Biology Kevin Dees. Chapter 7 Membrane Structure and Function

Transcription

1 Chapter 7 Membrane Structure and Function

2 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) This means:

3 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, and (Dutch) determined that phospholipids could form bilayers in an aqueous solution

4 and proposed in 1935 a structure of a membrane that was basically a protein sandwich with phospholipid bread But some were still skeptical Especially about the proteins Large molecules Some are amphipathic as well!!!

5 Fluid Mosaic Model In 1972 and proposed that the membrane proteins were scattered, floating in the phospholipid bilayer Hydrophilic portions exposed through the membrane

6 Membrane fluidity The fluid mosaic model allowed for the membranes to be very fluid (flexible) What holds the phospholipid bilayer together?

7 Membrane proteins The portion of the model Many, many different proteins At least 50 different in just RBCs alone!!!!

8 Two broad categories proteins pass through the hydrophobic portions of the phospholipid bilayer proteins attached to the surface of the membrane or to integral proteins

9 Integral protein Peripheral protein Extracellular matrix proteins Cytoskeleton proteins Cell-cell recognition Glycolipids Glycoprotein

10 Functions of membrane proteins 1. Transport passive or active 2. Enzymes catalyze rxns 3. Signal proteins - hormones

11 Functions of membrane proteins 4. cell-cell recognition glycoproteins 5. Intercellular linkages gap junction or tight junctions 6. Attachment cytoskeleton or extracellular matrix

12 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!

13 Membrane transport Selective permeability Transport proteins discovered by Peter Agre in 2003 won Nobel Prize transport of water across plasma membrane

14 Transport can be : requires no expenditure of cellular energy requires cellular energy expense - ATP

15 Passive Transport the tendency of molecules of any substance to spread out evenly into the available space Substances move along a from areas of high concentration to low concentration Net diffusion ceases when in reached The movement does not cease however

16 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

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18 Effects of tonicity on plant and Osmoregulation maintain proper osmotic balance Most animal cells fare best in isotonic environment (not lysed) Most plants fare best in hypotonic environment (not plasmolyzed) animal cells

19 What about a single celled living thing that lives in the water?? Contractile vacuole

20 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

21 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 -

22 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).

23 Sodium-potassium pump classic example of active transport

24 Bulk transport movement of larger molecules proteins, polysaccharides Exocytosis vesicles fuse with plasma membrane of cell and release contents into extracellular space

25 Endocytosis materials enter the cell by formation of vesicles from plasma membrane Three types Phagocytosis cell eating Pinocytosis cell drinking Receptor-mediated endocytosis

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28 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 Biology Kevin Dees