Plants form the basis of all life on earth. They are known as producers. Plant cells contain structures known as plastids which are absent in animal cells. There are three types of plastids –chromoplast, chloroplast, and leucoplast. Chromoplast the colored plastids other than the green ones; chloroplasts which are the green colored plastids and the colorless plastids are known as leucoplasts.

Plants, especially the green ones prepare their own food by a process called photosynthesis.They are therefore known as autotrophs. The specialized structure known as chloroplast helps them for this. Inside these, green pigments called chlorophyll are present which are responsible for the food preparation.

Chloroplast Structure

The chloroplast is bounded by a double membrane layer. There are two distinct regions present inside the chloroplasts. One is grana while the other is stroma. Grana is made up of stacks of disc-shaped structures known as thylakoids. These contain the molecule chlorophyll and are the functional units of chloroplasts. Stroma is the matrix which contains grana and is similar to the cytoplasm in cells in which all the organelles are embedded. Stroma also contains various enzymes, DNA, ribosomes, and other substances. Stroma lamellae connect the stacks of thylakoid sacs. There are two types of reactions by which photosynthesis occur, light reaction and dark reaction. Light reaction occurs in grana while dark reaction takes place in the stroma of chloroplasts.


Photosynthesis is the process in which plants obtain energy from the sun and food in the form of sugar is prepared. When sun’s energy reaches the chlorophyll molecules of chloroplasts, light energy is converted into the chemical energy and this energy is found in compounds such as ATP and NADPH. These compounds which are rich in energy, move into the stroma where carbon atoms from carbon dioxide molecules are fixed by enzymes. These molecular reactions create sugar and release oxygen.

Functions of Plastids

  1. Chloroplasts have a structure called chlorophyll which trap solar energy and this is utilized to manufacture food for the plant.
  2. The flowers have different colors because of the pigments called chromoplast. These colors attract insects for pollination.
  3. Leucoplasts are the colorless plastids which store food in the form of carbohydrates, fats, and proteins.

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Practise This Question

Which of the following pairs of cell organelles work together to package macromolecules?