Prokaryotic and Eukaryotic Cells

Structure and Function

In general microbes or microorganisms may be either prokaryotic (bacteria) or eukaryotic (protists, fungi, and some animals).

However, there are some microbial organisms that appear to be intermediates between prokaryotes and eukaryotes (they possess a nucleus but do not have mitochondria or chloroplasts, an example is Giardia intestinalis.

Prokaryotes differ from eukaryotes in several ways including but not limited to:

CharacteristicsProkaryoticEukaryoticTypesbacteria (monerans)protists, fungi, plants, and animalsOrganizationunicellularusually multicellular (exception some protists)Cell sizesmall (0.1-10um)larger (10-100um)Membrane-bound organellesabsentpresentReproductionasexualasexual and sexualDNAcircularlinearProteins assoc. with DNA BasicHistonePlasma membrane No sterolsSterolsRibosomes70S80SCytoskeletonAbsentpresent

PROKARYOTIC CELLS

Key Functions of Cells

A boundary that keeps the cellular contents separate from the external environment but allows for the transfer of some substances into and out of the cell.

Replication of DNA

Synthesis of cellular components

The ability to obtain energy through metabolic processes

Appendages are basically involved in movement or adhesion

Flagella (singular flagellum) are cellular appendages that consist of three parts:a filament that rotates for movementa hook where the filament attachesa basal body that anchors the hook to the cellthe arrangement of the hook/basal body articulation allows the hook with its filament to rotate 360o

Arrangements of Flagellapolar - flagella are attached at one or both ends of the cell monotrichous - a single flagellum at one endlophotrichous - multiple flagella arising from one or both endsamphitrichous a single flagellum arises from each endperitrichous - flagella are randomly dispersed over the cell surface

Axial filaments

are modified flagella that occur in spirochetes

Non-locomotor appendagesPili are longer and sparser than fimbriae.

fimbriae are involved in adhesion and pili (found only in Gram - bacteria) are involved in conjugation ( a mating process).

Cell envelopes differ between taxa but they basically consist of three layers:

The capsule or slime layer (outermost layer) differs greatly in thickness, organization and chemical composition depending on the bacterial species. T

Beneath the outer layer lies the cell wall.

The cell membrane is a thin flexible sheet that surrounds the contents of the bacterial cell. Its functions include: transport, energy extraction, nutrient processing, and synthesis

The Gram StainAn important tool in the identification of bacteria is the Gram stain. Some bacteria are Gram + while others are Gram - (some however, do not show a Gram reaction). Gram + bacteria stain purple (the color of the primary dye) and Gram - bacteria stain pink (the color of the counterstain or second dye). The Gram stain is a differential staining technique because different species of bacteria stain differently. The difference is a result of the composition of the cell wall.

The protoplasm or cytoplasmis the dense gelatinous solution within the cell membrane that is the primary site for the cells biochemical and synthetic processes. The following are found within the protoplasm of the prokaryotic cell:

chromatin body or the bacterial chromosome

nucleoid or nuclear region of the cell that is associated with the chromatin body

plasmids are tiny circular extrachromosomal strands of DNA

ribosomes are small structures consisting of RNA and proteins that are involved in protein synthesis

inclusions or granules are areas where nutrients are concentrated

Endospores are dormant structures produced by some species of Bacillus and Clostridium.

Shapes and arrangements of bacteriaThere are six common shapes of bacteria: coccus, bacillus, coccobacillus, vibrio, spirochete, and spirullum and there are several arrangements of these cells: single, chains (strepto-), clusters (staphylo-), pairs (diplo-) etc.

Eukaryotic Cell

Cell MembraneCell membraneStructureComponentsArrangement Functions includeBarrierTransport (know diffusion, osmosis, facilitated diffusion and active transport)Recognition (e.g., self vs. non-self)Reception (for protein hormones)Adhesion

NucleusStructure and Functionmembrane similar to cell membrane (similar function)Nucleolus (formation of ribosomes)Chromosomes (gene expression)Nucleoplasm (matrix)

RibosomesStructurerRNAProteinsFunctionSite of protein formation (translation)Found in both prokaryotes and eukaryotes (different structurally)

Endoplasmic ReticulumStructure membranous system of tunnels and sacsRough with ribosomes on surfaceSmooth- no ribosomes on surfaceFunctionRough protein synthesisSmooth- lipid synthesis

Golgi ApparatusStructure also membranous, kind of like a stack of pancakes

Function processing of lipids and proteins

Lysosomes

Structure membrane bound sac containing hydrolytic enzymes

Function digestion

MitochondriaStructure cigar-shaped, double membrane-bound organelleFunction Energy transfer by ATP synthesis

ChloroplastStructureAlso cigar or spindle shaped, double membrane-bound, greenFunctionSite of photosynthesis

OTHER STUCTURESCell walls, not in animal cellsVacoulesCytoskeletonCytoplasm