An organ, system, or other discrete element of an organism the part of the plant that carries out photosynthesis. EXPLANATION: The chemical compounds of living things are known as organic compounds because of their association with organisms. Organic compounds, which are the compounds associated with life processes, are the subject matter of organic chemistry. Among the numerous types of organic compounds, four major categories are found in all living things: carbohydrates, lipids, protein, and nucleic acids. Carbohydrates
Almost all organisms use carbohydrates as sources of energy. In addition, some carbohydrates serve as structural materials. Carbohydrates are molecules composed of carbon, hydrogen, and oxygen; the ratio of hydrogen atoms to oxygen atoms is 2:1. Simple carbohydrates, commonly referred to as sugars, can be monosaccharides if they are composed of single molecules, or disaccharides if they are composed of two molecules. The most important monosaccharide is glucose, a carbohydrate with the molecular formula C6H12O6. Glucose is the basic form of fuel in living things. It is soluble and is transported by body fluids to all cells, where it is metabolized to release its energy. Glucose is the starting material for cellular respiration, and it is the main product of photosynthesis. Complex carbohydrates are known as polysaccharides. Polysaccharides are formed by linking innumerable monosaccharides. Among the most important polysaccharides are the starches, which are composed of hundreds or thousands of glucose units linked to one another. Two other important polysaccharides are glycogen and cellulose. Glycogen is also composed of thousands of glucose units, but the units are bonded in a different pattern than in starches. Proteins
Proteins, among the most complex of all organic compounds, are composed of amino acids(see Figure 3 ), which contain carbon, hydrogen, oxygen, and nitrogen atoms. Certain amino acids also have sulfur atoms, phosphorous, or other trace elements such as iron or copper. Figure 3 The structure and chemistry of amino acids. When two amino acids are joined in a dipeptide, the —OH of one amino acid is removed, and the —H of the second is removed. A dipeptide bond (right) forms to join the amino acids together. Many proteins are immense in size and extremely complex. However, all proteins are composed of long chains of relatively simple amino acids. There are 20 kinds of amino acids. Each amino acid (see the left illustration in Figure 3 ) has an amino (—NH2) group, a carboxyl (—COOH) group, and a group of atoms called an —R group (where R stands for radical). The amino acids differ depending on the nature of the —R group, as shown in the middle illustration of Figure 3. Examples of amino acids are alanine, valine, glutamic acid, tryptophan, tyrosine, and histidine. One essential use of proteins is in the construction of enzymes. Enzymes catalyze the chemical reactions that take place within cells. They are not used up in a reaction; rather, they remain available to catalyze succeeding reactions. Every species manufactures proteins unique to that species. The information for synthesizing the unique proteins is located in the nucleus of the cell. Nucleic acids
Like proteins, nucleic acids are very large molecules. The nucleic acids are composed of smaller units called nucleotides. Each nucleotide contains a carbohydrate molecule, a phosphate group, and a nitrogen-containing molecule that because of its properties is a nitrogenous base. Living organisms have two important nucleic acids. One type is deoxyribonucleic acid, or DNA. The other is ribonucleic acid, or RNA. DNA is found primarily in the nucleus of the cell, while RNA is found in both the nucleus and the cytoplasm, a semi-liquid substance that composes the foundation of the cell. Further...