Prokaryotes

Prokaryotes

• First fossil evidence approx. 3.5 Billion Years Ago (BYA)
• First Unicellular Organism: Single celled organism
• Polyphyletic group
• Archaea and Eukarya are sister to Bacteria
• Possibly a bad classification
• Earliest fossils are Stromatolites: Rock-like layers of bacterial mats and sediment

Photo by Paul Harrison | CC BY

Structure of Prokaryotes (Bacteria)

Simple

• No nucleus
• DNA is circular
• Also may have plasmids: Additional DNA
• Specialized functions (antibiotic resistance)
• No membrane-enclosed organelles
• No mitochondria
• All have cell walls
• Peptidoglycogen: modified sugars used for cell wall
• not cellulose (used by plants)
• not chitin (used by fungi)
• Used for shape and protection
• prevents cell from bursting in hypotonic solution (a solution with lower concentration of ions than the cell)
• Absent in Archaea
• Prokaryotes are small
• size limited by diffusion
• all nutrients must diffuse passively across the cell wall
• Small cells have greater Surface Area to Volume ratio
• Take up nutrients more rapidly and efficiently

Photo by Ali Zifan | CC BY

Prokaryotic Reproduction

• Binary Fission: form of asexual reproduction, one cell splits into two
• Reproduce rapidly
• Short generation time
• High mutation rate
• Increases genetic diversity
• Exchange genetic information through horizontal gene transfer
• Horizontal gene transfer: absorb DNA from the environment for genetic recombination
• Transformation: uptake of foreign DNA from the environment (dead bacteria)
• Transduction: bacterial viruses (bacteriophage) insert new DNA from past hosts
• Conjugation: transfer of DNA directly from one prokaryote to another
• DNA from donor is transferred to recipient cell using the pilus
• Unidirectional
• Often transfer plasmids
• Frequently used to transfer antibiotic resistance

Photo by JWSchmidt | CC BY

How Organisms Obtain Nutrients

• All organisms fall into the following categories:

• Phototrophs: energy originates from the sun
• Photo-auto-trophs: produce energy directly from the sun (like photosynthesis)
• Carbon comes from CO2
• Photo-hetero-trophs: produce energy directly from the sun (like photosynthesis)
• Rare
• Carbon comes from organic compounds ( not CO2)
• This is a rare metabolic trait found in few prokaryotes

• Chemotrophs: energy comes from chemicals (not the sun)
• Chemo-auto-trophs: produce energy directly from chemicals in the environment
• Rare
• Carbon comes from CO2
• Chemo-hetero-troph: Get energy by consuming other organisms
• Carbon comes from organic compounds