A Tour of the Cell: Structure, Function, and Microscopy

Chapter 6: A Tour of the Cell

Overview: The Fundamental Units of Life

Cells are the basic structural and functional units of all living organisms. They are typically too small to be seen by the naked eye, requiring specialized tools for observation and study. This chapter explores the diversity of cell types, their internal organization, and the methods used to study them.

Microscopy and Cell Study

Concept 6.1: Biologists Use Microscopes and Biochemistry to Study Cells

Microscopes are essential for visualizing cells and their components. The development of microscopy has enabled scientists to explore cell structure and function in detail.

• Light Microscopes (LM): Use visible light to illuminate specimens. They allow observation of live cells and can magnify up to about 1000x.

• Electron Microscopes (EM): Use beams of electrons for much higher resolution, revealing subcellular structures. Types include Scanning Electron Microscopes (SEM) for surface details and Transmission Electron Microscopes (TEM) for internal structures.

• Specimen vs. Image: Images viewed through a microscope are reversed, magnified, and inverted compared to the specimen.

• Key Parameters of Microscopy:

  • Magnification: The ratio of an object's image size to its real size (e.g., 400x means 400 times larger).

  • Resolution: The ability to distinguish two close points as separate; higher resolution reveals more detail.

  • Contrast: The difference in brightness between parts of the image; stains and dyes can enhance contrast.

Electron Microscopy

• SEM: Provides 3D images of cell surfaces.

• TEM: Reveals internal cell structures by passing electrons through thin sections of specimens.

• Electron microscopes can magnify up to 100,000x but cannot be used with live cells.

Cell Types and Internal Organization

Concept 6.2: Eukaryotic Cells Have Internal Membranes That Compartmentalize Their Functions

Cells are classified as prokaryotic or eukaryotic based on their internal structure.

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles. DNA is located in the nucleoid region. Found in Bacteria and Archaea.

• Eukaryotic Cells: Have a nucleus and membrane-bound organelles. Found in protists, fungi, animals, and plants.

• Common Features of All Cells:

  • Plasma membrane

  • Cytosol (semifluid substance)

  • Chromosomes (genetic material)

  • Ribosomes (protein synthesis)

Cell Size and Surface Area-to-Volume Ratio

Cell size is limited by the surface area-to-volume ratio. As cells grow, volume increases faster than surface area, limiting efficient exchange of materials.

Cell Structure: Prokaryotic vs. Eukaryotic

Prokaryotic Cell Structure

• No nucleus; DNA in the nucleoid region

• No membrane-bound organelles

• Cell wall, plasma membrane, ribosomes, and sometimes flagella or fimbriae

Eukaryotic Cell Structure

• Membrane-bound nucleus

• Membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus)

• Larger and more complex than prokaryotic cells

Comparison Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cell	Eukaryotic Cell
Nucleus	Absent	Present
Organelles	Absent	Present
Size	1–10 µm	10–100 µm
Domains	Bacteria, Archaea	Eukarya

Summary

• Microscopy is essential for studying cells, with light and electron microscopes providing different levels of detail.

• Cells are classified as prokaryotic or eukaryotic based on the presence of a nucleus and organelles.

• Cell size is constrained by the surface area-to-volume ratio, influencing the efficiency of material exchange.

• All cells share basic features, but eukaryotic cells are more complex and compartmentalized.