MOLECULAR COMPOSITION OF PLANT CELLS
Raven et al. (1999), p. 17
Six chemical elements make up 99% of the weight of all living matter: carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur.
Water makes up more than half of all living matter and more than 90% of the weight of most plant tissues.
Organic molecules: Contain carbon; synthesized by living cells; probably 10,000 different kinds of organic molecules are found in each living plant and animal cell; about 5000 in each bacterial cell.There a four great classes of organic compounds (primary compounds): carbohydrates (composed of sugars), lipids (most contain fatty acids), proteins (composed of amino acids), and nucleic acids (DNA and RNA, made up of nucleotides). CARBOHYDRATES: the most abundant organic molecules in nature; the primary energy-storage molecules in living organisms; important part of cell structure.Carbohydrates (saccharides) are formed from small molecules known as sugars: there are three main kinds of carbohydrates, classified according to the number of sugar subunits they contain: Monosaccharides: one sugar molecule; examples include glyceraldehyde, ribose, glucose, and fructose (see p. 19 for molecular structure of monosaccharides). Disaccharides: two sugar subunits linked together; examples include sucrose (table sugar), maltose (malt sugar), and lactose (milk sugar). Polysaccharides: contain 1OO's or 1000's of sugar subunits linked together; examples include starch, cellulose, and glycogen.Polymers are macromolecules that are made up of many similar or identical subunits; each subunit is a monomer; polymerization is the stepwise linking of monomers into polymers.Monosaccharides function as building blocks and as sources of energy; monosaccharides are the simplest carbohydrates; they are made up of linked carbon atoms to which hydrogen and oxygen atoms are attached in the proportion of one carbon atom: two hydrogen atoms: one oxygen atom; different monosaccharide molecules may contain from 3 to 7 carbon atoms (see p. 19 for molecular structure); monosaccharides are hydrophilic ("water-loving"), or soluble in water; the monosaccharide glucose in the form in which sugar is transported in the circulatory system of vertebrate animals.The disaccharide sucrose is a transport form of sugar in plants; sucrose is hydrophilic; a disaccharide is synthesized when two monosaccharides are linked together; the linking to two sugars is called dehydration synthesis, a condensation reaction, where a molecule or water is removed and a new bond is formed between the two sugars; the reverse reaction, hydrolysis, occurs when a disaccharide (or polysaccharide) is split into its monosaccharide subunits when a water molecule in added between two sugars.Polysaccharides function as storage forms of energy or as structural materials; starch is the primary storage polysaccharide in plants; starch consists of chains of glucose molecules; starch is stored as starch grain is cells; glycogen is the common storage polysaccharide in prokaryotes, fungi, and animals; glycogen consists of chains of glucose molecules; fructans are the principal storage polysaccharides in leaves and stems (fructans are water-soluble); cellulose, a polysaccharide, is the principal component of plant cell walls; half of all the organic carbon in the biosphere is contained in cellulose; cellulose is the most abundant organic compound known; wood is about 50% cellulose; cotton fibers are nearly pure cellulose; only a few organisms can produce the enzyme cellulase, which hydrolyzes cellulose ( certain prokaryotes, protozoans, and fungi, and very few animals, such as silverfish); chitin is the principal structural component of fungal cell walls. LIPIDS: fats and fatlike substances; generally they are hydrophobic and thus insoluble in water; lipids serve as energy storage molecules (fats and oils), and as structural molecules in the case of phospholipids and waxes.Fats and oils are triglycerides that store energy; fats and oils contain about three times as much energy as in carbohydrates and proteins; in plants fats and oils often are stored in seeds and fruits.Each different fat or oil (triglycerides) consist of three fatty acid molecules bonded to one glycerol molecule (see p. 23 for molecular structure); fatty acids are long carbon chains, perhaps 14-18 carbons with their hydrogens; a glycerol molecule is a three carbon alcohol.Different fatty acid molecules may be saturated or unsaturated; saturated fatty acids have no double bonds between the carbon atoms; unsaturated fatty acids have one or more double bonds. Phospholipids are modified triglycerides that are principal components of cell membranes; phospholipids are amphipathic molecules, i.e., they have a water-soluble polar "head" and a water-insoluble "tail" (see p. 24 for molecular structure). Cutin, suberin, and waxes are lipids that form barriers to water loss; cutin and suberin are unique lipids that are components of many plant cell walls; waxes are embedded in cutin and suberin; many outer plant surfaces have a protective cuticle composed of wax embedded in cutin (cuticular wax); the cuticular wax may be covered by a layer of epicuticular wax; suberin is a major component of the walls of cork; waxes are the most water-repellent lipids, and the best example is carnauba wax, used for car and floor polishes. Steroids have four interconnected hydrocarbon rings, and playa variety of roles in plants; sterols are kinds of steroids, e.g., sitosterol in green algae and plants, ergosterol in fungi, and cholesterol in animals; steroids may function as hormones. PROTEINS: among the most abundant molecules, making up perhaps 50% or more of dry weight; proteins are incredibly diverse; they are all made of amino acids, some 20 different kinds of them, arranged in linear sequence; a single plant or animal cell may contain thousands of different kinds of proteins; proteins are concentrated in seeds where they function in storage of amino acids that are available for the growing embryo. Amino acids are the building blocks of proteins; amino acids have the same basic structure (see p. 27), an amino group, a carboxyl group, and a hydrogen atom all bonded to a central carbon atom; the differences arise from the fact that every amino acid has an "R " group bonded the central carbon. Protein synthesis is yet another kind of dehydration synthesis, where the amino group of one amino acid links to the carboxyl group of the adjacent amino acid by removal of a molecule of water. The bond formed between two amino acids is known as a peptide bond; the molecule that results from the linking of many amino acids is known as a polypeptide, or protein. Protein structure = levels of organization; all different proteins have a different linear sequence of amino acids; the levels of organization include primary, secondary, tertiary, and quaternary . Primary Structure = the specific linear sequence of amino acids Secondary Structure = partially folded polypeptide chain, caused by interactions between the various amino acids along the chain; two of the most common shapes of secondary structure are the alpha helix and beta pleated sheet (see pp. 28-29); proteins predominated by primary or secondary structure are called fibrous proteins. Tertiary Structure = the secondary structure folds more elaborately, as a result of complex interactions between the R groups in the individual amino acids, to form tertiary structure; tertiary proteins are known as globular proteins; globular proteins are the most biologically active proteins, such as enzymes, hormones, membrane proteins, and transport proteins; some globular proteins are important structural proteins, such as microtubules. Quaternary Structure = proteins that are composed of more than one polypeptide chain, the chains held together by various kinds of bonds. Protein Denaturation; the breakdown of bonds causing tertiary folding, resulting in a loss of biological activity of the protein; environmental factors such as heat and acidity can cause denaturation. Enzymes are proteins that catalyze chemical reactions in cells; enzymes are large complex globular proteins; catalysts are substances that accelerate the rate of a chemical reaction but remain unchanged in the process; enzyme names end in -ase; -ase usually is added the name of the substrate, thus amylase catalyzes the hydrolysis of amylose (starch) into glucose molecules, and sucrase catalyzes the hydrolysis of sucrose into glucose and fructose; nearly 2000 enzymes are now known, and each of them is capable of catalyzing some specific chemical reaction. NUCLEIC ACIDS: information dictating protein structure is encoded in and translated by nucleic acids; nucleic acids consist of long chains of molecules known as nucleotides. Nucleotide: consists of three subunits; a phosphate group, a five-carbon sugar (either ribose or deoxyribose ), and a nitrogenous base; five different nitrogenous bases occur in the nucleotides that are the building blocks of nucleic acids; they are adenine-thymine and cytosine-guanine in DNA; in RNA uracil replaces thymine. Deoxyribonucleic Acid (DNA) and ribonucleic acid (RNA) are two types of nucleic acids found in living organisms. DNA is the carrier of the genetic message; it contains the information, organized in units known as genes, that we and other organisms inherit from our parents. A DNA molecule has the overall structure of a very long double helix; RNA molecules are involved in the synthesis of proteins based on the genetic information provided by DNA. The structure of an RNA molecule is that of a long single helix. Some RNA molecules function as enzyme-like catalysts (ribozymes). Independent nucleotides have a crucial function in living systems; the molecule ATP is the cell's energy currency; the principal energy carrier for most processes in living organisms is the molecule adenosine triphosphate, or ATP (see p. 31 for structure); the structure of ATP is that of a nucleotide with three phosphates attached; most of the energy in ATP is associated with the terminal phosphate.
The primary metabolites (carbohydrates, lipids, proteins, nucleic acids) are found in all plant cells. Secondary metabolites are restricted in their distribution, both within the plant and among different species of plants. Secondary metabolites were once considered to be waste products of primary metabolism. Now we know that secondary compounds are important for the survival and propagation of plants that produce them. Many serve as chemical signals that enable the plant to respond to environmental cues. Others function in the defense of the producer against herbivores, pathogens (disease-causing organisms), or competitors. Some provide protection from radiation from the sun, while others aid in pollen and seed dispersal.There are three major classes of secondary plant compounds: alkaloids, terpenoids, and phenolics (see p. 32-37).
Alkaloids: classes of molecules that include morphine, cocaine, caffeine, nicotine, and atropine; alkaloids are among the most important pharmacologically, or medicinally, active compounds; interest in them has traditionally stemmed from their dramatic physiological or psychological effect on humans; alkaloids are bitter-tasting nitrogenous compounds that are basic (alkaline) in their chemical properties.Terpenoids: composed of isoprene units and include essential oils, taxol, rubber, and cardiac glycosides; terpenoids, also called terpenes, occur in all plants and are by far the largest class of secondary metabolites, with over 22,000 terpenoid compounds described; the simplest of the terpenoids is the hydrocarbon isoprene (CSH8); all terpenoids are classified according to their number of isoprene units, e.g., monoterpenoids consist of two isoprene units, sesquiterpenoids consist of three ioprene units, and diterpenoids consist of four isoprene units; the largest known terpenoid compound is natural rubber (400-100,000) isoprene units.Phenolics: include flavonoids, tannins, lignins, ans salicylic acid; phenolics include a broad range of compounds, all ofwhich have a hydroxyl group (-OH) attached to an aromatic ring; phenolics are almost universally present in plants, and are known to accumulate in all plant parts; the function of many phenolic compounds is still unknown; flavonoids are water-soluble pigments, including anthocyanins; flower pigments act as visual signals to attract pollinating birds and bees; tannins are the most important deterrents to herbivore feeding in the angiosperms; they are extremely astringent; lignins are deposited in cell walls; the compressive strength of lignin allowed terrestrial plants to evolve size; lignin physically protects plants from fungal attacks; salicylic acid, the active ingredient in aspirin, was discovered by the ancient Greeks and by Native Americans who obtained it for pain relief from tea brewed from willow bark; this phenolic acid is now known to protect plants from pathogenic bacteria, fungi, and viruses.