Can someone explain concentration gradient to me?

Answer 1

Concentration gradient is simply areas of differing concentration of solute particles. Say that you have one salt solution that is not very concentrated and separate it from a highly concentrated salt solution with simple saran wrap. The water will move through the wrap from the solution of lower salt concentration to the solution of higher salt concentration. You can think of the concentrated solution as "pulling" more water from the less concentrated solution until the two have a roughly equal concentration.

Answer 2

Passive transport is accomplished by diffusion, the spontaneous movement of a substance from a region of greater concentration to a region of lesser concentration. The difference between the concentration of a substance in two different areas is known as a concentration gradient. Diffusion moves molecules down a concentration gradient in a manner that does not require the cell to expend energy. Water, oxygen, carbon dioxide, and a few other small molecules diffuse directly across the plasma membrane by passing between phospholipid molecules. Substances that cannot pass directly through the plasma membrane diffuse into or out of cells with the aid of hollow, channel-like proteins in a process known as facilitated diffusion. These channel proteins are shaped so that only one substance, or a small group of closely related substances, can pass through each type of protein. This specificity enables a cell to control precisely the molecules that travel in and out of the cell.

In order to move substances against a concentration gradient—that is, from the side of the plasma membrane where the concentration of a substance is lower to the side where it is already higher—a cell must expend energy in a process known as active transport. Active transport is achieved by membrane proteins called pumps, which have a docking site that is shaped to fit a specific substance. These pumps are open on either the inside or the outside of the cell. When the proper molecule or ion attaches to the docking site, the pump changes shape so that the docking site moves its opening to the other side of the plasma membrane, releasing the molecular cargo. Many pumps obtain the energy necessary to perform this work from adenosine triphosphate (ATP), a molecule that serves as the main energy currency of living cells.

Two additional transport mechanisms provide pathways for large molecules to pass in and out of cells. In endocytosis, the plasma membrane folds inward, forming a pouch that traps molecules. The pouch continues to press inward until it forms a closed sac that breaks loose from the plasma membrane and sinks into the cell. The second mechanism, exocytosis, is a reversal of endocytosis. A sac inside the cell containing proteins and other molecules moves toward the outer edge of the cell until it touches the plasma membrane. The membrane of the sac then joins with the plasma membrane, and the contents of the sac are released from the cell. Most of the proteins released by animal cells, such as hormones and antibodies, exit the cells where they are made through exocytosis.

In multicellular organisms, the plasma membrane also plays a critical role in communication between cells. Proteins embedded in the plasma membrane act as receptors, binding to hormones and other molecules sent as signals from other cells. In animal cells, certain membrane proteins also act as markers that help the immune system distinguish the body’s own cells from foreign cells. These marker proteins help trigger the immune reaction that protects humans and other animals from disease-causing organisms such as bacteria, viruses, and fungi. These markers also play a role in the rejection of transplanted tissues and organs.

In certain types of cells, the plasma membrane has a wide variety of additional functions. Some membrane proteins are involved in holding neighboring cells together. In bacteria, plasma membrane proteins participate in photosynthesis and other reactions supplying the cell with energy.

Answer 3

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