B2. Diffusion

Question 1

The process of diffusion

What is diffusion?
What is a concentration gradient?
What makes it ‘simple’ diffusion?

Answer 1

Diffusion is the net movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration. Molecules will diffuse both ways, but the net movement will be to the area of lower concentration. This continues until particles are evenly distributed throughout the liquid or gas. The concentration gradient is the path from an area of higher concentration to an area of lower concentration. Particles diffuse down a concentration gradient.
Diffusion is a passive process - no energy is needed for it to happen. Particles can diffuse across cell membranes, as long as they can move freely through the membrane. When molecules diffuse directly through a cell membrane, it’s also known as simple diffusion.

Question 2

The process of diffusion - Example

Answer 2

Oxygen and carbon dioxide can diffuse easily through cell membranes because they’re small, so they can pass through spaces between the phospholipids. They’re also non-polar, which makes them soluble in lipids, so they can dissolve in the hydrophobic bilayer.

Question 3

Factors affecting the rate of diffusion
The rate of diffusion across both external and internal cell membranes can vary. Some specialised cells are adapted for rapid transport across their membranes. The rate of diffusion depends on:…

Answer 3

• The concentration gradient- the higher it is, the faster the rate of diffusion. As diffusion takes place, the difference in concentration between the two sides of the membrane decreases until it reaches an equilibrium (i.e. the concentration on both sides is equal). This means that diffusion slows down over time.
• The thickness of the exchange surface the thinner the exchange surface (i.e. the shorter the distance the particles have to travel), the faster the rate of diffusion.
• The surface area - the larger the surface area (e.g. of the cell-surface membrane), the faster the rate of diffusion.

Question 4

Factors affecting the rate of diffusion - Example

Answer 4

Some specialised cells (e.g. epithelial cells in the small intestine) have microvilli-projections formed by the cell-surface membrane folding up on itself. Microvilli give the cell a larger surface area in human cells microvilli can increase the surface area by about 600 times. A larger surface area means that more particles can be exchanged in the same amount of time-increasing the rate of diffusion.

Question 5

Facilitated diffusion

What type of molecules does it transport and why with 4 examples?
What speeds up diffusion?

How is it similar to simple diffusion?
What protein is involved?

Answer 5

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

Like diffusion, facilitated diffusion moves particles down a concentration gradient, from a higher to a lower concentration. It’s also a passive process-it doesn’t use energy. There are two types of protein involved-carrier proteins and channel proteins.

Question 6

Tip: Remember - small, non-polar substances and water …

Answer 6

Tip: Remember - small, non-polar substances and water can diffuse directly through the membrane.

Question 7

Facilitated diffusion - Carrier proteins

What do they move?
How do they work in 3 steps?

Answer 7

Carrier proteins move large molecules across the membrane, down their concentration gradient. Different carrier proteins facilitate the diffusion of different molecules.

Here’s how they work:
1) First, a large molecule binds to a carrier protein in the membrane.
2) Then, the protein changes shape.
3) This releases the molecule on the opposite side of the membrane

Question 8

Facilitated diffusion - Channel proteins

What do they transport and how?

Answer 8

Channel proteins form pores in the membrane for charged particles to diffuse through (down their concentration gradient). Different channel proteins facilitate the diffusion of different charged particles.

Question 9

Factors affecting the rate of facilitated diffusion
The rate of facilitated diffusion depends on:…

Answer 9

• The concentration gradient- the higher the concentration gradient, the faster the rate of facilitated diffusion (up to a point).
  As equilibrium is reached, the rate of facilitated diffusion will level off.
• The number of channel or carrier proteins—once all the proteins in a membrane are in use, facilitated diffusion can’t happen any faster, even if you increase the concentration gradient.

Question 10

Calculating the rate of diffusion
In the exams, you might be asked to calculate the rate of diffusion (or any other form of transport across a membrane) from a graph. For a straight line graph, this means finding the gradient of the line. For a curved graph, it means drawing a ___________and finding the __________of the tangent.

Answer 10

Calculating the rate of diffusion
In the exams, you might be asked to calculate the rate of diffusion (or any other form of transport across a membrane) from a graph. For a straight line graph, this means finding the gradient of the line. For a curved graph, it means drawing a tangent and finding the gradient of the tangent.