Light and Dark Reactions in Photosynthesis

Written by tutor Kathie Z.

Photosynthesis is the process by which green plants absorb light energy from the sun with the assistance of water and carbon dioxide, and transform it into chemical energy to make (synthesize) carbohydrate (specifically glucose) and oxygen. Photosynthesis can be summarized with this formula:

6CO2 + 6H2O + sunlight (light energy) → C6H12O6 + 6O2

If you need a longer review, check out our lesson on photosynthesis before reading on.

The “light-independent” or dark reactions happen in the stroma of the chloroplasts. This is also known as the Calvin Cycle. Since these processes can only happen in the chloroplast (a chlorophyll filled plastid in green plants), photosynthesis can only happen in green plants!

The first overall principle of photosynthesis is that the light energy from the sun is transformed into chemical energy and stored in the bonds of glucose (the sugar carbohydrate) for later use by the plant and/or organism that eats the plant.

The second overall principle of photosynthesis is that carbon, oxygen, and hydrogen atoms are taken from carbon dioxide and water molecules and are broken up and rearranged into new substances: carbohydrate (specifically glucose) and oxygen gas (so we can breathe, whew!). This reaction represents the transfer of matter: carbon dioxide from the atmosphere, water from the soil or atmosphere, into sugar in the plant and oxygen back into the atmosphere.

Light-Dependent Reactions

The first part of the process happens in the thylakoids of the chloroplasts and are the “light-dependent” reactions: The photosystems I and II absorb the photons from the sunlight and process them through the membranes of the thylakoids simultaneously. The photons excite electrons in the chlorphyll which then move through the electron transport chain and causes NADP- to combine with H+ forming NADPH. At the same time, ADP (adenosine diphosphate) has come from the dark reaction and a third phosphate chain is bonded forming ATP (adenosine triphosphate) to feed the Calvin Cycle next. Remember that ATP is the important source of all cellular energy.

We now believe that all the oxygen released in photosynthesis comes from the water molecules and all oxygen atoms that form the carbohydrates come from the carbon dioxide molecules. So, in other words during the light-dependent reaction a water molecule is broken down producing two H+ ions and half an oxygen molecule. We get the rest of the oxygen molecule when another water molecule is broken down.

Dark Reactions

Dark reactions are also known as the Calvin Cycle, the Calvin-Benson cycle, and light-independent reactions. The point is that they do not require sunlight to complete their process.

After ATP is formed in the first part of photosynthesis, for living things to grow, reproduce and repair themselves, the inorganic form of CO2 must be transformed into carbohydrate. This happens during the Calvin Cycle in the stroma (the fluid filled interior of the chloroplast). ATP and NADPH combine with CO2 and water to make the end product of glucose. The ADP and NADPH+ are recycled to the light-dependent side to start the process over.

Remember that during hours of darkness, plants cannot perform photosynthesis so they do cellular respiration in the mitochondria just as all living organisms do.


  • chlorophyll: green pigment in plant that can trap photons
  • chloroplast: plastid containing chlorophyll
  • plastid: organelle in the cell
  • rubisco: important enzyme for attaching CO2 to 5-carbon sugar
  • stomate: small pores in the plant leaf
  • stroma: fluid filled interior of the chloroplast
  • thylakoid: small sac in the chloroplast that stack up like coins


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