(IGP) GS Paper 1 – General Science – “Photosynthesis”

Integrated Guidance Programme of General Studies for IAS
(Pre) – 2013

Subject – General Science
Chapter : Photosynthesis

Photosynthesis Process

Photosynthesis is the process by which plants, some bacteria,
and some protistans use the energy from sunlight to produce sugar, which
cellular respiration converts into ATP, the “fuel” used by all living things.
The conversion of unusable sunlight energy (solar energy) into usable chemical
energy, is associated with the actions of the green pigment chlorophyll. Most of
the time, the photosynthetic process uses water and releases the oxygen.

Stages of Photosynthesis

Photosynthesis is a two stage process. The first process is
the Light Dependent Process (Light Reactions), requires the direct energy of
light to make energy carrier molecules that are used in the second process. The
Light Independent Process (or Dark Reactions) occurs when the products of the
Light Reaction are used to form C-C covalent bonds of carbohydrates. The Dark
Reactions can usually occur in the dark, if the energy carriers from the light
process are present. Recent evidence suggests that a major enzyme of the Dark
Reaction is indirectly stimulated by light, thus the term Dark Reaction is
somewhat of a misnomer. The Light Reactions occur in the grana and the Dark
Reactions take place in the stroma of the chloroplasts.

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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 leaf, 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 xylem.

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

  • 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.

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