Unit 5 — Biology: The Science of Living Systems

Welcome to Biology!

Biology is the scientific study of life.

Every living organism—from microscopic bacteria to giant sequoia trees and blue whales—shares certain characteristics that distinguish life from nonliving matter.

By studying biology, we learn how living systems function, interact, adapt, and evolve over time.

Biology also helps us improve medicine, agriculture, environmental stewardship, and our understanding of humanity's place within Earth's ecosystems.

Characteristics of Life

Although living organisms are extraordinarily diverse, they share several common characteristics.

Living things:

• are composed of cells
• use energy
• grow and develop
• respond to their environment
• maintain internal stability
• reproduce
• pass hereditary information to offspring
• evolve over generations

These characteristics help scientists distinguish living organisms from nonliving matter.

Cell Theory

The cell is the fundamental unit of life.

Cell Theory states that:

• all living things are made of one or more cells
• the cell is the basic unit of structure and function
• all cells arise from preexisting cells

Modern biology is built upon these foundational principles.

Cell Structure

Although cells vary greatly, many contain similar structures.

Important cell structures include:

• cell membrane
• cytoplasm
• nucleus
• ribosomes
• mitochondria

Plant cells also contain:

• cell walls
• chloroplasts
• large central vacuoles

Each structure performs specialized functions that help maintain life.

DNA and Heredity

Every organism contains genetic information.

This information is stored primarily in DNA (deoxyribonucleic acid).

DNA provides instructions for:

• growth
• development
• cellular function
• reproduction

Genes are sections of DNA that influence inherited traits.

The study of heredity is known as genetics.

Evolution

Over long periods of time, populations of organisms may change.

This process is called evolution.

Evidence supporting evolution includes:

• fossils
• comparative anatomy
• embryology
• genetics
• observed changes in living populations

Evolution helps explain both the diversity and the shared ancestry of life on Earth.

Ecosystems

Organisms do not live in isolation.

An ecosystem includes:

• living organisms
• soil
• water
• air
• climate
• interactions among organisms

Healthy ecosystems depend upon complex relationships among producers, consumers, decomposers, and the physical environment.

Biodiversity

Biodiversity refers to the variety of living organisms on Earth.

High biodiversity often contributes to:

• ecosystem stability
• resilience
• food security
• scientific discovery
• environmental health

Protecting biodiversity helps sustain ecosystems for future generations.

Scientific Investigation

Biologists investigate questions through careful observation and experimentation.

Scientific investigations typically include:

• asking a question
• forming a hypothesis
• designing an investigation
• collecting data
• analyzing results
• drawing conclusions
• communicating findings

Scientific knowledge grows through continual testing and refinement.

Activity

Complete the following:

• label the major parts of plant and animal cells
• compare prokaryotic and eukaryotic cells
• investigate one ecosystem in your region
• research one inherited genetic trait
• analyze evidence supporting evolutionary change
• design a simple biological investigation and explain your methods

Support your conclusions with scientific evidence.

Practice

Answer these questions.

What characteristics define living organisms?

What are the three principles of Cell Theory?

What role does DNA play in living organisms?

What is biodiversity?

Why are ecosystems important?

How does scientific investigation improve biological knowledge?

Review

Today you learned:

• biology is the scientific study of life
• all living things are composed of cells
• DNA carries hereditary information
• evolution helps explain biological diversity
• ecosystems depend upon interactions among living and nonliving components
• biodiversity strengthens environmental stability
• scientific investigation advances biological understanding

Biology helps us understand the remarkable complexity of living systems and our responsibility to study and protect the natural world.

Looking Ahead

Next we will continue Biology by exploring genetics, natural selection, ecology, human body systems, and the remarkable interconnectedness of life on Earth.