Created by: CK-12/Adapted by Christine Miller
Like Pushing a Humvee Uphill
You can tell by their faces that these airmen (Figure 4.8.1) are expending a lot of energy 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 plasma membrane 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 plasma membrane without any energy required because they are moving from an area of higher concentration to an area of lower concentration. This type of transport is called passive transport. 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 active transport. The energy for active transport comes from the energy-carrying molecule called ATP (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 sodium-potassium pump is a mechanism of active transport that moves sodium ions out of the cell and potassium ions into the cells — in all the trillions of cells 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 ATP. The sodium-potassium pump also requires carrier proteins. 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.
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 cell membrane, called the membrane potential. Tightly controlling the membrane potential is critical for vital body functions, including the transmission of nerve impulses 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 sodium-potassium pump.
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. 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.
Endocytosis
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 phagocytosis. When fluid is engulfed, the process is called pinocytosis.
Exocytosis
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 cell membrane. Because the vesicle membrane is a phospholipid bilayer like the plasma membrane, the vesicle membrane fuses with the cell membrane, and the substance is released outside the cell.
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 pressure, heart disease, diabetes, 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
- Active transport requires energy to move substances across a plasma membrane, 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 sodium-potassium pump 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.
- Vesicle transport is a type of active transport that uses vesicles to move large molecules into or out of cells.
4.8 Review Questions
- Define active transport.
-
- What is the sodium-potassium pump? Why is it so important?
- 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.
Figure 4.8.6 Use this image to answer question #4 - What are the similarities and differences between phagocytosis and pinocytosis?
- What is the functional significance of the shape change of the carrier protein in the sodium-potassium pump after the sodium ions bind?
- 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
- Canteloupes. Image Number K7355-11 by Scott Bauer/ USDA on Wikimedia Commons is in the public domain (https://en.wikipedia.org/wiki/Public_domain).
- Spinach by chiara conti on Unsplash is used under the Unsplash license (https://unsplash.com/license).
- Eleven long purple eggplants by JVRKPRASAD on Wikimedia commons is used under a CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en) license.
- Bananas by Marco Antonio Victorino on Pexels is used under the Pexels license (https://www.pexels.com/license/).
- Potato picking by Nic D on Unsplash is used under the Unsplash license (https://unsplash.com/license).
- Maldives by Sebastian Pena Lambarri on Unsplash is used under the Unsplash license (https://unsplash.com/license).
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
The ability to do work.
A semi-permeable lipid bilayer that separates the interior of all cells from their surroundings.
a type of movement of substances across the cell membrane which does not require energy because the substances are moving with the concentration gradient (from high to low concentration).
The movement of ions or molecules across a cell membrane into a region of higher concentration, assisted by enzymes and requiring energy.
A complex organic chemical that provides energy to drive many processes in living cells, e.g. muscle contraction, nerve impulse propagation, and chemical synthesis. Found in all forms of life, ATP is often referred to as the "molecular unit of currency" of intracellular energy transfer.
A solute pump that pumps potassium into cells while pumping sodium out of cells, both against their concentration gradients. This pumping is active and occurs at the ratio of 2 potassium for every 3 calcium.
The smallest unit of life, consisting of at least a membrane, cytoplasm, and genetic material.
Proteins that carry substances from one side of a biological membrane to the other. Many carrier proteins are found in a cell's membrane, though they may also be found in the membranes of internal organelles such as the mitochondria, chloroplasts, nucleolus, and others.
The semipermeable membrane surrounding the cytoplasm of a cell.
The difference in electric potential between the interior and the exterior of a cell due to differences in the concentrations of ions on opposite sides of a cellular membrane.
A signal transmitted along a nerve fiber.
A form of active transport in which substances cross the plasma membrane with the help of a vesicle.
Endocytosis is a cellular process in which substances are brought into the cell. The material to be internalized is surrounded by an area of cell membrane, which then buds off inside the cell to form a vesicle containing the ingested material. Endocytosis includes pinocytosis and phagocytosis.
The process by which a cell uses its plasma membrane to engulf a large particle, giving rise to an internal compartment called the phagosome.
The ingestion of liquid into a cell by the budding of small vesicles from the cell membrane.
An important process of plant and animal cells as it performs the opposite function of endocytosis. In exocytosis, membrane-bound vesicles containing cellular molecules are transported to the cell membrane and released into the area surrounding the cell.
A thin polar membrane made of two layers of phospholipid molecules. These membranes are flat sheets that form a continuous barrier around all cells.
A structure within a cell, consisting of lipid bilayer. Vesicles form naturally during the processes of secretion, uptake and transport of materials within the plasma membrane.
