Chlorophyll

Chlorophyll is a green pigment contained in the foliage of plants, giving them their notable coloration. This pigment is responsible for absorbing sunlight required for the production of sugar molecules, and ultimately of all biochemicals, in the plant.

Chlorophyll is found in the thylakoid sacs of the chloroplast . The chloroplast is a specialized part of the cell that functions as an organelle. Once the appropriate wavelengths of light are absorbed by the chlorophyll into the thylakoid sacs, the important process of photosynthesis is able to begin. In photosynthesis, the chloroplast absorbs light energy, and converts it into the chemical energy of simple sugars.

Vascular plants, which can absorb and conduct moisture and nutrients through specialized systems, have two different types of chlorophyll. The two types of chlorophyll, designated as chlorophyll a and b, differ slightly in chemical makeup and in color. These chlorophyll molecules are associated with specialized proteins that are able to penetrate into or span the membrane of the thylakoid sac.

When a chlorophyll molecule absorbs light energy, it becomes an excited state, which allows the initial chain reaction of photosynthesis to occur. The pigment molecules cluster together in what is called a photosynthetic unit. Several hundred chlorophyll a and chlorophyll b molecules are found in one photosynthetic unit.

A photosynthetic unit absorbs light energy. Red and blue wavelengths of light are absorbed. Green light cannot be absorbed by the chlorophyll and the light is reflected, making the plant appear green. Once the light energy penetrates these pigment molecules, the energy is passed to one chlorophyll molecule, called the reaction center chlorophyll. When this molecule becomes excited, the light reactions of photosynthesis can proceed. With carbon dioxide, water, and the help of specialized enzymes , the light energy absorbed creates chemical energy in a form the cell can use to carry on its processes.

In addition to chlorophyll, there are other pigments known as accessory pigments that are able to absorb light where the chlorophyll is unable to. Carotenoids, like B-carotenoid, are also located in the thylakoid membrane. Carotenoids give carrots and some autumn leaves their color. Several different pigments are found in the chloroplasts of algae, bacteria , and diatoms , coloring them varying shades of red, orange, blue, and violet.

See also Autotrophic bacteria; Blue-green algae

chlorophyll , green pigment that gives most plants their color and enables them to carry on the process of photosynthesis . Chemically, chlorophyll has several similar forms, each containing a complex ring structure and a long hydrocarbon tail. The molecular structure of the chlorophylls is similar to that of the heme portion of hemoglobin, except that the latter contains iron in place of magnesium. Within the photosynthetic cells of plants the chlorophyll is in the chloroplasts—small, roundish, dense protoplasmic bodies that contain the grana, or disks, where the chlorophyll molecules are located. Most forms of chlorophyll absorb light in the red and blue-violet portions of the visible spectrum; the green portion is not absorbed and, reflected, gives chlorophyll its characteristic color. Chlorophyll f absorbs near infrared wavelengths that are slightly beyond the red portion of the visible spectrum. Chlorophyll tends to mask the presence of colors in plants from other substances, such as the carotenoids. When the amount of chlorophyll decreases, the other colors become apparent. This effect can be seen most dramatically every autumn when the leaves of trees "turn color."

Chlorophyta , phylum (division) of the kingdom Protista consisting of the photosynthetic organisms commonly known as green algae . The organisms are largely aquatic or marine. The various species can be unicellular, multicellular, coenocytic (having more than one nucleus in a cell), or colonial. Those that are motile have two apical or subapical flagella. A few types are terrestrial, occurring on moist soil, on the trunks of trees, on moist rocks, and even in snowbanks. Various species are highly specialized, some living exclusively on turtles, sloths, or within the gill mantles of marine mollusks.

It is generally accepted that early chlorophytes gave rise to the plants. Cells of the Chlorophyta contain organelles called chloroplasts in which photosynthesis occurs; the photosynthetic pigments chlorophyll a and chlorophyll b, and various carotenoids, are the same as those found in plants and are found in similar proportions. Chlorophytes store their food in the form of starch in plastids and, in many, the cell walls consist of cellulose. Unlike in plants, there is no differentiation into specialized tissues among members of the division, even though the body, or thallus, may consist of several different kinds of cells. There are four evolutionary lineages of green algae. Most living species are grouped in classes that are coextensive with three of these lineages.

Class Chlorophyceae

This group contains the largest number of species of the division. They can have two or more flagella, near the apex of the cell. Mitosis in this class involves phycoplasts, microtubules that develop between and separate the daughter nuclei. This characteristic is not seen in any other organism, implying that no organisms have descended from this class. There are a variety of asexual and sexual reproductive techniques. Sexual reproduction is characterized by the formation of a zygospore (a dormant diploid zygote protected by a thick wall) that later undergoes meiosis.

The class includes unicellular organisms such as those in the genus Chlamydomonas with their two apical flagella and nonmotile organisms such as Chlorella, which is being cultivated for use as a dietary supplement. Colonial genera of Chlorophyceae include Hydrodictyon (the "water net" ) and the so-called volvocine line of flagellated specimens that range from simple colonies of Gonium to the intricate spinning spheres of Volvox, which can consist of up to 60,000 cells and exhibit some cellular specialization. The most complex of the class are the filamentous members, some of which exhibit features that are seen primarily in plants. Despite this similarity the class is not believed to have been the evolutionary source of plants.

Class Charophyceae

Charophyceae are of great fossil age; the stoneworts date as far back as the late Silurian period. Cells of this class are asymetrical. Those that are motile have two flagella, at right angles near the apex of the cell. Sexual reproduction in this class, as in Chlorophyceae, is characterized by the formation of a zygospore and zygotic meiosis. Unlike in the other two common classes of green algae, but as with plants, the nuclear envelope disintegrates when mitosis begins. During cell division the mitotic spindle is present; in some a phragmoplast similar to those seen in plants aids in the formation of a cell plate. Plants are thought to have evolved from early species of Charophyceae.

The class includes Spirogyra, familiar filamentous algae that float on ponds and lakes in slimy masses. The desmids are single cells noted for their extraordinary symmetry and geometrical beauty. They are found only in fresh (usually still) water and often take an important place in the food chains of small nutrient-poor ponds and peat bogs. The stoneworts consist of a complex branched thallus with an erect stemlike structure and many whorls of short branches. They occur in shallow fresh or brackish water and especially in water rich in calcium, where they become stiff and lime-encrusted, a characteristic that has made them plentiful in the fossil record.

Class Ulvophyceae

Ulvophyceae contains marine organisms that take a variety of shapes that may consist of a few cells, long filaments, thin sheets of cells, or coenocytic cells. Most approach being radially symmetrical. They have an alternation of generations and unlike in the other classes, meiosis occurs in the spores rather than the zygotes. When present, there can be two or more apical flagella. During mitosis, the nuclear envelope and the mitotic spindle persist, as they do in the Charophyceae.

The class Ulvophyceae includes sea lettuce, or Ulva, bright green, leaflike algae that grows in shallow waters on rocks and piers. Ventricaria is an egg-shaped, coenocytic alga, familiar in warm seas. Some organisms of Ulvophyceae produce toxins that discourage predation. The chloroplasts of some others become symbionts after they are retained in the bodies of sea slugs that eat the algae. They continue to perform photosynthesis, providing the slug with needed oxygen.

See also seaweed .

Bibliography

See H. C. Bold and M. J. Wynne, Introduction to the Algae: Structure and Reproduction (1985); C. A. Lembi and J. R. Waaland, Algae and Human Affairs (1988); C. van den Hoek, Algae: an Introduction to Phycology (1994).

Chlorophyll

All forms of life on the surface of Earth are powered, directly or indirectly, by absorption of the energy in sunlight by chlorophyll molecules in plant cells. The subsequent processes of photosynthesis convert light energy to electrical and then chemical energy, which the cell uses for growth. The minimal absorption of green light by chlorophyll causes plants to have a green color (see accompanying graph).

Chlorophylls are cyclic tetrapyrroles, that is, molecules made by connecting four 5-membered pyrrole rings into a macrocycle. The initial biosynthetic precursor, 5-aminolevulinic acid (ALA), is made from the abundant amino acid glutamic acid. Condensation of two ALA molecules produces the 5-membered ring compound porphobilinogen. Four of these molecules are joined into a large ring structure, some of the side chains are modified, and the compound is oxidized to generate the fully conjugated double-bond arrangement that allows efficient absorption of light energy. At this stage, Mg²⁺ is inserted into the center of the large ring structure, and the fifth ring is formed.

The long hydrocarbon side chain causes chlorophyll to act as a lipid, allowing it to become embedded in thylakoid membranes. Chlorophyll a can be oxidized to chlorophyll b, which differs only in the presence of an alde-hyde group on ring B. All chlorophyll molecules are bound to protein molecules and incorporated into complexes that allow energy absorbed by the molecules to be trapped in reaction centers of photosynthesis. In eukaryotic photosynthetic organisms, all these reactions occur in the chloroplast .

Other forms of chlorophyll also are found in nature. Some families of algae contain chlorophyll c, which does not have a long lipid tail and differs in several other respects. Chlorophyll d, which was found recently as the major chlorophyll in a photosynthetic prokaryote living inside ascidians in the Pacific Ocean, is similar to chlorophyll b but with the aldehyde on ring A. Bacteriochlorophylls, possibly the evolutionary ancestors of chlorophylls, occur in photosynthetic bacteria. Unlike other chlorophylls, bacteriochlorophylls absorb light in the infrared region, near 800 nanometers (nm).

see also Chloroplasts; Photosynthesis, Light Reactions and; Pigments.

J. Kenneth Hoober

Bibliography

Beale, Samuel I. "Enzymes of Chlorophyll Biosynthesis." Photosynthesis Research 60 (1999): 43-73.

chlorophyll The green pigment in plants that functions in photosynthesis by absorbing radiant energy from the Sun, predominantly from blue (435–438 nm) and red (670–680 nm) regions of the spectrum. The light removes an electron from the chlorophyll molecule. This is used to produce either ATP or NADP (nicotinamide adenine dinucleotide phosphate) for carbon-dioxide fixation. Chlorophylls are magnesium-porphyrin derivatives, the principal variants in land plants being designated chlorophylls a and b, and in marine algae c and d.

chlorophyll The green pigment in plants that functions in photosynthesis by absorbing radiant energy from the Sun, predominantly from blue (435–438 nm) and red (670–680 nm) regions of the spectrum. The light removes an electron from the chlorophyll molecule. This is used to produce either ATP or NADP (nicotinamide adenine dinucleotide phosphate) for carbon dioxide fixation. Chlorophylls are magnesium-porphyrin derivatives, the principal variants in land plants being designated chlorophylls a and b, and in marine algae c and d.

chlorophyll The green pigment of plant materials which is responsible for the trapping of light energy for photosynthesis, the formation of carbohydrates from carbon dioxide and water. Both α‐ and β‐chlorophylls occur in leaves, together with the carotenoids xanthophyll and carotene. Chlorophyll has no nutritional value, although it does contain magnesium as part of its molecule, and although it is used in breath‐fresheners and toothpaste, there is no evidence that it has any useful action.

chlorophyll Any one of a class of pigments found in all photosynthetic organisms; the most important members are chlorophyll a (see formula) and chlorophyll b, which occur in all land plants and are responsible for their green colour. Chlorophyll molecules are the principal sites of light absorption in the light-dependent reactions of photosynthesis (see photosystems I and II). They are magnesium-containing porphyrins, chemically related to cytochrome and haemoglobin. See also bacteriochlorophyll.

chlo·ro·phyll / ˈklôrəˌfil/ • n. a green pigment, present in all green plants and in cyanobacteria, responsible for the absorption of light to provide energy for photosynthesis. Its molecule contains a magnesium atom held in a porphyrin ring. DERIVATIVES: chlo·ro·phyl·lous / ˌklôrəˈfiləs/ adj.

chlorophyll Group of green pigments present in the chloroplasts of plants and algae that absorb light for photosynthesis. There are five types: chlorophyll a is present in all photosynthetic organisms except bacteria; chlorophyll b, in plants and green algae; and chlorophylls c, d and e, in some algae. It is similar in structure to haemoglobin, with a magnesium atom replacing the iron atom.

chlorophyll •bill, Brazil, brill, Camille, chill, cookchill, dill, distil (US distill), downhill, drill, Edgehill, Estoril, fill, freewill, frill, fulfil (US fulfill), Gill, goodwill, grill, grille, hill, ill, instil, kill, krill, mil, mill, nil, Phil, pill, quadrille, quill, rill, Seville, shill, shrill, sill, skill, spadille, spill, squill, still, stock-still, swill, thill, thrill, till, trill, twill, until, uphill, will •hwyl • bank bill • handbill • waxbill •playbill, waybill •cranesbill • sibyl • crossbill • sawbill •hornbill • storksbill • shoebill •spoonbill • duckbill • razorbill •gerbil • wind chill • Churchill • idyll •daffodil • back-fill • landfill • monofil •fibrefill (US fiberfill) • chlorophyll •bluegill