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A short class with Sarah and her students. The other ways are under More.
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Diffusion moves particles from high to low concentration. What happens if a cell needs to collect a substance it already has more of than its surroundings?
Diffusion can't do that, because it would go the wrong way.
Exactly. The cell has to move it against the gradient, from low to high. That is active transport.
Like pushing a ball uphill instead of letting it roll down?
A good picture. Rolling down needs no effort, but pushing uphill needs energy.
So where does the cell get that energy?
From ATP, made in respiration, mostly in the mitochondria.
Active transport needs special proteins in the membrane. What do you think they do?
They grab the particle and carry it across?
Yes. The particle fits into a carrier protein, energy from ATP changes the protein's shape, and the particle is released on the other side.
Is that what the sodium-potassium pump does?
Exactly. For each ATP, it pumps three sodium ions out of a nerve cell and two potassium ions in.
So if the cell stopped making ATP, the pump would stop.
Right, and the nerve cell could no longer send impulses properly.
How could we test whether roots take up potassium ions by active transport?
Stop them respiring and see if uptake falls.
Good. We can bubble nitrogen through one solution so there's no oxygen, and air through another. What stays the same?
The plants, the mass of roots, the starting concentration, the temperature and the time.
Excellent. In our sample results, uptake fell from 3.2 to 0.6 without oxygen. What does that tell us?
That about 81% of the uptake depends on aerobic respiration, so it must be active transport.
That is exactly the conclusion the data support.
Active transport moves particles from a lower to a higher concentration.
The energy comes from ATP made in respiration, so active transport stops when respiration stops.
Specific proteins in the membrane change shape to carry particles across.
Mineral ion uptake by roots, the sodium-potassium pump, and glucose absorption in the gut and kidneys.
Stopping aerobic respiration greatly reduces ion uptake by roots, supporting active transport.
A root hair cell absorbs nitrate ions from the soil even though the nitrate concentration inside the cell is already higher than in the soil water.
The sodium-potassium pump in a nerve cell uses one ATP molecule to pump three sodium ions out and two potassium ions in.
Roots kept in a solution without oxygen take up far fewer potassium ions, because they cannot make enough ATP for active transport.
Low to high takes ATP; high to low comes free.
A nightclub bouncer only lets people into an already packed club when they pay at the door. The bouncer is the carrier protein, the payment is ATP, and the crowd inside keeps growing even though it is already fuller than the street.
Active needs ATP: both start with A.
C.A.U. - Carrier protein, ATP, Uphill: the three signs of active transport.
Term
Define active transport.
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Term
Where does the energy for active transport come from?
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What does a carrier protein do?
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Sodium-potassium pump
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Why do root hair cells have many mitochondria?
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The movement of particles across a membrane from a lower to a higher concentration, using energy from ATP.
The difference in concentration of a substance between two regions.
A protein in the cell membrane that binds a specific particle and carries it across.
The molecule that supplies energy for cell processes, made in respiration.
The release of energy from glucose using oxygen, mainly in the mitochondria.
A carrier protein that uses ATP to pump three sodium ions out of a cell and two potassium ions in.
Charged particles such as nitrate and potassium that plants absorb from the soil.
Active transport moves molecules or ions across a cell membrane from a lower concentration to a higher concentration, against the concentration gradient, using carrier proteins and energy from ATP made in respiration.
This lesson explains how cells move substances "uphill", against their concentration gradient. It compares active transport with diffusion and osmosis, shows how carrier proteins use energy from ATP to change shape and carry particles across the membrane, and explores examples in root hair cells, nerve cells (the sodium-potassium pump), the small intestine and the kidneys. It then works through an investigation into whether ion uptake by roots depends on respiration, from the question to the interpretation of sample results.
Diffusion and osmosis move particles "downhill", from where they are more concentrated to where they are less concentrated, and need no energy. Cells often need the opposite: to collect a substance they already hold more of, or to push out a substance that is already more concentrated outside. Moving particles from a lower to a higher concentration is active transport, and it needs energy.
ATP (adenosine triphosphate) is made by respiration, mainly aerobic respiration in the mitochondria. So cells that carry out a lot of active transport usually contain many mitochondria, and anything that stops respiration, such as a lack of oxygen or a poison like cyanide, soon stops active transport too.
1. Formulating a scientific question. Does the uptake of potassium ions by plant roots depend on aerobic respiration? Hypothesis: roots in a solution bubbled with air will take up far more potassium ions than roots in a solution bubbled with nitrogen (no oxygen), because active transport needs ATP from aerobic respiration.
2. Planning the investigation. Independent variable: whether air or nitrogen is bubbled through the solution. Dependent variable: the uptake of potassium ions per gram of root per hour, worked out from the fall in the potassium concentration of the solution. Control variables: the same species and age of plant, the same mass of roots, the same starting concentration and volume of solution, the same temperature and the same time. Three repeats of each condition.
3. Recording and presenting data. Sample results (for practice), in micromoles of potassium ions per gram of root per hour:
A bar chart of the two means, with the range of the repeats shown, presents the comparison clearly.
4. Interpreting the results. Without oxygen, uptake fell from a mean of 3.2 to 0.6, a reduction of (3.2 − 0.6) ÷ 3.2 × 100, about 81%. Without oxygen the roots cannot carry out aerobic respiration, so they make far less ATP, and the carrier proteins cannot work as fast. This supports the hypothesis that ion uptake by roots is mainly active transport. The small uptake that remains could come from ATP made by anaerobic respiration.