Cell Theory Overview

Cell Theory

• Cells are the fundamental building blocks of all living organisms, varying in shape and size.
• Robert Hooke first observed cells in 1665 by examining cork through a microscope, coining the term “cellulae.”
• Key contributors to cell theory: Theodor Schwann, Matthias Schleiden, Rudolph Virchow.
• Three main postulates of cell theory:
  • All living things are made of one or more cells.
  • Cells are the basic unit of life.
  • All cells arise from preexisting cells.

Discovery Timeline

• Robert Hooke (1665): First recorded cells in cork.
• Antoine van Leeuwenhoek (1660s): Observed protists and bacteria, describing them as "animacules."
• Matthias Schleiden (1838): Established that all plants are composed of cells.
• Theodor Schwann (1839): Stated that animals consist of cells and their products.
• Robert Remak (1842): Identified three embryonic layers: ectoderm, mesoderm, endoderm.
• Rudolph Virchow (1855): Concluded "omnis cellula e cellula" - all cells originate from existing cells.

Organelles in Plant & Animal Cells

• Organelles perform specific functions within cells and include:
  • Cell Membrane: Protective barrier assisting in transport.
  • Cell Wall (Plant): Provides structural protection.
  • Cytoplasm: Gel-like fluid containing dissolved substances.
  • Nucleus: Control center for growth and reproduction.
  • Nucleolus: Produces ribosomal RNA within the nucleus.
  • Chloroplast (Plant): Site of photosynthesis, converting sunlight to sugar.
  • Mitochondria: Energy production through cellular respiration.
  • Golgi Apparatus: Processes and packages proteins for export.
  • Smooth ER: Synthesizes and stores lipids.
  • Rough ER: Involved in protein synthesis with ribosomes.
  • Vacuole: Storage for nutrients and waste, larger in plants.
  • Ribosomes: Assemble proteins from amino acids.
  • Peroxisome: Breaks down fatty acids and detoxifies.
  • Lysosome (Animal): Digests macromolecules and recycles cellular waste.
  • Microtubules (Animal): Support structure and assist in cell division.
  • Centrioles (Animal): Aid in cell division; found in pairs.

Cell Types

• Organisms can be unicellular or multicellular, with humans composed of trillions of cells.
• Two major cell types:
  • Prokaryotes: Simple, no membrane-bound organelles, no nucleus, oldest cell type (3.5 billion years).
  • Eukaryotes: Complex, membrane-bound organelles, contain a nucleus, appeared 1.5 billion years ago.
• Similarities: Both have DNA and a cell membrane, containing essential biomolecules.
• Differences: Eukaryotes are significantly larger than prokaryotes.

Cell Cycle

• Comprises three main stages: Interphase, Mitosis, Cytokinesis.
• Interphase:
  • G1 Phase: Rapid growth and routine functions, longest duration.
  • S Phase: DNA replication occurs.
  • G2 Phase: Prepares for mitosis, assembles microtubules.
• Mitosis: Division of the nucleus into two identical nuclei, includes prophase, metaphase, anaphase, and telophase.
• Cytokinesis: Division of the cytoplasm resulting in two daughter cells.

Control of the Cell Cycle

• Three checkpoints monitor cell conditions:
  • G1 Checkpoint: Assesses size, health, and environmental factors before S phase.
  • G2 Checkpoint: Checks for DNA errors prior to mitosis.
  • Mitosis Checkpoint: Verifies successful completion of mitosis before returning to G1.
• Checkpoint failures can lead to cancer or uncontrollable cell growth.

Meiosis

• A specialized type of cell division producing four haploid daughter cells (sex cells).
• Ensures organisms from sexual reproduction have proper chromosome numbers.
• Stages:
  • Prophase 1: Chromosome condensation and crossing over occur.
  • Metaphase 1: Chromosome pairs align randomly.
  • Anaphase 1: Chromosome pairs separate to opposite sides.
  • Telophase 1: New nuclei form, resulting in two haploid cells.
  • Prophase 2: Prepares for formation of four haploid cells.
  • Metaphase 2: Chromosomes align individually for separation in the next step.