The Cytoplasmic Membrane

The cytoplasmic membrane is a delicate thin fluid structure that surrounds the cytoplasm and defines the boundary of the cell. It serves as an important semipermeable barrier between the cell and its external environment. Although the membrane's chemical composition primarily allows only water, gases, and some small hydrophobic molecules to pass through freely, specific proteins are embedded within the membrane that act as selective gates. These permit desirable nutrients to enter the cell, and waste products to exit. Other proteins within the membrane serve as sensors of environmental conditions. Thus, while the cytoplasmic membrane acts as a barrier, it also functions as an effective and highly discriminating conduit between the cell and its surroundings.

Structure and Chemistry of the Cytoplasmic Membrane

The structure of the cytoplasmic membrane is typical of other biological membranes—a lipid bilayer embedded with proteins (figure 3.24). The bilayer consists of two opposing leaflets composed of phospholipids. At one end of each phospholipid molecule are two fatty acid chains, which act as hydrophobic tails. The other end, containing glycerol, a phosphate group, and other polar molecules, functions as a hydrophilic head. The phospholipid molecules are arranged in each leaflet of the bilayer so that their hydrophobic tails face in, toward the other leaflet. Their hydrophilic heads face outward. As a consequence, the inside of the bilayer is water insoluble whereas the two surfaces, which interface the internal and external environments, interact freely with aqueous solutions. ■ phospholipids, p. 35 ■ hydrophobic, p. 27 ■ hydrophilic, p. 27

More than 200 different membrane proteins have been found in E. coli. Many function as receptors, binding to specific molecules in the environment. This, in turn, provides a mechanism for the cell to sense and adjust to its surroundings. Proteins are not stationary within the fluid bilayer; rather, they are constantly changing position. Such movement is necessary for the important functions the membrane performs. This structure, with its resulting dynamic nature, is called the fluid mosaic model.

Members of the Bacteria and Archaea have the same general structure of their cytoplasmic membranes, but the lipid composition is distinctly different. The side chains of the membrane lipids of Archaea are connected to glycerol by a different type of chemical linkage. In addition, the side chains are hydrocarbons rather than fatty acids. These differences represent important distinguishing characteristics between these two domains of prokaryotes. ■ hydrocarbon, p. 34

54 Chapter 3 Microscopy and Cell Structure

Cytoplasmic membrane

Cytoplasmic membrane

_ Phospholipid bilayer

_Hydrophilic head

— Hydrophobic tail

Figure 3.24 The Structure of the Cytoplasmic Membrane Two opposing leaflets make up the phospholipid bilayer. Embedded within the bilayer are a variety of different proteins, some of which span the membrane.

Figure 3.24 The Structure of the Cytoplasmic Membrane Two opposing leaflets make up the phospholipid bilayer. Embedded within the bilayer are a variety of different proteins, some of which span the membrane.

Permeability of the Cytoplasmic Membrane

The cytoplasmic membrane is selectively permeable; relatively few types of molecules can pass through freely. These move through the membrane by a process called simple diffusion, whereas other molecules must be transported across it by specific mechanisms. The transport mechanisms, which generally require an expenditure of energy, will be discussed in detail later.

Simple Diffusion

Simple diffusion is the process by which some molecules move freely into and out of the cell. Water, small hydrophobic molecules, and gases such as oxygen and carbon dioxide are among the few compounds that move through the cytoplasmic membrane by simple diffusion. The speed and direction of diffusion depend on the relative concentration of molecules on each side of the membrane. The greater the difference in concentration, the higher the rate of diffusion. The molecules continue to pass through at a diminishing rate until their concentration is the same on both sides of the membrane.

The ability of water to move freely through the membrane has important biological consequences. The cytoplasm of a cell is a concentrated solution of inorganic salts, sugars, amino acids, and various other molecules. However, the environments in which prokaryotes normally grow contain only small amounts of some salts and other small molecules. Since the concentration of dissolved molecules, or solute, tends to equalize inside and outside the cell, water flows from the surrounding medium into the cell, thereby reducing the concentration of solute inside the cell (figure 3.25). This is the process of osmosis. This inflow of water exerts tremendous osmotic pressure on the cytoplas-mic membrane, much more than it generally can resist. However, the rigid cell wall enveloping the membrane generally withstands such high pressure. The cytoplasmic membrane is forced up against the wall but cannot balloon further. Damage to the cell wall weakens the structure, and consequently, cells may burst or lyse.

The Role of the Cytoplasmic Membrane in Energy Transformation

The cytoplasmic membrane of a prokaryotic cell plays an indispensable role in converting energy to a usable form. This is an important distinction between prokaryotic and eukary-otic cells; in eukaryotic cells energy is transformed in membrane-bound organelles, which will be discussed later in this chapter.

As part of their energy-harvesting processes, most prokaryotes have a series of compounds, the electron transport chain, embedded in their membrane. These sequentially transfer electrons and, in the process, eject protons from the cell. The details of these processes will be explained in chapter 6. The expulsion of protons by the electron transport chain results in the formation of a proton gradient across the cell membrane. Positively charged protons are concentrated immediately outside the membrane, whereas negatively charged hydroxyl ions accumulate directly inside the membrane (figure 3.26). This separation

_ Phospholipid bilayer

_Hydrophilic head

— Hydrophobic tail

Water flow

Higher solute • concentration \ inside cell ".

Higher solute • concentration \ inside cell ".

■ H2O flows into cell

Cytoplasmic membrane is forced against cell wall

Higher solute concentration outside cell

Higher solute concentration outside cell

Figure 3.25 Osmosis (a) Water flows across a membrane toward the side that has the highest concentration of molecules and ions, thereby equalizing the concentrations on both sides. (b) The effect of osmosis on cells.

Cytoplasmic membrane pulls away from cell wall

Figure 3.25 Osmosis (a) Water flows across a membrane toward the side that has the highest concentration of molecules and ions, thereby equalizing the concentrations on both sides. (b) The effect of osmosis on cells.

Nester-Anderson-Roberts: I I. Life and Death of I 3. Microscopy and Cell I I © The McGraw-Hill

Microbiology, A Human Microorganisms Structure Companies, 2003

Perspective, Fourth Edition

Bilayer membrane

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