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17.1: Introduction
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Small molecules like O2 or CO2 can cross cellular membranes unassisted; neither the hydrophilic surfaces nor the hydrophobic interior of the phospholipid bilayer are barriers to their transit. On the other hand, most molecules (even water!) need the help of membrane transport proteins to get in or out of cells and organelles.
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17.2: Membrane Transport
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Only a few small, relatively uncharged molecules can cross a membrane unassisted (i.e., by passive diffusion). Hydrophilic molecules that must enter or leave cells do so with help, i.e., by facilitated transport. Passive and facilitated transport release the free energy inherent in concentration gradients as molecules diffuse across a membrane.
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17.3: Ligand and Voltage Gated Channels in Neurotransmission
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When neurotransmitters bind to their receptors, ion channels in responding neuron or muscle cells open. The resulting influx of Na+ ions disrupts the resting potential of the target cell. The effect is only transient if the membrane potential remains negative. However, if enough Na+ ions enter the cell, the membrane becomes depolarized. If the cell experiences hyperpolarization, a localized reversal of normal membrane polarity (say from –70 mV to +65mV or more) will generate an action potential.
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17.4: Endocytosis and Exocytosis
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Endocytosis is a mechanism for internalizing large extracellular molecules (e.g., proteins), insoluble particles, or even microorganisms. The three main types of exocytosis are phagocytosis, pinocytosis and receptor-mediated endocytosis. Pinocytosis is non-specific. Phagocytosis targets large structures (e.g., bacteria, food particles…) and is not particularly specific. As its name suggests, receptor-mediated endocytosis is specific for substances recognized by a cell-surface receptor.
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17.5: Directing the Traffic of Proteins in Cells
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Each polypeptide protein translated by ribosomes from a sequence of bases in an mRNA has a specific functional location, either in the cytoplasm, on cellular membranes, inside organelles or in extracellular fluids. In this section we consider the movement and sorting of proteins in the endomembrane system as well as the transport of proteins into and out of organelles.
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17.6: How Cells are Held Together and How they Communicate
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Proteins and glycoproteins on cell surfaces play a major role in how cells interact with their surroundings and with other cells. Some of the proteins in the glycocalyx of adjacent cells interact to form cell-cell junctions, while others interact with extracellular proteins and carbohydrates to form the extracellular matrix (ECM). Still others are part of receptor systems that bind hormones and other signaling molecules at the cell surface, conveying information into the cell by signal moves.
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17.7: 17.7 Signal Transduction
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When hydrophobic chemical effector molecules such as steroid hormones reach a target cell they can cross the hydrophobic membrane and bind to an intracellular receptor to initiate a response. When large effector molecules (e.g., protein hormones) or highly polar hormones (e.g., adrenalin) reach a target cell, they can’t cross the cell membrane. Instead, they bind to transmembrane protein receptors on cell surfaces.
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17.8: Key Words and Terms
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