Energy Systems in Plants: Photosynthesis 🪴☀️

• Types of CO, fixation

  Calvin cycle or Calvin-Benson cycle or C, cycle
• Researches in the field of photosynthesis have led to the conclusion that carbon in the carbohydrates, synthesised in the green cells of plants comes from carbon dioxide and hydrogen from water. Further, Ruben and Kamen (1941) confirmed that all the oxygen evolved in photosynthesis comes from water and not from carbon dioxide.
• Photosynthesis is the only process on the earth by which sun's energy is trapped by autotrophic organisms and is converted into food for all the living organisms.
• The energy locked up in the chemical compounds is released by oxidation through respiration for carrying out all the vital functions of life. Hence, it is said that 'life on the earth is bottled sun's energy'.
• Though only 0.2% of the incident light energy on the earth is trapped by photosynthetic organisms, this is able to meet the food requirement of all the heterotrophs.
• Chloroplasts


  The site of photosynthesis
• Chloroplasts of higher plants


  • They show phototactic movement (a movement in response to light)
  • In strong light, they are aligned with the longitudinal walls of mesophyll cells with their edges only towards light
  • In moderate light, chloroplasts arrange themselves perpendicular to the incident light
  • In poor light, chloroplasts are arranged variously
• Structure of chloroplast

  • It is bounded by a double-membrane envelope
  • Its interior is filled with proteinaceous, colloidal matrix, called stroma and an elaborated system of membranous lamellae, the thylakoids
• Stroma


  • It contains enzymes, DNA, RNAs and 70S ribosomes
  • Enzyme Ribulose biphosphate carboxylase (RaB? carboxylase), the most abundant enzyme is also present in the stroma
  • It is the site of dark reaction of photosynthesis
• Thylakoids


  • They run parallel in the stroma and occur as flattened sacs of two types: grana thylakoids and stroma thylakoids
  • Thylakoid membranes possess photosynthetic pigments-chlorophyll a and chlorophyll b, carotenoids, cytochromes (b and f), ATP synthetase and enzymes needed in photochemical reactions or light reactions of photosynthesis
  • Thylakoids are the site of light reaction of photosynthesis
• Photosynthetic pigments


  • They have the ability to absorb light at specific wavelengths
  • They can be separated through paper chromatography
  • The chromatogram of leaf pigments shows three groups of pigments: chlorophylls, carotenoids and phycobilins
• Types of chlorophylls

  • Chlorophyll a
  • Chlorophyll b
  • Chlorophyll c
  • Chlorophyll d
  • Chlorophyll e
• Chlorophyll a


  • It is bluish-green in colour
  • It is the universal photosynthetic pigment and is responsible for the conversion of light energy (photons) into chemical energy and for the emission of electrons during both cyclic and noncyclic photophosphorylation
  • It is more soluble in petroleum ether and is highly fluorescent in solution
• Chlorophyll b


  • It is yellow-green or olive-green in colour
  • It occurs along with chlorophyll a in all green plants, algae and euglenophytes
  • It is more soluble in 90% methyl alcohol
• Chlorophyll c


  • It occurs in brown algae, diatoms and dinoflagellates
  • It is soluble in aqueous alcohols
• Chlorophyll d


  It occurs in red algae
• Chlorophyll e


  It is present in xanthophyceae (a class of yellow-green algae) along with chlorophyll a
• Photosynthetic pigments in cyanobacteria and other photosynthetic bacteria


  • They possess bacteriochlorophyll and chlorobium chlorophyll (also called bacterioviridin)
  • Bacteriochlorophyll occurs in a, b, c and d forms and is found in green and purple bacteria, whereas chlorobium chlorophyll occurs in green sulphur bacteria, Chlorobium
• Fluorescence in chlorophylls


  • Absorption of light or photons causes release the energy and immediately drop back to the ground state within 10 seconds
  • This transition of chlorophyll molecules from their ground state to their excited state releases radiation energy, which is called fluorescence
• Carotenoids


  • They are of two types: carotenes (unsaturated hydrocarbons) and xanthophylls (oxygen-derivatives of carotenes)
  • They absorb light between the red and blue regions of spectrum
  • They act as antenna complexes, capturing light from different regions of spectrum and funneling it into chlorophyll a (the reaction centre)
• Functions of carotenoids


  • They absorb light in the violet to blue regions of the spectrum
  • They protect chlorophyll molecule from photo-oxidation
  • They provide colouration to flowers and fruits
  • ẞ carotene produces vitamin A in animals
  • Three xanthophylls, antheroxanthin, zeaxanthin and violoxanthin take part in dissipation of excess energy by converting it to heat
• Phycobilins


  • They are water-soluble pigments with open tetrapyrrolic structure but do not bear magnesium and phytol tail
  • The common Phycobilins are phycocyanin, phycoerythrin and allophycocyanin
  • They are protein-linked pigments and get destroyed by heat
  • They harvest the light energy and transfer it to chlorophyll a to be used in photosynthesis, just like carotenoids
• Absorption spectrum


  A curve obtained by plotting the absorbed amount of light of different wavelengths by a particular pigment
• Action spectrum


  The curve showing the rate of photosynthesis at different wavelengths of light
• The action spectrum of photosynthesis corresponds closely to absorption spectra of chlorophylls a and b showing that the latter are the main photosynthetic pigments. But, it is seen that sufficient photosynthesis occurs in the mid part of the light spectrum where carotenoids are active.
• Red drop: Emerson effect

  Emerson and his co-workers exposed Chlorella to only one wavelength of light at a time and measured the quantum yield (i.e., the number of O2 molecules evolved per light quanta absorbed)
• Photosynthetic units or light harvesting complexes


  • A number of pigment molecules that work together to carry out a photochemical cycle for releasing one molecule of oxygen
  • Each photosynthetic unit has three parts: reaction centre, antenna molecules, and reaction centre chlorophyll
• Reaction centre


  • It is formed of a single molecule of chlorophyll a which absorbs light of longer wavelengths, i.e., 680 nm or 700 nm and is accordingly represented by P680 or P700
  • Each reaction centre is surrounded by about 250-400 light harvesting pigment molecules
• Antenna molecules


  The light harvesting pigment molecules that absorb light and transfer the energy to the reaction centre
• Emerson effect


  Exposing Chlorella to only one wavelength of light at a time and measuring the quantum yield (i.e., the number of O2 molecules evolved per light quanta absorbed)
• Monochromatic light


  Light with one wavelength
• Photosynthetic units or light harvesting complexes


  • They carry out a photochemical cycle for releasing one molecule of oxygen
  • They have three parts: reaction centre, antenna molecules, and core molecules
• Reaction centre


  It is formed of a single molecule of chlorophyll a which absorbs light of longer wavelengths, i.e., 680 nm or 700 nm and is accordingly represented by P680 or P700
• Antenna molecules


  Molecules of chlorophyll a, chlorophyll b, carotene and anthophyll that lie outer to the core molecules and absorb light of different wavelengths but shorter than the wavelengths absorbed by core molecules and reaction centre
• Core molecules


  Chlorophyll a and chlorophyll b molecules that lie around the reaction centre and harvest light energy directly as well as from the antenna molecules and transfer to reaction centre
• Photosystem I (PS I)


  • It has a reaction centre made up of a dimer of chlorophyll a molecules called P700
  • It has a proximal antenna complex of about 90 chlorophyll a and 12-15 carotene molecules, two molecules of vitamin K (phylloquinone), FeS, FeSA, FeS (iron-sulphur proteins), Fd (ferredoxin), cytochrome b-f and plastocyanin
  • It has more chlorophyll a than chlorophyll b and carotenoids
• Operation of photosystem I

  1. Light quanta are absorbed by photosystem I (P700)

  2. Energy rich electrons are emitted from the reaction centre

  3. These flow down along a chain of electron carriers to NADP with the protons generated by splitting of water

  4. This results in the formation of NADPH
• Photosystem II (PS II)


  • It possesses chlorophylls a, b and carotenoids
  • Chlorophyll a and chlorophyll b contents are equal
  • It has a higher carotenoid content compared to PS I
  • It consists of a photocentre, oxygen evolving complex, light harvesting complex (LHC II) and some electron carriers
  • Photocentre has a dimer of special chlorophyll a molecules called P680
• Photolysis of water (Splitting of water by light energy)


  1. When PS II absorbs light, electrons are released and chlorophyll molecule is oxidised

  2. The oxidised P680 regains its electrons by the photolysis of water

  3. Light-induced splitting (photo-oxidation of water) results in the release of H, e- and O2

  4. It is assisted by a water-oxidising enzyme, an unknown electron acceptor Z, manganese (Mn) and chloride (CI) ions
• Light-dependent reaction or Hill reaction or photophosphorylation


  1. Absorption of light by pigments of light harvesting complex (LHC) of PS II (P680) and PS I (P700)

  2. Excitation of reaction centres of PS I and PS II

  3. Photosynthetic electron transport and formation of assimilatory energy - ATP

  4. Photophosphorylation and ATP Synthesis