Mitochondria and chloroplasts: the energy factories

The mitochondrion, present in almost everyone

The mitochondrion is a small rod-shaped organelle, present in almost all eukaryotic cells, both animal and plant. Its role is to produce energy usable by the cell, through cellular respiration: it consumes oxygen and nutrients (such as glucose) to produce energy, and releases carbon dioxide and water.

The more energy a cell needs, the more mitochondria it contains: a highly active muscle cell can contain several thousand, while a less active cell contains far fewer.

The chloroplast, exclusive to plants

The chloroplast is a green organelle, present only in plant cells exposed to light (leaves, green stems). It carries out photosynthesis: from carbon dioxide, water and light, it manufactures organic matter (glucose) and releases oxygen.

Its green color comes from chlorophyll, a pigment that captures light energy.

Two complementary functions, not opposing ones

• Photosynthesis (chloroplasts) produces organic matter by storing energy.
• Cellular respiration (mitochondria) releases this stored energy so the cell can use it.

Beware of the misconception that plants do not breathe: this is false. Plant cells have both chloroplasts AND mitochondria. They carry out photosynthesis during the day, in light, but breathe continuously, day and night, like all living cells.

Simplified diagram

(photosynthesis, in the chloroplast):
CO2 + water + light --> organic matter + O2

(cellular respiration, in the mitochondrion):
organic matter + O2 --> usable energy + CO2 + water