A single substance moves from a high concentration to a low concentration area until the concentration is equal across a space. Diffusionĭiffusion is a passive process of transport. Simple sugars and amino acids also need the help of various transmembrane proteins (channels) to transport themselves across plasma membranes. 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 membrane's mosaic, their charge prevents them from doing so. While some polar molecules connect easily with the cell's outside, they cannot readily pass through the plasma membrane's lipid core. Polar substances present problems for the membrane. Oxygen and carbon dioxide molecules have no charge and pass through membranes by simple diffusion. Fat-soluble drugs and hormones also gain easy entry into cells and readily transport themselves 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. Non-polar and lipid-soluble material with a low molecular weight can easily slip through the membrane's hydrophobic lipid core. This characteristic helps move some materials through the membrane and hinders the movement of others. Recall that plasma membranes are amphiphilic: They have hydrophilic and hydrophobic regions. Department of Biology, Texas A&M University. Credit: Rao, A., Ryan, K., Fletcher, S., Hawkins, A. This adds considerably to plasma membrane's selective nature ( Figure 5.7).įigure 5.7 The plasma membrane's exterior surface is not identical to its interior surface. These carbohydrate complexes help the cell bind required substances in the extracellular fluid. Carbohydrates, attached to lipids or proteins, are also on the plasma membrane's exterior surface. There are peripheral proteins on the membrane's exterior that bind extracellular matrix elements. On the membrane's interior, some proteins serve to anchor the membrane to cytoskeleton's fibers. 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 membrane's interior is not identical to its exterior. A physical space in which there is a single substance concentration range has a concentration gradient. In passive transport, substances move from an area of higher concentration to an area of lower concentration. Passive transport is a naturally occurring phenomenon and does not require the cell to exert any of its energy to accomplish the movement. The most direct forms of membrane transport are passive. Most cells spend the majority of their energy to maintain an imbalance of sodium and potassium ions between the cell's interior and exterior, as well as on protein synthesis. 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. They must have a way of obtaining these materials from extracellular fluids. Some cells require larger amounts of specific substances. 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 (semipermeable)-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. Define tonicity and its relevance to passive transport.Understand the osmosis and diffusion processes.Explain why and how passive transport occurs.By the end of this section, you will be able to do the following:
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