UPSC Biology Photosynthesis in Higher Plants Photosynthesis


Category : UPSC




Structure of leaf

  • Plants are the only photosynthetic organisms to have leaves (and not all plants have leaves). A leaf may be viewed as a solar collector crammed full of photosynthetic cells.




  • The raw materials of photosynthesis, water and carbon dioxide, enter the cells of the Seat, and the products of photosynthesis, sugar and oxygen, leave the leaf.


  • Water enters the root and is transported up to the leaves through specialized plant cells known as xylene.


  • Land plants must guard against drying out (desiccation) and so have evolved specialized structures known as stomata to allow gas to enter and leave the leaf. Carbon dioxide cannot pass through the protective waxy layer covering the leaf (cuticle), but it can enter the leaf through an opening (the stoma; plural = stomata; Greek for hole) flanked by two guard cells.


  • Likewise, oxygen produced during photosynthesis can only pass out of the leaf through the opened stomata.


  • Unfortunately for the plant, while these gases are moving between the inside and outside of the leaf, a great deal water is also lost.


  • Cottonwood trees, for example, will lose 100 gallons of water per hour during hot desert days. Carbon dioxide enters single-celled and aquatic autotrophs through no specialized structures.


Chlorophyll and Accessory Pigments

  • A pigment is any substance that absorbs light-The color of the pigment comes from the wavelengths of light reflected (in other words, those not absorbed).


  • Chlorophyll, the green pigment common to all photosynthetic cells, absorbs all wavelengths of visible light except green, which it reflects to be detected by our eyes.


  • Black pigments absorb all of the wavelengths that strike them.


  • White pigments/lighter colors reflect all or almost all of the energy striking them. Pigments have their own characteristic absorption spectra, the absorption pattern of a given pigment.


  • Chlorophyll is a complex molecule. Several modifications of chlorophyll occur among plants and other photosynthetic organisms. All photosynthetic organisms (plants, certain protistans, prochlorobacteria, and cyanobacteria) have chlorophyll a. Accessory pigments absorb energy that chlorophyll a does not absorb. Accessory pigments include chlorophyll b (also c, d, and e in algae and protistans), xanthophyll’s, and carotenoids (such as beta-carotene). Chlorophyll absorbs its energy from the Violet-Blue and Reddish orange-Red wavelengths, and little from the intermediate (Green-Yellow-Orange) wavelengths.


The Carbon Cycle

  • Plants may be viewed as carbon sinks, removing carbon dioxide from the atmosphere and oceans by fixing it into organic chemicals. Plants also produce some carbon dioxide by their respiration, but this is quickly used by photosynthesis. Plants also convert energy from light into chemical energy of C-C covalent bonds. Animals are carbon dioxide producers that derive their energy from carbohydrates and other chemicals produced by plants by the process of photosynthesis.


  • The balance between the plant carbon dioxide removal and animal carbon dioxide generation is equalized also by the formation of carbonates in the oceans. This removes excess carbon dioxide from the air and water (both of which are in equilibrium with regard to carbon dioxide). Fossil fuels, such as petroleum and coal, as well as more recent fuels such as peat and wood generate carbon dioxide when burned. Fossil fuels are formed ultimately by organic processes, and represent also a tremendous, carbon sink. Human activity has greatly increased the concentration of carbon dioxide in air.

Other Topics

NCERT Summary - Photosynthesis

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