Vacuoles

Vacuoles – Storage Sacs of the Cell

1. Location and Structure
Vacuoles are membrane-bound sacs found inside cells. They are prominently seen in plant cells, whereas in animal cells, they are fewer in number and smaller in size.

In most plant cells, there are two or three permanent vacuoles, but in some large plant cells, a single large vacuole fills the central region of the cell. This is known as the central vacuole. It can take up to 90% of the total cell volume, making it a very important organelle in plant cells.

---

2. Tonoplast Membrane
The vacuole is surrounded by a semipermeable membrane known as the tonoplast membrane.
This membrane controls the movement of substances in and out of the vacuole and maintains the unique composition of the liquid inside, which is called cell sap.

The composition of the cell sap varies depending on the type of cell. In most cases, the central vacuole's sap contains a high concentration of ions, making it hypertonic (more concentrated) than the cytosol (the fluid outside the vacuole).

---

3. Storage Function
The vacuoles act as storage units for many substances. For example:

• Small vacuoles in seeds store organic materials like proteins.

• Vacuoles also store excretory products and chemical compounds that are toxic or unpleasant to herbivores, which helps protect the plant.

• Some vacuoles store pigments that give bright colours to petals, which help in attracting pollinators.

---

4. Role in Food Intake and Excretion

• When a cell takes in food or foreign particles by a process called phagocytosis, it forms a food vacuole to enclose the ingested material.

• In freshwater unicellular organisms like Paramoecium, a special type of vacuole called the contractile vacuole helps in excretion and osmoregulation (regulating water balance in the cell).
  

  
  
  
  

---

5. Maintaining Turgidity
One of the most important functions of vacuoles in plant cells is to maintain turgidity (firmness of the cell).
By storing water and applying pressure against the cell wall, the vacuole helps the plant cell remain rigid and upright.

---

Conclusion
Vacuoles are multi-functional organelles mainly seen in plant cells. They store nutrients, waste, pigments, and defensive chemicals, while also helping in maintaining cell structure, pressure, and water balance. They play an important role in both plant protection and survival.

---

Other Membrane-Bound Cell Organelles (Microbodies)

In addition to the endomembrane system, there are several other cell organelles that are also bound by a single layer of plasma membrane. These are known as microbodies.

---

Microbodies – Tiny Functional Sacs

1. Presence and Structure
Microbodies are small, membrane-bound sacs found in both plant and animal cells.
Each microbody is surrounded by a single unit membrane, and contains specific enzymes that carry out important biochemical processes in the cell.

Based on the type of enzymes they contain, microbodies are classified into different types. Some important types are explained below:

---

2. Sphaerosomes – Fat Storage Bodies

• Sphaerosomes are microbodies mainly found in plant cells that are involved in synthesis and storage of fats.

• For example, they are seen in the endosperm of oil seeds, where fat is stored for the growing plant embryo.

• The membrane of sphaerosomes is known as a half unit membrane, which means it is made up of only one phospholipid layer, unlike most organelles which have a double layer.

---

3. Peroxisomes – Detox Centres of the Cell

• Peroxisomes are microbodies found in both plant and animal cells.

• They contain enzymes that remove hydrogen atoms from various substrates (chemicals inside the cell). This process uses oxygen and produces hydrogen peroxide (H₂O₂), which is a toxic substance.

• However, peroxisomes also contain another set of enzymes that convert hydrogen peroxide into water, making it harmless.

• Peroxisomes in liver cells play a key role in detoxifying alcohol and other harmful substances in the body.

---

Conclusion
Microbodies like sphaerosomes and peroxisomes may be small, but they play big roles in maintaining the health of plant and animal cells. They help in fat storage, detoxification, and breakdown of harmful substances, making them essential components of the cell.

---