OP- L4+L5

Question 1

3 Factors that determine whether a substance will cross a membrane

Answer 1

• Membrane permeability
• Chemical gradiant
• Electrical gradient

Question 2

Passive Diffusion

Answer 2

Simple diffusion:
- Non polar molecules:
— O2, CO2, NO
— Steroid hormones, ethanol
- only occurs down conc. gradient
- No ATP or carrier proteins needed
EXAMPLES:
- O2 from lung to blood to muscle
- Ethanol from mouth to blood to brain
(in both the substance needs to cross 4 lipid memb. layers each time)

Simple diffusion thru channels:
– Inorganic ions
– Water

Facilitated diffusion:
– Small organic molecules
– Glucose

• Occurs via protein channels and carriers
• Its specific to substances and the process is saturable (continues until equilibrium is reached)

CHANNELS:
- Open channels – open on both sides
- Gated channels – usually closed and open in response to certain stimuli

CARRIERS:
- Passage only opens to one side

Question 3

Brownian Motion

Answer 3

Molecules of a solution are in constant motion due to their thermal energy.

Question 4

What substances does the lipid memb. allow to pass thru? And role of carrier proteins in memb?

Answer 4

– gases + small hydrophobic substances down conc. gradient
– Ions and non-lipid soluble molecules cant cross unless theres a memb. protein present
– Carrier proteins make membrane permeable - specific for diff substrates

Question 5

Fick’s law of diffusion

Answer 5

Rate of diffusion depends on:

Membrane thickness
Membrane permeability (lipid solubility/molecular size)
Surface area
Concentration gradient

Fick’s law = (Memb. per. x SA x conc. gradient)/ memb. thickness

Question 6

Types of carrier proteins and the two modes of movement?

Answer 6

Active and passive transport

Carrier proteins:

• Channels:
  — Regulated (P) - Gated
  — Non-regulated (P) - always open
• Transporters:
  — Uniporters (P) - transport 1 mol. down conc. gradient - regulated by removal/insertion of channel
• EXAMPLE: GLUT1 glucose transporter in kidney

— Symporters (A) - 2 mols. in same direction

— Antiporters (A) - 2 mols in diff directions

• Pumps:
  — ATP pump (A)

Question 7

Active transport

Answer 7

Active T:
Moves mols against conc. gradient
ATP needed

Primary active transport:
- Ion Pumps
- ATP dependent
EX. Na+/K+ pump

Secondary active T:
Transporters:
— Symporters (A) - 2 mols. in same direction
— Antiporters (A) - 2 mols in diff directions
- Use energy stored in conc. gradients established by primary transport
- thus Indirectly dependent on ATP

Question 8

Osmosis

Answer 8

• Movement of water across a permeable cell memb.
• Water channels (aquaporins) allow for permeability
• Moves from region of low osmolarity to high

Question 9

Osmolarity

Answer 9

Amount of solute dissolved in 1L of water/solvent

1mol NaCl in 1L water = 2Osm

Question 10

Osmolality

Answer 10

The amount of solute particles in 1KG of solvent

Question 11

How does glucose move into the blood in the intestines

Answer 11

In lumen (Apical memb)– low Glu and high Na
In cell – High Glu and low Na
In interstitial space (basolateral BM) – low Glu and high Na

• Glu can move down conc gradient from cell to interstitial space
• Na in lumen moves down conc gradient into the cell
• Na cant move out into IS - needs active transport – Na+ pump moves Na from cell into IS
• This reduces Na level in cell - Creates a conc. gradient for Na from lumen to enter cell - but no Na channel
• However Na/Glu symporter on apical memb uses energy created by Na conc. gradient to move Na and Glu into the cell
• The glu uniporter on the basement membrane then transports Glu out into IS

LOCATION OF CHNNELS:
- apical memb: Na/Glu symporter
- Basement memb : Glu uniporter and Na+ pump

Question 12

Transcellular and paracellular absorption of water

Answer 12

Transcellular – thru aquaporins on epithelial membrane
Paracellular – thru tight junctions of epithelial memb.

Type of movement depends on the location of the epithelial memb.

Question 13

How is water moved into cells thru paracellular tight leaky junctions? Why is glucose important in oral rehydration therapy

Answer 13

• Na+ moves from into the IS from the cell via an Na+ pump
• This decreased Na conc. in cell – thus conc. gradient created btwn cell and apical memb
• No Na channels on apical memb. thus Na alone cant move into the cell
• However theres a Na/Glu symporter present
• Na and Glu thus move thru it together into the cell
• Glu transported into IS via Glu uniporter on basolateral memb.
• Na pumped out via the Na pump on basolateral memb.
• Creates a conc. gradient (high osm. in IS and low in lumen) thus water moves paracellularly from lumen into the IS via tight leaky junctions

> When a person is dehydrated - not enough Na+ in IS thus administration of water alone not enough
Giving Na alone wont help either as no Na transport channels on apical memb.
However theres an Na/Glu symporter – thus Na moves with glucose
Creates a conc. gradient allowing for paracellular movement of water into the IS