Cells the Unit of Life Complete Overview for Competitive Exam

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Prepare for competitive exams with our in-depth overview of cells, providing a comprehensive understanding of their definition, structure, types, and functions. Master the unit of life with ease.

Cells are like the building blocks of life. Imagine if you could break down any living thing to its smallest part – that would be the cell. A cell is the basic and essential unit of all living things.

We often call cells the ‘Building Blocks of Life’ or the ‘Basic units of Life.‘ When an organism is made of only one cell, we call it ‘unicellular,’ and if it’s made of many cells, it’s ‘multicellular.’

Cells play various roles in a living organism, like handling digestion, breathing, and reproduction. Cells are super tiny, ranging from 0.0001 mm to almost 150 mm across.

For instance, in our bodies, many cells come together to form a tissue. When multiple tissues work together, they create an organ. Several organs form an organ system, and when these systems collaborate, they make up the entire human body.

For example 👇

Cell → Tissue (Multiple Cells) → Organ (Multiple Tissues) → Organ System (Multiple organs)  → Human body (Several organ systems functioning together)

Check out these cell notes to understand what a cell is, its definition, structure, types, and what it does. These notes go deep into explaining everything about cells.

Cells the Unit of Life

Cell Definition

“A cell is defined as the smallest, basic unit of life that is responsible for all of life’s processes.”

Cells are like the tiny, functional units that make up all living things.

What’s interesting is that a cell can make copies of itself on its own, making it the basic building block of life.

Inside each cell, there’s a fluid called cytoplasm surrounded by a membrane. In this cytoplasm, you’ll find important biomolecules like proteins, nucleic acids, and lipids. And there are also small structures inside the cell called cell organelles.

What is a Cell?

A cell is like the essential unit of life – its structure and foundation. The study of cells, covering everything from their basic structure to the functions of each tiny cell part, is known as Cell Biology. The credit for discovering cells goes to Robert Hooke, the first biologist.

All living things are made up of cells, and they can either be just one cell (unicellular) or many cells (multicellular). Mycoplasmas holds the record for being the tiniest cells known. Think of cells as the building blocks of living beings – they give structure to the body and turn the nutrients from food into energy.

Cells are pretty complex, and their different parts have specific jobs in an organism. They come in various shapes and sizes, just like the bricks of a building. Our bodies are a mix of cells with different shapes and sizes.

When it comes to the organization of life, cells are at the bottom level. The count of cells can vary from one organism to another. Humans, for instance, have more cells compared to bacteria.

Inside cells, there are special cell parts called organelles, each with a specific function to keep life processes going. And don’t forget, the genetic material of living things is also inside these cells.

Discovery of Cells

The discovery of cells stands out as a remarkable achievement in the world of science. It reveals that every living thing is composed of cells, and these cells play a crucial role in performing different life processes. By exploring the structure and functions of cells, we’ve gained a deeper understanding of life itself.

Who discovered cells?

In the year 1665, Robert Hooke made a significant discovery by finding the cell. While observing a piece of cork through a compound microscope, he noticed tiny structures that looked like small rooms. He named these structures “cells.” However, due to the limitations of his microscope’s magnification, Hooke couldn’t see detailed structures and concluded that these were non-living entities.

Later on, Anton Van Leeuwenhoek used a different compound microscope with higher magnification and observed cells again. This time, he noticed some movement (motility) in the cells. This led Leeuwenhoek to conclude that these microscopic entities were indeed “alive.” Over time, after various observations, these entities were named animalcules.

In 1838, Robert Brown, a Scottish botanist, provided the first insights into cell structure. He described the nucleus present in the cells of orchids, contributing to our understanding of cells.

Characteristics of Cells

Cells exhibit various crucial characteristics:

  • Structural Support: Cells provide the essential structure and support for the organism’s body.
  • Organelle Organization: Inside the cell, various individual organelles are neatly organized, each surrounded by its membrane.
  • Nucleus Significance: The nucleus, a major organelle, contains genetic information vital for cell growth and reproduction.
  • Unified Structure: Each cell contains one nucleus and organelles surrounded by membranes in the cytoplasm.
  • Mitochondria’s Energy Role: Mitochondria, a double membrane-bound organelle, plays a crucial role in ensuring energy transactions vital for the cell’s survival.
  • Lysosomes for Cleanup: Lysosomes function to digest unwanted materials within the cell, contributing to cellular cleanliness.
  • Endoplasmic Reticulum’s Role: The endoplasmic reticulum is pivotal in organizing the cell internally. It synthesizes specific molecules and processes, directs, and sorts them to their designated locations.

Types of Cells

Cells can be likened to factories where different workers and departments collaborate for a shared goal. Different types of cells carry out distinct functions. Considering cellular structure, there are two main types of cells:

  • Prokaryotes
  • Eukaryotes

Prokaryotic Cells

  • Prokaryotic cells don’t have a nucleus. Instead, some prokaryotes like bacteria have a region inside the cell where the genetic material is freely suspended, known as the nucleoid.
  • These are all single-celled microorganisms, including archaea, bacteria, and cyanobacteria. Their cell size ranges from 0.1 to 0.5 µm in diameter.
  • The hereditary material in prokaryotes can be either DNA or RNA.
  • Prokaryotes typically reproduce through binary fission, a type of asexual reproduction. Additionally, they can engage in conjugation, which is often likened to sexual reproduction (although it’s not the same).

Eukaryotic Cells

  • Eukaryotic cells stand out for having a true nucleus.
  • These cells typically range in size from 10 to 100 µm in diameter.
  • Eukaryotes include a wide range of living organisms such as plants, fungi, protozoans, and animals.
  • The plasma membrane is a crucial component, overseeing the transport of nutrients and electrolytes in and out of cells. It also plays a key role in cell-to-cell communication.
  • Eukaryotic cells can reproduce both sexually and asexually.
  • There are notable differences between plant and animal cells. For instance, plant cells have chloroplasts, central vacuoles, and other plastids, features absent in animal cells.

Cell Structure

The structure of a cell consists of distinct components, each with specific functions crucial for carrying out life’s processes. These components include the cell wall, cell membrane, cytoplasm, nucleus, and various cell organelles. Keep reading to delve into more details about the structure and function of cells.

Cell Membrane

  • The cell membrane plays a vital role in supporting and safeguarding the cell. It regulates the movement of substances in and out of the cell, creating a boundary between the cell and its external environment. This protective layer is present in all cells.
  • The cell membrane serves as the outer covering of a cell, encompassing other organelles like the cytoplasm and nucleus. Sometimes, it’s also called the plasma membrane.
  • In terms of structure, it’s a porous membrane with tiny openings, allowing specific substances to move in and out of the cell. Additionally, the cell membrane acts as a shield, safeguarding the cell’s components from potential damage and leakage.
  • This membrane forms a barrier between two cells and between the cell and its surroundings.
  • Plants, being stationary, have cell structures well-adapted to shield them from external factors. The cell wall is instrumental in reinforcing this protective function.

Cell Wall

  • The cell wall stands out as a prominent component in the structure of plant cells. Composed of cellulose, hemicellulose, and pectin, it is an essential part of the plant’s cellular makeup.
  • Exclusive to plant cells, the cell wall serves as a protective layer for the plasma membrane and other cellular components. Positioned as the outermost layer of plant cells, it has a rigid and stiff structure surrounding the cell membrane.
  • Apart from protection, the cell wall plays a crucial role in providing shape and support to plant cells. It acts as a shield against mechanical shocks and injuries, contributing to the overall robustness of the cell structure.


  • The cytoplasm is a dense, transparent, jelly-like substance found within the cell membrane.
  • Numerous chemical reactions crucial for the cell occur in this cytoplasm.
  • Cell organelles like the endoplasmic reticulum, vacuoles, mitochondria, and ribosomes are all suspended in this cytoplasm.


  • The nucleus holds the cell’s hereditary material, the DNA.
  • It communicates signals to the cells, directing them to grow, mature, divide, and undergo programmed cell death.
  • Enclosed by the nuclear envelope, which acts as a barrier, the nucleus keeps the DNA isolated from the rest of the cell.
  • The nucleus serves as a protective shield for the DNA and is a crucial component in the structure of a plant cell.
Read More about the Structure and Functions of Nucleus  

Cell Organelles

Cells consist of a variety of cell organelles, each with specific functions crucial for life processes. The diverse cell organelles, along with their primary functions, include:

Cell OrganelleFunction
NucleolusSite for ribosome synthesis; involved in controlling cellular activities and reproduction.
Nuclear MembraneProtects the nucleus by forming a boundary between the nucleus and other organelles.
ChromosomesCrucial for determining an individual’s sex; human cells contain 23 pairs.
Endoplasmic ReticulumInvolved in substance transport throughout the cell; plays a key role in carbohydrate, lipid, steroid, and protein metabolism.
Golgi BodiesActs as the cell’s post office, facilitating material transportation within the cell.
RibosomesProtein synthesis within the cell.
MitochondriaKnown as the “powerhouse of the cell”; produces ATP, the cell’s energy currency.
LysosomesProtects the cell by engulfing foreign bodies; assists in cell renewal, earning the nickname “cell’s suicide bags.”
ChloroplastsPrimary organelles for photosynthesis; contain the pigment chlorophyll.
VacuolesStores food, water, and waste materials within the cell.

Cell Theory

The Cell Theory was put forward by German scientists Theodor Schwann, Matthias Schleiden, and Rudolf Virchow. This theory explains:

  • Every living thing on Earth is made up of cells.
  • A cell is the fundamental building block of life.
  • All cells come from existing cells.

A more recent version of the cell theory includes these points:

  • Energy moves around inside cells.
  • Genetic information is passed from one cell to another.
  • The chemical makeup of all cells is similar.

Time Line of Cell Theory

In 1838, a German botanist named Matthias Schleiden stated that cells are the basic elements of all plants. The next year, another German botanist, Theodor Schwann, extended this idea to animals, highlighting that cells are the fundamental units for them too. These revelations challenged the belief that plants and animals have distinct structural differences.

Their findings gave rise to the ‘Cell Theory,’ which asserts that cells are the fundamental units of all living beings. However, it didn’t explain how new cells form. In 1855, Rudolf Virchow, a German physiologist, added a crucial point: new cells come from existing ones.

So, the modified cell theory includes three key points:

  1. Cells are the basic functional and structural units of all living organisms.
  2. All living organisms are composed of cells.
  3. Every cell comes from pre-existing cells.

Later, scientists also discovered that the units responsible for reproduction in the body are cells. Here’s an interesting fact: the female egg (Ovum) is the largest cell in the human body, while the smallest cell is the sperm.

Functions of Cell

Cells play crucial roles in the growth and development of living organisms by performing essential functions:

  • Support and Structure: Cells make up the structural foundation of all living things. The cell wall and membrane offer support and structure. For instance, the skin consists of numerous cells, and in plants, the xylem cells provide structural support.
  • Facilitate Growth through Mitosis: Cells undergo mitosis, a process where a parent cell divides into daughter cells, promoting organismal growth.
  • Transport of Substances: Cells import nutrients for various chemical processes and eliminate waste through active and passive transport. Small molecules like oxygen, carbon dioxide, and ethanol diffuse passively, while larger molecules require active transport, involving significant energy.
  • Energy Production: Cells generate energy, vital for chemical processes, through photosynthesis in plants and respiration in animals.
  • Reproduction: Cells contribute to reproduction through mitosis and meiosis. Mitosis involves asexual reproduction, where parent cells divide to form genetically identical daughter cells. Meiosis leads to genetically diverse daughter cells.

In summary, cells serve as the structural and functional units of life, offering support, carrying out growth, facilitating substance transport, producing energy, and contributing to reproduction. Their multifunctional roles make them integral to life processes.

Frequently Asked Questions on Cell

Q1. What is a Cell?

Answer: A cell is the basic, structural, and functional unit of all living organisms.

Q2. What are the characteristics of cells?


  • Cells offer structural support to an organism.
  • Genetic information for reproduction is found in the nucleus.
  • Cells contain organelles suspended in the cytoplasm.
  • Mitochondria produce cell energy.
  • Lysosomes digest the waste in the cell.
  • The endoplasmic reticulum synthesizes and processes molecules.

Q3. Highlight the cell structure and its components.


  • The cell structure includes essential components:
  • Cell membrane
  • Cell wall
  • Cell organelles
  • Nucleolus
  • Nuclear membrane
  • Endoplasmic reticulum
  • Golgi Bodies
  • Ribosome
  • Mitochondria
  • Lysosomes
  • Chloroplast
  • Vacuoles

Q4. what are the types of cells?

Answer: Cells are classified into two types:

  • Prokaryotic cells
  • Eukaryotic cells

Q5. Elaborate Cell Theory.


  • Proposed by German scientists Matthias Schleiden, Theodor Schwann, and Rudolf Virchow.
  • All living species are made of cells.
  • A cell is the basic unit of life.
  • All cells come from pre-existing cells.

Q6. What is the function of mitochondria in the cells?


  • Mitochondria, known as the powerhouse, produce ATP, the cell’s energy currency, and regulate cellular metabolism.

Q7. What are the functions of the cell?


  • Provides support and structure.
  • Facilitates growth through mitosis.
  • Aids in reproduction.
  • Produces energy and allows substance transport.

Q8. What is the function of Golgi bodies?

Answer: Golgi bodies pack and sort proteins for secretion, create lysosomes, and transport lipids within cells.

Q9. Who discovered the cell and how?

Answer: Robert Hooke discovered cells in 1665 by observing cork under a microscope, noting structures resembling small rooms.

Q10. Name the cell organelle that contains hydrolytic enzymes capable of breaking down organic matter.

Answer: Lysosomes.

Q11. Which cellular structure regulates the entry and exit of molecules to and from the cell?

Answer: A cell membrane, is a selectively permeable structure controlling molecule entry and exit.

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As a professional blogger and passionate educator, I am driven by a deep-seated desire to share knowledge and empower others. With years of experience in the field, I am committed to providing valuable insights and guidance to aspiring learners. My passion lies in helping individuals discover their potential and achieve their goals. I am also a firm believer in the power of motivation and strive to inspire others to pursue their dreams with unwavering determination.

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