Chapter 3

Cells and Their Structures

3.1 Lesson 3.1: Introduction to Cells

Lesson Objectives

• Explain how cells are observed.


• Recall the cell theory.


• Explain the levels of organization in an organism.

Check Your Understanding

• What are the main characteristics of living things?


• Name the four main classes of organic molecules that are building blocks of life.

Introduction

How do lipids, carbohydrates, proteins, and nucleic acids come together to form a living
organism? By forming a cell. These organic compounds are the raw materials needed for
life, and acellis the smallest unit of an organism that is still considered living. Cells are the
basic units that make up every type of organism. Some organisms, like bacteria, consist of
only one cell. Other organisms, like humans, consist of trillions of specialized cells working
together. Even if organisms look very different from each other, if you look close enough
you’ll see that their cells have much in common. (Use of a microscope inFigure3.1helps
to illustrate this.)