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๐ฌ Think about it
A factory has many rooms — one builds, one packs, one cleans, one powers the machines. If each room works well, the factory runs smoothly. A cell has its own little "rooms" called organelles. Let us meet them one by one.
Why does a eukaryotic cell need organelles?
- They carry out many life processes at the same time.
- They build new materials and remove waste.
- They provide energy to the cell.
- Together they run the cell like a tiny living factory.
What does the nucleus do?
- The nucleus is the control centre of the cell.
- It has a double-layered nuclear membrane with pores.
- Pores allow transfer of material with the cytoplasm.
- The nucleolus inside makes ribosomal subunits.
What is inside the nucleus?
- It contains chromosomes, seen only when a cell divides.
- Chromosomes are made of DNA and proteins.
- DNA carries genetic information; its parts are genes.
- In a resting cell, DNA stays as chromatin threads.
From Cell to DNA
Cell
↓
Nucleus
Control centre holding genetic material.
↓
Chromosome
Made of DNA and proteins.
↓
DNA
Carries the genetic information; its parts are genes.
Where is the genetic material in a prokaryotic cell?
- Prokaryotic cells have no well-defined nucleus.
- Their DNA is a single circular molecule with proteins.
- The region holding this DNA is called the nucleoid.
What do ribosomes do?
- Ribosomes are tiny structures in the cell.
- They are free in the cytoplasm or on the ER.
- They are the sites of protein synthesis.
Example:
Ribosomes are like the cell's protein factories.
What is the endoplasmic reticulum (ER)?
- The ER is a large network spread through the cytoplasm.
- It is continuous with the outer nuclear membrane.
- It helps make and transport proteins, fats and some hormones.
How are rough ER and smooth ER different?
- Rough ER has ribosomes on its surface.
- It mainly makes and secretes proteins.
- Smooth ER has no ribosomes, so it looks smooth.
- It makes and stores fats and hormones.
Rough ER (RER)
Smooth ER (SER)
Has ribosomes on its surface.
Has no ribosomes; looks smooth.
Makes and secretes proteins.
Makes and stores fats and hormones.
Example: gland cells like pancreas.
Involved in fat and hormone work.
What does the Golgi apparatus do?
- It is a stack of flattened, sac-like structures.
- It acts like the cell's post office.
- It modifies, sorts and packs proteins and lipids.
- It packs them into vesicles for transport or secretion.
Example:
Like a post office, it packs and ships parcels (proteins) to the right place.
What is the job of lysosomes?
- Lysosomes are single membrane-bound sacs of enzymes.
- They break down unwanted proteins, carbohydrates and fats.
- They clear damaged parts, keeping the cell clean and healthy.
- The broken products go back to the cytoplasm to be reused.
Why are mitochondria called the powerhouse of the cell?
- They supply the energy needed for most cell activities.
- Each is surrounded by two membranes.
- The outer membrane is smooth and porous.
- The inner membrane folds into cristae for more reactions.
How do mitochondria release energy?
- Glucose and other molecules are broken down inside them.
- This happens during cellular respiration.
- The energy is stored in a molecule called ATP.
- ATP acts as the energy currency of the cell.
Example:
ATP is like a charged battery the cell spends to do work.
What are plastids and what do they do?
- Plastids are organelles in plant cells for food synthesis and storage.
- Chloroplasts contain the green pigment chlorophyll.
- Chlorophyll absorbs sunlight for photosynthesis.
- Chloroplasts are double-membrane-bound, like mitochondria.
How do flowers, fruits and vegetables get their colours?
- Some plastids hold pigments other than chlorophyll.
- These are called chromoplasts (chroma means colour).
- Their yellow, orange or red colours attract pollinators.
- Colourless plastids that store food are called leucoplasts.
| Plastid | Pigment | Job |
|---|---|---|
| Chloroplast | Green (chlorophyll) | Makes food by photosynthesis. |
| Chromoplast | Yellow, orange or red | Gives colour to flowers and fruits. |
| Leucoplast | No pigment (colourless) | Stores starch, oils or proteins. |
Why are mitochondria and plastids special?
- They both have their own DNA and ribosomes.
- So they can make some of their own proteins.
- This is also true of certain bacteria.
- It suggests they share an evolutionary history with single-celled organisms.
Example:
Like bacteria, both these organelles carry their own DNA.
What is the job of vacuoles?
- A mature plant cell usually has one large central vacuole.
- It is filled with a watery fluid called cell sap.
- It stores water, minerals, sugars and waste.
- By storing water, it keeps the plant cell firm.
Why do plants wilt without enough water?
- The vacuole stores water and keeps up pressure.
- Without water, the vacuole loses water.
- So the cells become less firm and the plant wilts.
- In animal cells vacuoles are small and sometimes present.
Example:
A plant left without water on a hot day droops and wilts.
Important Points
- The nucleus controls the cell and holds the DNA.
- Ribosomes make proteins; the ER makes and moves proteins and fats.
- The Golgi apparatus packs and ships; lysosomes clean up.
- Mitochondria release energy; plastids and vacuoles serve plant cells.
๐งฌ Threads of Curiosity
- Mature red blood cells (RBCs) have no nucleus.
- This gives more space for haemoglobin to carry oxygen.
- Without a nucleus they cannot repair or divide.
- So their lifespan is short, about 120 days.
๐ฌ Meet a Scientist
- The Golgi apparatus was first seen in 1898.
- Italian scientist Camillo Golgi observed it.
- He used special staining on barn owl nerve cells.
- Electron microscopes later confirmed it; it was named in his honour.
๐งฌ Threads of Curiosity
- Human sperm cells contain lysosomal enzymes.
- When a sperm meets an egg, these enzymes act.
- They break down the egg's outer layer.
- This allows fertilisation to take place.
๐งฌ Threads of Curiosity
- In 2010, J. Craig Venter's team studied a bacterium's DNA.
- They chemically made an exact copy of this DNA.
- They put the synthetic DNA into another cell.
- The cell grew and divided — showing DNA controls the cell.
โ Test Yourself
-
Which organelle is the control centre of the cell?
View Answer
The nucleus. It holds the DNA. -
Where does protein synthesis happen?
View Answer
On the ribosomes. -
Which organelle is the "post office" of the cell?
View Answer
The Golgi apparatus — it packs and ships proteins and lipids. -
What is the energy currency of the cell?
View Answer
ATP (Adenosine Triphosphate), made in mitochondria. -
Which plastid gives fruits their red and orange colour?
View Answer
The chromoplast.
Important Definitions
- Nucleus — the control centre of the cell that holds the DNA.
- Nucleoid — the region holding the DNA in a prokaryotic cell, without a membrane.
- Ribosomes — tiny structures that are the sites of protein synthesis.
- Endoplasmic reticulum — a network that makes and transports proteins and fats.
- Golgi apparatus — stacked sacs that modify, sort and pack materials into vesicles.
- Lysosomes — enzyme-filled sacs that break down waste and worn-out parts.
- Mitochondria — double-membrane organelles that release energy as ATP.
- Plastids — plant organelles for food synthesis and storage (chloroplast, chromoplast, leucoplast).
- Vacuole — a sap-filled sac that stores materials and keeps a plant cell firm.
๐ NCERT Question 3 — Look at the diagram
Identify parts (a) to (g) in the cell diagram and match them with their functions.
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๐ NCERT Question 4 — Which of the following option(s)
Which option correctly places organelles present in plant cells and absent in animal cells?
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๐ NCERT Question 5 — Two students, Renu and Rohit
Renu and Rohit argue about whether roots contain plastids — who is correct?
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๐ NCERT Question 6 — Mitochondria and chloroplasts are
Discuss how mitochondria and chloroplasts are similar and different.
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๐ NCERT Question 7 — Which of the following pairs
Which pair of cell organelles contains DNA?
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๐ NCERT Question 11 — Identify the pair that
Identify the pair that incorrectly matches an organelle with its function.
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๐ NCERT Question 12 — What outcome do you
What outcome do you expect if all mitochondria are removed from a eukaryotic cell?
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๐ NCERT Question 14 — The cell membrane of
Which organelles help make the cell membrane, and what path do the compounds follow?
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