Unit 1: Historical background of Cell Biology

Broad Classification of Cell Types (Prokaryotes and Eukaryotes)

The most fundamental division of life is based on cell structure. All living organisms are classified into two main groups: Prokaryotes and Eukaryotes.

Prokaryotic Cells

From Greek, pro- (before) and -karyon (kernel or nucleus). These are structurally simpler and smaller than eukaryotic cells.

• No Nucleus: Genetic material (a single circular DNA molecule) is located in a region called the nucleoid, which is not enclosed by a membrane.
• Organelles: Lack membrane-bound organelles (like mitochondria, ER, Golgi).
• Ribosomes: Have 70S ribosomes, which are smaller than those in eukaryotes.
• Cell Wall: Most have a cell wall, typically made of peptidoglycan (in bacteria).
• Examples: Bacteria (Eubacteria) and Archaea (Archaebacteria).

Eukaryotic Cells

From Greek, eu- (true) and -karyon (kernel or nucleus). These cells are larger, more complex, and have a "true" nucleus.

• True Nucleus: Genetic material (multiple linear DNA molecules as chromosomes) is enclosed within a double-membraned nucleus.
• Organelles: Possess numerous membrane-bound organelles (e.g., mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus, lysosomes).
• Ribosomes: Have 80S ribosomes (in the cytosol and on the RER).
• Cell Wall: If present, it's made of cellulose (in plants and algae) or chitin (in fungi). Animal cells lack a cell wall.
• Examples: Plants, Animals, Fungi, and Protists.

Comparison of Prokaryotic and Eukaryotic Cells

Cell as Basic Unit of Life & Cell Theory

The concept of the cell as the basic unit of life is the foundation of modern biology. This idea was formalized into the Cell Theory by Matthias Schleiden and Theodor Schwann (1839), and later modified by Rudolf Virchow (1855).

The Three Tenets of Classical Cell Theory:

1. All living organisms are composed of one or more cells.
2. The cell is the basic structural and functional unit of life.
3. All cells arise from pre-existing cells. (Virchow's contribution: Omnis cellula e cellula)

Modern additions to the Cell Theory include:

• Cells contain hereditary information (DNA) which is passed from cell to cell during division.
• All cells are basically the same in chemical composition and metabolic activities.
• Energy flow (metabolism and biochemistry) occurs within cells.

Pre-cellular Evolution and Artificial Creation of Cell

Pre-cellular Evolution

This field explores how non-living matter could have given rise to the first living cells. Key concepts include:

• Chemical Evolution: The idea that simple organic molecules (like amino acids, nucleotides) formed from inorganic precursors on early Earth (e.g., Miller-Urey experiment).
• Polymerization: These simple monomers joined to form complex polymers (proteins, nucleic acids).
• Protocells: These polymers became enclosed within a lipid membrane, forming a "protocell" (or coacervate). This structure could separate its internal environment from the outside world.
• RNA World Hypothesis: The theory that RNA, not DNA, was the first genetic material. RNA can both store information (like DNA) and act as a catalyst (like an enzyme), solving the "chicken-and-egg" problem of which came first, DNA or proteins.

Artificial Creation of Cell

This refers to the field of synthetic biology. It's not about creating life from scratch (like pre-cellular evolution) but rather about building a new cell from known, existing biological parts.

A famous example is the work of J. Craig Venter, whose team synthesized an entire bacterial chromosome (genome) from chemicals in the lab and transplanted it into an empty host bacterial cell. This new "synthetic" cell booted up and began to replicate, controlled entirely by the lab-made DNA.

Characteristic Features of Cell Types (Acellular & Cellular)

This section covers both true cells and acellular (non-cellular) infectious agents.

Cellular (Prokaryotic)

• Archaea bacteria (Archaea): These are prokaryotes that are biochemically and genetically distinct from bacteria. They are often **extremophiles**, living in harsh conditions. Their cell walls lack peptidoglycan, and their cell membranes have unique lipids.
• Eubacteria (Bacteria): These are the "true" bacteria. They are found everywhere and have cell walls made of peptidoglycan. This group includes most of the familiar bacteria, like E. coli and Staphylococcus.
• Mycoplasma (PPLO): These are a type of bacteria that lack a cell wall.
  • They are the smallest known free-living cells.
  • Because they have no cell wall, they are pleomorphic (can change their shape).
  • The term PPLO stands for Pleuropneumonia-like organisms, as they were first identified as causing lung disease in cattle.

Acellular (Non-cellular)

These are infectious agents that are not cells. They lack cytoplasm, organelles, and the ability to reproduce on their own (they are obligate intracellular parasites).

• Viroids:
  • Structure: Just a short, circular, single-stranded RNA molecule.
  • No Protein: They have no protein coat (capsid).
  • Host: They are known to infect plants only (e.g., Potato spindle tuber viroid).
• Prions:
  • Structure: "Proteinaceous infectious particle." They are a misfolded version of a normal host protein (PrP).
  • No Nucleic Acid: They contain no DNA or RNA.
  • Mechanism: The misfolded prion protein (PrP-Sc) induces other normal PrP proteins to misfold, creating a chain reaction that destroys nerve tissue.
  • Host: They cause fatal neurodegenerative diseases in mammals (e.g., Mad Cow Disease in cattle, Creutzfeldt-Jakob Disease (CJD) in humans).

Eukaryotic Microbes

These are microorganisms (microbes) that have a eukaryotic cell structure. They are a diverse group, often single-celled, and form the kingdom Protista, as well as including some fungi.

• Protozoa: Animal-like protists (e.g., Amoeba, Paramecium).
• Algae: Plant-like protists (e.g., Chlamydomonas, Diatoms).
• Fungi (Microscopic): E.g., Ex: Yeast (which is single-celled).
• Slime Molds: Fungi-like protists.

Ecological Amplitude of Cell (Extremophiles)

Ecological Amplitude refers to the range of environmental conditions that an organism (or cell) can tolerate and survive in.

Cells that thrive in extreme environments are called Extremophiles. These are most often Archaea but can also be bacteria or other microbes.

• High Altitude: Tolerate low oxygen, low temperature, and high UV radiation.
• Arctic (Psychrophiles): "Cold-loving." Grow and reproduce at very low temperatures (e.g., near 0°C).
• Hot Spring (Thermophiles): "Heat-loving." Thrive at high temperatures (50-80°C).
  • Hyperthermophiles: Thrive at extreme high temperatures (80-120°C), like in deep-sea hydrothermal vents.
• Arid (Xerophiles): "Dry-loving." Survive in environments with very low water availability.
• Brackish (Halotolerant): Tolerate a wide range of salt concentrations (salinity).
• Freshwater (Hypotonic): Have adaptations (like contractile vacuoles in Paramecium) to prevent bursting from water influx (osmosis).