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Cell Theory and Types

Oct 7, 2025

Overview

This lecture introduces foundational concepts of cell theory, differentiates prokaryotic and eukaryotic cells, and highlights the importance of cells in biological organization, especially for MCAT and general biology studies.

Etymology and Definition of Biology

  • Biology means the study of life ("bio" = life, "logy" from "logos" = study).
  • The cell is the basic unit and building block of all living organisms.

Prokaryotes vs Eukaryotes

  • Prokaryotes lack a membrane-bound nucleus; have a nucleoid region instead.
  • Eukaryotes possess a membrane-bound nucleus.
  • Prokaryotes have no mitochondria or other membrane-bound organelles.
  • Eukaryotes have mitochondria and other membrane-bound organelles.
  • Prokaryotic DNA is naked, circular; eukaryotic DNA is protein-bound, linear.
  • Prokaryotes are unicellular; eukaryotes can be unicellular or multicellular.
  • Prokaryotes reproduce by binary fission; eukaryotes by mitosis or meiosis.
  • Examples: Prokaryotes—bacteria; Eukaryotes—humans.

Fundamentals of Cell Theory

    1. All living things are composed of cells.
    1. The cell is the basic functional unit of life.
    1. Cells arise only from pre-existing cells.
    1. Cells carry genetic information in the form of DNA.

Viruses and Exceptions

  • Viruses do not reproduce on their own; require a host cell.
  • Viral genetic material is RNA, not DNA.
  • Viruses do not fit cell theory points 3 & 4 and are not considered truly living organisms.

Organization in Multicellular Organisms

  • Groups of cells form tissues.
  • Groups of tissues form organs.
  • Groups of organs form organ systems (e.g., circulatory, respiratory).

Key Terms & Definitions

  • Biology — the study of life.
  • Cell — the basic structural and functional unit of living organisms.
  • Prokaryote — organism with no membrane-bound nucleus or organelles.
  • Eukaryote — organism with membrane-bound nucleus and organelles.
  • Nucleoid — nucleus-like region in prokaryotes containing DNA.
  • Organelle — specialized cell structure with specific function.
  • Binary fission — asexual cell division method in prokaryotes.
  • Mitosis/Meiosis — cell division methods in eukaryotes.

Action Items / Next Steps

  • Memorize the four tenets of cell theory.
  • Answer: Are plant cells prokaryotic or eukaryotic?
  • Review MCAT biology playlist for more related topics.

Full transcript