In primary active transport, energy from ATP is directly used to transport molecules against their concentration gradient. For example, the Na+/K+-ATPase is a membrane spanning protein carrier. Please note the –ase ending, so it is also an enzyme that hydrolyzes (breaks bonds with water) ATP to get its energy. It is also referred to as the ‘Na+/K+ pump’. This is because it acts much like a pump that is bailing out a leaky ship. It works non-stop to continuously expel 3 Na+ ions out of the cell and at the same time import 2 K+ ions into the cell per cycle. Each cycle of the pump requires 1 ATP molecule. Both Na+ and K+ are being moved against their concentration gradients, therefore we know that ATP must be required because this is active transport. The ATP is hydrolyzed to ADP + Pi + Energy! This is an antiport mechanism, as both molecules are being transported in opposite directions. The Na+/K+ pump helps to maintain the resting membrane potential (RMP) across the plasma membrane of all living cells. Draw a diagram of the Na+/K+ pump.
2) Secondary active transport (indirect)
In secondary active transport, the ATP is used indirectly to move molecules across membranes. Essentially what this means is the potential energy that is stored in a concentration gradient is used to help move molecules across a membrane. An excellent illustration of how this is done is seen in the Na+/glucose transporter. The relative concentration of Na+ is low on the inside, high on the outside of the cell. When Na+ moves down its concentration gradient (into the cell) this force is harnessed to move glucose against its concentration gradient (also into the cell). While the Na+ goes down its gradient, the glucose can be dragged along with it, up hill, so to speak. The original source of ATP that allows this to occur is the one used in the Na+/K+ pump described above, as it maintains a low Na+ concentration inside the cell. This is a symport mechanism, as both molecules are being transported in the same direction. Draw a diagram of the Na+/glucose transporter.
3) Vesicular Transport
Vesicular transport is used to move large macromolecules or large quantities of a molecule across the plasma membrane. Vesicles are like mini lipid bilayer bubbles that bud off from plasma membrane and encapsulate large molecules. This is an active form of transport that directly requires energy in the form of ATP for the maneuvering of the cytoskeleton.
There are two main forms of Vesicular transport
- Endocytosis – bringing material into the cell, inward vesicular transport.
- Exocytosis – releasing material from the cell, outward vesicular transport.
- Endocytosis: There are three general kinds of endocytosis.
1) Pinocytosis (cell drinking): relatively unselective whereby ECF is transported into the cell.
2) Phagocytosis is a process by which cells engulf a particle or another cell into a much larger vesicle, e.g., certain types of WBC (called Macrophages) engulf bacteria this way.