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Plastids in Plant Cells: Types, Structure & Functions

 

Plastids in Plant Cells: Types, Structure & Functions

Introduction

Plastids are essential organelles found in plant cells and some algae. Coined by A.F.W. Schimper, the term "plastid" refers to various cell structures responsible for photosynthesis, pigment storage, and food storage. Understanding plastids—such as chloroplasts, chromoplasts, and leucoplasts—is fundamental in biology, especially for students preparing for exams in the UK and USA.

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What Are Plastids? | Definition & Importance

  • Plastids are large organelles found in all plant cells.

  • They are easily visible under a microscope.

  • Plastids contain pigments and contribute to essential plant processes like photosynthesis, color production, and storage of food.

Types of Plastids:

  1. Chloroplasts – for photosynthesis.

  2. Chromoplasts – store pigments responsible for red, orange, and yellow colors.

  3. Leucoplasts – colorless plastids for storage of starch, proteins, and fats.

Chloroplast: Structure, Function, and Importance in Photosynthesis

What Is a Chloroplast?

Chloroplasts are double-membrane, self-replicating organelles found in green plants. They contain chlorophyll pigments, DNA, and RNA, and they act as the plant's "photosynthetic kitchen."

Functions:

  • Main site for photosynthesis.

  • Synthesizes food using light energy.

  • Contains enzymes for carbohydrate and protein synthesis.

Structure:

  • Lens-shaped or oval.

  • Surrounded by a double membrane.

  • Contains thylakoids arranged in stacks called grana, suspended in a fluid called stroma.

  • Encloses a space called the lumen.

  • Ribosomes of 70S type present.

Chromoplast: The Pigment-Storing Plastids

Chromoplasts are responsible for the vibrant colors seen in fruits, flowers, and autumn leaves.

Key Features:

  • Found in light-exposed parts of the plant.

  • Contain carotenoids like carotene, xanthophyll, etc.

  • Store fat-soluble pigments giving red, orange, and yellow hues.

Leucoplast: The Storage Plastids in Plant Cells

Leucoplasts are colorless plastids that store nutrients in plant cells.

Types of Leucoplasts:

  • Amyloplasts – store starch.

  • Elaioplasts – store oils and fats.

  • Aleuroplasts – store proteins.

Additional Organelles: Peroxisomes and Glyoxysomes

Peroxisomes:

  • Contain enzymes like catalase.

  • Detoxify harmful substances like hydrogen peroxide.

Glyoxysomes:

  • Specialized peroxisomes found in fat-storing seeds (soybean, sunflower, peanut).

  • Convert fats into sugars during germination.

Spherosomes:

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