An organelle is a tiny structure inside a cell that performs a particular function. Organelles are only found in eukaryotic cells (those with a distinct nucleus), and are not found in prokaryotic cells (those without a distinct nucleus). Both animal and plant contain many types of organelles (or "little organs").
Just as any organ has a specialized, particular function to perform as part of a larger system, so these "little organs" within a cell have certain tasks they perform. Organelles are bounded by a membrane and are run by the cell's control center, the nucleus (which itself is an organelle). Each organelle has a job to do that is crucial to maintaining the life of the cell and in most eukaryotic cells, organelles can be grouped into three categories according to their general function. The organelle that directs a cell's activities and holds the cell's genetic information is the nucleus. Both plant and animal cells have a nucleus. It is the largest structure in animal cells and is separated from the rest of the cell's cytoplasm (jelly-like fluid) by a double membrane called a nuclear envelope.
Organelles also function in transport, synthesis (making things), storage, and recycling. In plant and animal cells, the organelles responsible for these activities are called the endoplasmic reticulum, ribosomes, Golgi bodies, and lysosomes. Plant cells also have organelles called vacuoles. The endoplasmic reticulum is a complex network of folded membranes that form tubes and transport, or move, materials to all parts of the cell. They are something like a pipeline. Ribosomes play an important role in the synthesis, or making of, proteins. Golgi bodies look like a stack of flattened pancakes. They put the finishing touches on proteins, and then sort the proteins and pack them for transport. Lysosomes are bags of enzymes that help a cell digest the food it takes in. Only plant cells have a large sac called a vacuole that they use for storage.
Other organelles function to produce energy. Called mitochondria and chloroplasts, these organelles are responsible for changing energy from one form to another. Mitochondria are found in both plant and animal cells and are called the powerhouses of the cell because these organelles break down a cell's food and release energy.
Although chloroplasts also produce energy for cell, they are a little different than mitochondria for two reasons. First, chloroplasts are not found in animal cells, they are only found in plant cells. Second, unlike animals that must take food into their bodies, plants can make their own food from which they obtain their energy. Plants create this food using small, green organelles called chloroplasts that capture the energy in sunlight. Chloroplasts use this trapped radiant energy of the Sun and turn it into chemical energy for use or storage. When the plant is ready to use the energy, the mitochondria take over and release the stored energy.
An organelle is a specialized cellular structure in eukaryotic cells analogous to an organ in the body. Organelles are discrete structures within the cell that perform a specialized function. Most are surrounded by internal membranes and can be seen in the light or the electron microscope. Organelles increase the efficiency of cellular processes by concentrating the factors necessary to carry out specific biochemical reactions separate from the rest of the cell. Bacterial cells do not contain organelles or intracellular membrane-bound structures. Examples of organelles are lysosomes, nucleus , mitochondria , and the endoplasmic reticulum .
see also Cell; Chloroplast; Endoplasmic Reticulum; Golgi; Lysosomes; Mitochondrion; Nucleus; Ribosome; Vacuole
Stephen A. Adam
or·gan·elle / ˌôrgəˈnel/ • n. Biol. any of a number of organized or specialized structures within a living cell.