Key Difference – Active Transport vs Group Translocation
Molecules pass in and out from the cells via cell membranes. The cell membrane is a selectively permeable membrane which controls the movement of molecules. Molecules naturally move from a higher concentration to a lower concentration along the concentration gradient. It occurs passively without an energy input. However, there are also some situations where molecules travel across the membrane against the concentration gradient, from a lower concentration to a higher concentration. This process requires an energy input, which is known as active transport. Group translocation is another form of active transport where certain molecules are transported to cells using energy derived from phosphorylation. The key difference between active transport and group translocation is that in active transport, substances are not chemically modified during the movement across the membrane while, in group, translocation substances are chemically modified.
CONTENTS
1. Overview and Key Difference
2. What is Active Transport
3. What is Group Translocation
4. Side by Side Comparison – Active Transport vs Group Translocation
5. Summary
What is Active Transport?
Active transport is a method of transporting molecules across the semipermeable membrane against the concentration gradient or electrochemical gradient by utilizing the energy released from ATP hydrolysis. There are numerous situations where cells require certain substances such as ions, glucose, amino acids, etc. at higher or proper concentrations. In these occasions, active transport carries substances from a lower concentration to a higher concentration against the concentration gradient utilizing energy and accumulates inside the cells. Therefore, this process is always associated with a spontaneous exergonic reaction such as ATP hydrolysis, which provides energy to work against the positive Gibbs energy of the transportation process.
Active transport can be divided into two forms: primary active transport and secondary active transport. Primary active transport is driven using the chemical energy derived from ATP. Secondary active transport uses potential energy derived from the electrochemical gradient.
Specific transmembrane carrier proteins and channel proteins facilitate active transport. Active transport process depends on the conformational changes of the carrier or pore proteins of the membrane. As an example, sodium potassium ion pump shows repeated conformational changes when potassium ions and sodium ions are transported in and out of the cell respectively by active transport.
There are many primary and secondary active transporters in the cell membranes. Among them, sodium-potassium pump, calcium pump, proton pump, ABC transporter and glucose symporter are some examples.
Figure 01: Active transport via sodium-potassium pump
What is Group Translocation?
Group translocation is another form of active transport in which substances are subjected to covalent modification during the movement across the membrane. Phosphorylation is the main modification undergone by transported substances. During phosphorylation, a phosphate group is transferred from one molecule to another. Phosphate groups are joined by high energy bonds. Hence, when a phosphate bond breaks, a relatively large amount of energy is released and is used for the active transport. Phosphate groups are added to the molecules which enter the cell. Once they cross the cell membrane, they are returned to the unmodified form.
PEP phosphotransferase system is a good example for group translocation shown by bacteria for sugar uptake. By this system, sugar molecules such as glucose, mannose, and fructose are transported into the cell while being chemically modified. Sugar molecules become phosphorylated when entering the cell. The energy and the phosphoryl group are provided by PEP.
Figure 02: PEP phosphotransferase system
What is the difference between Active Transport and Group Translocation?
Active Transport vs Group Translocation |
|
| Active transport is the movement of ions or molecules through a semipermeable membrane from a lower concentration to a higher concentration, consuming energy. | Group translocation is an active transport mechanism in which molecules are chemically modified during the movement across the membrane. |
| Chemical Modification | |
| Molecules are not normally modified during the transportation. | Molecules are phosphorylated and chemically modified during group translocation. |
| Examples | |
| Sodium-potassium ion pump is a good example for active transport. | PEP phosphotransferase system in bacteria is a good example for group translocation. |
Summary – Active Transport vs Group Translocation
The cell membrane is a selectively permeable barrier, which facilitates the passage of ions and molecules. Molecules move from a high concentration to a low concentration along the concentration gradient. When the molecules are required to travel from a lower concentration to a higher concentration against the concentration gradient, it is necessary to provide an energy input. The movement of ions or molecules across a semipermeable membrane against the concentration gradient with the aid of proteins and energy is known as active transport. Group translocation is a kind of active transport which transports molecules after being chemically modified. This is the difference between active transport and group translocation.
Reference:
1. Metzler, David E., and Carol M. Metzler. “Biochemistry.” Google Books. N.p., n.d. Web. 17 May 2017.
2. “Active transport.” Wikipedia. Wikimedia Foundation, 14 May 2017. Web. 18 May 2017. <https://en.wikipedia.org/wiki/Active_transport>.
3. “Group Translocation – PEP:PTS.” Encyclopedia of Life Sciences. N.p., n.d. Web. 18 May 2017. <http://www.els.net/WileyCDA/ElsArticle/refId-a0001423.html>.
Image Courtesy:
1. “Scheme sodium-potassium pump-en” By LadyofHats Mariana Ruiz Villarreal – Own work (Public Domain) via Commons Wikimedia
2. “Phosphotransferase system” By Yikrazuul – Own work; ISBN 978-3-13-444608-1; S. 505 (CC BY-SA 3.0) via Commons Wikimedia