Cell evolution/origin and features

Common features of all cells

  • There are four things that every cell has in common, regardless of whether it’s in your body, in a tree, or a bacterium on a table:

    • Cytoplasm

    • Ribosomes

    • Genetic material (typically DNA)

    • Plasma membrane

  • Cytoplasm- The fluid-like filling inside the cell, often referred to as the cytosol, which is primarily water, ions, and macromolecules.

    • Holds organelles in place, suspending them within the cell.

    • A major site for biochemical reactions, including glycolysis and many metabolic pathways.

  • Ribosomes- Specialized molecular machines whose sole job is to synthesize proteins through a process called translation.

    • Proteins carry out virtually all cellular functions, acting as enzymes, structural components, transport molecules, and signaling agents.

    • Composed of ribosomal RNA (rRNA) and proteins, found in both prokaryotes and eukaryotes.

  • Genetic material (DNA)- Provides the complete set of instructions for making all the proteins and running all cellular processes.

    • In its stable double-helix form, DNA stores the hereditary information passed from one generation to the next.

  • Plasma membrane- The selectively permeable boundary that separates the internal cellular environment from the external world.

    • Regulates what gets into and out of the cell through various transport mechanisms (e.g., passive diffusion, active transport, facilitated diffusion).

    • Composed of a phospholipid bilayer with embedded proteins, providing flexibility and dynamic functionality.

Prokaryotic cells: overview and basic features

  • Prokaryotic cells fall into two main categories: archaea and bacteria; this lecture focuses primarily on bacteria (archaea are not addressed further here).

  • Key characteristics of prokaryotes:

    • Do not have a membrane-bound nucleus; their genetic material is located in a specific region of the cytoplasm.

    • Very small and simple compared to eukaryotic cells, lacking complex membrane-bound organelles.

    • Typical size of bacteria: 1-10\ \mu m in diameter.

    • Eukaryotic cells are significantly larger, typically many times bigger: roughly 10-100\ \mu m in diameter, making them approximately 10-15\times larger than most prokaryotic cells.

  • Fundamental components shared with other cells (despite being prokaryotic):

    • Cytoplasm, ribosomes, genetic material (DNA), plasma membrane—all essential for basic life functions.

  • How prokaryotes store DNA:

    • DNA is free-floating in a region called the nucleoid, which is a non-membrane-bound area where the bacterial chromosome aggregates.

    • In addition to the main chromosome, many bacteria also contain plasmids, small circular DNA molecules that carry non-essential but often beneficial genes (e.g., antibiotic resistance).

  • Cell wall and glycocalyx:

    • Most bacteria have a rigid cell wall, external to the plasma membrane, primarily made of peptidoglycan (a polymer of sugars and amino acids). This wall provides structural support and protection against osmotic lysis.

    • Outside the cell wall is a glycoprotein-rich layer called the glycocalyx. It forms a waxy, waterproof coating that helps prevent desiccation (drying out) and aids in adhesion to surfaces and other cells.

      • The glycocalyx can be a well-organized capsule (tightly attached) or a looser slime layer.

  • Surface structures and movement:

    • Pili (or pilus): Thin, hair-like appendages made of protein. They are used to connect cells, facilitating the exchange of DNA/nucleic acids in a process called conjugation. Pili also enable adhesion to host tissues.

    • Cilia and flagella for motion: Flagella are common in many bacteria; they are long, whip-like appendages that rotate like a propeller to drive the cell through liquid environments. The bacterial flagellum is a complex structure powered by a proton motive force.

      • While the term 'cilia' is more often associated with eukaryotic cells, some bacterial structures or terms have been historically used in a broader sense for motility.

  • Environmental interactions and common misconceptions:

    • Bacteria are remarkably adaptable and can survive on surfaces and in various extreme environments. Understanding their resilience affects disinfection strategies.

    • Disinfection that kills bacteria does not necessarily kill viruses (e.g., Clorox wipes are effective against bacteria but not all viruses, which have different structures and replication mechanisms).

    • Waterborne bacteria vary significantly by geographical region; exposure to different strains can impact human digestion and health if ingested, as the immune system may not be accustomed to them.

  • Microbiome and host interaction:

    • The human digestive tract hosts a vast and diverse population of bacteria, collectively known as the gut microbiome.

    • The microbiome is the collective community of all microorganisms (bacteria, archaea, fungi, viruses) within a particular environment, especially within the human body.

    • In the human body, there are more prokaryotic cells than eukaryotic cells, with an approximate ratio of \frac{N{\text{prok}}}{N{\text{euk}}}\approx 2:1 to 10:1.

    • The gut (stomach and intestines) is a major reservoir for these prokaryotes, playing crucial roles in digestion, vitamin synthesis, and immune system development.

  • Crohn’s disease and fecal transplants (clinical relevance):

    • Crohn’s disease is a chronic inflammatory condition primarily affecting the lining of the intestinal tract, often linked to imbalances in the gut microbiome.

    • Fecal microbiota transplantation (FMT) has been used as a therapeutic approach to introduce a healthier and more balanced bacterial population, aiming to reverse some symptoms by replacing dysbiotic (harmful) bacteria with more benign and beneficial ones.

  • Real-world example: microbiome and exposure to different bacteria:

    • People moving between different environments (e.g., from a parent’s house to a university campus) can experience significant shifts in their gut microbiome composition due to changes in diet, exposure to new bacterial populations, and varying environmental factors.

  • Notable takeaway about bacteria vs viruses:

    • Bacteria are living, single-celled organisms with their own metabolism, containing DNA, ribosomes, cytoplasm, and a plasma membrane. They can reproduce independently.

    • Viruses are not cells; they are obligate intracellular parasites, consisting of genetic material (DNA or RNA) enclosed in a protein coat. They lack ribosomes and cytoplasm and must hijack host cell machinery to replicate. This fundamental difference dictates different requirements for their disinfection and treatment.

The membrane and immune recognition

  • The plasma membrane is more than a passive boundary; it is a dynamic structure containing a diverse array of proteins with crucial roles in communication, transport, and recognition.

    • Channel proteins: Specific integral membrane proteins that form hydrophilic pores, allowing particular ions or small molecules to cross the membrane down their concentration gradient.

    • Enzymes: Membrane-bound enzymes catalyze specific chemical reactions at or near the membrane surface, often participating in signaling pathways or metabolic processes.

    • Receptor proteins: Bind specific signaling molecules (ligands) and transmit signals into the cell, initiating a cellular response.

    • Membrane proteins often have sugars attached (glycosylation), forming glycoproteins or glycolipids on the cell surface. This glycosylation creates a unique 'sugar coat' or glycocalyx (distinct from bacterial glycocalyx) on eukaryotic cells.

  • Immune recognition and self-markers:

    • These sugar molecules on the cell surface act like unique identifiers or molecular 'fingerprints' that help the immune system exquisitely distinguish