Cell Membrane Transport

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. 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 Factors affecting transport: cell membrane

4. Stable phospholipid organizations Figure 2.4

5. The membrane is permeable to: H2O Gases (O2, CO2, N2) Lipids Small, neutral molecules (such as urea) 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

6. 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 Factors affecting transport: Chemical gradient

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

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

12. 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 Osmosis

13. 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 Solution tonicity

14. 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 Facilitated diffusion

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

18. Phagocytosis

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

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