Biology (Holt)

Scanning electron microscopeIn a scanning electron microscope,

shown in Figure 5,the electron beam is focused on a specimen

coated with a very thin layer of metal. The electrons that bounce off

the specimen form an image on a fluorescent screen. A scanning

electron micrograph (SEM) shows three-dimensional images of cell

surfaces, such as the image of sperm cells shown in Figure 5. As

with the transmission electron microscope, images produced by the

scanning electron microscope are black and white, but often they

are artificially colored.

Scanning Tunneling Microscope

New video and computer techniques are increasing the resolution and

magnification of microscopes. The

uses a needle-like probe to measure differences in voltage caused by elec-

trons that leak, or tunnel, from the surface of the object being viewed. A

computer tracks the movement of the probe across the object, enabling

objects as small as individual atoms to be viewed. The computer

generates a three-dimensional image of the specimen’s surface. The

scanning tunneling microscope can be used to study living organisms.

scanning tunneling microscope

54 CHAPTER 3Cell Structure

In a scanning electron micro-
scope, electrons bounce off a
specimen, forming a three-
dimensional image of the
specimen on a fluorescent
screen.

Figure 5 Scanning
electron microscope

SEM of sperm

Describethe relationship between a meter, a

millimeter, and a micrometer.

Describehow magnification and resolution

affect the appearance of objects viewed under

a microscope.

Comparethe magnifying power of a light micro-

scope with the magnifying power of an electron

microscope.

Critical ThinkingRecognizing Differences

Explain why electron microscopes cannot be

used to view the structure of living cells.

Critical ThinkingComparing Functions

Assume that for the purposes of your investiga-

tion, you need detailed images of the internal

structure of a bacterium. What type of micro-

scope would you select for that that task?

Explain your answer.

The English scientist Robert

Hooke used a crude microscope to examine

Aindividual atoms Csingle-celled organisms

Belectrons Dcork cells

TAKS Test PrepTAKS Test Prep

Section 1 Review

2A

2A

2A

2A

2A

3F