MEMBRANE STRUCTURE AND FUNCTION

1. MEMBRANE STRUCTURE AND FUNCTION • Membranes organize the chemical activities of cells

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

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

4. Water Hydrophilicheads Hydrophobictails Water • Phospholipids form a two-layer sheet called a phospholipid bilayer, with the heads facing outward and the tails facing inward Figure 5.11B

5. Fibers of the extracellular matrix Carbohydrate(of glycoprotein) Glycoprotein Glycolipid Plasmamembrane Phospholipid Proteins Cholesterol Microfilamentsof cytoskeleton Cytoplasm • The membrane is a fluid mosaic of phospholipids and proteins Figure 5.12

6. Proteins can associate with the membrane several ways • Peripheral proteins only associate with the hydrophilic heads • Integral proteins also have a hydrophobic part that associated with the hydrophobic tails • Some integral proteins go all the way through and are called transmembrane proteins

7. Proteins make the membrane a mosaic of function • Many membrane proteins function as enzymes cartalyzing chemical reactions Figure 5.13A

8. Messenger molecule Receptor Activatedmolecule • Other membrane proteins • Function as receptors for chemical messages from other cells Figure 5.13B

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

10. Proteins and glycoproteins are also important for cell recognition

11. Cholesterol • In the cell membrane Cholesterol helps control fluidity

12. Molecules of dye Membrane Equilibrium Equilibrium • 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 Figure 5.14A Figure 5.14B

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

14. Solutemolecule Transportprotein • Facilitated diffusion • 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 Figure 5.15

15. Equalconcentrationof solute Higherconcentrationof solute Lowerconcentrationof solute H2O Solutemolecule Selectivelypermeablemembrane Watermolecule Solute molecule with cluster of water molecules Net flow of water • 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 Figure 5.16

16. Hypertonic solution Hypotonic solution Isotonic solution H2O H2O H2O H2O Animalcell (3) Shriveled (2) Lysed (1) Normal Plasmamembrane H2O H2O H2O H2O Plantcell (6) Shriveled (plasmolyzed) (5) Turgid (4) Flaccid • 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 Figure 5.17

17. The control of water balance • Is called osmoregulation

18. Transportprotein P P P Phosphatedetaches Proteinchanges shape ATP Solute ADP Transport 1 Solute binding 2 Phosphorylation 3 4 Protein reversion • Cells expend energy for active transport • Transport proteins can move solutes against a concentration gradient, which requires ATP Figure 5.18

19. Pumps • One big example is the sodium potassium pump

20. Coupled Transport

21. Fluid outside cell Vesicle Protein Cytoplasm • 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) Figure 5.19A

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

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