CHAPTER 4 FUNCTIONAL ANATOMY OF PROKARYOTIC AND EUKARYOTIC CELLS

All cells share numerous characteristics, but cells of living things can be divided into 2 basic types. Here is a quick comparison.

CHARACTERISTIC	PROKARYOTIC	EUKARYOTIC
Size	Smaller---typical size is 0.2 - 2 mm in diameter	Larger---typical size is 10 - 100 mm in diameter
Nucleus	No nuclear membrane or nucleoli (nucleoid)	True nucleus with nuclear membrane and nucleoli
Membrane-enclosed organelles	Absent	Many, including lysosomes, Golgi complex, ER, mitochondria & chloroplasts
Flagella	Consist of 2 protein building blocks	Complex; consist of multiple microtubules
Phagocytosis	None	Some can carry on phagocytosis
Glycocalyx	Present as a capsule or slime layer	Present in some cells that lack a cell wall
Cell wall	Usually present, chemically complex (peptidoglycan)	When present, usually simple, NO peptidoglycan
Plasma membrane	No carbohydrates, almost all lack sterols	Sterols and carbohydrates incorporated
Cytoplasm	No cytoskeleton	Cytoskeleton
Ribosomes	Smaller size (70S)	Larger size (80S) except those within organelles
Chromosome (DNA) arrangement	Single circular chromosome, proteins associated with DNA are not histones	Multiple linear chromosomes with histone proteins
Cell division	Binary fission	Mitosis
Sexual reproduction	No meiosis; transfer fragments of DNA only	Meiosis

Since bacteria are all prokaryotic, and are the only prokaryocytes, the following terms can be used interchangeably:

Bacteria (both eubacteria and archaea)

Prokaryocytes

Prokaryotes

Prokaryotic organisms

Prokaryotic cells

PROKARYOTIC CELLS

Size----most range from 0.2 to 2 mm in diameter, 2 - 8 mm in length

Shape----Coccus (sphere), Bacillus (rod), Spiral

Arrangement:

a. Cocci---many will be single, but sometimes when cells divide they may remain attached to form:

1) Diplococci (pairs)

2) Streptococci (chains)

3) Tetrads (groups of 4)

4) Sarcinae (groups of 8)

5) Staphylococci (grapelike clusters)

b. Bacilli----mostly single, but some form:

1) Diplobacilli (pairs)

2) Streptobacilli (chains)

3) Palisades (rods side by side)

Rods may vary in shape-long rods to just slightly oval (coccobacilli). Some are irregularly shaped----club-shaped, for example

Some species may exhibit the ability to have different shapes under different conditions. Some that are normally quite rod-shaped may change to very short rods, so short they look almost round, for example. This is frequently seen in cultures grown in artificial media and is called pleomorphism.

c. Spirals are always single. They vary from:

1) Vibrios-----just somewhat curved

2) Spirilla----spirals like a cork-screw and stiff

3) Spirochetes----spirals but flexible

A few bacteria do not fit into the 3 basic shapes: star-shaped, square, triangular. Some are filamentous (threadlike). These are mostly soil bacteria.

PROKARYOTIC CELLS IN DETAIL

A. STRUCTURES EXTERNAL TO THE CELL WALL

1. GLYCOCALYX--this is a sticky mass that surrounds many prokaryocytes. It may be made of polysaccharides or polypeptides.

a. Capsule-glycocalyx that is organized and firmly attached to the cell wall.

b. Slime layer-unorganized and loosely attached.

c. Functions of glycocalyx

1) May protect bacteria from phagocytosis, making them more virulent (able to cause more serious disease)—Bacillus anthracis, Streptococcus pneumoniae

2) May allow bacteria to attach to a surface---Streptococcus mutans on teeth

3) May serve as a source of nutrients and protect against dehydration

2. FLAGELLA-long filamentous (thread-like) appendages used by bacteria for movement

a. Arrangement of flagella

1) Monotrichous--- 1 flagellum at one end

2) Amphitrichous- cluster of flagella at each end

3) Lophotrichous—2 or more flagella at one or both ends

4) Peritrichous- over entire cell

5) Atrichous—no flagella

b. Structure of flagella (3 parts):

1) Filament-- long, threadlike, outermost part. Made of a protein called flagellin (which may not be exactly the same in all bacteria). Chains of this protein twist around a hollow core.

2) Hook--- connects filament and basal body

3) Basal body---anchors flagellum to cell wall and plasma membrane. Consists of a central rod inserted into rings.

a. 2 pairs of rings in gram-negative bacteria; 1 pair in gram-positive

c. Movement is produced by rotation of the basal body. Bacteria possessing flagella can use them to move toward a favorable environment or away from harm.

d. Since the proteins of the flagella may vary slightly species or even among different strains of the same species, this can be used for identification.

1) The flagellar protein called H antigen can be used for distinguishing serovars, or variations within a species of gram-negative bacteria.

2) Escherichia coli has at least 50 possible H antigens. One strain identified this way, E. coli 0157:H7 is the one in the news for causing severe food poisoning

3. AXIAL FILAMENTS (ENDOFLAGELLA)--- these are found only on spirochetes. These are bundles of fibrils that arise at the ends of the cell and wrap around the length of the cell under a layer called the outer sheath. These axial filaments rotate like flagella, but since they are under the outer sheath this causes the outer sheath to move and gives the bacteria a corkscrew motion.

4. FIMBRIAE AND PILI--these are made of the protein pilin. They are shorter and thinner than flagella, and are used for attachment (not motility). They are found on some gram-negative bacteria.

a. Fimbriae-vary from a few to several hundred---used for attachment to surfaces, such as mucous membranes. Neisseria gonorrhoeae (gonorrhea) and Eshcerichia coli use these in “hanging on” to establish urinary tract infections.

b. Pili—longer--only one or two per cell. Used to join bacterial cells in preparation for the process of conjugation (transfer of DNA). They are also sometimes called conjugation pili.

B. CELL WALL

Complex, semi-rigid structure that gives shape and protection to the cell.

1. Functions

► a. Prevents cell from rupturing due to entrance of too much water.

b. Helps give shape

c. Point of anchorage for flagella

d. May contribute to ability to cause disease

e. Site of action for some antibiotics

f. Chemical composition may be used in classification

2. Composition and Characteristics

a. Eubacterial cell walls are mainly made of peptidoglycan, a substance found nowhere else in nature. It is in the form of one macromolecule that surrounds the entire cell. It consists of disaccharide units connected by polypeptides to form a lattice.

1) Gram-positive cell walls

a) These consist of many layers of peptidoglycan, forming a thick, rigid structure.

b) Also included are teichoic acids, which consist of an alcohol and phosphate.

c) Exact structure of the teichoic acids varies among species. This is the part that often acts as an antigen and is used for identification.

d) Cell walls of Streptococcus are covered with various polysaccharides, which are used to sort them into groups.

e) Cell walls of acid-fast bacteria contain large amounts of mycolic acid and this is what makes them acid-fast.

2. Gram-negative cell walls--these have a much thinner layer of peptidoglycan. Outside the peptidoglycan layer is the outer membrane, which consists of lipoproteins, lipopolysaccharides and phospholipids. The fluid-filled space between the outer membrane and the plasma membrane is called the periplasm. The peptidoglycan is in this space.

a. Functions of outer membrane

1) Help the bacterium avoid phagocytosis and effects of complement

2) Provides a barrier to certain antibiotics and harmful chemicals

3) Allows nutrients to enter through channels called porins

4) Lipopolysaccharides of the membrane are important in 2 ways:

a) O polysaccharides act as antigens—this is the rest of the E. coli strain O157:H7

b) Lipid A is an endotoxin----causes fever and shock

TABLE 4.1 P. 88 COMPARES TYPES OF CELL WALLS

3. Atypical cell walls

a. A few prokaryocytes do not have a cell wall.

1) Members of the genus Mycoplasma have no cell walls. These organisms are also extremely tiny.

2) Since they lack a cell wall, their plasma membranes need extra strength. To provide this, Mycoplasma organisms incorporate sterols into the membrane.

b. Cell walls of archaea do not contain peptidoglycan.

1) Most have a cell wall which contains pseudomurein instead.

2) A few archaea lack a cell wall.

3) Most would stain gram-negative

4. Damage to the Cell Wall

a. Cell walls can be damaged by exposure to the enzyme lysozyme (found in tears, mucus, saliva). Cell walls of Gram-positive bacteria are more susceptible.

b. If the Gram-positive cell loses its wall and is in hypotonic surroundings, it will undergo lysis (burst) due to entrance of water

c. If the surroundings are isotonic, the gram-positive cell can exist with no cell wall as a protoplast.

d. Lysozyme does not affect Gram-negative cell walls to the same extent. A Gram-negative cell treated with lysozyme becomes a spheroplast. It retains its outer membrane and remnants of the cell wall.

e. Since animal cells do not have cell wall, antimicrobials which target cell walls often do not harm the cells of the patient (host). This makes the cell wall an ideal target for antibiotics.

f. Penicillin is an example of an antibiotic that works this way. Gram -negative organisms are not usually highly susceptible to penicillin because the outer membrane gives them some protection.

C. STRUCTURES INTERNAL TO THE CELL WALL

1. PLASMA (CYTOPLASMIC) MEMBRANE-----lies inside the cell wall and encloses the cytoplasm.

a. Consists of phospolipids and proteins. Phospholipids form a bilayer that consists of 2 layers of phospholipid molecules. A phospholipid molecule looks like this:

Head-polar and water soluble, hydrophilic (glycerol + phosphate)

2 tails, non-polar, hydrophobic, not water soluble (fatty acids)

They line up like this to form the phospholipid bilayer:

b. Membrane proteins are placed in 2 groups

1) Peripheral proteins---loosely attached to the inner or outer surface of the membrane. Some are enzymes and some give support to the membrane. These are easily removed.

2) Integral proteins are built into the membrane and cannot be removed without damage. Some of these extend from one side of the membrane to the other (transmembrane proteins). Some proteins contain channels through which substances entering/leaving the cell can pass.

c. The membrane is highly flexible and allows membrane proteins to change position if necessary. This idea of the structure of the membrane is known as the fluid mosaic model.

d. Functions:

1) The plasma membrane serves as a selective barrier to substances entering or leaving the cell. Some things are allowed to cross while others are not. This is selective permeability.

Permeable to a substance-----it can cross

Impermeable to a substance-----it cannot cross

Selectively permeable---some can cross and some cannot

Some general statements regarding permeability:

a) Large molecules mostly cannot cross (proteins would be an example). Smaller molecules cross more easily.

b) Lipid-soluble substances cross easily since they just dissolve in the phospholipid of the membrane. (O₂ and CO₂ are examples)

c) Some non-lipid-soluble smaller molecules cross with the aid of integral protein channels. (Some simple sugars, water)

d) Ions usually require the aid of a transporter protein and cross slowly.

2) In bacteria, the plasma membrane is associated with the breakdown of nutrients and the synthesis of ATP. In photosynthetic bacteria. Required pigments and enzymes for the process are associated with the plasma membrane.

DAMAGE TO THE PLASMA MEMBRANE BY ANTIMICROBIALS

The plasma membrane is vital to the survival of the bacterial cell. Some chemical disinfectants act by damaging the plasma membrane. This includes aldohols and quaternary ammoniums. In addition, polymixin antibiotics also act on the plasma membrane.

Before discussing movement across the plasma membrane, we need to briefly review solutions. A solution consists of a dissolving medium, called the solvent, and one or more dissolved substances, called the solutes. In biological solutions, the solvent is water. In crossing cell membranes, substances are moving from one solution to another.

MOVMENT OF MATERIALS ACROSS MEMBRANES

Processes are divided according to whether or not they require energy.

1) Passive processes---- all share 2 major characteristics:

· Do not require energy

· Can move only from greater concentration to lesser concentration (with the concentration gradient)

a) Simple Diffusion----- net movement of molecules or ions from an area of higher concentration to an area of lower concentration. This movement continues until the 2 concentrations become equal.

b) Osmosis- this is really still simple diffusion but this time water (solvent) molecules do the moving instead of the solutes (dissolved material). Osmosis occurs across a membrane that is permeable to water but relatively impermeable to the solutes. Water molecules move from an area of higher water concentration to an area of lower water concentration. Here are the 3 ways osmosis can affect a bacterial cell:

1] Isotonic surroundings-water and solute concentration are equal inside the cell and in the solution that surrounds the cell. Since concentrations are already equal, no net movement occurs.

Water = Water

Solutes = Solutes

2] Hypotonic surroundings---- water concentration is higher and solute concentration is lower outside the cell than inside. Bacteria often find themselves in these surroundings. In these conditions, water would enter the cell, even enough to cause the cell to burst, if not for the cell wall.

Lower Water Higher Water

Higher Solutes Lower solutes

3] Hypertonic surroundings- water concentration is lower and solute concentration higher outside the cell than inside. Water would tend to leave the cell. If enough leaves, this could kill the cell. This is the principal behind salted meats and jellies.

Higher Water Lower Water

Lower Solutes Higher Solutes

c) Facilitated diffusion---substance to be transported combines with a plasma membrane carrier protein called a transporter. Transporters are specific to one substance. As the substance molecule moves into the open end of the transporter molecule, its presence causes the carrier molecule to change shape. The open end of the carrier in now on the inside of the cell, so the substance moves on in. The carrier releases the substance and returns to the original shape, ready to repeat the process. Movement can continue as long as the substance is moving from higher concentration to lower concentration.

d) Passive processes are less important in bacterial cells than in eukaryotic cells. One reason is that bacterial cells frequently find themselves in surroundings where nutrients are in low concentration, so passive processes are unable to meet their needs. Another point is that only relatively small molecules can enter cells by passive processes. In some cases, bacteria produce enzymes and release them out into their surroundings (extracellular enzymes). These enzymes break down larger molecules into a size that can enter passively.

2) Active Processes---all share 2 major characteristics

· Require energy from ATP

· Can move substances from lower concentration to higher concentration (against the gradient)

a) Active transport---this is quite similar to facilitated diffusion in that a specific transporter protein molecule in the plasma membrane is required. The difference is that ATP energy is required, and that substances can be moved from lower concentrations to higher concentrations. In this process the substance is not changed as it enters the cell.

b) Group translocation- this occurs only in prokaryocytes. It is a special form of active transport in which the substance being transported is chemically changed as it is transported. One example is glucose, which is changed to glucose phosphate as it is brought in. The plasma membrane is completely impermeable to glucose phosphate, so it cannot leave the cell, even if the concentration inside the cell becomes quite high.

c) Phagocytosis and pinocytosis do not occur in prokaryotic cells.

2. CYTOPLASM----(prokaryotic definition)---cellular material inside the plasma membrane. (There is no nuclear membrane for it to be outside of!) It is about 80% water and is thick, semitransparent and gel-like. It contains dissolved and suspended materials:

Proteins (many of these are enzymes)

Carbohydrates

Lipids

Inorganic ions

It is the site of many chemical reactions.

3. NUCLEAR AREA (NUCLEIOD)---prokaryotic cells do not have a separate, membrane-enclosed area for the DNA. Their DNA, in the form of one circular chromosome without associated histones, is not separated from the cytoplasm. The chromosome’s DNA twists into a tight helix and is connected to the plasma membrane. As in eucaryocytes, the chromosome must replicate (make a copy of itself) before the cell divides, so that each daughter cell can receive one. Proteins of the plasma membrane are believed to be responsible for this replication of the DNA.

4. PLASMIDS--not all bacterial cells have these, but many do. Plasmids are small, circular DNA molecules separate from the chromosome. They usually contain 5-100 genes, which code for proteins that may give the cell resistance to certain antibiotics, resistance to toxic metals, ability to produce certain toxins, or ability to synthesize enzymes not included in the chromosomal genes. Proteins coded on plasmid DNA are usually not required for cells under ideal conditions, but they can give the cell that possesses them advantages for survival and also makes those cells more dangerous as pathogens. Copies of plasmids can be transferred from one cell to another and once a cell has a plasmid, it will replicate the plasmid and pass a copy on to each daughter cell during cell division.

5. RIBOSOMES---ribosomes are the site of protein synthesis. In a living cell, nothing but a ribosome can make a protein. In prokaryotic cells, the ribosomes are mostly scattered in the cytoplasm.

a. Ribosomes are composed of two subunits, which fit together. Each subunit consists of protein and ribosomal RNA.

b. Bacterial ribosomes are described as 70 S ribosomes. The two subunits are one 30 S subunit and one 50 S subunit. Ribosomes of eukaryocytes are slightly different.

c. The S stands for Svedberg unit. This measures the rate at which particles sink when centrifuged.

d. Since bacterial ribosomes are not quite the same as eukaryotic ribosomes, this makes another possible target for antibiotics.

Streptomycin and gentamicin---30 S subunit

Erythromycin and chloramphenicol---50 S subunit

6. INCLUSIONS----these are reserve deposits of chemicals within the cytoplasm. They tend to accumulate when nutrients are plentiful.

a. Metachromatic granules (volutin granules)---phosphate reserve that can be used in synthesizing ATP. These large granules stain red with methylene blue. They are characteristic of Corynebacterium diphtheriae. These granules are also found in algae, fungi, and protozoa.

b. Polysaccharide granules---glycogen granules (which iodine stains reddish brown) and starch granules (which iodine stains blue).

c. Lipid inclusions- most common is poly-b-hydroxybutyric acid (PHB). Found in Mycobacterium, Bacillus and others.

d. Sulfur granules- bacteria whose metabolism involves oxidizing sulfur may deposit these as an energy reserve. Thiobacillus is an example.

e. Carboxysomes-bacteria that use CO₂ as their only carbon source form these granules containing an enzyme required in the process.

f. Gas vacuoles- these are adjusted so that the cell that contains them can float at the proper depth in water.

g. Magnetosomes- inclusions of iron oxide that act like magnets. Function uncertain, but they apparently can decompose hydrogen peroxide.

7. ENDOSPORES- these are dormant cells, formed when certain genera (mainly Clostridium and Bacillus) find themselves facing adverse conditions such as lack of nutrients, drying, presence of toxic materials, etc. Once formed, endospores can withstand extreme conditions and survive for apparently unlimited periods of time.

a. Regular forms of bacteria are known as vegetative cells. These are the ones that take in nutrients, carry out metabolic activities, reproduce, cause disease, etc. They are susceptible to heat, drying, lack of nutrients, etc., and most of them are relatively easy to kill.

b. Sporulation or sporogenesis is the process of endospore formation.

1) Bacterial chromosome replicates. One copy plus a little cytoplasm are isolated in a small area of the cell as the plasma membrane forms a partition called the spore septum.

2) Spore septum becomes a double-layered membrane that surrounds the chromosome and bit of cytoplasm and this is now called the forespore. This is inside the original cell.

3) Thick layers of peptidoglycan are produced between the two membrane layers of the spore septum.

4) A thick protein spore coat forms around the outside membrane.

5) Water in the cytoplasm of the spore is removed throughout the process, leaving very little in the cytoplasm. Metabolic reactions cease. This cytoplasm will contain high levels of dipicolinic acid and calcium ions.

6) When the endospore is complete, the vegetative cell that formed it will rupture and release the endospore. While it is still contained in the vegetative cell, the endospore will be located in a characteristic part of the cell: