32 Eukaryotic Cells
Learning Objectives
By the end of this section, you will be able to do the following:
- Describe the structure of eukaryotic cells
- Compare animal cells with plant cells
- State the role of the plasma membrane
- Summarize the functions of the major cell organelles
Have you ever heard the phrase “form follows function?” It’s a philosophy that many industries follow. In architecture, this means that buildings should be constructed to support the activities that will be carried out inside them. For example, a skyscraper should include several elevator banks. A hospital should have its emergency room easily accessible.
Our natural world also utilizes the principle of form following function, especially in cell biology, and this will become clear as we explore eukaryotic cells (Figure 4.8). Unlike prokaryotic cells, eukaryotic cells have: 1) a membrane-bound nucleus; 2) numerous membrane-bound organelles such as the endoplasmic reticulum, Golgi apparatus, chloroplasts, mitochondria, and others; and 3) several rod-shaped chromosomes. Because a membrane surrounds a eukaryotic cell’s nucleus, it has a “true nucleus.” The word “organelle” means “little organ,” and as we already mentioned, organelles have specialized cellular functions, just as your body’s organs have specialized functions. Eukaryotic cells are also, on average, about ten times larger than prokaryotic cells. Eukaryotic organisms include plants, animals, fungi, and protists.
At this point, it should be clear to you that eukaryotic cells have a more complex structure than prokaryotic cells. Organelles allow different functions to be compartmentalized in different areas of the cell. Before turning to organelles, let’s first examine two important components of the cell: the plasma membrane and the cytoplasm.
Visual Connection
If the nucleolus were not able to carry out its function, what other cellular organelles would be affected?
The Plasma Membrane
Like prokaryotes, eukaryotic cells have a plasma membrane (Figure 4.9), a phospholipid bilayer with embedded proteins that separates the internal contents of the cell from its surrounding environment. The plasma membrane controls the passage of organic molecules, ions, water, and oxygen into and out of the cell. Wastes (such as carbon dioxide and ammonia) also leave the cell by passing through the plasma membrane. The plasma membrane also allows the cell to interact with and respond to its environment.
The plasma membranes of cells that specialize in absorption fold into fingerlike projections that we call microvilli (singular = microvillus) (Figure 4.10). These work to increase surface area. Such cells typically line the small intestine, the organ that absorbs nutrients from digested food. This is an excellent example of form following function. People with celiac disease have an immune response to gluten, which is a protein in wheat, barley, and rye. The immune response damages microvilli, and thus, afflicted individuals c