Photosynthesis Overview for Microbiology Exams

Hi, I'm Kathy with Level Up RN! In this article, I'll provide a high-level review of photosynthesis to help you with your microbiology class and exams.

At the end of this video, I'll quiz you on some key concepts, so be sure to stick around and test your understanding! If you have our Level Up RN Microbiology Flashcards, grab the ones on photosynthesis so you can follow along. And if you prefer digital resources, our flashcards are also available online.


Where Does Photosynthesis Happen?

Photosynthesis takes place inside the chloroplast, which contains stacks of grana. Each granum is made up of thylakoid membranes, where photosynthetic pigments like chlorophyll are arranged into photosystems.

The fluid surrounding the thylakoids is called the stroma. The two main stages of photosynthesis occur in different parts of the chloroplast:

  • Light-dependent reactions occur in the thylakoid membrane.
  • Light-independent reactions (Calvin cycle) occur in the stroma.

Light-Dependent Reactions

During the light-dependent reactions, pigments in the thylakoid membrane absorb energy from sunlight, exciting high-energy electrons in the reaction center of the photosystem.

These electrons move through the electron transport chain, leading to the production of ATP and NADPH via a process called photophosphorylation.

  • Photophosphorylation is similar to oxidative phosphorylation, except the energy comes from sunlight instead of breaking down biological molecules.
  • The electrons lost from the reaction center need to be replaced. In oxygenic photosynthesis, water donates electrons, and oxygen gas is released as a byproduct.

Oxygenic vs. Anoxygenic Photosynthesis

  • Oxygenic photosynthesis (used by cyanobacteria, algae, and plants) produces oxygen as a byproduct.
  • Anoxygenic photosynthesis (used by green and purple sulfur bacteria) does not produce oxygen because it uses molecules other than water (such as hydrogen sulfide) as an electron donor.

Interestingly, oxygenic photosynthesis is essentially the reverse of aerobic respiration:

  • Photosynthesis: CO₂ + H₂O → Glucose + O₂
  • Aerobic respiration: Glucose + O₂ → CO₂ + H₂O

Light-Independent Reactions (Calvin Cycle)

The Calvin cycle (light-independent reactions) takes place in the stroma and uses ATP and NADPH from the light-dependent reactions to fix carbon dioxide—meaning CO₂ is used to build glucose molecules.

Most microbiology professors don’t expect students to memorize the detailed steps of the Calvin cycle, but you should understand its main purpose: carbon fixation using the energy generated in the first stage of photosynthesis.

Quiz Time!

Let’s see how much you remember!

  1. What are the two stages of photosynthesis?

    • Answer: Light-dependent reactions and the Calvin cycle (light-independent reactions).

  2. During which stage of photosynthesis is chemical energy used to fix carbon dioxide?

    • Answer: The Calvin cycle (light-independent reactions).

  3. In which type of photosynthesis is water the electron donor and oxygen a byproduct?

    • Answer: Oxygenic photosynthesis.

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