Anatomy & Physiology

Anatomy & Physiology Study Guide concentrations is the concentration gradient . Diffusion tends to eliminate that gradient. Important factors that influence diffusion rates include the following: • Distance • Molecule size • Temperature: The higher the temperature, the faster the diffusion rate • Gradient size: The larger the concentration gradient, the faster diffusion proceeds Osmosis Osmosis is the net diffusion of water across a membrane. The term osmosis will always refer to the movement of water, and the term diffusion for the movement of solutes. Whenever a solute concentration gradient exists, a concentration gradient for water exists also. Because dissolved solute molecules occupy space that would otherwise be taken up by water molecules, the higher the solute concentration, the lower the water concentration. As a result, water molecules tend to flow across a membrane toward the solution containing the higher solute concentration because this movement is down the concentration gradient for water. Water movement will continue until water concentrations—and thus solute concentrations—are the same on either side of the membrane. Carrier-Mediated Transport In carrier-mediated transport, integral proteins bind specific ions or organic substrates and carry them across the plasma membrane. Many examples of carrier-mediated transport involve the movement of a single substrate molecule across the plasma membrane. A few carrier mechanisms transport more than one substrate at a time. In cotransport or symport , the carrier transports two substances in the same direction simultaneously, either into or out of the cell. In countertransport or antiport , one substance moves into the cell and the other moves out. There are two types of carrier- mediated transport: facilitated diffusion and active transport. Facilitated Diffusion Many essential nutrients, such as glucose and amino acids, are insoluble in lipids and too large to fit through membrane channels. These substances can be passively transported across the membrane by carrier proteins in a process called facilitated diffusion. The molecule to be transported must first bind to a receptor site on the protein. The shape of the protein then changes and moves the molecule across the plasma membrane. As in the case of simple or channel-mediated diffusion, no ATP is expended in facilitated diffusion. The molecules merely move from an area of higher concentration to one of lower concentration. Active Transport In active transport, a high-energy bond (in ATP or another high-energy compound) provides the energy needed to move ions or molecules across the membrane. All cells contain carrier proteins called ion pumps. These pumps actively transport the cations sodium (Na+), potassium (K+), calcium (Ca2+), and magnesium (Mg2+) across their plasma membranes.

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