Dictionary of Chemistry [6th Ed.]

2H++ 2e–+ NADP+→NADPH + H+.

The ATP and NADPH produced dur-

ing the light-dependent reactions

provide energy and reducing power,

respectively, for the ensuing light-

independent reactions (formerly

called the ‘dark reaction’), which

nevertheless cannot be sustained

without the ATP generated by the

light-dependent reactions. During

these reactions carbon dioxide is re-

duced to carbohydrate in a metabolic

pathway known as the *Calvin cycle.

Photosynthesis can be summarized

by the equation:

CO 2 + 2H 2 O →[CH 2 O] + H 2 O + O 2.

Since virtually all other forms of

life are directly or indirectly depend-

ent on plants for food, photosynthe-

sis is the basis for all life on earth.

Furthermore virtually all the atmos-

pheric oxygen has originated from

oxygen released during photosynthe-

sis.

photosynthetic pigmentsThe

plant pigments responsible for the

capture of light energy during the

light-dependent reactions of *photo-

synthesis. The green pigment

*chlorophyll is the principal light re-

ceptor, absorbing blue and red light.

pH scaleA logarithmic scale for ex-

pressing the acidity or alkalinity of a

solution. To aÜrst approximation,

the pH of a solution can be deÜned as

–log 10 c, where c is the concentration

of hydrogen ions in moles per cubic

decimetre. A neutral solution at 25°C

has a hydrogen-ion concentration of

10 –7mol dm–3, so the pH is 7. A pH

below 7 indicates an acid solution;

one above 7 indicates an alkaline so-

lution. More accurately, the pH de-

pends not on the concentration of

hydrogen ions but on their *activity,

which cannot be measured experi-

mentally. For practical purposes, the

pH scale is deÜned by using a hydro-

gen electrode in the solution of inter-

est as one half of a cell, with a

reference electrode (e.g. a calomel

electrode) as the other half cell. The

pH is then given by (E – ER)F/2.303RT,

where E is the e.m.f. of the cell and

ERthe standard electrode potential of

the reference electrode, and F the

Faraday constant. In practice, a glass

electrode is more convenient than a

hydrogen electrode.

pH stands for ‘potential of hydro-

gen’. The scale was introduced by

Søren Sørensen (1868–1939) in 1909.

phthalic acid A colourless

crystalline dicarboxylic acid,

C 6 H 4 (COOH) 2 ; r.d. 1.6; m.p. 207°C.

The two –COOH groups are substi-

tuted on adjacent carbon atoms of

the ring, the technical name being

benzene-1,2-dicarboxylic acid. The

acid is made from phthalic anhydride

(benzene-1,2-dicarboxylic anhydride,

C 8 H 4 O 3 ), which is made by the cata-

lytic oxidation of naphthalene. The

anhydride is used in making plasti-

cizers and polyester resins.

phthalic anhydrideSee phthalic

acid.

photosynthetic pigments 418

p

O

O

OH

OH

O

O

O

phthalic acid phthalic anhydride

Phthalic acid