Photosynthesis: An Overview


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My AP Biology Thoughts

Unit 3 Cellular Energetics

Welcome to My AP Biology Thoughts podcast, my name is Shriya Karthikvatsan and I am your host for episode # called Unit 3 Cellular Energetics: Overview of Photosynthesis. Today we will be discussing what exactly Photosynthesis is, its importance to life, and the processes that make up it.

Segment 1: Introduction to Photosynthesis

  • We’re going to start off by discussing exactly what photosynthesis is, its importance, and a brief overview of the different stages of it
  • Photosynthesis is when light energy is captured by photosynthetic molecules that are broken down in cellular respiration to obtain energy (ATP)
  • Essentially, glucose molecules (or other sugars) are constructed from water and carbon dioxide, and oxygen is released as a byproduct
  • Also, the carbon in CO2 is used to make carbs and other organic molecules that the organism requires to survive
  • The process of producing the organic molecules is called carbon fixation where the carbon in CO2 is “fixed” and incorporated into sugars to be used to build the organic molecules
  • Photosynthesis is important because photosynthetic organisms such as plants, algae, and bacteria play a key role
  • They introduce chemical energy and help fix carbon by utilizing light energy, meaning they can produce their own food and are therefore called photoautotrophs
  • Since humans and other organism cannot convert carbon dioxide to organic compounds, they rely on the autotrophs for these compounds and to receive energy
  • Also, photosynthesis is important because it produces oxygen as a byproduct which affects the Earth’s atmosphere and removes carbon dioxide from the atmosphere, preventing a gas buildup that would be detrimental to us
  • Photosynthesis simplified can be divided into two stages: the light-dependent reactions and the Calvin Cycle
  • The light-dependent reactions occur in the thylakoid membrane and require light energy
  • The Calvin Cycle, or light-independent reactions, takes place in the stroma and doesn’t require light
  • In the next segment, we are going to go more in depth about these two processes and the specific significant parts of a plant involved in them

Segment 2: More About Photosynthesis

  • Okay to start we are going to go over the important parts of a plant involved in photosynthesis you should know
  • Photosynthesis itself takes place in the chloroplast of a plant which contains chlorophyll which is the pigment that absorbs different wavelengths of light from the energy source
  • The chloroplasts are surrounded by a double membrane and have a third inner membrane called the thylakoid membrane where light-dependent reactions take place
  • The stroma is the fluid and space in the chloroplast
  • Here is a diagram of both parts of photosynthesis
  • The first part of photosynthesis, the light-dependent reactions, are when light energy is converted into chemical energy
  • This energy conversion occurs through the formation of two compounds: ATP (energy storage molecule) and NADPH (electron carrier)
  • Water molecules are also converted into oxygen gas
  • This entire process wouldn’t occur without help from photosystems 1 and 2, and the electron transport system which produces NADPH for the Calvin cycle
  • PS 2 absorbs light energy to excite electrons to produce ATP
  • The electrons come from splitting hydrogen atoms in H2O
  • The protons activate ATP Synthase, an enzyme, and form a gradient
  • PS 1 excites electrons again to reduce NADP+ to NADPH
  • The next part of photosynthesis, is the Calvin cycle, where carbon is fixed using NADPH and ATP produced by the light-dependent reactions
  • It takes place in the stroma and doesn’t require light
  • It begins with carbon fixation which attaches to RuBP, an organic substance, which is catalyzed by the enzyme rubisco
  • It produces an unstable 6-Carbon molecule that splits into 2 3-PGA
  • Then, the Calvin cycle continues on with reduction where the 3-PGA is reduced, and NADPH is oxudized to form G3P
  • Finally, regeneration occurs in the cycle of RuBP because 5G3P molecules are used for RuBP and 1G3P is used in other processes, hence why it is a cyclic process

Segment 3: Connection to the Course

  • Overall, this unit was about cellular energetics and how all living things require energy to function
  • The energy itself can come from a variety of sources, and it is neither created nor destroyed, it is transformed
  • In photosynthesis, we viewed light energy transformed into chemical energy
  • Photosynthesis is just one of the processes that produces energy for organisms and that ATP is created in both the light-dependent reactions and Calvin Cycle
  • Like photosynthesis, cellular respiration is another source of energy for living organisms
  • They differ in the form of energy absorbed or released
  • However, they both involve a series of redox reactions where molecules are reduced and involve an electron transport chain
  • In cellular respiration, electrons flow from glucose to oxygen, forming water and releasing energy
  • In photosynthesis, they go in the opposite direction, starting in water and winding up in glucose through an energy-requiring process powered by light
  • As you can see, photosynthesis is just one of the many processes that produce energy and organic compounds for living organisms to function, but it is different in the specific molecules it utilizes

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

Music Credits:

  • "Ice Flow" Kevin MacLeod (
  • Licensed under Creative Commons: By Attribution 4.0 License

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