This phagosome can then unite with a lysosome inside the cell, and the engulfed material can be digested for use within the cell. As these pseudopodia surround the particle, they eventually fuse, creating a vesicle containing the particle. In this process, the cell sends extensions of its plasma membrane, called pseudopodia, out and around the particle to be phagocytized. Only a limited number of cells are capable of phagocytosis, specifically cells of the immune system.

First, let's discuss phagocytosis (see figure below), which means cell eating. There are several types of endocytosis, and we will briefly explore each one. EndocytosisĮndocytosis is the bulk transport of materials into the cell. These processes also require ATP and are, therefore, examples of active transport, but they move materials in a very different way. There are instances, however, when it is necessary to move much larger materials across the membrane, like when a macrophage engulfs a bacterium or when larger amounts of a given material are released from a cell, such as the release of a hormone. To this point, we have been talking about the movement of relatively small solutes across the cell membranes (i.e. It is the dissipation of this sodium gradient that provides the energy required for secondary active transport. ATP energy is required to generate the sodium concentration gradient but is not directly involved in moving the desired solute across the membrane. A number of organic molecules are transported across membranes by this process, such as glucose and amino acids. Once both solutes have bound, sodium moves down its concentration gradient into the cell, much like what happens with carrier-mediated diffusion and in the process provides the energy required for the other solute to be transported into the cell ( symport) or out of the cell ( antiport), against its concentration gradient. In this process, the carrier protein has a binding site for the solute to be transported against its concentration gradient and a binding site for sodium. Since sodium is always in higher concentration outside of the cell (due to primary active transport), the sodium gradient is often used to power secondary active transport. Instead, this process uses the energy stored in concentration gradients to move the solute. However, with secondary active transport, ATP is not directly involved in the pumping of the solute. Like primary active transport, secondary active transport also moves solutes against their concentration gradients. Image created at BYU-Idaho by MG 2013 Three Na + ions are moved out of the cell in exchange for two K + ions with the aid of ATP. Potassium is the primary intracellular cation in the body while sodium is the primary extracellular cation, and the Na +, K +-ATPase is responsible for maintaining this distribution. This protein moves three sodium ions out of the cell and two potassium ions into the cell for each ATP hydrolyzed. One of the most important primary active transport proteins is the Na +, K +-ATPase. These transport systems can move one or multiple ions across the membrane. However in this case, the carrier has an ATP binding site, which upon hydrolysis into ADP and inorganic phosphate (Pi) provides the energy to move solute against its concentration gradient. Primary active transport requires a carrier protein that is much like the proteins involved in carrier-mediated diffusion mentioned above. Just like moving water from the spillway of a dam back to the reservoir, these processes require an energy source and are called active transport processes. moving a solute across the membrane where it is higher in concentration). However, there are times when it is important for the cell to be able to move solutes against their concentration gradient (i.e. To this point, the transport processes we have discussed have all been passive processes in which the solute movement has been down a concentration gradient with no input of energy required.

Active transport processes definition