1. Cell Membrane Transport

2. Cell Membrane Transport
1- Factors affecting transport
- cell membrane
- Chemical gradient
- Electrical gradient
- Rate of transport
2- Passive transport
- Diffusion
- Osmosis
- Facilitated diffusion
3- Active transport
- Pumps
- phagocytosis
- Endocytosis/exocytosis

3. Factors affecting transport: cell membrane
The cell needs to absorb and excrete various compounds throughout its life.
These compounds need to pass through the membrane which is made from a phospholipid bilayer
The phospholipid bilayer is formed by phospholipid molecules bipolar molecule: the fatty acid side is hydrophobic, the phosphoric side is hydrophilic

4. Stable phospholipid organizations
Figure 2.4

5. The membrane is impermeable to:
- Small, charged molecules
“large molecules” such as amino acids, glucose and larger
 these compounds must go through channels present in the membrane in order to enter or exit the cell
The membrane is permeable to:
H2O
Gases (O2, CO2, N2)
Lipids
Small, neutral molecules (such as urea)

6. Factors affecting transport: Chemical gradient
Compound moves from an area of high concentration to low concentration (or concentration gradient)
All compounds permeable to the phospholipid bilayer will move this way

7. Factors affecting transport: Electrical force
Positive ions are attracted to negative ions and vice versa
Ions are repelled by ions of the same charge (+ against + and – against -)
Figure 4.3

8. Movement across the cell membrane
Both chemical and electrical forces (electrochemical force) drive the movement of compounds across the cell membrane

9. Factors affecting the rate of transport
The rate of transport will depend on:
- the concentration gradient
- the compound permeability to the membrane
- the type and number of charges present
on the compound

10. Passive transport
Compounds will move from area of high concentration toward area of lower concentration
No ATP is needed for this type of transport

11. Diffusion Compounds move toward the area of lower concentration
Compounds permeable to the cell membrane will move through diffusion. (Compounds unable to pass through the membrane will only pass if membrane channels open)

12. Osmosis Each compound obeys the law of diffusion
However, some compounds are unable to cross the cell membrane (glucose, electrolytes…)
Water can cross  will enter or exit the cell depending its concentration gradient
Note: the cell membrane is a semipermeable membrane

13. Solution tonicity
Isotonic solution: solution which has the same compound concentration as the cell
Hypotonic solution: solution having a compound in lower concentration compared to the cell
Hypertonic solution: solution having a compound in higher concentration compared to the cell

14. Facilitated diffusion
Some compounds are unable to diffuse through the membrane.
They will be allow to cross if the membrane has proteins that can bind these compounds and enable to cross toward the area of lower concentration

15. Figure 4.11a

16. Active transport
Compounds move from area of low concentration toward area of higher concentration
ATP (energy) is needed  pump

17. ATPase pumps
The most common: Na/K pumps  reestablish membrane potential. Present in all cells.
Two K+ ions are exchanged with 3 Na + ions

18. Phagocytosis

19. Receptor-mediated endocytosis
Cell receptors bind to a compound  initiate endocytosis
Figure 4.21c

20. Readings: Chp. 4: p. 95-124. Clinical connections, p.121.
Not expected: Toolbox, p. 96, p. 100, p. 105.