B2.1 Membranes and membrane transport

What is the structure of a phospholipid?

• 2 fatty acids chains and a phosphate bonded to glycerol molecule

• Amphipathic:

  • Nonpolar, hydrophobic fatty acid tails

  • Charged, hydrophilic phosphate head

How do phospholipids structure themselves when added to water?

Form bilayers:

• Hydrophilic phosphate heads face water

• Hydrophobic fatty acid tails in the middle

What is the role of the phospholipid bilayer?

• Separates the cytoplasm and cell contents from the environment

• Acts as a barrier for materials entering and exiting the cell

• Only hydrophobic/uncharged particles can pass through hydrophobic fatty acid tails

• Large/hydrophilic/charged particles cannot pass directly through

What is the kinetic theory?

• Particles are in constant motion

• Particles in gases, liquids, solutes in aqueous solutions move in random directions

• Random movement of particles leads to diffusion and osmosis

What is diffusion?

Passive transport of particles from high to low concentration

What are integral proteins?

• Permanently attached to the plasma membrane

• Penetrate into the centre of phospholipid bilayer

• Contain 1 hydrophobic section and 2 hydrophilic sections

• Can be transmembrane or only partially penetrate bilayer

• Can be glycoproteins,channels/protein pumps

What are peripheral proteins?

• Temporarily attached to membrane surface/integral proteins through electrostatic interactions

• Charged peripheral proteins attracted to charged sections of integral proteins and phosphate heads

• Hydrophilic, do not penetrate the phospholipid bilayer

• Can be receptors and enzymes

What is osmosis?

Passive transport of water molecules from low to high solute concentration through a semipermeable membrane

What are aquaporin?

• Integral channel proteins that selectively transport water rapidly through membranes

• Significantly increases membrane permeability to water

What is facilitated diffusion?

Passive transport of molecules from high to low concentration through channel proteins

What are channel proteins?

• Specific to the molecule that can pass through them - selectively permeable

• Have central pore which allows specific particles to move through - lined with hydrophilic amino acid R groups that allow one type of molecule to pass through

• Some are gated - only open to allow facilitated diffusion in response to a stimulus

What is Adenosine Triphosphate (ATP)?

Provides energy required to change shape of protein pumps for active transport in cells

What is active transport?

Movement of particles from low concentration to high concentration using protein pumps and ATP energy

What is the process of active transport?

• Particle binds to binding site on protein pump

• ATP binds to protein pump, hydrolyses → ADP

• Phosphate attaches to protein pump, causes pump to change shape

• Particle moved against concentration gradient and released

• Phosphate released → protein pump returns to original shape

What are examples of selectivity in membrane permeability?

• Facilitated diffusion: selective

• Active transport: selective

• Simple diffusion: not selective

What are glycoproteins?

Membrane proteins with carbohydrate chain attached

What are glycolipids?

Phospholipids with carbohydrate chain attached

What are the roles of glycoproteins and glycolipids?

• Receptors for hormones

• Cell to cell communication: bind to neurotransmitters

• Immune response: act as markers on cells

• Cell to cell adhesion: form tissues

How are glycoproteins and glycolipids involved in cell recognition?

• Carbohydrate chains have specific shapes that allow immune system to recognise cells as self

• Act as antigens if carbohydrate chain is not recognised as self

What is the fluid mosaic model?

• Phospholipids and proteins can move around

• Proteins embedded like mosaic

• Helps to explain passive + active movement between cells and surroundings, cell to cell interactions, cell signalling

What is the role of cholesterol in animal membranes?

• Stabilises by reducing extremes of fluidity

• Prevents too much permeability at high temps

• Prevents freezing at low temps

• Positioned between phospholipids with OH group near heads