Diffusion and osmosis only ever go down a gradient. When a cell needs to gather a substance it already has more of, it must push particles the other way and pay for it with energy from respiration. The role of protein carriers and ion uptake by root hairs is Extended content.
Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (against a concentration gradient), using energy from respiration. Extended: protein carriers in the membrane move the molecules or ions across, which lets cells such as root hair cells take up mineral ions from very dilute soil water.
Fig. 1: a carrier protein moves a nitrate ion into a root hair cell, against the concentration gradient, using energy from respiration
Section 01
Against the gradient.
In the sim there are already more nitrate ions inside the root hair cell than in the soil water, yet the cell keeps taking more in. Diffusion would carry them the wrong way. Turn the oxygen supply off and watch what happens.
Diffusion
Osmosis
Active transport
What moves
any particles (gases, solutes)
water only
molecules or ions
Direction
down a concentration gradient
higher to lower water potential
against a concentration gradient
Energy from respiration
not needed
not needed
needed
Membrane needed
no (but can cross one)
yes, partially permeable
yes, with carrier proteins
Oxygen off: without oxygen the cell cannot respire aerobically, so little energy is released, the carriers stop and some ions leak back out by diffusion.
Waterlogged soil has little air, so roots cannot respire well and mineral ion uptake falls.
Examiner trap
Active transport needs energy from respiration. Do not say the energy comes from the kinetic energy of particles (that is diffusion), and do not say active transport "makes" energy: it uses it.
Stage 1 · Learn
Check what the sim just showed you
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In the sim, what happens to ion uptake when the oxygen supply is turned off?
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In which direction does active transport move ions?
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Where does the energy for active transport come from?
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Which process needs energy from respiration?
Section 02
Carriers and root hairs.
Extended. Active transport lets cells collect molecules or ions even when the concentration outside is lower than inside. It is carried out by protein carriers in the cell membrane.
Protein carriers: each carrier picks up a specific molecule or ion on one side of the membrane, uses energy from respiration to change shape, and releases it on the other side.
Root hair cells: mineral ions such as nitrate and magnesium are very dilute in soil water, so root hairs absorb them by active transport. Nitrate is needed to make amino acids and proteins; magnesium to make chlorophyll.
Many mitochondria: cells that carry out a lot of active transport, such as root hair cells, contain many mitochondria to release the energy needed.
In humans: glucose is absorbed from the small intestine into the blood by active transport as well as by diffusion, so none is left behind in the gut.
Examiner trap
Root hair cells take up water by osmosis but mineral ions by active transport. Keep the two processes separate in your answers.
Stage 2 · Exam
Exam-style questions
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Extended: which structures in the cell membrane move ions during active transport?
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Extended: root hair cells take up nitrate ions from soil water by active transport because:
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Roots are kept in aerated and in unaerated (no oxygen) solutions. Uptake of potassium ions is much lower without oxygen. The best explanation is:
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Extended: a cell that carries out a lot of active transport is most likely to contain many:
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Active transport, mastered.
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