Man's physical universe

xanabras

UNIT X

SECTION 1

PHOTOSYNTHESIS STORES ENERGY FOR LIVING

ORGANISMS

Introduction.

It has been calculated that the gasoline burned up in automobiles

in one year produces 17,500,000,000 gallons of water, which would be

enough to fill a canal 6 feet deep and 25 feet wide extending across the

continent from New York to San Francisco. At the same time,

sufficient carbon dioxide is produced to make 160,000,000 tons of dryice.

Some people fear that if we continue burning our fuels at this rate

the time will soon come when there will be no fuels. They ask, "Is

there no way in which this reaction could be reversed?" The answer

is, "Yes, such a process is taking place all the time, and the sun furnishes

the energy." This process is photosynthesis, and it is this

process that is believed to have indirectly produced all the fuel that

we are now using.

To photosynthesis we are indebted for the cotton and linen for our

clothing, our foods, the lumber for our homes, as well as for our fuels.

Truly, photosynthesis might be called

the most important chemical

reaction known to man, although combustion is of nearly equal importance

for it enables man to release this stored energy.

Photosynthesis Is an Endothermic Reaction.

Many reactions continue by themselves once they have been started

by activation with light. Thus a mixture of hydrogen and chlorine

gases will explode when any portion of it is exposed to sunlight.

Photosynthesis (i.e.,

the synthesis of organic compounds in the presence

of light) is an endothermic process and therefore requires continuous

'

irradiation. Photosynthesis cannot take place in the absence of light,

and its rate increases with increasing exposure to light; hence, plants

grow more rapidly in greenhouses where artificial lighting supplements

daylight. The rate of photosynthesis depends upon the wave length

* An endothermic process is one which uses up heat or energy.

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