Cells: Transport Across Membranes - Diffusion

Question 1

Process of Diffusion

Answer 1

• Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration
• Diffusion is a passive process - no energy is needed for it to occur
• When particles diffuse directly through a cell membrane, it’s also known as simple diffusion

Question 2

Factors Affecting the Rate of Diffusion

Answer 2

• Temperature
- Particles will be affected by kinetic energy

• Concentration gradient

• The higher it is, the faster the rate of diffusion
• As diffusion occurs, difference in concentration decreases until it reaches an equilibrium
• Diffusion slows down over time

• Thickness of exchange surface
- The thinner the exchange surface (the shorther the distance the particles have to travel), the faster the rate of diffusion

• Surface area
- The larger the surface area, the faster the rate of diffusion

Question 3

What is the purpose of facilitated diffusion?

Answer 3

• Some larger molecules (e.g amino acids, glucose) would diffuse extremely slowly through phospholipid bilayer because they’re so big
• Charged, water-soluble particles (e.g ions and polar molecules) would diffuse slowly because centre of phospholipid is hydrophobic
• To speed things up, large or charged particles diffuse through carrier or channel proteins in the cell membrane instead

Question 4

How is facilitated diffusion similar to simple diffusion?

Answer 4

• Facilitated diffusion also moves particles down a concentration gradient
• It’s also a passive process
• Difference is that facilitated diffusion involves 2 proteins: carrier and channel proteins

Question 5

Carrier Proteins

Answer 5

• Carrier proteins move large molecules across a membrane
• First, a large molecule attaches to a carrier protein in the membrane
• Then, the protein changes shape
• This releases the molecule on the opposite side of the membrane

Question 6

Channel Proteins

Answer 6

• Channel proteins form pores in the membrane for charged particles to diffuse through
• Different channel proteins are needed for the facilitated diffusion of different charged particles

Question 7

Factors Affecting the Rate of Facilitated Diffusion

Answer 7

• Concentration gradient

• The higher the concentration gradient, the faster the rate of facilitated diffusion
• As equilibrium is reached, the rate of facilitated diffusion will level off

• Number of channel or carrier proteins

• The greater the number of channel or carrier proteins, the faster the rate of facilitated diffusion
• Once all the proteins in a membrane are in use, facilitated diffusion can’t happen any faster

Question 8

Calculating the Rate of Diffusion

Answer 8

• You may be asked to calculate the rate of diffusion from a graph
• For a straight line graph, find the gradient of the line
• For curved graph, draw a tangent and find gradient of tangent