Cells-The Basic Unit of Life
I. Cell Theory--Represents the collective synthesis of over 300 years of observation and experimentation.
The cell is the basic living unit of organization for living things.
All cells arise from pre-existing cells--no spontaneous generation. There is no
All living things are composed of one or more cells.
II. So what is a cell?--It can vary in size from invisible bacteria to chicken eggs and nerve cells several feet in length. A cell is characterized by three fundamental features:
It is surrounded by a plasma membrane.
It has a region of DNA containing instructions for growth, development, and reproduction.
It contains cytoplasm--a water-based assemblage of the basic molecules and strutures that carry out the internal energy transformations necessary to sustain life. These energy transformations are collectively refered to as metabolism. The rate at which these energy transformations occur is regulated by special proteins called enzymes.
III. Types of cells--There are two major types of cells:
--Include nearly 3,000 different species of single-celled organisms (mostly bacteria) that are characterized by the absence of a distinct
(defined as a region of DNA bounded by a membrane). The DNA of prokaryotes is found in the cytoplasm as a single circular strand. In addition, prokaryotes lack membrane-bound organelles. This group includes two of the major domains of the living world, the bacteria and the archaea. The archaea are believed to be directly related to the world's most ancient living forms and inhabit the most inhospitable environments on earth (hot springs, volcanic craters, mineral springs, and the human gastrointestinal tract).
IV. Structures common to all cells
--In contrast to bacteria and other prokaryotes,eukaryotes do possess a true membrane-bound region of DNA, the nucleus, andthey also have other membrane-bound organelles. Most eukaryotic cells are 10 to 30 times larger than prokaryotes. The eukaryotes constitute the third major domain of life and include plants, animals, fungi, and single-celled protists. At the molecular level, they are more closely related to the Archaea than they are to the Bacteria.
What about viruses and prions?
Viruses are actually on the "boundary of life," and strictly speaking they should not be considered as "living organisms." They are not cells because they lack a plasma membrane, and they do not have cytoplasm and hence are not capable of independent metabolism. Viruses consist of little more than a core of nucleic acid (either DNA or RNA) and a protective coat of proteins and sometimes lipids. In isolation, viruses cannot do most of the things we normally associate with life. They can, however, reproduce by invading cells and using the host "machinery" to produce macromolecules that ultimately are used to make more viruses. Prions are also difficult to classify in the context of cell theory. Prions are proteins that are involved in diseases such as Mad Cow Disease (Creutzfeldt-Jakob disease), Kuru, and Scrapie. They appear to lack gentic material entirely and exact mechanism by which they cause disease is not known. However, the association of these dangerous proteins with fatal symptoms in the nervous system (spongiform encephalopathy) is now well-established. The semantic argument of whether viruses and prions are truly living or simply very dangerous biochemicals does not change the fact they are responsible for causing many important diseases in living things.
Cytoplasm--the water based assemblage of molecules and structures within which energy transformations (metabolism) occur. Ribosomes--the site of protein synthesis. Plasma membrane--the phospholipid bilayer that surrounds all cells. The "fluid mosaic model" of the cell membrane describes a structure in which the relatively fluid phospholipids form a matrix within which are found proteins that serve as receptor molecules at the cell surface. The receptor molecules are held in place by ionic bonds, hydrogen bonds, and hydrophobic interactions. Because of the structure of the membrane, it is permeable to some molecules (very small polar molecules or ions and nonpolar compounds like the sex hormones and ethyl alcohol), and not to others (large polar compounds, like glucose). This is called selective or differential permeability. The three principle ways of moving materials into or out of a cell are:
V. Cell organelles found in eukaryotes
Simple diffusion--requires only a concentration gradient to move compounds from an area of high concentration to an area of low concentration, assuming the molecule can get across the cell membrane (i.e. it's either small or nonpolar). Facilitated diffusion--requires a concentration gradient and also a receptor molecule to serve as a gate or channel across the otherwise impermeable membrane. Active transport--can be used to move a compound against a concentration gradient, but it requires a receptor molecule and energy.
Nucleus--consists of DNA in the form of chromosomes surrounded by the nuclear membrane.
The cytomembrane system--a network of membranes that extends through the cell. It is continuous with the nuclear membrane, and is often found associated with ribosomes cytomembrane system serves as a conduit for the transfer of information from the nucleus, where the DNA is located, to the ribosomes, where the proteins are synthesized.
Vesicles--These are the packages produced by the cell that may contain enzymes. Some vesicles (lysosomes) remain within the cell and are important for "cleaning up the cell" in the form of breaking down proteins, lipids, etc. Lysosomes contain digestive enzymes to do this job. A number of congenital diseases in humans are caused by lysosome dysfunction. Included is Tay Sachs disease, in which an enzyme important in the breakdown of lipids does not work properly, the consequence is the ultimaltely fatal buildup of lipids in nerve cells.
Mitochondria--Site of aerobic respiration and the production of ATP
Chloroplasts--found only in plant cells, the site of photosynthesis.
Central vacuole--also found in plant cells, a large vesicle that serves as a storage site for numerous compounds, including secondary plant compounds.
Cell wall--not actually an organelle, because it is not surrounded by a membrane, but the cell wall is often the most conspicuous feature of plant cells. In plants it is made up of cellulose (glucose molecules), and in bacteria it is made up of a combination of protein and carbohydrate.
The endosymbiotic theory has been proposed to account for the evolution of eukaryotes from prokaryotic ancestors. This theory postulates that ancestral eukaryotes were actually a symbiotic assemblage of once free-living prokaryotes that had been incorporated into the cytoplasm of a large host cell. Evidence for this theory includes the observation that both mitochondria and chloroplasts contain their own nucleic acids which produce enzymes necessary for these organelles to function. In humans, mitochondrial DNA is inherited maternally, which means that all of the mitochondria within a person's body were derived from the mitochondria of that person's mother--the father contributes nothing to the mitochondrial pool. If a woman has only male offspring, then her mitochondrial lineage dies out. Because mitochondrial DNA undergoes mutations through time, and because it is maternally inherited, it is possible to use variation in mitochondrial DNA as a tool in exploring human ancestry. Using this approach, some scientists have suggested that modern humans (loosely called "Eve" in popular accounts of this research) first emerged about 150,000 years ago in southern Africa. All modern humans are descended from this small group of ancestors. In a simlar manner, studies of the ancestry of plants have benefitted from the analysis of chloroplast DNA.