Transport across membrane
3.1 BIOLOGICAL MEMBRANES
3.1.1 Properties of Cell Membranes
• Separates living cell from its nonliving surroundings. • 8 nm thick.
• Selectively permeable - allows some substances to cross more easily than others. 4.1.2 Fluid Mosaic Model
• Singer and Nicolson (1972) - plasma membrane is a mosaic of proteins dispersed within lipid bilayer, with only bilayer, the hydrophilic regions exposed to water. Hydrophilic region of protein
Phospholipid bilayer Hydrophobic region of protein
• Plasma membrane - continuous, fluid, double layer of phospholipids
= lipid bilayer.
• Phospholipids & most other membrane constituents are amphipathic - have hydrophobic & hydrophilic regions.
–Hydrophobic tails face inside of bilayer. bilayer. –Hydrophilic head faces exterior (extracellular fluid) and interior (cytosol).
(cytosol).
WATER
Hydrophilic
head
Hydrophobic tail WATER
• Proteins - embedded in bilayer or associated with cytoplasmic or extracellular face.
• Carbohydrates - linked to proteins
(glycoproteins) or lipids (glycolipids) glycoproteins) (glycolipids) on extracellular side. side. • Cholesterol - within membrane.
• Membrane molecules held in place by weak hydrophobic interactions.
• Most lipids & some proteins drift laterally.
• Rarely flip-flop from one layer to the other. flip- Lateral movement
(~107 times per second)
Movement of phospholipids
Flip-flop
(~ once per month)
• As temperatures cool, membranes switch from fluid to solid state
• Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids
• Membranes must be fluid to work properly.
Fluid
Unsaturated hydrocarbon tails with kinks
Membrane fluidity
Viscous
Saturated hydrocarbon tails
• Cholesterol has different effects on membrane fluidity at different temperatures:
– Warm temperatures (e.g., 37°C): restrains
37°
movement of phospholipids
– Cool temperatures:
3.1.1 Properties of Cell Membranes
• Separates living cell from its nonliving surroundings. • 8 nm thick.
• Selectively permeable - allows some substances to cross more easily than others. 4.1.2 Fluid Mosaic Model
• Singer and Nicolson (1972) - plasma membrane is a mosaic of proteins dispersed within lipid bilayer, with only bilayer, the hydrophilic regions exposed to water. Hydrophilic region of protein
Phospholipid bilayer Hydrophobic region of protein
• Plasma membrane - continuous, fluid, double layer of phospholipids
= lipid bilayer.
• Phospholipids & most other membrane constituents are amphipathic - have hydrophobic & hydrophilic regions.
–Hydrophobic tails face inside of bilayer. bilayer. –Hydrophilic head faces exterior (extracellular fluid) and interior (cytosol).
(cytosol).
WATER
Hydrophilic
head
Hydrophobic tail WATER
• Proteins - embedded in bilayer or associated with cytoplasmic or extracellular face.
• Carbohydrates - linked to proteins
(glycoproteins) or lipids (glycolipids) glycoproteins) (glycolipids) on extracellular side. side. • Cholesterol - within membrane.
• Membrane molecules held in place by weak hydrophobic interactions.
• Most lipids & some proteins drift laterally.
• Rarely flip-flop from one layer to the other. flip- Lateral movement
(~107 times per second)
Movement of phospholipids
Flip-flop
(~ once per month)
• As temperatures cool, membranes switch from fluid to solid state
• Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids
• Membranes must be fluid to work properly.
Fluid
Unsaturated hydrocarbon tails with kinks
Membrane fluidity
Viscous
Saturated hydrocarbon tails
• Cholesterol has different effects on membrane fluidity at different temperatures:
– Warm temperatures (e.g., 37°C): restrains
37°
movement of phospholipids
– Cool temperatures: