Active Transport

Essential Question

How do cells move substances against a concentration gradient?

Passive transport only moves substances down a gradient, but cells often need to move substances the other way. In this lesson, you’ll learn how pumps use ATP to move substances against their gradients, how cells move large particles in vesicles, and how to tell passive and active transport apart.

Moving Against the Gradient

• Passive transport moves substances down the gradient
• Active transport moves them against the gradient
• Moving up a gradient needs energy
• The energy comes from ATP

Cells often need a higher concentration of a substance inside than outside, or the other way around. Moving particles up a concentration gradient is like pushing a ball uphill, and it takes energy. In active transport, a membrane protein uses energy from ATP to move a substance from a low concentration to a high concentration. Passive transport moves substances down the gradient with no energy from the cell, so the direction of movement compared with the gradient tells you which kind it is.

Protein Pumps

• Pumps are carrier proteins that use ATP
• The sodium-potassium pump moves 3 Na+ out
• It moves 2 K+ in for each ATP used
• The pump changes shape with each cycle

A pump is a carrier protein that uses ATP. The sodium-potassium pump, found in many animal cells, binds three sodium ions (Na+) inside the cell. ATP gives up energy, and the pump changes shape and releases the sodium ions outside. It then binds two potassium ions (K+) from outside, changes back, and releases the potassium inside. Each cycle uses one ATP, moves 3 Na+ out and 2 K+ in, and keeps sodium low and potassium high inside the cell.

Endocytosis and Exocytosis

• Large particles move in vesicles
• Endocytosis takes material into the cell
• Exocytosis sends material out of the cell
• Both processes use ATP

Particles too large to cross through a protein move in vesicles, which is called bulk transport. In endocytosis, the membrane folds inward around a particle and pinches off to form a vesicle inside the cell. In exocytosis, a vesicle fuses with the membrane and releases its contents outside the cell. This is how the secreted proteins in lesson 6.3 leave the cell. Both processes use ATP.

Active Transport Needs ATP

• Active transport stops when ATP is not available
• ATP comes from cellular respiration
• Respiration in most cells needs oxygen
• Passive transport does not depend on ATP

Because active transport uses ATP, anything that lowers the ATP supply slows it down. Most cells make ATP by cellular respiration, which needs oxygen, so low oxygen can reduce active transport. In the graph, root cells took up nitrate faster as oxygen increased. A substance that moves by passive transport would not show this pattern, because passive transport does not use ATP. You’ll study how cells make ATP in Unit 8.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Count the Pump Cycles

A sodium-potassium pump moves 3 Na+ out of a cell and 2 K+ into the cell for each ATP it uses. How many ATP are needed to move 600 Na+ out of the cell, and how many K+ enter the cell during that time?

Show the solution

1. Per cycle: 1 ATP moves 3 Na+ out and 2 K+ in.
2. ATP needed: 600 Na+ / 3 Na+ per ATP = 200 ATP.
3. K+ moved in: 200 ATP x 2 K+ per ATP = 400 K+.
4. Answer: 200 ATP are used, and 400 K+ enter the cell.

Example 2: Oxygen and Uptake

Root cells took up nitrate at different oxygen levels (sample data). Write a claim, evidence, and reasoning about how oxygen affects uptake.

Show the solution

1. Claim: nitrate uptake increases as oxygen increases and then levels off.
2. Evidence: uptake rose from 3 units at 0% oxygen to 24 at 5% and 33 at 10%, and was nearly level at 35 at 21%.
3. Reasoning: nitrate is taken up by active transport, which needs ATP. More oxygen allows more cellular respiration and more ATP, until another factor, such as the number of pumps, limits the rate.
4. Answer: Uptake rises with oxygen and levels off, which fits active transport.

Example 3: Passive or Active?

For each substance, compare the concentrations outside and inside the cell, and decide whether the movement is passive or active.

Substance	Outside (mM)	Inside (mM)	Moves
Glucose (through a carrier)	5	1	Into the cell
Potassium ion	5	150	Into the cell
Sodium ion	145	15	Out of the cell

Show the solution

1. Rule: down the gradient (high to low) is passive, and up the gradient (low to high) is active.
2. Glucose: moves from 5 mM to 1 mM, which is down the gradient, so it is passive (facilitated diffusion).
3. Potassium: moves from 5 mM to 150 mM, which is against the gradient (30 times higher inside), so it is active.
4. Sodium: moves from 15 mM inside to 145 mM outside, which is against the gradient, so it is active.
5. Answer: Glucose: passive. Potassium: active. Sodium: active.

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Practice

Classify movement as passive or active, calculate how many ATP a pump uses, read data on how oxygen affects uptake, and compare endocytosis with exocytosis. Try it on your own first, then check your work with the answer key.

• Active Transport Practice Worksheet (PDF)
• Active Transport Practice Answer Key (PDF)

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Lesson Slides

• 6.7 Active Transport Slides (PDF)

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Big Ideas

• Active transport moves substances against their gradient
• It uses ATP, usually through pumps
• The sodium-potassium pump moves 3 Na+ out and 2 K+ in per ATP
• Endocytosis and exocytosis move large materials in vesicles
• Passive transport is down the gradient and uses no ATP

Back to the essential question: passive transport moves substances down their gradient with no ATP, while active transport uses ATP to move them against it. Pumps handle ions, and vesicles handle large particles through endocytosis and exocytosis. Next, you’ll see why most cells are small.

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