Agriculture Reference
In-Depth Information
plant's manufacturing biomass (see the discussion box: Photosynthesis: Nature's
Green Chemistry). When photosynthesis stops, less CO 2 is extracted from the
environment and less carbon is stored in the soil. For example, much of the
biomass of a tree is in its root systems (more than half for many species). When
the tree is cut down, not only does the harvested biomass release carbon, such
as in the smoke when the wood is burned, but gradually the underground stores
of carbon in the root systems migrate from the soil to the troposphere (see the
discussion box: The Tree).
Photosynthesis: Nature's Green Chemistry
Organic material generated when plants and animals use stored solar energy is
known as biomass . Photosynthesis is the process by which green plants * absorb
the sun's energy, convert it to chemical energy, and store the energy in the
bonds of sugar molecules. The process of photosynthesis takes place in the
chloroplasts, which are organelles ( chloro
=
green; plasti
=
formed, molded),
using the green pigment chlorophyll ( chloro
=
green; phyll
=
leaf ), which has
a porphyrin ring with magnesium in the center.
The simplest sugars are monosaccharides, which have the molecular formula
(CH 2 O) n , where n may be any integer from 3 to 8. Monosaccharides contain
hydroxyl groups and either a ketone or an aldehyde group (see the discussion
of organic chemistry earlier in the Chapter). These functional groups are
polar, rendering sugars very soluble in water. Fructose has the same molecular
formula as glucose, but the atoms of carbon, hydrogen, and oxygen are arranged
a little differently (i.e., they are isomers). Glucose has an aldehyde group;
fructose has a ketone group. This structural nuance imparts different physical
and chemical properties to the two monosaccharides.
These monosaccharides link by a dehydration synthesis reaction to form
disaccharides, forming one water molecule in the process. Maltose is formed
by joining two glucose molecules. Sucrose is formed by combining glucose
and fructose. Lactose is formed by combining glucose and the monosaccha-
ride galactose. Maltose, sucrose, and lactose have the same molecular formula,
C 12 H 22 O 11 , but are each isomers with unique physical and chemical properties.
The energy in these sugars' chemical bonds moves through the food web,
being passed on to animals that consume the plants. Although numerous
chemical reactions occur in photosynthesis, the process can be seen a very
simple reaction with water and carbon dioxide reacting in the presence of
radiant energy to form sugars (e.g., glucose) and molecular oxygen:
6H 2 O
+
6CO 2 (
+
radiant energy)
C 6 H 12 O 6
+
6O 2
* In fact, photosynthesis occurs in two taxonomical kingdoms: Plantae and Protista (Protoctista).
Algae fall in the latter kingdom.
Search WWH ::




Custom Search