1. Chapter 4: Membrane Structure & Function
“Fluid mosaic model”
Chapter 4: Membrane Structure & Function

2. Cell Membrane
Cell membrane controls movement of substances in/out of cell
acts as gatekeeper – “selectively permeable”
Also…
provides structural support
recognizes foreign material
communicates with other cells

3. Structure
Phospholipid bilayer with imbedded or partially imbedded proteins
Only certain substances can pass through the lipid bilayer
For example: small, nonpolar substances
Ions & polar molecules repelled by nonpolar interior of lipid bilayer

4. Lipid Bilayer
The head is polar and is attracted to water (Note: there is water inside and outside the cell)
 Hydrophilic
The fatty acid tails are nonpolar and are repelled by water
 Hydrophobic

6. Act as markers identifying the cell
Structure continued…
Glycoproteins = carbohydrate chains attached to the outside surface of proteins
Glycolipids – carbohydrate chains attached to the outside surface of phospholipids
Act as markers identifying the cell

7. Structure continued…. Membrane proteins:
Can face inside or outside the cell
Can stretch across membrane
Remember that proteins are made of amino acids
Some of these amino acids are polar and some are nonpolar

8. Membrane Proteins 5 Types:
Channel proteins: move substances in/out freely (Cystic fibrosis is cause by a faulty Cl- channel)
Carrier proteins: interacts with certain molecules/ions to move them in/out of cell
Cell Recognition: glycoproteins that identify the cell as “self” (issues with transplants)
Receptor Proteins: receive signals from surroundings (some types of dwarfism)
Enzymatic: biochemical reactions (cholera bacteria releases toxin that interferes with one)

9. Membrane Proteins

10. Ways for Substance to Cross the Membrane
1. Passively
Requires NO energy
Includes: diffusion, osmosis, & faciliated transport
2. Actively
Requires ATP energy
Includes: active transport, exocytosis & endocytosis

11. Diffusion

12. Molecules move from high concentration to low concentration in between phospholipids molecules
Moves down its concentration gradient
small, nonpolar molecules pass through membrane
Examples: O2 in, CO2 out, H2O in/out

13. Osmosis Water diffusing across the cell membrane
Type of facilitated diffusion (i.e. needs a transport protein—channel)

14. Possible situations with osmosis:
1. Isotonic solutions
No net movement of water across membrane.
Same concentration of water outside the cell as in the cytoplasm
Cells placed in an isotonic solution stay the same size

15. 2. Hypertonic solutions
lower concentration of water outside the cell as in the cytoplasm
water will leave the cell & the cell will shrivel

16. 3. Hypotonic solutions
Higher concentration of water outside the cell as in the cytoplasm
water will enter cell & the cell will swell and possibly burst

17. Summary of what happens to animal cells placed in different solutions:

18. Summary of what happens to plant cells placed in different solutions:

19. Facilitated Diffusion
movement through a channel protein down a concentration gradient
Polar substances, ions, etc.

20. Active Transport
Moves molecules from low concentration to high concentration
Requires ATP energy
Examples: sodium-potassium pump (important for nerve impulses & muscle contractions)

22. Endocytosis Cell membrane forms a vesicle to bring in materials
2 types:
Pinocytosis = when substance entering cell is small or liquid
Phagocytosis = when substance entering cell is large

23. Exocytosis
Internal vesicles fuse with cell membrane releasing substances out of the cell