Environmental Engineering Reference
In-Depth Information
DID YOU KNOW?
Charles Darwin was the first to discuss how plants respond to light. He found
that new shoots of grasses bend toward the light because the cells on the dark
side grow faster than those on the lighted side.
are attached to the membranes of disc-like structures, called
chloroplasts
, inside the
cells. Chloroplasts are the site of photosynthesis, the process in which light energy
is converted to chemical energy. In chloroplasts, the light absorbed by chlorophyll
supplies the energy used by plants to transform carbon dioxide and water into oxygen
and carbohydrates. Chlorophyll is not a very stable compound; bright sunlight causes
it to decompose. To maintain the amount of chlorophyll in their leaves, plants con-
tinuously synthesize it. The synthesis of chlorophyll in plants requires sunlight and
warm temperatures; therefore, during summer chlorophyll is continuously broken
down and regenerated in the leaves of trees.
Photosynthesis
Because our quality of life, and indeed our very existence, depends on photosynthe-
sis, it is essential to understand it. In photosynthesis, plants (and other photosynthetic
autotrophs) use the energy from sunlight to create the carbohydrates necessary for
cell respiration. More specifically, plants take water and carbon dioxide and trans-
form them into glucose and oxygen:
6CO
2
+ 6H
2
O + Light energy → C
6
H
12
O
6
+ 6O
2
This general equation of photosynthesis represents the combined effects of two dif-
ferent stages. The first stage is called the
light reaction
and the second stage is called
the
dark reaction
. The light reaction is the photosynthesis process in which solar
energy is harvested and ultimately converted into NADPH and adenosine triphos-
phate (ATP); the light reaction can only occur in light. In the dark reaction, the
NADPH and ATP formed by the light reaction reduce carbon dioxide and convert it
into carbohydrates; the dark reaction can occur in the dark as long as ATP is present.
r
oots
Roots absorb nutrients and water, anchor the plant in the soil, provide support for
the stem, and store food. They are usually below ground and lack nodes, shoots,
and leaves. There are two major types of root systems in plants.
Taproot systems
have a stout main root with a limited number of side-branching roots. Examples
of taproot system plants are nut trees, carrots, radishes, parsnips, and dandelions.
Taproots make transplanting difficult. The second type of root system,
fibrous
, has
many branched roots. Examples of fibrous root plants are most grasses, marigolds,
and beans. Radiating from the roots is a system of root hairs, which vastly increase
the absorptive surface area of the roots. Roots also anchor the plant in the soil.
