In a way, chloroplasts complement mitochondria because their processes achieve the very opposite goals. Whereas mitochondria consume O2 and C6H12O6 to produce ATP (in the process of cellular respiration) with a byproduct of CO2 and H2O, chloroplasts do the opposite. Photosynthesis uses CO2 and H2O to create C6H12O6, and O2 is the byproduct. Even when we take a closer look at the electric transport chains integrated in the interior membranes of these organelles, we see that the whole process is similar yet inverted.

Although I do emphasize the contrast between animal and plant cells, beware not to be led astray by naive misconceptions, such as the notion that mitochondria are absent in plant cells. Plants are inanimate, admittedly not mobile like animals, but they do have mitochondria because ATP is needed for all energy-requiring cell processes. Plants cells have some mitochondria, but animal cells do not have any chloroplasts.


A chloroplast has an outer membrane, an inner membrane, and a thylakoid membrane. The stacks of thylakoids are termed ‘grana’ (singular: ‘granum’), and the fluid between grana is called the stroma. Within the thylakoid is empty space called the lumen, and molecules flow between the lumen and stroma during the light-dependent reactions of photosynthesis.

Side view; the darker green areas are grana



Photosynthesis can be divided into two parts, the light-dependent reactions and the light-independent reactions.

Light-dependent reactions:

12 H2O + 12 NADP+ + 18 ADP + 18 Pi + Light → 6 O2 + 12 NADPH + 18 ATP

Light-independent reactions:

6 CO2 + 18 ATP + 12 NADPH2 → C6H12O6 + 18 ADP + 18 Pi + 12 NADP + 6 H2O


6 C02 + 6 H2O  C6H12O6 + 6 O2
Carbon Dioxide + Water + Light → Glucose + Oxygen


Chloroplasts contain the pigment chlorophyll, which are found embedded in the thylakoid membrane. Chlorophyll absorb sunlight and transfer this energy to the reaction centers of photosystems. Because green light is not absorbed very well in the visible light spectrum, the pigment appears green to our eyes.


Here are some videos provided by the NDSU Virtual Cell Animation Project:

Light Reactions
Photosystem II

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