11th Class Biology Photosynthesis in Higher Plants Photosynthesis In Higher Plants

Chloroplast (The site of photosynthesis) : Chloroplast are green plastids which function as the site of photosynthesis in eukaryotic photoautotrops.

Photosynthetic unit can be defined as number of pigment molecules required to affect a photochemical act, that is the release of a molecule of oxygen. Park and Biggins (1964) gave the term quantasome for photosynthetic units is equivalent to 230 chlorophyll molecules.

Chloroplast pigments : Pigments are the organic molecules that absorb light of specific wavelengths in the visible region due to presence of conjugated double bonds in their structures. The chloroplast pigments are fat soluble and are located in the lipid part of the thylakoid membranes. There is a wide range of chloroplastic pigments which constitute more than 5% of the total dry weight of the chloroplast. They are grouped under two main categories :

(1) Chlorophylls : Chlorophyll 'a' is found in all the oxygen evolving photosynthetic plants except photosynthetic bacteria. Reaction centre of photosynthesis is formed of chlorophyll a. It occurs in several spectrally distinct forms which perform distinct roles in photosynthesis (e.g., \[Chl{{a}_{680}}\,\,or\,\,{{P}_{680}},\text{ }Chl{{a}_{700}}\,\,or\,\,{{P}_{700}},\]etc.). It directly takes part in photochemical reaction. Hence, it is termed as primary photosynthetic pigment. Other photosynthetic pigments including chlorophyll b, c, d and e ; carotenoids and phycobilins are called accessory pigments because they do not directly take part in photochemical act. They absorb specific wavelengths of light and transfer energy finally to chlorophyll a through electron spin resonance.

Chlorophyll a is bluish-green while chlorophyll b is olive-green. Both are soluble in organic solvents like alcohol, acetone etc. Chlorophyll is a green pigment because it does not absorb green light (but reflect green light) Chlorophyll \[a\,\,({{C}_{55}}{{H}_{72}}{{O}_{5}}{{N}_{4}}Mg)\] possesses \[C{{H}_{3}}\](methyl group), which is replaced by \[CHO\](an aldehyde) group in chlorophyll \[b\,\,({{C}_{55}}{{H}_{70}}{{O}_{6}}{{N}_{4}}Mg).\] Chlorophyll molecule is made up of a squarish tetrapyrrolic ring known as head and a phytol alcohol called tail. The magnesium atom is present in the central position of tetrapyrrolic ring. The four pyrrole rings of porphyrin head are linked together by methine \[(CH=)\] groups forming a ring system.

When central Mg is replaced by Fe, the chlorophyll becomes a green pigment called 'cytochrome' which is used in photosynthesis (Photophosphorylation) and respiration both.

(2) Carotenoids : They are sometimes called lipochromes due to their fat soluble nature. They are lipids and found in non-green parts of plants. Light is not necessary for their biosynthesis. Carotenoids absorb light energy and transfer it to Chl. a and thus act as accessory pigments. They protect the chlorophyll molecules from photo-oxidation by picking up nascent oxygen and converting it into harmless molecular stage. Carotenoids can be classified into two groups namely carotenes and xanthophyll.

(i) Carotenes : They are orange red in colour and have general formula \[{{C}_{40}}{{H}_{56}}.\]They are isolated from carrot.

They are found in all groups of plants i.e., from algae to angiosperms. Some of the common carotenes are \[\alpha ,\beta ,\gamma \]  and \[\delta \] carotene; phytotene, lycopene, neurosporene etc. The lycopene is a red pigment found in ripe tomato and red pepper fruits. The \[\beta -\]carotene on hydrolysis gives vitamin A, hence the carotenes are also called provitamin A. \[\beta -\]carotene is black yellow pigment of carrot roots.

(ii) Xanthophylls : They are yellow coloured carotenoid also called xanthols or carotenols. They contains oxygen also along with carbon and hydrogen and have general formula \[{{C}_{40}}{{H}_{56}}{{O}_{2}}.\]

Lutein \[({{C}_{40}}{{H}_{56}}{{O}_{2}})\] a widely distributed xanthophyll which is responsible for yellow colour in autumn foliage. Fucoxanthin \[({{C}_{40}}{{H}_{56}}{{O}_{6}})\] is another important xanthophyll present in Phaeophyceae (Brown algae).

(3) Phycobilins : These pigments are mainly found in blue-green algae (Cyanobacteria) and red algae. These pigments have open tetrapyrrolic in structure and do not bear magnesium and phytol chain.

Blue-green algae have more quantity of phycocyanin and red algae have more phycoerythrin. Phycocyanin and phycoerythrin together form phycobilins. These water soluble pigments are thought to be associated with small granules attached with lamellae. Like carotenoids, phycobilins are accessory pigments i.e., they absorb light and transfer it to chlorophyll a.

Nature of light : Sunlight is a type of energy called radiant energy or electromagnetic energy. This energy, according to electromagnetic wave theory (Proposed by James Clark Maxwell, 1960), travels in space as waves. The distance between the crest of two adjacent waves is called a wavelength \[(\lambda ).\] Shorter the wavelength greater the energy.

The unit quantity of light energy in the quantum theory is called quantum (hn), whereas the same of the electromagnetic field is called photon. Solar radiation can be divided on the basis of wavelengths. Radiation of shortest wavelength belongs to cosmic rays whereas that of longest wavelength belong to radio waves. Visible light lies between wavelengths of ultra-violet and infra-red. The visible spectrum of solar radiations are primarily absorbed by carotenoids of the higher plants are violet and blue. However, out of blue and red wavelengths, blue light carry more energy.

\[\underset{\text{Maximum}\,\text{energy}}{\mathop{\text{Shortest}\,\text{wavelength}}}\,\xrightarrow{\,\,\,\,\,\,\,\,}\underset{\text{Minimum}\,\text{energy}}{\mathop{\text{Longest}\,\text{wavelength}}}\,\]

Visible light : \[390nm\,\,(3900{AA})\]to \[760nm\,(7600{AA}).\] Violet \[(390430nm),\] blue \[(430470nm),\] blue-green \[(470500nm),\] green \[(500580nm),\] yellow \[(580600nm),\] orange \[(600650nm),\] orange-red\[(650660nm)\] and red \[(660760nm)\] Far-red \[(700760nm).\] Infra-red \[760nm\text{ }100\,\mu m.\] Ultraviolet \[100390nm.\] Solar Radiations \[300nm\] (ultraviolet) to 2600nm (infra-red). Photosynthetically active radiation (PAR) is \[400700nm.\] Leaves appear green because chlorophylls do not absorb green light. The same is reflected and transmitted through leaves.

Absorption and action spectra : The curve representing the light absorbed at each wavelength by pigment is called absorption spectrum. Curve showing rate of photosynthesis at different wavelengths of light is called action spectrum.

Absorption spectrum is studied with the help of spectrophotometer. The absorption spectrum of chlorophyll a and chlorophyll b indicate that these pigments mainly absorb blue and red lights. Action spectrum shows that maximum photosynthesis takes place in blue and red regions of spectrum. The first action spectrum of photosynthesis was studied by T.W. Engelmann (1882) using green alga Spirogyra and oxygen seeking bacteria.

In this case actual rate of photosynthesis in terms of oxygen evolution or carbon dioxide utilisation is measured as a function of wavelength.