Unit 1 - Membrane Proteins and Signalling

Question 1

Cyclic GMP (cGMP)

Answer 1

Binds to ligand gated sodium channels and keeps them open. When hydrolysed to GMP by the enzyme PDE the sodium channels close. This causes a build up of sodium ions ouside the cell resulting in hyperpolarisation and the generation of a nerve impulse.

Question 2

Transducin

Answer 2

A G-protein that is activated by photoexcited rhodopsin. One rhodopsin activates many G-proteins, creating a cascade. Each transduction activates one molecule of the enzyme phosphodiesterase (PDE).

Question 3

Rhodopsin

Answer 3

A retinal-opsin complex embedded in photoreceptor membranes (rods and cones) in the retina. Changes conformation to become photoexcited rhodopsin when retinal absorbs light, triggering a cascade that amplifies the photon signal.

Question 4

Retinal

Answer 4

A light sensitive molecule derived from vitamin A that captures light energy. It is a prosthetic group which binds to the protein opsin.

Question 5

Transporter proteins

Answer 5

Bind to the ions or molecules to be transported and undergo a conformational change. Can be facilitated diffusion or active transport.

Question 6

Voltage gated channels

Answer 6

Change conformation as a result of a change in ion concentration (voltage) across a membrane eg. during nerve impulse transmission

Question 7

Ligand gated channels

Answer 7

Specific channels that change conformation and open when a signal molecule bonds to it. eg. sodium channels in the retina and at synapses

Question 8

Channel proteins

Answer 8

Selective proteins that allow facilitated diffusion of ions and polar molecules through the membrane without changing their conformation.

Question 9

Specific transmembrane proteins

Answer 9

Channels or transporter proteins that span the membrane and allow ions or polar molecules through.

Question 10

Small non-polar molecules

Answer 10

Include oxygen and carbon dioxide. They are able to pass freely through the phospholipid bilayer. Ions and polar molecules cannot.

Question 11

Peripheral proteins

Answer 11

Are not embedded in the phospholipid bilayer but are bound to the membrane surface by ionic and hydrogen bonding with hydrophilic R groups on their surface.

Question 12

Integral proteins

Answer 12

Penetrate the hydrophobic interior of the membrane and are held there by strong hydrophobic interactions between the phospholipid tails and hydrophobic R groups of the amino acids.

Question 13

Phospholipid bilayer

Answer 13

A double layer of phospholipids with the hydrophobic tails pointing inwards and the hydrophilic heads pointing towards the aqueous cytoplasm and external fluid surrounding the cell.

Question 14

Fluid mosaic model

Answer 14

Cell membranes have this structure - a phospholipid bilayer containing integral or peripheral globular proteins arranged in a ‘patchwork’ or ‘mosaic’

Question 15

Sodium-potassium pump (Na+/K+ATPase)

Answer 15

Transmembrane transporter found in most animal cells.

Actively transports sodium ions out of cells and potassium ions into cells along steep concentration gradients, using energy from hydrolysis of ATP.

3 sodium go out, 2 potassium come in (3-S-O-P-I-2)

The pump has 2 stable conformational states with differing affinities for the 2 ions.

Question 16

Glucose symport

Answer 16

Found in the small intestine. The sodium-potassium pump generates a sodium ion gradient across the membrane, which drives the active transport of glucose.

The glucose transporter transports sodium ions and glucose at the same time, in the same direction (from outside the cell, back in) but sodium ions are moving down a concentration gradient, glucose against.

Question 17

Receptor

Answer 17

Globular protein with a binding site for a specific signaling molecule.

Found on target cells. Binding of the signalling molecule changes the conformation of the receptor, and initiates a response in the target cell.

Question 18

Hormone

Answer 18

Extracellular signalling molecule secreted by an endocrine gland into the blood.

The hormone circulates in the blood until it reaches the receptor of its target cell, or is broken down.

Question 19

Hydrophobic hormones

Answer 19

Hydrophobic hormones are steroid hormones such as oestrogen and testosterone.

Question 20

Hydrophilic hormones

Answer 20

Peptide hormones such as insulin, glucagon, growth hormone (somatotrophin) and ADH (anti-diuretic hormone).

Question 21

Neurotransmitters

Answer 21

Substances released into the synaptic cleft (the gap between nerve cells).

They are hydrophilic signalling molecules that allow transmission of a nerve impulse. eg acetylcholine and noradrenaline.

Question 22

Hydrophobic signals

Answer 22

They are lipid soluble and pass through membranes, binding to intracellular receptors in the cytosol or nucleus of the target cell. eg steroid hormones

The receptors bind to their specific hormone and act as transcription factors, binding to DNA and stimulating or inhibiting transcription.

The hormone-receptor complex binds to specific DNA sequences - hormone response elements (HRES) which influences the rate of transcription.

Question 23

Hydrophilic signals

Answer 23

Peptide hormones and neurotransmitters.

They are unable to cross the hydrophobic part of the plasma membrane, so bind instead to transmembrane receptor molecules.

This results in signal transduction.

Question 24

Signal transduction

Answer 24

Transmembrane receptor molecules for specific hydrophilic signalling molecules change conformation when the signal ligand binds.

The signal is transduced - the behaviour of the target cell changes in response to the binding of the signalling molecule. The signal does not enter the cell.

Transduced signals may involve G-proteins or cascades of phosphorylation by kinases.

Question 25

G-proteins

Answer 25

Relay signals from activated receptors (receptors with a signalling molecule attached) to target proteins such as enzymes and ion channels.

This activates a signalling pathway inside the cell.

Question 26

Phosphorylation cascade

Answer 26

Allows more than one signalling pathway to be activated inside the cell.

One kinase activates the next one in the sequence and so on, resulting in the phosphorylation of many proteins.

Question 27

Insulin receptor

Answer 27

A kinase linked receptor in the plasma membrane of fat and muscle cells.

Once insulin binds, the signal is transduced triggering a phosphorylation cascade that triggers the recruitment of GLUT4 glucose transporters to the plasma membrane.

The transporters allow glucose to enter the cell, reducing blood glucose level.

Question 28

Diabetes mellitus - type 1

Answer 28

Loss of control of blood glucose level caused by a failure to produce insulin.

Type 1 is treated with insulin injections.

Question 29

Diabetes mellitus - type 2

Answer 29

Caused by loss of receptor function (type 2).

Associated with obesity - cells are no longer sensitive to insulin.

Exercise triggers recruitment of GLUT4, therefore improving uptake of glucose to fat and muscle cells, thus reducing blood glucose levels.

Question 30

Resting membrane potential

Answer 30

A state where there is no net flow of ions across a membrane.

The sodium-potassium pump moves 3 sodium ions out, and 2 potassium ions into a neuron.
Potassium channels allow some of the potassium to leak back out.

This results in a positive charge outside the cell, compared to the inside.

Resting potential = -70mV

Question 31

Action potential

Answer 31

A wave of electrical excitation along the plasma membrane of a neuron.

Question 32

Depolarisation

Answer 32

A sudden reversal of the resting potential, caused by an influx of sodium ions through a ligand gated ion channel.

The ligand gated channel opens due to the binding of a neurotransmitter.

If there is sufficient ion movement, it triggers voltage gated sodium channels in the next section of membrane to open, causing it to depolarise. The effect travels like a wave along the neuron.

Question 33

Repolarisation

Answer 33

Once a critical voltage is reached, the voltage gated sodium ion channels close, voltage gated potassium ion channels open and potassium ions diffuse out of the neuron restoring resting potential.

Once resting potential is reached, the potassium channels close, and the sodium-potassium pump (which keeps working throughout) resets the ion gradients.

Question 34

Synapse

Answer 34

The gap between neurons. The wave of depolarisation triggers voltage gated calcium ion channels to open.

The influx of calcium ions triggers the release of hydrophilic neurotransmitters into the gap, via the secretory pathway.

The neurotransmitters bind to ligand gated sodium channels in the membrane of the target cell, opening them and causing depolarisation and transmission of the nerve impulse.

Question 35

Photopsin

Answer 35

Retinal combines with different forms of opsin, so absorb light of different wavelengths. This results in colour vision in cone cells.