CONCEPTS IN BIOLOGY

PART II. CORNERSTONES: CHEMISTRY, CELLS, AND METABOLISM

 

4. Cell Structure and Function

 

4.2. Cell Size

Cells of different kinds vary greatly in size (figure 4.4). In general, the cells of Bacteria and Archaea are much smaller than those of eukaryotic organisms. Prokaryotic cells are typically 1-2 micrometers in diameter, whereas eukaryotic cells are typically 10-100 times larger.

FIGURE 4.4. Comparing Cell Sizes

Most cells are too small to be seen with the naked eye. Bacteria and Archaea cells are generally about 1-2 micrometers in diameter. Eukaryotic cells are much larger and generally range between 10 and 100 micrometers. A micrometer is 1/1,000 of a millimeter. A sheet of paper is about 1/10 of a millimeter thick, which is about 100 micrometers. Therefore, some of the largest eukaryotic cells are just visible to the naked eye.

Some basic physical principles determine how large a cell can be. A cell must transport all of its nutrients and all of its wastes through its outer membrane to stay alive. Cells are limited in size because, as a cell becomes larger, adequate transport of materials through the membrane becomes more difficult. The difficulty arises because, as the size of a cell increases, the amount of living material (the cell’s volume) increases more quickly than the size of the outer membrane (the cell’s surface area). As cells grow, the amount of surface area increases by the square (X2) but volume increases by the cube (X3). This mathematical relationship between the surface area and volume is called the surface area-to-volume ratio and is shown for a cube in figure 4.5. Notice that, as the cell becomes larger, both surface area and volume increase. Most important, volume increases more quickly than surface area, causing the surface area-to-volume ratio to decrease. As the cell’s volume increases, the cell’s metabolic requirements increase but its ability to satisfy those requirements is limited by the surface area through which the needed materials must pass. Consequently, most cells are very small.

FIGURE 4.5. Surface Area-to-Volume Ratio

As the size of an object increases, its volume increases faster than its surface area. Therefore, the surface area- to-volume ratio decreases.

There are a few exceptions to this general rule, but they are easily explained. For example, what we call the yolk of a chicken’s egg cell is a single cell. However, the only part of an egg cell that is metabolically active is a small spot on its surface. The largest portion of the egg cell is simply inactive stored food called yolk. Similarly, some plant cells are very large but consist of a large, centrally located region filled with water. Again, the metabolically active portion of the cell is at the surface, where exchange of materials with the surroundings is possible.

4.2. CONCEPT REVIEW

3. On the basis of surface area-to-volume ratio, why do cells tend to remain small?

4. What happens to the surface-to-volume ratio when folds are made in a cell’s outer membrane?