Cell Biology: Prokaryotic and Eukaryotic Cells

Cell Definition and Characteristics

• A cell is the smallest membrane-enclosed unit of life built with macromolecules.
• Cells possess characteristics such as:
  • Responsiveness: Reacting to environmental stimuli.
  • Growth and Reproduction: Enlarging and making copies of themselves using nutrients.
  • Metabolism: Taking in or producing nutrients for survival through chemical reactions.
  • Homeostasis: Maintaining a constant internal environment.
  • Organization: Arranging atoms into molecules and macromolecules (carbohydrates, proteins, nucleic acids, and lipids).

Uniformity and Diversity Among Cells

• Cells exhibit both uniformity (shared characteristics) and diversity (different structures and functions).
• Diversity includes variations in shapes, sizes, nutritional requirements, aerobic/anaerobic nature, movement behavior, and tolerance to extreme temperatures or environments.
• Despite diversity, all cells share a basic chemistry, being built upon the same macromolecules: carbohydrates, lipids, nucleic acids, and proteins.
• All cells utilize the central dogma of biology: DNA is transcribed into RNA, which is then translated into proteins.
• All cells descend from a previous cell, indicating relatedness and forming the basis for evolutionary trees.
• A cell's genome dictates its proteins, structure, functions, and behavior.
• Mutations in the genome can lead to changes in a cell's structure, function, or behavior over time.

Evolutionary Trees and Major Cell Types

• Evolutionary trees illustrate an ancestral cell from which all other cells have diverged.
• The trees show two major branches: prokaryotic (bacteria) and eukaryotic cells.
• Prokaryotic cells: Generally smaller than eukaryotic cells.
• Eukaryotic cells: Contain numerous internal structures.

Prokaryotic vs. Eukaryotic Cells

• Prokaryotic Cells
  • Unicellular organisms.
  • Small and simple.
  • Most abundant cell type.
• Eukaryotic Cells
  • Large and complex.
  • Can be unicellular or multicellular.
  • Found in plants, animals, and fungi.
• Eukaryotic Cell Structures
  • Nucleus: Contains and protects DNA.
  • Organelles: Membrane-bound structures with specific functions.
• Prokaryotic Cell Structures
  • No nucleus; DNA is in the cytoplasm, often circular (plasmid).
• Commonalities:
  • Both cell types are built from the same macromolecules (carbohydrates, lipids, nucleic acids).
  • Cytosol.
  • Chromosomes (DNA).
  • Ribosomes (for protein synthesis).

Detailed Look at Prokaryotic Cells

• Simplified cell type, smaller in size, and unicellular.
• Found in almost any environment and can be virulent or pathogenic.
• Size ranges from 2 to 10 micrometers in length $(\mu m)$.
• Outer layers:
  • Cell Wall: Semi-rigid peptidoglycan layer that provides shape, attachment for flagella, and prevents rupturing from osmotic pressure changes.
  • Glycocalyx: Sugar-protein coating that provides virulence by preventing attack from the host's immune system.
• DNA in chromosomes, sometimes circular (plasmids).
• Appendages:
  • Flagella: Long, whiplike extensions for locomotion; a cell may have one to a few.
  • Fimbrae: Short, hair-like extensions for attachment, not locomotion.

Prokaryotic Cell Shapes

• Rod Shape: E. Coli bacterium, approximately 2.5 $(\mu m)$ long, with DNA distributed in the cytoplasm.
• Basic Morphological Categories:
  • Cocci: Spherical or ball-shaped cells.
  • Spirulia: Helical or corkscrew-shaped structures.
  • Verbio: Comma-shaped or curved rod.
• Archaea: known as extremophiles, found in harsh environments due to newer branch, that shares the same characteristics as prokaryotes but can survive in extreme temperatures, oxygen-poor environments, freezing pools, or cows stomachs.

Eukaryotic Cells

• Give rise to single and multicellular organisms, including plants, animals, fungi, and protists.
• Animalia (Animals):
  • Multicellular, with cells forming tissues and organs.
  • Heterotrophic, consuming nutrients from the environment.
  • Certain cells have motility.
  • Do not have a regular shape.
• Planete (Plants):
  • Carry out photosynthesis, converting light energy to chemical energy.
  • Autotrophic, using inorganic materials and sunlight to generate energy.
  • Cell wall made of cellulose (not peptidoglycan).
  • Specialized organelles: plastids (storage vesicles), central vacuole (for turgor), and chloroplasts (with chlorophyll for photosynthesis).
• Fungi:
  • Include mushrooms, molds, and unicellular fungi like yeast.
  • Most pathogenic species attack plants.
  • Eukaryotic cells with a nucleus.
  • Cell wall contains chitin, a glucose derivative.
  • Saprotrophic, externally digesting nutrients and absorbing them from decaying matter.
• Protista (Protists):
  • Catch-all kingdom for diverse unicellular organisms with plant-like or animal-like characteristics.
  • Some are heterotrophic, others autotrophic.
  • Most live in water or as parasites.
  • High mobility.
  • Examples: euglena (with an eye spot for light detection), amoebas (with pseudopods for movement), and paramecium (with an oral groove for channeling food).

Viruses

• Non-living infectious microbes that replicate within a host cell.
• Do not reproduce on their own, take up or produce nutrients, or have metabolism.
• Basic structure: genetic material (DNA or RNA), capsid (protein coat), and sometimes a viral envelope (membrane with host cell components).
• Need a host cell to replicate.
• Lack organelles and the characteristics of life.

Comparison of Characteristics of Life