Introduction to Cell
A cell is the smallest unit of life capable of independent existence. It is the structural and functional unit of all living organisms.
Every living being — from the simplest bacteria to the complex human body — is made up of cells.
Each cell consists of three main parts:
- Plasma membrane
- Cytoplasm
- Nucleus
1. Plasma Membrane
The plasma membrane forms the flexible outer covering of the cell. It separates the internal contents of the cell from the external environment.
This membrane acts as a selective barrier, allowing certain substances to enter or leave the cell while maintaining the proper environment inside for all cellular activities.
2. Cytoplasm
The cytoplasm includes everything inside the cell between the plasma membrane and the nucleus.
It is a jelly-like material that holds the cell organelles in place and provides a medium for the exchange of materials within the cell.
3. Nucleus
The nucleus is the largest organelle and acts as the control center of the cell.
It contains the cell’s genetic material — DNA — which is organized into chromosomes. Each chromosome holds many genes that determine the characteristics and functions of the cell.
Structure of Cell – Components and Their Functions
Below are the main cell organelles and their roles within the cell.
1. Cell Membrane
The cell membrane is made up of a lipid bilayer composed of phospholipids, glycolipids, and cholesterol.
It keeps the cell together and controls the movement of substances in and out.
Because it allows only selected materials to pass, it is called a selectively permeable membrane.
2. Centrosomes
Centrosomes contain small cylindrical structures called centrioles, which play a key role in cell division.
3. Cytoplasm
The cytoplasm, sometimes called the “cell matrix,” is a thick fluid that fills the inside of the cell and supports all organelles.
4. Golgi Apparatus
The Golgi apparatus works closely with the endoplasmic reticulum (ER).
It modifies, stores, and transports proteins and lipids produced in the ER to different parts of the cell or outside it.
5. Lysosomes
Lysosomes are small sacs filled with enzymes that digest and recycle old or damaged cell parts.
They also break down nutrients for the cell to use.
When a cell dies, lysosomes help destroy it — which is why they are sometimes called the “suicidal bags” of the cell.
6. Microvilli
Microvilli are tiny, finger-like projections found on the outer surface of some cells.
They increase the surface area of the cell, allowing more absorption and faster exchange of materials.
For example, cells lining the intestines have microvilli for better nutrient absorption.
7. Mitochondria
The mitochondria are known as the “powerhouses of the cell”.
They produce energy in the form of ATP (Adenosine Triphosphate), which powers all cell activities.
Cells that need more energy, like muscle cells, contain more mitochondria.
8. Nuclear Membrane
The nuclear membrane surrounds the nucleus and separates it from the rest of the cell. It protects the genetic material inside.
9. Nuclear Pores
These small openings on the nuclear membrane allow the movement of materials such as nutrients, waste, and genetic signals in and out of the nucleus.
10. Nucleolus
The nucleolus is a dense region inside the nucleus. It helps produce cell organelles like ribosomes, which are essential for protein synthesis.
11. Nucleus
The nucleus stores genetic information in the form of DNA and controls all cell activities, including growth, metabolism, and reproduction.
It also contains chromosomes that carry hereditary information.
12. Ribosomes
Ribosomes are small structures responsible for making proteins.
They read genetic information from the nucleus and assemble amino acids into specific proteins needed by the cell.
13. Rough Endoplasmic Reticulum (RER)
The RER has ribosomes attached to its surface, giving it a “rough” appearance.
It is the site for protein synthesis and helps transport these proteins throughout the cell.
14. Smooth Endoplasmic Reticulum (SER)
The SER lacks ribosomes on its surface, making it “smooth.”
It helps in the production of lipids (fats) and also assists in detoxifying harmful substances in the cell.
Detailed Notes
For pdf style full-color notes, open the complete study material below: