Plant Ecology of the Sonoran Desert Region - Design with the Desert: Conservation and Sustainable Development - page 152

8.3 Photosynthesis
Chlorophyll is one of the most consequential chemicals in the biosphere. Nearly all life
on the planet depends on it. Living organisms seem to defy the law of entropy, which
describes a universal tendency toward increasing disorder. By using energy acquired
from outside they prevent themselves—temporarily—from dying and disintegrating into
simple, dissociated molecules (becoming disordered). A small number of species derive
their energy from metabolizing sulfur compounds. All others, including all the organisms
that we encounter in everyday life, depend on solar energy (light) to maintain their orderly
existence. Light, however, is unmanageable; it can't be concentrated and stored for later use.
(Outside of science fiction there is no such thing as a photon battery.) Enter photosynthesis.
Green plants use light energy to combine low-energy molecules (carbon dioxide and water)
into high-energy molecules (carbohydrates), which they accumulate and store as energy
reserves. Chlorophyll—the green pigment in plants—is the only known substance in the
universe that can capture volatile light energy and convert it into a stable form usable for
biological processes (chemical energy).
See it with your eyes: Earth reenergized by the sun's rays every day.—Moody Blues
Almost without exception, living organisms—plants, animals, and the other three
kingdoms—obtain energy for sustaining life from carbohydrates (sugars and starches)
by the metabolic process of respiration. (Respiration is colloquially and medically used
to mean breathing. The mechanical act of breathing, however, is only the first step in the
physiological process of respiration—the intake of oxygen.) Respiration is the chemical
pathway through which carbohydrate is broken down (oxidized) into carbon dioxide and
water, releasing the energy stored in the carbohydrate molecules. This is represented by
the formula: carbohydrate + O 2 → H 2 O + CO 2 + energy. (The multiple arrows indicate many
sequential chemical reactions.) Green plants manufacture carbohydrates by photosynthesis.
Animals acquire their carbohydrates by eating plants or other animals.
Photosynthesis is the opposite of respiration: Carbon dioxide and water are combined
to form larger molecules of carbohydrate, with the addition of energy from sunlight:
H 2 O + CO 2 + energy → carbohydrate + O 2 . Water is absorbed through the roots, and CO 2
diffuses into the leaves through the stomates (valved pores in leaf and stem surfaces). The
plant joins several carbon dioxide molecules and adds hydrogen atoms split from water
molecules to form molecules of sugar (simple carbohydrate). Surplus oxygen atoms from
the water molecules are released through the stomates as oxygen gas (O 2 ).
When you see the word “carbohydrate,” think “stored energy” and “calories.” Plants store
energy for long-term use in the form of starch, which is a complex carbohydrate consisting
of long chains of sugar molecules. When a plant needs energy to grow new leaves or flowers,
it does exactly what animals do—it respires carbohydrate to release the stored energy. The
complex respiratory pathway of scores of individual chemical reactions is nearly identical in
all life forms: bacteria, mushrooms, higher plants, up to the highest life forms such as toads.
In contrast to plants, animals use fat as their main energy store; it has twice the calories
per gram as carbohydrate and protein. When animals in need of energy run low on the
small amount of carbohydrate stored in the liver or circulating in the blood, they convert
fat (or protein if they run out of fat) into carbohydrate and then respire it.
The most common form of photosynthesis creates a three-carbon sugar as its first stable
product, so it's called C3 photosynthesis. Other sugars with more carbon atoms are later