B2.1 MEMBRANES & TRANSPORT

Draw the fluid mosaic model of cell membranes

Include:

1. PHOSPHOLIPID BILAYER

• 10nm

• hydrophobic tail: saturated is straight, unsaturated is bent, established by hydrophilic interactions, inside membrane-

• ydrophilic head: glycerol, phosphate, choline, established by reactions between water & head

• phospholipids: flluid and move laterally

2. PROTEINS — Integral/channel proteins:

• permeate surface/width of the membrane

• Peripheral protein: outside of bilayer, globular-

• Glycoproteins/glycolipid: carbohydrate groups attached to protein or lipid

Glucose: hexagons, glycosidic bonds on top of peripheral protein/fatty acid tail -> branches- OH group & ring structure groupEnsure:1. at least three phospholipids in uniform size2. no gap between heads and tail

Outline the basis of cell membranes

1. Lipid bilayers are the basis of cell membranes

2. Phospholipids are amphipathic (hydrophilic head and hydrophobic fatty acid tails)

3. Phospholipids and other amphipathic lipids naturally from continues sheet like bilayers when mixed water

4. Phospholipids form monolayers when in contact in water

Describe the formation of phospholipid bilayers

Cell membranes are formed from phospholipid bilayers

1. Hydrophobic fatty acid tails:

• orient inside, away from water

• not attracted to water, attracted to each other - forms the lipid bilayer

• held together via hydrophobic interactions

2. Hydrophilic phosphate heads:

• orient outside, associated with water

• head (phosphate, glycerol, choline) is attracted to water

• stability in double layer because heads on outer edge are attracted to water, whilst the tails are attracted to each other in the middle

Describe the structure and function of the membrane proteins

INTEGRAL/INTRINSIC PROTEINS:

1. Permanently attached to the membrane

2. Typically polytopic (go all the way through the membrane), can be monotopic (penetrate on the surface)

3. Channel proteins to transport substances

4. Channel (facilitated diffusion) or pump proteins (active transport)

GLYCOPROTEINS (and glycolipids)

1. Carbohydrate chains attached to proteins (or lipids) of the cell membrane

2. Glycolayx: glycoproteins + glycolipids are hydrophilic and located on exterior

3. Role in cell recognition of ABO blood groups

4. Cell adhesion molecule (CAM) and glycoproteins form tight junctions/gap junctions/desmosomes

Outline the overall function of ALL membrane proteins

Membrane proteins have diverse structures, locations, and functions. Remember it by TRACIE!

Transport: Protein channels (facilitated) and protein pumps (active)

Receptors: peptide hormones bind to receptors

Anchorage: Cytoskeleton attachments and extracellular matrix

Cell to cell recognition: MHC proteins and antigens

Intercellular joining: Tight junctions and plasmodesmata

Enzyme activity: Catalyse reactions and electron carrier molecules

Describe how channel protein's structure and function (with an example!)

STRUCTURE:

1. Integral protein embedded in bilayer

2. Selective filter that regulates the movement of specific ions based on size/charge

3. Polar amino acids on the interior of channel/associated phosphate heads

4. Nonpolar amino acids in lipid bilayer

FUNCTION:

1. OPEN to ALLOW diffusion of molecules through membrane, CLOSE to PREVENT diffusion

2. Facilitated diffusion: movement of molecules down the concentration gradient (high to low) through protein

3. Passive transport, doesn't require ATP

4. Transports molecules that aren't permeable (ie. hydrophilic, ionic)

5. Channel proteins are specific to the molecule

EXAMPLE: impulse along a neuron

1. Sodium channel protein: NA diffuses into axon through NA channel protein

2. Potassium channel protein: K diffuses out through a K channel protein

Outline gated ion channels in neurons

LIGAND GATED ION CHANNELS:

1. Nicotinic acetcholine (ligand) binds to a receptor on the post synaptic membrane

2. Conformation change in the channel

3. Na channel opens so that Na can diffuse into cell

VOLTAGE GATED ION CHANNELS:

1. Threshold voltage is reached

2. Voltage-gated NA+ channel opens

3. Na diffuses into axon

Outline the structure and function of pump proteins. Give an example

STRUCTURE:

1. Integral protein embedded in the bilayer

2. Specific for molecule being pumped

3. Binding site for molecule and ATP

FUNCTION:

1. Creates concentration gradient

2. Moves molecules against/up the gradient from low to high concentration

3. Requires ATP and pump protein

4. Phosphorylation and dephosphorylation results in a conformational change in the shape of protein

EXAMPLE: Sodium-potassium pump

1. 3 Na+ ions are pumped out of the cell, higher Na ion concentrations out of cell

2. 2 K+ ions are pumped into the cell, higher K ion concentration inside cell

3. ATP Is used = Na/K pump is phosphorylated

4. Pump changes its conformation

5. Creates concentration gradients

Outline how sodium-dependent glucose cotransporters are an example of indirect active transport

In Intestinal epithelial cells:

1. Na/K pump is used to pump sodium out of cell via active transport (requires ATP)

2. Sodium moves down concentration gradient from the lumen into the cell via diffusion

3. Glucose is brought into the cell with Na through a cotransport protein

4. Glucose is brought UP the glucose concentration gradient through indirect active transport

Describe how lipid bilayers act as barriers

1. Lipid bilayers act as barriers between aqueous solutions: cytoplasm and extracellular fluid

2. Hydrophobic hydrocarbon chains/fatty acid chains have LOW permeability to large molecules and hydrophilic particles

Describe the selectivity in membrane permeability

1. Permeability depends on size and hydrophilic/hydrophobic properties

2. Permeable to SMALL, HYDROPHOBIC molecules (oxygen, carbon dixide) via simple diffusion and SMALL, POLAR (h2o, glycerol)

3. Not permeable to LARGE (glucose, amino acids) IONIC (H+, Na+) substances

Describe the movement of water by osmosis and aquaraporins

OSMOSIS:

1. Movement of water from low to high SOLUTE concentration through membrane by diffusion

2. Passive transport, doesn't require ATP

3. Diffusion accurs through the random movement of water from high to low WATER potential

4. Solutes are not permeable to membrane -> lowers water potential

5. Water is nearly always permeable to membranes

AQUAPORINS:

1. Aquaporins are channel proteins that move water across the membrane by facilitated diffusion

2. Transports specific molecules

Outline simple diffusion

1. Movement of molecules down the concentration gradient from high to low concentration

2. Passive transport, doesn't require ATP

3. Molecule must be permeable

• small & hydrophobic (o2, co2)

• small & polar (h2o)

• non-polar molecules (LIKE STEROIDS!)

Describe the relationship between fatty acid composition and their fluidity

1. SATURATED fatty acids DECREASE fluidity, INCREASES hydrophobic reactions (straight)

2. UNSATURATED fatty acids INCREASE fluidity, decreases hydrophobic interactions (bent)

Describe the role of cholesterol in membrane fluidity in animal cells

1. Modulator of membrane fluidity

2. @ LOW temp, it INCREASES fluidity: prevents stiffening of the membrane, disrupts intermolecular forces between hydrocarbon tails

3. @ HIGH temp, REDUCES fluidity, stabilising membrane, restrict phospholipid movement

4. Polar hydroxyl group associates with hydrophilic phosphate head

5. Hydrocarbon rings associates with hydrophobic fatty acid tails

Outline the funcitons of membrane fluidity

1. Fluidity of memrbane allows for materials to be taken in by cells (endocytosis) or released (exocytosis) via vesicles

2. Exocytosis: - vesicles fuse with plasma membrane

• materials are released

• exocytosis of proteins/enzymes/hormones synthesised on rough endoplasmic reticulum

3. Endocytosis: plasma membrane fluidity pinches/invaginates membranes

• vesicle is formed

• material taken into cell