1. Biological Membranes
Chapter 5

2. Fluid Mosaic Model
Amphipathic molecules: nonpolar, hydrophobic portion (2 fatty acid chains in a phospholipid) linked to a polar, hydrophilic portion (glycerol head in a phospholipid)
Cylindrical shape – allows the phospholipid to orient itself with tails toward the center of the bilayer and heads out
The embedded proteins in the bilayer are free to move about like icebergs floating on the sea
This can be thought of as a liquid crystal

3. Figure 5-4 Page 98
Lateral movement only
Time

4. Other membrane lipids:
Cholesterol – in animal cell membranes. At low temperatures it helps prevent solidifying. At higher temps it helps maintain stability
Glycolipids – carbohydrates combined with lipids; found on cell surfaces of animal cells; allow cells to recognize and interact with each other

5. In addition to phospholipids
Integral proteins – tightly bound to the membrane. Some do not extend all the way through the membrane. Transmembrane proteins do
Peripheral proteins – not embedded in the lipid bilayer and are located on the inner or outer surface
Glycoproteins – carbohydrates and proteins; on the outer surface of cells; may allow cells to adhere or provide protection

6. a Carbohydrate chains Glycoprotein Carbohydrate chain
Extracellular fluid
Hydrophobic
Hydrophilic
Glycolipid
Cholesterol
Hydrophilic
helix a
Peripheral
protein
Integral
proteins
Cytosol

7. Functions of membrane proteins
Anchoring: anchor the cell to the extracellular matrix and connect to microfilaments within the cell
Passive transport: for passage of certain ions or molecules
Active transport: used to pump solutes across the membrane opposite to diffusion

8. Functions of membrane proteins…
Enzymatic activity: catalyze reactions that occur along the surface
Signal transduction: bind to signal molecules such as hormones
Cell recognition: identification tags
Intercellular junctions: attach membranes of adjacent cells

9. Passage through the cell membrane
Diffusion concepts:
Kinetic energy of all matter
Concentration gradient
Equilibrium
Cellular diffusion:
Passive transport
Osmosis (water + plasma membrane)
Facilitated diffusion

10. Membranes are selectively permeable
Physical processes
Diffusion
Osmosis
Carrier-mediated processes
Channel proteins
Carrier proteins

11. Diffusion 1 2 3

12. Osmosis: water passes through selectively permeable membrane from region of higher concentration to lower

13. Osmosis Isotonic Hypertonic Hypotonic Equal solute concentration
Cell remains in equilibrium
Hypertonic
High in solute, low in water
Water moves out of cell
Cell shrinks
Hypotonic
Low in solute, high in water
Water moves into cell
Cell swells

14. Effects of osmosis In plant cells: In animal cells:
Turgor pressure – cells push against rigid cell walls
Plasmolysis – cell contents pull away from cell wall  wilting
In animal cells:
Cells may swell to bursting - cytolysis
Some have contractile vacuoles to remove excess water to prevent bursting

15. (a)
Plasma
membrane
(b)
Nucleus
(c)
Vacuole
Vacuole
Vacuolar
membrane
(tonoplast)
Plasma
membrane
Cytoplasm

16. (a) (b) (c) 10µm Outside cell Inside cell Outside cell Inside cell
No net water
movement
Net water movement
out of the cell
Net water movement
into the cell
(b)
(c)
10µm
Isotonic solution
Hypertonic solution
Hypotonic solution

17. Carrier-mediated transport
Uses carrier proteins to allow substances to cross the nonpolar interior of the phospholipid bilayer
Ions, large polar molecules (glucose, amino acids)
Two types:
Facilitated diffusion – passive transport; does not require an additional source of energy; works with the concentration gradient
Carrier-mediated active transport – requires an additional energy source, usually ATP; works against the concentration gradient

18. Facilitated Diffusion
Makes use of the potential energy of the concentration gradient
As a molecule moves from a high concentration to a low concentration – energy is released
Specific carrier proteins are involved – the shape of the protein determines the solute particle it can transport and changes as it moves the particle across the membrane
This is a form of passive transport

19. Carrier-mediated active transport
For materials that the cell requires in high concentrations
The cell must ‘pump’ the material from low to high concentration – up the concentration gradient
An example is the sodium-potassium pump found in animal cells

20. Sodium-potassium pump
Uses energy in the form of ATP
Pumps two K+ ions into the cell for every three Na+ ions it pumps out
This causes an electrical as well as chemical gradient across the cell membrane – an electrochemical gradient
This gradient stores energy for the cell and can be used to help drive other transport systems

21. Na+/K+ pump

22. Transportation in vesicles
Exocytosis – cells eject wastes or specific products such as hormones
The vesicle fuses with the cell membrane
Endocytosis – extracellular materials are incorporated into the cell:
Phagocytosis
Pinocytosis
Receptor-mediated endocytosis

23. Exocytosis

24. Phagocytosis ‘cell eating’ large, solid materials such as other cells
White blood cells ingest whole bacteria

25. Phagocytosis

26. Pinocytosis ‘cell drinking’
Droplets of fluid collect in a fold of the plasma membrane which pinches off into the cytosol in a vesicle
Contents of the vesicle diffuse into the cytosol and the vesicle shrinks

27. Pinocytosis

28. Receptor-mediated endocytosis
Specific molecules combine with receptor molecules on the plasma membrane
Cholesterol is taken up this way

29. Receptor-mediated endocytosis