Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

2. Structural Components of the Plant Cell
2.9 General Characteristics of Proteins, Carbohydrates, Lipids, and Nucleic Acids

Proteins. Features of the Regulation of Protein Synthesis. Proteins are the core components of all cellular structural elements. Proteins are macromolecules with molecular weights ranging from 10 thousand to several millions. Protein molecules are built from Amino Acids. Although over 100 different Amino acids have been identified in plants, only 20 of them are used to build proteins; these are known as Proteinogenic Amino Acids.

Amino acids are amphoteric compounds containing carboxyl and amino groups. They are linked together into a protein molecule via a peptide bond, where the amino group of one acid combines with the carboxyl group of another. The formation of a peptide bond requires an expenditure of energy. For each type of protein, There is a specific order in which amino acids are arranged within the polypeptide chain. The sequence of amino acid residues in a protein molecule is called the Primary Cell/13.html">Protein Structure, which is genetically determined.

The Introduction/11.html">Secondary structure of proteins is formed through additional bonds, namely Hydrogen Bonds. Hydrogen bonds form between the hydrogen atom of the amino group of one peptide bond and the oxygen atom of the carboxyl group of another. When hydrogen bridges form within a single peptide chain, Helical structures emerge; when they form between two peptide chains, folded sheet structures are created.

The Tertiary Structure of a protein molecule arises when the coiled polypeptide chains adopt a specific, unique spatial conformation for each protein. The tertiary structure is stabilized by four types of bonds between the side chains of The polypeptide chains: interactions between nonpolar hydrocarbon residues (homeopolar interactions, Van der Waals forces), hydrogen bonds between polar groups, ionic bonds between dissociating acidic and basic groups, and disulfide bridges formed between two Cysteine residues via SH groups to create an S-S group.

The quaternary structure is formed when several spatially organized polypeptide chains combine to form a large, biologically active protein molecule. The stabilization of The quaternary structure involves the same types of bonds as those in the tertiary structure, with the exception of disulfide bridges.

All proteins are divided into simple (consisting solely of amino acid residues) and complex (proteids).

Depending on their solubility in various Solvents, proteids are classified into six groups:

1. Albumins — soluble in Water, acids, and alkalis (leucosin in wheat, rye, and barley germs; legumelins in soybean and pea seeds; ricin in castor bean seeds).

2. Globulins — insoluble in water, soluble in weak solutions of neutral salts (edestin in hemp seeds; phaseolin in bean seeds; glycinin in soybeans; vicilin in peas; conglutin in lupines).

3. Prolamins — soluble in 70% alcohol solution, characteristic of cereals (gliadin in wheat and rye grain; hordein in barley; zein in corn; avenin in oats).

4. Glutelins — soluble in weak alkaline solutions (glutenin in wheat seeds; oryzin in rice).

5. Protamines — Proteins of the simplest structure, found in fish sperm and the pollen of various plants.

6. Histones — proteins with weakly alkaline properties.

Proteids (comprising a protein and non-protein part, or prosthetic group) include Phosphoproteins, Lipoproteins, Glycoproteins, Metalloproteins, Chromoproteins, and Nucleoproteins.

Based on the conformation of the polypeptide chain, they are distinguished into fibrous (elongated) and globular (rounded).

Nucleic Acids and Protein Synthesis. The blueprint for building a protein is encoded in DNA, and its synthesis takes place on Ribosomes. The transmission of information from DNA is carried out by mRNA. At certain moments in the life of a cell, the DNA double helix unwinds, and an mRNA molecule is synthesized upon it as a template — a process known as METABOLISM/31.html">Transcription, or copying. The formation of RNA on the DNA template is mediated by a special enzyme, DNA-dependent RNA polymerase. By binding with a protein (informosome), mRNA passes through the nuclear pores into the Cytoplasm, carrying the information for protein synthesis. Afterward, the mRNA is released from the informosome, and the single-stranded, uncoiled molecule attaches to the small subunit of the ribosome, specifically at the site that connects with the large subunit. The First stage of synthesis consists of the formation of a complex between the mRNA and the ribosome.

Amino acids are transported to the ribosomes by tRNA. Each amino acid corresponds to its own tRNA. It has a cloverleaf shape and dual Specificity: it carries a specific triplet (anticodon) responsible for attaching to a specific site on the mRNA (the codon), and simultaneously, tRNA is specific with respect to Enzymes. The formation of a polypeptide chain requires energy. This energy is released during the interaction of amino acids with ATP and is subsequently transferred to the tRNA molecule. Via its anticodon, the aminoacyl-tRNA binds to the mRNA codon on the small ribosomal subunit.

Translation is the decoding of The nucleotide sequence of an RNA molecule into the Amino Acid Sequence of a protein.

Protein synthesis begins with The process of initiation. The initiator amino acid enters the small subunit, after which the large subunit joins the small one, the mRNA shifts by one codon, and the next codon (encoding the subsequent amino acid) enters the small subunit.

The addition of amino acid residues is called elongation, and the completion of the polypeptide chain formation is termed termination.

The segment of DNA that contains the information for building a single protein is called a Gene.

Nucleic acids (DNA and RNA) participate in PROTEIN SYNTHESIS AND carry the hereditary information of The Cell. DNA is double-stranded, consisting of a nitrogenous base (purine bases — adenine and guanine; pyrimidine bases — cytosine, thymine, and methylcytosine), a pentose sugar (deoxyribose), and a phosphoric acid residue. RNA is single-stranded, consisting of a nitrogenous base (adenine, guanine, cytosine, and uracil), a pentose sugar (ribose), and a phosphoric acid residue.

CARBOHYDRATES. The dry matter of a plant consists of nearly 90% carbohydrates. They are Components of the protoplasm and form The basis of The Cell wall. In addition, plants store carbohydrates as reserves (in tubers, ROOT vegetables, fruits, and seeds).

Carbohydrates consist of carbon, hydrogen, and oxygen. They are divided into two groups: 1) simple — Monosaccharides, 2) complex — Polysaccharides.

Simple carbohydrates are derivatives of polyhydric alcohols containing aldehyde or ketone groups (aldoses and ketoses). Simple carbohydrates are classified According to the number of carbon atoms in their molecules: trioses — dihydroxyacetone; tetroses — erythrose; pentoses — ribose, deoxyribose; hexoses — glucose.

Monosaccharides are white, crystalline substances that are readily soluble in water and have a sweet taste.

Due to their free aldehyde or ketone group, they exhibit reducing properties in an alkaline environment. In an acidic environment, The oxidation of the aldehyde group in monosaccharides yields aldonic acids. Polyhydric alcohols can be derived from monosaccharides: sorbitol from glucose or fructose, and mannitol from mannose. When interacting with acids, monosaccharides form esters, which play a part in plant metabolic processes. When interacting with alcohols, monosaccharides form Glycosides, which play a specific role in plant metabolism.

Complex carbohydrates are subdivided into Disaccharides (Oligosaccharides) and polysaccharides. Their molecules are polymers. Oligosaccharides dissolve in water, have a sweet taste, and are capable of crystallization. Representative Examples of this group include: sucrose (most abundant in sugar beet roots at 14-20% and sugar cane stalks at 14-25%), which consists of glucose and fructose; maltose (malt sugar), consisting of two glucose molecules; trehalose (mushroom sugar), which yields galactose upon Hydrolysis; lactose (milk sugar), consisting of glucose and galactose; and cellobiose, which consists of two glucose molecules and is a structural component of Cellulose.

Among polyoses, the most widespread are starch, cellulose, inulin, and Glycogen.

Polysaccharides are divided into two groups: Homopolysaccharides and Heteropolysaccharides. The molecules of the former consist of residues of a single monosaccharide only (starch, glycogen, lichenin, and cellulose consist of glucose residues; inulin and polyfructosides of fructose; Mannans of mannose). Hemicellulose, Gums, and mucilages belong to heteropolysaccharides. Their molecules consist of residues of different monosaccharide molecules.

Rice seeds contain 60-80%, wheat grain 60-75%, and potato tubers 12-24% starch.

Starch is deposited in the form of starch grains (simple and compound). The carbohydrate portion of starch consists of amylose (which turns blue with iodine) and amylopectin (which turns violet-brown with iodine). Amylose dissolves in warm water, whereas amylopectin is insoluble and forms a paste. Amylose has a linear structure, while amylopectin is branched. Under the action of the enzyme amylase, starch hydrolyzes into dextrins: amylodextrins turn a blue-violet color with Lugol's solution, erythrodextrins reddish-brown, achrodextrins brown, followed by maltodextrins and maltose, which turns yellow.

The highest proportion of inulin is found in dahlia tubers (12%) and chicory roots (up to 10%), as well as in Jerusalem artichoke and dandelion. Hydrolysis yields fructofuranose. Irisin is a component of iris rhizomes; asparagosin is found in the roots of asparagus (Siberian asparagus), graminin in ripe rye grains; secalin in young rye plants; and lichenin in Lichens.

Cellulose - (C6H10O5). Cotton fibers consist of 95-98% cellulose; flax of 80-90%; and the wood of various plants contains 40-50% cellulose. Acid hydrolysis of cellulose yields glucose, whereas Enzymatic hydrolysis (using the enzyme cellulase) yields cellobiose. Cellulose is insoluble in water, but swells in concentrated acids upon heating.

Hemicellulose is insoluble in water but soluble in alkalis. It is found in various parts of plants (bran, straw, seeds, nuts, wood, and corn cobs).

Pectin substances are high-molecular-weight compounds of a carbohydrate nature, particularly abundant in fruits and root crops. The intercellular layer that joins the walls of adjacent Cells is composed of Pectins. Pectin substances are part of The plant cell wall and play a crucial role in fruit ripening, storage, and Processing. During fruit ripening, protopectin (the precursor of pectin) is converted into soluble pectin. The Separation of bast fibers during retting is based on the hydrolysis of pectin substances by specialized microorganisms.

Gums and mucilages are found in seeds (flax, rye) and are formed in various plant Organs upon injury (cherry gum); they swell in water and become viscous.

Thus, carbohydrates perform building (structural), energetic, and reserve Functions.

Lipids. Fats and fat-like substances—compounds that are insoluble in water but soluble in organic solvents (ether, acetone, benzene, chloroform)—belong to the group of lipids. Lipids are divided into fats and fat-like compounds, or lipoids. Lipoids include phosphatides, Waxes, fat-soluble pigments (chlorophylls, carotenoids), sterides, and Fat-soluble Vitamins (A, E, D, K). Chemically, fats are a mixture of esters of glycerol and high-molecular-weight Fatty acids. Unsaturated fatty acids include oleic, linoleic, and linolenic, while saturated fatty acids include palmitic and stearic. Linoleic and linolenic acids are synthesized exclusively by plants; they are essential for animals and are sometimes referred to as vitamin F.

Plant fats can be monoacid or polyacid.

Fats and lipoids are constituents of membranes, cytoplasm, and various Organelles, serve as energy reserves, and represent a high-quality energy source. The oxidation of fat releases twice as much energy as the oxidation of carbohydrates.

Phosphatides are glycerides that contain phosphoric acid and other substances, often including nitrogen. They are most abundant in oilseeds and legumes. Choline phosphatides, or lecithins, contain choline; colamine phosphatides, or cephalins, contain colamine; and Serine phosphatides contain serine.

Chemically, waxes are esters of Fatty Acids and high-molecular-weight monohydric fatty or aromatic alcohols. Waxes coat the leaves, fruits, and young stems of plants, thereby performing a protective function by retarding water evaporation and preventing the penetration of pathogenic microorganisms. Cutin and suberin are insoluble lipid polymers that impregnate the cell wall. Suberin is found in the Casparian strips of endodermal cells.

Sterides are esters of fatty acids and high-molecular-weight cyclic alcohols known as sterols. In the Cell Cytoplasm, they form complex structures with proteins and are substances capable of inducing malignant tumors.

Steroids and Terpenes do not contain fatty acids and are built from five-carbon isoprene units. Steroids are components of certain glycosides, whereas terpenes are constituents of Essential Oils, such as menthol, camphor, and natural rubber.