Biology (Holt)

Table 1 Metric Units of Length and Equivalents

Unit Prefix Metric equivalent Real-life equivalent

Kilometer (km) Kilo- 1,000 m About two-thirds of a mile

Meter (m) 1 m (SI base unit) A little more than a yard

Centimeter (cm) Centi- 0.01 m About half the diameter of a Lincoln penny

Millimeter (mm) Milli- 0.001 m About the width of a pencil tip

Micrometer (μm) Micro- 0.000001 m About the length of an average bacterial cell

Nanometer (nm) Nano- 0.000000001 m About the length of a water molecule

Section 1 Looking at Cells

50 CHAPTER 3Cell Structure

Cells Under the Microscope

Most cells are too small to see with the naked eye; a typical human

body cell is many times smaller than a grain of sand. Scientists

became aware of cells only after microscopes were invented, in the

1600s. When the English scientist Robert Hooke used a crude

microscope to observe a thin slice of cork in 1665, he saw “a lot of

little boxes.” The boxes reminded him of the small rooms in which

monks lived, so he called them cells. Hooke later observed cells in

the stems and roots of plants. Ten years later, the Dutch scientist

Anton van Leeuwenhoek used a microscope to view water from a

pond, and he discovered many living creatures. He named them

“animalcules,” or tiny animals. Today we know that they were not

animals but single-celled organisms.

Measuring Cell Structures

Measurements taken by scientists are expressed in metric units.

Scientists throughout the world use the metric system. The official

name of the metric system is the International System of

Measurements, abbreviated as SI. SI is a decimal system, so all rela-

tionships between SI units are based on powers of 10. For example,

scientists measure the sizes of objects viewed under a microscope

using the SI base unit for length, which is the meter. A meter, which

is about 3.28 ft (a little more than a yard), equals 100 centimeters

(cm), or 1,000 millimeters (mm). A meter also equals 0.001 kilome-

ter (km). Most SI units have a prefix that indicates the relationship

of that unit to a base unit. For example, the symbol “μ” stands for

the metric prefix micro. A micrometer (μm) is a unit of linear meas-

urement equal to one-millionth of a meter, or one-thousandth of a

millimeter. Table 1summarizes the SI units used to measure length.

Objectives

Describehow scientists
measure the length of
objects.

Relatemagnification and
resolution in the use of
microscopes.

Analyzehow light
microscopes function.

Comparelight
microscopes with electron
microscopes.

Describethe scanning
tunneling microscope.

Key Terms

light microscope

electron microscope

magnification

resolution

scanning tunneling

microscope

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