Lecture 22: Transport Across Cell Membranes

Question 1

types of transport across plasma membrane

Answer 1

non mediated transport
mediated protein
passive transport
active transport
vesicular transport

Question 2

non-mediated transport

Answer 2

important for absorption of nutrients and excretion of waste

non polar, hydrophobic molecules; fatty acids, small alcohols, o2, co2, nitrogen, steroids, ammonia, fat-soluble vitamins aedk

Question 3

ion channels

Answer 3

ionic selectivity: negative ring attracts positive ions. bigger ring = bigger attraction for bigger ions. being selective can harness energy stored in different ion gradients

gating: controls opening and closing of the pore to maintain electrochemical gradient. stimuli = voltage, ligand binding, cell volume ph, phosphorylation

measurement: glass pipette. current can be measured and recorded. fluctuations represent the opening and closing of single ion channels (represent conformational changes in channel structure associated with channel gating)

Question 4

carrier mediated transport

Answer 4

protein undergoes a conformational change –> transport rates are slower –> molecules flow through

Question 5

carrier mediated transport properties

Answer 5

specificity, inhibition, competition, saturation. transport proteins do not catalyse chemical reactions.

Question 6

facilitated diffusion of glucose

Answer 6

1. glucose binds to transport protein (GLUT 9)
2. transport protein changes shape
3. glucose moves across cell membrane (only down the concentration gradient)
4. kinase enzyme reduces glucose conc inside the cell by transforming glucose into glucose-6-photsphate
5. conversion of glucose maintains concentration gradient for glucose entry

Question 7

active transport

Answer 7

1. primary active transport (energy derived from the hydrolysis of atp. typical cell uses 30% of energy on primary active transport
2. secondary active transport: uses energy stored in ionic concentration gradient to drive active transport against gradient.

Question 8

primary active transporters

Answer 8

na/katpase
3na removed from cell, 2k+ into the cell.
pump generates a net current and is electrogenic.

other examples = ca/k (muscle), h/k (stomach)

-maintains a low conc of na+ and high conc of k+ in the cytosol.

Question 9

what is the difference in ionic concentration important for?

Answer 9

maintain resting membrane potential
electrical excitability
contraction of muscle
maintenance of steady state cell volume
uptake of nutrients via secondary active transporters
maintenance of intracellular ph by secondary active transporters.

Question 10

what are secondary active transpoers?

Answer 10

use energy stored in an ion gradient created by primary active transporters to move other substance against their own conc gradient.

• these transporters indirectly use energy obtained by hydrolysis of atp
• cells have many secondary active transporters which are powered by the Na+ gradient initially established by the Na pump

1. antiporter/exchanger = na+ ions rush inward, ca2+/H+ pushed out
2. na symporters or cotransporters
   glucose amino acids rush inward together with na+ ions.