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 surfaceAnimal 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