MODERN BOTANY - P. RAVEN - 1990

SECTION I. THE PLANT CELL

CHAPTER 3. MOLECULAR COMPOSITION OF CELLS

Conclusions

Living matter is constructed from only a small number of naturally occurring elements. Water constitutes the bulk of its mass, while the remainder consists primarily of Organic compounds—CARBOHYDRATES, Lipids, Proteins, and Nucleic Acids.

Carbohydrates serve as a primary energy source for living systems and function as vital Structural components of Cells. The simplest carbohydrates are Monosaccharides, such as glucose and fructose. Monosaccharides can link together to form Disaccharides, such as sucrose, and Polysaccharides (which are chains composed of many monosaccharide units)—namely starch and Cellulose. Disaccharides and Polysaccharides can be broken down into their constituent monomers via Hydrolysis.

Lipids also serve as an energy source and structural material for cells. Compounds of this group—fats, cutin, suberin, Waxes, and Phospholipids—are generally insoluble in water.

Proteins are very large molecules built from long chains of Amino Acids called Polypeptides. Only 20 Amino acids are found in proteins; nonetheless, these are sufficient to form an enormous variety of protein molecules. The primary Levels of Protein Organization are: (1) Primary Structure, or the linear sequence of amino acids; (2) Secondary structure, or the manner in which the polypeptide chain is folded; (3) tertiary structure, or the way the folded chain is packed into a globular structure; (4) quaternary structure, or the spatial arrangement and combination of two or more polypeptide chains.

Enzymes are Globular proteins that act as catalysts for Chemical Reactions. With the help of enzymes, cells are able to accelerate chemical reactions at relatively low temperatures.

Nucleic acids are built from NUCLEOTIDES joined into long chains. Each nucleotide consists of three subunits: a nitrogenous base, a five-carbon sugar, and a phosphate group. Nucleotides containing the sugar deoxyribose form DNA, while those containing the sugar ribose form RNA.

Two nucleotide derivatives, ATP and ADP, commonly participate in energy transfer processes.

Appendix: Radiocarbon Dating

All organic substances contain carbon that previously existed as carbon dioxide and entered living organisms through Photosynthesis. Most carbon atoms (including those in СО₂) have an atomic mass of 12 (¹²С), but a certain proportion consists of the radioactive carbon isotope ¹⁴С, which is formed in outer space and exists in small quantities as heavy carbon dioxide. Plant cells utilize carbon dioxide to synthesize glucose and other organic molecules, absorbing ¹⁴СО₂ almost as readily as ¹²СО₂. All animals depend directly or indirectly on plants for their Nutrition; consequently, a certain fraction of the carbon atoms in the Tissues of all living creatures is represented by radioactive carbon ¹⁴С. Upon the death of a living Organism, the uptake of carbon naturally ceases, and the ratio between radioactive and non-radioactive carbon changes: ¹⁴С slowly decays, while ¹²С remains. The half-life is 5,730 years, meaning a fossil of that age should contain exactly half the ¹⁴С found in a living organism. Fossils or artifacts made by humans from wood or any other organic material can be dated with high precision by measuring the ¹⁴С/¹²С ratio. Radiocarbon dating has been applied with great success to The Study of archaeological finds. The method relies on the assumption that The ratio of ¹⁴С to ¹²С in Earth's atmosphere has remained constant throughout the period under study. Atmospheric nuclear testing has made radiocarbon dating impossible for future archaeologists.