You Gotta Know These Organelles
The word “organelle” comes from the Latin for “little organ,” which fits their function as organized structures within cells that allow the cell to survive.
- Nucleus: The nucleus is the “command central” of the cell because it contains almost all of the cell’s DNA, which encodes the information needed to make all the proteins that the cell uses. The DNA appears as chromatin through most of the cell cycle, but condenses to form chromosomes when the cell is undergoing mitosis. Within the nucleus there are dense bodies called nucleoli, which contain ribosomal RNA. In eukaryotes, the nucleus is surrounded by a selectively-permeable nuclear envelope.
- Ribosomes: Ribosomes are the machines that coordinate protein synthesis, or translation. They consist of several RNA and protein molecules arranged into two subunits. Ribosomes read the messenger RNA copy of the DNA and assemble the appropriate amino acids into protein chains.
- Mitochondria: The powerhouses of the cell. Mitochondria are double-membrane-bound organelles that are the site of respiration and oxidative phosphorylation, processes that produce energy for the cell in the form of ATP. The inner membrane of a mitochondrion forms folds called cristae, which are suspended in a fluid called the matrix. The mitochondrial matrix contains DNA and ribosomes.
- Endoplasmic reticulum: The ER is a network of tube-like membranes continuous with the nuclear envelope. Part of it are “rough” because they are covered in ribosomes, while other parts are “smooth” because they aren’t. In the ER, proteins undergo modifications and folding to yield the final, functional protein structures.
- Golgi apparatus: The stack of flattened, folded membranes that forms the Golgi apparatus acts as the “post office of the cell.” Here proteins from the ribosomes are stored, chemically modified, “addressed” with carbohydrate tags, and packaged in vesicles for delivery.
- Lysosomes: Lysosomes are membrane-bound organelles that contain digestive enzymes that break down proteins, lipids, carbohydrates, and nucleic acids. They are important in processing the contents of vesicles taken in from outside the cell. It is crucial to maintain the integrity of the lysosomal membranes because the enzymes they contain can digest cellular components as well.
- Chloroplasts: Found only in plants and certain protists, the chloroplast contains the green pigment chlorophyll and is the site of photosynthesis. Like the mitochondrion, a chloroplast is a double-membrane-bound organelle, and it has its own DNA and ribosomes in the stroma. Chloroplasts contain grana, which are stacks of single membrane structures called thylakoids on which the reactions of photosynthesis occur.
- Vacuoles: Found mainly in plants and protists, vacuoles are liquid-filled cavities enclosed by a single membrane. They serve as storage bins for food and waste products. Contractile vacuoles are important for freshwater protists to rid their cells of excess water that accumulates because of salt imbalance with the environment.
- Cilia and flagella: Cilia and flagella are important organelles of motility, that is, they allow the cell to move. Flagella are long, whip-like structures, while cilia are short, hair-like projections. Both contain a 9 + 2 arrangement of microtubules in cross-section (two microtubules in the middle, nine pairs in a circle around the outside) and are powered by molecular motors of kinesin and dynein molecules.
- Centrioles: Not found in plant cells, centrioles are paired organelles with nine sets of microtubule triplets in cross section. They are important in organizing the microtubule spindle needed to move the chromosomes during mitosis.