The cell

History of the Cell

The following scientists contributed to the history of the cell:

  1. Robert Hooke (1665): An English scientist known as the father of cells, he discovered the cell while examining thin slices of cork from an oak tree.
  2. Felix Dujardin (1835): A French biologist who discovered that the cell is made up of a living substance called protoplasm.
  3. Robert Brown: He discovered the nucleus in plant cells.
  4. Matthias Schleiden (1838): A German botanist who stated that all plant tissues are composed of cells and that the cell is the basic unit of all plants and animals.
  5. Theodor Schwann (1839): A German zoologist who discovered that the bodies of animals are composed of cells.
  6. Rudolf Von Virchow (1855): A German biologist who concluded that all cells come from previously existing cells.

Cell Theory

The cell theory was formulated in 1838, primarily credited to Matthias Schleiden and Theodor Schwann. However, several other scientists, including Rudolf Virchow, contributed to the development of this theory.

The cell theory states that:

  1. All living organisms are composed of one or more cells.
  2. The cell is the structural and functional unit of life.
  3. All cells arise from pre-existing, living cells by biogenesis.
  4. Information present in the nucleic acids (DNA/RNA) of a cell is passed down to offspring cells.

Forms in Which Living Cells Exist

  1. Single and Free Living (Independent): These are organisms that possess only one cell and can live freely on their own.
    • Chlamydomonas: A unicellular, plant-like protist that is motile using its flagellum and has a cup-shaped chloroplast. It is sensitive to light due to its eyespot. In the presence of light, starch grains appear in the cytoplasm, disappearing in darkness. Chlamydomonas responds to environmental changes by moving toward light and disposes of excess water using contractile vacuoles. It reproduces both asexually and sexually when it reaches maximum size.
    • Amoeba: A unicellular, animal-like protist and one of the simplest organisms with a heterotrophic mode of feeding. Amoeba is shapeless and digests complex food using a food vacuole. Undigested food and excess water are expelled through its contractile vacuole. It captures food using pseudopodia ("false feet") and reproduces asexually by binary fission.
    • Paramecium: A more complex unicellular organism compared to Amoeba, Paramecium is slipper-shaped and moves using cilia, which also help direct food into its gullet. Undigested food is removed through the anal pore, and excess water is expelled through contractile vacuoles. It has two types of nuclei: a micronucleus and a macronucleus.
    • Euglena (Euglena viridis): A protist with both plant and animal characteristics. It has star-shaped chloroplasts for photosynthesis, pyrenoid for starch storage, and paramylum granules (a form of stored starch).
  2. As a Colony: These are organisms made up of similar cells joined together, which cannot be differentiated from each other. Examples include Volvox, sponges, Pandorina, and Eudorina.
    • Volvox: A colony made up of several Chlamydomonas-like cells arranged in an angular layer to form a hollow ball-like structure. The cells are connected by cytoplasmic strands and move through the beating of their flagella. Most Volvox cells lose their ability to reproduce, with only a few able to form daughter cells.
  3. As a Filament: These are similar or identical cells joined end-to-end to form unbranched filaments, such as Spirogyra, Oscillatoria, Oedogonium, and Zygnema.
    • Spirogyra: A filament of identical cells joined end-to-end without branching. Each cell carries out its function independently. Spirogyra has spirally arranged chloroplasts for photosynthesis, and the filament can grow indefinitely. Break-off cells can reproduce asexually to form new filaments and can also reproduce sexually.
  4. As Part of Multicellular Organisms: In multicellular organisms, groups of similar cells come together to form tissues that perform specific functions. These tissues form organs, and the organs form systems. This relationship is found in higher animals, including humans.