Author:
Tina B. Jones
Subject:
Applied Science, Life Science, Biology
Material Type:
Module
Level:
Community College / Lower Division
Provider:
Rice University
Provider Set:
OpenStax College
Tags:
  • Calvin Cycle
  • Calvin-Benson Cycle
  • Carbon Fixation
  • Dark Reaction
  • Energy Cycle
  • Fixation
  • Organic Molecule
  • Photosynthesis
  • Prokaryote
  • Reduction
  • Regeneration
  • RuBP
  • RubisCO
  • Tina B. Jones
    License:
    Creative Commons Attribution Non-Commercial Share Alike
    Language:
    English

    Using Light Energy to Make Organic Molecules

    Using Light Energy to Make Organic Molecules

    Overview

    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

    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 and packaged into ATP and NADPH, the cell has the fuel needed to build food in the form of carbohydrate molecules. The carbohydrate molecules made will have a backbone of carbon atoms. Where does the carbon come from? The carbon atoms used to build carbohydrate molecules comes from carbon dioxide, the gas that animals exhale with each breath. The Calvin cycle is the term used for the reactions of photosynthesis that use the energy stored by the light-dependent reactions to form glucose and other carbohydrate molecules.

    The Calvin Cycle

    The Interworkings of the Calvin Cycle

    In plants, carbon dioxide (CO2) enters the chloroplast through the stomata and diffuses into the stroma of the chloroplast—the site of the Calvin cycle reactions where sugar is synthesized. The reactions are named after the scientist who discovered them, and reference the fact that the reactions function as a cycle. Others call it the Calvin-Benson cycle to include the name of another scientist involved in its discovery (Figure 5.14).

    This illustration shows that ATP and NADPH produced in the light reactions are used in the Calvin cycle to make sugar.

    Light-dependent reactions harness energy from the sun to produce ATP and NADPH. These energy-carrying molecules travel into the stroma where the Calvin cycle reactions take place.

     

    The Calvin cycle reactions (Figure 5.15) can be organized into three basic stages: fixation, reduction, and regeneration. In the stroma, in addition to CO2, two other chemicals are present to initiate the Calvin cycle: an enzyme abbreviated RuBisCO, and the molecule ribulose bisphosphate (RuBP). RuBP has five atoms of carbon and a phosphate group on each end.

    RuBisCO catalyzes a reaction between CO2 and RuBP, which forms a six-carbon compound that is immediately converted into two three-carbon compounds. This process is called carbon fixation, because CO2 is “fixed” from its inorganic form into organic molecules.

    ATP and NADPH use their stored energy to convert the three-carbon compound, 3-PGA, into another three-carbon compound called G3P. This type of reaction is called a reduction reaction, because it involves the gain of electrons. A reduction is the gain of an electron by an atom or molecule. The molecules of ADP and NAD+, resulting from the reduction reaction, return to the light-dependent reactions to be re-energized.

    One of the G3P molecules leaves the Calvin cycle to contribute to the formation of the carbohydrate molecule, which is commonly glucose (C6H12O6). Because the carbohydrate molecule has six carbon atoms, it takes six turns of the Calvin cycle to make one carbohydrate molecule (one for each carbon dioxide molecule fixed). The remaining G3P molecules regenerate RuBP, which enables the system to prepare for the carbon-fixation step. ATP is also used in the regeneration of RuBP.

    Art Connection

    This illustration shows a circular cycle with three stages. Three molecules of carbon dioxide enter the cycle. In the first stage, the enzyme RuBisCO incorporates the carbon dioxide into an organic molecule. Six ATP molecules are converted into six ADP molecules. In the second stage, the organic molecule is reduced. Six NADPH molecules are converted into six NADP+ ions and one hydrogen ion. Sugar is produced. In stage three, RuBP is regenerated, and three ATP molecules are converted into three ADP molecules. RuBP then starts the cycle again.The Calvin cycle has three stages. In stage 1, the enzyme RuBisCO incorporates carbon dioxide into an organic molecule. In stage 2, the organic molecule is reduced. In stage 3, RuBP, the molecule that starts the cycle, is regenerated so that the cycle can continue.

     

    Link to Learning

    QR Code representing a URL

    This link leads to an animation of the Calvin cycle. Click stage 1, stage 2, and then stage 3 to see G3P and ATP regenerate to form RuBP.

     

    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 CO2, 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 CO2 allows plants to adapt to living with less water. Some plants such as cacti (Figure) 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 short, round prickly cacti 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: Piotr Wojtkowski)

    Photosynthesis in Prokaryotes

    Photosynthesis in Prokaryotes The two parts of photosynthesis—the light-dependent reactions and the Calvin cycle—have been described, as they take place in chloroplasts. However, prokaryotes, such as cyanobacteria, lack membrane-bound organelles. Prokaryotic photosynthetic autotrophic organisms have infoldings of the plasma membrane for chlorophyll attachment and photosynthesis (Figure 5.17). It is here that organisms like cyanobacteria can carry out photosynthesis.  This illustration shows a green ribbon, representing a folded membrane, with many folds stacked on top of another like a rope or hose. The photo shows an electron micrograph of a cleaved thylakoid membrane with similar folds from a unicellular organism

     

    This illustration shows a green ribbon, representing a folded membrane, with many folds stacked on top of another like a rope or hose. The photo shows an electron micrograph of a cleaved thylakoid membrane with similar folds from a unicellular organism

    A photosynthetic prokaryote has infolded regions of the plasma membrane that function like thylakoids. Although these are not contained in an organelle, such as a chloroplast, all of the necessary components are present to carry out photosynthesis. (credit: scale-bar data from Matt Russell)

    The Energy Cycle

    The Energy Cycle

    Living things access energy by breaking down carbohydrate molecules. However, if plants make carbohydrate molecules, why would they need to break them down? Carbohydrates are storage molecules for energy in all living things. Although energy can be stored in molecules like ATP, carbohydrates are much more stable and efficient reservoirs for chemical energy. Photosynthetic organisms also carry out the reactions of respiration to harvest the energy that they have stored in carbohydrates, for example, plants have mitochondria in addition to chloroplasts.

    You may have noticed that the overall reaction for photosynthesis:

    6CO2   +   6H2O    →    C6H12O6    +    6O2  

    is the reverse of the overall reaction for cellular respiration:

    C6H12O6    +    6O2    →    6CO2    +    6H2O   

     

    Photosynthesis produces oxygen as a byproduct, and respiration produces carbon dioxide as a byproduct.

    In nature, there is no such thing as waste. Every single atom of matter is conserved, recycling indefinitely. Substances change form or move from one type of molecule to another, but never disappear (Figure 5.18).

    CO2 is no more a form of waste produced by respiration than oxygen is a waste product of photosynthesis. Both are byproducts of reactions that move on to other reactions. Photosynthesis absorbs energy to build carbohydrates in chloroplasts, and aerobic cellular respiration releases energy by using oxygen to break down carbohydrates. Both organelles use electron transport chains to generate the energy necessary to drive other reactions. Photosynthesis and cellular respiration function in a biological cycle, allowing organisms to access life-sustaining energy that originates millions of miles away in a star.

    This photograph shows a giraffe eating leaves from a tree. Labels indicate that the giraffe consumes oxygen and releases carbon dioxide, whereas the tree consumes carbon dioxide and releases oxygen.

    Photosynthesis consumes carbon dioxide and produces oxygen. Aerobic respiration consumes oxygen and produces carbon dioxide. These two processes play an important role in the carbon cycle. (credit: modification of work by Stuart Bassil)

    Section Summary

    Using the energy carriers formed in the first steps of photosynthesis, the light-independent reactions, or the Calvin cycle, take in CO2 from the environment. An enzyme, RuBisCO, catalyzes a reaction with CO2 and another molecule, RuBP. After three cycles, a three-carbon molecule of G3P leaves the cycle to become part of a carbohydrate molecule. The remaining G3P molecules stay in the cycle to be regenerated into RuBP, which is then ready to react with more CO2. Photosynthesis forms an energy cycle with the process of cellular respiration. Plants need both photosynthesis and respiration for their ability to function in both the light and dark, and to be able to interconvert essential metabolites. Therefore, plants contain both chloroplasts and mitochondria.

    Art Connections

    Figure Which of the following statements is true?

    A.  In photosynthesis, oxygen, carbon dioxide, ATP, and NADPH are reactants. G3P and water are      products.

    B.  In photosynthesis, chlorophyll, water, and carbon dioxide are reactants. G3P and oxygen are products.

    C.  In photosynthesis, water, carbon dioxide, ATP, and NADPH are reactants. RuBP and oxygen are      products.

    D.  In photosynthesis, water and carbon dioxide are reactants. G3P and oxygen are products.

    Hint:

    Figure D

    Review Questions

    Which molecule must enter the Calvin cycle continually for the light-independent reactions to take place?

    A.  RuBisCO

    B.  RuBP

    C.  3-PGA

    D.  CO2

    Hint:

    D

    Where in plant cells does the Calvin cycle take place?

    A.  thylakoid membrane

    B.  thylakoid space

    C.  stroma

    D.  granum

    Hint:

    C

    Which statement correctly describes carbon fixation?

    A. the production of carbohydrate molecules from G3P

    B.  the use of RuBisCO to form 3-PGA

    C.  the conversion of CO2 into an organic compound

    D.  the formation of RuBP from G3P molecules

    E.  the use of ATP and NADPH to reduce CO2

    Hint:

    C

    The overall reaction for photosynthesis is the reverse of the overall reaction for cellular respiration.

    A.  TRUE

    B.  FALSE

    Hint:

    A

    Free Response

    Why is the third stage of the Calvin cycle called the regeneration stage?

    Hint:

    Because RuBP, the molecule needed at the start of the cycle, is regenerated from G3P.

    Which part of the light-independent reactions would be affected if a cell could not produce the enzyme RuBisCO?

    Hint:

    None of the cycle could take place, because RuBisCO is essential in fixing carbon dioxide. Specifically, RuBisCO catalyzes the reaction between carbon dioxide and RuBP at the start of the cycle.

    Why does it take three turns of the Calvin cycle to produce G3P, the initial product of photosynthesis?

    Hint:

    Because G3P has three carbon atoms, and each turn of the cycle takes in one carbon atom in the form of carbon dioxide.