Unit 11 — Cells, Living Organisms, and Biological Organization

Welcome back!

Every living thing on Earth, from tiny bacteria to towering trees and human beings, is made of cells.

Cells are the basic building blocks of life.

By studying cells, scientists learn how organisms grow, obtain energy, reproduce, respond to their environment, and maintain life.

Today you will explore the remarkable organization of living things.

Characteristics of Living Things

Scientists identify several characteristics shared by all living organisms.

Living things:

• are made of one or more cells
• obtain and use energy
• grow and develop
• respond to their environment
• reproduce
• maintain internal balance
• pass hereditary information to offspring
• adapt over generations

These characteristics distinguish living organisms from nonliving matter.

The Cell

A cell is the smallest unit capable of carrying out all the processes necessary for life.

Some organisms consist of only one cell.

Others contain trillions of cells working together.

Although cells are microscopic, they perform complex and highly organized functions.

Cell Theory

Modern biology is built upon cell theory.

Cell theory states:

• all living things are made of cells
• the cell is the basic unit of life
• all cells come from preexisting cells

These ideas form one of the foundations of biological science.

Plant and Animal Cells

Plant and animal cells share many structures, including:

• cell membrane
• cytoplasm
• nucleus
• mitochondria

Plant cells also contain:

• a rigid cell wall
• chloroplasts
• a large central vacuole

These structures help plants produce food through photosynthesis and maintain their shape.

Specialized Cells

In multicellular organisms, many cells become specialized.

Examples include:

• muscle cells
• nerve cells
• red blood cells
• skin cells
• leaf cells
• root cells

Each type of cell performs specific functions that help the organism survive.

Levels of Biological Organization

Living things become increasingly organized.

The major levels include:

• cells
• tissues
• organs
• organ systems
• organisms

Each level builds upon the previous one.

Working together, these levels allow complex organisms to function efficiently.

Microscopes

Most cells are too small to see without magnification.

Scientists use microscopes to observe:

• cell structures
• microorganisms
• tissues
• tiny organisms

Improved microscopes have greatly expanded our understanding of biology.

Unicellular and Multicellular Organisms

Some organisms contain only one cell.

Examples include:

• bacteria
• many protists

These are called unicellular organisms.

Plants, animals, and most fungi consist of many specialized cells.

These are multicellular organisms.

Both forms of life successfully survive in many environments.

Activity

Observe prepared microscope slides or high-quality images of:

• plant cells
• animal cells
• bacteria
• other microorganisms

Create labeled drawings showing the visible structures.

Then construct a diagram illustrating the levels of biological organization from cell to organism.

Practice

Answer these questions.

What is a cell?

What does cell theory state?

How do plant cells differ from animal cells?

What are specialized cells?

What are the levels of biological organization?

Why are microscopes important?

Review

Today you learned:

• cells are the basic units of life
• all living things are composed of cells
• plant and animal cells have both similarities and differences
• specialized cells perform specific functions
• complex organisms are organized into tissues, organs, and organ systems
• microscopes allow scientists to study structures too small to see with the naked eye

Understanding cells provides the foundation for studying all living organisms.

Looking Ahead

Next we will explore human body systems, learning how organs work together to support movement, circulation, breathing, digestion, and overall health.