Membrane Structure & Function (Outline)

Transcription

1 Membrane Structure & Function (Outline) Basic function, biochemical composition, and physical fluid state Role of cholesterol for animal cells membranes. Summarize the functions of membrane proteins. Traffic of small and large molecules across the plasma membrane. Chemical and physical basis of selective membrane permeability Passive (diffusion and facilitated diffusion) and active transport. Osmosis (passive transport of water) Role of protein transporters in passive and active transport Role of ATP and phosphorylation in active transport. Movement of macromolecules and particles in and out of the cell

2 MEMBRANE STRUCTURE AND FUNCTION Membranes divide the cell into compartments The plasma membrane: outer boundary of the cell selectively permeable (controls the flow of substances into or out of the cell)

3 Membranes are made of phospholipids bilayer One hydrophilic head Two hydrophobic tails Hydrophilic head + N CH Phosphate 2 O group O P O O CH O O C O C O CH 3 CH 3 CH 3 Hydrophilic heads Hydrophobic tails Water Water CH 3 CH CH Symbol CH 3 Hydrophobic tails

4 In addition to phospholipids, membranes contain proteins that determine the function of the membrane Hydrophilic region of protein Phospholipid bilayer Hydrophobic region of protein

5 Cholesterol is a steroid lipid with a carbon skeleton consisting of four fused carbon rings present in plasma membranes of animal cells

6 Cholesterol is wedged between phospholipid molecules in the plasma membrane of animals cells. At warm temperatures, it restrains the movement of phospholipids and reduces fluidity. At cool temperatures, it maintains fluidity by preventing tight packing. Cholesterol Cholesterol within the animal cell membrane

7 A fluid mosaic of phospholipids and proteins Membrane fluidity affects activity of membrane-bound enzymes. Fibers of the extracellular matrix Carbohydrate (of glycoprotein) Glycoprotein Plasma membrane Glycolipid Phospholipid Figure 5.12 Microfilaments of cytoskeleton Cholesterol Proteins Cytoplasm

8 Functions of plasma membrane proteins Enzymes Signal ATP Receptor Transport Enzymatic activity Signal transduction

9 Receptors receive chemical messages from other cells Messenger molecule Receptor Activated molecule

10 Traffic Across Membranes A. Non-polar, polar, ions and monomers (small molecules) Move physically through the membrane B. Macromolecules and large particles move across inside vacuoles & vesicles

11 Traffic of ions and monomers Selective permeability depends on interaction of that molecule with the hydrophobic core and presence of specific proteins non-polar molecules can pass unassisted polar and ionic molecules and inorganic ions are assisted by membrane proteins

12 Identify, by number, the non-polar molecules or structures. Do they pass through the plasma membrane unassisted? What about the remaining molecules? Na

13 Traffic of ions and monomers Selective permeability depends on interaction of that molecule with the hydrophobic core and presence of specific proteins non-polar molecules can pass: hydrocarbons, CO 2, and O 2 polar and ionic molecules and inorganic ions are assisted by membrane proteins nutrients (monomers of sugars and amino acids) and metabolic waste products Na +, K +, Ca 2+, and Cl -

14 Both diffusion and facilitated diffusion are forms of passive transport of molecules down their concentration gradient, while active transport requires an investment of energy to move molecules against their concentration gradient.

15 Movement across membranes I. Passive transport From an area of high concentration to one with lower concentration, down a concentration gradient, no ATP required II. Active transport From an area of low concentration to one with higher concentration, requires ATP

16 I. Passive transport is diffusion across a membrane - without work by the cell - Spreading from areas of high concentration to areas of low concentration Molecules of dye Membrane Equilibrium Figure 5.14A Equilibrium Figure 5.14B

17 I. Passive Transport 1. Simple diffusion (gases & hydrocarbons) 2. Osmosis diffusion of solvent (H 2 O) 3. Facilitated diffusion (via protein transporters)

18 Solvation of ionic compounds in water Biology1111/animations/dissolve.html

19 Osmosis is the diffusion of water across a membrane Water travels from a solution of lower solute concentration to one of higher solute concentration Lower Higher concentration concentration of solute of solute Equal concentration of solute Solute molecule H 2 O Selectively permeable membrane Water molecule Figure 5.16 Net flow of water Solute molecule with cluster of water molecules

20 Water balance between cells and their surroundings is crucial to organisms Osmosis causes cells to shrink in hypertonic solutions swell in hypotonic solutions not change in isotonic solutions

21 In isotonic solutions Animal cells are normal, but plant cells are limp Isotonic solution H 2 O H 2 O Hypotonic solution Hypertonic solution H 2 O H 2 O Animal cell (1) Normal (2) Lysed (3) Shriveled H 2 O H 2 O H 2 O Plasma H 2 O membrane Plant cell Figure 5.17 (4) Flaccid (5) Turgid (6) Shriveled (plasmolyzed)

22 Transport proteins may facilitate diffusion across membranes by providing passages or channels Solute molecule Figure 5.15 Transport protein

23 II. Active transport uses cellular energy Transport proteins can move solutes against a concentration gradient ATP provides a phosphate group to change the shape of the transporter protein Transport protein P ATP P Protein Phosphate P Solute ADP changes shape detaches 1 Solute binding 2 Phosphorylation 3 Transport 4 Protein reversion Figure 5.18

24 B. Transport of large macromolecules across plasma membrane (Exocytosis and Endocytosis) Uptake of macromolecules is known as endocytosis Secretion or excretion of macromolecules is known as exocytosis e.g. insulin (protein hormone) by pancreatic cells. Endocytosis and exocytosis r6/animations.html#

25 In exocytosis, a vesicle fuses with the membrane and expels its contents Fluid outside cell Vesicle Protein Figure 5.19A Cytoplasm

26 In endocytosis, a membrane vesicle folds inward enclosing material from the outside Vesicle forming Figure 5.19B

27 Endocytosis can occur in three ways Phagocytosis Pinocytosis Receptor-mediated endocytosis Pseudopodium of amoeba Food being ingested Plasma membrane Material bound to receptor proteins LM 230 Cytoplasm Phagocytosis Pinocytosis Receptor-mediated endocytosis TEM 54,000 PIT TEM 96,500 Figure 5.19C

28 Faulty membranes can overload the blood with cholesterol Harmful levels of cholesterol can accumulate in the blood if membranes lack cholesterol receptors LDL particle Cholesterol Phospholipid outer layer Vesicle Protein Figure 5.20 Plasma membrane Receptor protein Cytoplasm