General Cell Structure

In Schwann's time, little was known about cells except that they were enclosed in a membrane and contained a nucleus. The fluid between the nucleus and surface membrane, called cytoplasm, was thought to be little more than a gelatinous mixture of chemicals. The transmission electron microscope (TEM), invented in the mid-twentieth century, radically changed this concept. Using a beam of electrons in place of light, the TEM enabled biologists to see a cell's ultrastructure (fig. 3.3), a fine degree of detail extending even to the molecular level. The most important thing about a good microscope is not magnification but resolution—the ability to reveal detail. Any image can be photographed and enlarged as much as we wish, but if enlargement fails to reveal any more useful detail, it is empty magnification. A big fuzzy image is not nearly as informative as one that is small and sharp. The TEM reveals far more detail than the light microscope (LM), even at the same magnification (fig. 3.4). A later

Microscope Cell Detail Image
2.0 |m
Figure 3.3 Ultrastructure of a White Blood Cell as Seen by TEM.

<+ * %: v ' '.J * ¿0 * • "» -

(a)

* mBm y ■ vv - * ■ „ „ ' - * 0

(b) ' 2.0 |m

Figure 3.4 Magnification Versus Resolution. These cells were photographed at the same magnification (about X750) through (a) a light microscope and (b) a transmission electron microscope.

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

Chapter 3 Cellular Form and Function 97

invention, the scanning electron microscope (SEM), produces dramatic three-dimensional images at high magnification and resolution (see fig. 3.12), but can only view surface features.

Table 3.1 gives the sizes of some cells and subcellu-lar objects relative to the resolution of the naked eye, light microscope, and electron microscope. You can see why the very existence of cells was unsuspected until the light microscope was invented and why little was known about their internal components until the TEM became available.

Figure 3.5 shows some major constituents of a typical cell. The cell is surrounded by a plasma (cell) membrane made of proteins and lipids. The composition and functions of this membrane can differ significantly from one region of a cell to another, especially between the basal, lateral, and apical (upper) surfaces of cells like the one pictured.

The cytoplasm is crowded with fibers, tubules, passageways, and compartments (see photographs on pp. 93 and 985). It includes several kinds of organelles and a supportive framework called the cytoskeleton—all of which we will study in this chapter. A cell may have 10 billion protein molecules, including potent enzymes with the potential to destroy the cell if they are not contained and isolated from other cellular components. You can imagine the enormous problem of keeping track of all this material, directing molecules to the correct destinations, and maintaining order against the incessant trend toward disorder. Cells maintain order partly by compartmentalizing their contents in the organelles. The organelles and cytoskele-ton are embedded in a clear gel called the cytosol or intracellular fluid (ICF). The fluid outside the cell is extracellular fluid (ECF).

Before You Go On

Answer the following questions to test your understanding of the preceding section:

1. What are the basic principles of the cell theory?

2. What does it mean to say a cell is squamous, stellate, columnar, or fusiform?

3. Why can cells not grow to unlimited size?

4. What is the difference between cytoplasm and cytosol?

5. Define intracellular fluid (ICF) and extracellular fluid (ECF).

Table 3.1 Sizes of Some Biological Structures in Relation to the Resolving Power of the Human Eye, Light Microscope (LM), and Transmission Electron Microscope (TEM)

Object

Size

Human egg, diameter

100 ^m

Resolution of the unaided eye

70-100 ^m

Most human cells, diameter

10-15 ^m

Cilia, length

7-10 ^m

Mitochondria, width x length

0.2 x 4 ^m

Bacteria (E. coli), length

1-3 ^m

Microvilli, length

1-2 ^m

Lysosomes, diameter

0.5 ^m = 500 nm

Resolution of the light microscope

200 nm

Nuclear pores, diameter

30-100 nm

Centriole, diameter x length

20 x 50 nm

Polio virus, diameter

30 nm

Ribosomes, diameter

15 nm

Globular proteins, diameter

5-10 nm

Plasma membrane, thickness

7.5 nm

DNA molecule, diameter

2.0 nm

Plasma membrane channels, diameter

0.8 nm

Resolution of the TEM

0.5 nm

Carbon atom, diameter

0.15 nm

Hydrogen atom, diameter

0.07 nm

Saladin: Anatomy & I 3. Cellular Form and I Text I © The McGraw-Hill

Physiology: The Unity of Function Companies, 2003 Form and Function, Third Edition

98 Part One Organization of the Body

Apical surface of cell

Microvillus -

Terminal web -

Lipid droplet -

(an inclusion)

Centrosome Centriole

Ribosomes free in cytosol

Microtubule

Nuclear pore

Nucleus

Nuclear envelope

Rough endoplasmic reticulum

Ribosomes on rough ER

Basement membrane

Terminal web -

Ribosomes free in cytosol

Microtubule

Nuclear pore

Nucleus

Nuclear envelope

Rough endoplasmic reticulum

Ultrastructure Nuclear Envelope

Secretory vesicles -Cytosol

Lateral surface of cell

Plasma membrane Golgi complex Golgi vesicle

Smooth endoplasmic reticulum

Lysosome

Microfilament Mitochondrion

Basal surface of cell

Figure 3.5 Structure of a Representative Cell.

Secretory vesicles -Cytosol

Lateral surface of cell

Plasma membrane Golgi complex Golgi vesicle

Smooth endoplasmic reticulum

Lysosome

Microfilament Mitochondrion

Basal surface of cell

Figure 3.5 Structure of a Representative Cell.

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Responses

  • Alexander Bey
    What does it mean to say a cell is squamous, stellate, columnar, or fusiform?
    7 years ago
  • Razanur Smallburrow
    What does it mean to say a cell is squamous, stellate, collumnar, or fusiform?
    4 years ago
  • Isengrin
    What does it mean to say a cell is squamous, stellate, columnar, or fusiorm?
    3 years ago

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