1. Diffusion and Osmosis

2. Diffusion
Solute molecules moving from an area of high concentration to an area of low concentration
Random motion drives diffusion
Movement is based on kinetic energy (speed), charge, and mass of molecules
Equilibrium is reached when there is an even distribution of solute molecules
(water) 2 3 1 4

3. Osmosis Diffusion of water through a semi-permeable membrane
Semi-permeable: permeable to solvents (WATER), but not to large molecules
High [water] to low [water]
Dissolved molecules (i.e. glucose, starch) are called solutes
REMEMBER:
Water = solvent
Glucose, Starch = solutes

4. Effect of Water on Cells
Hypertonic Environment
High [solute], low [water]
Hypotonic Environment
High [water], low [solute]
Isotonic Environment
[water] = [solute]
Hypertonic
Hypotonic
Isotonic

5. Osmosis in Living Cells
Cellulose in cell wall

6. Osmosis in Plant Cells
Plasmolysis
Hypertonic
Hypotonic

7. Osmosis in Red Blood Cells
Isotonic
Hypotonic
Hypertonic

8. Tonicity Lab
Hypotonic
Isotonic
Hypertonic

9. Membrane Permeability
The cell membrane is a selectively permeable membrane; it lets some molecules pass through by passive diffusion, but not others.
Four factors:
*Lipid solubility
*Molecular size
*Polarity
*Charge

10. Transportation of Molecules
Passive Transport
Movement of molecules across a semi-permeable membrane
no energy required
• Facilitated Diffusion
Movement of molecules across a
semi-permeable membrane
protein
- no energy required
Active Transport
Movement of molecules across a semi-permeable membrane against a concentration gradient with a protein
ENERGY – ATP

11. Passive Transport
Simple diffusion: membrane does not influence the direction of movement
(high conc. → low conc).
Facilitated diffusion: a molecule that is too big or too polar to diffuse across
the membrane combines with a specific transport carrier protein and is released in the cytoplasm.
No addition of energy (ATP) is required.

12. Transport by Carrier Proteins
Some biologically useful molecules pass through the plasma membrane because of channel proteins and carrier proteins that span the membrane.
Carrier proteins are specific and combine with only a certain type of molecule.
Facilitated transport and active transport both require carrier proteins.

13. Facilitated transport
During facilitated transport, substances pass through a carrier protein following their concentration gradients.
Facilitated transport does not require energy.
The carrier protein for glucose has two conformations and switches back and forth between the two, carrying glucose across the membrane.
After glucose binds to the open end of the carrier, the carrier closes behind the glucose molecule. As glucose moves along, the constricted end of the carrier opens in front of the glucose molecule. After glucose is released into the cytoplasm, the carrier returns to its former conformation. This process can occur as often as 100 times per second.

14. Facilitated diffusion of glucose
A carrier protein speeds the rate at which a solute crosses a membrane from higher solute concentration to lower solute concentration. At (1), the molecule enters the carrier protein. At (2), the carrier undergoes a change in shape (conformation) that releases the molecule to the other side of the membrane. At (3), the carrier returns to its former shape.
This model of facilitated transport suggests that after a carrier has assisted the movement of a molecule to the other side of the membrane, it is free to help other similar molecules pass through the membrane. Because the glucose carrier is designed mainly for glucose, glucose can cross the membrane many times faster than the other sugars. This is a good example of differential permeability of a plasma membrane.

15. Active transport
During active transport, ions or molecules are moved across the membrane against the concentration gradient – from an area of lower to higher concentration.
Energy in the form of ATP is required for the carrier protein to combine with the transported molecule.

16. Active Transport

17. Exocytosis and Endocytosis
During exocytosis, vesicles fuse with the plasma membrane for secretion.
Some cells are specialized to produce and release specific molecules.
Examples include release of digestive enzymes from cells of the pancreas, or secretion of the hormone insulin in response to rising blood glucose levels.
Release of the hormone insulin from the pancreas is called regulated secretion, because vesicles fuse with the plasma membrane only when insulin is needed to reduce blood glucose.

18. Exocytosis
Exocytosis deposits substances on the outside of the cell and secretion occurs.

19. Endocytosis
During endocytosis, cells take in substances by invaginating a portion of the plasma membrane, and forming a vesicle around the substance.
Endocytosis occurs as:
Phagocytosis – large particles
Pinocytosis – small particles
Receptor-mediated endocytosis – specific particles

20. Phagocytosis
Phagocytosis occurs when the substance to be transported into the cell is large; amoebas ingest food by phagocytosis. Certain types of human white blood cells are amoeboid and engulf worn-out cellular debris or bacteria using phagocytosis. When an endocytic vesicle fuses with a lysosome, digestion of the vesicle contents occurs.

21. Pinocytosis
Pinocytosis occurs when a macromolecule, such as a polypeptide, is to be transported into the cell. The resulting vesicle or vacuole is small. Pinocytosis occurs continuously, but the loss of plasma membrane due to vesicle formation is offset by exocytosis.

22. Receptor-mediated endocytosis
Receptor-mediated endocytosis is a form of pinocytosis. The substance to be taken in binds with a specific receptor protein, which migrates to a pit or is already in a coated pit. The resulting vesicle contains the substance and the receptor. Receptor-mediated endocytosis is responsible for cells taking up low-density lipoprotein (LDL) when LDL receptors gather in a coated pit. In individuals with a genetic disorder called familial hypercholesterolemia, the LDL receptor is unable to properly bind to the coated pit, and cells are unable to take up cholesterol. Cholesterol accumulates in the walls of arterial blood vessels, causing severe health problems.