sis.) The photolysis of water, using
energy from sunlight absorbed by
chlorophyll, produces gaseous oxygen, electrons, and hydrogen ions
and is a key reaction in *photosynthesis.
photon A particle with zero rest
mass consisting of a *quantum of
electromagnetic radiation. The photon may also be regarded as a unit of
energy equal to hf, where h is the
*Planck constant and f is the frequency of the radiation in hertz. Photons travel at the speed of light. They
are required to explain the photoelectric effect and other phenomena
that require light to have particle
character.
photosensitive substance 1. Any
substance that when exposed to electromagnetic radiation produces a
photoconductive, photoelectric, or
photovoltaic effect. 2. Any substance, such as the emulsion of a
photographic Ülm, in which electromagnetic radiation produces a chemical change.
photosynthesis The chemical
process by which green plants synthesize organic compounds from
carbon dioxide and water in the presence of sunlight. It occurs in the
chloroplasts (most of which are in
the leaves) and there are two principal types of reactions. In the lightdependent reactions, which require
the presence of light, energy from
sunlight is absorbed by *photosynthetic pigments (chieÛy the green
pigment *chlorophyll) and used to
bring about the photolysis of water:
H 2 O → 2H
+ + 2e
– + ½O 2 .
The electrons released by this reaction pass along a series of electron
carriers (see electron transport
chain); as they do so they lose their
energy, which is used to convert ADP
to ATP in the process of photophosphorylation. The electrons and protons produced by the photolysis of
water are used to reduce NADP:
417
photosynthesis
p
ATP
ADP
electrons
Light-dependent reactions
electron
transport
system
e –
e –
Cyclic
photophosphorylation
chlorophyll
light
ATP
ADP
oxygen
Photolysis
of water
water
Noncyclic
photophosphorylation
2H
2H +
H 2 O
2OH –
H 2 O
2OH
O 2
NADPH+H +
ATP
ADP
3C
ATP
ADP
glucose
1
2
NADP +
2H
ADP
ATP
5C
ribulose
bisphosphate
carbon
dioxide
CO 2
glyceraldehyde
3-phosphate
Calvin
cycle
glycerate
3-phosphate
Light-independent reactions
Photosynthesis
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energy from sunlight absorbed by
chlorophyll, produces gaseous oxygen, electrons, and hydrogen ions
and is a key reaction in *photosynthesis.
photon A particle with zero rest
mass consisting of a *quantum of
electromagnetic radiation. The photon may also be regarded as a unit of
energy equal to hf, where h is the
*Planck constant and f is the frequency of the radiation in hertz. Photons travel at the speed of light. They
are required to explain the photoelectric effect and other phenomena
that require light to have particle
character.
photosensitive substance 1. Any
substance that when exposed to electromagnetic radiation produces a
photoconductive, photoelectric, or
photovoltaic effect. 2. Any substance, such as the emulsion of a
photographic Ülm, in which electromagnetic radiation produces a chemical change.
photosynthesis The chemical
process by which green plants synthesize organic compounds from
carbon dioxide and water in the presence of sunlight. It occurs in the
chloroplasts (most of which are in
the leaves) and there are two principal types of reactions. In the lightdependent reactions, which require
the presence of light, energy from
sunlight is absorbed by *photosynthetic pigments (chieÛy the green
pigment *chlorophyll) and used to
bring about the photolysis of water:
H 2 O → 2H
+ + 2e
– + ½O 2 .
The electrons released by this reaction pass along a series of electron
carriers (see electron transport
chain); as they do so they lose their
energy, which is used to convert ADP
to ATP in the process of photophosphorylation. The electrons and protons produced by the photolysis of
water are used to reduce NADP:
417
photosynthesis
p
ATP
ADP
electrons
Light-dependent reactions
electron
transport
system
e –
e –
Cyclic
photophosphorylation
chlorophyll
light
ATP
ADP
oxygen
Photolysis
of water
water
Noncyclic
photophosphorylation
2H
2H +
H 2 O
2OH –
H 2 O
2OH
O 2
NADPH+H +
ATP
ADP
3C
ATP
ADP
glucose
1
2
NADP +
2H
ADP
ATP
5C
ribulose
bisphosphate
carbon
dioxide
CO 2
glyceraldehyde
3-phosphate
Calvin
cycle
glycerate
3-phosphate
Light-independent reactions
Photosynthesis
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