Cell Structure and Taxonomy

Cell Structure and Taxonomy

• Cells are the fundamental units of all organisms.
• Cells exhibit basic characteristics of life, including metabolism (all chemical reactions in a cell), growth and reproduction, response to stimuli (light, heat, cold, and chemicals), and mutations.
• Robert Hooke invented the compound microscope and coined the term "cell."
• Matthias Schleiden (botanist) proposed plant cell theory.
• Theodor Schwann (zoologist) proposed animal cell theory.
• Rudolf Virchow proposed biogenesis (the idea that cells come from other cells).
• Eukaryotes have a true nucleus and complex membrane-bound organelles; examples include algae, protozoa, fungi, plants, animals, and humans.
• Prokaryotes lack membrane-bound organelles and a true nucleus.
  • Bacteria and archaea are examples.
• Prokaryotes are 10x smaller than eukaryotes.

Eukaryotic Cell Structure

• Cell membrane (AKA plasma or cytoplasmic membrane) regulates the passage of nutrients, waste, and secretions into and out of the cell.
• The cell membrane is a mosaic of proteins and phospholipids.
• Nucleus: controls all cellular functions.
  • Parts include chromosomes (DNA and histones), nucleoplasm (gelatinous matrix), nuclear membrane, nuclear pores, nucleolus (rRNA production).
• Cytoplasm is a semi-fluid, gelatinous, nutrient matrix containing organelles.
• Cytosol is cytoplasm without the organelles or storage granules.
• The endoplasmic reticulum is a highly convoluted system of interconnected membranes (rough and smooth).
  • Rough ER has ribosomes for protein synthesis (40, 60, and 80S - Svedberg units).
• Ribosomes synthesize proteins.
• Golgi complex packages proteins into membrane-enclosed vesicles for storage or export.
• Lysosomes contain lysozyme and break down foreign material; autolysis is the process where the cell breaks down internal components.
• Peroxisomes degrade hydrogen peroxide and contain catalase (2H₂O₂ → 2H₂O + O₂).
• Mitochondria produce adenosine triphosphate (ATP).
• Plastids are energy-producing organelles in plants and algae; chloroplasts are involved in photosynthesis (CO₂ + H₂O → CHO + O₂).
• The cytoskeleton is a system of fibers throughout the cytoplasm that provide structure, cell division, contraction, motility, and chromosome movement.

Prokaryotic Cell Structure

• Cell membrane, cell wall, and glycocalyx form the prokaryotic cell envelope.
• The cell wall is thinner in gram-negative bacteria compared to gram-positive bacteria.
• Gram-positive cell walls have thick peptidoglycan and lipo/teichoic acid molecules.
• Gram-negative cell walls have thinner peptidoglycan and lipid outer membrane.
• Mycoplasma species lack a cell wall.
• Archaeans have peptidoglycan-less cell walls.
• Glycocalyx (capsule or slime layer) can be firmly or loosely attached to the cell wall; the capsule is anti-phagocytic.
• Pili or fimbriae are hair-like structures aiding in attachment and transfer of genetic material (conjugation).
• Prokaryotic cells contain a nucleoid (DNA-occupied area in the cytoplasm), plasmids (extrachromosomal circular DNA), cytoplasm containing enzymes, and dissolved oxygen, waste products, and essential nutrients. Cytoplasmic particles include ribosomes (30, 50, and 70S).
• Flagella are protein structures for movement (peritrichous, lophotrichous, amphitrichous, and monotrichous).
• Axial filaments are found in spirochete bacteria.
• Endospores (spores) are resistant to heat, cold, drying, and chemicals; found in Bacillus and Clostridium.

Taxonomy

• Taxonomy is the classification, nomenclature, and identification of organisms.
• Organisms are categorized into taxa (e.g., groups) based on similarities.
• Binomial nomenclature (e.g., Escherichia coli) is a standardized naming system.
• Classification schemes include five-kingdom and three-domain systems.

Cell Reproduction

• Mitosis is somatic cell division; resulting in two identical daughter cells
• Meiosis is gamete (sex cell) division. Results in four genetically distinct daughter cells.
• Binary fission is prokaryotic cell reproduction; resulting in two identical daughter cells. A circular chromosome replicates and the cell divides.