University:
California State University, Northridge
Course:
BIOL 100 | Introductory Biology
Academic year:

2021
Views:

118

Pages:

2
Author:

B. Coleman

TRANSPORT OF SUBSTANCES THROUGH THE CELL MEMBRANE The transport of substances through the cell membrane is very dependent on the type of molecule, the size of the molecule, and the cell's need for the molecule. With its semi-permeability properties, the cell membrane prevents many polar ions and molecules from entering the cell, so it requires the help of proteins in the membrane so they can enter the cell. Proteins in cell membranes can be specific as transport proteins for specific solutes and even have special binding sites like enzymes. These transport proteins can function to help the entry of molecules or ions because they have hydrophilic channels that are used by certain molecules as channels to pass through the membrane. Membrane proteins have many roles in the transport of many molecules and ions across cell membranes. The presence of these transport proteins will help the diffusion process, especially when active transport occurs which requires energy to fight the concentration gradient. Active transport is the main factor that determines a cell's ability to maintain an internal concentration of small molecules that is different from its environmental concentration. Protein channels from a hydrophilic state through the cell membrane through which water or ions can move continuously. Some channels will be open for a certain time and some other ion channels only open when there is a specific response or electrical signal for that channel. Active transport is carried out by specific proteins embedded in the membrane and requires energy in the form of ATP. An example of an active transport mechanism with ATP energy is the sodium-potassium pump. In this pump mechanism, Na+ and K+ are not translocated one to one between inside the cell and outside the cell, but instead three sodium ions must be pumped out of the cell for every two potassium ions pumped into the cell. With each pumping activity, there will be a difference in the transfer of one positive charge from the cytoplasm to the extracellular fluid, which will thus store energy in the form of voltage. The protein that plays a role in generating voltage across the membrane by transferring Na+ and K+ ions is called an electrogenic pump. By generating voltage across the membrane, the electrogenic pump protein will store energy that can be used for cellular work, including a type of membrane traffic called cotransport. Transporters, which can also be called carriers, will move ions and molecules across the cell membrane. There are three types of transporter proteins, namely uniport, which functions to transport only one type of molecule at different concentration conditions by means of facilitated diffusion. In contrast, antiport and symport both move one type of ion or molecule at different concentrations. This type of protein is called cotransport, which has the ability to move molecules from two different solutions continuously.