Cells are the basic units of all living things. Each one of these microscopic structures exhibits all of the properties associated with being alive in the scientific sense, and in fact, many organisms consist of only a single cell. Almost all of these single-celled organisms belong to a broad class of organisms known as prokaryotes – creatures in the taxonomic domains Bacteria and Archaea.
In contrast, Eukaryota, the domain that includes animals, plants and fungi, has cells that are far more complex and that feature numerous organelles, which are internal membrane-bound structures that display specialized functions. The nucleus is perhaps the most striking feature of eukaryotic cells, owing to its size and more-or-less-central location inside the cell; the cell's mitochondria, on the other hand, both present a unique appearance and stand as an evolutionary and metabolic marvel.
Components of the Cell
All cells have a number of components in common. These include a cell membrane, which acts as a selectively permeable barrier to molecules entering or leaving the cell; cytoplasm, which is a jellylike substance that forms the bulk of a cell's mass and serves as a medium in which organelles can sit and for reactions to occur; ribosomes, which are protein-nucleic acid complexes whose sole job is manufacturing proteins; and deoxyribonucleic acid (DNA), which contains the cell's genetic information.
Eukaryotes are generally far larger and more complex than prokaryotes; accordingly, their cells are more complicated and contain a variety of organelles. These are specialized inclusions that allow the cell to grow and prosper from the time it is created until the time it divides (which may be a day or less). Foremost among these visually on a microscope image of a cell are the nucleus, which is the cell's "brain" that holds the DNA in the form of chromosomes, and the mitochondria, which are needed for the complete breakdown of glucose using oxygen (i.e., aerobic respiration).
Other critical organelles include the endoplasmic reticulum, a sort of membranous "road system" that packages and processes proteins while moving them between the cell exterior, the cytoplasm and the nucleus; the Golgi apparatus, which are vesicles serving as miniature taxis for these substances and which can "dock" with the endoplasmic reticulum; and lysosomes, which serve as the cell's waste-management system by dissolving old, worn-out molecules.
Two characteristics that make mitochondria different from other organelles are the Krebs cycle, which is hosted by the mitochondrial matrix, and the electron transport chain, which takes place on the inner mitochondrial membrane.
Mitochondria are football-shaped and rather look like bacteria themselves, which as you'll see is no accident. They are found in higher density in places where oxygen requirements are high, such in the leg muscles of endurance athletes like distance runners and cyclists. The whole reason they exist is the fact that eukaryotes have energy requirements far in excess of those of prokaryotes, and mitochondria are the machinery that allow them to meet those requirements.
Read more about the structure and function of the mitochondria.
Origins of Mitochondria
Most molecular biologists adhere to the endosymbiont theory. In this framework, over 2 billion years ago, certain early eukaryotes, which ingested food by taking in sizable molecules across the cell membrane, in effect "ate" a bacteria that had already evolved to carry out aerobic metabolism. (Prokaryotes capable of this are comparatively rare but continue to exist today.)
Over time, the ingested life form, which reproduced on its own, came to rely exclusively on its intracellular environment, which offered a ready supply of glucose at all times and protected the "cell" from external threats. In return, the engulfed life form allowed their host organisms to grow and thrive over generations beyond anything seen at that point in zoological history on Earth.
"Symbionts" are organisms that share an environment in a mutually beneficial way. At other times, such sharing arrangements involve parasitism, where one organism is harmed to allow the other to thrive.
In any narrative about a eukaryotic cell, the nucleus takes center stage. The nucleus is surrounded by a nuclear membrane, also called the nuclear envelope. During most of the cell cycle, the DNA is diffusely spread throughout the nucleus. Only at the beginning of mitosis do the chromosomes condense into the forms most students associate with these structures: those tiny little "X" forms.
Once the chromosomes, which were copied in interphase during the cell cycle, separate during the M phase, the entire cell is ready to divide (cytokinesis). The mitochondria, meanwhile, have increased in number through dividing in half early in interphase, along with the cell's other cytoplasmic contents (i.e., anything outside the nucleus).
Read more about the structure and function of the nucleus.
The Nucleus and DNA
The nucleus goes into mitosis with two identical copies of each chromosome, linked together at a structure called the centriole. Humans have 46 chromosomes, so at the start of mitosis, each nucleus has 92 individual DNA molecules, arranged in identical-twin sets. Each twin in a set is called a sister chromatid.
When the nucleus divides, the chromatids in every pair are pulled to opposite sides of the cell. This creates identical daughter nuclei. It is important to note that the nucleus of every cell contains all of the DNA needed to reproduce the organism as a whole.
Mitochondria and Aerobic Respiration
Mitochondria host the Krebs cycle, in which acetyl CoA combines with oxaloacetate to create citrate, a six-carbon molecule that is reduced to oxaloacetate in a series of steps that generate two ATP per glucose molecule, feeding the process upstream along with a host of molecules that carry electrons to the electron chain transport reactions.
The electron chain transport system also occurs in mitochondria. This series of cascading reactions uses energy from electrons stripped from the substances NADH and FADH2 to drive the synthesis of a great deal of ATP (32 to 34 molecules per glucose upstream).
Mitochondria vs. Chloroplasts
Similar to the nucleus, chloroplasts and mitochondria are membrane-bound and stocked with a strategic set of enzymes. Do not fall into the common trap, however, of thinking that chloroplasts are "the mitochondria of plants." Plants have chloroplasts because they cannot ingest glucose and must make it instead from carbon dioxide gas brought into the plant through its leaves.
Both plant and animal cells have mitochondria because both participate in aerobic respiration. Much of the glucose a plant makes is eaten by animals in the environment or simply rots eventually, but most plants manage to dip heavily into their own stash, too.
Nucleus and Mitochondria: Similarities
The main difference between nuclear DNA and mitochondrial DNA is simply the amount of it and the specific products produced. Also, the structures have very different jobs. Both of these entities, however, reproduce by splitting in half and direct their own division.
The cells we think of when considering eukaryotic cells could not survive without mitochondria. To simplify greatly, the nucleus is the brains of the cell operation, while mitochondria are the muscle.
Nucleus and Mitochondria: Differences
Now that you're an expert on eukaryotic organelles, which of the following is a difference between the nucleus and a mitochondrion?
- Only the nucleus contains DNA.
- Only the nucleus is surrounded by a double plasma membrane.
- Only the nucleus divides in two during the cell cycle.
- Only the nucleus hosts chemical reactions that do not occur elsewhere in the cell.
In fact, none of these statements are true. Mitochondria, as you've seen, possess their own DNA, and furthermore, this DNA contains genes that nuclear (regular) DNA does not. Mitochondria and nuclei, along with organelles such as the endoplasmic reticulum, have their own membrane. As noted, each body organizes and conducts its own process of division, and each structure hosts reactions that do not occur anywhere else in the cell (e.g., RNA transcription in the nucleus, the electron transport chain reactions in mitochondria).
- University of California at Berkeley: Marshalling the Evidence
- University of Utah: Genetic Science Learning Center: The Evolution of the Cell
- Scitable by Nature Education: Eukaryotic Cells
- U.S. National Library of Medicine: Genetics Home Reference: What Is a Cell?
- Scitable by Nature Education: The Origin of Mitochondria
About the Author
Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.