History of Cell Discoveries and Theories

History of Cell Discoveries

• Robert Hooke (1665) was the first to observe and name cells, using a microscope to examine non-living cork.
• Anton Van Leeuwenhoek (1673) is considered the "Father of Microscopy". He examined pond water and samples from a human mouth, drawing organisms that are now known as bacteria.

The Cell Theory (1800s)

• All living things are made up of cells (Schleiden & Schwann).
• Cells are the basic unit of life.
• Cells come only from other cells (Virchow), verifying Lorenz Oken's suggestion from 50 years prior.

Microscopes

• Light microscopes allow light to pass through a sample to create an image.
• Stains are sometimes used with light microscopes to reveal important structures.
• Electron microscopes utilize a beam of electrons focused by magnetic fields, providing higher resolution images.
• Electron microscopes are only used to observe non-living organisms.

Prokaryotic Cells

• Lack organelles and a nucleus.
• Always unicellular.
• Cells are not specialized.
• The only examples are bacteria.

Eukaryotic Cells

• Have organelles and a nucleus.
• Usually multicellular.
• Cells can be specialized, having a specific function (e.g. nerve cell, muscle cell).
• Examples include animals, plants, fungi, and protists.

Prokaryotic vs. Eukaryotic

• Prokaryotes are small and primitive with cell walls and cell membranes.
• Prokaryotes contain DNA (nucleoid and plasmid), ribosomes, and cytoplasm, but lack a true nucleus and membrane-bound organelles.
• Prokaryotes divide by binary fission.
• Eukaryotes have a true nucleus and membrane-bound organelles.
• Eukaryotes, like prokaryotes, possess cell membranes and may have cell walls.
• Eukaryotes can be unicellular or multicellular.
• Some eukaryotes have cilia.
• Eukaryotic cells divide through mitosis or meiosis.

Cytoplasm

• The fluid portion of the cell outside the nucleus is called the cytosol.
• Organelles are "little organs" within the cytoplasm.

Organelles

• Nucleus - Controls cell activities and contains DNA.
  • Contains a nuclear envelope with two membranes (each a lipid bilayer).
    • The outer layer is continuous with the rough ER, while the inner layer has nuclear pores that regulate the movement of proteins, RNA, and molecules in and out.
  • The nucleolus is the site of ribosome production.
• Endoplasmic Reticulum (ER)
  • Rough ER: Contains ribosomes, responsible for producing products for export.
  • Smooth ER: Lacks ribosomes and synthesizes membrane lipids, detoxifies drugs (found in the liver).
• Ribosomes - Produce proteins using coded instructions from DNA, either attached to the ER or free in the cytoplasm.
• Golgi Bodies/Apparatus - Package and transport cell products out of the cell, and produce lysosomes.
• Mitochondria - Site of energy production (ATP) and contain their own DNA. Mitochondrial DNA can self-replicate.
• Lysosomes - Filled with enzymes that remove cellular "junk".
  • Autolysis: Self-destruction.
  • Autophagy: Recycling cell parts.
• Peroxisomes - Site of filtering toxic materials.
  • Contain catalase.
  • Naturally found in the liver.
• Cytoskeleton - Involved in support, transport, and reproduction of the cell.
• Vacuoles - Sac-like structures that store extra sugar, water, salt, and waste.
  • Plant cells have a central vacuole that holds water and increases rigidity.
  • Unicellular organisms have a contractile vacuole that pumps excess water out of the cell.

Chloroplasts (Plants Only)

• Convert solar energy and CO2 into chemical energy (glucose).

The Cell Wall

• Tough, outer layer outside the cell membrane.
• Found in plants (cellulose), fungi (chitin), and some bacteria (peptidoglycan).
• Provides support and protection.
• Not found in animal cells.

The Cell Membrane (Lipid Bilayer)/Plasma Membrane

• Thin, flexible barrier that separates, regulates, and communicates.
• Separates: Forms a boundary around the cell.
• Regulates: Controls what enters and leaves the cell.
  • Semi-permeable/selectively permeable: Selects which molecules can pass in an out.
• Communicates: Allows the cell to interact with its environment.
• Structure: Composed of two organic compounds:
  • Phospholipids: A double layer of lipids with hydrophilic heads facing towards water and hydrophobic tails facing inwards away from water.
  • Proteins:
    • Integral proteins: Span the entire cell membrane.
      • Channel proteins: Open all the time.
      • Carrier proteins: Open and close to carry large molecules in and out (e.g. glucose).
    • Peripheral proteins: Located on the periphery (inside and out).
      • Act as receptors and cell markers.
• Fluid Mosaic Model: The cell membrane acts like a fluid, composed of multiple components.