Dictionary of Chemistry [6th Ed.]

sis.) The photolysis of water, using
energy from sunlight absorbed by
chlorophyll, produces gaseous oxy-
gen, electrons, and hydrogen ions
and is a key reaction in *photosyn-
thesis.

photon A particle with zero rest
mass consisting of a quantum of
electromagnetic radiation. The pho-
ton may also be regarded as a unit of
energy equal to hf, where h is the
Planck constant and f is the fre-
quency of the radiation in hertz. Pho-
tons travel at the speed of light. They
are required to explain the photo-
electric effect and other phenomena
that require light to have particle
character.

photosensitive substance 1.Any
substance that when exposed to elec-
tromagnetic radiation produces a
photoconductive, photoelectric, or
photovoltaic effect. 2.Any sub-
stance, such as the emulsion of a
photographicÜlm, in which electro-

magnetic radiation produces a chem-

ical change.

photosynthesisThe chemical

process by which green plants syn-

thesize organic compounds from

carbon dioxide and water in the pres-

ence of sunlight. It occurs in the

chloroplasts (most of which are in

the leaves) and there are two princi-

pal types of reactions. In the light-

dependent reactions, which require

the presence of light, energy from

sunlight is absorbed by *photosyn-

thetic 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 reac-

tion 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 photophos-

phorylation. The electrons and pro-

tons 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

photophos-

phorylation

chlorophyll

light

ATP

ADP

oxygen

Photolysis

of water

water

Noncyclic

photophos-

phorylation

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