The microscopic containers known as cells are the basic units of living things on Earth; each one boasts all of the characteristics that scientists ascribe to life. In fact, some living things consist of only a single cell; your own body, on the other hand, has in the range of 100 trillion. Almost all single-celled organisms are prokaryotes, and in the grand classification-of-life scheme, these belong to either the Bacteria domain or the Archaea domain; humans, along with all other animals, plants and fungi, are eukaryotes.
These tiny structures perform the same tasks on a "micro" scale to keep themselves intact that you and other full-sized organisms do on a "macro" scale to remain alive. And obviously, if enough individual cells fail at these tasks, the parent organism will fail along with it.
Structures within cells have individual functions, and in general, no matter the structure, these can be reduced to three essential jobs: A physical interface or boundary with specific molecules; a systematic means of shuttling chemicals into, along or out of the structure; and a specific, unique metabolic or reproductive function.
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Prokaryotic Cells vs. Eukaryotic Cells
As mentioned, while cells are generally regarded as tiny components of living things, a lot of cells are living things. Bacteria, which cannot be seen but certainly make their presence felt in the world (e.g., some cause infectious diseases, others help foods such as cheese and yogurt age properly and still others play a role in maintaining the health of the human digestive tract), are an example of single-celled organisms, and of prokaryotes.
Prokaryotic cells have a limited number of internal components compared to their eukaryotic counterparts. These include a cell membrane, ribosomes, deoxyribonucleic acid (DNA) and cytoplasm, the four essential features of all living cells; these are described in detail later. Bacteria also have cell walls outside the cell membrane for added support, and some of these also have structures called flagella, whiplike constructs that are made of protein and that help the organisms to which they're attached move about in their environment.
Eukaryotic cells have a host of structures that prokaryotic cells do not, and accordingly, these cells enjoy a wider range of functions. Perhaps the most important are the nucleus and the mitochondria.
Cell Structures and Their Functions
Before digging deep into how individual cell structures handle these functions, it's helpful to review what those structures are and where they can be found. The first four structures in the following list are common to all cells in nature; the others are found in eukaryotes, and if a structure is only found in certain eukaryotic cells, this information is noted.
The cell membrane: This is also called the plasma membrane, but this can cause confusion because eukaryotic cells actually have plasma membranes around their organelles, many of which are detailed below. This consists of a phospholipid bilayer, or two identically constructed layers facing each other in a "mirror image" manner. It is as much a dynamic machine as it is a simple barrier.
Cytoplasm: Also called cytosol, this gel-like matrix is the substance in which the nucleus, organelles and other cell structures sit, like pieces of fruit in a classic gelatin dessert. Substances move through the cytoplasm by diffusion, or from areas of higher concentrations of those substances to areas of lower concentration.
Ribosomes: These structures, which do not have their own membranes and are thus not considered true organelles, are the sites of protein synthesis in cells and are themselves made of protein subunits. They have "docking stations" for messenger ribonucleic acid (mRNA), which carries DNA instructions from the nucleus, and amino acids, the "building blocks" of proteins.
DNA: The cell's genetic material sits in the cytoplasm of prokaryotic cells, but in the nuclei (the plural of "nucleus") of eukaryotic cells. Consisting of monomers – that is, repeating subunits – called nucleotides, of which there are four basic kinds, DNA is packaged along with supporting proteins called histones into a long, stringy substance called chromatin, which is itself divided into chromosomes in eukaryotes.
Organelles of Eukaryotic Cells
Organelles provide great examples of cell structures that serve distinct, necessary and unique purposes that rely on maintaining transport mechanisms that in turn depend on how these structures physically relate to the rest of the cell.
Mitochondria are perhaps the most prominent molecules in terms of both their distinctive appearance under a microscope and their function, which is to use the products of the chemical reactions that break down glucose in the cytoplasm to extract a great deal of adenosine triphosphate (ATP) as long as oxygen is present. This is known as cellular respiration and takes place mainly on the mitochondrial membrane.
Other key organelles include the endoplasmic reticulum, a sort of cellular "highway" that packages and moves molecules between ribosomes, the nucleus, the cytoplasm and the cell exterior; Golgi bodies, or "discs" that break off from the endoplasmic reticulum like small taxicabs; and lysosomes, which are hollow, spherical bodies that break down the waste products formed during the cell's metabolic reactions.
Plasma Membranes Are the Gatekeepers of Cells
The three jobs of the cell membrane are preserving the integrity of the cell itself, serving as a semipermeable membrane across which small molecules may pass and facilitating the active transport of substances via "pumps" embedded in the membrane.
The molecules that make up each of the two layers of the membrane are phospholipids, which have hydrophobic "tails" made of fat that face inward (and hence toward each other) and hydrophilic phosphorus-containing "heads" that face outward (and this toward the inside and outside of the organelle itself, or in the case of the cell membrane proper, the inside and outside of the cell itself). These are linear and perpendicular to the overall sheet-like structure of the membrane as a whole.
A Closer Look at Phospholipids
The phospholipids are close enough together to keep out toxins, or large molecules that would harm the interior if granted passage. But they are far enough apart to allow small molecules needed for metabolic processes, such as water, glucose (the sugar all cells use for energy) and nucleic acids (which are used to build nucleotides and thus DNA and ATP, the "energy currency" in all cells).
The membrane has "pumps" embedded among the phospholipids that make use of ATP to bring in or move out molecules that would not ordinarily pass through, either because of their size or because their concentration is greater on the side the molecules are pumped toward. This process called active transport.
The Nucleus Is the Brain of the Cell
The nucleus of each cell contains a complete copy of all of the DNA of an organism in the form of chromosomes; humans have 46 chromosomes, with 23 inherited from each parent. The nucleus is surrounded by a plasma membrane called the nuclear envelope.
During a process called mitosis, the nuclear envelope is dissolved, and the nucleus splits in two after all of the chromosomes are copied, or replicated. This is followed shortly by the division of the entire cell, a process known as cytokinesis. This results in the creation of two daughter cells that are identical to each other as well as to the parent cell.