Membrane Proteins

Question 1

What is the fluid mosaic model ?

Answer 1

The model of the cell membrane

Fluid = the membrane is constantly moving and can change shape.
Mosaic = refers to the patchy arrangement of protein molecules dispersed throughout.

Question 2

What are the two types of membrane proteins

Answer 2

Peripheral and Integral

Question 3

What are peripheral proteins ?

Answer 3

Peripheral proteins are not a part of or sunk into the the plasma membrane, they only stick on.

-Peripheral membrane proteins have hydrophilic R groups on their surface and are bound to the surface of membranes, mainly by ionic and hydrogen bond interactions

-Many peripheral membrane proteins interact with the surfaces of integral membrane proteins

Question 4

What are integral proteins ?

Answer 4

Proteins are a part of the plasma membrane and are sunk into it.

-Regions of hydrophobic R groups allow strong hydrophobic interactions that hold integral membrane proteins within the phospholipid bilayer

-Integral membrane proteins interact extensively with the hydrophobic region of membrane phospholipids.

-Some integral membrane proteins are transmembrane proteins.

Question 5

What are the types of transmembrane proteins ?

Answer 5

Proteins which act as channels and as transporters.

-They act to control the concentration of ions and other molecules within the cell.

-To perform specialised functions, different cell types have different channel and transporter proteins

Question 6

What is the phospholipid bilayer ?

Answer 6

-The phospholipid bilayer is a barrier to ions and most uncharged polar molecules, due to its hydrophobic nature.

-Some small molecules, such as oxygen and carbon dioxide, pass through the bilayer by simple diffusion.

Question 7

What is facilitated diffusion ?

Answer 7

-Some molecules (too big, ions, most uncharged polar, etc) require specific transmembrane proteins to allow them to diffuse across the membrane.

-Facilitated diffusion is the passive transport of substances across the membrane through specific transmembrane proteins

Question 8

What do transmembrane proteins which allow for facilitated diffusion act as ?

Answer 8

Channels or transporters.

Question 9

What are channel proteins ?

Answer 9

Channels are multi-subunit (transmembrane) proteins with the subunits arranged to form water-filled pores that extend across the membrane.

-Most channel proteins in animal and plant cells are highly selective

-Some channel proteins are gated and change conformation to allow or prevent diffusion (Ligand and voltage-gated channels)

-Allow for facilitated diffusion

Question 10

What are gated channel proteins ?

Answer 10

-Some channel proteins are gated. Of these, some change conformation to open or close to allow or prevent the diffusion of their ion.

-Gated channels respond to a stimulus which causes them to open or close.

Question 11

What are ligand and voltage-gated ion channels ?

Answer 11

-Ligand-gated channels are controlled by the binding of signal molecules, to the receptor protein.

-Voltage-gated channels are controlled by changes in ion concentration (and thus, the membrane potential)

Question 12

What are examples of channel proteins ?

Answer 12

1) Aquaporins (ungated water channels)

2) Ligand-gated ion channels

3) Voltage-gated ion channels

Question 13

What are transporter proteins ?

Answer 13

-Transporter proteins bind to the specific substance to be transported and undergo a conformational change to transfer the solute across the membrane

-Transporters alternate between two conformations so that the binding site for a solute is sequentially exposed on one side of the bilayer, then the other.

-Also allows for facilitated diffusion and can also act as pumps

Transporter proteins are specific to to one type of ion or molecule

Question 14

What transmembrane proteins does active transport use ?

Answer 14

-Active transport uses pump proteins that transfer substances across the membrane against their concentration gradient

-Pumps that mediate active transport are transporter proteins coupled to an energy source.

-A source of metabolic energy is required for active transport

Question 15

How to pump proteins get energy for active transport ?

Answer 15

-Some active transport proteins hydrolyse ATP directly to provide the energy for the conformational change required to move substances across the membrane

-ATPases hydrolyse ATP.

Question 16

What is the electrochemical gradient ?

Answer 16

For a solute carrying a net charge, the concentration gradient and the electrical potential difference combine to form the electrochemical gradient that determines the transport of the solute

Question 17

What is a membrane potential ?

Answer 17

A membrane potential (an electrical potential difference) is created when there is a difference in electrical charge on the two sides of the membrane.

Question 18

What is electrical potential

Answer 18

All cells have an electrical potential difference (voltage) across their plasma membrane

(Minus sign inside cell means inside of cell is negative relative to outside)

Question 19

What do ion pumps do ?

Answer 19

-Ion pumps, such as the sodium-potassium pump, use energy from the hydrolysis of ATP to establish and maintain ion gradients.

-ATP is hydrolysed directly to ADP and an inorganic phosphate, the energy released is used to move one or more ions across the cell membrane;

Question 20

What does the sodium-potassium pump (Na/K ATPase) do ?

Answer 20

-The sodium-potassium pump (transporter protein) transports ions against a steep concentration gradient using energy directly from ATP hydrolysis

-It actively transports sodium ions out of the cell and potassium ions into the cell; for each ATP hydrolysed, three sodium ions are transported out of the cell and two potassium ions are transported into the cell. This establishes and maintains both concentration gradients and an electrical gradient.

Question 21

Describe cell ions and charges at the beginning of action of the sodium-potassium pump

Answer 21

All cells have a higher concentration of Na+ outside the cell than inside, and a higher concentration of K+ inside the cell than outside.

The inside of the cell is relatively negative, and the outside is relatively positive.

Question 22

Explain the action of the sodium potassium pump

Answer 22

1) The pump has high affinity for sodium ions inside the cell; binding of 3 sodium ions occurs at binding sites.

2) Stage 1 stimulates phosphorylation by ATP; addition of phosphate group causes conformational change which opens protein to extracellular side.

3) Second conformation has a decreased affinity for sodium ions; sodium ions released outside of the cell.

4) Second conformation has a higher affinity for potassium ions, 2 K+ ions bind to specific sites exposed outside the cell.

5) Stage 4 causes dephosphorylation of the transporter protein; conformation changes back again - opening on intracellular side.

6) Original conformation has low affinity for potassium ions so are taken into cell cytosol; affinity for sodium returns to start.

The pump (Na/K ATPase) has two stable conformational states; one has high affinity for sodium ions inside the cell, one has high affinity for potassium ions outside the cell.

Question 23

Where are Na/K pumps found ?

Answer 23

The sodium-potassium pump is found in most animal cells, accounting for a high proportion of the basal metabolic rate (25%) in many organisms

Question 24

Describe the role of the sodium-potassium pump in the epithelial cells lining the small intestine

Answer 24

In the small intestine, the sodium gradient created by the sodium-potassium pump drives the active transport of glucose.

-In intestinal epithelial cells the sodium-potassium pump generates a sodium ion gradient across the plasma membrane, actively transports sodium ions into bloodstream so creates low concentration in cytosol. (Also pumps K+ in, not relevant)

-Sodium ions enter the cell down their concentration gradient; this is coupled to and simultaneous with the transport of glucose being pumped into the cell against its concentration gradient (AT), from the intestine lumen into the epithelial cells, by a glucose symporter protein .

-The glucose transporter responsible for this glucose symport transports sodium ions and glucose at the same time and in the same direction

-A GLUT glucose transporter removes glucose from the epithelial cell cytosol into the bloodstream (by diffusion)

Question 25

What do the different structure in villi absorb ?

Answer 25

Lacteal - Absorbs fatty acids and glycerol

Blood capillary - Absorbs glucose and amino acids, by active transport