Biology 3.1

1. Biology 3.1 Looking at Cells

2. Measuring Cell Structure • Measurements taken by scientists are expressed in metric units. • The official name of the metric system is the International System of Measurementsabbreviated as SI.

3. Measuring Cell Structure • SI is a decimal system so all units are based upon powers of 10. • A micrometer is a unit of measure in the SI system that stands for one-millionth of a meter.

4. Characteristics of Microscopes • Since Robert Hooke first observed cork cells, microscopes have unveiled the details of cell structure. • Biologists use different types of microscopes depending on the organisms they wish to study • Two common types of microscopes are light microscopes and electron microscopes

5. Light Microscope • In a light microscope, light passes through one or more lenses to produce an enlarged image of a specimen.

6. Electron Microscopes • An electron microscope forms an image of a specimen using a beam of electrons rather than a light. Electron microscope image

7. Characteristics of Microscopes • An image produced by a microscope is called a micrograph. • Many micrographs are labeled with the kind of microscope that produced the image. • These microscopes include light micrograph (LM), a transmission electron microscope (TEM), or a scanning electron microscope (SEM).

8. Characteristics of Microscopes • Resolution is a measure of the clarity of an image. • Both high magnification and good resolution are needed to view the details of extremely small objects clearly. • Electron microscopes have much higher magnifying and resolving powers than light microscopes. Electron microscope image of a fly’s foot

9. Characteristics of Microscopes • Micrographs are often labeled with the magnification value of the image. • Magnification is the quality of making an image appear larger than it’s actual size. • For example, a magnification value of 200X indicates that an image is 200 times larger than the object’s actual size.

10. Compound Light Microscopes • Light microscopes that use two lenses are called compound light microscopes. • In a typical compound light microscope, a light bulb in the base shines light up through the specimen, which is mounted on a glass slide.

11. Compound Light Microscopes • The objective lens, closest to the specimen, collects the light, which than travels to the ocular lens, closest to the viewers eye. • Both lenses magnify the image. • Thus a 40X objective lens and a 10X ocular lens produces a total magnification of 400X. • (40 X 10 = 400)

12. Electron Microscopes • Electron microscopes can magnify an image up to 200,000 times allowing them to study structure inside cells or on cell surfaces. • In electron microscopes, both the electron beam and the specimen must be placed in a vacuum chamber so that the electrons in the beam will not bounce off gas molecules in the air.

13. Electron Microscopes • Because living cells can not survive in a vacuum, they cannot be viewed using electron microscopes!!! • There are two types of primary electron microscopes; the Transmission Electron Microscope (TEM) and the Scanning Electron Microscope (SEM)

14. Electron Microscopes • In a Transmission Electron Microscope (TEM) the electron beam is directed at a thin slice of a specimen stained with metal ions. • Some structures in the specimen become more stained than others as they absorb more of the metal ions.

15. Electron Microscopes • The heavily stained parts of the specimen absorb electrons, while those that are lightly stained allow electrons to pass through. • The electrons that pass through a specimen strike a fluorescent screen, forming an image on the screen.

16. Electron Microscopes • In a Scanning Electron Microscope (SEM) 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

17. Electron Microscopes • A Scanning Electron Microscope (SEM) shows three-dimensional images of cell surfaces. • As with the Transmission Electron Microscope (TEM), the images produced are black and white but often they are artificially colored afterwards.

18. Scanning Tunnel Microscope • The Scanning Tunnel Microscope uses a needle-like probe to measure differences in voltage caused by electrons that leak, or tunnel, from the surface of an object being viewed.

19. Scanning Tunnel Microscope • 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 tunnel organism can be used to study living organisms.