Diffusion, osmosis and active transport
By the end of this chapter you will be able to
- Explain diffusion using a concentration gradient, and say how temperature, gradient, surface area and distance change its rate
- Explain osmosis and predict what happens to a plant or animal cell in a dilute or concentrated solution
- Explain why active transport needs energy, and give two examples
- Calculate surface area to volume ratios, and explain how exchange surfaces are adapted
- Plan and interpret the potato osmosis practical, including the isotonic concentration
You should already know
- Particles in solids, liquids and gases (KS3 Chemistry and Biology)
- Cells and their parts (Chapter 2)
- Calculating percentage change (Chapter 1)
Where this leads
- Gas exchange in the lungs (Chapter 7)
- Absorption in the digestive system (Chapter 5)
- Water and mineral uptake by roots (Chapter 8)
GCSE Biology. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Describe diffusion and the factors that affect its rate, Describe osmosis across a partially permeable membrane, and interpret results from potato tissue, [H] Explain water potential, Explain active transport against a concentration gradient, using energy from respiration, Explain how surface area to volume ratio and exchange surfaces help substances move in and out of cells.
Exercise P Do you remember?
Method. Recall the direction of diffusion.
- Recall. Diffusion is the net movement of particles from a high concentration to a low concentration.high to low
- Write the answer. from high to low concentration
Check. Particles spread out until they are evenly mixed. ✓
Method. Divide by 100 by moving the point two places left.
- Divide. 12 / 100 = 0.12.0.12
- Write the answer. 0.12
Check. 0.12 x 100 = 12. ✓
Method. Divide the gain by the starting mass.
- Divide. 0.6 / 5 = 0.12.0.6 / 5 = 0.12
- Write the answer. 0.12
Check. 0.12 x 5 = 0.6. ✓
Method. Recall the site of aerobic respiration.
- Recall. Aerobic respiration releases energy in the mitochondria.releases energy
- Write the answer. mitochondria
Check. Mitochondria are found in plant and animal cells. ✓
Method. Multiply the two numbers.
- Multiply. 5 x 6 = 30.30
- Write the answer. 30
Check. 30 divided by 6 is 5. ✓
Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. The difference in concentration is called the concentration gradient. Diffusion happens in gases and in liquids, and it does not need energy. It is passive, because the particles move because they have kinetic energy, not because a cell uses energy to push them.
Substances that move by diffusion in living things include oxygen and carbon dioxide across cell membranes, and some small molecules such as glucose across membranes.
Key ideas
- Diffusion is net movement from high to low concentration.
- It is passive: it does not need energy from respiration.
- A steeper concentration gradient makes diffusion faster.
- Higher temperature, larger surface area and a shorter distance all speed up diffusion.
Method. Recall the process.
- Recall. The movement down a concentration gradient, with no energy, is called diffusion.no energy needed
- Name it. So the process is diffusion.diffusion
- Write the answer. diffusion
Check. Active transport is the process that needs energy. ✓
Method. Think about the particles.
- Think. At higher temperatures the particles have more kinetic energy.more kinetic energy
- Conclude. So they move faster and spread out faster.move faster
- Write the answer. Particles have more kinetic energy at a higher temperature, so they move faster and diffuse faster.
Check. Temperature changes the speed of the particles. ✓
Method. Recall the factors that speed diffusion up.
- Recall. A steeper gradient, a higher temperature, a larger surface area and a shorter distance all speed up diffusion.speed it up
- Choose two. Any two of these answer the question.any two
- Write the answer. Accept any two of: a steeper concentration gradient, a higher temperature, a larger surface area, a shorter distance.
Check. Each factor works in the same direction. ✓
Watch out
Watch out: diffusion goes from high to low concentration, not the other way. It is the net movement that matters, because particles move in both directions.
Exam tip
Exam tip. When you explain a rate, say which factor changes, then say what that does to the particles.
Try it (with your teacher). Put a few drops of food colouring in a beaker of warm water and in a beaker of cold water, at the same time, with a teacher. Watch how quickly the colour spreads. Describe the effect of temperature.
Exercise 3A Diffusion
Method. Recall the direction.
- Recall. Particles move down the concentration gradient.down the gradient
- Write the answer. from high to low concentration
Check. Down means from high to low. ✓
Method. Recall whether diffusion is passive.
- Recall. Diffusion is a passive process.passive
- Write the answer. So diffusion does not need energy from respiration.no
- Write the answer. no
Check. Active transport is the process that needs energy. ✓
Method. Recall the name for the difference.
- Recall. The difference in concentration between two regions is the concentration gradient.difference in concentration
- Write the answer. concentration gradient
Check. The gradient can be steep or shallow. ✓
Method. Recall the gas that photosynthesis uses.
- Recall. Photosynthesis uses carbon dioxide from the air.uses carbon dioxide
- Write the answer. carbon dioxide
Check. Oxygen is made by photosynthesis and leaves the leaf. ✓
Method. Recall how distance affects the rate.
- Check (a). A shorter distance means particles arrive sooner.arrive sooner
- Check (b). A longer distance slows diffusion down.slows
- Decide. So a shorter distance makes diffusion faster.a
- Write the answer. a
Check. Diffusion speed depends on distance. ✓
Method. Think about the particles at each temperature.
- Think. At higher temperatures particles have more kinetic energy.more kinetic energy
- Conclude. So they move faster and diffusion is faster.diffusion is faster
- Write the answer. higher temperature
Check. Lower temperatures slow particles down. ✓
Method. Recall the effect of surface area.
- Recall. A larger surface area gives more space for particles to pass through.more space
- Conclude. So diffusion is faster.faster
- Write the answer. faster
Check. Exchange surfaces are large for this reason. ✓
Method. Think about where oxygen is used up.
- Think. The cell uses oxygen, so the inside of the cell has a lower concentration.lower concentration inside
- Conclude. So the concentration outside is higher, and oxygen diffuses in.diffuses in
- Write the answer. higher
Check. Oxygen moves from high outside to low inside. ✓
Method. Think about the particles.
- Think. Heating gives the water particles more energy.more energy
- Conclude. So the colour particles also move faster and spread quicker.move faster
- Write the answer. Accept any sentence that says the particles have more kinetic energy at a higher temperature.
Check. A fair test keeps the volume and the drop size the same. ✓
Method. Recall the factors.
- Recall. Any of the four factors makes diffusion faster.four factors
- Choose one. So one of these is the answer.one of these
- Write the answer. Accept any one: a steeper gradient, a higher temperature, a larger surface area, or a shorter distance.
Check. Each factor increases the speed of diffusion. ✓
Method. Link surface area and distance to the rate of diffusion.
- Surface area. A large surface area gives more space for oxygen to pass through.more space
- Distance. Thin walls mean a shorter distance for oxygen to diffuse.shorter distance
- Conclude. Both factors speed up diffusion, so oxygen is taken in faster.speed up
- Write the answer. Accept any sentence that says the large surface area gives more space for oxygen to diffuse, and the thin walls shorten the distance.
Check. Two factors act together to increase the rate. ✓
Osmosis, and water potential [H]
Osmosis is the net movement of water molecules across a partially permeable membrane, from a dilute solution (a high concentration of water) to a concentrated solution (a low concentration of water). Sugar molecules are too large to pass through the membrane, but water molecules can pass through it.
In a plant cell, water entering by osmosis makes the cell turgid (firm), which supports the plant. In a concentrated solution, water leaves the cell and the cell becomes flaccid (limp). Animal cells have no cell wall, so in pure water they can swell and burst, and in concentrated solution they shrink.
Key ideas
- Osmosis is the net movement of water across a partially permeable membrane.
- Water moves from a dilute solution to a concentrated solution.
- A plant cell in pure water becomes turgid; in concentrated solution it becomes flaccid.
- [H] Pure water has the highest water potential (0); solutions have lower (negative) water potentials.
Method. Recall the direction of osmosis.
- Recall. Water moves from a dilute solution to a concentrated solution.dilute to concentrated
- Name the direction. So water moves from the dilute solution into the concentrated solution.dilute into concentrated
- Write the answer. from the dilute solution into the concentrated solution
Check. Water moves down the water potential gradient. ✓
Method. Think about where water is more concentrated.
- Think. Pure water is dilute, so water moves into the cell by osmosis.water moves into the cell
- Conclude. The cell swells and becomes turgid.becomes turgid
- Write the answer. The cell takes in water by osmosis, so it swells and becomes turgid.
Check. The cell wall stops the cell bursting. ✓
Method. Compare the amount of water in each.
- Compare. Pure water contains only water, so it has the higher water potential.higher water potential
- Name it. A sugar solution has less free water, so its water potential is lower.lower water potential
- Write the answer. pure water
Check. Water moves from the higher to the lower water potential. ✓
Watch out
Watch out: osmosis is about water, not sugar. The sugar molecules do not move across the membrane; the water does.
Exam tip
Exam tip. Name the solution as dilute or concentrated, then say which way water moves, then say what happens to the cell.
Try it (with your teacher). With your teacher, look at a labelled diagram or a slide of plant cells in pure water and in concentrated salt or sugar solution. Describe what has changed in each cell.
Exercise 3B Osmosis, and water potential [H]
Method. Recall the definition.
- Recall. Osmosis is the net movement of water across a partially permeable membrane, from dilute to concentrated.net movement of water
- Write the answer. net movement of water across a partially permeable membrane
Check. Osmosis is a type of diffusion for water. ✓
Method. Recall the process.
- Recall. The movement of water across a partially permeable membrane is osmosis.osmosis
- Write the answer. osmosis
Check. Osmosis is the movement of water only. ✓
Method. Recall the size of sugar molecules.
- Recall. Sugar molecules are too large to pass through the membrane.too large
- Write the answer. So the answer is no.no
- Write the answer. no
Check. Water molecules are small enough to pass through. ✓
Method. Think about where water enters.
- Think. Water enters the cell by osmosis when the solution outside is more dilute.more dilute
- Conclude. Pure water is the most dilute, so the cell becomes turgid in pure water.turgid
- Write the answer. pure water
Check. The cell is firm when it has water. ✓
Method. Think about the water concentration.
- Think. The solution outside is more concentrated, so it has less free water than the cell.less free water
- Conclude. Water moves out of the cell by osmosis.moves out
- Write the answer. So yes, water leaves the cell.yes
- Write the answer. yes
Check. The cell becomes flaccid. ✓
Method. Recall the term.
- Recall. A limp cell that has lost water is flaccid.limp
- Write the answer. flaccid
Check. Turgid is the firm state. ✓
Method. Think about the water entering.
- Think. Water enters the animal cell by osmosis because pure water is dilute.water enters
- Conclude. An animal cell has no cell wall, so it can swell and burst.swell and burst
- Write the answer. So yes.yes
- Write the answer. yes
Check. A plant cell has a cell wall to stop this. ✓
Method. Compare the amount of free water.
- Compare. A dilute solution contains more free water than a concentrated solution.more free water
- Conclude. So the dilute solution has the higher water potential.higher water potential
- Write the answer. dilute solution
Check. Pure water has the highest water potential. ✓
Method. Think about the direction of the gradient.
- Think. Water moves down a water potential gradient.down a gradient
- Conclude. So it moves from the less concentrated side to the more concentrated side.less concentrated
- Write the answer. Accept any sentence that says water moves down a water potential gradient, from the less concentrated side to the more concentrated side.
Check. The gradient points from high to low water potential. ✓
Method. Think about where the water goes.
- Think. Water leaves the cell by osmosis, because the solution is more concentrated.water leaves
- Conclude. So the animal cell shrinks.shrinks
- Write the answer. shrinks
Check. A plant cell becomes flaccid instead. ✓
Method. Think about what the cell wall does.
- Think. Water enters the cell, so the vacuole and cell contents press outwards.press outwards
- Conclude. The cell wall stops the cell bursting, so it becomes turgid.becomes turgid
- Write the answer. Accept any sentence that says the cell swells and becomes turgid.
Check. The cell wall gives support. ✓
Method. Compare the structures of the two cells.
- Plant. A plant cell has a strong cell wall that resists the pressure from the water.strong cell wall
- Animal. An animal cell has no cell wall, so it can burst.no cell wall
- Write the answer. Accept any sentence that says the plant cell wall is strong and resists the pressure of the water, while the animal cell has no cell wall.
Check. The cell wall is the difference. ✓
Active transport
Active transport moves particles from a region of lower concentration to a region of higher concentration, against the concentration gradient. It needs energy from respiration. Proteins in the cell membrane act as carrier proteins, which bind the particles and move them across.
Two examples are: mineral ions moving into root hair cells from the soil, and glucose moving from the gut into the blood when the gut contains less glucose than the blood.
Key ideas
- Active transport moves particles against the concentration gradient.
- It needs energy from respiration, so cells with many mitochondria can do more.
- Carrier proteins in the membrane move the particles.
- Examples: mineral ions into root hair cells; glucose from the gut into the blood.
Method. Recall the process that needs energy.
- Recall. Moving particles against the gradient needs energy, so it is active transport.needs energy
- Name it. The answer is active transport.active transport
- Write the answer. active transport
Check. Diffusion moves particles down the gradient. ✓
Method. Think about the direction of the movement.
- Think. The particles move against the concentration gradient.against the gradient
- Conclude. Energy from respiration is needed to push them against the gradient.energy from respiration
- Write the answer. Particles are moved against the gradient, so energy from respiration is needed.
Check. Energy comes from aerobic respiration in the mitochondria. ✓
Method. Compare the two concentrations.
- Compare. The ions are at a higher concentration inside the cell than in the soil.higher inside
- Name the direction. By active transport they move into the cell, from the soil.into the cell
- Write the answer. into the cell
Check. Moving them in is against the gradient, so energy is used. ✓
Watch out
Watch out: active transport is not the same as diffusion. Diffusion needs no energy and goes down the gradient; active transport needs energy and goes up it.
Exam tip
Exam tip. Say “against the concentration gradient” and “energy from respiration” together for full marks.
Try it (with your teacher). Think about why a root hair cell needs many mitochondria. Write one sentence, and check it with your teacher. No practical is needed.
Exercise 3C Active transport
Method. Recall the process.
- Recall. Active transport moves particles against the gradient, using energy.using energy
- Write the answer. active transport
Check. It is not diffusion. ✓
Method. Recall where the energy comes from.
- Recall. The energy for active transport comes from respiration.comes from respiration
- Write the answer. respiration
Check. Respiration releases energy in the mitochondria. ✓
Method. Recall the direction.
- Recall. Active transport moves particles from low to high concentration, against the gradient.low to high
- Name it. That is up the gradient.up the gradient
- Write the answer. up the gradient
Check. Diffusion goes down the gradient. ✓
Method. Recall the proteins.
- Recall. Carrier proteins in the membrane bind particles and move them across.bind particles
- Write the answer. carrier proteins
Check. Carrier proteins are part of the cell membrane. ✓
Method. Recall an example in the root.
- Recall. Root hair cells take in mineral ions from the soil by active transport.root hair cells
- Write the answer. mineral ions into root hair cells
Check. Water enters roots by osmosis instead. ✓
Method. Recall an example from the gut.
- Recall. Glucose is absorbed from the small intestine into the blood by active transport when the gut contains less glucose.absorbed from the gut
- Write the answer. glucose from the gut into the blood
Check. This example is a case where the gut is less concentrated. ✓
Method. Compare the two concentrations.
- Compare. The ions are at a higher concentration inside the cell than outside.higher inside
- Decide. Moving them into the cell goes from low to high, against the gradient.against the gradient
- Write the answer. So the answer is yes.yes
- Write the answer. yes
Check. Against the gradient means energy is needed. ✓
Method. Think about what diffusion does not need.
- Think. Diffusion does not need energy or carrier proteins.does not need energy
- Conclude. So a process that does not need energy is not active transport.not active transport
- Write the answer. Accept any sensible answer: a concentration gradient in the same direction, or no energy.
Check. Compare the two processes side by side. ✓
Method. Link the energy to the organelle.
- Link. Active transport needs energy from respiration.needs energy
- Conclude. Mitochondria are where respiration releases this energy, so more mitochondria means more energy.more energy
- Write the answer. Accept any sentence that says mitochondria release the energy from respiration that active transport needs.
Check. More respiration means more energy for active transport. ✓
Method. Separate the direction from the speed.
- Direction. Active transport moves particles up the gradient, which needs energy.up the gradient
- Speed. Being able to go up a gradient does not mean it is faster than diffusion.not faster
- Conclude. The student has confused the direction with the speed.confused
- Write the answer. Accept any sentence that says active transport is slow in comparison, and it moves particles up a gradient only because energy is used to do this, not because it is faster.
Check. Direction and speed are different properties. ✓
Exchange surfaces and surface area to volume ratio
The surface area to volume ratio (SA:V) of an object tells us how much surface there is for each unit of volume. A small object has a large SA:V, so substances can reach all its cells quickly. As an object gets bigger, its SA:V gets smaller, so larger organisms need specialised exchange surfaces.
An exchange surface has a large surface area, a thin layer (a short distance), and a good supply of blood or air to keep the gradient steep.
| Cube side | Surface area (cm2) | Volume (cm3) | Surface area : volume |
|---|---|---|---|
| 1 cm | 6 | 1 | 6 : 1 |
| 2 cm | 24 | 8 | 3 : 1 |
| 3 cm | 54 | 27 | 2 : 1 |
| Exchange surface | Where | How it is adapted |
|---|---|---|
| villi | small intestine | many folds and projections; thin walls; blood supply |
| root hair cells | roots | long, thin extensions give a large surface area |
| alveoli | lungs | very many tiny sacs; thin walls; good blood supply |
Key ideas
- SA:V = surface area divided by volume.
- A smaller object has a larger SA:V.
- Exchange surfaces have a large surface area and thin walls.
- Villi, root hair cells and alveoli are all exchange surfaces.
Method. Divide the surface area by the volume.
- Divide. SA:V = 24 / 8.24 / 8
- Simplify. So the ratio is 3 : 1.3 : 1
- Write the answer. 3 : 1
Check. 3 x 8 = 24. ✓
Method. Think about the surface area.
- Think. The folds increase the surface area.increase the surface area
- Conclude. A larger surface area means more food can be absorbed by diffusion.more food absorbed
- Write the answer. The folds increase the surface area, so more food can be absorbed.
Check. Large surface area speeds up absorption. ✓
Method. Think about the distance.
- Think. The walls are one cell thick.one cell thick
- Conclude. So the gases have a short distance to diffuse.short distance
- Write the answer. The walls are thin, so gases diffuse over a short distance.
Check. A short distance speeds up diffusion. ✓
Watch out
Watch out: a bigger object has a bigger surface area, but a smaller SA:V. Do not write that a large animal has a large SA:V.
Exam tip
Exam tip. Give the ratio as a : 1 when a ratio is asked for. Divide the larger number by the smaller number if you need to simplify.
Try it (with your teacher). With your teacher, cut three cubes of jelly or sponge (with a grown-up or teacher only), of sides 1 cm, 2 cm and 3 cm. Work out the surface area to volume ratio of each cube. Describe the pattern.
Exercise 3D Exchange surfaces and surface area to volume ratio
Method. A cube has six faces.
- Find the area of one face. Each face is 1 cm x 1 cm = 1 cm2.1 cm2
- Multiply by six. Six faces give 6 x 1 = 6 cm2.6 x 1 = 6
- Write the answer. 6
Check. Six faces, each 1 cm2, make 6 cm2. ✓
Method. Volume is side multiplied by side multiplied by side.
- Multiply. 2 x 2 x 2 = 8.2 x 2 x 2 = 8
- Write the answer. 8
Check. 8 is 2 cubed. ✓
Method. Divide the surface area by the volume.
- Divide. SA:V = 24 / 8 = 3.24 / 8 = 3
- Write the answer. 3
Check. 3 x 8 = 24. ✓
Method. Divide the surface area by the volume.
- Divide. SA:V = 54 / 27 = 2.54 / 27 = 2
- Write the answer. 2
Check. 2 x 27 = 54. ✓
Method. Compare the ratios of the cubes.
- Compare. The ratios are 6, then 3, then 2 as the cube gets bigger.6, 3, 2
- Conclude. So the ratio decreases as the size increases.decreases
- Write the answer. decrease
Check. Bigger objects have a smaller SA:V. ✓
Method. Use the pattern from the cubes.
- Use the pattern. A small object has a larger SA:V than a large one.larger SA:V
- Choose. So the small single-celled organism has the larger SA:V.small single-celled organism
- Write the answer. small single-celled organism
Check. Small objects reach all their cells quickly. ✓
Method. Recall the structure with many finger-like projections.
- Recall. Villi are many finger-like projections on the lining of the small intestine.finger-like projections
- Write the answer. villi
Check. Villi also have thin walls and a blood supply. ✓
Method. Recall the root structure.
- Recall. Root hair cells are long thin extensions that increase the surface area.long thin extensions
- Write the answer. root hair cells
Check. Root hairs take in water by osmosis. ✓
Method. Recall the gas exchange sacs.
- Recall. The alveoli are tiny air sacs where oxygen and carbon dioxide are exchanged.air sacs
- Write the answer. alveoli
Check. The alveoli have thin walls and a blood supply. ✓
Method. Think about how the gradient is kept steep.
- Think. A thin wall gives a short distance; a blood supply keeps the gradient steep.short distance
- Choose one. So one of these features is the answer.one of these
- Write the answer. Accept any one: thin walls, a good blood supply, or a steep gradient maintained.
Check. More than one feature helps. ✓
Method. Area of one face, then six faces.
- One face. Each face is 4 x 4 = 16 cm2.4 x 4 = 16
- Six faces. Six faces give 6 x 16 = 96 cm2.6 x 16 = 96
- Write the answer. 96
Check. 96 divided by 6 is 16. ✓
Method. Find the surface area and the volume, then compare the ratio.
- Surface area. The surface area is 96 cm2, from six faces of 16 cm2.96
- Ratio. SA:V = 96 / 64 = 1.5, so the ratio is 1.5 : 1.96 / 64 = 1.5
- Compare. The 2 cm cube had a ratio of 3 : 1, so the bigger cube has a smaller ratio.smaller ratio
- Write the answer. Accept 1.5 : 1 and a sentence that says the ratio is smaller for the bigger cube.
Check. The ratio falls as the cube gets bigger. ✓
Required practical: osmosis in potato tissue
In this practical the independent variable is the concentration of the sucrose solution. The dependent variable is the percentage change in mass of each potato cylinder. Control variables include the size of the cylinders, the volume of solution, the time in solution and the temperature.
The cylinders are blotted dry before each weighing, so that only the water taken up by osmosis is measured. The concentration at which the mass does not change is the isotonic concentration: the potato cells and the solution have the same water potential.
Percentage change in mass = (final mass - starting mass) / starting mass x 100.
| Sucrose (mol/dm3) | Percentage change in mass (%) |
|---|---|
| 0.0 | +12 |
| 0.1 | +7 |
| 0.2 | +3 |
| 0.3 | 0 |
| 0.4 | -4 |
| 0.5 | -8 |
| 0.6 | -11 |
Key ideas
- Independent variable: sucrose concentration. Dependent variable: percentage change in mass.
- Keep the cylinder size, volume, time and temperature the same.
- A gain in mass means water entered by osmosis; a loss means water left.
- The isotonic concentration is where the percentage change is zero.
Method. Use percentage change = (final - starting) / starting x 100.
- Change. The change is 5.6 - 5.0 = 0.6 g.0.6 g
- Divide. Divide the change by the starting mass: 0.6 / 5.0 = 0.12.0.6 / 5.0 = 0.12
- Multiply. Multiply by 100 to get a percentage.0.12 x 100 = 12
- Write the answer. 12%
Check. 12% of 5.0 g is 0.6 g. ✓
Method. Find the concentration where the line crosses zero.
- Find zero. The line crosses the zero percentage change at one value.crosses zero
- Read the value. That value on the x-axis is 0.3 mol/dm3.0.3 mol/dm3
- Write the answer. 0.3
Check. At 0.3 mol/dm3 the mass does not change. ✓
Method. Subtract the two values.
- Read. At 0.0 the change is +12%; at 0.6 it is -11%.+12 and -11
- Subtract. The difference is 12 - (-11) = 23.12 + 11 = 23
- Write the answer. 23
Check. 23 percentage points between the two extremes. ✓
Watch out
Watch out: blot each cylinder dry before weighing, but do it gently and the same way every time. Skin oils or water left on the surface change the mass.
Exam tip
Safety. Cutting potato cylinders needs a teacher or a grown-up to supervise the cork borer and the scalpel. Keep the cutting board flat and use a dry surface.
Try it (with your teacher). With your teacher, cut potato cylinders of the same size, place them in sucrose solutions of two different concentrations, and weigh them before and after one hour. Calculate the percentage change in mass for each.
Exercise 3E Required practical: osmosis in potato tissue
Method. Find the variable changed on purpose.
- Find it. The concentration of the sucrose solution is changed on purpose.changed on purpose
- Write the answer. sucrose concentration
Check. The percentage change is the dependent variable. ✓
Method. Find the variable measured.
- Find it. The mass of each cylinder is measured before and after.measured before and after
- Write the answer. percentage change in mass
Check. It depends on the sucrose concentration. ✓
Method. Think about what must stay the same.
- Think. Each cylinder must be treated the same way.treated the same way
- Name one. So the size of the cylinders is a control variable.size of the cylinders
- Write the answer. Accept any one: the size of the cylinders, the volume of solution, the time, or the temperature.
Check. Only the concentration should change. ✓
Method. Think about what extra mass would be measured.
- Think. Water on the surface adds mass that did not come from osmosis.not from osmosis
- Conclude. So blotting removes this water, so the mass is measured accurately.measured accurately
- Write the answer. Accept any sentence that says surface water would add mass that is not from osmosis.
Check. Blot each one the same way. ✓
Method. Percentage change = change / starting mass x 100.
- Change. The change is 4.8 - 4.0 = 0.8 g.0.8 g
- Divide. Divide by the starting mass: 0.8 / 4.0 = 0.2.0.2
- Multiply. Multiply by 100: 0.2 x 100 = 20.20%
- Write the answer. 20%
Check. 20% of 4.0 g is 0.8 g. ✓
Method. Percentage change = change / starting mass x 100.
- Change. The change is 5.4 - 6.0 = -0.6 g.-0.6 g
- Divide. Divide by the starting mass: -0.6 / 6.0 = -0.1.-0.1
- Multiply. Multiply by 100: -0.1 x 100 = -10.-10%
- Write the answer. -10%
Check. The negative sign shows a loss of mass. ✓
Method. Read the value at 0.2 mol/dm3 from the table.
- Read. At 0.2 mol/dm3 the change is +3%.+3
- Interpret. A positive change means the cylinder gained mass.gained mass
- Write the answer. +3%
Check. Compare with the zero change at 0.3 mol/dm3. ✓
Method. Look for the negative values in the table.
- Look. The values -4%, -8% and -11% are negative.-4, -8, -11
- Match. These are at 0.4, 0.5 and 0.6 mol/dm3.0.4, 0.5, 0.6
- Write the answer. Accept 0.4 and 0.5 and 0.6 (any of these three).
Check. More concentrated solutions cause water to leave the potato. ✓
Method. Check the isotonic point.
- Check. At 0.3 mol/dm3 the mass does not change, so there is no net movement of water.no net movement
- Conclude. So the potato cells and this solution have the same water potential.same water potential
- Write the answer. So yes, Ruth is right to say this.yes
- Write the answer. yes
Check. No change in mass means the solutions are equal in water potential. ✓
Method. Find a weakness of the method.
- Weakness. Only one cylinder may be used for each concentration, so one result could be unreliable.one result
- Improve. Repeat readings for each concentration and take the mean, which makes the graph more reliable.repeat readings
- Write the answer. Accept any sensible improvement, for example more concentrations, or repeat readings for each concentration, to make the graph more reliable.
Check. Repeats show the spread and reduce the chance of an anomaly. ✓
Method. Compare the water potential inside and outside.
- Pure water. Pure water has a higher water potential than the potato, so water enters by osmosis.enters by osmosis
- Concentrated. The 0.6 mol/dm3 solution has a lower water potential than the potato, so water leaves by osmosis.leaves by osmosis
- Conclude. So the mass gains in pure water and falls in the concentrated solution.gains and falls
- Write the answer. Accept any answer that says water enters by osmosis in pure water, and leaves by osmosis in the concentrated solution because water potential is higher inside the potato.
Check. Water always moves down its water potential gradient. ✓
Chapter summary
| Skill | Method |
|---|---|
| Diffusion | Net movement from high to low concentration; passive; faster with a steeper gradient, higher temperature, larger surface area and shorter distance. |
| Osmosis | Net movement of water across a partially permeable membrane, from dilute to concentrated. |
| [H] Water potential | Pure water has the highest water potential; water moves down the gradient. |
| Active transport | Movement against the gradient; needs energy from respiration and carrier proteins. |
| Exchange surfaces | Large surface area, thin walls, and a good blood or air supply (villi, root hairs, alveoli). |
| SA:V ratio | Surface area divided by volume; falls as an object gets bigger. |
| Potato practical | Percentage change in mass against sucrose concentration; zero change is the isotonic point. |
Can you…?
- I can explain diffusion and the factors that change its rate
- I can explain osmosis and predict what happens to plant and animal cells
- I can explain why active transport needs energy, and give two examples
- I can calculate a surface area to volume ratio and explain how exchange surfaces are adapted
- I can plan the potato practical and interpret its graph, including the isotonic point
- [H] I can explain water potential
Key terms
- diffusion
- the net movement of particles from high concentration to low concentration
- concentration gradient
- the difference in concentration between two regions
- osmosis
- the net movement of water across a partially permeable membrane
- partially permeable membrane
- a membrane that lets some particles through but not others
- active transport
- movement of particles against a concentration gradient, using energy from respiration
- carrier protein
- a protein in the membrane that moves particles across it
- turgid
- firm, because the cell has taken in water by osmosis
- flaccid
- limp, because the cell has lost water
- isotonic
- when a solution and a cell have the same water potential, so no net movement of water
- surface area to volume ratio
- the surface area of an object divided by its volume
- exchange surface
- a surface with a large area and thin walls, where substances are exchanged
- [H] water potential
- a measure of how likely water is to move; pure water has the highest value
Exercise 3Z Check-up
Method. Recall the process.
- Recall. Movement down the concentration gradient, with no energy, is diffusion.no energy
- Write the answer. diffusion
Check. Active transport is the opposite case. ✓
Method. Recall the process for water.
- Recall. The movement of water across a partially permeable membrane is osmosis.movement of water
- Write the answer. osmosis
Check. Osmosis is for water only. ✓
Method. Recall the energy-using process.
- Recall. Active transport moves particles up the gradient, using energy.up the gradient
- Write the answer. active transport
Check. Diffusion does not need energy. ✓
Method. Think about the direction of water.
- Think. Pure water is dilute, so water enters the cell by osmosis.enters the cell
- Conclude. The cell becomes firm, which is turgid.turgid
- Write the answer. So yes.yes
- Write the answer. yes
Check. Turgid cells are firm. ✓
Method. Recall the factors.
- Recall. Four factors speed up diffusion.four factors
- Choose one. Any one of them is correct.any one
- Write the answer. Accept any one: a steeper gradient, a higher temperature, a larger surface area, or a shorter distance.
Check. Each factor increases the rate. ✓
Method. Surface area divided by volume.
- Divide. The surface area is 6 cm2 and the volume is 1 cm3, so 6 / 1 = 6.6 / 1 = 6
- Write the answer. 6
Check. 6 : 1 for the smallest cube. ✓
Method. Recall the finger-like projections.
- Recall. Villi are finger-like projections of the small intestine lining.finger-like
- Write the answer. villi
Check. Villi are exchange surfaces. ✓
Method. Change divided by the starting mass, times 100.
- Change. The change is 4.5 - 5.0 = -0.5 g.-0.5
- Divide. Divide by 5.0: -0.5 / 5.0 = -0.1.-0.1
- Multiply. Multiply by 100: -10%.-10%
- Write the answer. -10%
Check. The cylinder lost mass. ✓
Method. Compare the two concentrations.
- Compare. The ions move from low to high concentration, against the gradient.against the gradient
- Name it. Movement against the gradient is active transport.active transport
- Write the answer. active transport
Check. Energy from respiration is needed. ✓
Method. Think about the direction of the movement.
- Think. The particles move against the concentration gradient.against the gradient
- Conclude. Energy from respiration is needed to do this.energy from respiration
- Write the answer. Accept any sentence that says particles are moved against the concentration gradient, which needs energy from respiration.
Check. Diffusion needs no energy. ✓
Total 24 marks. Allow about 24 minutes.
Exercise 3X Exam-style questions
Method. Recall the process for water.
- Recall. The movement of water across a partially permeable membrane is osmosis.osmosis
- Write the answer. osmosis
Check. Osmosis is the movement of water only. ✓
Method. Recall the direction.
- Recall. Water moves from a higher water potential to a lower one.higher to lower water potential
- Name it. So water moves from the dilute solution to the concentrated solution.dilute to concentrated
- Write the answer. from a dilute solution to a concentrated solution
Check. Dilute solutions have a higher water potential. ✓
Method. Recall a factor.
- Recall. Any of these four factors speeds up diffusion.four factors
- Choose one. Any one is a correct answer.any one
- Write the answer. Accept any one: a higher temperature, a steeper gradient, a larger surface area, or a shorter distance.
Check. Each factor increases the rate. ✓
Method. Compare the concentrations, then explain the energy.
- Concentration. The ions are at a higher concentration in the cell than in the soil.higher in the cell
- Direction. Moving them into the cell goes against the gradient.against the gradient
- Energy. This needs energy from respiration.energy from respiration
- Write the answer. Accept any answer that says the ions are at a higher concentration inside the cell, so they move against the gradient, which needs energy from respiration.
Check. Diffusion would move them the other way. ✓
Method. Change divided by the starting mass, times 100.
- Change. The change is 5.8 - 5.0 = 0.8 g.0.8
- Divide. Divide by 5.0: 0.8 / 5.0 = 0.16.0.16
- Multiply. Multiply by 100: 16%.16%
- Write the answer. 16%
Check. 16% of 5.0 g is 0.8 g. ✓
Method. Calculate the ratio, then compare with a larger organism.
- Ratio. SA:V = 54 / 27 = 2, so the ratio is 2 : 1.2 : 1
- Pattern. A larger object has a smaller SA:V than a small object.smaller SA:V
- Conclude. So a large animal needs specialised exchange surfaces to supply its cells.specialised exchange surfaces
- Write the answer. Accept any answer that gives the ratio 2 : 1 and says a large animal has a smaller SA:V, so it needs specialised exchange surfaces.
Check. Compare with the 1 cm cube, which had 6 : 1. ✓
Method. List the adaptations of the villi.
- Point 1. Many finger-like projections increase the surface area.large surface area
- Point 2. Thin walls give a short distance for absorption.thin walls
- Point 3. A good blood supply keeps the concentration gradient steep.blood supply
- Write the answer. Accept any three of: many finger-like projections, large surface area, thin walls, and a good blood supply.
Check. Any three points make a full answer. ✓
Method. Explain the two results using water potential.
- Pure water. Pure water has a higher water potential, so water enters the potato by osmosis.enters by osmosis
- Concentrated. The 0.6 mol/dm3 solution has a lower water potential, so water leaves the potato.leaves the potato
- Conclude. The mass gain and loss follow the water potential gradient.water potential gradient
- Write the answer. Accept any answer that says water enters by osmosis in pure water and leaves in the more concentrated solution, because water potential inside the potato is between the two.
Check. Water always moves down its water potential gradient. ✓
Method. Set out the plan in four parts.
- Independent. The sucrose concentration is changed on purpose.sucrose concentration
- Dependent. The percentage change in mass is measured.percentage change in mass
- Controls. Keep the size of the cylinders and the volume of solution the same.size and volume
- Calculation. Use (final mass - starting mass) / starting mass x 100.(final - starting) / starting x 100
- Write the answer. Accept any plan that names sucrose concentration, percentage change in mass, and two sensible controls, with the formula (final mass - starting mass) / starting mass x 100.
Check. Blot dry and weigh each cylinder the same way. ✓
Method. Describe the water movement, then the wall.
- Osmosis. Pure water is dilute, so water enters the cell by osmosis.enters the cell by osmosis
- Turgid. The cell swells and the contents press outwards.press outwards
- Wall. The cell wall stops the cell bursting, so it becomes turgid.becomes turgid
- Write the answer. Accept any answer that says water enters the cell by osmosis, and the cell wall stops the cell bursting, so the cell becomes firm.
Check. Turgid means firm. ✓