The Cell is the fundamental unit of life.
Cells posses a genetic program and the means to use it.
Cells can replicate
Cells acquire and utilize energy
Cells carry out a variety of chemical reactions
Cells respond to stimuli
Cells self-regulate
The two fundamentally different classes of cells are the prokaryotic cells and the eukaryotic cells.
Eukaryotes differ from the prokaryotes in the following:
The organelles within eukaryotic cells are the following:
Plasma membrane (PM)
It is a selectively permeable bilipid layer membrane. It allows passive diffusion, active transport and vesicular transport of compounds in and out of the cell as needed. The plasma membrane is also instrumental in cell-to-cell communication.
Endoplasmic Reticulum (ER)
It is an intracellular membrane continuous with the outer layer of the nuclear membrane. Synthesis and transport of proteins and lipids occurs within the ER. N-type glycosylation of proteins also occurs in ER. Proteins are transported to and from the ER via vesicles to the Golgi.
Golgi Complex
It is closely associated with the ER and is a stack of membranes involved in processing (i.e. O-type glycosylation), sorting and packaging of newly synthesized proteins into membrane vesicles for storage or secretion. The Golgi also serves as site for lipid synthesis and (in plant cells) as a site of synthesis of some polysaccharides that compose the cell wall.
Lysosomes
These are small membrane enclosed structures that have a digestive function. They breakdown foreign materials and intracellular constituents that are no longer needed by the cell.
Peroxisomes
Are membrane-bounded vesicles containing oxidative enzymes that generate and destroy hydrogen peroxide. They also play a role in fatty acid metabolism.
Vacuoles
They are large vesicles especially prominent in plant cells. They have a role in maintaining water balance. They occupy up to 90% of the cell. Vacuoles can digest macromolecules and store both waste products and nutrients.
Nucleus
It is a membrane bound organelle housing chromatin fibers containing DNA.
Nucleolus
It is a small spherical structure within the nucleus that contains DNA information to form ribosomes.
Nucleoplasm
Fills the spaces around the chromatin fibers and nucleoli
(During cell division, the chromatin fibers and nucleoli condense to form chromosomes)
Nuclear Envelope
It consists of an outer and inner membrane. Occasional connections occur between the outer and inner membranes forming the nuclear pores. The nuclear pores help regulate flow of materials between the nucleus and the cytoplasm and serve as attachment points for chromatin fibers.
In prokaryotes, a circular single-DNA-containing chromosome folded into a compact structure occupies a region of the cell called the nucleoid.
Ribosomes
These are organelles on which protein synthesis occurs. Ribosomes are found free in the cytoplasm and attached to the ER. Also, small numbers of ribosomes resembling those of prokaryotes occur in mitochondria and chloroplasts.
The cytoskeleton is a network of filaments developed in eukaryotes to move components within the cell as well as the cell as a whole. The cytoskeleton consists of the following:
1. Microtubules are relatively rigid hollow structures of about 25 nm in diameter composed of tubulin. Microtubules form part of the mitotic spindle that moves the chromosomes during cell division. Microtubules also form the core of motile appendages known as cilia and flagella.
2. Actin filaments also generate movement within the cytosol of eukaryotes. They are about 6 nm in diameter and are formed from actin (a protein present in contractile fibers of muscle cells as well). Actin filaments generate bulk movement of the cytoplasm, as well as a crawling type of locomotion of the cell as a whole. They also move plasma membrane during cell division.
3. Intermediate filaments provide important structural support and anchoring in the cytoskeleton.
These tubules and filaments do not occur in prokaryotes. Prokaryotes can move due to flagella which differ in chemical makeup from those of eukaryotes.
Metabolic reactions occur everywhere in the cell, but those most central to the flow of energy are localized to the cytoplasm. Reactions involved in initial breakdown of energy-rich nutrients occur in the cytosol.
A second set of reactions occurs in which most of the energy contained in nutrients is released and used to drive the formation of ATP. In eukaryotes, this occurs in mitochondria.
Mitochondria
Mitochondria are as large as typical bacteria and are enclosed by two membranes. The inner membrane is folded into cristae that project into the matrix of the mitochondria. Within the matrix, there is also DNA and ribosomes similar to those found in bacteria.
Bacteria do not have mitochondria. The reactions that take place in the mitochondria in eukaryotes take place in the plasma membrane of prokaryotes.
Sunlight can also be used as a source of energy in some organisms:
Photosynthetic unicellular and multicellular plants (eukaryotes) and photosynthetic bacteria (prokaryotes) are such organisms that can obtain energy from sunlight and convert it into chemical energy. In eukaryotes, photosynthesis occurs in chloroplasts.
Chloroplasts
The chloroplast is enclosed by two membranes surrounding an internal compartment, the stoma. Within the stroma are thylakoid membranes that contain chlorophyl and other light absorbing pigments. Also, within the stroma there are ribosomes and DNA similar to those in bacteria.
The Endosymbiont theory refers to the idea that a single cell of greater complexity (e.g. the eukaryotic cell) could have evolved from two or more separate, simpler cells living in a symbiotic relationship with one another. For example, a primitive could have engulfed mitochondria and/or chloroplasts and become for complex.
Prokaryotic Cells
Prokaryotes are divided into two major groups: the Archaea (archaeons) and the Bacteria (eubacteria).
Living archaeons include the methanogens, the halophiles and the thermophiles.
All other prokaryotes are in the domain Bacteria.
Viruses
Viruses, like bacteria, are responsible for many diseases. Viruses are known to be responsible for AIDS, polio, influenza, cold sores, measles, and a few types of cancer. All viruses are obligatory intracellular parasites. They cannot reproduce unless present within a host cell, which depending on the specific virus may be a plant, animal or bacterial cell. Outside of a living cell, the virus exists as a particle, or virion, which is little more than a macromolecular package. The virion contains a small amount of genetic material that, depending on the virus, can be single stranded or double stranded, RNA or DNA. The genetic material of the virion is surrounded by a protein capsule. Each virus has on its surface a protein that is able to bind to a particular surface component of its host cell.
Viroids
Viroid is an infectious agent consisting of a small circular RNA molecule that totally lacks a protein coat. The RNAs of viroids range in size from about 240 to 600 nucleotides, tenth the size of the smaller viruses. It seems that the RNA does not code for any proteins, but that viroids utilize the hosts cell proteins for duplication. Viroids are thought to cause disease by interfering with the cell's normal path of gene expression.
Prions
A prion is a 28KDa glycoprotein that is an abnormal counterpart of a normal protein expressed in the brain by a gene called PRNP. Prion disorders cause plaque and holes within the brain resulting symptoms include dementia, extreme fatigue, or loss of balance, depending on whether one is human, bovine or ovine. In the bovine species, it is called spongiform encephalopathy (mad cow disease) and in humans Creutzfeldt-Jakob disease (CJD). CJD is a rare, fatal disorder that attacks the brain, causing a loss of motor coordination and dementia. The modified protein has the same amino acid sequence as its normal counterpart, but folds differently. As a result of the differences in folding, the normal protein remains soluble in the brain, whereas abnormal prions produce insoluble aggregates that kill the nervous cells.
E. coli DNA: 4.7X10(6) base pairs
This prokaryotic cell divides every 20-60 minutes. Most of our present day understanding of DNA replication, the genetic code, gene expression and protein synthesis derive from studies of this bacterium.
Yeasts DNA: 14X10(6) base pairs
It is the simplest eukaryote and divides every 2 hours. Yeasts mutants have been important in understanding many fundamental processes in eukaryotes, including DNA replication, transcription, RNA processing, protein sorting, and the regulation of cell division.
Dictyostelium discoideum
DNA: 70X10(6) base pairs
It is a single cell amoeba that has been an important model for studying the molecular mechanisms responsible for animal cell movements, such as study the roles of several genes in cell motility. It also provides a model to study cell signaling and cell-cell interactions when cells aggregate into multicellular structures.
Caenorhabditis elegans
DNA: 100X10(6) base pairs
This is a nematode with important features that make it a useful model for the study of animal development and cell differentiation.
Drosophila melanogaster
DNA: 165X10(6) base pairs
This organism has also been a crucial model organism in developmental biology.
Arabidopsis thaliana
DNA: 70X10(6) base pairs
It is a small flowering plant very useful model to study the molecular biology of plants.
Vertebrates Human and mouse DNA: 3000X10(6) base pairs
Since humans cannot be used as a total model system for ethical reasons, the culture of human cells has proven invaluable. Using cell culture, DNA replication, gene expression, protein synthesis and processing, cell division and cell signaling have been investigated.
The mouse has provided with a very useful animal model in which to do whole animal studies. Transgenic mice in which specific genes have been modified are very useful tools to elucidate the functional role of sgenes.
The frog Xenopus laevis has been an important model for studies of early vertebrate development. Thus, insights into the molecular mechanism that control development, differentiation and embryonic cell division have been obtained.