Why do plants have chlorophyll and chloroplasts?

Nature Study Tutorials > Why do plants have chloroplasts?

What are chloroplasts?

All green plants and algae harness energy from the sun using a process called photosynthesis, which takes place inside chloroplasts. Chloroplasts are like miniature cells inside plant cells, which in turn contain a special green pigment called chlorophyll. It is chlorophyll, which absorbs the sun’s energy for the plant.

It is thought that chloroplasts were once tiny bacteria that lived over a million years ago in the nutrient soup that formed Earth’s primeval seas. These bacteria were able to survive by harnessing sunlight through their chlorophyll, which they used to make sugar.
Over time, these bacteria became incorporated inside plant cells, allowing the plants to also harness the energy of the sun by making use of the chlorophyll pigment inside the chloroplasts.

What do plants need chloroplasts?

Nearly all life on Earth depends on the sun’s energy. Without chloroplasts, plants would not be able to survive and without plants, animals could not survive.

Chloroplasts are like little solar converters. They contain a green pigment called chlorophyll, which absorbs  sunlight and converts it into sugars, which the plant can use for energy – in other words, it is because of the chloroplasts that plants can live, breathe and grow.

Because, not only do plants store and use the sugars that are produced by the chloroplasts, but they can also convert those sugars into all the other nutrients they need, such as proteins, polysaccharides, nucleic acids and lipids.

How does chlorophyll work?

The process by which chloroplasts absorb sunlight and convert it into sugars, is called photosynthesis. And while sunlight is obviously important for photosynthesis, it is not the only ingredient that is needed. Because, in order for photosynthesis to occur, plants need not just sunlight, but carbon dioxide and water too.

Carbon dioxide diffuses into the plant through little holes (or pores) on the leaf, called stomata, before travelling through the leaf and into the chloroplasts. Water meanwhile, is absorbed through the roots of the plants and travels up through little channels called xylem, which go all the way up through the stem and into the leaves to the chloroplasts.
Once inside the chloroplast, a set of reactions occurs, one after the other, which uses sunlight absorbed by the chlorophyll, to convert the carbon dioxide and water into oxygen and sugar. The oxygen then diffuses back out of the leaf into the atmosphere, while the sugar is either stored, used or converted into something else needed by the plant.

How much energy can plants make through photosynthesis?

Despite photosynthesis being so important to life on Earth, the process itself is not very efficient, since only one to three percent of the sunlight which reaches a leaf will actually be absorbed. How much of this sunlight is then converted into energy then depends on the surrounding temperature and the amount of water available to the plant.

In hot, dry places for example, plants often close their stomata during the day, to stop water evaporating out of the plant, but this means they cannot absorb carbon dioxide either, which limits the amount of phytosynthesis that can take place – and so the amount of energy that can be produced, even though there is so much sunlight.

In addition, during the night and in autumn and winter, when sunlight is absent or of low intensity, even though there is plenty of water and the stomata are wide open to absorb carbon dioxide, significantly less sunlight is absorbed compared to normal, so energy production slows down or stops. (Although a different form of energy production can occur at night).


In summary, plants need chloroplasts (which were once ancient bacteria), because they contain a pigment called chlorophyll (which absorbs sunlight), which is used to convert carbon dioxide and water into sugars (through a process called phytosynthesis), which the plant then uses to live, breathe and grow.

And remember, since almost all life on Earth is dependent on plants (or algae – which also contain chloroplasts), then essentially, we all owe chloroplasts a deep debt of gratitude, since without them, we would not be here…

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