Cell Membrane: Structure & Function

1. Cell Membrane:Structure & Function Ms. Levensailor

2. Staple ingredients of membranes are: Lipids Proteins Phospholipids are most common Amphipathic molecule: it has both a hydrophilic and hydrophobic region Membrane Structure

3. Membranes are fluid • Membranes are held together by hydrophobic interactions (much weaker than covalent bonds). • They must be fluid to work properly • When solid permeability changes & enzymatic proteins become inactive.

4. Membranes are mosaics of structure & function • A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer.

5. Membranes are mosaics of structure & function • Lipid bilayer is the main fabric of the membrane. • Proteins determine most of the membrane’s functions.

6. Membrane Carbohydrates • Important in cell to cell recognition: • A cell’s ability to distinguish between 1 type of neighboring cell from another. • i.e. identifying foreign cells (immune system) • Recognize other cells by keying in on surface molecules, carbohydrates, on the plasma membrane.

7. Some Functions of the membrane proteins

8. Selective Permeability • Substances do not cross the barrier indiscriminately! • Hydrophobic nonpolar molecules (hydrocarbons, carbon dioxide, & oxygen) cross with ease. • Hydrophilic polar molecules (ions) are impeded by the hydrophobic core of the membrane.

9. Transport Proteins • Proteins built into the membrane • Allow passage of hydrophilic substances. • Can be so specific that they only allow 1 specific substance to move through. • i.e. glucose carried in blood to the human liver enters liver cells rapidly through a specific transport protein. • It is so selective it rejects fructose (structural isomer of glucose).

10. Passive transport • Molecules have an intrinsic kinetic energy called thermal motion (heat). • A result of this is diffusion: • Tendency of molecules of any substance to spread out into available space. • A substance will diffuse from areas of high concentration to areas of low concentration. • Passive transport does not require energy!

11. Passive Transport

12. Diffusion of one solute

13. Osmosis • The passive transport of water. • Hypertonic: solution with a higher concentration of solute. • Hypotonic: solution with a lower solute concentration. • Isotonic: solutions of equal solute concentration.

14. Osmosis

15. Cell survival depends on water uptake & loss • Cells w/out walls (animal cells) • If cell loses water it will shrivel • If cell takes up too much water it will burst • Cell w/ wall (plants & fungi) • If cell takes in water the wall exerts pressure back and prevents further uptake. • Isotonic solutions will make the plant limp. • If a cell loses water it will shrivel and the plasma membrane pulls away from the wall (plasmolysis).

16. Plasmolysis Hypertonic Hypotonic

17. Osmosis

18. Active Transport • The pumping of solutes against their gradients. • Across the plasma membrane from the side where they are less concentrated to the side where they are more concentrate. • Like moving “up hill” & requires work! • ATP supplies the energy for active transport. • i.e. sodium-potassium pump (online activity)

19. Active Transport

20. Exocytosis & Endocytosis • Transport large molecules (proteins & polysaccharides). • Exocytosis: cell secretes macromolecules by the fusion of vesicles with the plasma membrane. • Endocytosis: cell takes in macromolecules by forming new vesicles from the plasma membrane.

21. Exocytosis & Endocytosis