![]() ![]() ![]() A single substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across a space. Simple sugars and amino acids also need help with transport across plasma membranes, achieved by various transmembrane proteins (channels).ĭiffusion is a passive process of transport (see Figure 2). Ions such as sodium, potassium, calcium, and chloride must have special means of penetrating plasma membranes. Additionally, while 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 present problems for the membrane. Molecules of oxygen and carbon dioxide have no charge and so pass through membranes by simple diffusion. Fat-soluble drugs and hormones 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 with a low molecular weight can easily slip through the hydrophobic lipid core of the membrane. This characteristic helps the movement of some materials through the membrane and hinders the movement of others. Recall that plasma membranes are amphiphilic: they have hydrophilic and hydrophobic regions. The eukaryotic plasma membrane is a phospholipid bilayer with proteins and cholesterol embedded in it. This adds considerably to the selective nature of plasma membranes (Figure 1). 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. There are peripheral proteins on the exterior of the membrane that bind elements of the extracellular matrix. On the interior of the membrane, some proteins serve to anchor the membrane to fibers of the cytoskeleton. In fact, there is a considerable difference between the array of phospholipids and proteins between the two leaflets that form a membrane. Plasma membranes are asymmetric: the interior of the membrane is not identical to the exterior of the membrane. All cells spend the majority of their energy to maintain an imbalance of sodium and potassium ions between the interior and exterior of the cell. For example, red blood cells use some of their energy doing just that. Some materials are so important to a cell that it spends some of its energy, hydrolyzing adenosine triphosphate (ATP), to obtain these materials. This may happen passively, as certain materials move back and forth, or the cell may have special mechanisms that facilitate transport. Some cells require larger amounts of specific substances than do other cells they must have a way of obtaining these materials from 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 to pass through, but not others. Plasma membranes must allow certain substances to enter and leave a cell, and prevent some harmful materials from entering and some essential materials from leaving. What you’ll learn to do: Explain how substances are directly transported across a membrane ![]()
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