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:

• Involved in lipid storage and synthesis.
  
  

• Found only in plant cells.
  
  

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Explain What Is Pollination In Brief - Class 12th Bio Notes

   What is Pollination? Types, Mechanisms &                             Importance in Plants

Pollination is one of the most essential processes in the life cycle of flowering plants. It refers to the transfer of pollen grains from the anther (male part) to the stigma (female part) of a flower, enabling fertilization and seed formation. This process ensures the continuation of plant species and promotes genetic diversity when it occurs between different plants.


🌼 Types of Pollination in Plants


Pollination in plants mainly happens in two ways:

• Self-Pollination

• Cross-Pollination

1. Self-Pollination

In self-pollination, pollen grains are transferred from the anther to the stigma of the same flower or another flower on the same plant. This type of pollination maintains genetic purity but reduces diversity. Self-pollination occurs in two main forms:

A) Autogamy

Autogamy happens when the pollen grains from the anther are transferred to the stigma of the same flower. It occurs through three methods:

• Cleistogamy: In some plants, the flowers do not open, ensuring complete self-pollination.

Examples: Oxalis, Viola.

• Homogamy: The anther and stigma of the bisexual flower mature at the same time.

Examples: Mirabilis, Potato, Sunflower.

• Bud Pollination: Anthers and stigma mature before the flower bud opens.

Examples: Wheat, Rice, Pea.

B) Geitonogamy

In geitonogamy, pollen grains are transferred from one flower to another flower on the same plant, but genetically it is still considered self-pollination.

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Advantages of Self-Pollination

• Ensures higher chances of pollination.

• Maintains purity of the species and avoids unwanted mixing of traits.

• Plants do not need to produce a large number of pollen grains.

 Disadvantages of Self-Pollination

• No new desirable traits are introduced.

• Harmful characters cannot be easily removed.

• Does not support evolution and genetic improvement.

2. Cross-Pollination

Cross-pollination refers to the transfer of pollen grains from the flower of one plant to the stigma of another plant of the same species but genetically different individuals. It is also known as Xenogamy or Allogamy.

If you’d like to learn more about the structure of pollen grains, click here → Structure of Pollen Grains.

Natural Mechanisms That Promote Cross-Pollination:

a) Dichogamy

When the male and female parts mature at different times, preventing self-pollination.

Examples: Coriander, Jasmine, Sunflower.

b) Unisexuality

Male and female flowers occur on either the same plant (monoecious) or separate plants (dioecious).

c) Heterostyly

Flowers have different lengths of stamens and styles, preventing pollen from reaching the stigma of the same flower.

Examples: Primula, Linum.

d) Herkogamy

A physical barrier prevents self-pollination even when anther and stigma mature at the same time.

Examples: Calotropis, Orchids.

e) Self-Incompatibility

In some cases, pollen grains from the same flower fail to germinate on the stigma, preventing fertilization. This condition is called self-sterility.

🌸 Pollen-Pistil Interaction: The Journey of Pollen Grains

For pollination to result in fertilization, the pistil recognizes compatible pollen. Here’s how the process occurs:

1. Landing: Pollen grains land on a compatible stigma.

2. Germination: Pollen grains germinate, forming a pollen tube.

3. Pollen Tube Growth: The tube travels through the style toward the ovary.

4. Fertilization: Male gametes are released into the ovary, where fertilization takes place.


🌼 Artificial Cross-Pollination Techniques

Two important methods are used in plant breeding:

• Emasculation: Removal of stamens from a flower to prevent self-pollination.

• Bagging: Covering the flower with a bag to avoid unwanted pollination from other sources.

 Conclusion

Pollination is a crucial step in the life cycle of flowering plants. While self-pollination maintains genetic stability, cross-pollination brings genetic variation essential for evolution and adaptation. Understanding how plants control these processes helps in crop improvement and biodiversity conservation.

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