Main Difference – Active vs Passive Transport
Active and passive transport are two methods that transport molecules across the cell membrane. A cell membrane is a multi-task unit which gives structure to the cell while protecting the cytosolic content from the extracellular environment. The movement of molecules in and out of the cell is determined by the phospholipid bilayer, maintaining a delicate homeostasis of the cell. The phospholipid bilayer is semi-permeable, allowing some molecules to freely pass the membrane through a concentration gradient and some molecules to use special structures in order to pass the membrane and others to pass the membrane by utilizing cellular energy. The main difference between active and passive transport is that active transport pumps molecules against the concentration gradient using ATP energy whereas passive transport allows the molecules to pass the membrane through a concentration gradient, requiring no cellular energy.
This article looks at,
1. What is Active Transport
– Definition, Types, Function, How it works
2. What is Passive Transport
– Definition, Types, Function, How it works
3. What is the difference between Active and Passive Transport
What is Active Transport
Active transport is the movement of molecules across the membrane against the concentration gradient with the assistance of enzymes and usage of cellular energy. It is required for the accumulation of molecules like glucose, amino acids, and ions inside the cell in high concentrations. Two types of active transport can be identified: primary active transport and secondary active transport.
Primary Active Transport
During primary active transport, the presence of substances in the extracellular fluid that is required by the cell is recognized by the specialized trans-membrane proteins on the cell membrane, which serve as pumps of transporting molecules. These trans-membrane proteins are powered by ATP. The primary active transport is most obvious in the sodium/potassium pump (Na+/K+ ATPase), which maintains the resting potential of the cell. The energy released by the hydrolysis of ATP is used to pump three sodium ions out of the cell and two potassium ions into the cell. Here, sodium ions are transported from a lower concentration of 10 mM to a higher concentration of 145 mM. Potassium ions are transported from a 140 mM concentration inside the cell to a 5 mM concentration of extracellular fluid. The proton/potassium pump (H+/K+ ATPase) is found in the lining of the stomach, maintaining an acidic environment inside the stomach. Omeprazole is a proton/potassium pump inhibitor, reducing the acid reflux inside the stomach. During both oxidative phosphorylation and photophosphorylation of electron transport chain use primary active transport to create a reducing power as well. The action of sodium/potassium pump is shown in figure 1.
Figure 1: Sodium/Potassium Pump
Secondary Active Transport
Secondary active transport is powered by an electrochemical gradient. Here, channels are made by pore-forming proteins. A simultaneous movement of another substance against the concentration gradient is observed with the secondary active transport. Hence, the channel proteins involved in the secondary active transport can be identified as cotransporters. There are two types of cotransporters: antiporters and symporters. Particular ion and the solute are transported in the opposite directions by antiporters. Sodium/calcium exchanger, which allows the restoration of calcium ion concentration in the cardiomyocyte after the action potential, is the most common example for antiporters. Ions are transported through the concentration gradient while the solute is transported against the concentration gradient by symporters. Here, both molecules are transported in the same direction across the cell membrane. SGLT2 is a symporter that transports glucose into the cell along with the sodium ions. The function of symporter and antiporter is shown in figure 2.
Figure 2: The action of Symporter and Antiporter
What is Passive Transport
Passive transport is the movement of molecules across the membrane through a concentration gradient with no use of cellular energy by the movement. It uses natural entropy to move molecules from a higher concentration to a lower concentration until the concentration becomes equalized. Then, there will be no net movement of molecules at the equilibrium. Four main types of passive transport are found: osmosis, simple diffusion, facilitated diffusion, and filtration. The simple movement of molecules across a permeable membrane is called simple diffusion. Small, non-polar molecules use simple diffusion. The diffusion distance should be less in order to maintain a better flow. Passive transport across the membrane is shown in figure 3.
Figure 3: Passive Transport
During facilitated diffusion, special transport proteins are used to guide the movement of polar molecules and large ions. These transport proteins are glycoproteins and are specific to a particular protein. The GLUT4 is a glucose transporter that transports glucose from the bloodstream into the cell. It is mostly found in fat and skeletal muscles. Three types of transport proteins are involved in the facilitated diffusion: channel proteins, aquaporins, and carrier proteins. Channel proteins make hydrophobic tunnels across the membrane, allowing the selected hydrophobic molecules to pass through the membrane. Some channel proteins are opened at all times, and some are gated like ion channel proteins. Aquaporins allow water to cross the membrane quickly. Carrier proteins change their shape, transporting target molecules across the membrane. Facilitated diffusion by carrier proteins is shown in figure 4.
Figure 4: Facilitated Diffusion
Filtration is the movement of solutes along with water due to the hydrostatic pressure generated by the cardiovascular system. It occurs in Bowman’s capsule in the kidney. Osmosis is the movement of water across a selectively permeable membrane. It occurs from a high water potential to a low water potential.
Difference Between Active and Passive Transport
Definition
Active Transport: Active transport pumps molecules through the cell membrane against the concentration gradient.
Passive Transport: Passive transport allows molecules to pass the cell membrane through a concentration gradient.
Cellular Energy Usage
Active Transport: Active transport utilizes cellular energy in the form of ATP.
Passive Transport: Passive transport does not require cellular energy.
Types of Transport
Active Transport: Endocytosis, exocytosis, secretion of substances into the bloodstream, and sodium/potassium pump are the types of active transport.
Passive Transport: Diffusion, facilitated diffusion, and osmosis are the types of passive transport.
Role
Active Transport: Active transport allows molecules to pass the cell membrane, disrupting the equilibrium established by the diffusion.
Passive Transport: A dynamic equilibrium of water, nutrients, gasses and wastes is maintained by passive transport between cytosol and extracellular environment.
Transporting Particles
Active Transport: Ions, large proteins, complex sugars as well as cells are transported by active transport.
Passive Transport: Water soluble molecules like small monosaccharides, lipids, sex hormones, carbon dioxide, oxygen, and water are transported by passive transport.
Importance
Active Transport: Active transport is required for the entrance of large, insoluble molecules into the cell.
Passive Transport: Passive transport allows the maintenance of a delicate homeostasis between the cytosol and extracellular fluid.
Conclusion
Active and passive transport are the two methods of transporting molecules across the cell membrane. Active transport pumps molecules against a concentration gradient using cellular energy. In primary active transport, ATP is used as the energy. In secondary active transport, the electrochemical gradient is used to move molecules across the membrane. Nutrients are concentrated into the cell by using active transport. Passive diffusion allows small, non-polar molecules to move across the membrane. It only occurs through a concentration gradient. Therefore, no energy is utilized by the process. Osmosis and filtration are also methods of passive diffusion. However, the main difference between active transport and passive transport is their mechanisms of transporting molecules across the membrane.
Reference:
1.”Passive transport and active transport across a cell membrane article (article).” Khan Academy. N.p., n.d. Web. 03 May 2017.
2.”Diffusion and passive transport.” Khan Academy. N.p., n.d. Web. 03 May 2017.
Image Courtesy:
1. “Scheme sodium-potassium pump-en” By LadyofHats Mariana Ruiz Villarreal – Own work. Image renamed from Image:Sodium-Potassium_pump.svg (Public Domain) via Commons Wikimedia
2. “Porters”By Lupask – Own work, Public Domain) via Commons Wikimedia
3. “Figure 05 02 02” By CNX OpenStax – (CC BY 4.0) via Commons Wikimedia
4. “Blausen 0213 CellularDiffusion” By Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. – Own work (CC BY 3.0) via Commons Wikimedia
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