Channel / Transporter / Pump

Question 1

How would you describe the permeability of cell membranes?

Answer 1

Cell membranes are selectively permeable. A given cell membrane’s lipid and protein composition determines which molecules will enter the cell.

• If a substance can pass through the cell, it is permeable.
• If a substance can’t pass through the cell, it is impermeable.

Non-polar compounds e.g. lipids, oxygen, and carbon dioxide move easily across the membrane however ions, most polar molecules, and large molecules such as proteins have a harder time entering.

Question 2

Describe ion movement

Answer 2

Ion movement is influenced by electrical gradients because of the attraction of opposite charges and repulsion of like charges. For this reason, ions move in response to combined electrical and concentration gradients, electrochemical gradients.

Question 3

What types of diffusion are there?

Answer 3

1. Simple diffusion:
   Diffusion directly across the phospholipid bilayer of a membrane
2. Facilitated diffusion:
   Mediated transport through proteins which can mediate the passive movement of molecules down their gradient stops when concentrations are equal on both sides.

Question 4

How do carriers and channels transport molecules?

Answer 4

Carriers use facilitated diffusion.

Facilitated diffusion carriers have the same properties as simple diffusion. The transported molecules move down their concentration gradient, and the process does not require input of outside energy and net movement stops at equilibrium.

Question 5

Which types of transport proteins exist in the membrane and how are they distinguished?

Answer 5

Channels:
Create water-filled passageways that directly link the intracellular and extracellular compartments

Carriers / Transporters:
Bind the specific solute to be transported and undergo a series of conformational changes to transfer the bound solute across the membrane. They are open on one side of the membrane where the substrate binds and upon binding turns to the other.
- Use facilitated transport

Pumps:
Actively pump certain solutes across the membrane against their electrochemical gradient (“uphill”); this process, known as active transport.

Question 6

How can active transport be divided?

Answer 6

Primary 1° direct active transport: directly from ATP.

Secondary 2° indirect active transport: uses potential energy stored in the concentration gradient of one molecule to push another molecule against their concentration gradient.
o Symport
o Antiport