MEMBRANE STRUCTURE AND FUNCTION

Transcription

1 MEMBRANE STRUCTURE AND FUNCTION

2 2.4.2 Membranes organize the chemical activities of cells Membranes provide structural order for metabolism Form most of the cell's organelles Compartmentalize chemical reactions The plasma membrane forms a boundary between a living cell and its surroundings Exhibits selective permeability Controls traffic of molecules in and out

3 LE 5-10 Outside of cell Cytoplasm

4 Hydrophilic head Phosphate group Symbol Membrane phospholipids form a bilayer Phospholipids are the main structural components of membranes Two nonpolar hydrophobic fatty acid "tails" One phosphate group attached to the hydrophilic glycerol "head" Hydrophobic tails

5 In membranes, phospholipids form a bilayer Two-layer sheet Phospholipid heads facing outward and tails facing inward Selectively permeable Polar lipid-soluble molecules pass through Nonpolar molecules not soluble in lipids do not pass through Hydrophilic heads Water Hydrophobic tails Water

6 2.4.3 The membrane is a fluid mosaic of phospholipids and proteins A membrane is a mosaic Proteins and other molecules are embedded in a framework of phospholipids A membrane is fluid Most protein and phospholipid molecules can move laterally Membrane glycoproteins and glycolipids function in cell identification

7 LE A.1 Cholesterol in mammalian membranes helps prevent extremes. Reducing fluidity, but also preventing fatty acid chain crystalizatioon Extracellular matrix Glycoprotein Carbohydrate Plasma membrane Glycolipid Phospholipid Proteins Microfilaments of cytoskeleton Cholesterol Cytoplasm

8 2.4.3 Proteins perform most membrane functions Transporters of substances across the membrane Receptors of chemical messages from other cells (signal transduction) Anchorage Cell recognition Intercellular junctions between adjacent cells Enzymes

9 LE 5-13a Enzyme activity

10 LE 5-13b Messenger molecule Receptor Activated molecule Signal transduction

11 LE 5-13c ATP Transport

12 2.4.4 Passive transport is diffusion across a membrane Diffusion is the tendency for particles to spread out evenly in an available space From an area of high concentration to an area of low concentration Passive transport across membranes occurs when a molecule diffuses down a concentration gradient Small nonpolar molecules such as O 2 and CO 2 diffuse easily across the phospholipid bilayer of a membrane

13 LE 5-14a Molecules of dye Membrane Equilibrium

14 LE 5-14b Equilibrium

15 2.4.5 Transport proteins may facilitate diffusion across membranes In facilitated diffusion Transport proteins that span the membrane bilayer help substances diffuse down a concentration gradient To transport the substance, a transport protein may Provide a pore for passage Bind the substance, change shape, and then release the substance

16 LE 5-15 Solute molecule Transport protein

17 2.4.6 Cells expend energy for active transport Active transport requires energy to move solutes against a concentration gradient ATP supplies the energy Transport proteins move solute molecules across the membrane Transport protein Solute ATP P ADP Protein changes shape P Phosphate detaches P Solute binding Phosphorylation Transport Protein reversion

18 2.4.7 Cells transport materials in vesicles Proteins made in ER are sent to the Golgi via vesicles Vesicles go from the Golgi to the plasma membrane

19 2.4.8 Exocytosis and endocytosis transport large molecules To move large molecules or particles through a cell membrane A vesicle may fuse with the membrane and expel its contents outside the cell (exocytosis) Membranes may fold inward, enclosing material from the outside (endocytosis)

20 LE 5-19a Fluid outside cell Vesicle Protein Cytoplasm

21 LE 5-19b Vesicle forming

22 Endocytosis can occur in three ways Phagocytosis ("cell eating") Surrounding food and engulfing it Pinocytosis ("cell drinking") Absorbing fluid into tiny vesicles Receptor-mediated endocytosis Like pinocytosis, but very specific

23 LE 5-19c Pseudopodium of amoeba Food being ingested Phagocytosis Plasma membrane Material bound to receptor proteins PIT Pinocytosis Cytoplasm Receptor-mediated endocytosis

24 Osmosis is the diffusion of water across a membrane In osmosis water, molecules diffuse across a selectively permeable membrane From an area of low solute concentration To an area of high solute concentration Until the solution is equally concentrated on both sides of the membrane The direction of movement is determined by the difference in total solute concentration Not by the nature of the solutes

25 LE 5-16 Lower concentration of solute Higher concentration of solute Equal concentration of solute Solute molecule H 2 O Selectively permeable membrane Water molecule Solute molecule with cluster of water molecules Net flow of water

26 Water balance between cells and their surroundings is crucial to organisms Osmoregulation is the control of water balance Tonicity is the tendency of a cell to lose or gain water in solution Isotonic solution: solute concentration is the same in the cell and in the solution No osmosis occurs Animal cell volume remains constant; plant cell becomes flaccid

27 Hypotonic solution: solute concentration is greater in the cell than in the solution Cell gains water through osmosis Animal cell lyses; plant cell becomes turgid Hypertonic solution: solute concentration is lower in the cell than in the solution Cell loses water through osmosis Animal cell shrivels; plant cell plasmolyzes

28 LE 5-17 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 membrane H 2 O Plant cell (4) Flaccid (5) Turgid (6) Shriveled (plasmolyzed)

29 The difference between passive and active transport is: A: Passive doesn t take energy B: Active transport is for larger molecules C: Passive transport aloows things to come in and out of the cell D: All of the above are true

30 Which of the following is not a form of active transport? A: Exocytosis B: Endocytosis C: Facilitated diffusion D: Protein pumps

31 A B C Which of the following represents a picture of a cell placed in an environment that has more solutes outside of the cell?