Course: SAT 170 Introductory Biotechnology

Unit 2 Learning Module

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Cells - The Basic Units of Life (continued)
Organelles

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Nucleus

Perhaps the largest and most obvious of all of the eukaryotic organelles is the nucleus. This structure plays a very important role as it houses the cell’s DNA and the nucleolus. This DNA is protected by the nuclear envelope which, as its name implies, is a double membrane that surrounds the nucleus. Within the nuclear envelope are nuclear pores which are holes that can regulate what enters and exits the nucleus. For example, also within the nucleus is the nucleolus. The nucleolus is the site where ribosomes are assembled. Ribosomes play an important role in the cell as they make proteins; however, they carry out this role in the cytoplasm. So once a ribosome is made in the nucleolus, it must be shipped out of the nucleus into the cytoplasm through the nuclear pores of the nuclear envelope.

See Figure 3: As is evident in this image, the nucleus is surrounded by a double membrane which contains small holes known as nuclear pores. In the center of the nucleus you can see the structure known as the nucleolus.

Endoplasmic Reticulum (ER)

The endoplasmic reticulum is a series of membrane bound flattened strictures which often lie just outside of the nucleus. There are two regions of the ER that are easily distinguishable based on their appearance. The rough ER, which have ribosomes on their surface, function in protein synthesis for the cell. The smooth ER, which lack ribosomes on their surface, function in lipid synthesis as well as the detoxification of drugs for the cell. In some instances, the smooth ER can also serve more specialized functions for the cell. For example, in muscle cells, the smooth ER serves as a site for calcium storage which is essential for muscle contractions.

See Figure 4: Note the flattened sacs that comprise the endoplasmic reticulum shown in this image. Observe the connection between the nucleus (left) and the ER (right); you can see materials moving through the nuclear pores of the nucleus into the ER. This particular region of ER contains ribosomes and thus represents the rough ER.

Golgi Apparatus

Once a product leaves the ER it is sent to a series of membrane bound structures where it will be modified, packaged and shipped to various locations throughout the cell. The organelle that accomplishes this is known as the golgi apparatus or golgi complex.

See Figure 5: This image is an electron micrograph of the golgi complex. You can see the vesicles on either side of the series of flattened sacs known as the golgi complex.

Lysosomes

Lysosomes are often referred to as the digestive system of the cell. They are membrane bound structures which contain digestive enzymes. This allows them to break down unwanted materials such as bacteria, food, and worn out cell parts.

Vacuoles

Large membrane bound structures are known as vacuoles. There are many types of vacuoles, but they commonly function in storage for the cell.

Mitochondria

The mitochondria are very important cell structures as they serve as the site of many of the reactions of cellular respiration in our cells. Cellular respiration is our main metabolic pathway which converts fats and sugars into energy (in the form of adenosine triphosphate or ATP) while using oxygen and producing carbon dioxide.

See Figure 6: Considered to be the powerhouse of the cell, the mitochondria has several unique features including a double membrane clearly shown in the image here.

Chloroplasts

Chloroplasts are structures that are found in plants and certain types of protists such as algae to serve as the site of photosynthesis. During the process of photosynthesis, light energy and carbon dioxide can be converted into valuable compounds such as sugars and oxygen. Chloroplasts contain the green pigment known as chlorophyll which is responsible for giving certain photosynthetic organisms their green color.

Cytoskeleton

The cytoskeleton is a complex network of protein fibers which forms the basic framework for our cells. It is involved in structural support, transport and movement for our cells, much like the skeleton of our bodies.

See Figure 7: Flourescent proteins have been used to highlight the different parts of the cytoskeleton. Notice how this network of fibers that create the basic shape of the cell.

Cilia and Flagella

Cilia and flagella are extracellular appendages which aid in movement, either by moving things across the surface of the cell or by moving the cell itself. While cilia are short, hair-like extensions which move back and forth, flagella are long, hair-like extensions which move in a whip-like fashion.

See Figure 8: Cilia are extracellular appendages that aid in cell movement. They tend to be short extensions and often cover the cell surface.

See Figure 9: In contrast to the cilia, the flagella, shown here, tend to be longer and are typically present in 1 to maybe a few per cell

It is important to understand that while these are the basic components of a eukaryotic cell, there are some features that we did not go over in detail. It is also important to understand, that even though we described the features of both eukaryotic and prokaryotic cells, different cells may exhibit different structure. Even though two cells may be both eukaryotes, they may not have the same exact cell structure. For example, a nerve cell has a very different structure than a muscle cell, and a plant cell is quite different from an animal cell. I tried to focus this material to that which is most relevant to understanding the current field of biotechnology.
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