Unit 2 Part 2: A Tour of the Cell

Introduction to Cell Theory

The cell is the fundamental unit of life, and cell theory forms the foundation of modern biology. This section introduces the historical development and key principles of cell theory.

• All organisms are made of cells: Every living thing is composed of one or more cells.

• The cell is the basic building block of life: Cells are the smallest units that carry out all life processes.

• All cells come from pre-existing cells: New cells are produced by the division of existing cells, as established by Rudolf Virchow in 1855.

Example: Multicellular organisms like humans are made up of trillions of cells, each specialized for different functions.

Fundamental Properties of Cells

Cells share several universal properties that are essential for life.

• Energy flows within cells: Cells carry out metabolic reactions to obtain and use energy.

• Similar species have similar cells: The basic structure and function of cells are conserved across related organisms.

• Cells divide and pass along genetic information: Genetic material is transmitted from parent to daughter cells during cell division.

Microscopy: How Do We See Cells?

Types of Microscopes

Microscopes are essential tools for studying cells and their structures. Different types of microscopes offer varying levels of magnification and resolution.

• Light Microscope: Uses visible light to illuminate specimens and can display living cells. Magnification is typically up to 1000x.

• Electron Microscopes: Use beams of electrons for much greater magnification and resolution. Two main types are:

  • Scanning Electron Microscope (SEM): Provides detailed images of cell surfaces.

  • Transmission Electron Microscope (TEM): Reveals internal cell structures (ultrastructure).

Example: Electron microscopes can reveal the intricate details of organelles such as mitochondria and chloroplasts.

Scale and Size of Cells

Cells and their components vary greatly in size, which affects how they are studied and visualized.

• Unassisted eye: Can see objects larger than about 0.1 mm (e.g., frog eggs).

• Light microscope: Can resolve most plant and animal cells, bacteria, and some organelles.

• Electron microscope: Required to see viruses, ribosomes, and molecular complexes.

Example: Most bacteria are about 1-10 micrometers in diameter, while animal cells are typically 10-100 micrometers.

Cell Surface Area and Volume

Importance of Surface Area-to-Volume Ratio

The size and structure of cells are constrained by the relationship between surface area and volume. This ratio affects the ability of cells to exchange materials with their environment.

• Surface area: The total area of the outside surfaces of a three-dimensional object.

• Volume: The amount of space a three-dimensional object occupies.

• Surface area-to-volume ratio (SA:V): As a cell grows, its volume increases faster than its surface area, limiting efficient exchange of materials.

Formula:

Example: Small cells have a higher SA:V ratio, which facilitates faster exchange of nutrients and waste products.

Additional info: The table above illustrates how smaller objects have a higher surface area-to-volume ratio, which is critical for cell function.