What are the 2 main types of transmembrane transport?
- non-mediated
(does not directly use a transport protein), - mediated and requires transport
proteins
What are the two main classes of transport proteins?
- Channels
2. Carriers
Define Ion channels
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.
Describe 3 features of ion channels
- Ions do not bind to the channel
pore, so transport can be very rapid (e.g., 1 million ions/sec). - The direction of ion flow is governed
by the laws of diffusion. - 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.
What different types of stimuli can control the opening and closing of a gate channel?
- Voltage;
- Ligand binding;
3, Cell volume (stretch);
4.pH; - Phosphorylation
How is the diffusion of ions through a channel measured?
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.
What types of carriers are there?
passive (facilitated) or active.
Why are carrier transporters slower than channels?
passive (facilitated) or active.
What are some features of carriers?
- Specificity;
- Inhibition; changing the binding pocket
- Competition (ie: comp: galactose and glucose - 6 carbons and non comp - 1 molecule)
- Saturation (transport maximum) - number of binding pockets
What is facilitated diffusion?
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)
What is Active transport?
Active transport systems utilise metabolic energy to drive substances against their electrochemical
potential gradient
There are two forms of active transport:
- primary active transport in which energy
is directly derived from the hydrolysis of ATP; - 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.
A typical cell uses x of its energy (ATP)
on primary active transport
30%
What is an example o Facilitated diffusion?
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.
What is an example of primary active transport?
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.
Why is the difference in ion concentrations important?
- Maintenance of the 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.
What is secondary active transport?
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
What is primary active transport?
primary active transport in which energy
is directly derived from the hydrolysis of ATP
Understand the difference between passive and active transport of solutes
refer to book
Understand the difference between channel and carrier mediated transport
refer to book
Describe the difference between primary and secondary active transport
refer to book
What are the main ways substances can be transported across a membrane?
- Non-mediated transport does not directly use a transport protein
- Mediated transport moves materials with the help of a transport protein
- Passive transport moves substances down their concentration or electrochemical gradients with only their kinetic energy
- Active transport uses energy to drive substances against their concentration or electrochemical gradients
- Vesicular transportmove materials across membranes in small vesicles either by exocytosis or endocytosis –not discussed
What is non mediated transport important for?
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
In ion channels, what is lined in the core?
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
