Ch 1 · Diffusion · IGCSE Biology 0610
Topic 1 of 3 in Movement into and out of Cells — IGCSE Biology 0610 — 58 lessons.
Introduction
Cells take in oxygen and glucose and get rid of carbon dioxide without any pump or moving part. They rely on diffusion, the spreading out of particles that happens on its own because particles are always moving. Diffusion runs gas exchange in lungs and leaves, absorption in the gut and exchange in every capillary. The definition is tested often, and the four factors that affect its rate turn up in practical questions.
Key ideas — What diffusion is
Diffusion is the net movement of particles from a region of their higher concentration to a region of their lower concentration, down a concentration gradient, as a result of their random movement.
🧑🏫 Teacher's explanation
Imagine a crowded room where everyone is milling about randomly. If one side of the room is packed with people and the other side is empty, even though people are moving in all directions, more people will naturally drift toward the empty space simply because there is more room to move into. This 'net' shift from the crowded area to the empty area is exactly how diffusion works.
📖 Definitions
🔍 Worked example — Easy
A cell has a concentration of glucose of 0.8 mol/dm^3 outside the membrane and 0.2 mol/dm^3 inside the membrane. Describe the direction of net movement of glucose.
🔍 Worked example — Standard
Calculate the percentage increase in the rate of diffusion if the temperature is increased from 20circC to 40circC, assuming the rate is directly proportional to the kinetic energy of the molecules.
🔍 Worked example — Exam-style
Explain why a large surface area increases the rate of diffusion in a cell. [3 marks]
⚠️ Common errors
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🎯 Exam question
Key ideas — Diffusion across cell membranes
Some substances move into and out of cells by diffusion through the cell membrane. Oxygen and carbon dioxide are small molecules that pass straight through. Water and small dissolved solutes such as glucose and ions also cross the membrane down their concentration gradients. Large molecules such as starch and proteins cannot diffuse through it.
🧑🏫 Teacher's explanation
Imagine a crowded dance floor where everyone is moving randomly. If one side of the room is packed with people and the other side is empty, people will naturally spread out until they are more evenly distributed. This isn't because they are 'trying' to get to the empty space, but simply because their random movements make it statistically likely that they will move from the crowded area to the empty one.
📖 Definitions
🔍 Worked example — Easy
A cell has an internal glucose concentration of 0.2 mol/dm^3 and the surrounding fluid has a concentration of 0.8 mol/dm^3. Calculate the percentage increase in external concentration compared to the internal concentration.
🔍 Worked example — Standard
Explain why an increase in temperature increases the rate of diffusion of oxygen into a plant cell.
🔍 Worked example — Exam-style
A student investigates the rate of diffusion of a dye in water. They compare a 10 cm long tube to a 20 cm long tube, keeping the diameter and concentration constant. Predict and explain the effect on the rate of diffusion.
⚠️ Common errors
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🎯 Exam question
Key ideas — Why diffusion matters in living organisms
In each case the cell or organ keeps the gradient steep by using up or carrying away the substance that has diffused.
🧑🏫 Teacher's explanation
Imagine you drop a single bead of blue ink into a glass of clear water. At first, the ink stays in one spot, but slowly, the blue color spreads out until the whole glass is a pale blue. This happens because the ink particles are constantly jiggling around randomly. Eventually, they spread out until they are evenly distributed. This is exactly how molecules behave in your body.
📖 Definitions
🔍 Worked example — Easy
A cell has a glucose concentration of 0.8 mol/dm^3 inside the cytoplasm and 0.2 mol/dm^3 in the surrounding extracellular fluid. Describe the direction of net movement of glucose.
🔍 Worked example — Standard
Explain why the rate of diffusion of oxygen into a red blood cell increases as the temperature increases from 25circC to 37circC.
🔍 Worked example — Exam-style
Calculate the percentage increase in the rate of diffusion if the concentration gradient is doubled (e.g., from 0.1 mol/dm^3 to 0.2 mol/dm^3) assuming all other factors remain constant.
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🎯 Exam question
Key ideas — Factors that affect the rate of diffusion
🧑🏫 Teacher's explanation
Imagine a crowded shopping mall where everyone is trying to get to the exit. If the exit is a tiny single door (small surface area), people move out slowly. If the exit is a massive wide gate (large surface area), people flow out much faster. This is exactly how diffusion works in your body.
📖 Definitions
🔍 Worked example — Easy
A student observes that a drop of blue food coloring spreads faster in a beaker of hot water than in a beaker of ice-cold water. Explain why using the concept of kinetic energy.
🔍 Worked example — Standard
Calculate the percentage increase in surface area if a spherical cell increases its radius from 1 mum to 2 mum. How does this affect the rate of diffusion?
🔍 Worked example — Exam-style
An examiner asks: 'Explain how the structure of the alveoli is adapted to maximize the rate of gas exchange.'
⚠️ Common errors
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🎯 Exam question
(a) Define diffusion. [2] (b) Explain how the rate of glucose uptake would change if the temperature of the solution is increased. [2] [4 marks]
Key ideas — Investigating diffusion
Keep all other variables constant and repeat.
🧑🏫 Teacher's explanation
Imagine you are at a crowded music festival. If everyone is packed tightly in one corner (high concentration) and the rest of the field is empty (low concentration), people will naturally spread out into the empty space simply because they are moving around randomly. This 'spreading out' is exactly what diffusion is: particles moving from where there are many to where there are few.
📖 Definitions
🔍 Worked example — Easy
A student uses agar cubes containing phenolphthalein indicator to investigate diffusion. Cube A has a side length of 1 cm and Cube B has a side length of 2 cm. If the acid penetrates 0.5 cm into both cubes, which cube will show a color change throughout its entire volume more quickly? Explain why.
🔍 Worked example — Standard
In an experiment measuring the rate of diffusion of potassium manganate(VII) in water, the time taken for the color to spread 2 cm was 15 minutes at 20^°C and 5 minutes at 40^°C. Calculate the factor by which the rate of diffusion increased.
🔍 Worked example — Exam-style
A student investigates the effect of concentration gradient using Visking tubing. They place tubing filled with 1.0 mol/dm^3 glucose into distilled water and another tubing filled with 0.1 mol/dm^3 glucose into distilled water. Predict the outcome of the Benedict's test on the water surrounding the tubing after 10 minutes and explain the result.
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🎯 Exam question
Worked example
A student cuts three cubes of agar containing an indicator, with sides of 5 mm, 10 mm and 20 mm, and places them in dilute acid at the same time. The acid turns the indicator colourless as it diffuses in.
🔍 Worked example — Easy
Calculate the percentage increase in the surface area of a cube if its side length is doubled from 2 mm to 4 mm.
🔍 Worked example — Standard
A cell is placed in a solution where the concentration of glucose is 0.05 mol/dm^3. The concentration inside the cell is 0.25 mol/dm^3. Describe the direction of net movement of glucose.
🔍 Worked example — Exam-style
An agar cube experiment is used to model diffusion. A cube with side length L is placed in a solution. If the side length is halved, how does the surface area to volume ratio (SA:V) change?
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Practice
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