P: Transport across cell membranes

Question 1

Types of passive transport

Answer 1

1. Simple diffusion: nonpolar molecules
2. Simple diffusion through protein channels: inorganic ions, water
3. Facilitated diffusion: small organic molecules, glucose.

Question 2

Brownian motion

Answer 2

molecules in solution are in a constant state of motion as a result of their thermal energy.

Question 3

Passive transport for gases + lipid-soluble molecules

Answer 3

• No carrier protein
• Only down concentration gradient
• No ATP

Question 4

Gated channels:

Answer 4

open In response to chemical, mechanical or electrical signals.

Question 5

Carrier proteins

Answer 5

Never open on two sides of membrane, passage opens on one side and provides a passage to other side.

Question 6

Fick’s law

Answer 6

Diffusion rate is proportional to:
(concentration gradient* membrane permeability * surface area)/ membrane thickness

Question 7

Factors affecting rate of diffusion

Answer 7

• Lipid solubility
  
  • Molecular size
  • Cell membrane thickness
  • Concentration gradient
  • Membrane surface area
    Composition of lipid layer.

Question 8

Secondary active transport:

Answer 8

• Symporters + antiporters
  Use energy stored in concentration gradients from primary active transport

Question 9

Sodium/ potassium pump

Answer 9

• Sodium ions are actively extruded from the cell
  
  • Uses ATP

Question 10

Osmolarity

Answer 10

Amount of solute dissolved in 1L of water

Question 11

Hypertonic cell

Answer 11

Solute concentration is higher outside the cell than inside, so water diffuses out of the cell.

Question 12

Isotonic cell

Answer 12

Similar concentration of fluid, sugars + salt than blood

Question 13

Hypotonic cell

Answer 13

Solute concentration of cell is higher than in solution, so water diffuses into cell.

Question 14

If a cell is placed in a hypertonic solution:

Answer 14

If a cell is placed in a hypertonic solution, there will be a net flow of water out of the cell, and the cell will lose volume. A solution will be hypertonic to a cell if its solute concentration is higher than that inside the cell, and the solutes cannot cross the membrane.

Question 15

If a cell is placed in a hypotonic solution:

Answer 15

If a cell is placed in a hypotonic solution, there will be a net flow of water into the cell, and the cell will gain volume. If the solute concentration outside the cell is lower than inside the cell, and the solutes cannot cross the membrane, then that solution is hypotonic to the cell.

Question 16

Osmolarity

Answer 16

is the measure of osmotic pressure (amount of solute in solution).

Question 17

Tonicity

Answer 17

The relative solute concentration of two solutions separated by a semi permeable membrane. By comparing tonicity of two environments, we can predict the direction that osmosis will occur.

Question 18

Hypo-osmotic solution

Answer 18

solutions with lower osmolarity than plasma

Question 19

Hyper-osmotic solution

Answer 19

solutions with higher osmolarity than plasma

Question 20

Tonicity vs osmolarity

Answer 20

Tonicity only measures concentration of non-penetrating solutes through a semipermeable membrane.
Osmolarity measures total concentration of penetrating and non penetrating solutes.

Question 21

Isosmotic vs isotonic

Answer 21

If solution is isosmotic + cannot cross cell membrane –> isotonic
If solution is isosmotic + can cross cell membrane –> not isotonic.

Question 22

How is glucose absorbed in the gut?

Answer 22

• Sodium ions are pumped out of ICF by active transport
• This leads to lower sodium ion concentration, so they look to diffuse into the cell
• Sodium enters cell via Na/glu co-transporter, where glucose is dragged into the cell following sodium on apical membrane
• Glucose leaves basolateral membrane through a glucose uniporter.

Question 23

Absorption of water in the gut?

Answer 23

• Na enters interstitial space via Na+ pump on basolateral membrane
• This lowers concentration of sodium in cell and creates an electrochemical gradient across apical membrane
• Na/Glu co-transporter uses energy from concentration gradient to allow Na and glucose to enter cell from gut.
• Glucose enters interstitial space via glucose uniporter
• Net effect: H2O moves paracellularly from lumen via leaky tight junctions.