L22

Question 1

What are the 2 main types of transmembrane transport?

Answer 1

1. non-mediated
   (does not directly use a transport protein),
2. mediated and requires transport
   proteins

Question 2

What are the two main classes of transport proteins?

Answer 2

1. Channels

2. Carriers

Question 3

Define Ion channels

Answer 3

Ion channels are water filled pores that span the hydrophobic core of the lipid bilayer and allow
passage of ions (and small molecules) across the cell membrane.

Question 4

Describe 3 features of ion channels

Answer 4

1. Ions do not bind to the channel
   pore, so transport can be very rapid (e.g., 1 million ions/sec).
2. The direction of ion flow is governed
   by the laws of diffusion.
3. Channels exhibit ion selectivity. Specific
   amino acids lining the pore determine the selectivity of the channel to ions. By being selective to a
   particular ion the channel can harness the energy stored in the different ion gradients.

Question 5

What different types of stimuli can control the opening and closing of a gate channel?

Answer 5

1. Voltage;
2. Ligand binding;
   3, Cell volume (stretch);
   4.pH;
3. Phosphorylation

Question 6

How is the diffusion of ions through a channel measured?

Answer 6

The current fl owing through an individual channel can be recorded using the patch
clamp technique.

Current fluctuations represent the opening and closing of single ion channels. The
current fluctuations represent the conformational changes in channel structure that are associated
with channel gating.

Question 7

What types of carriers are there?

Answer 7

passive (facilitated) or active.

Question 8

Why are carrier transporters slower than channels?

Answer 8

passive (facilitated) or active.

Question 9

What are some features of carriers?

Answer 9

1. Specificity;
2. Inhibition; changing the binding pocket
3. Competition (ie: comp: galactose and glucose - 6 carbons and non comp - 1 molecule)
4. Saturation (transport maximum) - number of binding pockets

Question 10

What is facilitated diffusion?

Answer 10

Mediated transport can be passive, in which case it is termed facilitated diffusion. In this case,  the transport protein changes shape and glucose moves across the
cell membrane (but only down its concentration gradient)

Question 11

What is Active transport?

Answer 11

Active transport systems utilise metabolic energy to drive substances against their electrochemical
potential gradient

Question 12

There are two forms of active transport:

Answer 12

1. primary active transport in which energy
   is directly derived from the hydrolysis of ATP;
2. secondary active transport in which the energy
   released by a substance diff using down its concentration gradient is used to actively transport
   another substance against its concentration gradient.

Question 13

A typical cell uses x of its energy (ATP)

on primary active transport

Answer 13

30%

Question 14

What is an example o Facilitated diffusion?

Answer 14

An example is glucose which binds
to a transport protein (GLUT). The transport protein changes shape and glucose moves across the
cell membrane (but only down its concentration gradient). Inside the cell, kinase enzymes reduces
glucose concentration by converting glucose into glucose-6-phosphate. This conversion of glucose
maintains the concentration gradient for continued glucose absorption.

Question 15

What is an example of primary active transport?

Answer 15

The sodium-potassium pump is an example of primary active transport. The Na/K pump maintains a
low concentration of Na+ and a high concentration of K+ in the cytosol. Three Na+ ions are pumped
out (removed from cell) for every two K+ ions pumped into the cell. Therefore the pump generates a
net current and is electrogenic.

Question 16

Why is the difference in ion concentrations important?

Answer 16

1. Maintenance of the resting membrane potential;
2. Electrical excitability;
3. Contraction of muscle;
4. Maintenance of steady state cell volume;
5. Uptake of nutrients via secondary active transporters;
6. Maintenance of intracellular pH by secondary active transporters.

Question 17

What is secondary active transport?

Answer 17

Secondary active transport uses the energy stored in an ion gradient created by primary active
transporters to move other substances against their own concentration gradient. Thus, these
transporters indirectly use the energy obtained by hydrolysis of ATP

Question 18

What is primary active transport?

Answer 18

primary active transport in which energy

is directly derived from the hydrolysis of ATP

Question 19

Understand the difference between passive and active transport of solutes

Answer 19

refer to book

Question 20

Understand the difference between channel and carrier mediated transport

Answer 20

refer to book

Question 21

Describe the difference between primary and secondary active transport

Answer 21

refer to book

Question 22

What are the main ways substances can be transported across a membrane?

Answer 22

1. Non-mediated transport does not directly use a transport protein
2. Mediated transport moves materials with the help of a transport protein
3. Passive transport moves substances down their concentration or electrochemical gradients with only their kinetic energy
4. Active transport uses energy to drive substances against their concentration or electrochemical gradients
5. Vesicular transportmove materials across membranes in small vesicles either by exocytosis or endocytosis –not discussed

Question 23

What is non mediated transport important for?

Answer 23

Important for absorption of nutrients –excretion of waste

For Nonpolar, hydrophobic molecules – oxygen, carbon dioxide, nitrogen, fatty acids, steroids, small alcohols, ammonia and fat-soluble vitamins

Question 24

In ion channels, what is lined in the core?

Answer 24

Water filled pore lined by hydrophilic amino acids

The channel forms a water filled pore that shields the ions from the hydophobic core of the lipid bilayer

Question 25

What determines the ionic selectivity of channels?

Answer 25

Specific amino acids lining the pore determine the selectivity of the channel to ions. By being selective to a particular ion the channel can harness the energy stored in the different ion gradients

Question 26

What are 2 properies of channels?

Answer 26

1. Ionic selectivity
2. gating
3. electrical current

Question 27

Why do transport proteins have properties similar to enzymes?

Answer 27

Transport proteins do not catalyze chemical reactions they mediate transport across the cell membrane at a faster than normal rate.

Also exhibits saturation. ie: Glucose transport occurs until all binding sites are saturated

Question 28

What are some examples of secondary transporters?

Answer 28

1. Na+ antiporter or exchangers.
   Na+ ions rush inward, Ca2+ or H+ pushed out
2. Na+ symporters or cotransporters
   Glucose or amino acids rush inward together with Na+ ions