Membrane Structure and Function

Membrane Models 1925 Gorter and Grendel – extracted phospholipids from rbc’s, noted enough for bilayer, hydrophobic tails, hydrophilic heads 1940’sDanielli 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 Functions to separate and regulate = homeostasis Phospholipids – polar heads, non-polar tails Cholesterol – lipid in animal membranes, regulate fluidity, stiffens and strengthens

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

Function of integral proteins Each membrane has its own set of proteins, according to its function Channel – passage of molecules and ions Carrier – passage of molecules by combining with protein and being carried across membrane ex. Na+, K+, selective Cell recognition – glycoproteins, recognize pathogens, stimulates immune system Receptor proteins – have specific shape for specific molecule to bind. Ex. Signal molecules (hormones) Enzymatic – carry out metabolic reactions directly

Plasma Membrane Selectively, semi, differentially permeable – only certain substances can move across. Free passage = passive transport Need assistance can be passive via a carrier protein need energy = active transport

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

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

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

Tonicity cont. Hypertonic solution – more solute in solution than in cell 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 Only carry specific molecules or ions across the membrane Facilitated diffusion – helping, no energy ex. Glucose and amino acids.

Active Transport Accumulate, low to high concentration 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 Transport of macromolecules Uses energy Exocytosis Golgi body produces vesicles, fuses with membrane, materials (hormones…) released outside of cell Part of cell growth

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

Modification of cell surface Animal cells Animal cells have junctions between their cells Anchoring junctions Mechanically attach adjacent cells Adhesion junctions – intercellular filaments Desmosome – single point of attachment, common in skin cells Sturdy but flexible connection Tight junctions Proteins from plasma membrane attach to each other, zipper like Ex. Barrier tissues, intestines, blood brain barrier

Junctions cont. Gap junction Allows cellular communication 2 identical channel proteins join Strengthens membrane, allows small molecules and ions to pass

Extracellular matrix Nonliving network of polysaccharides and proteins Collagen, elastin, fibronectins and laminins Cartilage (gel), bone (solid)

Plant cells Cell wall Primary (cellulose fibrils) Middle lamella – layer of adhesive substances, holds cells together. Secondary wall – only in few plant cells, forms inside the primary cell wall Plasmodesmata – membrane lined channels, connects cytoplasm of plant cells