"Form fits function" is a common refrain in the world of both the natural and human forms of engineering. When the purposeful construction of an everyday tool is at issue, this is often obvious: A young child given a shovel, a drinking glass, a pair of socks or a hammer could probably determine with relative ease what these implements are for, whereas in the case of, say, a bicycle chain or a dog collar in isolation, the puzzle is considerably more difficult to solve.
Natural structures, formed over the course of millions of years of evolution, remain in place because they have been selected because of the survival advantages they give the organisms that possess them. This is the case with cells, which are the simplest natural structures that have all of the properties of the dynamic entity known as life: reproduction, metabolism, the maintenance of chemical balance and physical solidity.
Cell Structures and Functions
As in the "macro" world, the way the parts of a cell speak to their functions – both those that stand alone and those that are integrated with the rest of the cell – is a fascinating subject of biology in its own right.
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Cell composition and function vary considerably both between organisms and, in the case of complex multicellular organisms, between different tissues and organs within the same organism. But all cells have a number of elements in common. These include:
- Cell membrane: This structure forms the outer lining of the cell and is responsible for both the cell's physical integrity and for allowing certain substances to pass in and out while denying others passage. It actually consists of a double plasma membrane.
- Cytoplasm: This forms the interior substance of cells and consists of a watery matrix that supports other interior cell contents, like a scaffolding. The liquid, non-organelle portion is called the cytosol, and most chemical reactions in the cell occur here with he help of proteins called enzymes.
- Genetic material: The genetic material, which almost every cell of the organism contains a complete copy of, carries the information needed for protein synthesis in the form of deoxyribonucleic acid (DNA). DNA is what is passed along to subsequent generations during the reproductive process.
- Ribosomes: These proteins are responsible for the manufacture of all proteins the organism needs. They take direction from messenger ribonucleic acid (mRNA). On ribosomes, individual amino acids are linked together to create chains, forming proteins. The mRNA is made by DNA in a process called transcription; the conversion of mRNA instructions into proteins on the ribosomes, which consist of two subunits, is known as translation.
Prokaryotic Cells vs. Eukaryotic Cells
Living things can be divided into two types: Prokaryotes, which include the domains Bacteria and Archaea, and eukaryotes, which consist of the domain Eukaryota. Most prokaryotes are single-celled organisms, whereas almost all eukaryotes – plants, animals and fungi – are multicellular.
Prokaryotic cells include the four structures already described, but not much else, although bacteria do have cell walls. Many of them also have a cell capsule; the primary function of these is protection. Some prokaryotes also have whiplike structures on their surface called flagella. As you can guess from their appearance, these are used mainly for locomotion.
Eukaryotic cells, in contrast, are rich in organelles, which are membrane-bound entities that serve the cell in particular ways. Importantly, eukaryotes house their DNA inside a nucleus, while in prokaryotes, which lack internal membrane-bound structures of any sort, DNA floats in a loose cluster in the cytoplasm called the nucleoid region.
Organelles and Membranes: General Characteristics
The relationship between the parts of a cell and their functions is exemplified with elegance and clarity in the organelles of eukaryotes. In turn, all organelles feature a plasma membrane. Every plasma membrane in cells -- including the outer, named cell membrane as well as the membranes enclosing organelles -- consists of a phospholipid bilayer.
This bilayer consists of two individual "sheets" facing each other in a mirror-image fashion. The inside features the hydrophobic, or water-repelling, portions of each layer, which consist of lipids in the form of fatty acids. The outer portions, in contrast, are hydrophilic, or water-seeking, and consist of the phosphate portions of the phospholipid molecules.
Thus one "wall" of hydrophilic phosphate heads faces the inside of the organelle (or in the case of the cell membrane per se, the cytoplasm) whereas the other faces the exterior, or cytoplasmic, side (or in the case of the cell membrane, the outside environment).
The structure of the membrane is such that small molecules like glucose and water can drift freely between the phospholipid molecules, whereas larger ones cannot and must be pumped actively in or out (or denied passage, period). Again, structure fits function.
While not usually termed an organelle because of its supreme importance, the nucleus is actually the embodiment of one. Its plasma membrane is called the nuclear envelope. The nucleus contains DNA packaged into chromatin, which is protein-rich matter split into chromosomes.
When the chromosomes divide, and the nucleus with them, the process is called mitosis. For this to happen, the mitotic spindle must be created within the nucleus, which is essentially the brain of the cell and consumes a significant fraction of the overall volume of most cells.
These roughly oval-shaped organelles are the power plants of eukaryotes, because they are the site of aerobic ("with oxygen") respiration, the source of most of the energy that eukaryotes derive from the fuel they eat (in the case of animals) or synthesize with the help of sunlight (in the case of plants).
Mitochondria are believed to have originated over 2 billion years ago when aerobic bacteria wound up inside existing, non-aerobic cells and began cooperating with them metabolically. The many folds in their membrane, where aerobic respiration actually occurs, is another example of the confluence of structure and function in cells.
This membranous structure is rather like a "highway" in that it reaches from the nucleus (and is in fact joined to its membrane), through the cell, out to the far reaches of the cytoplasm. It carries and modifies protein products made by the ribosomes.
Some endoplasmic reticulum is called rough endoplasmic reticulum because it is studded with ribosomes, as can be seen under a microscope; the forms lacking ribosomes are correspondingly called smooth endoplasmic reticulum.
The Golgi apparatus is similar to the endoplasmic reticulum in that it packages and processes proteins and other cell-generated substances, but it is arranged in round stacked discs, much like a roll of coins or a stack of tiny pancakes.
Lysosomes are the cell's waste-disposal centers, and accordingly, these small globular bodies have enzymes that dissolve and dispense of cell-breakdown products resulting from everyday metabolism. Lysosomes are actually a type of vacuole, a name for a hollow, membrane-bound unit in cells whose purpose is to serve as a container for chemicals of some sort.
The cytoskeleton is made of microtubules, proteins arranged like tiny bamboo shoots and serving as structural support girders and beams. These extend across the entire cytoplasm from the nucleus to the cell membrane.