Foundations of General Biology: Cells, Microscopy, and the Tree of Life

Introduction to Biology

Biology is the scientific study of living organisms. All living organisms share several fundamental characteristics that distinguish them from non-living matter.

• Cells: All organisms consist of at least one cell.

• Replication: All organisms reproduce.

• Information: Genetic information is passed to the next generation (hereditary information), and environmental information influences the next generation's traits.

• Energy: Organisms acquire and use energy to stay alive.

• Evolution: Populations of organisms evolve over successive generations, adapting to new changes.

Theories in Biology

Major Biological Theories

Theories in biology provide broad explanations for a wide range of phenomena, supported by substantial evidence. Three foundational theories are:

1. The Cell Theory: All living things are composed of cells. Cells are the basic unit of structure and function in organisms.

2. The Theory of Evolution by Natural Selection: Explains how organisms change over time due to heritable traits that enhance survival and reproduction.

3. The Chromosome Theory of Inheritance: Describes how genetic information is transmitted from one generation to the next via chromosomes.

• Example: A virus has genetic information but is not considered alive because it does not meet all five criteria of life.

Cell Theory

Key Concepts

• Cells are highly organized compartments separated from their environment by a membrane barrier.

• All organisms are made up of cells.

• All cells come from preexisting cells (Louis Pasteur's hypothesis disproved spontaneous generation).

How Big is a Cell and its Components?

Units of Measurement

• m = meter

• cm = centimeter (1/100 of a meter)

• mm = millimeter (1/1,000 of a meter)

• μm = micrometer (1/1,000,000 of a meter)

• nm = nanometer (1/1,000,000,000 of a meter)

Conversion examples:

• 1 m = 100 cm = 1,000 mm = 1,000,000 μm = 1,000,000,000 nm

• 1 cm = $10^{-2}$ m

• 1 mm = $10^{-3}$ m

• 1 μm = $10^{-6}$ m

• 1 nm = $10^{-9}$ m

Example: 750 cm = $\frac{750 \times 1,000,000}{1}$ μm = 7,500,000 μm

Microscopy

Using Microscopy

Microscopes are essential tools for studying cells and their components, as many are too small to be seen with the naked eye.

• Light Microscopy: Can resolve structures as small as 200 nm.

• Electron Microscopy: Uses a beam of electrons for much higher resolution, down to 0.2 nm.

Types of Electron Microscopy

• Scanning Electron Microscopy (SEM): Provides 3D images of surface features.

• Transmission Electron Microscopy (TEM): Reveals internal (inferior) structures in 2D.

Resolution

Resolution is the minimum distance at which two points can be distinguished as separate.

Examples of Microscopy

• Light microscopy can visualize stained tissues and living cells.

• Electron microscopy can reveal detailed structures of organelles, viruses, and cell surfaces.

The Central Dogma of Molecular Biology

Flow of Genetic Information

The Central Dogma describes how genetic information flows within a cell:

• Genes (segments of DNA) contain hereditary information and are located on chromosomes.

• Information flows from DNA to RNA to protein.

Diagram: DNA → RNA → Protein

Life Requires Energy

Energy in Living Systems

• Chemical reactions in cells require energy.

• Organisms acquire energy in the form of adenosine triphosphate (ATP).

• Energy is used for processes such as eating, sleeping, and building macromolecules (DNA, RNA, proteins).

Evolution and Natural Selection

Key Concepts

• Evolution is a change in the characteristics of a population over time.

• Natural selection explains how evolution occurs: individuals with advantageous traits are more likely to survive and reproduce.

• All species are related by common ancestry and have changed over time.

The Tree of Life

Classification of Life

• The cell theory and natural selection provide the foundation for classifying life.

• Organisms are classified into three domains based on genetic sequence:

Domain	Characteristics
Bacteria	Prokaryotic, unicellular
Archaea	Prokaryotic, unicellular, distinct from bacteria
Eukarya	Eukaryotic, includes plants, animals, fungi, and protists

Prokaryotes vs. Eukaryotes

Cellular Differences

Cell Type	Characteristics
Prokaryotic Cell	Homogenous, unicellular, lacks a nucleus
Eukaryotic Cell	Complex, most are multicellular, contains a nucleus and organelles

Additional info: Eukaryotic cells are typically larger and more structurally complex than prokaryotic cells. Prokaryotes include bacteria and archaea, while eukaryotes include plants, animals, fungi, and protists.