AH BIOLOGY: CELLS AND PROTEINS- PPT 6 MEMBRANE PROTEINS: CHANNEL AND TRANSPORT PROTEINS

THIS TOPIC LOOKS AT SPECIFIC FUNCTIONS OF MEMBRANE PROTEINS IN RELATION TO THE MOVEMENT OF MOLECULES THROUGH THE MEMBRANE

PHOSPHOLIPID BILAYER IS A BARRIER TO IONS AND MOST UNCHARGED POLAR MOLECULES. TRANSMEMBRANE PROTEINS INCLUDE CHANNEL PROTEINS AND TRANSPORT PROTEINS

CHANNEL PROTEINS allow passive movement of molecules. They include aquaporin (water transport), ligand gated ion channels or voltage gated ion channels.

AQUAPORINS

LIGAND GATED ION CHANNELS Opened by a signal molecule binding to the protein giving a conformational change. E.g. neuromuscular junctions and synaptic junctions between nerve cells. Transmitting an electrically based nerve action potential across the gap between nerves or neuromuscular junctions. The nerve action potential is transduced to a chemical signal that can cross the synapse gap, giving rise to an action potential in the adjoining nerve cell or triggering myofibril contraction in a muscle cell. Neurotransmitters? revise

VOLTAGE GATED ION CHANNELS Change in polarisation of a membrane results in the opening or closure of voltage-gated ion channels. Good examples are the Na + and K + voltage channels associated with delivering a nerve impulse along a nerve axon.

TRANSPORTER PROTEINS Involved in facilitated transport and active transport. Proteins change conformation to transport proteins Including facilitated glucose transport with sodium and the sodium/potassium pump (Na/K ATPase) where energy comes from hydrolysis of ATP to cause the change

TRANSPORT PROTEINS Transports single molecules down conc. gradient Transports molecules coupled so as one moves another one does too e.g. glucose and Na same direction Transports molecules coupled but in opposite directions e.g. Na/K

TRANSPORTER PROTEINS Passive Active

SODIUM POTASSIUM PUMP Involved in re-establishing the balance of sodium and potassium between extracellular and intracellular fluids.

The transporter protein has high affinity for 3 Na + ions. ATP is reduced adding phosphate to transporter (phosphorlyation). Protein opens on extracellular side, pumping out the 3 Na + ions. The 2 sites for K + become exposed and have high affinity for the ions.

2 K + ions attached to transport protein. The phosphate is released (dephosphorylation). The K + ions are released inside the cell. The protein now has a high affinity for Na + again. This is active and uses energy as both ions are being transported against the concentration gradient.

THIS MECHANISM IS INVOLVED IN The osmotic balance in animal cells. Glucose transport (symport) in small intestine. Generation and long term maintenance of the ion gradient in neuron resting potential. Ion gradient in the kidney tubule.

Na/K ATPase may account for 25% of the basal metabolic rate in humans maintaining ion gradients

Signals can be passed through membrane proteins in signal transduction. Connecting extracellular chemical signals to intracellular responses including Activation of enzymes or G proteins, Rearrangement of the cytoskeleton, Activation of proteins that regulate gene transcription.