Lecture 19

Question 1

What is diffusion?

Answer 1

the process which molecules move from an area of high concentration to an area of low concentration by random thermally driven motion

Question 2

Rate of diffusion across membranes depends on

Answer 2

1. Size: smaller = faster diffusion
2. solubility: hydrophobic = faster diffusion

Question 3

Order molecules on their membrane permeability

Answer 3

1. small non polar molecules (O2)
2. small uncharged polar molecule (H2O)
3. larger uncharged polar molecules (amino acid)
4. ions (Cl-)

Question 4

What ions concentration contribute the most in the cell

Answer 4

Na+ : in = [5-15], out = [145]
K+ : in = 140, out = [5]
Cl- : in = [5-15], out = [110]

Question 5

what is membrane potential

Answer 5

• it is the voltage difference across the membrane around -200mV

Question 6

What is the difference between transporters and channels

Answer 6

1. transporters: transport only molecules or ions that fit into specific binding sites
2. channels: transport solutes through channels, discriminating based on size and electric charge

Question 7

What is passive transport

Answer 7

• spontaneous movement of solute down its electrochemical gradient
• both channels and transporters can perform passive transport
• it is also possible that passive transport require energy

Question 8

What is active transport

Answer 8

• movement of a solute against its electrochemical gradient
• performed by transporters known as “pumps” which utilize energy

Question 9

what the the electrochemical gradient

Answer 9

• the driving force determining how an ion will move across a plasma membrane. it is a combined influence of ions concentration and membrane potential

Question 10

Why doesn’t K+ diffuse out of the cell spontaneously

Answer 10

• it doesn’t because the membrane potential is negative therefore the K+ ions are attracted back into the cell due to it’s electrochemical gradient

Question 11

what is osmosis

Answer 11

• osmosis is the movement of water down its concentration gradient from and area of low solute concentration to an area of high solute concentration

Question 12

what are aquaporins

Answer 12

• special channel proteins that allow the flow of water across the plasma membrane

Question 13

glucose transporter

Answer 13

• multipass transmembrane protein
• performs passive transport by changing conformations
• after a meal, concentration of glucose is high and bind to liver cell, causing its import into the cytosol
• when blood sugar is low, glucagon stimulates liver cell breakage of glycogen to release glucose in the blood resulting in a higher concentration of glucose in the cell than outside the cell
• the flow of glucose can go in either ways according the direction of the concentration gradient

Question 14

What are pumps? And provide examples

Answer 14

Pumps perform active transport in 3 way:
1. Coupled pump: link uphill transport of one solute with the downhill transport of another
2. ATP-driven pump: hydrolyze ATP to drive uphill transport
3. Light-driven pump: use energy derived from sunlight to drive uphill transport

Question 15

The Na+ - K+ pump

Answer 15

• pumps 3 Na+ out of the cell and 2 K+ into the cell at the expense of ATP. A phosphate is transiently added to the pump
• maintains a steep concentration gradient: high Na+ outside of the cell and high k+ inside the cell
• high [Na+] outside of the cell acts like a dam, storing energy. Influx of Na+ can be coupled with the transport of other molecules

Question 16

The Ca2+ pump

Answer 16

• pumps Ca2+ outside of the cell at the expense of ATP
• Ca2+ can modify the activity of a variety of proteins (involved in cell communication and muscle contraction)
• pump can be found in plasma membrane and the ER membrane (both pump the Ca2+ out of the cytosol)
• similar structure and mechanism to the Na+ pump

Question 17

Uniport

Answer 17

• only ferries one solute

Question 18

Symport

Answer 18

• moves two solutes in the same direction

Question 19

antiport

Answer 19

• moves two solutes in the opposite directions

Question 20

Glucose-Na+ symport

Answer 20

• found in apical side of gut cells
• utilizes import of Na+ to drive the import of glucose
• the binding of Na+ and glucose is cooperative meaning that both molecules need to be present for transport to occur
• passive glucose transporter on basal and lateral side of guy cells

Question 21

electrochemical H+ gradients

Answer 21

• plants, bacteria and fungi primarily use H+ gradients rather than Na+ gradients to transport molecules
• gradient is generated by H+ pump which pumps H+ outside of the cell
• some pumps are driven by ATP or energy from light
• H+ pumps are also found in membranes of intracellular organelles like lysosomes and vacuoles