Membrane Proteins Flashcards
(14 cards)
What is the fluid mosaic model?
The fluid mosaic model is the membrane of cells made up of a phospholipids bilayer with globular proteins penetrating the bilayer or attached to the surface.
Describe the structure of the phospholipid molecule.
A phospholipid molecule has a hydrophilic head and hydrophobic tails. When they form a bilayer, the heads face toward the cytoplasm and the aqueous external fluid and the hydrophobic tails stay on the inside of the bilayer.
In what two ways could proteins in the membranes be arranged?
INTEGRAL proteins penetrate the hydrophobic interior of the phospholipid bilayer and are folded so that they have regions of folded hydrophobic R groups. These form strong hydrophobic interactions which hold integral membrane proteins within the phospholipid bilayers. Some integral proteins only extend partly into the bilayer, however others are transmembrane proteins that span the width of the membrane.
PERIPHERAL proteins are not embedded into the bilayer, instead they have hydrophilic R groups on their surface and are bound to the surface of these membranes mainly by ionic and hydrogen bonds. Some peripheral proteins on the inside of the membrane are attached to the cytoskeleton, which helps give mechanical support and shape to the cells.
The hydrophobic centre of a phospholipid bilayer allows what to pass through?
The hydrophobic centre of the phospholipid bilayer allows oxygen and carbon dioxide to pass through directly as they are small non-polar molecules.
The phospholipid bilayer is a barrier to what?
The phospholipid bilayer is a barrier to ions and most uncharged polar molecules.
How can ions and polar molecules pass through the phospholipid bilayer?
Any ion and polar molecule that can pass through the membrane can only do so through a specific channel or transporter protein.
What are channel proteins and what are their function?
Channel proteins are multi-subunit proteins with the subunits arranged to form water filled pores that extend across the membrane.
Ions or molecules which pass through channel proteins require no change in conformation due to facilitated diffusion. The proteins make it easier for the ion or molecule to move passively across the membrane.
Describe in detail the two types of gated channels.
LIGAND GATED CHANNELS
- signal molecules bind to the ligand gated channels and change its conformation.
- in order to pass a signal, a neurotransmitter binds in order to trigger the ligand gated channel to open and let Na+ ions flow through.
- in rods and cones in the retina, the ligand keeps the sodium channel closed until light activates a pathway to release the ligand and the channels let ions through to generate a signal.
VOLTAGE GATED CHANNELS
- if there is a large enough change in ion concentrations across the membrane, the conformation of a voltage gated channel protein may change.
- in the transmission of a nerve impulse along a nerve, the movement of ions across the membrane causes voltage across the membrane to reach a critical level so sodium channels open. The voltage change continues as ions move across the membrane causing the gate to close again.
What are transporter proteins?
Transporter proteins are proteins which bind to ions or molecules to be transported causing the protein to undergo a conformational change. The proteins don’t just provide a route through the membrane, but carries them across.
Transporters alternate between 2 conformations so that the binding site for a solute is exposed on one side of the bilayer then the other.
How does active transport work?
Active transport uses pump proteins that transfer substances across the membrane against their concentration gradient. A source of metabolic energy is required for active transport.
Pumps that mediate active transport are transporter proteins coupled to an energy source. Some active transport proteins hydrolyse ATP directly to promote energy for the conformational change required to move substances across the membrane.
What is the sodium potassium pump?
The sodium potassium pump is a vital transmembrane transporter protein found in most animal cells. The pump does active transport, moving sodium ions out of cells and potassium into cells against steep concentration gradients by using ATP energy directly from ATP hydrolysis.
How does the sodium potassium pump work?
The sodium potassium pump has two stable conformational states.
One with high affinity for the sodium ions inside the cell
Another with high affinity for the potassium ions outside the cell.
For each ATP hydrolysed, 3 sodium’s are transported out and 2 potassium’s transported in. This generates an electrochemical gradient across the membrane.
Summarise the process mechanism of the sodium potassium pump.
STEP 1 - the transporter protein has its ion binding sites exposed to the cytoplasm. This protein has a high affinity for sodium ions and 3 Na+ ions bind to the binding sites.
STEP 2 - when sodium ions attach, the transporter protein is able to hydrolyse ATP to ADP and phosphate ATPases hydrolyse ATP. The phosphate attaches to the protein to phosphorylate it and this causes a conformational change in the protein.
STEP 3 - this new conformation has its ion binding sites exposed to the outside of the cell. As it now has decreased affinity for sodium ions, Na+ ions are released out.
STEP 4 - the new conformation however has a high affinity for potassium ions and two K+ ions bind to the exposed ion binding sites outside the cell to be taken in. This triggers de-phosphorylation and releases the phosphate group from the protein.
STEP 5 - the de-phosphorylation causes the protein to revert to its original confirmation with the binding sites exposed to the cytoplasm.
STEP 6 - this conformation has a low affinity for the potassium ions therefore releases the K+ ions into the cell. The transport proteins revert back to having a high affinity for sodium ions and the process repeats.
REFER TO DIAGRAM IN NOTEBOOK FOR VISUAL REPRESENTATION!
Why is the sodium potassium pump important?
The sodium potassium pump accounts for high properties of basal metabolic rate in many organisms and is found in most animal cells.
It’s important uses include:
1. General maintenance of ion gradients for resting potential in neurons.
2. Glucose transport in small intestine. The sodium potassium pump generates a sodium ion gradient across the plasma membrane of the intestinal epithelial cells which then drives the active transport of glucose.
—— the glucose transporter responsible for this glucose symport transports sodium and glucose at the same time in the same direction. As sodium ions enter the cell down their concentration gradient, the glucose is pumped against its concentration gradient into the cell and vice versa.
