Organelle (cell biology)

An organelle is a structure that performs a specific function in the body of a cell, the fundamental unit of all life. Each type of organelle is unique unto itself, though a cell could not survive if even one ceased performing its task. The cells of most complex living things contain the same standard set of organelles, but a few organelles are specific to particular organisms.

All cells may be classified into two broad categories: prokaryotic and eukaryotic. Prokaryotic cells are small and simple and lack any organelles whatsoever. Some bacteria are examples of prokaryotes. Eukaryotic cells are larger and more complex and operate with a number of important organelles. These cells compose multicellular organisms—living things made of more than one cell—such as animals and plants. The cells of these two groups contain a number of the same organelles but also maintain some important differences.

Animal Organelles

The first important organelle in an animal cell is the cell membrane. This is the outer casing that forms the physical structure of the cell itself. The membrane is made mostly of molecules of fat but also some portions of protein. The cell membrane protects the cell by preventing foreign materials from entering it. The membrane, however, is not impermeable; it allows important chemicals within the body to pass into the cell while also permitting materials the cell produces to be sent out to the rest of the body.

Cytoplasm takes up most of the space inside the cell. The rest of the cell's organelles are located within the cytoplasm. The cytoplasm contains a cytoskeleton, a tough protein network that supports the cell structure. It is located throughout the cytosol, the gel-type substance dispersed throughout the cell.

Within the animal cell's cytosol are two types of organelles: membranous and non-membranous. Membranous organelles are cell organs enclosed by a membrane. Non-membranous organelles are not separated from other cell parts in this way and function directly in the cytosol. The cells of human beings boast five membranous organelles: endoplasmic reticulum, Golgi complexes, lysosomes, peroxisomes, and mitochondria.

The endoplasmic reticulum is an intricate network of membrane tubes that carry proteins and lipids, or fats, to all parts of the cell. The Golgi complex (also known as the Golgi apparatus) is a conglomeration of flat membranous storage areas that prepare important chemicals such as proteins and send them to parts of the cell that need them. Lysosomes are small, irregularly shaped sacs that digest the chemical materials the cell absorbs.

Somewhat related to lysosomes are peroxisomes. These organs, smaller than lysosomes, break down fatty acids and the harmful materials that eventually become human waste. Finally, the oval-shaped mitochondria are responsible for deriving energy from food and allocating it to the necessary parts of the cell. Mitochondria are so important to cell life that up to several thousand may be found in one cell.

More numerous than membranous organelles, however, are non-membranous organelles. The most common of these are ribosomes, which create proteins for use by various parts of the cell. Other non-membranous organelles include cilia, sensory nerves that help cells move and detect nearby objects; flagella, longer cilia used for more advanced cell movement; and centrioles, which assist in constructing the new cell that forms after a singular cell divides itself.

The most important organ in an entire cell is the nucleus. Encased by a double-layered membrane, this large, complex, membranous organelle is located at the cell center and protects the genetic chemical deoxyribonucleic acid, or DNA. DNA controls the cell's protein creation and forms the genetic basis of new, identical cells, which are created from existing cells. It is vital that cell nuclei keep DNA safe because it is this genetic material that directly forms offspring in reproduction.

Plant Organelles

Plant cells function similarly to animal cells. They contain all the same membranous and non-membranous organelles, such as nuclei and cilia, but also house several more parts not present in animal cells. These include cell walls, vacuoles, and chloroplasts.

Cell wall are similar to the cell membranes that all plant and animal cells have. In plants, however, cells walls surround the cell membrane and give the cell added protection and structural support. Vacuoles are large storage sacs that engorge with water and other chemical materials to ensure that cells have access to these substances when needed. Finally, all plant cells contain chloroplasts, organelles that contain the material chlorophyll that uses water and light energy from the Sun to make food for the plant. Chloroplasts are the reason that plants are largely self-sustaining organisms.

Organelles in Specialized Cells

Not all animal or plant cells are exactly alike. Rather, many are tailored to carry out certain specific tasks. Such cells may contain more or fewer than the standard number and variety of organelles. In humans, for instance, red blood cells contain no nucleus because their only purpose is to transport oxygen to all regions of the body. Therefore, these cells are composed simply of a thin membrane that protects the copious amounts of hemoglobin inside. Hemoglobin holds the oxygen that will be delivered to the rest of the body.

Plants contain specialized cells as well. For example, a plant's leaf cells contain more chloroplasts than any other part of the plant body. This is because the leaves that absorb the Sun's light and provide sustenance for the plant. These cells differ drastically from a plant's root hair cells. These long, thin cells are located in a plant's roots, where they take in water and nutrients from the soil to keep the plant alive. All the organelles that a plant or animal cell contains play vital roles in keeping the organism alive and functioning properly.

Bibliography

“Cell Structure.” BBC Bitesize, 2024, www.bbc.co.uk/bitesize/guides/zpqpqhv/revision/7. Accessed 4 Dec. 2024.

Evans, Joanne, and Ania L. Manson. “Eukaryotic Organelles.” Cell Biology and Genetics, 3rd ed., Mosby Elsevier, 2008, pp. 23–29.

"Organelle." National Human Genome Research Institute, 4 Dec. 2024, www.genome.gov/genetics-glossary/Organelle. Accessed 4 Dec. 2024.

Sherwood, Lauralee. “Cell Physiology.” Human Physiology: From Cells to Systems, 8th ed., Brooks/Cole, Cengage Learning, 2013, pp. 22–33.