1. < BackNext >PreviewMain Section 1 The Characteristics of Cells Chapter 4 Bellringer Identify some of the functions that all cells have in common. Write your ideas in your science portfolio.

2. < BackNext >PreviewMain Chapter 4 What You Will Learn The cell theory explains why cells are important for living things. All cells have a cell membrane, cytoplasm, and DNA. Prokaryotic cells and eukaryotic cells differ in how their genetic information is contained. Section 1 The Characteristics of Cells

3. < BackNext >PreviewMain Chapter 4 Cells and Cell Theory Cells function similarly in all living things. A cell is the smallest structural and functional unit of living things. In 1665, Robert Hooke built a microscope and and observed box-like structures in a sample of cork. He called the structures cells. Section 1 The Characteristics of Cells

4. < BackNext >PreviewMain Chapter 4 Because animal cells lack cell walls, Hooke could not see them. He believed that only plants and fungi were made of cells. In 1673, Dutch merchant Anton van Leeuwenhoek made a microscope and observed swimming “animacules” in a sample of pond scum. Today we call these single-celled organisms protists. Section 1 The Characteristics of Cells Cells and Cell Theory, continued

5. < BackNext >PreviewMain Chapter 4 Leeuwenhooek also observed blood cells from different animals and was the first person to observe bacteria. Nearly 100 years later, Matthias Schleiden, a plant scientists, concluded that all plant parts were made of cells. A year later, in 1839, Theodor Schwann concluded that all animal tissues were made of cells. Section 1 The Characteristics of Cells Cells and Cell Theory, continued

6. < BackNext >PreviewMain Chapter 4 Schwann went on to write the first two parts of modern cell theory: All organisms are made up of one or more cells. The cell is the basic unit of all living things. Section 1 The Characteristics of Cells Cells and Cell Theory, continued

7. < BackNext >PreviewMain Chapter 4 Cells and Cell Theory, continued In 1858, Rudolf Virchow added the third part of the cell theory: All cells come from existing cells. Section 1 The Characteristics of Cells

8. < BackNext >PreviewMain Cells: The Basic Units of Life Cell Theory Chapter 4

9. < BackNext >PreviewMain Chapter 4 Cell Size The vast majority of cells are too small to be seen without a microscope. If the volume of a cell becomes too large, the surface area of its membrane will not be able to let in enough materials and let out wastes. Cells must remain small to maintain a proper surface- area-to-volume ratio. Section 1 The Characteristics of Cells

10. < BackNext >PreviewMain Chapter 4 Parts of a Cell Cells come in many shapes and sizes, and may be specialized for different functions. But all cells have some parts in common. All cells have a cell membrane. The cell membrane is a protective layer that covers the cell’s surface and acts a barrier. The cell membrane separates the cell’s contents from its surroundings. Section 1 The Characteristics of Cells

11. < BackNext >PreviewMain Chapter 4 The cell membrane also controls the materials going into and out of the cell. Most of the contents of a cell, including the fluid, is called the cytoplasm. Organelles are part of the cytoplasm. Organelles are structures that have specific jobs inside the cell. Section 1 The Characteristics of Cells Parts of a Cell, continued

12. < BackNext >PreviewMain Chapter 4 All cells have DNA (deoxyribonucleic acid) at some point in their lives. DNA is the genetic material that carries instructions for making new cells. In eukaryotic cells, including plants and animals, the DNA is found within the nucleus of the cell. The nucleus is an organelle specialized to hold the DNA. The nucleus plays a role in growth, metabolism, and reproduction. Section 1 The Characteristics of Cells Parts of a Cell, continued

13. < BackNext >PreviewMain Chapter 4 Parts of a Cell, continued Bacteria have DNA, but do not have a nucleus. Their DNA floats free in the cytoplasm. Human blood cells have a nucleus and DNA as they are growing. Once mature, they lose their DNA and nucleus. Most cells, however, need DNA throughout their lives. The DNA provides instructions for making proteins. Section 1 The Characteristics of Cells

14. < BackNext >PreviewMain Chapter 4 Two Kinds of Cells Cells that do not have a nucleus are called prokaryotes. Bacteria and archaea are prokaryotes. Prokaryotic DNA is a round molecule, twisted like a rubber band. Prokaryotes have cell walls. They lack the membrane-bound organelles found in other organisms. Section 1 The Characteristics of Cells

15. < BackNext >PreviewMain Chapter 4 Two Kinds of Cells, continued Eukaryotes are organisms made up of cells that have a nucleus enclosed by a membrane. Eukaryotic cells are 10 times as large as prokaryotic cells, although most eukaryotic cells are still microscopic. Eukaryotes have membrane-bound organelles to carry out the functions of the cell. Section 1 The Characteristics of Cells

16. < BackNext >PreviewMain Section 1 The Characteristics of Cells Chapter 4