Cell Organelles and its Function

CELL ORGANELLES

Organelles are specialized structures inside the cell that perform specific functions—just like organs in the human body. Most organelles are membrane-bound and are suspended in the cytoplasm.

Major organelles include:
Nucleus, Mitochondria, Ribosomes, Endoplasmic Reticulum, Golgi Apparatus, Lysosomes, Peroxisomes, Centrosome, and components of the Cytoskeleton.

1. NUCLEUS

The nucleus was discovered by Robert Brown (1831).

It is the largest and most important organelle in eukaryotic cells and serves as the control center of the cell.

Structure

• Surrounded by a double membrane called the nuclear envelope.
• Contains:
• Nucleolus – site of ribosome formation
• Chromatin – DNA + proteins
• Nuclear pores – allow exchange of materials
• Nucleoplasm – fluid inside the nucleus

Functions of Nucleus

1. Stores genetic material (DNA).
2. Controls protein synthesis.
3. Regulates cell growth and cell division.
4. Transfers hereditary information to the next generation.
5. Essential for cell reproduction—cells without a nucleus cannot divide.

2. MITOCHONDRIA

Discovered by Albert von Kolliker.

Known as the “Powerhouse of the Cell” because they produce ATP (energy).

Structure

• Double membrane organelle.
• Inner membrane folded into cristae.
• Contains its own DNA, ribosomes, and enzymes.
• Found abundantly in energy-demanding tissues like muscle and liver.

Functions

1. Produces ATP through cellular respiration.
2. Helps regulate metabolism.
3. Plays an important role in cell death (apoptosis).
4. Detoxifies ammonia in liver cells.
5. Essential for hormone synthesis (testosterone, estrogen).
6. Helps in formation of certain blood components.

3. RIBOSOMES

Discovered by George E. Palade (1955).

Ribosomes are non-membrane organelles made of RNA and proteins.

Types

• 70S ribosomes – found in prokaryotes
• 80S ribosomes – found in eukaryotes

Functions

• Site of protein synthesis.
• Convert genetic code (mRNA) into amino acid chains using tRNA.
• Essential for formation of enzymes, hormones, and structural proteins.

4. ENDOPLASMIC RETICULUM (ER)

The ER is a network of membranes forming channels throughout the cell.
It provides transportation, storage, and manufacturing functions.

There are two types:

i. Rough Endoplasmic Reticulum (RER)

• Has ribosomes on its surface (appears rough).
• Involved in protein synthesis.
• Helps in protein folding and sorting.

ii. Smooth Endoplasmic Reticulum (SER)

• No ribosomes (appears smooth).
• Involved in:
1. Lipid, phospholipid, and cholesterol synthesis
2. Production of steroid hormones
3. Carbohydrate metabolism
4. Detoxification of drugs in liver cells
5. Storage and release of calcium ions in muscle cells

5. GOLGI BODIES (GOLGI APPARATUS)

Discovered by Camillo Golgi (1898).

Appears as a stack of flattened membranes called cisternae.

Two Faces

• Cis-face: Receiving side
• Trans-face: Shipping side (releases vesicles)

Functions

1. Packages and modifies proteins and lipids.
2. Forms lysosomes.
3. Produces glycoproteins and glycolipids.
4. Helps form complex carbohydrates.
5. Important in forming plant cell walls.

6. LYSOSOMES

Discovered by Christian de Duve (1955).

Known as “Suicide bags of the cell” because they contain powerful digestive enzymes.

Functions

1. Break down waste materials (autophagy).
2. Digest materials taken in from outside (heterophagy).
3. Destroy harmful substances.
4. Release enzymes outside the cell (exocytosis) when needed.

Lysosomes protect the cell by ensuring harmful substances are broken down safely.

7. CYTOSOL AND CYTOSKELETON

Cytosol

The fluid portion of the cytoplasm where chemical reactions occur.

Functions:

• Site of metabolic reactions.
• Helps transport molecules.
• Supports signal transduction pathways.

Cytoskeleton

A network of protein fibres providing structural support.

Types of Cytoskeleton Fibers

8. PEROXISOMES

Discovered by Christian de Duve.

Small, spherical membrane-bound organelles containing the enzyme catalase.

Functions

1. Break down hydrogen peroxide (H₂O₂) into water + oxygen.
2. Protect cells from oxidative damage.
3. Perform β-oxidation of fatty acids.
4. Important in detoxification reactions.

9. CENTROSOME AND CENTRIOLES

The centrosome is located near the nucleus and contains a pair of centrioles.

Functions

1. Organizes microtubules.
2. Forms the mitotic spindle during cell division.
3. Helps with chromosome movement during mitosis.

Centrioles are essential for maintaining the internal organization and support of the cell.

10. CELL EXTENSIONS

Some cells have structures that extend from the plasma membrane for movement or increasing surface area.

Types of Cell Extensions