Introduction to Cells: Structure, Function, and Microscopy

Introduction to Cells

The Cell Theory

The cell is the fundamental unit of life. The development of the microscope allowed scientists to observe cells and led to the formulation of the cell theory, a cornerstone of modern biology.

• Cell Theory:

  1. Cells are the smallest unit of life. All living things are made up of cells, which are the basic structural and functional units.

  2. All organisms are composed of one or more cells. Organisms can be unicellular (single-celled) or multicellular (many-celled).

  3. Cells arise only by division from a pre-existing cell. ("Omnis cellula e cellula" – all cells come from cells.)

• Key Contributors:

  • Matthias Schleiden (Botanist)

  • Theodor Schwann (Physiologist)

  • Rudolph Virchow (Biologist)

Unicellular vs. Multicellular Organisms

Organisms can be classified based on the number of cells they possess.

• Unicellular Organisms: Consist of a single cell that performs all life functions. Examples include bacteria and many protists.

• Multicellular Organisms: Composed of multiple specialized cells that work together. Examples include plants, animals, and fungi.

• Illustration: The provided cartoon shows a 'married cell organism' (representing multicellularity) and 'single cell organism' (unicellularity), highlighting the difference in complexity.

Microscopy and Cell Size

Microscopes and Visualization of Cells

Cells are generally too small to be seen with the naked eye. Microscopes are essential tools for visualizing cells and their internal structures.

• Transmission Electron Microscope (TEM): Used to view thin sections of cells at very high resolution, revealing internal structures.

• Scanning Electron Microscope (SEM): Used to view the surface of cells in great detail.

Why Are Cells So Small?

Cell size is limited by the surface area-to-volume ratio, which affects the ability of the cell to transport materials in and out efficiently.

• Surface Area-to-Volume Ratio: As a cell grows, its volume increases faster than its surface area, limiting the rate of exchange with the environment.

• Typical Cell Sizes: Most plant and animal cells are between 10–100 μm in diameter.

• Scale of Biological Structures: Cells and their components vary greatly in size, from atoms (0.1 nm) to human eggs (100 μm) and beyond.

Structure	Approximate Size
Human height	~1–2 m
Chicken egg	~5 cm
Frog egg	~1 mm
Human egg	~100 μm
Most plant/animal cells	~10–100 μm
Nucleus	~5–10 μm
Most bacteria	~1–10 μm
Mitochondria	~1–2 μm
Viruses	~50–100 nm
Ribosomes	~20–30 nm
Proteins	~5–10 nm
Lipids	~2–5 nm
Small molecules	~1 nm
Atoms	~0.1 nm

Types of Cells: Prokaryotic vs. Eukaryotic

Prokaryotic Cells

Prokaryotic cells are simpler and lack a true nucleus. They are generally smaller and less complex than eukaryotic cells.

• No true nucleus: Genetic material is located in a region called the nucleoid.

• Generally unicellular: Examples include bacteria and archaea.

• Lack membrane-bound organelles.

Eukaryotic Cells

Eukaryotic cells are more complex and contain a true nucleus and various membrane-bound organelles.

• True nucleus: DNA is enclosed within a nuclear envelope.

• Specialized organelles: Such as mitochondria, endoplasmic reticulum, and Golgi apparatus.

• Can be unicellular or multicellular: Examples include protists, fungi, plants, and animals.

Common Features of All Cells

• Plasma membrane: Encloses the cell and separates it from the environment.

• Cytosol: The fluid component inside the cell where many metabolic reactions occur.

• Genetic material: DNA that stores hereditary information.

• Ribosomes: Sites of protein synthesis.

• Internal organization: Structures that organize cellular contents.

Cytosol and Cytoskeleton

Cytosol

The cytosol is the aqueous component of the cytoplasm, where many metabolic processes take place.

• Composition: Mostly water, with dissolved ions, small molecules, and large macromolecules.

• Functions: Site of glycolysis, protein synthesis, and other metabolic pathways.

Cytoskeleton

The cytoskeleton is a network of protein filaments that provides structural support, organization, and motility to the cell.

• Functions:

  • Maintains cell shape

  • Anchors organelles

  • Provides tracks for movement of materials

  • Facilitates cell movement

Components of the Cytoskeleton

• Microfilaments (Actin Filaments):

  • Diameter: 7 nm

  • Structure: Thin, intertwined threads

  • Protein subunit: Actin

  • Dynamic and involved in cell shape, movement, and division (e.g., muscle contraction, cytoplasmic streaming, cleavage furrow formation)

• Intermediate Filaments:

  • Diameter: 8–12 nm

  • Structure: Stretchy protein cables

  • Protein subunits: Various (e.g., vimentin, keratin, neurofilaments)

  • Stable, provide mechanical strength, anchor organelles, form nuclear lamina

• Microtubules:

  • Diameter: 25 nm

  • Structure: Hollow tubes

  • Protein subunits: α- and β-tubulin

  • Functions: Maintain cell shape, form cilia and flagella, serve as tracks for organelle movement, organize chromosome movement during cell division

Motor Proteins

Motor proteins use energy from ATP to move along cytoskeletal filaments, transporting vesicles and organelles within the cell.

• Examples: Kinesin and dynein (move along microtubules), myosin (moves along actin filaments)

• Function: Facilitate intracellular transport and cell movement

Additional info: The notes and images provided are foundational for understanding cell biology, including cell theory, cell types, microscopy, and the cytoskeleton. These concepts are essential for any introductory biology course.