Membrane Proteins

Question 1

What is facilitated transport?

Answer 1

movement of small water soluble molecules across the cell membrane membrane channels and transporters

Question 2

what does diffusion rate depend on? How do small non polar molecules, polar molecules and ions cross?

Answer 2

Size and solubility properties of the molecule:

• small non polar molecules will diffuse via concentration gradient
• uncharged polar molecules will diffuse if small or will be transported if large via concentration gradient
• ions will not cross the bilayer freely and will move based on electrochemical gradient

Question 3

What is membrane potential?

Answer 3

Voltage across the cell membrane ranging between -20 and -200 mV

Question 4

What does the membrane potential favour and oppose?

Answer 4

FAVOURS entry of positively charged ions

OPPOSES entry of negatively charged ions or exit of positively charged ions

Question 5

What is a membrane channel?

Answer 5

A transmembrane protein that select ions based on size and electric charge that exist in an open or closed state

Question 6

what is a membrane transporter?

Answer 6

Transmembrane protein that transports molecules or ions that fit into specific binding sites and change shape to move substances across the membrane

Question 7

What is passive transport?

Answer 7

Moving solutes down their concentration gradient without the use of energy (e.g. all channels, many transporters)

Question 8

What is active transport?

Answer 8

Moving a solute against its concentration gradient with the use of ATP, light, or ion gradient (e.g. specific transporters called pumps)

Question 9

What is the difference between transport of uncharged solutes and charged solutes?

Answer 9

Uncharged solutes will want to flow based on concentration gradient

Charged solutes will want to flow based on electrochemical gradient

Question 10

what are aquaporins?

Answer 10

channel proteins that move water across the cell membrane via osmosis

Question 11

How do protozoan, plants and animal cells avoid osmotic swelling?

Answer 11

Protozoan: eliminate excess water via contractile vacuole

Plant: Rigid cell wall

Animals: transmembrane pumps expel solutes shifting osmotic gradient out of cell

Question 12

What are passive transporters?

Answer 12

Transmembrane proteins that move a solute along its electrochemical gradient (glucose transporter)

Question 13

How does the glucose transporter work?

Answer 13

Glucose will bind to a specific binding site on the glucose transporter and will cause a conformational change in the transporter that will move glucose to the other side of the membrane. The direction is based on the concentration gradient.

Question 14

What are three types of pumps and examples?

Answer 14

ATP driven pump: uses ATP hydrolysis to move solutes to a higher concentration (Na+/K+ pump)

Gradient driven pump: energy from downhill transport of one solute is used for the uphill transport of another

Light-driven pump: uses energy from sunlight to move solutes uphill

Question 15

What does the Na+/K+ pump do?

Answer 15

Uses energy of ATP hydrolysis to pump 3 Na+ out and 2K+ in simultaneously to keep cytosolic Na+ low and K+ high

Question 16

What are the steps of the Na+/K+ pump?

Answer 16

1. Na+ binds
2. Binding causes pump to phosphorylate
3. Conformational change due to phosphorylation and ejection of Na+ outside the cell
4. New shape allows K+ to bind
5. Binding dephosphorylates
6. Dephosphorylation triggers conformational change back to original shape and ejects K+

Question 17

How does the Ca2+ work?

Answer 17

1. Ca2+ binds to site on pump
2. Binding causes phosphorylation
3. Phosphorylation changes shape and changes shape of protein
4. Change of shape ejects Ca2+ into the sarcoplasmic reticulum
5. Ejection of Ca2+ results in dephosphorylation

Question 18

What are the three types of gradient driven pumps?

Answer 18

Symport: pump that moves solute 1 downhill and solute 2 uphill in the same direction

Antiport: pump that moves solute 1 uphill and solute 2 down hill in opposite directioons

Uniport: transporter that moves one type oof solute downhill

Question 19

What is the Glucose-Na+ symport?

Answer 19

Symport pump that brings glucose against its concentration gradient into the cell by bringing Na+ down its electrochemical gradient.

Question 20

How does the Glucose-Na+ symport work?

Answer 20

1. Na+ binds to empty protein
2. Na+ binding allows glucose to bind
3. Both solutes occupying membrane allows it to eject both into cytosol

Question 21

How do gut epithelial cells take up glucose?

Answer 21

1. At apical side, glucose-Na+ symport actively takes up glucose against concentration gradient
2. At basal domain, glucose is taken down its concentration gradient via uniport via facilitated diffusion

Question 22

How is resting membrane potential established?

Answer 22

Na+/K+ keeps cytosolic Na+ low and K+ high

Question 23

How do Na+ ions stay outside the cell membrane if K+ channels are large enough for them to pass?

Answer 23

Narrow regions in the ion channel force the ions to touch the channel wall too allow them to pass. Na+ are to small to interact with the K+ channel walls.

Question 24

What is a patch clamp recording?

Answer 24

Type of mechanism that monitors ion flow through a single channel in a small isolated patch of membrane. Currents appear and disappear during opening and closing of channels.

Question 25

What are types of ion channels?

Answer 25

Voltage gated: controlled by membrane potential

Ligand gated: controlled by binding of ligand

Mechanically gated: controlled by mechanical force applied to the channel

Question 26

How do auditory hair cells work?

Answer 26

Sound vibrations tilt stereocilia in a hair cell, which stretches the mechanically gated channels open allowing positively charged ions to enter from the surrounding fluid and cause an action potential.