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By the end of this section, you will be able to:
  • Describe the Calvin cycle
  • Define carbon fixation
  • Explain how photosynthesis works in the energy cycle of all living organisms

After the energy from the sun is converted into chemical energy and temporarily stored in ATP and NADPH molecules, the cell has the fuel needed to build carbohydrate molecules for long-term energy storage. The products of the light-dependent reactions, ATP and NADPH, have lifespans in the range of millionths of seconds, whereas the products of the light-independent reactions (carbohydrates and other forms of reduced carbon) can survive for hundreds of millions of years. The carbohydrate molecules made will have a backbone of carbon atoms. Where does the carbon come from? It comes from carbon dioxide, the gas that is a waste product of respiration in microbes, fungi, plants, and animals.

The calvin cycle

In plants, carbon dioxide (CO 2 ) enters the leaves through stomata, where it diffuses over short distances through intercellular spaces until it reaches the mesophyll cells. Once in the mesophyll cells, CO 2 diffuses into the stroma of the chloroplast—the site of light-independent reactions of photosynthesis. These reactions actually have several names associated with them. Another term, the Calvin cycle    , is named for the man who discovered it, and because these reactions function as a cycle. Others call it the Calvin-Benson cycle to include the name of another scientist involved in its discovery. The most outdated name is dark reactions, because light is not directly required ( [link] ). However, the term dark reaction can be misleading because it implies incorrectly that the reaction only occurs at night or is independent of light, which is why most scientists and instructors no longer use it.

This illustration shows that ATP and NADPH produced in the light reactions are used in the Calvin cycle to make sugar.
Light reactions harness energy from the sun to produce chemical bonds, ATP, and NADPH. These energy-carrying molecules are made in the stroma where carbon fixation takes place.

The light-independent reactions of the Calvin cycle can be organized into three basic stages: fixation, reduction, and regeneration.

Stage 1: fixation

In the stroma, in addition to CO 2 , two other components are present to initiate the light-independent reactions: an enzyme called ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), and three molecules of ribulose bisphosphate (RuBP), as shown in [link] . RuBP has five atoms of carbon, flanked by two phosphates.

Art connection

A diagram of the Calvin cycle is shown with its three stages: carbon fixation, 3-PGA reduction, and regeneration of RuBP. In stage 1, the enzyme RuBisCO adds a carbon dioxide to the five-carbon molecule RuBP, producing two three-carbon 3-PGA molecules. In stage 2, two NADPH and two ATP are used to reduce 3-PGA to GA3P. In stage 3 RuBP is regenerated from GA3P. One ATP is used in the process. Three complete cycles produces one new GA3P, which is shunted out of the cycle and made into glucose (C6H12O6).
The Calvin cycle has three stages. In stage 1, the enzyme RuBisCO incorporates carbon dioxide into an organic molecule, 3-PGA. In stage 2, the organic molecule is reduced using electrons supplied by NADPH. In stage 3, RuBP, the molecule that starts the cycle, is regenerated so that the cycle can continue. Only one carbon dioxide molecule is incorporated at a time, so the cycle must be completed three times to produce a single three-carbon GA3P molecule, and six times to produce a six-carbon glucose molecule.

Which of the following statements is true?

  1. In photosynthesis, oxygen, carbon dioxide, ATP, and NADPH are reactants. GA3P and water are products.
  2. In photosynthesis, chlorophyll, water, and carbon dioxide are reactants. GA3P and oxygen are products.
  3. In photosynthesis, water, carbon dioxide, ATP, and NADPH are reactants. RuBP and oxygen are products.
  4. In photosynthesis, water and carbon dioxide are reactants. GA3P and oxygen are products.

Questions & Answers

hetreothalism in fungi
Lekhram Reply
there are 3 trimester in human pregnancy
ROHIN Reply
I don't know answer of this question can u help me
ROHIN
yes
Bisa
what is a cell
Fatima Reply
A cell is functional and structural unit of life.
Bisa
what is genetic
Janet Reply
I join
Janet
what are the branchas of biology
Prisca Reply
zoology, ecology
Millicent
biochemistry,cytology,herpetology...etc
R0se
genetics, microbiology,botany and embryology
Muhammad
what is a cell
Kulunbawi Reply
cell is smallest unit of life. cells are often cell the building blocks of life...
Muhammad
the first twenty element
Orapinega Reply
what are the characteristics of living things?
R0se
growth,respiration,nutrition,sensitivity, movement,irritability, excretion,death.
Obinna
What is the difference between adaptation and competition in animals
Adeyemi Reply
What is biology
Adeyemi
it is a natural science stadey about living things
Zamiil
Biology is the bronch of science which deals with the study of life is called biology
Aziz
what is the x in 300 stands for?
Ogbudu Reply
the properties of life
Clarinda Reply
response to the environment, reproduction, homeostasis, growth,energy processing etc.....
Pushpam
hello.
Daniela
hi
MacPeter
Good
Thomas
what is reproduction
Tims
Reproduction is a fundamental feature of all known life,each individual organism exist as a result of re production.....or else Multiplying...
R0se
a complete virus particle known as
Darlington Reply
These are formed from identical protein subunitscalled capsomeres.
Pushpam
fabace family plant name
Pushpam Reply
in eukaryotes ...protein channel name which transport protein ...
Pushpam Reply
in bacteria ...chromosomal dna duplicate structure called
Pushpam
what is a prokaryotic cell and a eukaryotic cell
Matilda Reply
There are two types of cells. Eukaryotic and Prokaryotic cells. Prokaryotic cells don't have a nucleus or membrane enclosed organelles (little organs within that cell). They do however carry genetic material but it's not maintained in the nucleus. Prokaryotic cells are also one celled.
juanita
Prokaryotic cells are one celled (single celled).
juanita
Prokaryotic cells are Bacteria and Archea
juanita
Prokaryotic cells are smaller than Eukaryotic cells.
juanita
Eukaryotic cells are more complex. They are much bigger than Prokaryotic cells.
juanita
Eukaryotic cells have a nucleus and membrane bound organelles.
juanita
Eukaryotic cells are animals cells which also includes us.
juanita
Eukaryotic cells are also multicellular.
juanita
nice explaination
Amna
eukaryotic cells are individual cells .. but eukaryotes are multicellular organisms which consist of many different types of eukaryotic cells
Will
also eukaryotic cells have mitochondria. prokaryotic cells do not
Will
Good
John
in prokaryotes only ribosomes are present... in eukaryotes mitochondria ...glogi bodies ..epidermis .....prokaryotes one envelop but eukaryotes compartment envelop....envelop mean membrane bound organelles......
Pushpam
prokaryotic cell are cells dat have no true nuclei i.e no cell membrane while eukaryotic cell are cell dat have true nuclei i.e have cell membrane
Divine
grt
Thomas
we have 46 pair of somatic cell and 23 pair of chromosomes in our body, pls can someone explain it to me. pls
Matilda Reply
we have 22 pairs of somatic chromosomes and one pair of sex chromosome
Amna
thanks
Matilda
we have 23 pairs of chromosomes,22 pairs of somatic and one pair of sex chromosomes
Amna
23 chromosomes from dad & 23 chromosomes from mom 23 +23=46 total chromosomes
juanita
X & Y chromosomes are called sex cells, the very presence of a Y chromosome means the person is Male.
juanita
XX Female XY Male
juanita
If a Karyotype has more than 46 Chromosomes then nondisjunction occured. For example, having an extra chromosome 21 will cause Down Syndrome.
juanita
am caira I want to join
Blessing
caira,whrere are u from
David
I'm a Ghanaian
Osei

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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