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2.2: The Cytoplasmic Membrane

Skills to Develop

  1. State the chemical composition and major function of the cytoplasmic membrane in bacteria.
  2. Briefly describe the fluid phospholipid bilayer arrangement of biological membranes.
  3. State the net flow of water when a cell is placed in an isotonic, hypertonic, or hypotonic environment and relate this to the solute concentration.
  4. Define the following means of transport:
    1. passive diffusion
    2. osmosis
    3. facilitated diffusion
    4. transport through channel proteins
    5. transport through uniporter 
    6. active transport
    7. transport through antiporter
    8. transport through symporter 
    9. the ABC transport system
    10. group translocation
  5. State how the antibiotic polymyxin and disinfectants such as orthophenylphenol, chlorhexidine, hexachlorophene, zephiran, and alcohol affect bacteria.
  6. Define binary fission and geometric progression and relate this to bacteria being able to astronomically increase their numbers in a relatively short period of time.
  7. Briefly describe the process of binary fission in bacteria, stating the functions of Par proteins, the divisome, and FtsZ proteins.

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 (Figure 1).

Structure and Composition

Like all biological membranes in nature, the bacterial cytoplasmic membrane is composed of phospholipid 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 (Figure 2). With the exception of the mycoplasmas, the only bacteria that lack a cell wall, prokaryotic membranes lack sterols. 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.

 

Figure 3: Diagram of a 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 (Figure 2).

Functions

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 phospholipid 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, facilitated diffusion, and active transport.

Passive Diffusion

Passive diffusion is the net movement of gases or small uncharged polar molecules across a phospholipid bilayer membrane from an area of higher concentration to an area of lower concentration (Figure 3A and 3B) . Examples of gases that cross membranes by passive diffusion include N2, O2, and CO2; examples of small polar molecules include ethanol, H2O, and urea.

 

    

Figure 3A: Passive Diffusion Steps. 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 . 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 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. The aquaporins form channels that span the cytoplasmic membrane and transport water in and out of the cytoplasm ( channel proteins below).

To understand osmosis, one must understand what is meant by a solution. A solution consists of a solute dissolved in a solvent. 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 (Figure 4).Therefore, the higher the solute concentration, the lower the concentration of free water molecules capable of passing through the membrane.

  

Figure 4: Osmosis. Free Water Passing Through Membrane Pores. (left) 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 the membrane and through membrane pores, water molecules bound to solute are not. (right) When an ionic solute such as NaCl dissolves in water, the Na+ ion attracts the partial negative charge of the oxygen atom in the water molecule while the Cl- ion attracts the partial positive charge of the warter's hydrogen. While free, unbound water molecules are small enough to pass through the membranr and through membrane pores, water molecules bound to solute are not.

A cell can find itself in one of three environments: isotonic, hypertonic, or hypotonic (the prefixes iso-, hyper-, and hypo- refer to the solute concentration).

  • In an isotonic environment (Figure 5) 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.

    

Figure 5: Osmosis (Cell in an Isotonic Environment). (left) In anisotonic environment, 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. (right) If the environment surrounding the cell is hypertonic, the solute concentration is higher outside the cell, while the water concentration is greater inside the cell. The cytoplasm of the cell is hypotonic to the surrounding hypertonic environment. Water goes out of the cell.

  • If the environment is hypertonic ( Figure 6A and Figure 6B) 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 (Figure 7) 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.

   

Figure 7: Osmosis (Cell in Hypotonic Environment).  If the environment surrounding the cell is hypotonic, the solute concentration is greater inside the cell, while the water concentration is greater outside the cell. (The cytoplasm of the cell is hypertonic to the surrounding hypotonic environment.) Water goes into the cell.

Facilitated Diffusion

Facilitated diffusion 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 are transport proteins that transport a substance from one side of the membrane to the other (Figure 1.1.8). Potassium ions (K+) can enter bacteria through uniporters.

    

Figure 2.2.8: Transport of Substances Across a Membrane by Uniporters. Uniporters are transport proteins that transport a substance across a membrane down a concentration gradient from an area of greater concentration to lesser concentration. The transport is powered by the potential energy of a concentration gradient and does not require metabolic energy.

 

2. Channel proteins 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 ( Figure 6B). While water molecules can directly cross the membrane by passive diffusion, as mentioned above, channel proteins called aquaporins can enhance their transport.

 

Active Transport 

Active transport 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, 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 sparse 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 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 (Figure 9A). 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 (Figure 9B). Sodium ions (Na+) and protons (H+), for example, are co-transported across bacterial membranes by antiporters.

b. Symporter: Symporters are transport proteins that simultaneously transport two substances across the membrane in the same direction (Figure 10A). 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 (Figure 10B). 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 (Figure 11A). Meanwhile, an ATP-hydrolyzing protein breaks ATP down into ADP, phosphate, and energy (Figure 11B). It is this energy that powers the transport of the substrate, by way of the membrane-binding transporter, across the membrane (Figure 11C and Fig. 11D) 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 (Figure 12A through 12D). (This is actually the first step in glycolysis.) Other sugars that are transported by group translocation are mannose and fructose.

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 ( Fig.) for bacteria with aerobic and anaerobic 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.
  3. Movie of motile Rhodobacter spheroides with fluorescent labelled-flagella. Courtesy of Dr. Howard C. Berg from the Roland Institute at Harvard.
  4. waste removal. Waste by products of metabolism within the bacterium must exit through the cytoplasmic membrane.
  5. formation of endospores (discussed later in this unit; see Fig. and animation).

 

Binary fission

Bacteria divide by binary fission wherein one bacterium splits into two. Therefore, bacteria increase their numbers by geometric progression 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.

Figure 8: Bacterial Divisome.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. The divisome is responsible for directing the synthesis of new cytoplasmic membrane and new peptidoglycan to form the division septum.

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 (Figure 1 and Fig. 13).

  • 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.

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 and antiseptics, such as orthophenylphenol, chlorhexidine, hexachlorophene, zephiran, alcohol, triclosans, etc., used during disinfection alter the microbial cytoplasmic membranes and cause leakage of cellular needs.

 

Summary

  1. The bacterial cytoplasmic membrane is a fluid phospholipid bilayer that encloses the bacterial cytoplasm.
  2. The cytoplasmic membrane is semipermeable and determines what molecules enter and leave the bacterial cell.
  3. Passive diffusion is the net movement of gases or small uncharged polar molecules such as water across a membrane from an area of higher concentration to an area of lower concentration.
  4. Passive diffusion is powered by the potential energy of a concentration gradient and does not require the expenditure of metabolic energy or the use of transport proteins.
  5. Facilitated diffusion is powered by the potential energy of a concentration gradient and does not require the expenditure of metabolic energy, but it does require the use of transport proteins.
  6. A solution refers to solute dissolved in a solvent.
  7. Osmosis is the movement of water across a membrane from an area of higher water (lower solute) concentration to an area of lower water (higher solute) concentration by both passive diffusion and facilitated diffusion.
  8. Active transport is a process whereby the cell uses both transport proteins and metabolic energy to transport substances across the membrane against the concentration gradient.
  9. Most molecules and ions that a cell needs to concentrate within the cytoplasm in order to support life require active transport for entry into the cell.
  10. In order to colonize any environment, a bacterium must be able to effectively use its transport systems to compete with other bacteria, as well as the cells of other organisms – such as human cells - for limited nutrients.
  11. Bacteria divide by binary fission and increase their numbers by geometric progression.
  12. Some antimicrobial agents alter the microbial cytoplasmic membranes and cause leakage of cellular needs.

Questions

Study the material in this section and then write out the answers to these questions. Do not just click on the answers and write them out. This will not test your understanding of this tutorial.

  1. Match the following descriptions with the best answer.

    _____ Proteins that, in the presence of energy, transport two substances simultaneously across the membrane in opposite directions. (ans)

    _____ Proteins that, in the presence of energy, transport two substances simultaneously across the membrane in the same directions. (ans)

    _____ The movement of water across a membrane from an area of higher water concentration (lower solute concentration) to lower water concentration (higher solute concentration). (ans)

    _____ 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. No metabolic energy is required. (ans)

    _____ A transport where the cell uses transport proteins such as antiporters or symporters and metabolic energy to transport substances across the membrane against the concentration gradient. (ans)

    _____ If the net flow of water is out of a cell, the cell is in ________________ environment. (ans)

    _____ If the net flow of water is into a cell, the cell is in ________________ environment. (ans)

     

    1. uniporters
    2. symporters
    3. antiporters
    4. active transport
    5. group translocation
    6. passive diffusion
    7. osmosis
    8. a hypotonic
    9. a hypertonic
    10. an isotonic
  2. Even though there is a lower concentration of a particular nutrient outside a bacterium than inside, the bacterium is still able to transport that nutrient into its cytoplasm. Explain how this might occur and what is required for this transport. (ans)
  3. A bacterium is placed in a new environment and subsequently water flows out of the bacterium. Is this new environment isotonic, hypotonic, or hypertonic to the bacterium? Is the solute concentration higher inside the bacterium or outside? (ans)
  4. Bacteria normally do not grow in jams and jellies. In terms of osmosis, what might explain this? (ans)
  5. Define the following:
    1. binary fission (ans)
    2. geometric progression (ans)
  6. State the functions of the following in bacterial cell division:
    1. Par proteins (ans)
    2. divisome (ans)
    3. FtsZ proteins (ans)
  7. Multiple Choice (ans)

Contributors

  • Dr. Gary Kaiser (COMMUNITY COLLEGE OF BALTIMORE COUNTY, CATONSVILLE CAMPUS)