Chloroplast Function in Photosynthesis

Moss Chloroplast

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Photosynthesis occurs in eukaryotic cell structures called chloroplasts. A chloroplast is a type of plant cell organelle known as a plastid. Plastids assist in storing and harvesting needed substances for energy production. A chloroplast contains a green pigment called chlorophyll, which absorbs light energy for photosynthesis. Hence, the name chloroplast indicates that these structures are chlorophyll-containing plastids.

Like mitochondria, chloroplasts have their own DNA, are responsible for energy production, and reproduce independently from the rest of the cell through a division process similar to bacterial binary fission. Chloroplasts are also responsible for producing amino acids and lipid components needed for chloroplast membrane production. Chloroplasts can also be found in other photosynthetic organisms, such as algae and cyanobacteria.

Plant Chloroplasts

Cross-section of a Chloroplast
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Plant chloroplasts are commonly found in guard cells located in plant leaves. Guard cells surround tiny pores called stomata, opening and closing them to allow for gas exchange required for photosynthesis. Chloroplasts and other plastids develop from cells called proplastids. Proplastids are immature, undifferentiated cells that develop into different types of plastids. A proplastid that develops into a chloroplast only does so in the presence of light. Chloroplasts contain several different structures, each having specialized functions.

Chloroplast structures include:

  • Membrane Envelope: contains inner and outer lipid bilayer membranes that act as protective coverings and keep chloroplast structures enclosed. The inner membrane separates the stroma from the intermembrane space and regulates the passage of molecules into and out of the chloroplast.
  • Intermembrane Space: space between the outer membrane and inner membrane.
  • Thylakoid System: internal membrane system consisting of flattened sac-like membrane structures called thylakoids that serve as the sites of conversion of light energy to chemical energy.
  • Thylakoid Lumen: compartment within each thylakoid.
  • Grana (singular granum): densely layered stacks of thylakoid sacs (10 to 20) that serve as the sites of conversion of light energy to chemical energy.
  • Stroma: dense fluid within the chloroplast that lies inside the envelope but outside the thylakoid membrane. This is the site of conversion of carbon dioxide to carbohydrates (sugar).
  • Chlorophyll: a green photosynthetic pigment within the chloroplast grana that absorbs light energy.

Chloroplast Function in Photosynthesis

Plant Chloroplasts

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In photosynthesis, the sun's solar energy is converted to chemical energy. The chemical energy is stored in the form of glucose (sugar). Carbon dioxide, water, and sunlight are used to produce glucose, oxygen, and water. Photosynthesis occurs in two stages. These stages are known as the light reaction stage and the dark reaction stage.

The light reaction stage takes place in the presence of light and occurs within the chloroplast grana. The primary pigment used to convert light energy into chemical energy is chlorophyll a. Other pigments involved in light absorption include chlorophyll b, xanthophyll, and carotene. In the light reaction stage, sunlight is converted to chemical energy in the form of ATP (free energy containing molecule) and NADPH (high energy electron carrying molecule). Protein complexes within the thylakoid membrane, known as photosystem I and photosystem II, mediate the conversion of light energy to chemical energy. Both ATP and NADPH are used in the dark reaction stage to produce sugar.

The dark reaction stage is also known as the carbon fixation stage or the Calvin cycle. Dark reactions occur in the stroma. The stroma contains enzymes which facilitate a series of reactions that use ATP, NADPH, and carbon dioxide to produce sugar. The sugar can be stored in the form of starch, used during respiration, or used in the production of cellulose.

Chloroplast Function Key Points

  • Chloroplasts are chlorophyll-containing organelles found in plants, algae, and cyanobacteria. Photosynthesis occurs in chloroplasts.
  • Chlorophyll is a green photosynthetic pigment within the chloroplast grana that absorbs light energy for photosynthesis.
  • Chloroplasts are found in plant leaves surrounded by guard cells. These cells open and close tiny pores allowing for the gas exchange needed for photosynthesis.
  • Photosynthesis occurs in two stages: the light reaction stage and the dark reaction stage.
  • ATP and NADPH are produced in the light reaction stage which occurs within chloroplast grana.
  • In the dark reaction stage or Calvin cycle, ATP and NADPH produced during the light reaction stage are used to generated sugar. This stage occurs in plant stroma.


Cooper, Geoffrey M. "Chloroplasts and Other Plastids." The Cell: A Molecular Approach, 2nd ed., Sunderland: Sinauer Associates, 2000,

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Bailey, Regina. "Chloroplast Function in Photosynthesis." ThoughtCo, Feb. 18, 2021, Bailey, Regina. (2021, February 18). Chloroplast Function in Photosynthesis. Retrieved from Bailey, Regina. "Chloroplast Function in Photosynthesis." ThoughtCo. (accessed March 20, 2023).