Cellular transport8/17/2023 Ions such as sodium, potassium, calcium, and chloride must have a special means of penetrating plasma membranes. Additionally, whereas small ions could easily slip through the spaces in the mosaic of the membrane, their charge prevents them from doing so. While some polar molecules connect easily with the outside of a cell, they cannot readily pass through the lipid core of the plasma membrane. Polar substances, with the exception of water, present problems for the membrane. Molecules of oxygen and carbon dioxide have no charge and pass through by simple diffusion. Fat-soluble drugs also gain easy entry into cells and are readily transported into the body’s tissues and organs. Substances such as the fat-soluble vitamins A, D, E, and K readily pass through the plasma membranes in the digestive tract and other tissues. Lipid-soluble material can easily slip through the hydrophobic lipid core of the membrane. This characteristic helps the movement of certain materials through the membrane and hinders the movement of others. Recall that plasma membranes have hydrophilic and hydrophobic regions. This adds considerably to the selective nature of plasma membranes. These carbohydrate complexes help the cell bind substances that the cell needs in the extracellular fluid. Carbohydrates, attached to lipids or proteins, are also found on the exterior surface of the plasma membrane. Integral proteins that act as channels or pumps work in one direction. Plasma membranes are asymmetric, meaning that despite the mirror image formed by the phospholipids, the interior of the membrane is not identical to the exterior of the membrane. A physical space in which there is a different concentration of a single substance is said to have a concentration gradient. In passive transport, substances move from an area of higher concentration to an area of lower concentration in a process called diffusion. Passive transport is a naturally occurring phenomenon and does not require the cell to expend energy to accomplish the movement. The most direct forms of membrane transport are passive. The structure of the plasma membrane contributes to these functions, but it also presents some problems. Most cells expend most of their energy, in the form of adenosine triphosphate (ATP), to create and maintain an uneven distribution of ions on the opposite sides of their membranes. This may happen passively, as certain materials move back and forth, or the cell may have special mechanisms that ensure transport. Some cells require larger amounts of specific substances than do other cells they must have a way of obtaining these materials from the extracellular fluids. If they were to lose this selectivity, the cell would no longer be able to sustain itself, and it would be destroyed. In other words, plasma membranes are selectively permeable-they allow some substances through but not others. Plasma membranes must allow certain substances to enter and leave a cell, while preventing harmful material from entering and essential material from leaving.
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