CHAPTER 3 SUMMARY
Membrane Structure and Composition
•All cells are bounded by a plasma membrane, a thin phospholipid bilayer in which proteins are interspersed and to which carbohydrates are attached on the outer surface. The phospholipids orient themselves in a bilayer with a hydrophobic interior sandwiched between the hydrophilic outer and inner surfaces. This lipid bilayer forms the structural boundary of the cell, serving as a barrier for water-soluble substances and being responsible for the fluid nature of the membrane.
•Cholesterol molecules tucked between the phospholipids contribute to the fluidity and stability of the membrane.
•Membrane proteins, which vary in type and distribution among cells, serve as:
•channels for passage of small ions across the membrane;
•carriers for transport of specific substances in or out of the cell;
•receptor sites for detecting and responding to chemical messengers that alter cell function;
•membrane-bound enzymes that govern specific chemical reactions;
•cell adhesion molecules (CAMs) that help hold cells together;
•a support meshwork on the inner-membrane surface to help maintain cell shape in association with the cytoskeleton.
•Membrane carbohydrates, short sugar chains that project from the outer surface only, serve as self-identity markers. They are important in recognition of “self” in cell-to-cell interactions such as tissue formation and tissue growth.
Membrane Transport
Materials can pass between the ECF and ICF by the following pathways:
•Nonpolar (lipid-soluble) molecules of almost any size can dissolve in and pass through the lipid bilayer, moving passively down chemical gradients.
•Small ions traverse through protein channels specific for them, moving down electrochemical gradients.
•Osmosis is a special case of water moving down its own concentration gradient.
•Other substances can be selectively transferred across the membrane by specific carrier proteins. Carrier mechanisms are important for transfer of small polar molecules and for selected movement of ions. A given carrier can move a single specific substance in one direction, two substances in opposite directions, or two substances in the same direction.
•Some substances are moved down a concentration gradient without the need for energy expenditure (facilitated diffusion).
•Others are moved against a concentration gradient at the expense of cellular energy (active transport). Primary active transport requires the direct utilization of ATP to drive the pump, whereas secondary active transport is driven by an ion concentration gradient established by a primary active transport system.
•Large polar molecules and multimolecular particles can leave or enter the cell by being wrapped in a piece of membrane to form vesicles that can be internalized (endocytosis) or externalized (exocytosis).
•Cells are differentially selective in what enters or leaves because they possess varying numbers and kinds of channels, carriers, and mechanisms for vesicular transport. Large polar molecules (too large for channels and not lipid soluble) for which there are no special transport mechanisms are unable to permeate.
Intercellular Communication and Signal Transduction
Cells communicate with each other to carry out various coordinated activities largely by dispatching extracellular chemical messengers, which act on particular target cells to bring about the desired response.
•Transferral of the signal carried by the extracellular messenger into the cell for execution is known as signal transduction.
•Attachment of an extracellular chemical messenger that cannot gain entry to the cell, such as a protein hormone (the first messenger), bind to a membrane receptor initiates one of several related intracellular pathways to bring about the desired response:
(1) activating a kinase that phosphorylates a protein in the cell cytosol;
(2) opening or closing specific channels or
(3) activating an intracellular messenger (the second messenger). Two commonly employed second messengers are cyclic AMP and Ca++. Once activated, these second messengers initiate a similar cascade of intracellular events that ultimately lead to a change in the shape and function of particular proteins to cause the appropriate cellular response.
•Despite the often widespread distribution of a single chemical messenger and the similarity of intracellular pathways employed, cells vary in their response because
(1) different cell types are equipped with different sets of receptors that can bind with only selected types of messengers from among the many that might come into contact with each cell; and
(2) various cell types contain different intracellular proteins, each of which responds uniquely to an identical second messenger.
•Many chemicals have evolved in nature that alter signaling mechanisms, typically for defense or prey capture. Antagonist agents, such as D-tubocurarine in curare frogs, block a normal signal step, while agonist, such as nicotine in tobacco, activate a signal step but usually for a longer period than normal.
Membrane Potential
All cells have a membrane potential, which is a separation of opposite charges across the plasma membrane.
•The Na+–K+ pump makes a small direct contribution to membrane potential through its unequal transport of positive ions; it transports more Na+ ions out than K+ ions in. The primary role of the Na+–K+ pump, however, is to actively maintain a greater concentration of Na+ outside the cell and a greater concentration of K+ inside the cell. K+ is also dissolves more readily in cell water than Na+, and proteins have a greater affinity for K+.
•These concentration gradients tend to passively move K+ out of the cell and Na+ into the cell. Because the resting membrane is much more permeable to K+ than to Na+, substantially more K+ leaves the cell than Na+ enters. This results in an excess of positive charges outside the cell and leaves an unbalanced excess of negative charges inside in the form of large protein anions (A-) that are trapped within the cell. An increase in ECF K+ is far more deadly than an increase in ECF Na+.
•When the resting membrane potential of -70 mV is achieved, no further net movement of K+ and Na+ takes place, because any further leaking of these ions down their concentration gradients is quickly reversed by the Na+–K+ pump.
•The distribution of Cl-across the membrane is passively driven by the established membrane potential so that Cl-is concentrated in the ECF.