34 4.8 Active Transport

Created by: CK-12/Adapted by Christine Miller

Four soldiers pushing a Humvee. Their backs are against the vehicle and their faces show that they are pushing as hard as they can.
Figure 4.8.1 The Humvee challenge – Active transport.

Like Pushing a Humvee Uphill

You can tell by their faces that these airmen (Figure 4.8.1) are expending a lot of trying to push this Humvee up a slope. The men are participating in a competition that tests their brute strength against that of other teams. The Humvee weighs about 13 thousand pounds (about 5,897 kilograms), so it takes every ounce of energy they can muster to move it uphill against the force of gravity. Transport of some substances across a is a little like pushing a Humvee uphill — it can’t be done without adding energy.

What Is Active Transport?

Some substances can pass into or out of a cell across the  without any  required because they are moving from an area of higher concentration to an area of lower concentration. This type of transport is called . Other substances require energy to cross a plasma membrane, often because they are moving from an area of lower concentration to an area of higher concentration, against the concentration gradient. This type of transport is called . The energy for active transport comes from the energy-carrying molecule called (adenosine triphosphate). Active transport may also require proteins called pumps, which are embedded in the plasma membrane. Two types of active transport are membrane pumps (such as the sodium-potassium pump) and vesicle transport.

The Sodium-Potassium Pump

The  is a mechanism of that moves sodium ions out of the cell and potassium ions into the cells — in all the trillions of in the body! Both ions are moved from areas of lower to higher concentration, so energy is needed for this “uphill” process. The energy is provided by . The sodium-potassium pump also requires . Carrier proteins bind with specific ions or molecules, and in doing so, they change shape. As carrier proteins change shape, they carry the ions or molecules across the membrane. Figure 4.8.2 shows in greater detail how the sodium-potassium pump works, as well as the specific roles played by carrier proteins in this process.

Image shows a diagram of a sodium potassium pump. The pump collects three sodium ions, and moves them out of the cell, against the concentration gradient by changing its shape. Then, the pump collects 2 potassium ions and by changing its shape, moves these two ions into the cell, also against the concentration gradient.
Figure 4.8.2 The sodium-potassium pump moves sodium ions (Na+) out of the cell and potassium ions (K+) into the cell. First, three sodium ions bind with a carrier protein in the cell membrane. The carrier protein then changes shape, powered by energy from ATP, and as it does, it pumps the three sodium ions out of the cell. At that point, two potassium ions bind to the carrier protein. The process is reversed, and the potassium ions are pumped into the cell.

To appreciate the importance of the sodium-potassium pump, you need to know more about the roles of sodium and potassium in the body. Both are essential dietary minerals. You need to get them from the foods you eat. Both sodium and potassium are also electrolytes, which means they dissociate into ions (charged particles) in solution, allowing them to conduct electricity. Normal body functions require a very narrow range of concentrations of sodium and potassium ions in body fluids, both inside and outside of cells.

  • Sodium is the principal ion in the fluid outside of cells. Normal sodium concentrations are about ten times higher outside of cells than inside of cells.  To move sodium out of the cell is moving it against the concentration gradient
  • Potassium is the principal ion in the fluid inside of cells. Normal potassium concentrations are about 30 times higher inside of cells than outside of cells. To move potassium into the cell is moving it against the concentration gradient.

These differences in concentration create an electrical and chemical gradient across the , called the . Tightly controlling the membrane potential is critical for vital body functions, including the transmission of and contraction of muscles. A large percentage of the body’s energy goes to maintaining this potential across the membranes of its trillions of cells with the .

Vesicle Transport

Some molecules, such as proteins, are too large to pass through the plasma membrane, regardless of their concentration inside and outside the cell. Very large molecules cross the plasma membrane with a different sort of help, called . Vesicle transport requires energy input from the cell, so it is also a form of active transport. There are two types of vesicle transport: endocytosis and exocytosis. Both types are shown in Figure 4.8.3.

Image shows a artist's rendition of a cell performing endo and exo cytosis. On the left side of the diagram, the cell is taking in large molecules through the plasma membrane by forming a vesicle around the particle. This is endocytosis. On the right side of the diagram, large molecules are exiting the cell by arriving in vesicles that fuse with the membrane to release their contents. This is exocytosis.
Figure 4.8.3 Large molecules can enter and exit the cell with the help of vesicles. On the left side of the diagram you can see exocytosis, as large molecules exit the cell through the plasma membrane. On the right side of the diagram you can see endocytosis, as large molecules enter the cell through the plasma membrane, via vesicle formation.

Endocytosis

 is a type of vesicle transport that moves a substance into the cell. The plasma membrane completely engulfs the substance, a vesicle pinches off from the membrane, and the vesicle carries the substance into the cell. When an entire cell or other solid particle is engulfed, the process is called . When fluid is engulfed, the process is called .

Exocytosis

 is a type of vesicle transport that moves a substance out of the cell (exo-, like “exit”). A vesicle containing the substance moves through the cytoplasm to the . Because the vesicle membrane is a  like the plasma membrane, the vesicle membrane fuses with the cell membrane, and the substance is released outside the cell.

Image shows a diagram of both endocytosis and exocytosis. On the left side of the diagram, and large particle is being brought into the cell by creating a pocket of plasma membrane around the particle. This pocket deepens and eventually pinches off from the rest of the membrane, forming a vesicle containing the particle. This process is called endocytosis. On the right side of the diagram, a vesicle containing substances for export out of the cell are contained in a vesicle. The vesicle travels to the cell membrane and the vesicular membrane fuses with the cell membrane, releasing the contents of the vesicle outside of the cell.
Figure 4.8.4 Endocytosis brings substances into the cell via vesicle formation. Exocytosis allows substances to exit the cell by merging a transport vesicle with the cell membrane.

Feature: My Human Body

Maintaining the proper balance of sodium and potassium in body fluids by active transport is necessary for life itself, so it’s no surprise that getting the right balance of sodium and potassium in the diet is important for good health. Imbalances may increase the risk of high blood pressureheart diseasediabetes, and other disorders.

If you are like the majority of North Americans, sodium and potassium are out of balance in your diet. You are likely to consume too much sodium and too little potassium. Follow these guidelines to help ensure that these minerals are balanced in the foods you eat:

  • Total sodium intake should be less than 2,300 mg/day. Most salt in the diet is found in processed foods, or added with a salt shaker. Stop adding salt and start checking food labels for sodium content. Foods considered low in sodium have less than 140 mg/serving (or 5 per cent daily value).
  • Total potassium intake should be 4,700 mg/day. It’s easy to add potassium to the diet by choosing the right foods — and there are plenty of choices! Most fruits and vegetables are high in potassium. Potatoes, bananas, oranges, apricots, plums, leafy greens, tomatoes, lima beans, and avocado are especially good sources. Other foods with substantial amounts of potassium are fish, meat, poultry, and whole grains. The collage below shows some of these potassium-rich foods.

Figure 4.8.5 Potassium power! 

4.8 Summary

  • requires to move substances across a , often because the substances are moving from an area of lower concentration to an area of higher concentration, or because of their large size. Two types of active transport are membrane pumps (such as the sodium-potassium pump) and vesicle transport.
  • The is a mechanism of active transport that moves sodium ions out of the cell and potassium ions into the cell against a concentration gradient, in order to maintain the proper concentrations of ions, both inside and outside the cell, and to thereby control membrane potential.
  • is a type of active transport that uses  to move large molecules into or out of cells.

4.8 Review Questions

  1. Define active transport.
  2. What is the sodium-potassium pump? Why is it so important?
  3. The drawing below shows the fluid inside and outside of a cell. The dots represent molecules of a substance needed by the cell. Explain which type of transport — active or passive — is needed to move the molecules into the cell.
    Image shows a cell with higher concentrations of a substance on the inside of the cell than on the outside of the cell. The cell is in a hypotonic solution
    Figure 4.8.6 Use this image to answer question #4
  4. What are the similarities and differences between phagocytosis and pinocytosis?
  5. What is the functional significance of the shape change of the carrier protein in the sodium-potassium pump after the sodium ions bind?
  6. A potentially deadly poison derived from plants called ouabain blocks the sodium-potassium pump and prevents it from working. What do you think this does to the sodium and potassium balance in cells? Explain your answer.

4.8 Explore More

Neutrophil Phagocytosis – White Blood Cell Eats Staphylococcus Aureus Bacteria,
ImmiflexImmuneSystem, 2013.

Cell Transport, The Amoeba Sisters, 2016.

Attributions

Figure 4.8.1

Humvee challenge by Airman 1st Class Collin Schmidt on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.8.2

Sodium Potassium Pump by Christine Miller is used under a CC BY 4.0  (https://creativecommons.org/licenses/by/4.0/) license.

Figure 4.8.3

Cytosis by Manu5 on Wikimedia Commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) license.

Figure 4.8.4 

Endocytosis and Exocytosis by Christine Miller is used under a CC BY 4.0  (https://creativecommons.org/licenses/by/4.0/) license.

Figure 4.8.5

Figure 4.8.6

Active Transport by Christine Miller is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

References

Amoeba Sisters. (2016, June 24). Cell transport [digital image]. YouTube. https://www.youtube.com/watch?v=Ptmlvtei8hw&feature=youtu.be

ImmiflexImmuneSystem. (2013). Neutrophil phagocytosis – White blood cell eats staphylococcus aureus bacteria. YouTube. https://www.youtube.com/watch?v=Z_mXDvZQ6dU

Mayo Clinic Staff. (n.d.). Diabetes [online]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/diabetes/symptoms-causes/syc-20371444

Mayo Clinic Staff. (n.d.). High blood pressure (hypertension) [online]. MayoClinic.org. https://www.mayoclinic.org/diseases-conditions/high-blood-pressure/symptoms-causes/syc-20373410

Mayo Clinic Staff. (n.d.). Heart disease [online]. MayoClinic.org.  https://www.mayoclinic.org/diseases-conditions/heart-disease/symptoms-causes/syc-20353118

Wikipedia contributors. (2020, June 19). Ouabain. In Wikipedia.  https://en.wikipedia.org/w/index.php?title=Ouabain&oldid=963440756

License

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Human Biology by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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