balance.