Membrane Structure & Function (Learning Objectives)

Transcription

1 Membrane Structure & Function (Learning Objectives) Review the basic function and biochemical composition of the plasma membrane. Learn the fluid state of membranes and the movement of its lipids and proteins. Explain the importance of cholesterol in maintaining fluidity of animal cells membranes. Review the mosaics of membrane structures that dictate its function. Summarize the functions of membrane proteins. Compare and contrast movement of small and large molecules across the plasma membrane. Explain the basis of selective membrane permeability and the distinguishing features of substances that can move across freely and those that require protein transporters. Compare and contrast passive transport (diffusion and facilitated diffusion) with active transport. Explain the driving force behind diffusion and osmosis. Anticipate the movement of water into & out of the cells as a function of the concentration of water soluble substances. Explain the role of protein ti transporters t in moving specific molecules l across the membrane down their concentration gradient and uphill against that gradient. Describe the role of ATP and phosphorylation in active transport. Compareand and contrast exocytosis with endocytosis and the three types of endocytosis.

2 MEMBRANE STRUCTURE AND FUNCTION Membranes divide the cell into compartments where different chemical cellular activities can take place.

3 Theplasma membrane is theouter boundary of the cell and is selectively permeable It controls the flow of substances bt into or out of the cell Outside of cell TEM 200,000 Cytoplasm Figure 5.10

4 Membranes are made of phospholipids h id bilayer One hydrophilic head Two hydrophobic tails Hydrophilic head CH 3 N + CH 3 CH 3 Phosphate O group O P O O CH O O C O C O Figure 5.11A Symbol CH CH CH CH 2 2 CH 2 CH 2 CH 2 CH 3 CH 3 Hydrophobic tails

5 The heads of the phospholipid bilayer, face outward and the tails face inward Hydrophilic heads Water Hydrophobic tails Water Figure 5.11B

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

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

8 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 l Cholesterol within the animal cell membrane

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

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

11 Figure 5.13A Enzymes catalyze chemical reactions

12 Receptors receive chemical messages from other cells Messenger molecule Receptor Figure 5.13B Activated molecule l

13 Transporters move substances across the membrane Figure 5.13C ATP

14 Traffic Across Membranes A. Ions and monomers (small molecules) Move physically through the membrane B Macromolecules and large particles B. Macromolecules and large particles move across inside vacuoles & vesicles

15 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: polar and ionic molecules and inorganic ions are assisted by membrane proteins

16 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

17 Fig 8 16 Both diffusion and facilitated diffusion are forms of passive transport of molecules Fig 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.

18 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

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

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

21 Solvation of ionic compounds in water Biology1111/animations/dissolve.html Osmosis p//p / y / osis.htm

22 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 concentration of solute Higher concentration 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

23 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

24 In isotonic solutions Animal cells are normal, but plant cells are limp Isotonic solution Hypotonic solution Hypertonic solution H 2 O H 2 O 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)

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

26 II. Activetransportuses transport 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 P Protein P ATP Phosphate Solute ADP changes shape detaches 1 Solute binding 2 Phosphorylation 3 Transport 4 Protein reversion Figure 5.18

27 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#

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

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

30 Endocytosis can occur in three ways Phagocytosis Pinocytosis Receptor mediated endocytosis Pseudopodium of amoeba Food being ingested Material Plasma membrane bound to receptor proteins LM 230 TEM 54,00 00 Cytoplasm PIT TEM 96,,500 Phagocytosis Pinocytosis Receptor mediated endocytosis Figure 5.19C

31 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 Protein Phospholipid h id outer layer Vesicle Figure 5.20 Plasma membrane Receptor protein Cytoplasm