Biology

Biology is the study of living organisms — what they are, how they work, how they reproduce, and how they relate to each other. Since Chad is living among biological creatures on a biological planet, this matters.

What is Life?

Scientists define living things by certain shared characteristics:

Cellular organization — made of one or more cells
Metabolism — takes in and uses energy
Growth — increases in size/complexity
Reproduction — makes copies of itself
Response to stimuli — reacts to the environment
Homeostasis — maintains internal stability

Viruses are a gray area — they reproduce but need a host cell to do so.

The Cell

The cell is the basic unit of life. Every living thing is made of cells.

Some organisms are unicellular (one cell): bacteria, some fungi
Others are multicellular (many cells): plants, animals, humans

Human body: ~37 trillion cells.

Key structures in most cells:

Cell membrane — the outer boundary
Nucleus — contains DNA (the genetic blueprint)
Mitochondria — produce energy (the “powerhouse” of the cell)

DNA and Genetics

DNA (deoxyribonucleic acid) is the molecule that stores genetic information. It is shaped like a double helix — a twisted ladder.

DNA contains genes — sections that code for specific proteins or traits. Humans have ~20,000–25,000 genes.

When organisms reproduce, they pass DNA to their offspring. This is why children resemble their parents. Variations in DNA are why organisms within a species differ from each other.

Evolution

Evolution is the process by which species change over time through natural selection:

Individuals in a population vary (due to genetic differences)
Some variations make individuals more likely to survive and reproduce
Those individuals pass on their genes more
Over generations, advantageous traits become more common
Over long periods, populations can diverge into new species

All life on Earth shares a common ancestor. Humans share ~98% of their DNA with chimpanzees.

The Tree of Life

Life is organized into domains and kingdoms:

Bacteria — single-celled organisms without a nucleus; enormous diversity
Archaea — similar to bacteria but distinct; often in extreme environments
Eukarya — cells with a nucleus; includes:
- Protists (e.g., amoeba)
- Fungi (mushrooms, molds, yeast)
- Plants
- Animals (including humans)

Ecosystems

An ecosystem is a community of living organisms interacting with each other and their environment. Key concepts:

Food web — who eats whom. Energy flows from plants → herbivores → carnivores
Biodiversity — variety of species in an area; higher is generally more stable and resilient
Decomposers — fungi and bacteria that break down dead matter, returning nutrients to the soil

Humans have significantly disrupted many ecosystems through deforestation, pollution, and climate change (Climate & Environment).

The Plant Kingdom

Plants are crucial to all life on Earth. They perform photosynthesis: Sunlight + Water + CO₂ → Glucose + Oxygen

This is the foundation of nearly all food chains. Forests (particularly rainforests) are called “the lungs of the Earth” because of the oxygen they produce and the CO₂ they absorb.

A tree (since Chad needs to know): a large perennial plant with a woody trunk. Trees grow for decades or centuries, producing oxygen, providing habitat, and stabilizing soil.