K101 LAB EXERCISE 7
PHOTOSYNTHESIS: SPINNING SUGAR OUT OF SUNLIGHT
During the process of photosynthesis, light energy from the sun is used to generate ATP
reactions, called the “
”, on the surface of
containing membranes, such as those found in the chloroplast. During this process water is “split” by a
process called “
”, to obtain the
) needed to start photosynthesis,
oxygen gas (O
as a byproduct.
The ATP made during the light reactions is then used,
during a series of enzymatic steps called the
, to convert inorganic carbon dioxide (CO
to an organic form rich in free energy: sugar (C
The first photosynthetic organisms (single-
) appeared on the Earth somewhere around
2 billion years
the present (YBP), and over the course of time, have raised the levels of atmospheric oxygen to today’s
current levels, about 21% of the gas in the atmosphere.
Without photosynthesis, levels of food (in the
form of plant and plankton biomass) and atmospheric oxygen would become depleted to a level that
would be incapable of supporting animal life, either on land or in the oceans.
Photosynthesis can be
summarized by the equation:
O + 6CO
(gas) + 6H
) is one of the primary products of photosynthesis.
However, most plants
convert a fair amount of glucose into the disaccharide
(glu-fru) as a liquid ‘transport’ form of
sugar (like maple syrup sap).
The majority of excess glucose in photosynthetic organisms, though, is
converted into the storage carbohydrate
, consisting of numerous glucose units linked together
with the chemical formula being (C
, where n is the number of subunits.
Starch can be stored
inside chloroplasts, in separate plastids within the cell just for starch storage, called
, or in entire organs of the plant that store large quantities of starch, like the
A plant can mobilize starch quickly if it has used up its cellular supplies of glucose or sucrose, such as
during a cold, rainy spell when very little photosynthesis is taking place.
Plants produce a variety of pigments for a variety of purposes.
is simply a
substance that absorbs light.
Plants make several green
pigments involved in
light strongly in the red and blue regions of the visible spectrum,
reflect or transmit
green wavelengths of light, giving us the green appearance we see when we
look at a plant.
As a result, red and blue light are the most effective wavelengths of light for
Plants ‘boost’ their ability to capture other wavelengths of light by also
having accessory pigments such as
carotenoids and xanthophylls
, which absorb a different range of
wavelengths of light, and reflect orange and yellow wavelengths, respectively.
In today’s lab, we will do four exercises relating to photosynthesis.