The Cell: Discovery, Structure, and Function

1. Prokaryotic Cells – Simpler, older cells

• No nucleus; DNA floats freely in the cytoplasm.

• No membrane-bound organelles.

• Cell wall made of peptidoglycan (in bacteria).

• Examples: Bacteria and Archaea.

2. Eukaryotic Cells – Complex and organized

• DNA is enclosed in a nucleus.

• Have organelles (mini-organs) with specific functions.

• Larger and more advanced.

• Examples: Plants, animals, fungi, and protists.

Types of Organisms Based on Cell Number

1.  Unicellular Organisms

• Organisms made up of only one cell that performs all life functions.

Characteristics:

• • Very small (microscopic)

  • Single cell carries out all life processes (nutrition, reproduction, excretion, etc.)

  • Reproduce asexually (mostly by binary fission)

  • Found in diverse environments (water, soil, even inside other organisms)

Examples:

• Domain	Example Organisms	Notes
  Bacteria	E. coli, Streptococcus	Prokaryotic; no nucleus
  Archaea	Halophiles, Thermophiles	Live in extreme environments
  Protista	Amoeba, Paramecium, Euglena	Eukaryotic; some can photosynthesize
  Fungi (few)	Yeast (e.g., Saccharomyces cerevisiae)	Only fungi that are unicellular

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2. Multicellular Organisms

• Organisms made up of more than one cell that are often specialized for specific functions.

Characteristics:

• • Cells differentiate into tissues and organs

  • More complex and organized

  • Can grow larger and perform more complex functions

  • Usually reproduce sexually (some also asexually)

Examples:

• Kingdom	Example Organisms	Notes
  Animals	Humans, Birds, Fish, Insects	Eukaryotic; no cell wall; complex organ systems
  Plants	Trees, Flowers, Grasses	Eukaryotic; photosynthetic; cell wall with cellulose
  Fungi	Mushrooms, Molds	Eukaryotic; decomposers; cell wall with chitin
  Protists (some)	Multicellular algae like kelp	Simple multicellular life forms

Deep Dive into Eukaryotic Cell Structure and Function

Plasma Membrane

• Semi-permeable barrier.

• Made of a phospholipid bilayer.

• Regulates entry and exit of substances.

Nucleus

• Control center of the cell.

• Contains chromatin (DNA + proteins).

• Nucleolus inside makes ribosomes.

Mitochondria

• The “powerhouse” of the cell.

• Site of aerobic respiration – converts glucose into ATP (energy).

• Has its own DNA (supports the endosymbiotic theory).

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; makes proteins.

• Smooth ER: Makes lipids, detoxifies chemicals.

Golgi Apparatus

• Modifies, sorts, and packages proteins and lipids for storage or transport.

Lysosomes

• Contain enzymes that break down waste and cellular debris.

• Important in apoptosis (programmed cell death).

Ribosomes

• Sites of protein synthesis.

• Found floating in cytoplasm or attached to rough ER.

Vacuoles

• Store materials like water, salts, proteins, and carbohydrates.

• Large central vacuole in plant cells helps maintain pressure.

Chloroplasts (in plant cells)

• Conduct photosynthesis.

• Contain chlorophyll (green pigment).

• Like mitochondria, they have their own DNA.

Cell Wall (in plants, fungi, and some protists)

• Provides structure, protection, and shape.

• Made of cellulose in plants.

Special Concepts and Theories

Endosymbiotic Theory

• Proposed by Lynn Margulis.

• Suggests that mitochondria and chloroplasts were once free-living prokaryotes that were engulfed by larger cells.

• Explains why they have their own DNA and double membranes.

Cell Specialization (Differentiation)

• In multicellular organisms, cells specialize to perform specific tasks:

  • Muscle cells: contraction

  • Nerve cells: transmission of impulses

  • Red blood cells: oxygen transport

Observing Cells: Microscopy

Light Microscope

• Can view living cells.

• Magnifies up to ~1000x.

Electron Microscope

• Provides much higher resolution.

• Can view ultrastructure (fine details).

• Two types:

  • Transmission Electron Microscope (TEM): for internal structures.

  • Scanning Electron Microscope (SEM): for 3D surface images.

Conclusion: Why the Cell Matters

Cells are the fundamental units of life. Every process in an organism—from metabolism to growth, response to stimuli, and reproduction—starts at the cellular level. Studying cells helps us:

• Understand diseases (e.g., cancer = uncontrolled cell division).

• Develop medicines (targeting organelles or pathways).

• Advance biotechnology (e.g., genetic engineering, cloning).

References & Sources

• 1. McGraw-Hill Education
     Biology and Geology – Secondary 4
     ➤ A widely used textbook in international secondary science curricula. McGraw-Hill provides comprehensive overviews of cell biology and its historical development.

  2. OpenStax Biology
     https://openstax.org/books/biology/pages/
     ➤ Open-access textbook with peer-reviewed content on cell structure, function, and scientific discoveries related to the cell theory.

  3. Cells Alive!
     https://www.cellsalive.com/
     ➤ Offers interactive models and explanations of both prokaryotic and eukaryotic cells, plus animated cell functions and microscopy tutorials.

  4. National Institutes of Health (NIH)
     https://www.nih.gov/
     ➤ Resources on cell biology and how understanding cell function informs biomedical research and healthcare.

  5. Biology Online
     https://www.biologyonline.com/
     ➤ Offers definitions and explanations of biological terms including organelles, microscopy, cell theory, and specialization.

  6. Khan Academy – Cell Biology
     https://www.khanacademy.org/science/biology/cells
     ➤ Free educational videos and interactive content explaining the structure and function of cells.

  7. ASU – Ask A Biologist
     https://askabiologist.asu.edu/
     ➤ Contains games like the “Cell Viewer Game” and comprehensive articles on cell components and history.

  8. Lynn Margulis' Endosymbiotic Theory
     Margulis, L. (1970). Origin of Eukaryotic Cells. Yale University Press.
     ➤ The original work proposing that mitochondria and chloroplasts originated from symbiotic bacteria.

  9. History of Cell Theory – Encyclopedia Britannica
     https://www.britannica.com/science/cell-biology
     ➤ Reliable encyclopedia covering the history and scientists involved in the discovery of cells.