B2.1 Membranes and membrane transport

What is the universal structure for plasma membranes and organelles?

Phospholipid bilayers

Which part of phospholipids are hydrophilic?

Heads → out

Which part of phospholipids are hydrophobic?

Tails → in

What are the structures within phospholipid bilayers?

• Glycoproteins

• Glycolipids

• Extrinsic proteins

• Intrinsic protein

• Cholesterol

How do lipid bilayers act as barriers?

• Hydrophobic core blocks ions and large polar molecules

• Small non-polar molecules (O2, CO2, steroid hormones) cross easily

• Smaller molecules diffuse faster than large ones

What is simple diffusion?

• Passive net movement from high → low concentration

• No ATP or proteins required

• Works for small non-polar (O2, CO2) and very small polar molecules (ethanol, urea)

What are the characteristics of integral proteins?

Inside, hydrophobic regions embedded, often transmembrane

What are the characteristics of peripheral proteins?

Outside, attached to surface, often via other proteins

What are the characteristics of membrane proteins?

Membrane activity = higher protein content (18% myelin, 75% mitochondria/chloroplasts)

What is osmosis?

• Net movement of water from low solute → high solute concentration

• Passive, no energy input

What is the role of aquaporins?

Massively increase water permeability

What is facilitated diffusion?

• Selective pores for ions or polar molecules

• Passive, down concentration gradient

• Gated channels open/close in response to signals (voltage, ligands)

What are protein channels?

Proteins that can generate hydrophilic holes in cell membranes, allowing molecules to go down a concentration gradient

What are carrier proteins?

Essential proteins that carry chemicals across the membrane in both directions, down and up the concentration gradient

What are pump proteins?

• Used in active transport

• Use ATP to push molecules against gradient

• Pumps work in 1 direction - each specific to a solute

What are characteristics of glycoproteins and glycolipids?

• Project outward, forming glycocalyx (meshwork of carbohydrates that covers the membrane)

• Involved in cell to cell recognition, adhesion, immune response

What is the role of glycoproteins?

ABO blood group antigens

What is the role of glycolipids?

Help immune system distinguish self vs non-self

What is the fluid mosaic model?

• Phospholipids provide fluid background

• Proteins embedded like a mosaic

• Lateral movement allows flexibility and vesicle formation

What is the composition of saturated fatty acid tails?

Rigid, less fluid

What is the composition of unsaturated fatty acid tails?

Kinks, more fluid, more permeable

What is the role of cholesterol in animal membranes?

• Stabilises by reducing extremes of fluidity

• Prevents too much permeability at high temp and freezing at low temp

What is endocytosis?

Plasma membrane pinches inward

What is exocytosis?

Vesicles fuse with membrane to release contents

What is required for vesicle fusion and formation?

ATP and fluidity of bilayer

What are vesicle fusion and formation used for?

Nutrient uptake, waste expulsion, neurotransmitter release