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Prokaryotes, Cell Types, and Prokaryotic Metabolism: Structured Study Notes

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Introduction to Prokaryotes

Prokaryotic and Eukaryotic Cells

All living organisms are classified into two fundamental cell types: prokaryotic and eukaryotic. These categories represent the broadest and most distinct groupings of life.

  • Prokaryotic Cells: Do not have a nucleus. Includes both Bacteria and Archaea.

  • Eukaryotic Cells: Have a nucleus and other membrane-bound organelles.

  • Example: Prokaryotic cells include bacteria and archaea; eukaryotic cells include plants, animals, fungi, and protists.

Diagram of a prokaryotic cellLabeled structure of a bacterial cell

Domains of Life and Cell Classification

The three domains of life are Bacteria, Archaea, and Eukarya. Bacteria and Archaea are classified as prokaryotes, while Eukarya are eukaryotes.

  • Bacteria: Prokaryotic, no nucleus, no membrane-bound organelles.

  • Archaea: Prokaryotic, no nucleus, no membrane-bound organelles.

  • Eukarya: Eukaryotic, nucleus present, membrane-bound organelles present.

Table comparing domains of life: cell type, nucleus, organelles

Features of Bacterial Cells

Bacteria are the most abundant and diverse organisms on Earth. Their cellular structure is distinct from eukaryotes.

  • Bacterial DNA: Circular in shape, found in a region called the nucleoid.

  • Ribosomes: Small (70S), responsible for protein synthesis.

  • Cell Division: Occurs by binary fission.

Bacterial cell with chromosome and ribosomeBacterial cell with DNA and ribosome

Prokaryotic vs. Eukaryotic Cells: Key Differences

Prokaryotic and eukaryotic cells differ in several fundamental ways:

  • Cell Size: Prokaryotes are smaller (1-10 μm); eukaryotes are larger (10-100 μm).

  • Nucleus: Prokaryotes lack a nucleus; eukaryotes have a nucleus.

  • Organelles: Prokaryotes lack membrane-bound organelles; eukaryotes possess them.

  • Ribosome Size: Prokaryotes have 70S ribosomes; eukaryotes have 80S ribosomes.

  • Motility: Prokaryotes may use flagella; eukaryotes have more complex motility structures.

Prokaryote vs. Eukaryote cell comparisonDomains of life: cell type, size, nucleus, organelles, cellularity

Introduction to Archaea

Archaea (singular: archaeon) are prokaryotic organisms with unique features distinguishing them from bacteria.

  • Archaea have unique ribosomal RNA (rRNA) sequences.

  • Cell walls lack peptidoglycan, unlike bacteria.

  • Many archaea are extremophiles, thriving in extreme environments (e.g., high salinity, temperature, acidity).

Archaea in extreme environments

Prokaryotic Metabolism

Nutritional Factors of Microbial Growth

Microbes are classified based on three key nutritional factors:

  • Energy Source: The source of energy used for metabolic pathways.

  • Electron Source: The molecule supplying electrons to the Electron Transport Chain (ETC).

  • Carbon Source: The molecule supplying carbon for cell components.

Nutritional factors of microbial growth

Energy Source: Phototrophs vs. Chemotrophs

Organisms are classified by their energy source:

  • Phototrophs: Obtain energy from sunlight.

  • Chemotrophs: Obtain energy from chemical compounds.

Phototrophs vs. Chemotrophs cartoon

Electron Source: Lithotrophs vs. Organotrophs

Organisms are categorized by their electron source:

  • Lithotrophs: Supply ETC with electrons from reduced inorganic molecules (e.g., H2O, Fe2+).

  • Organotrophs: Supply ETC with electrons from organic molecules (e.g., glucose).

  • Example: Plants are lithotrophs because they harvest electrons from splitting water during photosynthesis.

Lithotrophs vs. Organotrophs cartoon

Carbon Source: Heterotrophs vs. Autotrophs

Microorganisms are classified by their carbon source:

  • Autotrophs: Use carbon fixation to capture carbon (CO2) for making their own food and cell components.

  • Heterotrophs: Consume and use organic molecules to supply carbon for cell components.

Autotrophs vs. Heterotrophs cartoon

Nutritional Diversity Among Microbes

Microbes are categorized based on combinations of energy, electron, and carbon sources. All combinations are theoretically possible, though some have no known organisms.

Energy Source

Electron Source

Carbon Source

Example Organism

Sunlight (Phototroph)

Inorganic (Lithotroph)

CO2 (Autotroph)

Cyanobacteria

Chemicals (Chemotroph)

Organic (Organotroph)

Organic (Heterotroph)

Humans

Sunlight (Phototroph)

Organic (Organotroph)

Organic (Heterotroph)

Some photosynthetic bacteria

Chemicals (Chemotroph)

Inorganic (Lithotroph)

CO2 (Autotroph)

Nitrifying bacteria

Sunlight (Phototroph)

Inorganic (Lithotroph)

Organic (Heterotroph)

Rare/unknown

Table summarizing nutritional factors

Summary Table: Cell Types and Domains of Life

Domain

Cell Type

Nucleus

Organelles

Cell Size

Cellularity

Bacteria

Prokaryotic

Absent

Absent

Small (1-10 μm)

Unicellular

Archaea

Prokaryotic

Absent

Absent

Small (1-10 μm)

Unicellular

Eukarya

Eukaryotic

Present

Present

Large (10-100 μm)

Unicellular or multicellular

Domains of life: cell type, size, nucleus, organelles, cellularity

Practice Questions and Applications

  • Which domains of life are classified as prokaryotes? Bacteria and Archaea.

  • Where is DNA found in bacteria? In the nucleoid region.

  • What is a primary difference between prokaryotes and eukaryotes? Prokaryotes have DNA that floats freely in the cytoplasm and are significantly smaller.

  • Which feature is not common to all cells? Presence of a nucleus (only eukaryotes have a nucleus).

Additional info: These notes expand on the original content by providing definitions, examples, and structured tables for clarity and completeness.

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