Chapter 2. Molecular and Cellular Biology
2.2.2 Cell Life Cycle

We have, at last, completed the “molecular basis of Life.” Finally we can begin to look at the Biology of Biology. Much of what you could have learned from this course up until now is presumed to help you understand why Life works the way it does. In a few cases, the constraints on Life will be rather obvious, but more often than not, it may not be so clear that Life is, in fact, constrained by the laws of Physics and Chemistry. One rationale for this is that complex systems (those made of simpler systems) have “emergent properties:” properties not found in the less complex systems, but which appear in the more complex system. For example: molecular Hydrogen (2H₂) is a highly flammable gas; molecular Oxygen (O₂) is a gas that supports combustion; but when combined they form Water (2H₂O), a liquid which can be used to put out a fire. Nothing about the inherent properties of molecular Hydrogen or of molecular Oxygen would suggest the peculiar properties of Water.

Organization of systems above molecules

Molecules themselves are chemical systems of varying complexity from rather simple two identical atoms (such as O₂) to the very complicated DNA double helix (which you will soon learn more about in Chapter 2.3 Molecular basis of heredity). Among the perhaps surprising emergent properties of the more complex molecule systems is that the 3-dimensional folding and Hydrogen bonding of proteins creates enzymes capable of driving extremely complex chemical reactions necessary to support Life, in spite of the fact that these reactions are extremely unlikely to occur at all without the enzymes. This effectively adds a new, higher level of system involving enzyme-coenzyme complexes that carry out a sequence of reactions.

    By organizing enzyme-coenzyme complexes into higher level systems (structures, called “organelles,” large enough to be seen using student microscopes, such as chloroplasts, mitochondria, and even Bacteria), the system supports metabolic processes rather than merely driving the component reactions of the metabolic processes. As we have seen these structures have a plasma membrane defining the structure, and holding the enzyme complexes in sufficiently close proximity to allow efficient metabolic processes. In the case of the bacteria, these systems can include all of the metabolic processes necessary to support the simplest of living systems. Once we have a single system that supports all of the metabolic processes necessary to support Life, the structures are called “cells.”

    Above the cellular level of organization, the systems are multicellular organisms (multicellular algae [seaweeds], Fungi, Plants and Animals), which are organized into Populations, which, in turn, are organized into Ecosystems.

cell

The cell is “the smallest structure which exhibits all of the characteristics of Life.” Cells may be procaryotic (“cells without a nucleus,”) or eucaryotic (“cells with a true, membrane-bound nucleus [or nuclei].” The prefix “pro-” means “before,” and the prefix “eu-” means “true.”

Procaryotes

The procaryotic cell, found in the Archaea and Monera (the two Kingdoms of Bacteria), is the simplest type of cell. (The illustration of a generalized prokaryotic cell was downloaded 1 May 2011 from http://micro.magnet.fsu.edu/cells/bacteriacell.html) It is bounded by a plasma membrane; and often has a capsule and/or cell wall outside the membrane. Capsules, when present, frequently are made of polysaccharides or lipoproteins (a protein with a lipid attached to the COOH-end of the protein). Cell walls, when present, are made of a peptidoglycan, a polypeptide-polysaccharide molecule. Flagella (singular, flagellum) are present in some free-living Bacteria, and function for locomotion. The pili are hair-like structures used by parasitic (pathogenic, or disease producing) Bacteria to attach to the host Plant or Animal. The inner layer of phospholipids (in the plasma membrane) is sometimes folded inward (where the enzymes of cellular respiration are embedded in the membrane). The Blue-greens have the pigments and enzymes of photosynthesis embedded in the folds of the inner membrane as well. The cytosol (or cytoplasm) contains free ribosomes where protein synthesis occurs, and sometimes other inclusions (detectable by electron microscope) used for storage of energy dense compounds and some toxins.

    There is a clear area centrally located in the cell where the single, circular DNA chromosome (without associated proteins), or ‘Nucleoid’ is located along with several small DNA fragments and small RNAs (probably involved in reading the DNA and replicating the DNA). There is no membrane surrounding this nuclear region [hence the name “procaryote]. Cell division is achieved by “binary fission”, constricting the cell to an hourglass-shape, finally pinching the cell into two smaller cells. The chromosome is attached to the membrane and replicated with each replicate attached at a slightly different locations on the membrane. Binary fission begins between the attachment points of the chromosome replicates so each daughter cell gets one copy (and its share of the DNA fragments and small RNAs). Bacteria (Monera) can divide [reproduce] as soon as 20 to 30 minutes after the previous division.

Eucaryotes

(The illustration of a generalized Plant cell was downloaded 1 May 2011 from micro.magnet.fsu.edu/cells/plantcell.html).
    Plant cells are enclosed in cell walls made of cellulose, and sometimes lignin, a protein which adds strength to wood. The cells are bounded by a plasma membrane. In most Plant cells there is a large membrane-bound vacuole filled with water and dissolved solutes, and a membrane-bound nucleus containing Chromosomes (DNA and associated Chromatin protein) [hence the name eucaryote], which has a centrally located clear area called the Nucleolus. The other membrane-bound organelles include both mitochondria and, frequently, chloroplasts. The smaller, non-membraneous organelles include endoplasmic reticulum (e.r.), Golgi apparatus, attached ribosomes and free ribosomes.

(The illustration of a generalized Animal cell was downloaded 1 May 2011 from micro.magnet.fsu.edu/cells/animalcell.html).
Animal cells lack cell walls, but may have cillia (singular, cillium) or flagella for locomotion. The nucleus is membrane-bound, with a conspicuous nucleolus. Within the nucleus there are multiple chromosomes consisting of DNA and chromatin protein. There are mitochondria, but no chloroplasts. When vacuoles are present, they tend to be much smaller than Plant cell vacuoles. Animal cells tend to have more microtubules and microfilaments than do Plant cells.

Eucaryotic cell anatomy
Structure	Function
plasma membrane	boundary
endoplasmic reticulum (er)	internal transport
Golgi apparatus	export warehouse
mitachondrion	energy
chloroplast	energy
nuclear membrane	boundary
nucleolus	DNA transcrition
chromosomes	DNA storage
free ribosomes	construction
attached ribosomes	enzyme factories

    The “Eucaryotic Cell Anatomy table” lists the major eucaryotic cell organelles and a summary statement of their their functions.
Plasma membranes and nuclear membrane always serve as a boundary between the environment outside the membrane and the contents of the bounded structure. As such, it not only keeps the environment out, but also keeps the contents in (which is just as important to the organelle or to the cell).
Endoplasmic reticulum (e.r.) is made of plasma membrane, and forms a network of tubes (reminiscent of a maze), so no part of the cell is more than several nanometers from the e.r. It serves as a transportation system for chemicals to move around inside the cell. The e.r. connects to pores in the nuclear membrane allowing chemicals (such as mRNA) to move from the nucleus to the ribosomes. The e.r. also connects to pores in the cell membrane, allowing molecules (such as disaccharides) to move from outside the cell into the cytoplasm, and allowiing molecules (such as metabolic wastes) from the cytoplasm out to the enviroonment. Plant cells sometimes send hormone-like molecules from cell to cell via e.r. and aligned pores between adjacent cells.
Golgi apparatus is a region of dense e.r. and can be found only in cells which make molecules for excretion to the intracellular environment. This suggests that chemicals intended for excretion accumulate in Golgi apparatus awaiting excretion (analygous to an export warehouse).
Mitochondria and chloroplasts are the organelles of energy processing. Chloroplasts capture solar energy in disaccharides. Mitochondria release the energy from disaccharides to ATP for use by cells.
The nucleolus is currently believed to have some role in replicating and transcribing DNA, and RNA synthesis.
Chromosomes hold the DNA code for protein synthesis. This is the physical location of the genes which control all inheritable characteristics of all Plants and Animals.
ribosomes are known to synthesis proteins based on the genetic code carried in mRNA from the nucleus to the ribosomes. Ribosomes may be “attached ribosomes” or “free ribosomes.” When attached, ribosomes are attached to e.r. (rough e.r., e.r. without attached ribosomes is called smooth e.r.) where they manufacture proteins (enzymes) which are dumped into the e.r. tubes for transport to where they are needed. This is analogous to factory-style manufacture followed by shipping (or to the export warehouses if intended to leave the cell). Free ribosomes move about the cell manufacturing proteins when, and where, needed. To me this suggests that they are primarily involved in making structural proteins. This is analogous to on-site consruction of buildings and infrastructure.

Works Cited

Brown, Judith E. 2008. Nutrition Now, 5th ed., 9th Edition © Thompson Learning, Inc.

    Davidson, Michael W. 1995-2010.
“Animal Cell Structure,” “Plant Cell Structure,” and “Bacterial Cell Structure.” in Molecular Expressions, Cell Biology and Microscopy, Structure and function of cells & viruses, Florida State University. © 1995-2010.

    Kimball, John W. 2011. Kimball's Biology Pages, an online biology textbook, ©John W. Kimball, 2011 (downloaded 20 Apr 2011).
Most of the chemistry on this page either came from or was adapted from Kimball's Biology Pages. When his textbook was in print form only, I often used it as a required textbook for my Introductory Biology courses.

    LaFrance, Charles R. 2010. Nutrition for Nursing Students www.twooldguys.com/Nutrition201/Calorie_N.html.” § 2004-2010 Dr Charles R. LaFrance. The eTextbook for an on-line Nutrition course.

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