Lesson #5: Plastids and Vacuoles
Plastids
Plastids are double-membrane-bound organelles found in the cells of plants and photosynthetic protists (such as algae). These organelles play an essential role in photosynthesis, storage, and pigment production.
Types of Plastids
Plastids are classified into three main types based on their color and function:
- Chloroplasts – Green plastids that perform photosynthesis.
- Chromoplasts – Colored plastids that give bright colors to flowers and fruits.
- Leucoplasts – Colorless plastids that store food.
1. Chloroplasts
Chloroplasts are green plastids found in the cells of green parts of plants (such as leaves and stems) and in algae.
Functions of Chloroplasts
- Photosynthesis: Chloroplasts contain photosynthetic pigments (light-absorbing molecules) such as chlorophyll (a green pigment).
- They capture light energy from the sun and convert it into chemical energy in the form of glucose (a type of sugar).
- This energy is later used by the plant for growth, repair, and reproduction.
Structure of Chloroplasts
Chloroplasts have a double membrane similar to mitochondria. Inside, they contain the following structures:
- Grana (singular: Granum) – Stacks of flattened membrane sacs that look like a pile of coins. These sacs are called thylakoids.
- Thylakoids – Disc-shaped structures where photosynthesis takes place. Photosynthetic pigments, including chlorophyll, are embedded in the membrane of thylakoids.
- Stroma – A fluid-filled space that surrounds the grana. It contains enzymes, DNA, and ribosomes, allowing chloroplasts to make some of their own proteins.
🔹 Example:
- Leaf cells contain many chloroplasts to maximize photosynthesis.
- Algae like Chlamydomonas also have chloroplasts to produce their own food.
2. Chromoplasts
Chromoplasts are plastids that contain colored pigments other than chlorophyll, mainly carotenoids (orange and yellow pigments).
Functions of Chromoplasts
- They give bright colors (red, orange, yellow) to flowers, fruits, and some roots.
- The bright colors attract pollinators (like bees and butterflies) and help in the dispersal of seeds and fruits by animals.
🔹 Example:
- Carrots contain carotene (an orange pigment) in chromoplasts.
- Tomatoes turn red because of the pigment lycopene in chromoplasts.
3. Leucoplasts
Leucoplasts are colorless plastids that do not contain pigments.
Functions of Leucoplasts
- They are involved in storing food materials such as:
- Starch (e.g., in potatoes).
- Lipids (fats) (e.g., in seeds).
- Proteins (e.g., in legumes like beans and peas).
🔹 Example:
- Leucoplasts in potato tubers store starch.
- Leucoplasts in seeds store oil (e.g., sunflower seeds).
Vacuoles
Vacuoles are membrane-bound sacs filled with fluid that store water, nutrients, and waste materials.
💡 Why are vacuoles called the “waste bins” of the cell?
Because they store and break down waste materials, just like a trash bin collects and holds garbage.
Structure of Vacuoles
- Vacuoles are surrounded by a single membrane.
- They are filled with a watery fluid that contains salts, sugars, and waste materials.
- The structure and function of vacuoles vary between animal and plant cells.
1. Vacuoles in Animal Cells
- Animal cells have small and temporary vacuoles.
- These vacuoles store water, nutrients, and waste materials.
🔹 Special Types of Vacuoles in Animals:
- Contractile Vacuoles – Found in some freshwater organisms like Amoeba and Paramecium. These vacuoles help remove excess water from the cell to prevent bursting.
- Food Vacuoles – Found in organisms that engulf food particles. The food vacuole stores food until it is digested.
🔹 Example:
- In Amoeba, a contractile vacuole pumps out extra water to maintain balance.
- In sponges, vacuoles help in waste removal.
2. Vacuoles in Plant Cells
- Plant cells have a large, permanent central vacuole.
- The vacuole is formed by the fusion of many small vacuoles.
- The membrane of the vacuole is called the tonoplast.
- The fluid inside the vacuole is called cell sap, which contains water, salts, and organic molecules.
Functions of the Central Vacuole in Plants
- Storage – Stores water, salts, sugars, proteins, and waste products.
- Maintains Cell Shape – The vacuole pushes the cytoplasm toward the cell membrane, keeping the plant cell firm.
- Turgor Pressure – The pressure exerted by the vacuole on the cell wall, helping the plant stay upright.
- Waste Storage – Stores harmful metabolic byproducts to prevent damage to the cell.
🔹 Example:
- In cacti, vacuoles store water to help the plant survive in dry conditions.
- In onions, vacuoles store pigments that give them their color.
Summary Table
| Organelle | Type | Function | Example |
| Plastids | Chloroplasts | Photosynthesis | Found in leaf cells |
| Chromoplasts | Give colors to fruits & flowers | Carrots, tomatoes | |
| Leucoplasts | Store food (starch, lipids, proteins) | Potatoes, seeds | |
| Vacuoles | Animal Vacuole | Temporary storage of food & water | Amoeba (contractile vacuole) |
| Plant Vacuole | Stores nutrients & maintains cell shape | Large vacuole in plant cells |
Key Takeaways
✔ Plastids are double-membrane organelles found in plants and algae.
✔ Chloroplasts perform photosynthesis and contain chlorophyll.
✔ Chromoplasts provide bright colors to flowers and fruits.
✔ Leucoplasts are colorless and store food materials like starch and fats.
✔ Vacuoles are fluid-filled sacs that help in storage and waste disposal.
✔ Plant vacuoles are large and permanent, whereas animal vacuoles are small and temporary.