Maarg Manthan · Topic 1.3

Cell Structure: Prokaryotic and Eukaryotic Cells

Science & Technology › Cell Biology and Genetics · Topic 1.3

The cell is the basic structural and functional unit of life. Cell theory, developed by Matthias Schleiden and Theodor Schwann in 1838 to 1839 and completed by Rudolf Virchow in 1855, says that all living things are made of cells, the cell is the basic unit of life, and new cells arise only from existing cells. Robert Hooke first saw and named cells in cork in 1665. Cells are of two kinds. Prokaryotic cells, found in bacteria and archaea, have no membrane-bound nucleus and no membrane-bound organelles. Eukaryotic cells, found in protists, fungi, plants and animals, have a true nucleus and organelles such as mitochondria, the endoplasmic reticulum and the Golgi apparatus.

5 min read · UPSC Prelims · Science & Technology · Cell Biology and Genetics

At a Glance

Cell theory Schleiden, Schwann, Virchow
First seen Robert Hooke, 1665
Prokaryotes No nucleus; 70S ribosomes
Eukaryotes True nucleus; 80S ribosomes
Powerhouse Mitochondria
Exam link UPSC Prelims, Science & Technology: Cell Biology and Genetics

Where Does It Fit?

This is the third topic of the macro on cell biology and genetics. It covers the structure of the cell, which is the setting for the membrane, energy and division topics that follow. Previous topic: History and Origin of Life on Earth. Next topic: Cell Membranes and Cell-Cell Communication/Interactions.

How Do Prokaryotic and Eukaryotic Cells Differ?

Feature	Prokaryotic cell	Eukaryotic cell
Nucleus	No true nucleus; DNA lies in the nucleoid region	Nucleus with a double membrane
Organelles	No membrane-bound organelles	Mitochondria, endoplasmic reticulum, Golgi apparatus and others
Ribosomes	70S (smaller)	80S in the cytoplasm
DNA	Usually a single circular chromosome, often with plasmids	Several linear chromosomes with histone proteins
Size	Usually 1 to 10 micrometres	Usually 10 to 100 micrometres
Examples	Bacteria, archaea, cyanobacteria	Protists, fungi, plants, animals

What Do the Main Organelles Do?

Nucleus: holds the DNA and controls the activities of the cell. The nucleolus inside makes ribosomal RNA.
Mitochondria: have a double membrane and their own DNA, and make most of the ATP of the cell through respiration, so they are called the powerhouse of the cell.
Endoplasmic reticulum: the rough form with ribosomes makes and folds proteins, and the smooth form makes lipids and detoxifies substances.
Golgi apparatus: modifies, sorts and packs proteins and lipids into vesicles, discovered by Camillo Golgi in 1898.
Lysosomes: sacs of digestive enzymes that break down waste and worn-out parts, and are called suicide bags because they can destroy the cell if they burst.
Ribosomes: make proteins by reading mRNA. They are made of rRNA and protein.
Plastids: chloroplasts in plants contain chlorophyll and carry out photosynthesis, and they have their own DNA.
Vacuole: a large central vacuole in plant cells stores water and helps to keep the cell firm.

What Else Should You Remember?

Plant and animal cells: plant cells have a cell wall of cellulose, chloroplasts and a large vacuole. Animal cells have centrioles, small vacuoles and no cell wall.
Endomembrane system: the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes and vacuoles work as a linked system, passing materials in vesicles from one part to the next.
Peroxisomes: small organelles that break down fatty acids and neutralise hydrogen peroxide, which is a harmful by-product.
Centrosome and centrioles: found in animal cells and help to organise the spindle fibres during cell division.
Nuclear envelope: a double membrane with pores that control the movement of RNA and proteins between the nucleus and the cytoplasm.
Cell wall: in bacteria it is made of peptidoglycan, in fungi of chitin and in plants of cellulose.
Extremes of size: the smallest cells are mycoplasmas, the largest single cell is the ostrich egg, and nerve cells can be over a metre long.
Cytoskeleton: a network of protein fibres that gives shape, helps movement and moves organelles.
Cell theory exception: viruses are not cells and cannot reproduce on their own, as in Viruses: Structure and Classification.
Stem cells: unspecialised cells that can divide and become other cell types, which are used in regenerative medicine.
Microscopes: the electron microscope allows the study of organelles, which are not visible clearly in a light microscope.

Why Does It Matter for the Exam?

High-frequency topic: questions often ask for the function of an organelle or for the difference between plant and animal cells.
Concept link: mitochondria connect to respiration, as in Cellular Respiration and Energy Metabolism (ATP Production), and the nucleus connects to genes.
Health link: bacterial cell wall features explain how antibiotics such as penicillin act.
Biotech link: plasmids of bacteria are used as vectors in genetic engineering, as in Recombinant DNA Technology.
Exam pattern: true-false statements on organelles and on scientists who discovered them.

As an analytical point, the cell is a small and organised factory. Each organelle has a job, and knowing the job helps to answer questions on diseases and technology.

Why Is This Topic Important?

Facts Organelle functions asked
Contrast Prokaryote versus eukaryote
Link Respiration and biotech

What Came Next?

The next topic covers cell membranes and cell-cell communication. Next topic: Cell Membranes and Cell-Cell Communication/Interactions.

Key People and Terms

Organelle Specialised structure inside a cell
Nucleoid DNA region of a prokaryote
Plasmid Small circular DNA in bacteria
Cytoplasm Jelly-like content of a cell
Peptidoglycan Bacterial cell wall material
Cytoskeleton Protein framework of the cell

Exam Corner

Points to Remember

Cell seen by Hooke in 1665.
Cell theory: Schleiden, Schwann, Virchow.
Mitochondria: powerhouse; own DNA.
Lysosomes: suicide bags.
Prokaryote ribosomes 70S; eukaryote 80S.

Do Not Confuse With

Rough and smooth ER Protein making versus lipid making
Chloroplast and chromoplast Green plastid versus coloured plastid
Cell wall and cell membrane Outer rigid layer versus living boundary

Memory Hook

Hooke saw cork, three theorists the cell define; mitochondria power, lysosomes suicide, nucleus the design.

Mains Angle

Questions ask about cell organelles and their link to health. These points add depth.

Types Prokaryote and eukaryote
Organelles Functions
Link Health and biotech

A Question You May Face

An original practice question, not a past paper question.

“Distinguish between prokaryotic and eukaryotic cells and explain the functions of major cell organelles.”

How to Answer

Theory Cell theory.
Types Two kinds.
Organelles Jobs.
Link Disease, biotech.

GS Relevance

Prelims: organelles and functions, differences between cell types, scientists. GS Paper 3: science and technology.

Frequently Asked Questions

What is cell theory?

Cell theory says that all living things are made of cells, the cell is the basic unit of structure and function of life, and new cells arise only from existing cells. Schleiden and Schwann proposed the first two ideas in 1838 to 1839, and Rudolf Virchow added the third in 1855.

What is the difference between prokaryotic and eukaryotic cells?

Prokaryotic cells, as in bacteria and archaea, have no membrane-bound nucleus or organelles and have smaller 70S ribosomes, while eukaryotic cells, as in plants, animals, fungi and protists, have a true nucleus, membrane-bound organelles and 80S ribosomes. Eukaryotic cells are also usually larger.

Why are mitochondria called the powerhouse of the cell?

Mitochondria are called the powerhouse of the cell because they make most of the ATP, the energy currency of the cell, through aerobic respiration. They have a double membrane and their own DNA and ribosomes, which supports the idea that they came from bacteria through endosymbiosis.

What is the function of lysosomes?

Lysosomes are membrane-bound sacs filled with digestive enzymes that break down worn-out organelles, food particles and invading microbes inside the cell. They are called suicide bags because the release of their enzymes can destroy the cell, which is important in cell death and recycling.

How do plant and animal cells differ?

Plant cells have a rigid cell wall made of cellulose, chloroplasts for photosynthesis and a large central vacuole, while animal cells lack these and usually have centrioles and many small vacuoles. Both types have a nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus and ribosomes.

PYQ Practice — Statement Analysis

1 Mitochondria are the main sites of ATP production in eukaryotic cells.

True

Through aerobic respiration.

2 Prokaryotic cells have a membrane-bound nucleus.

False

They have a nucleoid.

3 Robert Hooke first observed cells in cork.