Active Transport
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Printed: June 27, 2013
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C ONCEPT
Concept 1. Active Transport
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Active Transport
• Describe the process of 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 high concentration (large amount) to an area of low concentration
(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
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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 1.1) is an example of an active transport pump. The sodium-potassium pump
uses ATP to move sodium (Na+ ) and potassium (K+ ) ions to where they are already highly concentrated. Sodium
ions move out of the cell, and potassium ions move into the cell.
FIGURE 1.1
The
sodium-potassium
pump
moves
sodium ions to the outside of the cell
and potassium ions to the inside of the
cell. ATP is required for the protein to
change shape.
ATP is converted into
ADP (adenosine diphosphate) during
active transport.
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.
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.
Practice
Use the resource below to answer the questions that follow.
• Osmosis and Active Transport at http://www.youtube.com/watch?v=6tVc5gyOzO4 (8:40)
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Concept 1. Active Transport
MEDIA
Click image to the left for more content.
1. What does a cell use active transport for? Why does a cell use this type of transport as well as passive
transport?
2. Where does a cell obtain the energy for active transport?
3. How does the body prevent the loss of sugar in urine? What effect would passing sugar in the urine have on
an organism?
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?
References
1. CK-12 Foundation - Hana Zavadska, based on image by Mariana Ruiz Villarreal (LadyofHats) (http://commons.wikimedia.
sodium-potassium_pump-en.svg). . CC-BY-NC-SA 3.0
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