PHOTOSYNTHESIS
IN HIGHER PLANTS
Important
Terms:
· Absorption Spectrum:
A curve showing the amount of different wavelengths of light absorbed by a
substance.
· Action Spectrum:
A curve depicting the relative rates of photosynthesis at different wavelengths
of light.
· Accessory Pigments:
The pigments which handover the energy absorbed by them to the chlorophyll.
· CAM Plants:
Refers to the succulent and some other plants which show crassulacean acid
metabolism.
· Chemosynthesis:
The process of carbohydrate synthesis in which the organisms use chemical
reactions to obtain energy from inorganic compounds.
· Fluorescence: A
property of emission of radiations absorbed by a substance.
· Photocentre:
(also known as reaction centre)
Refers to a primary pigment (chlorophyll) molecule where light energy is
converted into chemical or electron energy.
· Photolysis: Light
dependent splitting of water molecule in the chloroplasts.
· Photon:
A tiny particle of light containing energy called quantum (hv).
· Photophosphorylation:
A light driven or energized synthesis of ATP from ADP and inorganic phosphate.
· Photosystem:
Light harvesting systems of pigment molecules present in thylakoids of the chloroplast.
They are photosystem I and photosystem II.
·
Photosynthetically
Active Radiation (PAR): The visible spectrum of light
between 400 nm to 700 nm which can be absorbed by the chlorophyll molecules.
· Photosynthetic Carbon Oxidation
Cycle: Photorespiration or C2 cycle. Light
dependent utilization of O2 and release of CO2 by the
photosynthetic organs of a plant.
· Photosynthetic Carbon Reduction
Cycle: C3 cycle.
· Phosphorescence:
Delayed emission of long wave radiations from an irradiated and activated substance.
· Primary Pigments:
Photosynthetic pigments, which convert light energy into chemical or electron
energy.
· Quantum Yield:
Number of oxygen molecules released per photon or quantum of light. Its value
is 1/8 to 1/10 (evolution of one molecule of O2 requires 8-10
quanta).
ADDITIONAL
INFORMATION
·
Photosynthesis
is an oxidation reduction process
where water is oxidized to O2 and CO2 is reduced to (CH2O)n.
·
The
Redox Potential of H2O/O2 is +
0.8V and that of CO2/(CH2O)n is -0.4V. The
difference is 1.2 V per electron transferred. Redox potential is the tendency
of the system (atom/molecule) to give or take up electrons. The symbol used to
express the standard redox potential at pH 7 is E0.
The compound having low redox potential tend to lose electrons and those having
high redox potential gain electrons.
·
Herbicides such as CMU ( 3
p.chlorophenyl 1,1 dimethyl urea) and DCMC (3,4- Dichlorophenyl 1,1 dimethyl
urea) block electron transport between quinone (Q) to plastoquinone (PQ),
during non-cyclic electron transport.
·
Quantum
Yield: Number of oxygen molecules released per photon or quantum of light is
called quantum yield. Its value is 1/8 to 1/10 (10-12.5%). In other words,
evolution of one molecule of oxygen or consumption/ fixation of one molecule of
CO2 requires 8-10 quanta. The number of light quanta used for
producing one molecule of O2 or fixation of one molecule of CO2
is called quantum requirement.
·
Chemical
Coupling Hypothesis: Synthesis
of ATP is connected to liberation of energy while electrons pass down hill over
the electron transport chain.
·
Chemi-Osmotic
Hypothesis (Mitchell 1961): There is accumulation
of protons which pass through membrane bases ATPase (for coupling factor,
elementary particle) catalyzing synthesis of ATP from ADP and inorganic
phosphate.
·
P-protein:
It
is synthesized in companion cells ( absent in gymnosperms). In sieve tube cells
it has a role as defence against phloem feeding insects and sealing of damaged
sieve tubes by plugging sieve plates.
·
Thylakoid
Reactions: Reactions of photochemical phase as they occur over
the thylakoids.
· Stroma Reactions: Reactions
of biosynthetic phase, as they occur in stroma.
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