Movement of Substances

Question 1

What is diffusion?

Answer 1

1. Diffusion is the net movement of particles (molecules or ions) from a region of high concentration to a region of low concentration (down a concentration gradient). Another way to say this is that diffusion occurs when a substance is more concentrated in one place than another.
2. After some time equilibrium is reached and this means that the substance is evenly distributed in the solution/across the cell membrane (no more net movement).
3. Remember that no energy is required for diffusion as it is a passive form of transport.

Question 2

What are the factors affecting the rate of diffusion?

Answer 2

• Concentration gradient: diffusion happens more quickly when there is a steep concentration gradient (a big difference in concentrations between two areas).
• Surface area to volume ratio: a larger surface area in proportion to the volume will increase the rate.
• Distance: the rate is decreased if the distance over which diffusion has to take place is greater.
• Temperature: the rate is greater at higher temperatures – this is because a high temperature provides the particles with more kinetic energy.

Question 3

What is simple diffusion?

Answer 3

1. Simple diffusion occurs when substances diffuse directly across the cell membrane through the lipid bilayer (from high concentration to low concentration) with no assistance required (energy or otherwise). So the substances do not go through channels or transporters.
2. Simple diffusion is important in the exchange of O₂ and CO₂ between blood and body cells.
3. Remember that no energy is required for simple diffusion as it is a passive form of transport.

Question 4

What are some examples of substances that move across the cell membrane via simple diffusion?

Answer 4

Examples of substances that move across the cell membrane via simple diffusion are lipid-soluble substances (can dissolve in lipids) such as
1. fatty acids,
2. steroids,
3. fat-soluble vitamins,
4. oxygen and
5. carbon dioxide.

Question 5

What is facilitated diffusion?

Answer 5

1. Facilitated diffusion occurs when substances that cannot move through the lipid bilayer by simple diffusion cross over via a passive process.
2. Proteins within the cell membrane assist substances to move across the membrane into/out of the cell.
3. These proteins can be channels, which are “openings” cell membrane and assist in the movement of
   
   • K⁺,
   • Cl⁻,
   • Na⁺, and
   • Ca²⁺.
4. These proteins can also be carriers, which bind a substance on one side of the cell membrane and then change shape to release it on the other side and assist in the movement of
   
   • glucose,
   • fructose, and
   • some vitamins.
5. Remember that no energy is required for facilitated diffusion as it is a passive form of transport.

Question 6

What is osmosis?

Answer 6

1. Osmosis is a special kind of diffusion involving the movement of water. It is defined as the net movement of water from a dilute solution to a more concentrated solution across the selectively permeable cell membrane.
2. Another way of saying this is that water moves by osmosis from an area of higher water (or solute) concentration to an area of lower water (or solute) concentration.
3. It is important to note that osmosis can only happen if the cell membrane is permeable to water but not to some other solutes.
4. The water moves through water channels in the cell membrane.
5. Again, please note that no energy is required for osmosis as it is a passive form of transport.

Question 7

What is active transport?

Answer 7

1. Active transport is an active process
2. in which cellular energy is used
3. to transport substances across the cell membrane
4. against a concentration gradient (from an area of low concentration to an area of high concentration).

Question 8

What are the two types of active transport?

Answer 8

1. Pumps, which are large protein molecules, use energy derived from splitting ATP in order to change shape allowing them to move substances.
2. This change in shape moves the substance across a cellular membrane against its concentration gradient.
3. Remember that energy is required to allow pumps to transport substances, as they are an active form of transport.

Transport in vesicles is the other form of active transport. A vesicle is a small round sac formed by budding off from an existing membrane.

Question 9

What are some examples of pumps?

Answer 9

Examples of pumps include:
1. the sodium-potassium pump;
2. pumps found on cells in the human small intestine, where some glucose in the gut is absorbed into the cells lining the intestine; and
3. pumps on cells in the roots of plants which also take up certain mineral ions in this way.