What are three mechanisms of carrier-mediated transport?
Multiple select question.
Secondary active transport
Primary active transport
Simple diffusion
Endocytosis
Facilitated diffusion
The correct answer and explanation is :
The three mechanisms of carrier-mediated transport are:
- Secondary active transport
- Primary active transport
- Facilitated diffusion
Explanation:
Carrier-mediated transport refers to the process by which molecules or ions are moved across a biological membrane with the help of specific carrier proteins. This can happen through several mechanisms that vary based on whether energy is required, and the direction of transport.
- Secondary Active Transport:
In secondary active transport, molecules are moved against their concentration gradient (from low to high concentration) using the energy stored in the electrochemical gradient of another molecule. This energy is typically provided by ions like sodium (Na+) or hydrogen (H+), which have been pumped across the membrane using primary active transport. For example, the sodium-glucose cotransporter uses the energy from the sodium gradient (created by the sodium-potassium pump) to move glucose into the cell against its concentration gradient. Secondary active transport can be symport (both substances move in the same direction) or antiport (substances move in opposite directions). - Primary Active Transport:
Primary active transport involves the direct use of energy, typically from ATP hydrolysis, to transport molecules against their concentration gradient. The energy from ATP is used by a pump protein (such as the sodium-potassium pump, Na+/K+ ATPase) to move ions like sodium and potassium across the membrane. For example, the sodium-potassium pump moves sodium ions out of the cell and potassium ions into the cell, both against their concentration gradients. - Facilitated Diffusion:
Facilitated diffusion does not require energy and occurs when molecules move down their concentration gradient (from high to low concentration) through a carrier or channel protein. While it does not require cellular energy (ATP), it does require a carrier protein to facilitate the transport of larger or polar molecules that cannot easily pass through the lipid bilayer of the membrane. Examples include the transport of glucose into cells or the movement of ions through ion channels.
Excluded Mechanisms:
- Simple diffusion is not carrier-mediated because molecules move directly through the lipid bilayer down their concentration gradient without the help of a carrier.
- Endocytosis involves the engulfment of materials by the cell membrane to bring substances into the cell and does not involve carrier proteins.
These mechanisms of carrier-mediated transport ensure that essential substances like glucose, ions, and amino acids are properly transported into and out of cells to maintain homeostasis.