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 broad cell types: prokaryotic and eukaryotic. These categories represent the most fundamental division in cellular biology.

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 cell Labeled diagram of a bacterial cell

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 and found in a region called the nucleoid.
Ribosomes: Small (70S) ribosomes, compared to larger (80S) ribosomes in eukaryotes.
Cell Division: Divide by binary fission.

Bacterial cell with chromosome and ribosome Bacterial cell with DNA and ribosome

Comparison of Prokaryotic and Eukaryotic Cells

Prokaryotic and eukaryotic cells differ in several key aspects:

Size: Prokaryotes are generally smaller (1-10 μm) than eukaryotes (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 may use cilia, flagella, or other structures.

Eukaryotic cell diagram with organelles Table comparing domains of life: cell type, nucleus, organelles

Practice Questions

Which domains of life are classified as prokaryotes? Answer: Bacteria & Archaea
In bacteria, DNA will be found in the nucleoid.
Primary differences between prokaryotes and eukaryotes include DNA location, cell size, and presence of organelles.
All cells share features such as DNA, cell membrane, and cytoplasm, but only eukaryotes have a nucleus.

Introduction to Archaea

Characteristics of Archaea

Archaea (singular: archaeon) are one of the three domains of life. They share a prokaryotic cell structure with bacteria but have unique features.

Cell Structure: Prokaryotic, lacking a nucleus.
rRNA Sequences: Unique compared to bacteria.
Cell Wall: Lacks peptidoglycan, unlike bacteria.
Habitats: Many are extremophiles, thriving in extreme environments (e.g., high salinity, temperature, acidity), but some live in moderate environments.

Diagram showing extremophilic archaea

Practice Questions

Many species of Archaea are known as extremophiles.
Best site to obtain Archaea: 95°C hot spring in Yellowstone.

Prokaryotic Metabolism

Nutritional Factors of Microbial Growth

Microbes are classified based on three key nutritional factors: energy source, electron source, and carbon source.

Energy Source: Consumed for powering metabolic pathways.
Electron Source: Supplies electrons to the Electron Transport Chain (ETC).
Carbon Source: Supplies carbon for creating cell components.

Diagram of 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 carbon fixation.

Lithotrophs vs. Organotrophs cartoon

Carbon Source: Heterotrophs vs. Autotrophs

Microorganisms are classified by their carbon source:

Autotrophs: Use carbon fixation to capture carbon 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 the three nutritional factors. All combinations are theoretically possible, though some have no known organisms.

Table of energy, electron, and carbon sources Diagram of nutritional diversity among microbes

Practice Questions

Cyanobacteria use sunlight as their energy source and CO2 as their carbon source: Photoautotrophs.
Humans are chemoorganoheterotrophs (use organic molecules for both energy and carbon).

Summary Table: 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

Summary Table: Nutritional Types

Energy Source	Electron Source	Carbon Source	Example Organism
Phototroph	Lithotroph	Autotroph	Cyanobacteria
Chemotroph	Organotroph	Heterotroph	Humans