| slovo | definícia |
Calvin cycle
(gcide) | Photosynthesis \Pho`to*syn"the*sis\, n. (Plant Physiol.)
The process of constructive metabolism in which green plants
utilize the energy of sunlight to manufacture carbohydrates
from carbon dioxide and water in the presence of chlorophyll.
It was formerly called assimilation, but this is now
commonly used as in animal physiology. --
Pho`to*syn*thet"ic, a. -- Pho`to*syn*thet"ic*al*ly, adv.
[Webster 1913 Suppl.]
Note: In green plants water is absorbed by the roots and
carried to the leaves by the xylem, and carbon dioxide
is obtained from air that enters the leaves through the
stomata and diffuses to the cells containing
chlorophyll. The green pigment chlorophyll is uniquely
capable of converting the active energy of light into a
latent form that can be stored (in food) and used when
needed.
The initial process in photosynthesis is the
decomposition of water (H2O) into oxygen, which is
released, and hydrogen; direct light is required for
this process. The hydrogen and the carbon and oxygen of
carbon dioxide (CO2) are then converted into a series
of increasingly complex compounds that result finally
in a stable organic compound, glucose (C6H12O6 ), and
water. This phase of photosynthesis utilizes stored
energy and therefore can proceed in the dark. The
simplified equation used to represent this overall
process is 6CO2+12H2O+energy=C6H12O6+6O2+6H2 O. In
general, the results of this process are the reverse of
those in respiration, in which carbohydrates are
oxidized to release energy, with the production of
carbon dioxide and water.
The intermediary reactions before glucose is formed
involve several enzymes, which react with the coenzyme
ATP (see adenosine triphosphate ) to produce various
molecules. Studies using radioactive carbon have
indicated that among the intermediate products are
three-carbon molecules from which acids and amino
acids, as well as glucose, are derived.
--http://www.infoplease.com/ce6/sci/A0860378.html
Note: The role of chlorophyll
Chlorophyll contains a hydrophyllic head group and a
hydrophobic tail region. A magnesium atom is held in
the center of a cyclic, conjugated double bond
porphyrin ring which is responsible for absorbing red
light. (There also is an absorption band in the blue.
Thus red and blue are absorbed and green passes
through, giving plants a characteristic green color.)
Light is absorbed by antenna chlorophyll molecules,
then transferred to the reaction center chlorophylls.
Some hundreds of antenna chlorophyll molecules transfer
energy to a reaction center, with transfer times of
about 10-10 sec from the edge of the unit to the
center.
The energy from light is used to pump H+ ions from the
stroma into the thylakoid space and to reduce NADP+ to
NADPH. Flow of H+ back into the stroma releases energy
which is used to phosphorylate ADP to ATP. The
chemiosmotic coupling is working here in a similar way
to the mechanism of ATP generation used in
mitochondria.
Carbon Fixation Carbon fixation is catalyzed by
ribulose bisphosphate carboxylase (RuBP carboxylase),
the world's most abundent enzyme.
The
Calvin cycle combines three carbon dioxide molecules into
one molecule of three carbon glyceraldehyde 3-phosphate.
Some plants, particularly many which live in hot, dry
climates, have a mechanism for storing carbon dioxide by
combining it with a three carbon molecule to form a four
carbon molecule. This pathway is known as the C4 or
Hatch-Slack pathway.
--http://fig.cox.miami.edu/Faculty/Tom/bil255/bil255sum98/17_photo.html
[PJC] Phototaxis |
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