Cell Membrane Transport

Cell Membrane Transport1- Factors affecting transport - cell membrane - Chemical gradient - Electrical gradient - Rate of transport2- Passive transport - Diffusion - Osmosis - Facilitated diffusion3- Active transport - Pumps - phagocytosis - Endocytosis/exocytosis

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

Figure 2.4

Stable phospholipid organizations

• 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

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

Figure 4.3

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 -)

Movement across the cell membrane

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

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

Passive transport

• Compounds will move from area of high concentration toward area of lower concentration

• No ATP is needed for this type of transport

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)

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

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

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

Figure 4.11a

Active transport

• Compounds move from area of low concentration toward area of higher concentration

• ATP (energy) is needed pump

ATPase pumps

• The most common: Na/K pumps reestablish membrane potential. Present in all cells.

• Two K+ ions are exchanged with 3 Na + ions

Phagocytosis

Figure 4.21c

Receptor-mediated endocytosis

• Cell receptors bind to a compound initiate endocytosis

Readings:

• Chp. 4: p. 95-124.

• Clinical connections, p.121.

• Not expected: Toolbox, p. 96, p. 100, p. 105.