Porth's Essentials of Pathophysiology, 4e

13

Cell Structure and Function

C h a p t e r 1

Channel protein

Carrier protein

Lipid bilayer

Concentration gradient

ATP ADP

Diffusion through lipid bilayer

Diffusion through a channel

Facilitated diffusion

Active transport

Passive transport

Vesicular transport

FIGURE 1-10. Mechanisms of membrane transport. Passive transport represents the net movement from a region of higher to lower concentration and active transport (energy requiring) from a region of lower to higher concentration. Vesicular transport involves the formation of membrane-enclosed vesicles or sacs that serve as transport vehicles from extracellular materials that are being moved into the cell (endocytosis) or intracellular materials that are being moved out of the cell (exocytosis). ADP, adenosine diphosphate; ATP, adenosine triphosphate.

proteins with their ligand-binding site on the outer surface of the cell membrane. Instead of having a cytosolic domain that associates with a G protein, their cytosolic domain either has intrinsic enzyme activity or associates directly with an enzyme. There are several classes of enzyme- linked receptors, including one widely used in hormonal control of cell function. The binding of the hormone to a special transmembrane receptor results in activation of the enzyme adenylyl cyclase at the intracellular portion of the receptor. This enzyme then catalyzes the formation of the second messenger cAMP, which has multiple effects on cell function. Insulin, for example, acts by binding to an enzyme-linked receptor (see Chapter 33). Ion Channel–Linked Receptors. Ion channel–linked receptors are involved in the rapid synaptic signaling between electrically excitable cells. Many neurotransmit- ters mediate this type of signaling by transiently opening or closing ion channels formed by integral proteins in the cell membrane (to be discussed). This type of signaling is involved in the transmission of impulses in nerve and mus- cle cells. Intracellular Receptors Some messengers, such as thyroid hormone and steroid hormones, do not bind to membrane receptors but move directly across the lipid bilayer of the cell membrane and are transported to the cell nucleus, where they influence DNA activity (see Chapter 31). Many of these hormones bind to a receptor within the cytoplasm, and the receptor– hormone complex enters the nucleus. There it binds to DNA, initiating processes that increase the production of proteins that alter cell function. MembraneTransport Mechanisms The lipid layer of the cell membrane serves as a bar- rier against the movement of water and water-soluble substances between the intracellular and extracellular

fluids, while allowing a few lipid-soluble (e.g., alco- hols with lower numbers of hydrocarbons, oxygen, nitrogen) and uncharged molecules (glycerol, water) to cross the cell membrane by simple diffusion. The cell membrane also contains large numbers of protein mol- ecules, many of which insert completely through the membrane (Fig. 1-10). Most ions and small molecules rely on these proteins for transport. Different membrane proteins function in different ways. For example, channel proteins form water-lined passageways through the membrane and allow free movement of water as well as selected ions or molecules. Membrane transport proteins bind molecules or ions and undergo a series of conformational changes to transfer the bound solute across the membrane. Some transport proteins, called uniporters , simply mediate the move- ment of a single solute from one side of the membrane to the other, whereas others function as coupled trans- porters in which the transfer of one solute depends on the transfer of a second solute (Fig. 1-11). This coupled transport involves either the simultaneous transport in the same direction, performed by transporters called symporters , or the transport of a second solute in the opposite direction, by transporters called antiporters . All channels and many transporters allow solutes to cross the membrane only passively by passive transport or facilitated diffusion . Cells also require transport pro- teins that actively pump certain solutes across the mem- brane against an electrochemical gradient, in a process called active transport . Active transport is directional and requires an energy source such as ATP. The cell mem- brane can also engulf substances, forming a membrane- bound vesicle; this vesicle is brought into the cell by endocytosis . The process by which cellular vesicles fuse to the cell membrane releasing contents outside of the cell is called exocytosis . Many integral transmembrane proteins form the ion channels found on the cell surface. These channel pro- teins have a complex morphology and are selective with respect to the substances that can transverse the channel.