The Plasma Membrane

All eukaryotic cells have a cytoplasmic membrane, or plasma membrane, which is similar in chemical structure and fUnction to that of prokaryotic cells. It is a typical phospholipid bilayer embedded with proteins. The lipid and protein composition of the leaflet that faces the cytoplasm, however, differs significantly from that facing the outside of the cell. The same is true for membranes that surround the organelles. The leaflet facing the lumen of the organelle is similar to its counterpart facing the cell exterior. This lack of symmetry reflects the important role these membranes play in the complex processes occuring within the eukaryotic cell.

The proteins in the lipid bilayer perform a variety of functions. Some are involved in transport and others are attached to internal structures, helping to maintain cell integrity. Those in the outer leaflet often function as receptors. Typically, these receptors are glycoproteins, proteins that have various sugars attached. A given receptor binds a specific molecule, which is referred to as its ligand. These receptor-ligand interactions are extremely important in multicellular organisms because they allow cells to communicate with each other—a process called signaling. For example, in our bodies, some cells secrete a specific protein when they encounter certain compounds perceived as dangerous. Other cells of the immune system have receptors for that protein on their surface. When the protein binds its receptor, those cells recognize the signal as a call for help and respond accordingly. This type of cell-to-cell communication enables the multicellular organism to function as a cohesive unit. ■ phagocytic cells, p. 382 ■ glycoproteins, p. 29

Some of the lipids of the outer leaflet are lipids with various sugars attached, called glycolipids. Other short chains of sugars, oligosaccharides, are often attached to these. This creates a carbohydrate-rich layer, or a glycocalyx, surrounding the cell. This layer retains moisture and helps protect the cell from physical damage.

The membranes of many eukaryotic cells contain sterols, which provide strength to the otherwise fluid structure. Recall that Mycoplasma, a group of bacteria lacking a cell wall, also have sterols in their membranes. The sterol found in animal cell membranes is cholesterol, whereas fungal membranes contain ergos-terol. This difference is exploited by antifungal medications that act by interfering with ergosterol synthesis or function. ■ antifungal medications, p. 526 ■ oligosaccharides, p. 29

The plasma membrane plays no direct role in energy transformation; instead, that task is performed in an organelle. Although proton motive force is not generated across the membrane, an electrochemical gradient is maintained by energy-

3.11 Transfer of Molecules Across the Plasma Membrane 71

Table 3.6 A Summary of Eukaryotic Cell Structures


Plasma Membrane Internal Protein Structures


Cytoskeleton Flagella

Ribosomes Membrane-Bound Organelles Chloroplasts

Endoplasmic reticulum Rough Smooth Golgi apparatus

Lysosome Mitochondria

Nucleus Peroxisome

Asymmetric lipid bilayer embedded with proteins. Selective permeability, conduit to external environment.

Appear to project out of a cell. Beat in synchrony to provide movement. Composed of microtubules in a 9 + 2 arrangement. Dynamic filamentous network that provides structure to the cell.

Appear to project out of a cell. Propel or push the cell with a whiplike or thrashing motion. Composed of microtubules in a 9 + 2 arrangement.

Two subunits, 60S and 40S, join to form the 80S ribosome.

Harvest the energy of sunlight to generate ATPWithin the stroma are chlorophyll-containing, disclike thylakoids.The membranes of these contain the components of the electron transport chain and the proteins that use proton motive force to synthesize ATP. The ATP is then used to fuel synthesis of organic compounds.

Site of synthesis of macromolecules destined for other organelles or the external environment.

Attached ribosomes extrude the proteins they are synthesizing through pores that lead into the lumen of the organelle.

Site of lipid synthesis and degradation, and Ca2+ storage.

Site where macromolecules that are synthesized in the endoplasmic reticulum are modified before they are transported in vesicles to other destinations.

Digestion of foodstuffs.

Harvests the energy released during the degradation of organic compounds to generate ATP. Within the highly folded inner membrane are the components of the electron transport chain and the proteins that use proton motive force to synthesize ATP

Contains the DNA.

Oxidation of lipids and toxic chemicals occurs.

consuming mechanisms that expel either sodium ions or protons. ■ electrochemical gradient, p. 55

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