Photosynthesis: The Pigments and Light Absorption

Photosynthesis: The Pigments

Introduction

  • Autotrophic plants synthesize organic food using light energy from the sun.
  • Carbohydrates produced are the basic raw materials for all organic components of plants and animals.
  • Humanity depends on plants for food.
  • Approximately 200 billion tons of carbon go through photosynthesis annually, making it a massive chemical event.
  • Plants take up 7 \times 10^{11} tons of CO_2 to produce roughly 5 \times 10^{11} tons of solid plant material.
  • About 90% of the world's photosynthesis is carried out by marine and freshwater algae.

Historical Perspective

Early Beliefs

  • From Aristotle until the 17th century, it was believed that plants derived nutrition from soil debris.

17th Century Discoveries

  • J.B. van Helmont (1577-1644): Belgian physician who cultivated a willow plant for five years, concluding that water and soil contribute to plant growth.
  • Woodward (1699): Proposed that vegetables are formed from a peculiar terrestrial matter absorbed along with water.

18th Century Advances

  • Priestley (1772): Discovered gas exchange in photosynthesis; CO2-containing 'injured' air would get purified in contact with green mint plants, producing oxygen. However, he did not recognize the role of CO_2 or light.

Ingenhousz's Contributions

  • Ingenhousz (1779): Reported that plants purify air only in the presence of light and that the same tissue made air impure in the dark. He noted that only green parts (chlorophyll) produced oxygen, while non-green tissue contaminated the air, recognizing chlorophyll and light's participation.

Senebier and de Saussure

  • Jean Senebier (1782): Recognized that "fixed" air (CO2) was essential in photosynthesis. Observed that oxygen liberated from plants came from the absorbed CO2 and that red wavelengths of light were most effective.
  • de Saussure (1804): Confirmed Ingenhousz's findings regarding gas exchange differences in light (photosynthesis) and darkness (respiration); also discovered that water was utilized in the process.

Other Key Figures

  • Stephen Hales (1727): Referred to as the 'father of plant physiology,' pointed out that plants get nourishment through leaves from air and sunlight.
  • Dutrochet (1837): Confirmed that chlorophyll is essential for photosynthesis.
  • Liebig (1840): Reported that the source of carbon in plants was carbon dioxide and that oxygen was released from CO_2.
  • Robert Mayer (1845): Announced the law of conservation of energy and pointed out that the energy used by animals in their metabolism is obtained from plants via solar energy transformation. His ideas remain qualitatively complete and true.
  • Sachs (1877): Established the foundation for modern views of photosynthesis, discovering that green chloroplasts are the organs where carbon dioxide is used up and oxygen is released. Also found that starch was the first visible product of photosynthesis.
  • Englemann (1888): Gave the action spectrum of photosynthesis.
  • Warburg (1919): First to use the green alga Chlorella for studying photosynthesis.
  • Boussingault (1860-65): Showed that plants obtained total carbon requirements from carbon dioxide in the air.

Synthesis of Chlorophyll

Role of Protochlorophyll

  • Chlorophyll is normally formed from a precursor called protochlorophyll, which differs from chlorophyll by two hydrogen atoms in one of its pyrrole rings.
  • Protochlorophyllide-grown seedlings in darkness produce small amounts of protochlorophyll. When transferred to light, protochlorophyll is quantitatively converted to chlorophyll, which can be chemical rather than photochemical.

Key Contributors to Understanding Chlorophyll Synthesis

  • Granick (1954) and Shemin (1956): Explained the synthesis of chlorophyll, indicating that glycine and succinyl Co-A condense to form unstable α-amino-β-ketoadipic acid, which forms δ-aminolevulinic acid upon decarboxylation.
  • Gassman (1967) and Bogorad (1967): Suggested that the synthesis of δ-aminolevulinic acid requires light.

Biochemical Steps

  1. Two molecules of δ-aminolevulinic acid condense to form a monopyrrole porphobilinogen in the presence of the enzyme δ-amino-levulinase (δ-aminolevulinic acid dehydrase).
  2. Four molecules of porphobilinogen give rise to uroporphyrinogen III under the influence of uroporphyrinogen synthetase and urophyrinogen III cosynthetase.
  3. Uroporphyrinogen III is decarboxylated to coproporphyrinogen III in the presence of uroporphyrinogen decarboxylase.
  4. Coproporphyrinogen III gives rise to protoporphyrinogen IX in the presence of coproporphyrinogen III oxidase decarboxylase.
  5. Protoporphyrinogen IX is oxidized to protoporphyrin IX, which incorporates magnesium to produce Mg-protoporphyrin IX.
  6. Mg-protoporphyrin IX takes up a methyl group from S-adenosyl methionine via the enzyme Mg-protoporphyrin methyl esterase to form Mg-proto-porphyrin IX monomethyl ester.
  7. Mg-proto-porphyrin IX monomethyl ester is converted into protochlorophyllide, which takes up a phytol group to form protochlorophyll.
  8. Protochlorophyll gains hydrogen to become chlorophyll.

Chlorophyll Synthesis Variations

  • According to some, chlorophyllide is an immediate precursor of chlorophyll, not protochlorophyll.
  • Gassman and Bogorad (1967), and Akoyunoglon and Slegelman (1968) found that protochlorophyllide is reduced to chlorophyllide a in the presence of light.
  • In gymnosperms, some ferns, and many algae, light is not absolutely essential for chlorophyll synthesis.
  • Esterification of a phytol group to chlorophyllide a forms chlorophyll a in the presence of the enzyme chlorophyllase.
  • Chlorophyll a is believed to give rise to chlorophyll b.

Chlorophyll Pigments

Types of Chlorophylls

  • There are at least seven types of chlorophylls known: chlorophylls a, b, c, d, and e, bacteriochlorophyll, and bacterioviridin.

Molecular Structure

  • All chlorophyll molecules contain a tetrapyrrole skeleton formed into a ring with a magnesium atom in the center.
  • A pyrrole molecule contains a ring of five atoms (four carbon, one nitrogen).
  • Four pyrroles form the 'head' of a chlorophyll molecule. Attached to this porphyrin ring is an alcohol (phytol) 'tail'.
  • Minor variations in the groupings of atoms joined to the head and tail account for differences among chlorophylls.

Distribution of Chlorophylls

  • Chlorophylls a and b are the two most abundant chlorophylls in plants.
  • Chlorophyll a is found in all autotrophic plants except photosynthetic bacteria.
  • Chlorophyll b is absent in blue-green, brown, and red algae.
  • Chlorophylls c, d, and e are found only in algae and in combination with chlorophyll a.
  • Chlorophyll a possesses a -CH_3 (methyl) group, which is replaced by -CHO (aldehyde) group in chlorophyll b.
  • Molecular Formulas:
    • Chlorophyll a: C{55}H{72}O5N4Mg
    • Chlorophyll b: C{55}H{70}O6N4Mg

Location within Thylakoids

  • Chlorophylls are primarily located within the grana thylakoids.
  • The chlorophyll molecules form a monolayer between the protein and lipid layers of the thylakoid membranes.
  • Hydrophilic heads are embedded within the protein layer, while lipophilic tails are in the lipid layer.

Absorption Spectrum

Effective Wavelengths

  • The portion of the electromagnetic spectrum that participates in photosynthesis spans from 300 to 900 nm.
  • In green plants, only the visible spectrum (400-750 nm) is effective.
  • Photosynthetic green bacteria can absorb wavelengths from 375-800 nm, while purple photosynthetic bacteria absorb 300-950 nm.

Absorption Peaks and Forms

  • Chlorophyll a has main absorption peaks at 430 and 662 nm. Chlorophyll b occurs at 453 and 642 nm.
  • Variations in absorption peaks occur due to environmental changes.
  • Pigments in green sulfur bacteria are called chlorobium chlorophylls (650 and 660) because their absorption peaks are at 650 and 660 nm.
  • The pigment system of Chromatium is composed of three forms of bacteriochlorophyll, absorbing at 800, 850, and 890 nm.
  • Bacteriochlorophyll absorbing light at 890 nm is called B 890, present at a ratio of one B 890 to every 50 bacteriochlorophyll molecules; it acts as the reaction center similar to P700 of PSI.
  • Green sulfur bacteria have chlorophyll 770 as the reaction center.
  • The rate of photosynthesis is proportionate to the light energy absorbed by chlorophyll molecules.

Carotenoids

Role and Distribution

  • Carotenoids are accessory pigments in photosynthesis that transfer light energy to chlorophyll.
  • They are widely distributed in bacteria, algae, and higher plants.
  • Carotenoids include orange carotenes and yellow xanthophylls (oxygenated carotenes).

Absorption

  • Carotenoids absorb wavelengths from 400 nm to 500 nm, appearing orange.

Types and Functions

  • β-carotene is the most abundant carotene, absorbing blue light and appearing yellow.
  • α-carotene is present in small amounts in certain species.
  • During autumn, chlorophylls degenerate, and carotenoids become visible as orange and yellow colors in leaves.
  • Carotenoids are located in chloroplast membranes or within chromoplasts.
  • Functions include trapping light energy and transferring it to chlorophyll a, particularly in algae and higher plants (lutein of xanthophylls and β-carotene).
  • Carotenoids are lipid-soluble. Lutein and zeaxanthin are hydroxylated forms of a-carotene and β-carotene, respectively.

Protective Role

  • At high light intensities, the cell apparatus is oxidized by atmospheric oxygen into carbon dioxide (photo-oxidation).
  • Chlorophyll mutants are actually carotenoid mutants.
  • Carotenoids (β-carotene) protect the photosynthetic apparatus by trapping and dissipating excess excitation energy that would otherwise convert molecular oxygen to a highly reactive superoxide (O_2).
  • Dissipation of heat is facilitated by the xanthophyll cycle.

Structure

  • Carotenoids consist of long chains of carbon atoms linked by alternating single and double bonds with six-carbon rings (carotenols have oxygen).
  • Carotenoids are located in chloroplasts and named after 'carrot'.

Phycobilins

Discovery and Function

  • Engelmann found blue-green light to be very effective in increasing the rate of photosynthesis in brown and red algae; red algae gave the best result in green light.

Role in Photosynthesis

  • Demonstrated the role of phycobilins and carotenoids in photosynthesis.
  • Irradiation of carotenoids causes fluorescence of chlorophyll, suggesting energy transfer from accessory pigments to chlorophyll a.

Types and Characteristics

  • Phycobilins are present in blue-green and red algae and are tetrapyrroles like chlorophylls but with four joined pyrrole rings forming a straight chain.
  • Like anthocyanins, they mask the green color of chlorophylls but are intimately associated with chlorophylls; light absorbed can be used in photosynthesis.
  • Phycobilins include red-colored phycoerythrins and blue-colored phycocyanins found in red and blue-green algae, respectively.
  • Phycoerythrin absorbs green light the best, while phycocyanin absorbs blue light the best.

Light Harvesting

  • Phycobilins are supplementary accessory light-harvesting pigments, which in association with proteins constitute phycobilinosomes in cyanobacteria and red algae.

Absorption Range

  • They absorb light in the range of 520-530 nm, which is not absorbed by chlorophylls, supplementing them in trapping maximum energy of the visible spectrum.

Solubility

  • Carotenoids are soluble in organic solvents, while phycobilins are soluble in hot water.

Structure and Energy Transfer

  • Phycocyanobilin and phycoerythrobilin are chromophores that bind to apoproteins to form the pigments phycocyanin, phycoerythrin, and allophycocyanin; they are designed to cause 95% energy transfer.

Anthocyanins

Characteristics

  • The color of leaves is modified in certain plants due to the presence of purple pigments called anthocyanins, formed by several linked rings of atoms.

Solubility and Location

  • Anthocyanins are soluble in water and present in the vacuolar sap of the cells.

Role in Photosynthesis

  • Anthocyanins do not take part in photosynthesis and are not present in the cytoplasm.

The Role of Light

Light Intensity

  • Full sunlight is inhibitory for photosynthesis.
  • Leaves are oriented to reduce sunlight intensity for optimal photosynthesis.
  • In direct sunlight, leaves lie at an acute angle to the rays; in the shade, they lie at right angles.

Absorption and Reflection

  • About 80% of incident light is absorbed, 10% is reflected, and 10% is transmitted; these vary with wavelengths.

Pigment Absorption

  • Pigments absorb light only in certain regions, transmitting the remaining wavelengths.

Wavelength Absorption

  • Chlorophylls absorb shorter (blue and violet) and longer waves (orange and red); carotenoids absorb shorter wavelengths; phycoerythrin absorbs blue, green, and yellow, and phycocyanin absorbs longer wavelengths.
  • Photosynthetic bacteria absorb even infrared light.

Energy Usage

  • Part of the radiant energy absorbed by chlorophyll is used in producing a chemical change (photochemical effect), and part is re-emitted as light (fluorescence).

Chlorophyll's Key Role

  • Chlorophyll a is the key substance in the photochemical reaction of photosynthesis, indicating energy transfer from carotenoids to chlorophyll a.

Algae Benefits

  • Such transfer is of special biological advantage in algae (e.g., red seaweeds) growing in deep water.
  • Green light, which penetrates the farthest in clear water, is not well absorbed by chlorophyll but is absorbed by special accessory photosynthetic pigments that transfer energy to chlorophyll a.