Active transport is the process of moving molecules across a cellular membrane through the use of cellular energy. Show
Credit: Anna Kireieva/Shutterstock.com The alternative to active transport is passive transport, which uses kinetic energy only to move the molecules. Passive transport can only move molecules from a region of higher concentration to lower concentration, whereas active transport moves molecules from an area of lower concentration to higher concentration. Active transport involves molecules moving against a gradient or other form of resistance, such as from an area of lower to higher charge. Active transport is used by cells to accumulate needed molecules such as glucose and amino acids. Active transport powered by adenosine triphosphate (ATP) is known as primary active transport. Transport that uses an electrochemical gradient is called secondary transport. Primary active transportMost primary active transport is carried out by transmembrane ATPases, an enzyme that crosses the cell membrane. The sodium-potassium adenosine triphosphatase (ATPase) enzyme is found in all animal cells. It maintains cell membrane potential by pumping three sodium ions out of the cell for every two potassium ions it moves into the cell. Related Stories
Another important example of active transport is the mitochondrial electron transport chain, which is based on reduction of NADH. It moves protons across the mitochondrial membrane from lower to higher concentration. This generates energy used to power life. Photosynthesis is also based on active transport. It uses the energy of photons to move protons across the thylakoid membrane of the chloroplast, creating redox potential. Secondary active transportSecondary active transport, or coupled transport, is not coupled to ATP. Electrochemical potential is built up by pumping ions into or out of the cell. This potential can provide energy for metabolism. For example, sodium ions are transported across the plasma membrane, and the electrochemical gradient powers active transport of another ion or molecule. Hydrogen pumps build up an electrochemical gradient of H+ ions in cells to power cellular respiration. How active transport worksTransmembrane proteins are proteins that cross a cell membrane. They usually have one or more transmembrane domains that cross through the membrane lipid bilayer, as well as domains that bind with ligands inside and outside of the membrane. The protein recognizes the molecule or ion to be transported and passes it through its transmembrane domain. Bulk transportSome materials are moved across cell membranes through the processes of endocytosis or exocytosis. In endocytosis, the cellular membrane folds around the materials being transported to form vesicles that move from one side to the other. In some cases, enzymes may digest the molecules transported into the cell by endocytosis. Viruses enter the cell by a different mode of bulk transport. The viral membrane fuses with the cellular membrane, forcing the DNA into the host cell. There are two types of endocytosis: pinocytosis and phagocytosis. Pinocytosis is a process wherein cells engulf liquid particles, such as fat droplets in the small intestine. Phagocytosis is a process for cells to absorb solid particles. In exocytosis, cells, expel substances by fusing their outer cell membranes with a vesicle membrane. Neurotransmitters are transported between brain cells through exocytosis. Sources:
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Last Updated: Feb 26, 2019  Written by Dr. Catherine ShafferCatherine Shaffer is a freelance science and health writer from Michigan. She has written for a wide variety of trade and consumer publications on life sciences topics, particularly in the area of drug discovery and development. She holds a Ph.D. in Biological Chemistry and began her career as a laboratory researcher before transitioning to science writing. She also writes and publishes fiction, and in her free time enjoys yoga, biking, and taking care of her pets. CitationsPlease use one of the following formats to cite this article in your essay, paper or report:
Suggested ReadingWhat is the function of active transport and moving small molecules and ions across cell membranes?Summary. Active transport is the energy-requiring process of pumping molecules and ions across membranes against a concentration gradient. Active transport processes help maintain homeostasis.
How does ATP enable transport proteins to move ions across cell membranes?ATP molecules are constantly colliding with the cytoplasmic face of the membrane. When an ATP molecule strikes its binding site on the carrier proteins, it attaches there. The release of energy causes a conformational change in the carrier protein. Note: Active transport is a vital process.
What is the function of active transport?Active transport involves molecules moving against a gradient or other form of resistance, such as from an area of lower to higher charge. Active transport is used by cells to accumulate needed molecules such as glucose and amino acids.
What does active transport move across the cell membrane?In cellular biology, active transport is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration—against the concentration gradient. Active transport requires cellular energy to achieve this movement.
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What is the function of active transport in moving small molecules and ions across cell membranes
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