AQA Biology 4.1.3.1–4.1.3.3 · Required practical
Diffusion, osmosis and active transport
Three ways substances cross a cell membrane. Sort any example by three questions: which way is it going, is it water, and does it need energy?
Triple Science · Higher tier
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Someone opens a bag of chips.
At the far end of a classroom, someone opens a bag of hot chips. Nobody moves, there is no breeze, and a minute later the whole room can smell them.
If you had to guess, are the smell particles spreading from where there are lots of them to where there are few, or the other way?
Three processes · one membrane
Which way, what moves, and is energy needed?
Diffusion
The spreading out of particles of a gas or a solute, resulting in a net movement from an area of higher concentration to an area of lower concentration. No energy needed.
Oxygen and carbon dioxide in gas exchange; urea from cells into the blood plasma. Faster with a bigger concentration difference, a higher temperature, or a larger membrane surface area.
Osmosis
The diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane. Water only. No energy needed.
Active transport
Movement from a more dilute solution to a more concentrated solution, against the concentration gradient. This requires energy from respiration.
Root hairs taking in mineral ions from very dilute soil water; sugar molecules absorbed from the gut into the blood.
Osmosis in cells
Plant cells have a wall. Animal cells don't.
In a dilute solution
Water moves into the cell by osmosis. A plant cell swells until its membrane presses against the cell wall: it becomes turgid, but the wall stops it bursting. An animal cell swells and may burst.
In a concentrated solution
Water moves out of the cell by osmosis. A plant cell loses turgor and becomes flaccid, so the plant wilts; in the extreme, the membrane pulls away from the wall (plasmolysis). An animal cell shrinks.
Required practical 3 · the AQA method
Potato in sugar solutions: weigh, soak, weigh again.
- Cut potato cylinders of the same length and diameter. Blot dry and record the mass of each.
- Put one cylinder in each of a range of sugar (or salt) solutions, and one in distilled water. Same volume of solution each time.
- Leave them for the same time, for example 30 minutes, at the same temperature.
- Remove each cylinder, blot dry the same way, and record its final mass.
- Calculate the percentage change in mass for each and plot it against concentration.
Percentage change, not change in mass, lets you compare cylinders that started at different masses. Where the line crosses 0%, there is no net movement of water: the solution has the same concentration as the cell contents.
Surface area to volume · exchange surfaces
The bigger the organism, the smaller its SA:V.
A single-celled organism has a large surface area compared with its volume, so diffusion across its surface meets its needs. In a large multicellular organism, the surface area to volume ratio is small and the inner cells are far from the surface, so it needs exchange surfaces and a transport system.
An exchange surface works better with a large surface area, a thin membrane for a short diffusion path, an efficient blood supply (animals), and ventilation (animals, for gas exchange).
Lungs
Many alveoli, thin walls, dense capillaries, ventilated.
Small intestine
Villi give a large surface area; thin walls; good blood supply.
Gills in fish
Many thin filaments, large surface area, good blood supply; water flows over them.
Roots
Root hair cells greatly increase the surface area for water and mineral ions.
Leaves
Flat and thin; stomata let gases in and out; air spaces inside.
Think again · which way water goes
“In osmosis, water moves from the concentrated solution to the dilute one.”
Your call
PredictLettuce leaves are sprinkled with salt and go limp. Which way did the water move?
Sort
Diffusion, osmosis or active transport?
Ask: which way, is it water, does it need energy?
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Equations · processing the data
Tap the quantity you want to find. For percentage change, the × 100 is a separate line: work out change ÷ initial first, then × 100.
Equation
Percentage change in mass
Side by side: multiply.
One over the other: divide.
Tap the quantity you want to find.
Written out
% change = (final mass − initial mass) ÷ initial mass × 100
- Δm
- change in mass (final − initial)
- g
- m
- initial mass
- g
- f
- fraction changed (% change ÷ 100)
- no unit
Equation
Rate of water uptake
Side by side: multiply.
One over the other: divide.
Tap the quantity you want to find.
Written out
rate of water uptake = change in mass ÷ time
- Δm
- change in mass
- g
- rate
- rate of water uptake
- g/min
- t
- time
- min
Equation
Surface area to volume ratio
Side by side: multiply.
One over the other: divide.
Tap the quantity you want to find.
Written out
SA:V = surface area ÷ volume
- SA
- surface area
- cm²
- SA:V
- surface area to volume ratio
- written n : 1
- V
- volume
- cm³
Command words in this lesson
- Compare
- Say how the processes are alike and how they differ: direction, what moves, energy.
- Explain
- Say which solution is more concentrated, which way water moves, and the result.
- Calculate
- Same mass units; keep the minus sign for a loss; write SA:V as n : 1.
Key fact
Diffusion and osmosis go down the concentration gradient with no energy; osmosis is water only, through a partially permeable membrane. Active transport goes against the gradient and needs energy from respiration.
Key note · AQA 4.1.3.1–4.1.3.3
Diffusion, osmosis and active transport
- Diffusion: net movement from higher to lower concentration; no energy. Faster with a bigger difference, higher temperature, larger surface area.
- Osmosis: water only, dilute to more concentrated, through a partially permeable membrane; no energy.
- Active transport: against the gradient, dilute to more concentrated; needs energy from respiration.
- Plant cell: turgid in dilute solution (wall stops bursting); flaccid then plasmolysed in concentrated. Animal cell: may burst / shrinks.
- RP3: % change = (final − initial) ÷ initial × 100. Crossing 0% = no net movement of water.
- SA:V falls as size rises, so large organisms need exchange surfaces: large area, thin, blood supply, ventilation.
Photograph this card. Tomorrow, cover it and say each line first.
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Not sure which way the water goes?
The practical graph shows typical class results for potato in sugar solutions; your crossing point may differ. Diagrams are schematic and not to scale.