Photosynthesis uses light energy and carbon dioxide to make
triose phosphates (G3P). G3P is generally considered the first end-product of photosynthesis.[
citation needed] It can be used as a source of metabolic energy, or combined and rearranged to form
monosaccharide or
disaccharide sugars, such as
glucose or
sucrose, respectively, which can be transported to other cells, stored as insoluble
polysaccharides such as
starch, or converted to structural carbohydrates, such as
cellulose or
glucans.
A commonly used slightly simplified
equation for photosynthesis is:
6 CO2(g) + 12 H2O(l) +
photons →
C6H12O6(aq) + 6 O2(g) + 6 H2O(l)
carbon dioxide + water + light energy → glucose + oxygen + water
The equation is often presented in introductory chemistry texts in an even more simplified form as:
[2]6 CO2(g) + 6 H2O(l) +
photons → C6H12O6(aq) + 6 O2(g)
Photosynthesis occurs in two stages. In the first stage, light-dependent reactions or photosynthetic reactions (also called the Light Reactions) capture the energy of light and use it to make high-energy molecules. During the second stage, the light-independent reactions (also called the
Calvin-Benson Cycle, and formerly known as the Dark Reactions) use the high-energy molecules to capture and
chemically reduce carbon dioxide (CO2) (also called
carbon fixation) to make the
precursors of
carbohydrates.
In the
light reactions, one molecule of the
pigment chlorophyll absorbs one
photon and loses one
electron. This electron is passed to a modified form of chlorophyll called
pheophytin, which passes the electron to a
quinone molecule, allowing the start of a flow of electrons down an
electron transport chain that leads to the ultimate reduction of
NADP to
NADPH. In addition, this creates a
proton gradient across the
chloroplast membrane; its dissipation is used by
ATP Synthase for the concomitant synthesis of
ATP. The chlorophyll molecule regains the lost electron from a
water molecule through a process called
photolysis, which releases a
dioxygen (O2) molecule.
In the
Light-independent or dark reactions the
enzyme RuBisCO captures
CO2 from the
atmosphere and in a process that requires the newly formed NADPH, called the Calvin-Benson Cycle, releases three-carbon sugars, which are later combined to form sucrose and starch.
Photosynthesis may simply be defined as the conversion of light energy into
chemical energy by living
organisms. It is affected by its surroundings, and the rate of photosynthesis is affected by the concentration of carbon dioxide in the air, the light intensity, and the
temperature.
Photosynthesis uses only 1% of the entire electromagnetic spectrum, and 2% of the visible spectrum.[
citation needed] It has been estimated that the productivity of photosythesis is 115 petagrams (Pg, equals 1015 grams or 109 metric tons).[
citation needed]
In plants
Most plants are
photoautotrophs, which means that they are able to
synthesize food directly from
inorganic compounds using light energy - for example from the sun, instead of eating other organisms or relying on nutrients derived from them. This is distinct from
chemoautotrophs that do not depend on light energy, but use energy from inorganic compounds.
6 CO2 + 12 H2O → C6H12O6 + 6 O2 + 6 H2O
The energy for photosynthesis ultimately comes from absorbed
photons and involves a
reducing agent, which is
water in the case of plants, releasing
oxygen as
product. The light energy is converted to chemical energy (known as
light-dependent reactions), in the form of
ATP and
NADPH, which are used for synthetic reactions in photoautotrophs. The overall equation for the light-dependent reactions under the conditions of non-cyclic electron flow in green plants is:
[3]2 H2O + 2 NADP+ + 2 ADP + 2 Pi + light → 2 NADPH + 2 H+ + 2 ATP + O2
Most notably, plants use the chemical energy to fix
carbon dioxide into
carbohydrates and other organic compounds through
light-independent reactions. The overall equation for carbon fixation (sometimes referred to as carbon reduction) in green plants is:
[3]3 CO2 + 9 ATP + 6 NADPH + 6 H+ → C3H6O3-phosphate + 9 ADP + 8 Pi + 6 NADP+ + 3 H2O
To be more specific, carbon fixation produces an intermediate product, which is then converted to the final carbohydrate products. The carbon skeletons produced by photosynthesis are then variously used to form other organic compounds, such as the building material
cellulose, as precursors for
lipid and
amino acid biosynthesis, or as a fuel in
cellular respiration. The latter occurs not only in plants but also in
animals when the energy from plants gets passed through a
food chain. Organisms dependent on photosynthetic and
chemosynthetic organisms are called
heterotrophs. In general outline, cellular respiration is the opposite of photosynthesis: Glucose and other compounds are oxidized to produce carbon dioxide, water, and chemical energy. However, the two processes take place through a different sequence of chemical reactions and in different cellular compartments.
Plants absorb light primarily using the
pigment chlorophyll, which is the reason that most plants have a green color. The function of chlorophyll is often supported by other
accessory pigments such as
carotenes and
xanthophylls. Both chlorophyll and accessory pigments are contained in
organelles (compartments within the
cell) called
chloroplasts. Although all cells in the green parts of a plant have chloroplasts, most of the energy is captured in the
leaves. The cells in the interior tissues of a leaf, called the
mesophyll, can contain between 450,000 and 800,000 chloroplasts for every square millimeter of leaf. The surface of the leaf is uniformly coated with a water-resistant
waxy cuticle that protects the leaf from excessive
evaporation of water and decreases the absorption of
ultraviolet or
blue light to reduce
heating. The transparent
epidermis layer allows light to pass through to the
palisade mesophyll cells where most of the photosynthesis takes place.
Plants convert light into
chemical energy with a maximum
photosynthetic efficiency of approximately 6%.
[4][5][6] By comparison
solar panels convert light into
electric energy at a photosynthetic efficiency of approximately 10-20%. Actual plant's photosynthetic efficiency varies with the frequency of the light being converted, light intensity, temperature and proportion of
CO2 in atmosphere.
