b2.1 membranes and membrane transport

why are lipid bilayers effective barriers?

• larger molecules are unable to pass through the membrane as the phospholipids are packed very tightly together, smaller but polar molecules like ions are also unable to pass through the bilayer due to the hydrophobic region in the middle. since the cell needs large/polar molecules to be transported in and out of the cell, the bilayer controls the movement of substances through the membrane

which substances can have simple diffusion across the membrane

o2 and co2 molecules, they are small and uncharged

integral vs peripheral proteins

Integral proteins are

• embedded in one or both of the lipid layers of a membrane.

• have amphipathic character with hydrophobic in mid section and hydrophilic regions at the ends , it is embedded into the membrane

Peripheral proteins are

• attached to one or other surface of the bilayer.

• do not protrude into middle hydrophobic region of membrane, bound to outside of the membrane, inside or outside the cell. sometimes bound to integral proteins

Movement of water molecules across membranes by osmosis and the role of aquaporins

• Osmosis is a type of passive transport, which involves the passive movement of water across a partially permeable membrane

• Water molecules move from a hypotonic solution to a hypertonic solution, and osmosis occurs until equilibrium is reached, where there is no net movement of water

• Since water is a polar molecule, it cannot pass through the cell membrane of the hydrophobic properties of the middle membrane region. Hence, most cell membranes have protein channels called aquaporins which allow water molecules to pass through them, allowing water to quickly enter and exit the cell

channel proteins - facilitated diffusion

have hydrophilic pores by which molecules of appropriate size and charge can pass, have gates that open or close in response to chemical or mechanical stimulus, only carry water soluble molecules and are specific for the ion they carry

pumps for active transport

pumps use energy from adenosine triphosphate (ATP) to transfer specific particles across membranes and therefore that they can move particles against a concentration gradient

Selectivity in membrane permeability in which 2 methods of transport?

Facilitated diffusion and active transport allow selective permeability in membranes. Permeability by simple diffusion is not selective and depends only on the size and hydrophilic or hydrophobic properties of particles.

glycoproteins and glycolipids and their functions

• glycolipids→cell membrane phospholipids have carbohydrate chains attached to them

• glycoproteins→cell membrane proteins have carbohydrate chains attached to them

• allows for cell adhesion and cell recognition

factors affecting the fluidity of the membrane

• degree of saturation of fatty acid tails→when degree of saturation is lower, fluidity increases as unsaturated fatty acids have a double bond which forms a kink, preventing the fatty acids from packing more closely together, which decreases hydrophobic interactions and hence membrane is more fluid

• length of fatty acid tails→shorter fatty acid tails result in a more fluid membrane

• temperature (assume high)→at high temperatures, particles move further away from each other, due to high KE, disintegrating and increasing the fluidity of the membrane. however, this is COUNTERACTED by a high number of saturated fatty acid tails at high temperatures, which allows the fatty acid tails to be more closely packed, increasing hydrophobic interactions and increasing strength of the bonds, causing membrane to be less fluid

Cholesterol 

• cholesterol at high temp→maintains structural integrity as the bonds between cholestrol and phospholipids prevent them from moving too far away from each other

• cholesterol at low temp→increases fluidity by disrupting regular packing of hydrocarbon tails that prevent them from solidifying at low temp

endo and exocytosis

• endocytosis→portion of the plasma membrane is pinched off to enclose macromolecules in a vesicle (vesicle closes due to hydrophilic/hydrophobic properties of phospholipids in water)

• exocytosis→a vesicle fuses with plasma membrane to secrete contents from the cell

gated ion channels

channels that open or close due to chemical or voltage stimulus, allowing ions to quickly pass through them

neurotransmitter gated ion channel (nicotinic acetyl choline)

when acetylcholine binds to the nicotinic acetylcholine receptor, the channel opens and ions like Na+, K+ and Ca2+ can pass through. the membrane potential is changed and an impulse is triggered, causing muscle movement

voltage gated ion channel (sodium potassium)

• Electrical stimulus: membrane polarity changes

• Sodium channels open first – Na moves from outside of neurone to inside, depolarizing the membrane

• Sodium channels close quickly

• Potassium channels open – K moves from inside of cell to outside; membrane returns to normal potential (repolarization)

sodium potassium pump

pumps 3 na out and 2 k in to maintain concentration gradient for the generation of membrane potential

sodium dependent glucose cotransporters

• indirect active transport→use the energy produced by the movement of one molecule down a conc gradient to transport another molecule against conc gradient

• for transport of glucose into intestinal/nephron cells, there is a high conc of glucose in cell but glucose needs to be absorbed into the cells so ATP is required

• there are more sodium ions outside than inside the intestinal cells, sodium and glucose bind to a specific protein on the cell membrane

• sodium travels down the conc gradient into the cell through the carrier which captures the energy produced by the movement

• the captured energy is used to transport glucose against the concentration gradient into the cell through the same protein

glucose absorption by cells in the small intestine and glucose reabsorption by cells in the nephron

cell adhesion molecules

there are diff CAMs for diff cell to cell junctions

eg desmosomes, plasmodesmata