<< Chapter < Page Chapter >> Page >

Light-independent reactions

After the energy from the sun is converted into chemical energy and temporarily stored in ATP and NADPH molecules (having lifespans of millionths of a second), photoautotrophs have the fuel needed to build multicarbon carbohydrate molecules, which can survive for hundreds of millions of years, for long-term energy storage. The carbon comes from CO 2 , the gas that is a waste product of cellular respiration.

The Calvin-Benson cycle (named for Melvin Calvin [1911–1997] and Andrew Benson [1917–2015]), the biochemical pathway used for fixation of CO 2 , is located within the cytoplasm of photosynthetic bacteria and in the stroma of eukaryotic chloroplasts. The light-independent reaction s of the Calvin cycle can be organized into three basic stages: fixation, reduction, and regeneration (see Appendix C for a detailed illustration of the Calvin cycle).

  • Fixation : The enzyme ribulose bisphosphate carboxylase (RuBisCO) catalyzes the addition of a CO 2 to ribulose bisphosphate (RuBP) . This results in the production of 3-phosphoglycerate ( 3-PGA ).
  • Reduction : Six molecules of both ATP and NADPH (from the light-dependent reactions) are used to convert 3-PGA into glyceraldehyde 3-phosphate (G3P). Some G3P is then used to build glucose.
  • Regeneration : The remaining G3P not used to synthesize glucose is used to regenerate RuBP, enabling the system to continue CO 2 fixation. Three more molecules of ATP are used in these regeneration reactions.

The Calvin cycle is used extensively by plants and photoautotrophic bacteria, and the enzyme RuBisCO is said to be the most plentiful enzyme on earth, composing 30%–50% of the total soluble protein in plant chloroplasts. A. Dhingra et al. “Enhanced Translation of a Chloroplast-Expressed Rbc S Gene Restores Small Subunit Levels and Photosynthesis in Nuclear Rbc S Antisense Plants.” Proceedings of the National Academy of Sciences of the United States of America 101 no. 16 (2004):6315–6320. However, besides its prevalent use in photoautotrophs, the Calvin cycle is also used by many nonphotosynthetic chemoautotrophs to fix CO 2 . Additionally, other bacteria and archaea use alternative systems for CO 2 fixation. Although most bacteria using Calvin cycle alternatives are chemoautotrophic, certain green sulfur photoautotrophic bacteria have been also shown to use an alternative CO 2 fixation pathway.

  • Describe the three stages of the Calvin cycle.

Key concepts and summary

  • Heterotrophs depend on the carbohydrates produced by autotrophs, many of which are photosynthetic, converting solar energy into chemical energy.
  • Different photosynthetic organisms use different mixtures of photosynthetic pigments , which increase the range of the wavelengths of light an organism can absorb.
  • Photosystems (PSI and PSII) each contain a light-harvesting complex , composed of multiple proteins and associated pigments that absorb light energy. The light-dependent reactions of photosynthesis convert solar energy into chemical energy, producing ATP and NADPH or NADH to temporarily store this energy.
  • In oxygenic photosynthesis , H 2 O serves as the electron donor to replace the reaction center electron, and oxygen is formed as a byproduct. In anoxygenic photosynthesis , other reduced molecules like H 2 S or thiosulfate may be used as the electron donor; as such, oxygen is not formed as a byproduct.
  • Noncyclic photophosphorylation is used in oxygenic photosynthesis when there is a need for both ATP and NADPH production. If a cell’s needs for ATP outweigh its needs for NADPH, then it may carry out cyclic photophosphorylation instead, producing only ATP.
  • The light-independent reactions of photosynthesis use the ATP and NADPH from the light-dependent reactions to fix CO 2 into organic sugar molecules.

True/false

Photosynthesis always results in the formation of oxygen.

False

Got questions? Get instant answers now!

Fill in the blank

The enzyme responsible for CO 2 fixation during the Calvin cycle is called ________.

ribulose bisphosphate carboxylase (RuBisCO)

Got questions? Get instant answers now!

The types of pigment molecules found in plants, algae, and cyanobacteria are ________ and ________.

chlorophylls and carotenoids

Got questions? Get instant answers now!

Short answer

Why would an organism perform cyclic phosphorylation instead of noncyclic phosphorylation?

Got questions? Get instant answers now!

What is the function of photosynthetic pigments in the light-harvesting complex?

Got questions? Get instant answers now!

Questions & Answers

microbial genetics mcqs
kajal Reply
hii
Happy
what is the importance of learning microbiology in nursing?
Grace Reply
привет
__
it's about child-care?
__
the way of common children disease? hmm 🤔🤔
__
how do you mean child care?
Grace
Was ist die Wichtigkeit beim Lernen von Mikrtobiologie in der Pflege? Es ist notwendig dass die Schwestern allgemein und auch die Kinderschwestern eine Ausbildung in der Mikrobiologie erfahren. Sie sind in ihrem Beruf gefo
__
you need to know the truth about diseases. the microbes are have immunity to some pharmacy.
__
is base pairing rule states that adenine pair with thymine cytosine pair with guannie what will be the complementry strand to acggt
KooL Reply
tgcca
Tanaya
yes
Swetha
introduction to microbiology
Shweta Reply
Study the life of microorganisms present in the environment their survivals etc..
ayesha
Microscopic organisms, commonly known as microorganisms or microbes, are found all around us and even inside our bodies. The category 'Microbes' includes a massive range of organisms including bacteria, fungi, viruses, algae, archaea and protozoa.
Amre
fermented foods likely benefited ancestors to preserve foods and make other foods like milk, cheese, and bread using microbes.
Zinnia Reply
the evidence that I would support the statement regarding ancient people ideas that disease was transmitted by things they could not see is by the discoveries under a microscope. Many microbiologists have discovered certain diseases caused by microbes.
Zinnia
plz give the information about glyoxylate cycle
kamini Reply
what type of information?
Sapiens
I need medical microbiology mcqs books for Mbbs
Kisota
for pakistani mbbs or other?
Sapiens
what is microbial soup?
Osborn Reply
😃😂funny question isn't it?!!! sorry for inconvenience 😊
ayesha
most bacterial pathogens that cause disease in humans are classified in terms if their preferable ph as
Beesan Reply
what is micro
Kabul Reply
extremely small in size
Swetha
almost invisible to the naked eye
Peter
thanks
Sadiq
uwc
Peter
invisible sized objects
Sivasri
invisible sized object or specimens.which is used to see in only the microscope that specimens or objective called the micro
Sivasri
small organism that can be seen by use of Microbes
Manyang
why do you need to study microorganisms
Sala
what is microbiology?
Green Reply
microbiology is a branch of biology .it deals with the study of microorganisms life cycle , uses, disadvantages,and it impact in any other fields .which is used to view in microscope .
Sivasri
Microbiology is the study of microorganisms, those being unicellular, multicellular, or acellular. Microbiology encompasses numerous sub-disciplines including virology, bacteriology, protistology, mycology, immunology and parasitology. 
Raj
In short , it is the study of micro organisms and types , characteristics.
Kaviya
can you explain the structure of rna in detail
Dhanalakshmi Reply
RNA is of many types....like mRNA,tRNA,rRNA,snRNA,guideRNA..etc .
Swetha
but generally RNA is single stranded
Swetha
It contains adenine guanine cytosine , instead of thymine it contains uracil.
Swetha
ribonucleic acid present in RNA.. where as in DNA it is deoxyribonucleic acid...and ribonucleotide is present in RNA
Swetha
RNA is a blue print of DNA. it has the information from DNA....and we can predict the base panirs in DNA if we have the RNA....copy of that DNA
Swetha
can either have positive or negative polarity
Ernestine
Diatoms need..... With the help of which they can construct their beautiful cell wall
Prathmesh Reply
Microbial growth curve shows a.... Curve.
Shambhuraj Reply
diminishing curve
Oluwapamilerin
It represents the decreasing growth of an organism.
Swetha
what is meant by reverse isolation in tuberculosis treatment?
Swetha Reply
why do we study microbiology
HABIBA Reply
Microbiology is the way of gaining knowledge about those organisms which are surround us but we can't see them with our naked eye,such organisms called microorganisms..We can see them only under microscope.
ayesha
from our birth to death microorganisms are with us in the environment as well as on our body which are called Microflora.
ayesha
Microfloras are non pathogenic
ayesha

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play




Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Microbiology' conversation and receive update notifications?

Ask