• All living cells have a cell (plasma) membrane, genetic information is stored in DNA and all are classified either as prokaryotic or eukaryotic.– prokaryotes lack a nucleus and other membrane-

enclosed structures, all are bacteria (archeobacteria are important ecologically).

– eukaryotes have various membrane-enclosed structures, include all plants, animals, fungi, and protists. We will study some fungi and various parasites, plus interactions between microbes and eukaryotes.

– Differences between prokaryotes and eukaryotes are important when looking for ways to control disease-causing microbes. Treatments generally have to damage prokaryotic cells (bacteria) while leaving our eukaryotic cells unscathed.

– Viruses do not fit in either category, they are acellular– See Table 4.1 (page 73)

Prokaryotes are very small – most range from 0.5 to 2.0 micrometers – but there are always exceptions. Because of their small size, bacteria have a large surface-to-volume ratio. No internal part of the cell is very far from the surface and nutrients can reach. Example, bacteria with a diameter of 2μm have a surface area of 12μm2 and a volume of 4μm3 which give a surface-to-volume ratio of 3:1. In contrast, large eukaryotic cells need internal membrane since their surface-to-volume ratio is much smaller.

Most bacteria come in three basic shapes – spherical, rodlike, and spiral – with variations. Even bacteria of the same kind sometimes vary in size and shape due to conditions like availability of nutrients, or aging cultures with build up of waste products. Some bacteria vary widely even within a single culture – pleomorphism.

Many bacteria are found in characteristic arrangements due to the way they divide and whether they separate.

Cocci that divide in one plane form pairs or chains

Division in two planes forms tetrads.

Division in three planes froms sarcina

Random division planes form grapelike clusters (staphylo)

Bacilli divide in only one plane but can remain attached .

Prokaryotes divide by binary fission (not sexual), not mitosis or meiosis. (see Table 4.1)

Checklist page 76

Fig. 4-3

Bacteria cells consist of

1. A cell membrane, usually surrounded by a cell wall and sometimes by an outer layer

2. An internal cytoplasm with ribosomes, a nuclear region, and sometimes granules and/or vesicles

3. A variety of external structurescapsules, flagell, pili

A semi-rigid cell wall is found outside the cell membrane in nearly all bacteria.

Two major functions include

1. it maintains the shape of the cell

2. it prevents the cell from bursting due to osmotic pressure (described later).

The cell wall usually does not regulate the entry and exit of material from the cell. The plasma membrane does that. The cell wall is usually very porous.

Peptidoglycan is the most important component of the cell wall. It is extremely large. Gram positive bacteria can have up to 40 layers.

Fig. 4-6

Peptidoglycan is composed of sugar chains (glycan) joined by peptides (small sequences of amino acids).

The sugar backbone consists of two alternating sugar molecules.

The peptide crosslinker is a tetrapeptide.

Gram-positive organisms also have teichoic acidteichoic acid, a very long polymer that extends beyond the rest of the cell wall.

Fig. 4-6

Fig. 4-5

Lipopolysaccharide (LPS), also called endotoxin, can be used to identify gram-negative bacteria. It is released only when the cell wall is broken down. LPS consists of sugars (polysaccharides) sticking out from the cell wall and lipid A, which holds LPS in the outer membrane. Lipid A is toxic, it causes fever and dilates blood vessels, leading to a drop in blood pressure. LPS is released only when gram-negative bacteria are dying. So antibiotics given late in an infection can even lead to the death of the patient.

The outer membrane is found mainly in gram-negative bacteria. Proteins called porins form large channels. Little control over transport in and out of the cell. Again, the cell membrane does that

Fig. 4-6

Gram-positive bacteria have a thick layer of peptidoglycan closely attached to the cell membrane. If it is digested the cell becomes a protoplast (no cell wall).

Gram-negative bacteria have a thinner but more complex cell wall, including an outer membrane and a periplasmic space which contains enzymes that protect the bacteria – make it less susceptible to antibiotics. If cell wall is digested the cell becomes a spheroplast with both a cell membrane and most of the outer membrane.

Acid-fast bacteria have a thick wall made up of lots of lipid. They grow slowly and stain gram-positive.

• Mycoplasma have no cell walls. Their cell membranes are reinforced with sterols, like eukaryotes (we have cholesterol in our cell membranes).

• When treating with antibiotics that block cell wall construction, some bacteria may survive in the form of L-forms (bacteria that no longer form cell walls). After treatment is stopped these bacteria can revert back to walled form and go on to re-grow an infecting population.

• Checklist on page 80

• The cell membrane is dynamic and constantly changing, unlike the static cell wall.

• It is made up primarily of phospholipids and proteins (a mosaic), both constantly moving (fluid) – fluid-mosaic model.

• The main function of the cell membrane is to controls what goes in and what comes out of the cell.

• Bacterial cell membranes synthesize cell wall components, assist with DNA replication, secrete proteins, carry on respiration, capture energy as ATP, etc.

Fig. 4-7

Phospholipids form the lipid bilayer

Fluid Mosaic Model

Internal Structure: The cytoplasm of prokaryotic cells is semifluid, about 4/5 water plus enzymes and other proteins, carbohydrates, lipids, and ions. Chemical reactions take place in the cytoplasm, including protein syntheisis on ribosomes.

Ribosomes consist of RNA and protein. They are often seen in the cytoplasm as polyribosomes. They include two subunits, the large 70S subunit and the small subunit. Eukaryotic cells have a larger ribosome (80S). Here is a good target for antibiotics since it is different than our eukaryotic ribosomes.

Target the 70S ribosomal subunit and disrupt protein synthesis in bacteria.

A defining feature of prokaryotic cells is the lack of a nucleus. Bacteria have a nuclear region or nucleoid centrally located containing one circular DNA chromosome. There are exceptions. Figure 4.9 Some bacteria also contain smaller circular DNA molecules called plasmids. These often contain genes for antibiotic resistance.

Prokaryotes lack intracellular membrane-bound organelles. However, there are exceptions. Photosynthetic bacteria and cyanobacteria contain internal membrane systems derived from the cell membrane. These contain pigments used to capture energy from light for making sugars – carbohydrates. Figure 4.10

• Vegetative cells (cells that are actively metabolizing nutrients) produce resting stages called endospores. One bacteria makes one spore in its cytoplasm, often when nutrients are scarce.

• They are highly resistant to heat, drying, acids, bases, etc. and allow for the survival of bacteria when vegetative cells cannot survive. Killing endospores in food is important. Figure 4.11

• Endospores can survive adverse environmental conditions for long periods of time. When the conditions are more favorable, endospores germinate and begin to develop into functional vegetative cells.

Fig. 4-13

Exterior structures include flagella and pili, capsules and slime layers.

About ½ of known bacteria are motile, and move with speed and direction, usually by means of flagella. Bacteria with a single polar flagellum are monotrichous; with two, one at each end are amphitrichous; with two or more at one or both ends are lophotrichous; and those with flagella all over are peritrichous. Bacteria with no flagella are atrichous. Cocci rarely have flagella. See Figure 4.12.

Prokaryotic flagellum have a diameter about 1/10th that of eukaryotes.

Gram positive

Gram negative

Fig. 4-14

Most flagella rotate. When they rotate counterclockwise, they bundle, and the bacteria move in a straight line (“run”). When the flagella rotate clockwise, the bundle comes apart, and the bacteria “tumble” randomly. Tumbles allow random changes in direction. Runs allow bacteria to cover more ground in the preferred direction (chemotaxis). Chemotaxis is moving toward (positive) or away from (negative) a high concentration of some substance or molecule.

Example: Bacteria will move toward a nutrient. They move up the concentration gradient toward the area of high glucose concentration

Molecules in the cell membrane detect attractants and repellants.

• Spirochetes have axial filaments which lie between the outer sheath and the cell wall. When these filaments rotate, the body of the spirochete moves like a cork screw (Figure 4.15).

• Pili are tiny, hollow projections used to attach bacteria to surfaces, not for movement.– Conjugation pili (sex), found in certain groups of bacteria, attach

two cells together and allow transfer of genetic DNA. This adds genetic variety and allows transfer of genes important to bacteria, including antibiotic resistant genes (Figure 4.16).

– Attachment pili help bacteria attach to surfaces. These pili contribute to pathogenicity since they allow bacteria to adhere and form colonies on cell surfaces within an organism.

• Example, Neisseria gonorrhoeae adheres to the epithelial cells of the urogenital system. N gonorrhoeae without pili rarely cause disease.

• Glycocalyx = all polysaccharide-containing substances found external to the cell wall, including the thick capsule to the thin slime layers.– A capsule is a protective structure outside the cell wall secreted by

the bacteria. Usually consist of complex polysaccharide molecules in a loose gel. Each capsule is unique to the strain of bacteria. Capsules also contribute to pathogenicity since they help the bacteria evade the immune responses such as phagocytosis.

• Example, when a bacteria loses its ability to secrete a capsule, it often loses its ability to cause disease.

– A slime layer is less tightly bound to the cell wall and is usually thinner than a capsule. It protects the cell from drying and allows bacteria to adhere to objects like your teeth (dental plaque).

– Checklist on page 89.

Fig. 4-18

Eukaryotic Cells are larger and more complex with a diameter of more than 10 microns. They contain a variety of highly specialized structures and compartments and are the basic unit of all organisms in the kingdoms protista, plantae, fungi, and animalia.

The plasma membrane has the same fluid-mosaic structure as prokaryotic cells. It contains a greater variety of lipids (sterols) and is functionally less versatile – specific functions are delegated to membrane-bound organelles. Example: Mitochondrial membranes contain the enzymes necessary for making ATP, not the plasma membrane of eukaryotic cells.

The cytoplasm is much like the prokaryotic cytoplasm except that it contains components of the cytoskeleton.

The nucleus is surrounded by the nuclear membrane (double) and contains nucleoplasm, nucleoli, and paired chromosomes.

The nuclear envelope consists of a double membrane with nuclear pores which allow specific molecules to enter and leave.

The gel within the nucleus is called nucleoplasm (instead of cytoplasm).

The nucleoli contain RNA and proteins needed to construct ribosomes (protein factories).

Also contains chromosomes (DNA plus structural and regulatory proteins) which are in the form of chromatin unless the cell is dividing.

Fig. 4-20The nuclei of eukaryotic cells divide by

The cytoskeleton forms a spindle apparatus which guides the movement of chromosomes.

Duplicated, condensed chromosomes

During sexual reproduction, the nuclei of sex cells divide by meiosis.

Since the two chromosome pairs are positioned together, they are separated into two nuclei with only one of each chromosomes (haploid). These cells go on to divide again, forming gametes with one copy of one of each of the chromosome pairs.

One replication, two divisions

Fig. 4-21

Mitochondria (power plant) have an inner and an outer membrane. The fluid-filled interior is called the matrix. The inner membrane is folded into cristae. The mitochondria contains some DNA, replicates on its own, and contains the enzymes needed for oxidation of “food” into ATP, the energy form used by the cell for most of their activities.

Fig. 4-22 a&b

Chloroplasts contain an inner and an outer membrane also. They also contain DNA and replicate independently. Chloroplasts capture the energy from light during photosynthesis.

Fig. 4-18

Ribosomes in eukaryotes are larger than in prokaryotes and are assembled in the nucleolus. Ribosomes are protein factories. Some make proteins in the cytosol, others make proteins on the ER – for export or for insertion into membrane.

Endoplasmic reticulum (ER) is an extensive system of membranes, smooth (lipid synthesis) and rough (protein synthesis). Vesicles transport lipids and proteins to the golgi.

Golgi Apparatus

Lysosomes

Cytoskeleton

Fig. 4-18

Golgi Apparatus receives vesicles from the ER. Modifies the proteins and the lipids, “labels and packages” them for export to the appropriate location via vesicles. If a protein is to be secreted, it is transported from the Golgi via secretory vesicles to the plasma membrane Lysosomes contain digestive enzymes. Lysosomes fuse with vesicles containing material to be digested. Cytoskeleton is a network of protein fibers including microtubules and microfilaments which givesShape and mechanical support and is involved in movement of organelles inside the cell and movement of the cell itself.

Fig. 4-23

External structure of eukaryotic cells include flagella, cilia, and cell walls.

Eukaryotic flagella are larger and more complex than prokaryotic flagellum. They consist of 2 central and 9 peripheral pairs of microtubules. Each microtubule is about the size of the prokaryotic flagellum. Dynein proteins use ATP energy to change shape, causing the microtubules to slide across each other and creating a wave like motion. Flagella are most common among protozoa. Spermatozoa are the only human flagellated.

Cilia are shorter and more numerous but have the same chemical composition and basic arrangement. Cilia are found mainly among protozoa. Cilia move in a coordinated manner, like oars on a large boat. Cilia are found on the cells of our bronchial tract.

Fig. 4-25

Eukaryotic cell walls do not contain petidoglycan like bacteria. Instead Algal cell walls consist of cellulose, fungal cell walls consist of cellulose and/or chitin. Cell walls give the cell rigidity and protect it from osmotic pressures which can burst a cell.

Cells without cell walls can move by forming pseudopodia. The plasma membrane extends in the direction of the movement. The cytoplasm streams into the new area and the trailing plasma membrane “pulls in”. The proteins in the plasma membrane must be able to “hold on to” a solid surface.

• The endosymbiotic theory offers a possible explanation for the evolution of prokaryotic cells to the more complex eukaryotic cells.– 1st a prokaryotic cell developed a nucleus when an invagination

surrounded the DNA– 2nd This new eukaryotic cell engulfed a bacteria which could use

oxygen to make ATP. This became mitochondria and/or a eukaryotic cell engulfed a photosynthetic bacteria, which eventually became a chloroplast.

– Evidence for this scenario includes• They are the same size as bacteria• They have their own DNA in a circle, like bacteria• They have their own ribosomes which are like prokaryotic ribosomes.• They carry out protein synthesis like bacteria , independently of the

cell.• They replicate by binary fission, independently of the cell.• They have a double-membrane.

Fig. 4-28

The movement of substances across membranes: a cell membrane separates a cell from its environment, carefully regulating what comes in and what goes out. Lipid membranes like the plasma membrane are semipermeable (selectively permeable). Very small polar substances, water, small ions, probably pass through pores. Some nonpolar substances, lipids dissolve in and pass through the membrane lipids. Most substances are moved through the membrane in a controlled way by specific carrier proteins. This process can be active (require energy) or passive.

Passive transport moves substances from high concentration to low concentration only – down its concentration gradient. This includes simple diffusion, facilitated diffusion and osmosis. Simple Diffusion

Simple diffusion occurs because of random movements of particles. As the molecules move randomly, they eventually reach equal concentration everywhere if there is no barrier. The net effect is that molecules diffuse from high concentration to low concentration.

Materials diffuse through small prokaryotic cells quickly, allowing nutrients in and wastes out. Eukaryotic cells are larger and have many membranes. Although many substances do diffuse in and out of the cell, much of the movement of molecules across the cell membrane is controlled by transport systems in these cells.

Fig. 4-29

Facilitated diffusion is diffusion down a concentration gradient and across a membrane with the assistance of special pores or carrier proteins. Saturation occurs when all the carriers are moving the diffusing molecules/ions as fast as they can. The rate of diffusion reaches a maximum, it can not increase further.

Many different kinds of pores and carrier proteins exist. These are specific for one or a few molecules or ions.

Remember passive transfer does not require energy from ATP.

Fig. 4-30

Osmosis is the diffusion of water molecules across a semi-permeable membrane.

b/c. The net movement of water is into the sugar solution because the concentration of water there is lower.

a. Sugar molecules can not pass through the membrane.

Fig. 4-31

The important thing for a microbiologist to know is how particles dissolved in fluid affect microorganisms.

Cells burst in hypotonic solutions and shrink in hypertonic solutions.

Bacteria have cell walls which protect them from these reactions to some degree.

Fig. 4-32

Active transport requires energy (usually ATP). Active transport uses energy to move molecules or ion against their concentration gradient. This is like moving something up a hill, it requires energy.

Active transport is important for microorganisms to move nutrients that are present in low concentrations in their environment. Membrane proteins act as carriers and enzymes. They are specific for a single or a few molecules or ions. The end result is that a gradient is set up and maintained. These carriers can be saturated.

Group translocation reactions move a substance from outside to inside a cell while modifying it at the same time. Ex. Glucose is phosphorylated and can not leave the cell.

Fig. 4-33

Eukaryotic cells move substances by forming membrane-enclosed vesicles – endocytosis and exocytosis.

Endocytosis is used for intake of liquid, small molecules, etc. For us the most important type of endocytosis is phagocytosis. This is the process in which white blood cells of our immune system surround and take in bacterium. The phagosome is then moved to and fused with a lysosome (digestive enzymes). Here the bacteria is digested. Useful molecules are moved into the cytoplasm while undigested parts are sent to the plasma membrane where the vesicle fuses and dumps its contents outside of the cell (exocytosis).