Membrane Structure and Function Membrane Models n n

Membrane Structure and Function

Membrane Models n n 1925 Gorter and Grendel – extracted phospholipids from rbc’s, noted enough for bilayer, hydrophobic tails, hydrophilic heads 1940’s. Danielli and Davson – sandwich model, phospholipids between 2 layers of proteins Robertson- Unit membrane model 1972 – Singer and Nicolson – Fluid Mosaic model, proteins are partially or wholly embedded in irregular pattern

Plasma Membrane Structure and Function n Functions to separate and regulate = homeostasis n Phospholipids – polar heads, non-polar tails n Cholesterol – lipid in animal membranes, regulate fluidity, stiffens and strengthens

Membrane proteins n n n Peripheral – inside surface, held by cytoskeleton, structural role, stabilize and shape membrane Integral – embedded in membrane, can move laterally back and forth Transmembrane – have hydrophilic and hydrophobic ends, span entire membrane Glycolipids – phospholipids with a carb chain attached Glycoproteins- carbs on proteins

Carbohydrate Chains n n Carbohydrate chains – only on outside of membrane Glycocalyx – “sugar” coating outside cell in animal cells – protection, adhesion between cells, cell recognition, reception of signal molecules Basis for blood types in humans Plays role in tissue rejection

Fluidity of Membrane n Body temperature – consistency of olive oil n Fluidity dependent on lipid components n Critical to proper functioning n Proteins tend to drift laterally

Function of integral proteins n n Each membrane has its own set of proteins, according to its function Transport q q Channel – passage of molecules and ions Carrier – passage of molecules by combining with protein and being carried across membrane ex. Na+, K+, selective n Enzymatic – carry out metabolic reactions directly n Integrin - binds to ECM to coordinate extracellular and intracellular changes.

Integral proteins continued n Cell to cell recognition – glycoproteins, recognize pathogens, stimulates immune system n Intercellular joining – adjacent membrane proteins may join together as in gap junctions or tight junctions. n Receptor proteins – have specific shape for specific molecule to bind. Ex. Signal molecules (hormones)

Plasma Membrane n n n Selectively, semi, differentially permeable – only certain substances can move across. Free passage = passive transport Need assistance q q q can be passive via a carrier protein Aquaporins – aid in passage of 3 billion water molecules across the membrane per second. need energy = active transport

Passive transport n n n Water, small, non charged molecules (carbon dioxide, oxygen, glycerol and alcohol) Move from a high concentration to a low concentration, following their concentration gradient Carrier proteins – glucose and amino acids, specific for substance it carries (ions and polar molecules)

Diffusion n Movement of molecules, high to low until equilibrium is reached q n Gases can diffuse through the lipid bilayer Solution – solvent and solute q Once equilibrium is met, still move, but not in any one direction

Osmosis n Movement of water across selectively permeable membrane due to concentration differences n Osmotic pressure – pressure that develops in a system due to osmosis.

Tonicity n n Tonicity – strength of solution Isotonic solution – equal concentrations of solute and water q n . 9% of Na. Cl solution is isotonic to rbc Hypotonic solution – solution with lower concentration of solute than in cell q q q Water will follow solute and go in cell, Animal cell – cytolysis, hemolysis Plant cell – turgor pressure

Tonicity cont. n Hypertonic solution – more solute in solution than in cell q q q Animal cells – cell shrink – rbc’s - crenation Plant cell – plasmolysis Animals have built in osmoregulators that allow them to lose salt/water when needed.

Carrier proteins n Only carry specific molecules or ions across the membrane n Facilitated diffusion – helping, no energy ex. Glucose and amino acids.

Active Transport n Accumulate, low to high concentration q q Carrier proteins and ATP are needed Usually find a lot of mitochondria near these membranes Pumps – sodium-potassium pump, Na out, K in Cystic fibrosis – faulty chloride channel

Vesicle formation n n Transport of macromolecules Uses energy Exocytosis Golgi body produces vesicles, fuses with membrane, materials (hormones…) released outside of cell Part of cell growth

Endocytosis n n Take in substances by vesicle Membrane invaginates, pinches off, intracellular vesicle q q q Phagocytosis – large molecules, food, another cell ex. Wbc, amoeba Pinocytosis – small molecules, liquids Receptor mediated endocytosis - specific