The Calvin Cycle

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My AP Biology Thoughts

Unit 3 Cellular Energetics

Welcome to My AP Biology Thoughts podcast, my name is Saarim Rizavi and I am your host for episode #79 called Unit 3 Cellular Energetics: The Calvin Cycle. Today we will be discussing the second stage of photosynthesis known as the Calvin Cycle (or the light-independent reactions). We will be talking about what the calvin cycle is in the first place and I will give a brief overview of the whole process. Then, I will go into the specific reactions of the calvin cycle and the three main steps of the calvin cycle which include carbon fixation, reduction, and regeneration. Finally, to end it off, I will place and discuss the calvin cycle in the scope of the broader topic of cellular energetics and just the overall importance of the calvin cycle to the environment. Before I begin, I would like to give credit to a couple of websites and resources that were used to create this podcast which include sciencing.com, national geographic, khan academy, biology libretexts, and Ms. Ribecca’s AP Biology Cellular Energetics Videos. So thank you to them for making this podcast possible.

Segment 1: Introduction to The Calvin Cycle

  • Calvin cycle is named Calvin cycle because it was named after Melvin C. Calvin who discovered it and won a Nobel Prize in Chemistry for it
  • What is the Calvin Cycle?
  • A process that plants and algae use to turn carbon dioxide from the air into sugar (glucose)
  • Every living thing on Earth depends on the Calvin cycle either directly or indirectly to survive
  • The calvin cycle takes place in the stroma - the inner space of the chloroplasts
  • Function - to create three carbon sugars which can be used to build other sugars such as glucose, starch, and cellulose that is then used by plants to function and survive
  • Steps of Calvin Cycle (quick overview)
  • Carbon fixation - organic carbon in the form of carbon dioxide in the air is incorporated into organic molecules
  • Inorganic carbon converted to organic compounds by living organisms
  • Reduction - the organic molecules produced in the first stage accept electrons and are reduced
  • Regeneration - the reduced organic molecules use energy from ATP to make RuBP to start the cycle all over again
  • Cycle is powered by ATP and NADPH from light dependent reactions

Segment 2: More About The Calvin Cycle

  • “Preliminary step”
  • In plants, carbon dioxide enters the leaves through the stomata, which is located on the underside of plant leaves
  • The CO2 diffuses through intercellular space until it reaches the mesophyll cells - CO2 then diffuses into the stroma of the chloroplast
  • Stroma - besides the CO2, there’s an enzyme called rubisco and three molecules of ribulose bisphosphate (RuBP) - RuBP = 5 carbon acceptor molecule
  • Carbon Fixation
  • A reaction between carbon dioxide and RUBP occurs and produces a 6 carbon compound that splits into 2 molecules of a three carbon compound known as 3-PGA, which has 3 carbons and one phosphate
  • 3 molecules of carbon dioxide react with 3 molecules of RuBP to produce 6 molecules of the 3 carbon molecules, 3-PGA - reaction catalyzed by rubisco
  • A turn of the calvin cycle involves only 1 RuBP and 1 carbon dioxide molecule forming 2 molecules of 3-PGA - it takes 3 turns of the calvin cycle to produce 6 molecules of 3-PGA
  • Reduction
  • The 3-PGA molecules created through fixation are converted into molecules of simple sugar, known as G3P
  • The 6 molecules of 3 PGA use 6 molecules of ATP and 6 molecules of NADPH, which store the light reactions, to generate 6 molecules of G3P, a 3 carbon sugar
  • Reduction reaction because the 3-PGA molecules gain electrons - the PGA is basically reduced to G3P
  • ATP - energy is released with the loss of the terminal phosphate group converting it to ADP
  • NADPH - both energy and a hydrogen atom are lost, converting it into NADP+; NADPH donates electron to a 3 carbon intermediate to make G3P
  • One turn of the calvin cycle would produce 2 G3P’s so it takes 3 turns to make 6 G3Ps
  • NADP+ and ADP molecules can return to the light-dependent reactions then to be reused and reenergized
  • Regeneration
  • 1 of the 6 G3P molecules leaves the Calvin cycle and is sent to the cytoplasm to contribute to the formation of other compounds, like glucose, needed by the plant
  • The other 5 of the G3P molecules use the energy from 3 molecules of ATP to produce 3 molecules of RuBP
  • Regeneration requires ATP and involves a complex network of reactions
  • Summary of key molecules that enter and exit the calvin cycle
  • In 3 turns of the calvin cycle:
  • 3 CO2 combine with 3 RuBP acceptors to make 6 molecules of G3P (reduction step)
  • 1 G3P exits the cycle and goes towards making glucose, 5 G3P molecules are recycled and regenerate 3 RuBP acceptor molecules
  • 9 ATP molecules are converted to 9 ADP during the fixation and regeneration steps
  • 6 NADPH are converted to 6 NADP+ during the reduction step
  • Glucose production
  • It takes 2 G3Ps to build a 6 carbon glucose molecule - It takes three turns of the carbon cycle to make 1 G3P so to make 2, it takes 6 turns of the cycle to produce 2 G3Ps and 1 molecule of glucose - this would take 6 carbon dioxide molecules, 18 ATP molecules, and 12 NADPH

Segment 3: Connection to the Course

  • Importance of glucose production as a result of the Calvin Cycle
  • Glucose is the plant’s food source and it enables the plant to survive, function, and grow
  • Glucose can be stored in plants - this stored glucose provides the energy to help plants flower
  • Glucose joins with oxygen in the process of cellular respiration in which glucose and oxygen react to produce energy for the plant in the form of ATP
  • The same applies for all other heterotrophs which depend on plants for food and their source of energy.
  • For plants: Glucose molecules form cellulose which builds and adds strength to the cell walls of plant cells.
  • Glucose molecules also form carbohydrates like starch to store energy for cell metabolism for plants
  • When glucose is combined with nitrates, glucose will form amino acids - when these amino acids join together, they form proteins, and many structural parts of the plant are made of protein - proteins which are stored in a plant’s embryo and vegetative cells are able to produce carbon, nitrogen, and sulfur resources for subsequent growth and development - protein can also be produced in the form of enzymes which help in catalyzing the reaction of photosynthesis
  • General importance of Calvin Cycle
  • Without the calvin cycle, organisms wouldn’t have the food, energy , and nutrients they need to survive
  • Photosynthesis and the calvin cycle enable cellular respiration because the products of photosynthesis, glucose and oxygen, are the reactants of cellular respiration
  • Carbon cycle could not occur without the calvin cycle
  • Relation to Cellular Energetics
  • Photosynthesis is an endergonic reaction which takes in energy in the form of sunlight to build organic compounds
  • Photosynthesis is an anabolic reaction since sugars are being formed and joined together during the calvin cycle - also considered nonspontaneous
  • Enzymes are important in the calvin cycle - the cycle couldn’t occur without the enzyme rubisco catalyzing the reaction between CO2 and RuBP
  • The calvin cycle and photosynthesis need cellular respiration to occur since cellular respiration produces CO2 and water which can then be used in photosynthesis - CO2 is used and reduced in the calvin cycle.

Thank you for listening to this episode of My AP Biology Thoughts. For more student-ran podcasts and digital content, make sure that you visit www.hvspn.com. (And always remember, learning new things doesn’t have to be challenging. It’s really easy once you have a goal in mind and a purpose for everything you do)!

Music Credits:

  • "Ice Flow" Kevin MacLeod (incompetech.com)
  • Licensed under Creative Commons: By Attribution 4.0 License
  • http://creativecommons.org/licenses/by/4.0/

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