II. THE PROKARYOTIC CELL: BACTERIA
B. PROKARYOTIC CELL STRUCTURE
1. The Cytoplasmic Membrane
The overall purpose of this Learning Object is:
1) to learn the chemical makeup and the functions associated with the bacterial cytoplasmic membrane;
2) to compare the various methods bacteria use to transport materials across their cytoplasmic membrane; and
3) to understand the process of bacterial binary fission.
LEARNING OBJECTIVES FOR THIS SECTION
In this section on Prokaryotic Cell
Structure we are going to look at the various organelles or structures that
make up a bacterium. As mentioned in the introduction to this section, a typical
bacterium usually consists of:
Because a cytoplasmic membrane surrounds all cells in nature, we will start with this structure.
The Cytoplasmic Membrane (def)
The cytoplasmic membrane, also called a cell membrane or plasma membrane, is about 7 nanometers (nm; 1/1,000,000,000 m) thick. It lies internal to the cell wall and encloses the cytoplasm of the bacterium (see Fig. 1).
A. Structure and Composition
Like all biological membranes in nature, the bacterial cytoplasmic membrane is composed of phospholipid (def) and protein molecules. In electron micrographs, it appears as 2 dark bands separated by a light band and is actually a fluid phospholipid bilayer imbedded with proteins (see Fig. 3). With the exception of the mycoplasmas, the only bacteria that lack a cell wall, prokaryotic membranes lack sterols (def). Many bacteria, however, do contain sterol-like molecules called hopanoids. Like the sterols found in eukaryotic cell membranes, the hopanoids most likely stabilize the bacterial cytoplasmic membrane.
The phospholipid bilayer is arranged so that the polar ends of the molecules (the phosphate and glycerol portion of the phospholipid that is soluble in water) form the outermost and innermost surface of the membrane while the non-polar ends (the fatty acid portions of the phospholipids that are insoluble in water) form the center of the membrane (see Fig. 3).
The cytoplasmic membrane is a selectively permeable membrane that determines what goes in and out of the organism. All cells must take in and retain all the various chemicals needed for metabolism. Water, dissolved gases such as carbon dioxide and oxygen, and lipid-soluble molecules simply diffuse across the phospholipd bilayer. Water-soluble ions generally pass through small pores - less than 0.8 nm in diameter - in the membrane . All other molecules require carrier molecules to transport them through the membrane.
Materials move across the bacterial cytoplasmic membrane by passive diffusion and active transport.
1. Passive Diffusion (def)
Passive diffusion is the net movement of gases or small uncharge polar molecules across a phospholipid bilayer membrane from an area of higher concentration to an area of lower concentration (see Fig. 4A and 4B) . Examples of gases that cross membranes by passive diffusion include N2, O2, and CO2; examples of small polar molecules include ethanol, H2O, and urea.
All molecules and atoms possess kinetic energy (energy of motion). If the molecules or atoms are not evenly distributed on both sides of a membrane, the difference in their concentration forms a concentration gradient that represents a form of potential energy (stored energy). The net movement of these particles will therefore be down their concentration gradient - from the area of higher concentration to the area of lower concentration. Diffusion is powered by the potential energy of a concentration gradient and does not require the expenditure of metabolic energy.
a. Osmosis (def) is the diffusion of water across a membrane from an area of higher water concentration (lower solute concentration) to lower water concentration (higher solute concentration). Osmosis is powered by the potential energy of a concentration gradient and does not require the expenditure of metabolic energy. While water molecules are small enough to pass between the phospholipids in the cytoplasmic membrane, their transport can be enhanced by water transporting transport proteins known as aquaporins (def). The aquaporins form channels that span the cytoplasmic membrane and transport water in and out of the cytoplasm (see channel proteins below).
To understand osmosis, one must understand what is meant by a solution (def). A solution consists of a solute (def) dissolved in a solvent (def). In terms of osmosis, solute refers to all the molecules or ions dissolved in the water (the solvent). When a solute such as sugar dissolves in water, it forms weak hydrogen bonds with water molecules. While free, unbound water molecules are small enough to pass through membrane pores, water molecules bound to solute are not (see Fig. 4C and Fig. 4D).Therefore, the higher the solute concentration, the lower the concentration of free water molecules capable of passing through the membrane.
A cell can find itself in one of three environments: isotonic (def), hypertonic (def), or hypotonic (def). (The prefixes iso-, hyper-, and hypo- refer to the solute concentration).
- In an isotonic environment (see Fig. 5A), both the water and solute concentration are the same inside and outside the cell and water goes into and out of the cell at an equal rate.
- If the environment is hypertonic (see Fig. 5B and Fig. 5D1) the water concentration is greater inside the cell while the solute concentration is higher outside (the interior of the cell is hypotonic to the surrounding hypertonic environment). Water goes out of the cell.
- In an environment that is hypotonic (see Fig. 5C and Fig. 5D2) the water concentration is greater outside the cell and the solute concentration is higher inside (the interior of the cell is hypertonic to the hypotonic surroundings). Water goes into the cell.
b. Facilitated Diffusion
Facilitated diffusion (def) is the transport of substances across a membrane by transport proteins, such as uniporters and channel proteins, along a concentration gradient from an area of higher concentration to lower concentration. Facilitated diffusion is powered by the potential energy of a concentration gradient and does not require the expenditure of metabolic energy.
1. Uniporter: Uniporters (def) are transport proteins that transport a substance from one side of the membrane to the other (see Fig. 6A1 and Fig. 6A2). Potassium ions (K+) can enter bacteria through uniporters.
2. Channel proteins (def) transport water or certain ions down either a concentration gradient, in the case of water, or an electric potential gradient in the case of certain ions, from an area of higher concentration to lower concentration (see Fig. 6B). While water molecules can directly cross the membrane by passive diffusion, as mentioned above, channel proteins called aquaporins can enhance their transport.
2. Active Transport
Active transport (def) is a process whereby the cell uses both transport proteins and metabolic energy to transport substances across the membrane against the concentration gradient. In this way, active transport allows cells to accumulate needed substances even when the concentration is lower outside.
Active transport enables bacteria to successfully compete with other organisms for limited nutrients in their natural habitat, and as will be seen in Unit 2, enables pathogens to compete with the body's own cells and normal flora bacteria for the same nutrients.
The energy is provided by proton motive force (def), the hydrolysis of ATP, or the breakdown of some other high-energy compound such as phosphoenolpyruvate (PEP).
Proton motive force is an energy gradient resulting from hydrogen ions (protons) moving across the membrane from greater to lesser hydrogen ion concentration. ATP is the form of energy cells most commonly use to do cellular work. PEP is one of the intermediate high-energy phosphate compounds produced at the end of glycolysis.
Specific transport proteins (carrier proteins) are required in order to transport the majority of molecules a cell requires across its cytoplasmic membrane. This is because the concentration of nutrients in most natural environments is typically quite low. Transport proteins allow cells to accumulate nutrients from even a sparce environment.
Transport proteins involved in active transport include antiporters, symporters, the proteins of the ATP-binding cassette (ABC) system, and the proteins involved in group translocation.
a. Antiporter: Antiporters (def) are transport proteins that transport one substance across the membrane in one direction while simultaneously transporting a second substance across the membrane in the opposite direction (see Fig. 6C). Antiporters in bacteria generally use the potential energy of electrochemical gradients from protons (H+), that is, proton motive force to co-transport ions, glucose, and amino acids against their concentration gradient (see Fig. 6E1). Sodium ions (Na+) and protons (H+), for example, are co-transported across bacterial membranes by antiporters.
b. Symporter: Symporters (def) are transport proteins that simultaneously transport two substances across the membrane in the same direction (see Fig. 6D). Symporters use the potential energy of electrochemical gradients from protons (H+), that is, proton motive force to co-transport ions, glucose, and amino acids against their concentration gradient (see Fig. 6E2). Sulfate (HSO4-) and protons (H+) as well as phosphate (HPO4-) and protons (H+) are co-transported across bacterial membranes by symporters.
c. ATP-binding cassette (ABC) system: An example of an ATP-dependent active transport found in various gram-negative bacteria is the ATP-binding cassette (ABC) system. This involves substrate-specific binding proteins located in the bacterial periplasm, the gel-like substance between the bacterial cell wall and cytoplasmic membrane. The periplasmic-binding protein picks up the substance to be transported and carries it to a membrane-spanning transport protein (see Fig. 7A). Meanwhile, an ATP-hydrolyzing protein breaks ATP down into ADP, phosphate, and energy (see Fig. 7B). It is this energy that powers the transport of the substrate, by way of the membrane-binding transporter, across the membrane (see Fig. 7C) and into the cytoplasm. Examples of active transport include the transport of certain sugars and amino acids. Over 200 different ABC transport systems have been found in bacteria.
d. Group translocation is another form of active transport that can occur in prokaryotes. In this case, a substance is chemically altered during its transport across a membrane so that once inside, the cytoplasmic membrane becomes impermeable to that substance and it remains within the cell.
An example of group translocation in bacteria is the phosphotransferase system. A high-energy phosphate group from phosphoenolpyruvate (PEP) is transferred by a series of enzymes to glucose. The final enzyme both phosphorylates the glucose and transports it across the membrane as glucose 6-phosphate (see Fig. 7E through 7H). (This is actually the first step in glycolysis.) Other sugars that are transported by group translocation are mannose and fructose.
C. Functions of the cytoplasmic membrane other than selective permeability
A number of other functions are associated with the bacterial cytoplasmic membrane and associated proteins of a collection of cell division machinery known as the divisome. In fact, many of the functions associated with specialized internal membrane-bound organelles in eukaryotic cells are carried out generically in bacteria by the cytoplasmic membrane. Functions associated with the bacterial cytoplasmic membrane and/or the divisome include:
1. energy production. The electron transport system (see Fig.) for bacteria with aerobic (def) and anaerobic (def) respiration, as well as photosynthesis for bacteria converting light energy into chemical energy is located in the cytoplasmic membrane.
2. motility. The motor that drives rotation of bacterial flagella ( see Fig.) is located in the cytoplasmic membrane.
- Movie of motile Rhodobacter spheroides with fluorescent labelled-flagella.Courtesy of Dr. Howard C. Berg from the Roland Institute at Harvard.
3. waste removal. Waste byproducts of metabolism within the bacterium must exit through the cytoplasmic membrane.
4. formation of endospores (def) (discussed later in this unit; see Fig. and animation).
D. Binary fission
Bacteria divide by binary fission (def) wherein one bacterium splits into two. Therefore, bacteria increase their numbers by geometric progression (def) whereby their population doubles every generation time.
In general it is thought that during DNA replication (discussed in Unit 6), each strand of the replicating bacterial DNA attaches to proteins at what will become the cell division plane. For example, Par proteins function to separate bacterial chromosomes to opposite poles of the cell during cell division. They bind to the origin of replication of the DNA and physically pull or push the chromosomes apart, similar to the mitotic apparatus of eukaryotic cells.
In the center of the bacterium, a group of proteins called Fts (filamentous temperature sensitive) proteins interact to form a ring at the cell division plane. These proteins form the cell division apparatus known as the divisome and are directly involved in bacterial cell division by binary fission (see Fig. 1 and Fig. 2).
- electron micrograph of a divisome: see under Bacterial Cell Division, Jon Beckwith's Lab.
The divisome is responsible for directing the synthesis of new cytoplasmic membrane and new peptidoglycan to form the division septum. The function of a number of divisome proteins have been identified, including:
- MinE: Directs formation of the FtsZ ring and divisome complex at the bacterium's division plane.
- FtsZ: Similar to tubulin in eukaryotic cells, FtsZ forms a constricting ring at the division site. As FtsZ depolymerizes, it directs an inward growth of the cell wall to form the division septum. It is found in both Bacteria and Archaea, as well as in mitochondria and chloroplasts.
- ZipA: A protein that connects the FtsZ ring to the bacterial cytoplasmic membrane.
- FtsA: An ATPase that breaks down ATP to provide energy for cell division and also helps connect the FtsZ ring to the bacterial cytoplasmic membrane.
- FtsK: Helps in separating the replicated bacterial chromosome.
- FtsI: Needed for peptidoglycan synthesis.
- Scanning electron micrograph of dividing Escherichia coli; courtesy of CDC.
- Scanning electron micrograph of dividing Salmonella typhimurium; courtesy of CDC.
- To view an transmission electron micrograph of dividing streptococci, see the Rockefeller University home page.
E. Using Antimicrobial Agents that Alter the Cytoplasmic Membrane to Control Bacteria
As will be discussed later in Unit 2, a very few antibiotics, such as polymyxins and tyrocidins as well as many disinfectants (def) and antiseptics (def), such as orthophenylphenol, chlorhexidine, hexachlorophene, zephiran, alcohol, triclosans, etc., used during disinfection (def) alter the microbial cytoplasmic membranes (def) and cause leakage of cellular needs.
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Updated: Aug., 2012
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