O-Level Biology · Secondary 3 and 4
Movement of Substances
Every cell has to take in what it needs and get rid of waste across its cell membrane. This topic covers the three ways substances move: diffusion, osmosis and active transport. You will also see what happens to plant and animal cells in solutions of different water potential. These ideas come back again and again, in gas exchange in the lungs and leaves, absorption in the small intestine, and water uptake by root hair cells, so it pays to learn the definitions precisely.
What the syllabus expects
- Define diffusion and describe its importance in living organisms
- Define osmosis in terms of water potential and a partially permeable membrane
- Describe and explain the effects of osmosis on plant and animal cells
- Define active transport and explain why it needs energy from respiration
- Explain how surface area to volume ratio affects the rate of movement of substances
- Calculate and interpret percentage change in mass in osmosis experiments
Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. It happens because particles move randomly, and it does not need energy from the cell. Oxygen diffuses from the alveoli into the blood, carbon dioxide diffuses into a leaf through the stomata, and digested food such as glucose diffuses from the small intestine into the blood. The rate of diffusion is faster when the concentration gradient is steeper, when the surface area is larger, when the distance the particles travel is shorter, and when the temperature is higher. This is why exchange surfaces such as alveoli and villi are thin, numerous and well supplied with blood, which carries substances away and keeps the gradient steep.
Osmosis and water potential
Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane. A dilute solution has a high water potential because it contains a lot of free water molecules. A concentrated solution has a low water potential. Pure water has the highest water potential of all. The partially permeable membrane lets small water molecules through but not larger solute molecules such as sucrose. You can show osmosis with Visking tubing filled with sugar solution and placed in water. Water enters the tubing by osmosis, so the tubing swells and the liquid level in a capillary tube attached to it rises. Always describe osmosis in terms of water potential, not the concentration of water.
Effects of osmosis on plant and animal cells
When a plant cell is placed in a solution of higher water potential than its cell sap, water enters by osmosis. The vacuole swells and pushes the cytoplasm against the cell wall, and the cell becomes turgid. The cell wall stops it from bursting. Turgor keeps leaves and young stems firm. In a solution of lower water potential, water leaves the cell, the vacuole shrinks and the cell becomes flaccid. If more water is lost, the cell membrane pulls away from the cell wall. This is plasmolysis. Animal cells have no cell wall, so they react more strongly. A red blood cell in distilled water takes in water until it swells and bursts. In a concentrated salt solution it loses water and shrinks, becoming crenated.
Active transport and surface area to volume ratio
Active transport is the movement of substances against a concentration gradient, from lower to higher concentration, across a membrane. It uses energy released by respiration and needs carrier proteins in the membrane. Root hair cells use it to absorb mineral ions such as nitrate from soil where the ions are less concentrated than inside the cell, and cells lining the small intestine use it to absorb glucose that remains after diffusion slows down. Anything that stops respiration, such as lack of oxygen or a poison, stops active transport. Size matters too. A 1 cm cube has a surface area of 6 cm² and a volume of 1 cm³, a ratio of 6 : 1. A 2 cm cube has 24 cm² and 8 cm³, a ratio of 3 : 1. Larger organisms have a smaller ratio, so they need specialised exchange surfaces and transport systems.
Worked example
Two potato strips, each with an initial mass of 5.0 g, were placed for 30 minutes in distilled water (strip A) and in concentrated sucrose solution (strip B). The final masses were 5.6 g for A and 4.6 g for B. Calculate the percentage change in mass of each strip and explain the results.
- Write the formula: percentage change = (final mass − initial mass) ÷ initial mass × 100%.
- Strip A: (5.6 − 5.0) ÷ 5.0 × 100% = 0.6 ÷ 5.0 × 100% = +12%.
- Strip B: (4.6 − 5.0) ÷ 5.0 × 100% = −0.4 ÷ 5.0 × 100% = −8%.
- Explain A: distilled water has a higher water potential than the cell sap, so water enters the potato cells by osmosis through their partially permeable membranes. The cells become turgid and the strip gains mass.
- Explain B: the sucrose solution has a lower water potential than the cell sap, so water leaves the cells by osmosis. The cells become flaccid or plasmolysed and the strip loses mass.
- Note why percentage change is used: it allows a fair comparison even if strips start with different masses.
Answer: Strip A gained 12% of its mass and strip B lost 8%. A gained water by osmosis from a solution of higher water potential, B lost water by osmosis to a solution of lower water potential.
Common mistakes
- Describing osmosis as the movement of solute, or of 'particles'. Osmosis is only the movement of water molecules across a partially permeable membrane.
- Saying water moves from a high concentration to a low concentration solution. Water moves from a dilute solution (higher water potential) to a concentrated solution (lower water potential).
- Writing that a plant cell bursts in pure water. The cellulose cell wall prevents bursting, so the cell becomes turgid instead. Only animal cells burst.
- Forgetting the membrane in an osmosis definition. Without 'partially permeable membrane' the definition is incomplete and loses the mark.
- Saying active transport goes 'down' a gradient or does not need energy. It moves substances against the gradient using energy from respiration.
- Leaving out the negative sign when a strip loses mass. A loss must be written as a negative percentage change, such as −8%.
Exam tips
- Learn the three definitions word for word. Examiners look for the key phrases: 'net movement', 'concentration gradient', 'water potential', 'partially permeable membrane' and 'energy from respiration'.
- In osmosis explanations, always compare the water potential of the solution with that of the cell sap or cytoplasm, then state the direction of water movement and its effect on the cell.
- If a graph of percentage change in mass crosses zero, the concentration at that point has the same water potential as the cell contents. Read it off the graph and say so.
Practice questions
1. Easy: Define diffusion and give one example in the human body.
Show answer
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. Example: oxygen diffuses from the air in the alveoli into the blood in the capillaries.
2. Medium: A cube of agar measures 3 cm on each side. Calculate its surface area to volume ratio and compare it with a 1 cm cube.
Show answer
Surface area = 6 × 3 × 3 = 54 cm². Volume = 3 × 3 × 3 = 27 cm³. Ratio = 54 : 27 = 2 : 1. A 1 cm cube has 6 cm² to 1 cm³, a ratio of 6 : 1, so the smaller cube has the larger ratio and substances reach its centre faster.
3. Harder: Root hair cells take in nitrate ions even when the soil contains fewer nitrate ions than the cell. Explain how, and why waterlogged soil reduces this uptake.
Show answer
Nitrate ions are absorbed by active transport, against the concentration gradient, using carrier proteins and energy released by aerobic respiration in the root cells. Waterlogged soil has little air, so the roots get less oxygen, aerobic respiration slows, less energy is released and active transport of ions decreases.
Related topics
Want a tutor for Biology?
Shortlist on WhatsApp within 16 hours. You choose the tutor. About O-Level Biology tuition
Get matched with a tutor