Chapter 7 Membrane Structure and Function. The plasma membrane surrounds the living cells from their surroundings.

Transcription

1 Chapter 7 Membrane Structure and Function The plasma membrane surrounds the living cells from their surroundings. Only 8 nm thick (8,000 to equal the thickness of a sheet of paper) Controls passage of materials in and out of cell Selectively permeable (selective permeability) Some substances cross easier than others Origins of life on earth?? Membranes could have formed that contained a solution while still allowing the passage of certain materials 1

2 Brief history of the discovery of the structure of the plasma membrane In 1915, chemical analysis was done on RBCs and the basic components were discovered no structure! Phospholipids (amphipathic molecules remember?!!) Proteins In 1925, Gorter and Grendel (Dutch) determined that phospholipids could form bilayers in an aqueous solution Davson and Danelli porposed in 1935 a structure of a membrane that was basically a protein sandwich with phospholipid bread Their work was initially supported by electron microscope images in the 1950s But some were still skeptical Especially about the proteins Large molecules Some are amphipathic as well!!! 2

3 In 1972, Singer and Nicolson proposed that the membrane proteins were scattered, floating in the phospholipid bilayer Hydrophilic portions exposed through the membrane Fluid Mosaic Model Membrane fluidity The fluid mosaic model allowed for the membranes to be very fluid The phospholipid bilayer held together by hydrophobic interactions Membrane proteins allowed to drift slowly Phospholipids also drift laterally very rapidly; rarely flip-flop 3

4 Membrane fluidity The phospholipid bilayer s fluidity is maintained by: unsaturated hydrocarbon tails in many membranes cholesterol in animal cells; reduces fluidity and acts as a temperature buffer Membrane proteins The mosaic portion of the model Many, many different proteins At least 50 different in just RBCs alone!!!! Two broad categories Integral proteins pass through the hydrophobic portions of the phospholipid bilayer Peripheral proteins attached to the surface of the membrane or to integral proteins 4

5 Integral protein Peripheral protein Extracellular matrix proteins Cytoskeleton proteins Cell-cell recognition Glycolipids Glycoprotein Functions of membrane proteins 1. Transport passive or active 2. Enzymes catalyze rxns 3. Signal proteins - hormones 5

6 Functions of membrane proteins 4. cell-cell recognition glycoproteins 5. Intercellular linkages gap junction or tight junctions 6. Attachment cytoskeleton or extracellular matrix Synthesis of membranes ER to Golgi to Vesicle to Membrane Inside of the membrane in the ER will become the outside of the cell membrane! 6

7 Membrane transport Selective permeability Transport proteins aquaporins discovered by Peter Agre in 2003 won Nobel Prize transport of water across plasma membrane Transport can be Passive requires no expenditure of cellular energy Active requires cellular energy expense -ATP Passive Transport Diffusion the tendency of molecules of any substance to spread out evenly into the available space Substances move along a concentration gradient from areas of high concentration to low concentration Net diffusion ceases when equilibrium in reached The movement does not cease however 7

8 Osmosis diffusion of water Solute abundance in solutions can affect the movement of water Tonicity the ability of a solution to cause a cell to gain or lose water. Hypotonic solution less solute (more water) Hypertonic solution more solute (less water) Isotonic solution equal concentrations of solute Hypotonic solution Net water movement Hypertonic solution 8

9 Effects of tonicity on plant and animal cells Osmoregulation maintain proper osmotic balance Most animal cells fare best in isotonic environment (not lysed) Most plants fare best in hypotonic environment (not plasmolyzed) What about a single celled living thing that lives in the water?? Contractile vacuole 9

10 So, if particles can move passively along a concentration gradient how do they cross a plasma membrane? Diffusion through the phospholipid bilayer Hydrophobic molecules Very small uncharged particles O 2 for example So, if particles can move passively along a concentration gradient how do they cross a plasma membrane? Facilitated diffusion passively with the help of transport proteins Channel protein Carrier protein - 10

11 There are instances where particles need to be transported backwards along the concentration gradient; from low concentration to high concentration. This type of transport is known as active transport because it requires the cell to expend cellular energy (ATP). Sodium-potassium pump classic example of active transport 11

12 Bulk transport movement of larger molecules proteins, polysaccharides Exocytosis vesicles fuse with plasma membrane of cell and release contents into extracellular space Endocytosis materials enter the cell by formation of vesicles from plasma membrane Three types Phagocytosis cell eating Pinocytosis cell drinking Receptor-mediated endocytosis 12

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14 LDL in blood low density lipoproteins (transport form of cholesterol) Hypercholesterolemia LDLs are ligands whch will enter the cell after attaching to a receptor protein. Genetic disposition for defective or missing receptor proteins. Causes high levels of LDL in blood (they cannot enter the cells) and may lead to deposits in the walls of blood vessels 14