Edexcel IGCSE Biology · Spec 2.15-2.16
Movement of Substances
Diffusion, osmosis and active transport compared — what moves, in which direction, and whether energy is needed.
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Movement of Substances
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Explained
Three ways in and out of a cell
Substances cross cell membranes in three ways. Two of them are passive, meaning they need no energy from the cell, and one is active, meaning it does. Sorting a process into the right category is usually the first mark.
Diffusion
The net movement of particles from a region of higher concentration to a region of lower concentration, down the concentration gradient.
It happens because the particles are already moving randomly, so no energy is needed. Oxygen entering a cell and carbon dioxide leaving it both work this way.
Osmosis
The movement of water across a partially permeable membrane, from a dilute solution to a more concentrated one.
Osmosis is really a special case of diffusion, and the special part is the membrane. It lets water through but not the dissolved solute, so only the water can move to even things out. A dilute solution has more water molecules, so water moves from where there is more of it to where there is less. Still passive, still no energy.
The word water belongs in every osmosis answer. Saying that a substance moves across the membrane describes diffusion; osmosis is about water specifically.
Active transport
The movement of particles from a lower concentration to a higher one, against the concentration gradient, using energy from respiration.
Because it works against the gradient, it cannot happen by itself. It needs carrier proteins in the membrane and it needs energy, which comes from ATP released by aerobic respiration.
Root hair cells absorbing mineral ions from dilute soil water do this, and so do the cells of the small intestine taking up the last of the glucose. Both are places where the useful substance is more concentrated inside the cell than outside.
Cells that do a lot of active transport contain many mitochondria, because that is where the energy comes from. If a question shows a cell with unusually many mitochondria, active transport is usually the reason.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme asks why a plant in waterlogged soil takes up fewer mineral ions, for three marks. It credits active transport, then less oxygen, then less respiration, then less energy or ATP.
Beside it is a short instruction: no credit for no air. Air is a mixture, and the part that matters is the oxygen, so the word oxygen is what earns the mark. This is the same precision the mark schemes ask for throughout biology, and it costs nothing to supply.
On another question, comparing the number of mitochondria in different cells, the mark scheme credits that mitochondria carry out aerobic respiration and release energy or make ATP, that villus cells carry out active transport and so need a lot of energy, that sperm cells swim and so need a lot of energy, and that skin cells have few active processes and use little energy.
Every one of those points links a structure to an energy requirement. Counting the mitochondria is the observation; saying what the cell does with the energy is the answer.
Comparing the three
Build the comparison on three questions. What moves? Diffusion moves any particle, osmosis moves water only, active transport moves dissolved particles such as ions and glucose.
Which way? Diffusion and osmosis go down the gradient, active transport goes against it.
Is energy needed? No for the first two, yes for the third.
Answering all three for each process gives a complete comparison, and it is far more reliable than trying to describe the processes one after another.
Spec 2.15-2.16
What you need to know
- Describe diffusion and osmosis
- Describe active transport
- Compare the three processes
Active recall
Quick check
Answer each question before opening the answer.
Define diffusion, osmosis and active transport.
Diffusion — net movement of particles from high to low concentration. Osmosis — movement of water across a partially permeable membrane from high to low water potential. Active transport — movement from low to high concentration, using energy.
Which of these processes requires energy from respiration?
Active transport.
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