Ch 1 · Diffusion · O Level Biology 5090
Topic 1 of 3 in Movement into and out of Cells — O Level Biology 5090 — 56 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 dancing randomly. Even though people are moving in every direction, if one side of the room is packed and the other is empty, more people will naturally drift toward the empty space simply because there is more room to move into. This 'drift' is what we call net movement.
📖 Definitions
🔍 Worked example — Easy
A cell is placed in a solution where the concentration of glucose is 0.5 mol/dm^3 outside and 0.1 mol/dm^3 inside. Describe the direction of net movement of glucose.
🔍 Worked example — Exam-style
Explain why a large surface area increases the rate of diffusion in the alveoli of the lungs.
⚠️ 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 nearly empty, people will naturally drift toward the empty space simply because they are moving around randomly. This isn't because they 'want' to go there, but because they are just as likely to move into an empty space as they are to stay in a crowded one. This 'drifting' is what we call diffusion.
📖 Definitions
🔍 Worked example — Easy
A cell is placed in a solution where the concentration of glucose is 0.5 mol/dm^3. The concentration of glucose inside the cell is 0.1 mol/dm^3. Describe the direction of net movement of glucose.
🔍 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 find that when the distance the dye travels is halved, the rate of diffusion doubles. Calculate the percentage change in the rate of diffusion if the distance is increased from 2 cm to 4 cm.
⚠️ 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 a crowded room where everyone is milling about randomly. If one side of the room is much more packed than the other, people will naturally drift toward the empty space simply because they are moving around randomly. This is exactly how diffusion works. It isn't a 'pulling' force; it is just the statistical result of particles bumping into each other and moving into spaces where there is more room.
📖 Definitions
🔍 Worked example — Easy
A cell has an internal glucose concentration of 2 mmol/dm^3. The surrounding blood plasma has a glucose concentration of 8 mmol/dm^3. Describe the direction of net movement of glucose.
🔍 Worked example — Standard
Explain why the rate of diffusion of oxygen into a leaf cell increases as the temperature increases from 20circC to 30circC.
🔍 Worked example — Exam-style
A student investigates the rate of diffusion of a dye in water. At 10circC, the dye travels 2 cm in 5 minutes. At 40circC, the dye travels 8 cm in 5 minutes. Calculate the percentage increase in the rate of diffusion.
⚠️ Common errors
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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 move toward an exit. If the exit is a tiny door (small surface area), people move slowly. If the exit is a massive wide gate (large surface area), people flow out much faster. This is exactly how diffusion works: particles want to spread out from where they are crowded to where they are less crowded.
📖 Definitions
🔍 Worked example — Easy
A student observes that a drop of food coloring spreads faster in a beaker of hot water than in a beaker of cold water. Explain why using the concept of kinetic energy.
🔍 Worked example — Standard
Calculate the percentage increase in the rate of diffusion if the concentration gradient is doubled, assuming all other factors remain constant.
🔍 Worked example — Exam-style
An organism relies on diffusion across its body surface. Compare the efficiency of diffusion in a spherical cell versus a cell with many finger-like projections (microvilli).
⚠️ Common errors
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🎯 Exam question
Key ideas — Investigating diffusion
Keep all other variables constant and repeat.
🧑🏫 Teacher's explanation
Imagine you are standing in a crowded hallway and suddenly someone opens a door to an empty room. People will naturally drift toward that empty space simply because there is more room to move. This is the essence of diffusion: particles don't 'know' where to go; they just move randomly, and because there are more particles in one spot, the net movement ends up being toward the area where they are less crowded.
📖 Definitions
🔍 Worked example — Standard
Describe how a student could investigate the effect of temperature on the rate of diffusion using potassium manganate(VII) crystals.
🔍 Worked example — Exam-style
A student uses dialysis tubing to model a cell membrane. They place tubing filled with 1.0 mol/dm^3 glucose solution into distilled water. After 30 minutes, the water outside the tubing is tested with Benedict's solution. Explain why the Benedict's test is positive and how the concentration gradient affects this.
⚠️ Common errors
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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
A student observes a cell where the concentration of glucose outside the cell is 0.5 mol/dm^3 and the concentration inside the cell is 0.1 mol/dm^3. Describe the direction of net movement of glucose.
🔍 Worked example — Standard
Calculate the surface area to volume ratio (SA:V) for a spherical cell with a radius of r = 0.01 mm. (Use SA = 4π r^2 and V = 4/3π r^3)
🔍 Worked example — Exam-style
An agar cube of side length 10 mm is placed in acid. After 5 minutes, the acid has penetrated 2 mm into the cube from all faces. Calculate the volume of the cube that has NOT yet been reached by the acid.
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Practice
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