Page 354 - Instant notes

Physical Chemistry 340

(O 3) layer in the stratosphere, which is the region of the Earth’s atmosphere extending
between about 15 and 50 km above the surface.
Molecular oxygen, O 2, in the stratosphere absorbs solar ultraviolet photons of
wavelength<240 nm and dissociates into O atoms which rapidly combine with
surrounding undissociated O 2 to form O 3:
O 2+hv→O+O
O+O 2→O 3
The ozone concentration is maintained at a steady state in the stratosphere by the
photodissociation of O 3 itself back to an O atom and the reformation of diatomic O 2 by
reaction of O with O 3:
O 3+hv→O 2+O
O+O 3→O 2+O 2

It is the absorption of solar near-ultraviolet radiation (280–380 nm) through the
photodissociation of O 3 in the stratosphere that prevents these wavelengths from
penetrating to the Earth’s surface where they can cause biological damage.

Photosynthesis

The most important biological example of an electronic excitation induced by absorption
of a photon is the transition that initiates the process of photosynthesis in plants. In broad
terms, photosynthesis uses the energy of solar photons to drive a series of reactions which
produce sugars and carbohydrates from carbon dioxide and water. The photons are
absorbed by chlorophyll molecules (Fig. 1) located in the membranes of the chloroplast
structures found in the cells of all green vegetation. A chlorophyll molecule consists of a
metal atom surrounded by a conjugated ring system (a porphyrin) and has an absorption
transition in the visible part of the electromagnetic spectrum.