The word organelle means “little organ.” Organelles are much smaller than plant or animal organs, though. Much like an organ serves a specific function in an organism, such as an eye helps a fish see or a stamen helps a flower reproduce, organelles each have specific functions within cells. Cells are self-contained systems within their respective organisms, and the organelles inside of them work together like components of an automated machine to keep things operating smoothly. When things don’t operate smoothly, there are organelles responsible for cellular self-destruction, also known as programmed cell death.
Many things float around in a cell, and not all of them are organelles. Some are called inclusions, which is a category for items such as stored cell products or foreign bodies that made their way into the cell, like viruses or debris. Most, but not all organelles are surrounded by a membrane to protect them from the cytoplasm they are floating in, but this is usually not true of cellular inclusions. In addition, inclusions are not essential for the cell’s survival, or at least functioning, in the way that organelles are.
TL;DR (Too Long; Didn't Read)
Cells are the building blocks of all living organisms. They are self-contained systems within their respective organisms, and the organelles inside of them work together like components of an automated machine to keep things operating smoothly. Organelle means “little organ.” Each organelle has a distinct function. Most are bound in one or two membranes to separate it from the cytoplasm that fills the cell. Some of the most vital organelles are the nucleus, the endoplasmic reticulum, the Golgi apparatus, the lysosomes and the mitochondria, although there are many more.
Cells' First Sightings
In 1665, an English natural philosopher named Robert Hooke examined thin slices of cork, as well as wood pulp from several kinds of trees and other plants, under a microscope. He was astonished to find marked similarities between such different materials, which all reminded him of a honeycomb. In all of the samples, he saw many adjoining pores, or “a great many little boxes,” which he likened to the rooms monks lived in. He coined them cellulae, which translated from the Latin, means little rooms; in modern English, these pores are familiar to students and scientists as cells. Nearly 200 years after Hooke’s discovery, the Scottish botanist Robert Brown observed a dark spot in the cells of orchids viewed under a microscope. He named this part of the cell the nucleus, the Latin word for kernel.
A few years later, the German botanist Matthias Schleiden renamed the nucleus the cytoblast. He stated that the cytoblast was the most important part of the cell, since he believed it formed the rest of the parts of the cell. He theorized that the nucleus – as it is again referred to today – was responsible for the varying appearances of cells in different species of plant and in different parts of an individual plant. As a botanist, Schleiden studied plants exclusively, but when he collaborated with the German physiologist Theodor Schwann, his ideas about the nucleus would be shown to hold true about animal and other species cells as well. They jointly developed a cell theory, which sought to describe universal features of all cells, regardless of what animal’s organ system, fungus or edible fruit they were found in.
Building Blocks of Life
Unlike Schleiden, Schwann studied animal tissue. He had been laboring to come up with a unifying theory that explained the variations in all the cells of living things; like so many other scientists of the time, he sought a theory that encompassed the differences in all of the many types of cells he was viewing under the microscope, but one that still allowed them all to be counted as cells. Animal cells come in a great many structures. He could not be sure that all of the “little rooms” he saw under the microscope were even cells, without a proper cell theory. Upon hearing about Schleiden’s theories about the nucleus (cytoblast) being the locus of cell formation, he felt like he had the key for a cell theory that explained animal and other living cells. Together, they proposed a cell theory with the following tenets:
- Cells are the building blocks of all living organisms.
- Regardless of how different individual species’ are, they all develop by the formation of cells.
- As Schwann noted, “Each cell is, within certain limits, an individual, an independent whole. The vital phenomena of one are repeated, entirely or in part, in all the rest.”
- All cells develop the same way, and so are all the same, regardless of appearance.
The Contents of Cells
Building upon Schleiden and Schwann’s cell theory, a great many scientists contributed discoveries – many made through the microscope – and theories about what went on inside of cells. For the next few decades, their cell theory was debated, and other theories were put forth. To this day, however, much of what the two German scientists posited in the 1830s is considered accurate in the biological fields. In the following years, microscopy allowed the discovery of more details of the insides of cells. Another German botanist named Hugo von Mohl discovered that the nucleus was not fixed to the inside of the plant’s cell wall, but floated within the cell, held aloft by a semi-viscous, jellylike substance. He called this substance protoplasm. He and other scientists noted that protoplasm contained small, suspended items within it. A period of great interest in the protoplasm, which came to be called cytoplasm, began. In time, using improving methods of microscopy, scientists would enumerate the organelles of the cell and their functions.
The Largest Organelle
The largest organelle in a cell is the nucleus. As Matthias Schleiden discovered in the early 19th century, the nucleus serves as the center of cell operations. Deoxyribose nucleic acid, better known as deoxyribonucleic acid or DNA, holds the genetic information for the organism and is transcribed and stored in the nucleus. The nucleus is also the locus of cell division, which is how new cells are formed. The nucleus is separated from the surrounding cytoplasm that fills the cell by a nuclear envelope. This is a double membrane that is periodically interrupted by pores through which genes that have been transcribed into strands of ribonucleic acid, or RNA – that becomes messenger RNA, or mRNA – pass to other organelles called endoplasmic reticulum outside the nucleus. The outer membrane of the nuclear membrane is connected to the membrane that surrounds the endoplasmic membrane, which facilitates the transfer of the genes. This is the endomembrane system, and it also includes the Golgi apparatus, lysosomes, vacuoles, vesicles and the cell membrane. The inner membrane of the nuclear envelope does the primary work of protecting the nucleus.
Protein Synthesis Network
The endoplasmic reticulum is a network of channels extending from the nucleus, and which is enclosed in a membrane. The channels are called cisternae. There are two types of endoplasmic reticulum: the rough and smooth endoplasmic reticulum. They are connected and are part of the same network, but the two types of endoplasmic reticulum have different functions. The smooth endoplasmic reticulum’s cisternae are rounded tubules with many branches. The smooth endoplasmic reticulum synthesizes lipids, especially steroids. It helps in the breakdown of steroids and carbohydrates as well, and it detoxifies alcohol and other drugs that enter the cell. It also contains proteins that move calcium ions into the cisternae, allowing the smooth endoplasmic reticulum to serve as a storage location for calcium ions and as a regulator of their concentrations.
The rough endoplasmic reticulum is connected to the outer membrane of the nuclear membrane. Its cisternae are not tubules, but flattened sacs that are studded with small organelles called ribosomes, which is where it gets the “rough” designation. Ribosomes are not enclosed in membranes. The rough endoplasmic reticulum synthesizes proteins that get sent outside of the cell, or packaged inside other organelles inside the cell. The ribosomes that sit on the rough endoplasmic reticulum read the genetic information encoded in the mRNA. The ribosomes then use that information to build proteins out of amino acids. The transcription of DNA to RNA to protein is known in biology as "The Central Dogma." The rough endoplasmic reticulum also makes the proteins and phospholipids that form the cell’s plasma membrane.
Protein Distribution Center
The Golgi complex, which is also known as the Golgi body or Golgi apparatus, is another network of cisternae, and like the nucleus and the endoplasmic reticulum, it is enclosed in a membrane. The organelle’s job is to process proteins that were synthesized in the endoplasmic reticulum and distribute them to other parts of the cell, or prepare them to be exported outside the cell. It also helps in the transport of lipids around the cell. When it processes materials to be transported, it packages them in something called a Golgi vesicle. The material is bound in a membrane and sent along the microtubules of the cell’s cytoskeleton, so it can travel to its destination through the cytoplasm. Some of the Golgi vesicles leave the cell, and some store a protein to release later. Others become lysosomes, which is another type of organelle.
Recycle, Detoxify and Self-Destruct
Lysosomes are a round, membrane-bound vesicle created by the Golgi apparatus. They are filled with enzymes that break down a number of molecules, such as complex carbohydrates, amino acids and phospholipids. Lysosomes are part of the endomembrane system like the Golgi apparatus and the endoplasmic reticulum. When a cell no longer needs a certain organelle, a lysosome digests it in a process called autophagy. When a cell is malfunctioning or is no longer needed for any other reason, it engages in programmed cell death, a phenomenon also known as apoptosis. The cell digests itself by means of its own lysosome, in a process called autolysis.
A similar organelle to the lysosome is the proteasome, which is also used to break down unneeded cell materials. When the cell needs a rapid reduction in the concentration of a certain protein, it can tag the protein molecules with a signal by attaching ubiquitin to them, which will send them to the proteasome to be digested. Another organelle in this group is called a peroxisome. Peroxisomes are not manufactured in the Golgi apparatus like lysosomes are, but in the endoplasmic reticulum. Their main function is to detoxify harmful drugs such as alcohol and toxins that travel in the blood.
An Ancient Bacterial Descendent as a Fuel Source
Mitochondria, the singular of which is mitochondrion, are organelles responsible for using organic molecules to synthesize adenosine triphosphate, or ATP, which is the source of energy for the cell. Because of this, the mitochondrion is widely known as the “powerhouse” of the cell. Mitochondria are continually shifting between a threadlike shape and a spheroidal shape. They are surrounded by a double membrane. The inner membrane has many folds in it, so that it looks like a maze. The folds are called cristae, the singular of which is crista, and the space between them is called the matrix. The matrix contains enzymes that mitochondria use to synthesize ATP, as well as ribosomes, like those studding the surface of rough endoplasmic reticulum. The matrix also contains small, round molecules of mtDNA, which is short for mitochondrial DNA.
Unlike other organelles, mitochondria have their own DNA that are separate and different from the DNA of the organism, which is in each cell’s nucleus (nuclear DNA). In the 1960s, an evolutionary scientist named Lynn Margulis proposed a theory of endosymbiosis, which is still today commonly thought to explain mtDNA. She believed that mitochondria evolved from bacteria that lived in a symbiotic relationship inside the cells of a host species about 2 billion years ago. Eventually, the result was the mitochondrion, not as its own species, but as an organelle with its own DNA. Mitochondrial DNA is inherited from the mother and mutates more rapidly than nuclear DNA.