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Active Transport

Discusses how cells can concentrate molecules inside themselves by expending energy.

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Active Transport

What does it take to roll a stone uphill?

This round stone tends to roll downhill due to the force of gravity. It takes an input of energy to push it uphill. Due to diffusion, molecules tend to move from an area of a large amount to an area of a small amount. So guess what it takes to move molecules the opposite way, from an area of low concentration to an area of high concentration? Energy, of course!

Active Transport

During active transport, molecules move from an area of low concentration to an area of high concentration. This is the opposite of diffusion, and these molecules are said to flow against their concentration gradient. Active transport is called "active" because this type of transport requires energy to move molecules. ATP is the most common source of energy for active transport.

As molecules are moving against their concentration gradients, active transport cannot occur without assistance. A carrier protein is always required in this process. Like facilitated diffusion, a protein in the membrane carries the molecules across the membrane, except this protein moves the molecules from a low concentration to a high concentration. These proteins are often called "pumps" because they use energy to pump the molecules across the membrane. There are many cells in your body that use pumps to move molecules. For example, your nerve cells (neurons) would not send messages to your brain unless you had protein pumps moving molecules by active transport.

The sodium-potassium pump (Figure below) is an example of an active transport pump. The sodium-potassium pump uses ATP to move three sodium (Na+) ions and two potassium (K+) ions to where they are already highly concentrated. Sodium ions move out of the cell, and potassium ions move into the cell. How do these ions then return to their original positions? As the ions now can flow down their concentration gradients, facilitated diffusion returns the ions to their original positions either inside or outside the cell.

Diagram of the sodium-potassium pump

The sodium-potassium pump moves sodium ions to the outside of the cell and potassium ions to the inside of the cell, areas where these ions are already highly concentrated. ATP is required for the protein to change shape. ATP is converted into ADP (adenosine diphosphate) during active transport.

Summary

  • During active transport, a protein pump uses energy, in the form of ATP, to move molecules from an area of low concentration to an area of high concentration.
  • An example of active transport is the sodium-potassium pump, which moves sodium ions to the outside of the cell and potassium ions to the inside of the cell.

Explore More

Use the resource below to answer the questions that follow.

  1. What is active transport?
  2. Where does a cell obtain the energy for active transport?
  3. How does the body prevent the loss of sugar in urine?
  4. List three factors that affect the movement of materials across a membrane. Explain how these factors affect the movement of matter.

Review

  1. How is active transport different from passive transport?
  2. What form of energy is usually used in active transport?
  3. Give an example of active transport. Explain what occurs during this process.

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Vocabulary

active transport

Movement across a membrane during which molecules move from an area of low concentration to an area of high concentration.

ATP

Adenosine triphosphate; usable form of energy inside the cell.

carrier protein

Transport protein that aids in diffusion by carrying a molecule across the membrane.

diffusion

Movement of molecules from an area of high concentration to an area of low concentration.

sodium-potassium pump

Protein in the membrane that moves sodium ions to the outside of the cell and potassium ions to the inside of the cell with the input of energy.

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