Main Difference – Primary vs Secondary Active Transport
Active transport is the movement of molecules across the cell membrane against the concentration gradient with the assistance of enzymes and usage of cellular energy. Active transport is divided into two types known as primary and secondary active transport depending on the source of energy used in the transport of molecules. The main difference between primary and secondary active transport is that molecules are transported by the breakdown of ATP in primary active transport, whereas in secondary active transport, the concentration gradient of one molecule provides the energy for the transport of another molecule against the latter’s concentration gradient.
Key Areas Covered
1. What is Primary Active Transport
– Definition, Types, Characteristics
2. What is Secondary Active Transport
– Definition, Types, Characteristics
3. What are the similarities between Primary and Secondary Active Transport
– Outline of Common Features
4. What is the difference between Primary and Secondary Active Transport
– Comparison of Key Differences
Key Terms: Antiport, ATP, Concentration Gradient, Electrochemical Gradient, Ion-coupled Transport, Primary Active Transport, Secondary Active Transport, Symport, Transmembrane Proteins
What is Primary Active Transport
Primary active transport is the transport of molecules against a concentration gradient by the use of energy from ATP. Transmembrane proteins are involved in governing the passage of molecules across the cell membrane. They contain one or more ATP binding sites on their cytosolic face. During primary active transport, the energy is transferred to the transmembrane protein and then to the concentration gradient. The primary active transport is most obvious in sodium/potassium pump (Na+/K+ ATPase), which maintains the resting potential of cells. Hydrolysis of an ATP pumps 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 the extracellular fluid. The proton/potassium pump (H+/K+ ATPase) is found in the lining of the stomach, maintaining an acidic environment in the stomach.
Figure 1: Sodium-Potassium Pump
What is Secondary Active Transport
Secondary active transport is the another type of active transport that uses the electrochemical gradient in the transport of molecules against the concentration gradient. Transmembrane proteins involved in the secondary active transport are identified as cotransporters since they transport two types of molecules at the same time. On that account, secondary active transport is referred to the ion-coupled transport. Typically, an ion (driving ion) is transported through its electrochemical gradient while another ion or solute (driven ion/molecule) is transported against the electrochemical gradient. The concentration gradient of the driving ion provides the energy for the transport of driven ion/molecule against latter’s concentration gradient. The maintenance of the concentration gradient of the driving ion is accomplished by primary active transport.
Symport and antiport are the two types of secondary active transport. In symport, both driving and driven molecules are transported in the same direction. The Na+/glucose cotransporter (SGLT1) is an example for symports in which both sodium and glucose are transported into the cell. It is found in kidney proximal tubules and the small intestine. In antiport, driving and driven ions are transported in the opposite directions. The Na+/Ca2+ exchanger (NCX) in muscle cells transports sodium ions into the cell while calcium ions are transported out of the cell. The sodium/glucose pump is shown in figure 2.
Figure 2: Sodium/Glucose Pump
Similarities Between Primary and Secondary Active Transport
- Both primary active transport and secondary active transport are two active transport methods.
- Both methods are involved in the pumping of molecules against the concentration gradient, from a low concentration to a high concentration.
- Transmembrane proteins are involved in facilitating both primary and secondary active transport.
- Transmembrane proteins are specific to the molecules transported across the membrane.
- The main purpose of both transportation methods is to speed up the movement of molecules across the cell membrane.
Difference Between Primary and Secondary Active Transport
Definition
Primary Active Transport: Primary active transport is the transport of molecules against a concentration gradient by the use of energy from ATP.
Secondary Active Transport: Secondary active transport is the transport of two different molecules across a transport membrane using energy in other forms than ATP.
Coupled Transport
Primary Active Transport: A single molecule is transported in primary active transport.
Secondary Active Transport: Two types of molecules are transported at once in secondary active transport.
Source of Energy
Primary Active Transport: Primary active transport uses energy from ATP.
Secondary Active Transport: The concentration gradient of the driving ion provides the energy for the transport of driven ion/molecule against its concentration gradient in secondary active transport.
Types of Molecules
Primary Active Transport: Ions like sodium, potassium, and calcium are transported across the cell membrane in primary active transport.
Secondary Active Transport: Ions serve as driving molecules, while ions or other molecules serve as driven molecules.
Types
Primary Active Transport: Transmembrane proteins are unique to the ion transported through them.
Secondary Active Transport: Symports and antiports are the two types of transmembrane proteins in secondary active transport.
Examples
Primary Active Transport: Sodium-potassium pump, calcium pump in the muscles, and proton pump in the stomach are the examples of the primary active transport.
Secondary Active Transport: Glucose-sodium pump, Na+/Ca2+ exchanger, and sodium/phosphate cotransporter are the examples of secondary active transport.
Conclusion
Primary and secondary active transport are the two variations of active transport of molecules across biological membranes. The main difference between primary and secondary active transport is the source of energy used by each transport method in order to transport molecules across the cell membrane. Primary active transport utilizes the energy of ATP to transport a single molecule at a time across the cell membrane. Secondary active transport transports two molecules across the membrane at a time; the concentration gradient of the driving ion provides energy for the transport of driven molecule in secondary active transport.
Reference:
1. “Active transport.” Khan Academy. N.p., n.d. Web. Available here. 15 June 2017.
2. “Primary Active Transport.” Boundless. N.p., 26 May 2016. Web. Available here. 15 June 2017.
3. “Secondary Active Transport.” PhysiologyWeb. N.p., n.d. Web. Available here. 15 June 2017.
Image Courtesy:
1. “Blausen 0818 Sodium-PotassiumPump” 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
2. “Figure 05 03 04” By CNX OpenStax –(CC BY 4.0) via Commons Wikimedia
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