Cell Structure, Classification, and the Domains of Life

Warm-Up Activity: Consider the number of cells in the human body.
- The average human body is composed of approximately $100 \text{ trillion}$ ( $100,000,000,000,000$ ) cells.
- However, there are even more bacterial cells living within a human body than the body's own cells.
Cellularity Spectrum:
- Some organisms are unicellular, meaning they are composed of just one cell (e.g., bacteria).
- Other organisms are multicellular, meaning they are composed of trillions of cells (e.g., plants, animals, humans).

Life is broadly classified into a hierarchical system, starting with three fundamental Domains.
Domains:
- Archaea
- Eukaryota (or Eukarya)
- Bacteria
Hierarchical Classification (from broadest to most specific):
- Life
- Domain
- Kingdom
- Phylum
- Class
- Order
- Family
- Genus
- Species

Universal Principle: All organisms (living things) are made of cells.
Cellular Composition:
- Most familiar organisms are multicellular, composed of many cells.
- Some organisms are unicellular, made up of only one cell, which constitutes their entire, tiny body.
Diversity of Life: The vast diversity of life is organized into the three domains: Bacteria, Archaea, and Eukarya.
- Prokaryotic Domains: Bacteria and Archaea.
- Eukaryotic Domain: Eukarya.

Domains Bacteria and Archaea: These domains consist of prokaryotic organisms.
- Represented by microscopic images (e.g., SEM at $3,250 \times$ and $25,000 \times$ magnification).
Domain Eukarya: This domain includes organisms composed of eukaryotic cells.
- Protists: A diverse group including protozoans and algae, categorized into multiple kingdoms.
- Kingdom Fungi: (e.g., represented by a mushroom, $275 \times$ magnification on Figure 1.5C).
- Kingdom Plantae: (e.g., represented by a plant, Figure 1.5C).
- Kingdom Animalia: (e.g., represented by an animal, Figure 1.5C).

Every organism on the planet, including humans, falls into one of two fundamental cell categories:
- Prokaryotes (Prokaryotic Cells):
  - Lack membrane-enclosed organelles.
  - Do not contain a DNA-containing nucleus.
  - Mnemonic: "Pro means NO!" (no true nucleus, no membrane-bound organelles).
- Eukaryotes (Eukaryotic Cells):
  - Contain membrane-enclosed organelles.
  - Possess a distinct DNA-containing nucleus.
  - Mnemonic: "Eu means True" (have a true nucleus).
Visual Distinction: Eukaryotic cells shown with a nucleus (containing DNA) and organelles, while Prokaryotic cells show DNA without a nucleus, and no organelles (e.g., Figure 1.3, $25,000 \times$ magnification).

Definition: Unicellular microorganisms.
Key Structural Features:
- Lack a distinct nucleus.
- Lack membrane-bound organelles.
- DNA is free-floating in the cytoplasm (often in a region called the nucleoid).
Classification: Always bacteria (or archaea).
Complexity & Size: Very simple and small.
Typical Components (from diagrams):
- Fimbriae
- Nucleoid (contains the bacterial chromosome, DNA)
- Ribosomes
- Plasma membrane
- Cell wall
- Glycocalyx (capsule)
- Flagella (for motility)
- Cytoplasm

Definition: Can be unicellular or multicellular organisms.
Key Structural Features:
- Have a distinct nucleus.
- Have membrane-bound organelles.
- DNA is enclosed within the nucleus.
Complexity & Size: Larger and more complex (contain more internal structures).
Examples: Animals, plants, fungi, and protists.
Exclusion: Never bacteria.
Members of Domain Eukarya: Protists (protozoans and algae, falling into multiple kingdoms), Fungi, Plantae, and Animalia.
Role of Organelles: Like organs in the human body, each organelle within a eukaryotic cell performs a specific function. These functions collectively help the cell maintain homeostasis (a state of balance).
Typical Components (from diagrams - Animal Cell vs. Plant Cell):
- Nucleus
- Cytoplasm
- Ribosomes
- Cell membrane
- Mitochondrion
- Plant Cell Specific: Cell wall of cellulose, Chloroplast, Vacuole

Both possess DNA (Deoxyribonucleic Acid) as their genetic material.
Both have a cell membrane, a lipid (fat) layer that surrounds the cell, protecting it from the external environment and controlling substance entry and exit.
Both contain similar basic biological molecules:
- Carbohydrates
- Proteins
- Nucleic acids
- Minerals
- Fats (lipids)
- Vitamins

Feature	Prokaryotic Cell	Eukaryotic Cell
Nucleus	No nucleus	Has a nucleus
Organelles	No membrane-bound organelles	Has membrane-bound organelles
DNA Location	DNA is free-floating	DNA is in the nucleus
Cellularity	Always unicellular	Can be unicellular or multicellular
Examples	Always bacteria	Plants, Animals, Fungi, Protists
Size	Smaller than Eukaryote	Much larger than Prokaryote
Reproduction	Asexual (exact copies/clones)	Can be asexual (clones) or sexual (variations)
Age	Older (around $3.5 \text{ billion years}$ old)	Younger (around $1.5 \text{ billion years}$ old)

Size Dimensionally: Prokaryotes are generally about $10 \text{X}$ ( $10$ times) smaller than eukaryotes.
- Prokaryotic Cell size: $0.1-10 \text{ μm}$
- Eukaryotic Cell size: $10-100 \text{ μm}$

Genetic Information Storage: Almost all cells contain the genetic information (DNA) essential for the survival of the cell and, in multicellular organisms, the entire organism.
Eukaryotes' Nucleus: Eukaryotic cells separate this genetic information from the rest of the cell's cytoplasm within a specialized membrane-bound organelle called the nucleus.
- The term "Eukaryote" literally means "having a true nucleus."
Prokaryotes' DNA: Prokaryotic cells do NOT have a nucleus, so their genetic material is not physically separated from the rest of the cell's internal contents.
- The term "Prokaryote" literally means "before a nucleus."

Defining Life: Scientists use several criteria to define life, including:
- Metabolism
- Reproduction
- Response to stimuli
- Evolution
How Viruses Fit/Don't Fit the Criteria:
- Viruses DO:
  - Have genetic material (either DNA or RNA).
  - Evolve through natural selection.
- Viruses DO NOT:
  - Have a cellular structure (they are acellular).
  - Grow or respond independently to environmental stimuli.
  - Reproduce on their own; they must hijack a host cell to replicate.
  - Carry out metabolism (they do not eat, use oxygen, convert energy, or produce waste by themselves).
Discussion Questions Regarding Viruses and Life:
- If viruses cannot reproduce independently but can do so inside a host cell, does this qualify as being alive?
- Is the ability to evolve sufficient to classify something as living?
- If viruses do not use or produce energy, how does this differentiate them from non-living, abiotic factors (such as rocks) that also do not use or produce energy?