Another Snippet from My New Book, >Fundamentals of an Excellent Liberal Arts Education< >>>>> Chapter Twelve: Biology (Section on Cell Structure)

Cell Structure


The microscope--- as developed by Antonie van Leeuwenhoek and improved over the years via stereomicroscopes, electron microscopes (EM), transmissions electron microscopes (TEM), scanning electron microscopes (SEM), contact ray microscopes, and scanning tunneling microscopes--- allowed humankind to observe and investigate tiny internal structures of organisms, including the most fundamentally important of all: the cell.


The cell, ranging in size from 0.5 to 40 microns, is the internal bodily structure responsible for the development and functioning of living things. A cell membrane (also called plasma membrane, covering just 80 angstroms [eight-millionths of a millimeter) encompasses the cytoplasm (material within the cell), within which is the vital nucleus that governs developmental processes for the cell, and therefore the organism, as a whole. Materials flow in and out of a cell through active transport (occurring when a cell expends energy to move molecules from one location of the organism to another) and passive transport (involving either diffusion [movement of particles from areas of high concentration to those of low concentration] or osmosis [movement of water across a cell membrane).


The nucleus contains chromosomes, long, thin structures composed of deoxyribonucleic acid (DNA) and protein; these chromosomes deliver all instructions concerning life processes of the cell. Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are crucial to the storage and transmission of genetic information that controls cell functions and interactions. Vital DNA and RNA are among those nucleotides (containing carbon, hydrogen, phosphorus, and nitrogen) that compose and according to their sequence determine the distinctive properties of those organic compounds known as nucleic acids.


All nucleic structures are ensconced in viscous (dense) material called the nucleoplasm, which is bound by a nuclear membrane; the latter allows only selected material to enter the nucleus, obstructing many substances that were able to pass through the relatively permeable membrane encompassing the whole cell. Attached to the outside of the nuclear membrane is an endoplasmic reticulum, important in facilitating the flow of materials in and out of the nucleus. Most cells contain a structure known as the nucleolus, which contains high concentrations of DNA and proteins.


Contained either within the endoplasmic reticulum, or found as free-floating organelles (intracellular structures) within the cytoplasm, are ribosomes, tiny granules containing high concentrations of ribosomal RNA (rRNA), which abets synthesis of the protein also found in ribosomes. Other cellular organelles include the Golgi apparatus, lysosome, mitochondria, plastids, microfilaments, microtubules, centrioles, cilia, and flagella:


Golgi apparatus >>>>>   flat, smooth, oval organelle that receives protein, carbohydrate, and other molecules previously processed in ribosomes and transferred through the endoplasmic reticulum ; further processing proteins and carbohydrates into more complex molecules such as glycoproteins and mucopolysaccharides, the Golgi apparatus then emits these as secretions, including mucus.


Lysosome  >>>>>   single-membrane storage vesicle containing powerful enzymes for digesting materials that have penetrated the cell membrane; also breaks down older parts of the cell into constituent materials for reuse by the cell.


Mitochondria  >>>>>   the cell’s power station, supplying the high-energy molecule adenosine triphosphate (ATP , active in many cellular activities, including cell division and muscle contraction) ; most abundant in cells with great energy needs, as in a plant’s growing root tip and in animal’s muscle and liver cells.


Plastids  >>>>>   large cytoplasmic organelles that include three types:


Leucoplasts  >>>>>   and colorless plastids that store oils, protein granules, as well as energy-rich starch formed from simple sugars;


Amyloplasts  >>>>>   plastids that specialize in the storage of starch;


Chloroplasts  >>>>>   plastids that contain the green pigment chlorophyll (important to photosynthesis) and red, yellow, and orange pigments that remain in temperate zone autumnal leaves after green pigment has broken down, accounting for the arboreal (tree) color of autumn.


Vacuoles  >>>>>   cellular spaces found in both plants and animals that fill with fluids containing high concentrations of sugars, amino acids, some proteins, and several pigments; vacuoles protect the cell by safely storing toxic chemicals, expelling water and waste from the cell, and collecting material that has penetrated the cell membrane (passing this material on to lysosomes for the digestive action of enzymes).


Microfilaments  >>>>>   long protein fibers that maintain cell structure and movement, including the contraction of muscle cells.


Microtubules  >>>>>   hollow structures that act as tracts along which organelles move, and along which chromosomes move in cell division.


Centrioles  >>>>>   structures that occur in pairs--- each composed of nine sets of triplets that revolve in a cylindrical shape--- and play a role in the formation of microtubules, which in turn facilitate the movement of chromosomes in cell division.


Cilia  >>>>>   short, thin organelle that band with other cilia to beat back and forth or to move in corkscrew fashion so as to propel or pull a cell through liquids


Flagella  >>>>>   long, thin organelle similar to the cilia in action (but isolated rather than banded together with others), beating back and forth or moving in corkscrew fashion so as to propel or pull a cell through liquids.


Cells are of two general types, prokaryotic and eukaryotic:


Prokaryotic cells are the more primitive of the two types, occurring in organisms such as bacteria and algae (identified, therefore, among the prokaryotes). Unlike eukaryotic cells, prokaryotic cell walls are filled with muramic acid and may have an infolding of the plasma membrane that is given the particular term, mesosome. Prokaryotic cells have no nuclear membrane but rather feature a nucleoid, a nuclear area that typically includes a single circular chromosome. The chromosome is devoid of protein and composed entirely of nucleic acids. Ribosomes and flagella of prokaryotic cells are different in shape from those of eukaryotic cells, and they have no Golgi apparatus, mitochondria, lysosomes, or vacuoles. Photosynthetic prokaryotes contain photosynthetic membranes and chlorophyll, but these are not enveloped inside chloroplasts.


Eukaryotic cells are the more complex types found in all of those organisms all protists, fungi, 4 plants, and animals--- known collectively as the eukaryotes. Eurkaryotic cells are those with the multiple structures and organelles (Golgi apparatus, endoplasmic reticulum, mitochondria, lysosomes, and vacuoles) discussed above. In these cells, a nuclear membrane encloses a nucleus, within which are the long filamentous chromosomes composed of nucleic acids and proteins and along which occur genes responsible for inherited traits.