Membrane Structure and Function

1. Membrane Structure and Function

2. 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

3. 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

4. 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

5. 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

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

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

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

9. 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 • Aquaporins – aid in passage of 3 billion water molecules across the membrane per second. • need energy = active transport

10. 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)

11. 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

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

13. 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

14. 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.

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

16. 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

17. 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

18. 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