CELL STRUCTURE AND TAXONOMY

Cell is defined as the fundamental living unit of any organism. Metabolism refers to all of the chemical

reactions that occur within a cell. Deoxyribonucleic acid (DNA) is the

genetic material that makes up the genes of the cell’s

chromosomes.

Procaryotes are microorganisms that do not have the complex system of membranes and organelles.

Procaryotes include archaea, bacteria, and cyanobacteria.

Eucaryotes are organisms that have a true nucleus and many membrane-bound organelles.

Eucaryotes include algae, protozoa, fungi, plants, animals, and humans.

Acellular microorganisms (not composed of cells) include viroids, prions, and viruses.

EUCARYOTIC CELL STRUCTURE 1. Cell Membrane - encloses the cell - selectively permeable

2. Nucleus - controls the functions of the entire cell and is called the “command center” - nucleoplasm is the gelatinous matrix where the chromosomes are suspended

- the nuclear membrane encloses the nucleus - chromosomes consist of linear DNA molecules and proteins - genes are located along the DNA molecules - each gene contains the genetic information that enables the cell to produce a gene product

- the organism’s complete collection of genes is its genotype (genome) - the nucleolus is the dark area in the nucleus where ribosomal ribonucleic

acid (rRNA) is manufactured

3. Cytoplasm - semifluid, gelatinous, nutrient matrix where storage granules and organelles are suspended, including endoplasmic reticulum, ribosomes, Golgi complexes, mitochondria, centrioles, microtubules, lysosomes, and other membrane-bound vacuoles

4. Endoplasmic Reticulum (ER) - system of membranes that are interconnected and arranged to form a transport network of tubules and flattened sacs - rough endoplasmic reticulum (RER) has many ribosomes attached to its outer surface - smooth endoplasmic reticulum (SER)

has no attached ribosomes

5. Ribosomes - sites of protein synthesis

6. Golgi Complex - flattened, membranous sacs that communicates with the ER and completes the transformation of newly synthesized proteins into mature, functional ones and packages them into small vesicles for storage within the cell or export outside the cell

- Golgi complexes are referred to as “packaging plants”

7. Lysosomes - vesicles that contain lysozyme and other digestive enzymes that break down foreign material taken into the cell by phagocytosis

- these enzymes also aid in breaking down

worn out parts of the cell and may destroy the entire cell by a process called autolysis if the cell is damaged

8. Peroxisomes - vesicles that contain catalase which catalyzes the breakdown of hydrogen peroxide into water and oxygen

9. Mitochondria - sites of adenosine triphosphate (ATP) formation and are referred to as “powerhouses” of the cell

10. Plastids - sites of photosynthesis - a plastid called chloroplast contains a green, photosynthetic pigment called chlorophyll

11. Cytoskeleton - include microtubules, microfilaments, and intermediate filaments - strengthen, support, and stiffen the

cell, and give the cell its shape

12. Cell Wall - external structure that provides

rigidity, shape, and protection to the cell - the cell walls of algae and plants

contain cellulose - the cell walls of fungi contain chitin

13. Flagella - long, thin structures that are

organelles of locomotion

14. Cilia - short, thin, numerous organelles of locomotion

PROCARYOTIC CELL STRUCTURE 1. Cell Membrane - encloses the cell - selectively permeable

2. Chromosome - single, long, supercoiled, circular DNA molecule that is suspended in the cytoplasm

- a procaryotic cell contains neither nucleoplasm nor a nuclear membrane - small, circular molecules of double- stranded DNA that are not part of the chromosome (referred to as plasmids) may also be present in the cytoplasm

3. Cytoplasm - semiliquid matrix

4. Cytoplasmic Particles - most of these are ribosomes - occur also in the form of cytoplasmic granules

5. Bacterial Cell Wall - the main constituent is a polysaccharide called peptidoglycan (murein) - provides rigidity, strength, and

protection

6. Glycocalyx - slimy, gelatinous material produced by the cell membrane and secreted

outside of the cell wall - Two types: 1. Slime layer - not highly organized and not firmly attached to the cell wall

- enables certain bacteria to glide or slide along solid surfaces

2. Capsule - highly organized and firmly attached to the cell wall - serves an antiphagocytic function, protecting the encapsulated bacteria from being ingested by phagocytic white blood cells

- S-colonies are smooth, mucoid, and glistening colonies on nutrient agar produced by encapsulated bacteria - R-colonies are dry, rough colonies produced by nonencapsulated bacteria

7. Flagella - threadlike, protein appendages for locomotion - Types of bacteria based on their flagella: 1. Peritrichous bacteria - bacteria possessing flagella over their entire surface 2. Lophotrichous bacteria - bacteria with a tuft of flagella at one end

3. Amphitrichous bacteria - bacteria that have one or more flagella at each end 4. Monotrichous flagella - bacteria possessing a single polar flagellum

8. Pili (Fimbriae) - hairlike structures composed of protein molecules called pilin that arise from the cytoplasm - Two types of pili: 1. A type that enables bacteria to adhere to surfaces 2. A sex pilus that enables transfer of genetic material from one bacterial cell to another by conjugation

9. Spores (Endospores) - thick-walled structures that are a means of survival when their moisture or nutrient supply is low - the process of spore formation is called sporulation, whereby a copy of the chromosome becomes enclosed in

thick protein coats

- spores are resistant to heat, cold, drying,

and most chemicals - when the dried spore lands on a moist, nutrient-rich surface, it germinates, and a new vegetative bacterial cell emerges

PROCARYOTIC CELL REPRODUCTION Procaryotic cells reproduce by binary

fission, in which one cell (the parent cell) splits in half to become two daughter cells.

Generation time is the time it takes for binary fission to occur.

TAXONOMY Taxonomy is the science of classification of

living organisms. Taxonomy consists of three separate but

interrelated areas: 1. Classification - arrangement of organisms into taxonomic groups (taxa) on the basis of similarities or relationships

- taxa include kingdoms or domains, divisions or phyla, classes, orders, families, genera, and species

2. Nomenclature - assignment of names to the various

taxa according to international rules

3. Identification - process of determining whether an isolate belongs to one of the

established, named taxa or represents a previously unidentified species

MICROBIAL CLASSIFICATION The science of taxonomy is based on the binomial system of nomenclature

developed in the 18th century by the Swedish scientist Carolus Linnaeus.

In the binomial system, each organism is given two names. The first name is the genus, and the

second name is the specific epithet. The first and second names together are referred to as the species.

To express the genus, capitalize the first letter of the word and underline or italicize the whole word – for example, Escherichia.

To express the species, capitalize the first letter of the genus name (the specific epithet is not capitalized) and then underline or italicize the entire species name – for example, Escherichia coli.

Frequently, the genus is designated by a single letter abbreviation; for example, E. coli

indicates the species.

In an article about Escherichia coli, Escherichia would be spelled out the first time the organism is mentioned; thereafter, the abbreviated form, E. coli, could be used.

The abbreviation “sp.” is used to designate a single species, whereas the abbreviation “spp.” is used to designate more than one species.

Five-Kingdom System of Classification by Robert H. Whittaker 1. Bacteria and archaeans are in the Kingdom Procaryotae (or Monera) 2. Algae and protozoa are in the Kingdom Protista (organisms in this kingdom are referred to as protists) 3. Fungi are in the Kingdom Fungi 4. Plants are in the Kingdom Plantae 5. Animals Are in the Kingdom Animalia

Three-Domain System of Classification by Carl R. Woese 1. Domain Archaea 2. Domain Bacteria 3. Domain Eucarya