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RuBisCO catalyzes a reaction between CO 2 and RuBP. For each CO 2 molecule that reacts with one RuBP, two molecules of phosphoglycerate (PGA) form. PGA has three carbons and one phosphate. Each turn of the cycle involves only one RuBP and one carbon dioxide and forms two molecules of PGA. The number of carbon atoms remains the same, as the atoms move to form new bonds during the reactions (3 atoms from 3CO 2 + 15 atoms from 3RuBP = 18 atoms in 3 atoms of PGA). This process is called carbon fixation , because CO 2 is “fixed” from an inorganic form into organic molecules.

Stage 2: reduction

ATP and NADPH are used to convert the six molecules of PGA into six molecules of a chemical called phosphoglyceraldehyde (PGAL). That is a reduction reaction because it involves the gain of electrons (from NADPH) by PGA. Recall that a reduction is the gain of an electron by an atom or molecule. Six molecules of both ATP and NADPH are used; making glucose is obviously an energy-intensive activity. For ATP, energy is released with the loss of the terminal phosphate atom, converting it into ADP; for NADPH, both energy and a hydrogen atom are lost, converting it into NADP + . Both of these molecules return to the nearby site of the light-dependent reactions to be reused and re-energized.

Stage 3: regeneration

Interestingly, at this point, only one of the PGAL molecules leaves the Calvin cycle and is sent to the cytoplasm to contribute to the formation of other compounds needed by the plant. Because the PGAL exported from the chloroplast has three carbon atoms, it takes three “turns” of the Calvin cycle to fix enough net carbon to export one PGAL. But each turn makes two PGAL, thus three turns make six PGAL. One is exported while the remaining five PGAL molecules remain in the cycle and are used to regenerate RuBP, which enables the system to prepare for more CO 2 to be fixed. Three more molecules of ATP are used in these regeneration reactions.

Overview of calvin cycle

During the Calvin cycle, energy and electrons harvested in the light-dependent reactions are used to produce carbohydrates i.e. glucose. There are three stages in the Calvin cycle. The first stage is carbon fixation, CO 2 from the atmosphere is attached to an organic molecule RuBP during this stage. The second stage is the carbon reduction, the energy and electrons in ATP and NADPH are used to produce carbohydrates (glucose). The last stage is the regeneration stage, energy from ATP is used to regenerate the first substrate of the cycle (RUBP). See ( [link] ) below for a review of the Calvin Cycle.

An overview of Calvin Cycle.
This image represents the Calvin Cycle. (Image by Eva Horne and Robert Bear)

Evolution connection

Photosynthesis

During the evolution of photosynthesis, a major shift occurred from the bacterial type of photosynthesis that involves only one photosystem and is typically anoxygenic (does not generate oxygen) into modern oxygenic (does generate oxygen) photosynthesis, employing two photosystems. This modern oxygenic photosynthesis is used by many organisms—from giant tropical leaves in the rainforest to tiny cyanobacterial cells—and the process and components of this photosynthesis remain largely the same. Photosystems absorb light and use electron transport chains to convert energy into the chemical energy of ATP and NADH. The subsequent light-independent reactions then assemble carbohydrate molecules with this energy.

Photosynthesis in desert plants has evolved adaptations that conserve water. In the harsh dry heat, every drop of water must be used to survive. Because stomata must open to allow for the uptake of CO 2 , water escapes from the leaf during active photosynthesis. Desert plants have evolved processes to conserve water and deal with harsh conditions. A more efficient use of CO 2 allows plants to adapt to living with less water. Some plants such as cacti ( [link] ) can prepare materials for photosynthesis during the night by a temporary carbon fixation/storage process, because opening the stomata at this time conserves water due to cooler temperatures. In addition, cacti have evolved the ability to carry out low levels of photosynthesis without opening stomata at all, an extreme mechanism to face extremely dry periods.

This photo shows a blooming cactus growing in cracks in a rock.
The harsh conditions of the desert have led plants like these cacti to evolve variations of the light-independent reactions of photosynthesis. These variations increase the efficiency of water usage, helping to conserve water and energy. (credit: David A. Rintoul)

Overview of photosynthesis

Photosynthesis converts light energy to chemical energy in two stages, the light-dependent reactions and the Calvin cycle. By exploring these two sets of reactions, we learned how photons of light energy are turned into food by photosynthesis. The light-dependent reactions harvest the light energy to make ATP and to transfer electrons from H 2 O to NADP + forming NADPH and Oxygen gas. The energy and electrons in ATP and NADPH are used in the Calvin cycle to produce glucose from carbon dioxide. The sunlight energy entering the chloroplasts becomes stored as the chemical bonds in the organic molecules. See ( [link] ) below for a review of photosynthesis.

A summary of photosynthesis.
The process of photosynthesis is divided to two stages that are linked by the energy and electron carriers ATP and NADPH. The light-dependent reactions split the water produced by aerobic cellular respiration and releases oxygen as a byproduct, and these reactions also convert light energy to chemical energy (ATP and NADPH). The Calvin cycle uses the energy in ATP and NADPH and produces carbohydrates by fixing CO 2 a byproduct of aerobic cellular respiration. (Image by Eva Horne and Robert Bear)

What is the fate of the carbohydrates produced by photosynthesis? About 50% of the carbohydrates are used by the plant for aerobic cellular respiration in their mitochondria. The other 50% of the carbohydrates are the building blocks for the biological macromolecules the make up plant cells that you learned about in module 3. As you may realize, these biological macromolecules are the food we eat, and you are what you eat. Taking this a bit further, all the activities you do from reading this text to sleeping require energy and that energy comes from the Sun.

Questions & Answers

Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
research.net
kanaga
sciencedirect big data base
Ernesto
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
Maciej
characteristics of micro business
Abigail
for teaching engĺish at school how nano technology help us
Anassong
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
NANO
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
NANO
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
Sanket Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Principles of biology. OpenStax CNX. Aug 09, 2016 Download for free at http://legacy.cnx.org/content/col11569/1.25
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