Diffusion is an important process that occurs in all living organisms.

What is meant by the term diffusion?

Describe why diffusion is important in plants and animals.

Give examples of the substances that are diffusing.

State and explain the factors that affect the diffusion rate of molecules into and out of cells.

Osmosis is one of the most important processes that plants and animals use to keep a stable internal environment.

What is osmosis?

What is meant by a partially permeable membrane?

A 0.9% salt solution is isotonic to red blood cells. This means they have the same solute and water concentrations.

What will happen if red blood cells are placed into a 20% salt solution?

A student wanted to observe red blood cells under a microscope.

He placed a small sample of blood onto a microscope slide and added a drop of distilled water.

When viewed at high magnification, the student observed that the red blood cells had burst.

In a similar procedure using plant tissue, the student observed that the plant cells did not burst.

Explain the student's observations.

The lungs are part of the human respiratory system.

Describe at least three adaptations of the lungs that help to make them efficient at exchanging gas by diffusion. Explain how those adaptations aid the efficient exchange of gases via diffusion.

A student decided to investigate osmosis in living cells using potato cylinders. The student followed the method below.

1. Prepare a range of sugar solutions with varying concentrations (0, 0.2, 0.4, 0.6, 0.8 and 1 mol/dm³), and place equal volumes of the solutions into labelled boiling tubes.
2. Peel a potato and cut potato cylinders using a cork borer on a ceramic tile. Use a ruler to ensure the cylinders are approximately 2 cm long.
3. Record the initial mass of each potato cylinder before placing them in the sucrose solutions for 1 hour.
4. Remove the potato cylinders from the solutions and blot them gently.
5. Immediately measure and record the final mass of each potato cylinder.
6. Repeat the experiment 3 times.

Why did the student blot the potato cylinders dry when he removed them from the sugar solutions?

The method controls a number of variables.

Name one other variable that the student should have controlled in this experiment.

The student's results are shown in this table.

Calculate value X.

Why did the student calculate the percentage change in mass?

Use the data from the table to complete the graph below.

Draw a line of best fit.

Use the table and the graph to explain what happens to potato cells when they are placed in increasing sugar concentrations.

When each side of a membrane has equal sugar concentrations, the solution is said to be isotonic.

Water molecules will be equally likely to move in both directions across the membrane, and therefore the net movement is zero.

What is the isotonic point of this potato?

Root hair cells are specialised cells found in the roots of plants.

Root hair cells are adapted to absorb water and mineral ions from the soil.

Add arrows to the image below to show the direction of movement of mineral ion A.

Show clearly if the particles are moving into or out of the root hair cell.

Name the type of cell transport resulting in the movement of mineral A, and explain your choice.

The image below shows the movement of a different substance, mineral B, into the root hair cell.

What is the name of the type of cell transport in the image above?

Explain your choice.

Suggest why root hair cells have large numbers of mitochondria.

The image below shows the diffusion of water into the root hair cell.

What is the name given to the diffusion of water?

Villi are structures found in some parts of the digestive system.

Suggest why it is necessary to have large numbers of villi in the digestive system.

Through which process(es) do villi absorb nutrients?

Name the part of the digestive system where you would expect to find a large number of villi.

Give two products of digestion that are taken up by the villi into the blood stream.

Diffusion is the of particles from an area of high concentration to an area of low concentration.

Diffusion is influenced by several factors:

• The gradient, as a steeper gradient results in faster diffusion.
• Temperature, with temperatures increasing the kinetic energy of particles, leading to faster movement and therefore faster diffusion.
• Surface area and thickness of membranes. A surface area facilitates more interaction between particles, promoting faster diffusion, and membranes present greater distances to move, which slows down the process.

For an organism to survive, substances must constantly move into and out of the organism's cells.

Diffusion, osmosis and active transport are three types of cell transport processes.

Give an example of active transport.

Which one of the following statements is correct?

Plants need phosphate ions in order to make proteins.

A plant is growing in soil flooded with water.

Explain why the plant cannot absorb enough phosphate ions.

Give two ways a plant root is adapted for the absorption of water and minerals.

The digestive system and the breathing system both contain specialised exchange surfaces.

The diagram below shows the structure of villi and alveoli.

Explain how these two exchange surfaces are adapted to absorb different molecules into the bloodstream.

All living organisms need to transport substances into and out of their cells.

Explain why a bacterium like E. coli can rely on diffusion for gas exchange, but humans need a respiratory system.

Digested food molecules are absorbed into the blood in the small intestine.

Some of the food molecules are absorbed by active transport.

Explain why the rate of absorption of these molecules depends on the concentration of oxygen in the cells that line the small intestine.