Cell – Structure and Function
Subject: Science | Topic: Cell – Structure and Function
For TLT, ECT, PST, JEST & All Competitive Exams
A cell is the smallest and most basic unit of life. Every living thing in the world — whether it is a human, an animal, or a plant — is made up of cells. This topic is a fundamental part of General Science and appears frequently in competitive exams such as ECT, BPS-15, PST, JEST, and other similar tests.
What is a Cell?
A cell is known as the structural and functional unit of life. Every living organism, whether it is a tiny bacterium or a full human body, is made of cells. Some organisms are made of just one cell, called unicellular organisms, such as bacteria and amoeba. On the other hand, humans, animals, and plants are multicellular organisms, meaning their bodies contain millions and billions of cells that together form tissues, organs, and the whole body.
The human body contains approximately 37 trillion cells, and each cell performs its own specific function — some cells produce energy, some form the immune system for defense, and some transmit information, such as nerve cells.
Discovery of the Cell
The cell was first observed in 1665 by a scientist named Robert Hooke. Using a microscope, he examined a thin slice of cork (a plant material) and noticed small box-like structures that reminded him of a honeycomb. He named these structures "cells," meaning "small rooms."
Later, scientists Matthias Schleiden and Theodor Schwann together proposed the "Cell Theory," which has three basic principles:
- Every living thing is made up of one or more cells
- The cell is the basic unit of life
- New cells arise only from pre-existing cells through division
Types of Cells
Cells are divided into two basic types:
1. Prokaryotic Cell
These are simple, structurally basic cells that do not have a nucleus — their genetic material (DNA) floats freely in the cytoplasm, not enclosed by any membrane. Bacteria have this type of cell. Prokaryotic cells are small and simple in structure.
2. Eukaryotic Cell
These cells have a well-defined nucleus enclosed by a membrane, along with various other organelles. The cells of humans, animals, and plants are eukaryotic. These cells are larger and more complex than prokaryotic cells.
Main Parts of a Cell
1. Cell Membrane (Plasma Membrane)
This is the outermost layer of the cell that covers the entire cell. Its most important job is to protect the cell and control which substances can enter or leave the cell. It is also called a selectively permeable membrane because it does not let everything pass through — only the necessary substances are allowed.
2. Cytoplasm
Inside the cell membrane is a jelly-like, semi-liquid material called cytoplasm. All the other organelles are present within it, and many chemical reactions necessary for the cell's life take place here, such as using energy and preparing new molecules.
3. Nucleus
The nucleus is known as the control center or "brain" of the cell. It contains DNA (Deoxyribonucleic Acid), which carries genetic information and controls all the activities of the cell — such as growth, reproduction, and protein synthesis. The nucleus is surrounded by a nuclear membrane that separates it from the rest of the cytoplasm.
4. Mitochondria
Mitochondria are called the "Powerhouse of the Cell" because they produce energy for the cell. When we eat food, we get glucose from it, and the mitochondria combine this glucose with oxygen to produce energy (ATP) that powers the cell. Cells that do more energy-demanding work, like muscle cells, contain more mitochondria.
5. Ribosomes
Ribosomes are very small organelles responsible for protein synthesis (making proteins). They can be found freely in the cytoplasm or attached to the endoplasmic reticulum.
6. Endoplasmic Reticulum (ER)
This is a network of membranes connected to the nucleus. It has two types — Rough ER (which has ribosomes attached to it and helps in protein production) and Smooth ER (which helps produce lipids and hormones).
7. Golgi Body (Golgi Apparatus)
This organelle packages and modifies proteins and other materials, then sends them to their correct location inside or outside the cell — it is often called the "post office" of the cell.
8. Lysosomes
These are known as the cell's "cleaning system." They break down old or unnecessary waste materials and damaged organelles, keeping the cell clean and healthy.
9. Vacuoles
These are storage sacs that hold water, food, and waste materials. Plant cells have one large vacuole that stores water and keeps the cell firm, while animal cells have several smaller vacuoles.
Difference Between Plant Cell and Animal Cell
Although plant and animal cells share many common features, there are some basic differences:
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell Wall | Present (rigid, firm) | Not present |
| Chloroplast | Present | Not present |
| Shape | Fixed, rectangular | Irregular, round-ish |
| Vacuole | One large vacuole | Several small vacuoles |
| Energy Source | Makes its own food via photosynthesis | Obtains energy by eating food |
Chloroplast and Photosynthesis
The chloroplast is found only in plant cells, and it is the site where photosynthesis takes place — the process by which plants use sunlight, water, and carbon dioxide to make their own food (glucose) and release oxygen. The chloroplast contains a green pigment called chlorophyll, which absorbs sunlight and gives plants their green color.
Importance of the Cell
The cell is the foundation of life. Every function of the body — whether it is the heartbeat, the brain's thinking, or the body's growth — depends on cells working properly. When cells do not function correctly or become damaged, diseases occur, such as cancer, which results from the uncontrolled growth of cells.
Practice MCQs
1. What is the control center of the cell called?
a) Cytoplasm b) Nucleus c) Cell Membrane d) Cell Wall
2. Who first observed the cell?
a) Charles Darwin b) Robert Hooke c) Isaac Newton d) Louis Pasteur
3. What is called the "Powerhouse of the Cell"?
a) Nucleus b) Mitochondria c) Ribosome d) Golgi Body
4. In which organelle does photosynthesis take place?
a) Mitochondria b) Chloroplast c) Nucleus d) Vacuole
5. What is missing in prokaryotic cells?
a) Cytoplasm b) A well-defined nucleus c) Cell Membrane d) DNA
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