MEMBRANE STRUCTURE AND FUNCTION 5.10 Membranes organize the chemical activities of cells Membranes Provide structural order for metabolism

The plasma membrane of the cell is selectively permeable Controlling the flow of substances into or out of the cell Cytoplasm Outside of cell TEM 200,000  Figure 5.10

5.11 Membrane phospholipids form a bilayer Have a hydrophilic head and two hydrophobic tails Are the main structural components of membranes CH2 CH3 CH N + O O– P C Phosphate group Symbol Hydrophilic head Hydrophobic tails Figure 5.11A

Phospholipids form a two-layer sheet Called a phospholipid bilayer, with the heads facing outward and the tails facing inward Water Hydrophilic heads Hydrophobic tails Figure 5.11B

5.12 The membrane is a fluid mosaic of phospholipids and proteins A membrane is a fluid mosaic With proteins and other molecules embedded in a phospholipid bilayer Fibers of the extracellular matrix Carbohydrate (of glycoprotein) Glycoprotein Microfilaments of cytoskeleton Phospholipid Cholesterol Proteins Plasma membrane Glycolipid Cytoplasm Figure 5.12

5.13 Proteins make the membrane a mosaic of function Many membrane proteins Function as enzymes Figure 5.13A

Other membrane proteins Function as receptors for chemical messages from other cells Messenger molecule Receptor Activated molecule Figure 5.13B

Membrane proteins also function in transport Moving substances across the membrane ATP Figure 5.13C

5.14 Passive transport is diffusion across a membrane In passive transport, substances diffuse through membranes without work by the cell Spreading from areas of high concentration to areas of low concentration Equilibrium Membrane Molecules of dye Figure 5.14A Figure 5.14B

Small nonpolar molecules such as O2 and CO2 Diffuse easily across the phospholipid bilayer of a membrane

5.15 Transport proteins may facilitate diffusion across membranes Many kinds of molecules Do not diffuse freely across membranes For these molecules, transport proteins Provide passage across membranes through a process called facilitated diffusion Solute molecule Transport protein Figure 5.15

5.16 Osmosis is the diffusion of water across a membrane In osmosis 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 H2O Solute molecule Selectively permeable membrane Water molecule Solute molecule with cluster of water molecules Net flow of water Figure 5.16

5.17 Water balance between cells and their surroundings is crucial to organisms Osmosis causes cells to shrink in hypertonic solutions And swell in hypotonic solutions In isotonic solutions Animal cells are normal, but plant cells are limp Plant cell H2O Plasma membrane (1) Normal (2) Lysed (3) Shriveled (4) Flaccid (5) Turgid (6) Shriveled (plasmolyzed) Isotonic solution Hypotonic solution Hypertonic solution Animal cell Figure 5.17

The control of water balance Is called osmoregulation

5.18 Cells expend energy for active transport Transport proteins can move solutes against a concentration gradient Through active transport, which requires ATP P Protein changes shape Phosphate detaches ATP ADP Solute Transport protein Solute binding 1 Phosphorylation 2 Transport 3 Protein reversion 4 Figure 5.18

5.19 Exocytosis and endocytosis transport large molecules To move large molecules or particles through a membrane A vesicle may fuse with the membrane and expel its contents (exocytosis) Fluid outside cell Cytoplasm Protein Vesicle Figure 5.19A

Membranes may fold inward Enclosing material from the outside (endocytosis) Vesicle forming Figure 5.19B

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