Cell Diversity

Cells of different organisms and even cells within the same organism are very diverse in terms of shape, size, and internal organization. One theme that occurs again and again throughout biology is that form follows function. In other words, a cell's function influences its physical features.

Cell Shape

The diversity in cell shapes reflects the different functions of cells. Compare the cell shapes shown in Figure 4-4. The long extensions that reach out in various directions from the nerve cell shown in Figure 4-4a allow the cell to send and receive nerve impulses. The flat, platelike shape of skin cells in Figure 4-4b suits their function of covering and protecting the surface of the body. As shown below, a cell's shape can be simple or complex depending on the function of the cell. Each cell has a shape that has evolved to allow the cell to perform its function effectively.

figure 4-4

Cells have various shapes. (a) Nerve cells have long extensions. (b) Skin cells are flat and platelike. (c) Egg cells are spherical. (d) Some bacteria are rod shaped. (e) Some plant cells are rectangular.

(a) Nerve cell

(d) Bacterial cells

(d) Bacterial cells figure 4-4

Cells have various shapes. (a) Nerve cells have long extensions. (b) Skin cells are flat and platelike. (c) Egg cells are spherical. (d) Some bacteria are rod shaped. (e) Some plant cells are rectangular.

Diverse Thinking

(a) Nerve cell

(e) Plant cells

(e) Plant cells

Cell Size

Cells differ not only in their shape but also in their size. A few types of cells are large enough to be seen by the unaided human eye. For example, the nerve cells that extend from a giraffe's spinal cord to its foot can be 2 m (about 6 1/2 ft) long. A human egg cell is about the size of the period at the end of this sentence. Most cells, however, are only 10 to 50 pm in diameter, or about 1/500 the size of the period at the end of this sentence.

The size of a cell is limited by the relationship of the cell's outer surface area to its volume, or its surface area-to-volume ratio. As a cell grows, its volume increases much faster than its surface area does, as shown in Figure 4-5. This trend is important because the materials needed by a cell (such as nutrients and oxygen) and the wastes produced by a cell (such as carbon dioxide) must pass into and out of the cell through its surface. If a cell were to become very large, the volume would increase much more than the surface area. Therefore, the surface area would not allow materials to enter or leave the cell quickly enough to meet the cell's needs. As a result, most cells are microscopic in size.

Materials microscope, prepared slides of plant (dicot) stem and animal (human) skin, pencil, paper Procedure Examine slides by using medium magnification (100x). Observe and draw the surface cells of the plant stem and the animal skin.

Analysis How do the surface cells of each organism differ from the cells beneath the surface cells? What is the function of the surface cells? Explain how surface cells are suited to their function based on their shape.

Comparing Surface Cells

Materials microscope, prepared slides of plant (dicot) stem and animal (human) skin, pencil, paper Procedure Examine slides by using medium magnification (100x). Observe and draw the surface cells of the plant stem and the animal skin.

Analysis How do the surface cells of each organism differ from the cells beneath the surface cells? What is the function of the surface cells? Explain how surface cells are suited to their function based on their shape.

1. All cubes have volume and surface area. The total surface area is equal to the sum of the areas of each of the six sides (area = length X width).

figure 4-5

Small cells can exchange substances more readily than large cells because small objects have a higher surface area-to-volume ratio.

2. If you split the first cube into eight smaller cubes, you get 48 sides. The volume remains constant, but the total surface area doubles.

3. If you split each of the eight cubes into eight smaller cubes, you have 64 cubes that together contain the same volume as the first cube. The total surface area, however, has doubled again.

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