Plastid

Plastids are double-membrane bounded cell organelle which is mainly found in the plant cell.

Types of plastid

On the basis of color, there are three types of plastids they are;

• Leucoplasts
• Chromoplasts
• Chloroplasts

Leucoplasts

Leucoplasts are colorless or white plastid without any pigment. They occur in the underground parts of the plant like root, rhizome. They help in the storage of food like starch, protein, fat etc.

Chromoplasts

They have colored plastid other than green. They usually occur in red, yellow and orange color. They contain carotenoids pigment. They occur in flowers and fruits and helps in pollination and dispersal of fruits.

Chloroplasts

These are green colored plastid which helps in photosynthesis. They contain pigments like chlorophyll and carotenoid.

Shape, size, and number of Chloroplasts

The shape of a chloroplast is variable. In higher plants, it occurs in the disc-shaped or lens-shaped form. But in lower plants, there is great variation in shape. It occurs in cup-shaped, ribbon-shaped, star-shaped or in reticulate shape.

The number is also variable. In the leaf cells of higher plants about 20-40 chloroplasts occur in one cell and in lower plants like Spirogyra its number range from 1-16.

Its size may range from 4-10µm.

Structure of chloroplast

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fig:Structure of chloroplast

The structure of chloroplast consists of following important components;

• Envelope
• Stroma or Matrix
• Thylakoids or Granum

Envelope

The chloroplast is bounded by an envelope consisting of two membranes outer and inner. It protects the chloroplast inside.

Stroma

Stroma or Matrix is a semi-fluid ground substance in which thylakoids or granum are subtended. It contains DNA, ribosomes 70s type, RNA, stored starch, lipid and enzymes of dark reaction of photosynthesis(Rubisco).

Thylakoid

Thylakoid is the structural unit of the chloroplast. They are a membranous sac-like structure which often remain group together in a bundle called granum (plural grana). There are many grana inside chloroplast and they remain interconnected through intergranal thylakoid. The thylakoids contain photosynthetic pigment, chlorophylls, carotenoids and help in the light reaction of photosynthesis.

Semi-autonomous nature of Chloroplasts

Due to the presence of DNA, ribosomes and the capacity of energy generation they can synthesis some of the required protein but for other protein, they depend on upon nucleus and ribosomes of the cell organelles. Hence, they are called a semi-autonomous nature of chloroplast.

Functions of chloroplast

• Chloroplast helps in photosynthesis in which green plants synthesize food materials like glucose using CO₂ and H₂O in the presence of sunlight and chlorophyll pigment. During this process, oxygen is liberated as a byproduct.

• Chloroplast helps in energy transduction or light energy is converted into chemical energy.
• By releasing oxygen, chloroplast helps to maintain the concentration of O₂ in the atmosphere.
• Chloroplast absorbs CO₂ from the atmosphere and reduce the concentration of CO₂ in the atmosphere. This prevents greenhouse effect and global warming.
• They help in the synthesis of fatty acid.
• They form chromoplasts.
• They provide greenery to nature.