2.3.1 Why do eukaryotic cells need these organelles?

Last updated at July 10, 2026 by

๐Ÿ’ฌ Think about it

A factory has many rooms โ€” 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?
Why Cells Need Organelles Many life processes at once Each organelle has a job Cell runs like a factory
Image: A cell drawn like a tiny factory, with different organelles shown as little workshops โ€” one building materials, one packing, one cleaning, and one powering machines โ€” to show each organelle does a specific job.
  • 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?
Role of the Nucleus Control Centre Directs all activities of the cell. Nuclear Membrane Double-layered with pores that allow transfer of material with the cytoplasm. Nucleolus Lies inside and makes ribosomal subunits.
Image: A diagram of a cell nucleus showing the double-layered nuclear membrane with nuclear pores, the round dense nucleolus inside, and entangled chromatin threads, labelled to show the structure of a nucleus.
  • 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?
Inside the Nucleus Nuclear Membrane Double-layered boundary with pores controlling entry and exit. Nucleoplasm Jelly-like fluid that fills the nucleus. Nucleolus Small dense body that helps make ribosomes. Chromatin / Chromosomes Chromatin condenses into chromosomes during division. DNA & Genes DNA carries genetic information; its segments are genes.
Nucleolus Nucle- Latin: kernel/nucleus -olus Latin: small A tiny body inside the nucleus that makes ribosomes.
Chromosome Chroma colour (Greek) Soma body (Greek) Coloured thread-like structures made of DNA that carry the genetic information of the cell.
  • 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
โ†“
Nucleus
Control centre holding genetic material.
โ†“
Chromosome
Made of DNA and proteins.
โ†“
DNA
Carries the genetic information; its parts are genes.
Where is the genetic material in a prokaryotic cell?
Nucleoid In a prokaryotic cell there is no true nucleus; the DNA is a single circular molecule held in a region called the nucleoid.
Nucleoid Nucleo nucleus (Latin: kernel) oid resembling (Greek) A region in prokaryotic cells where DNA is held โ€” resembling a nucleus but without a membrane around it.
Image: A bacterial cell showing its single circular DNA molecule lying free in the cytoplasm in a region labelled nucleoid, with no membrane around it, unlike a true nucleus.
  • 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 Tiny structures, free in the cytoplasm or on the ER, that are the sites of protein synthesis. e.g. Ribosomes are like the cell's protein factories.
Ribosome Ribo ribose sugar / RNA Some body (Greek: soma) Tiny RNA-containing bodies in the cell that build proteins โ€” found in all living cells.
Image: Tiny ribosomes shown both floating free in the cytoplasm and attached to the endoplasmic reticulum, labelled as the sites where proteins are made in the cell.
  • 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)?
Endoplasmic Reticulum Endoplasmic within the cytoplasm Reticulum a network A large membrane network through the cytoplasm that makes and transports proteins, fats and hormones.
Image: A diagram of the endoplasmic reticulum showing rough ER with ribosomes dotted on its surface and smooth ER without ribosomes, both spreading like a network from the nuclear envelope.
  • 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 vs Smooth ER Rough ER (RER) Smooth ER (SER) Has ribosomes on its Has no ribosomes; looks surface. smooth. Makes and secretes Makes and stores fats and proteins. hormones. e.g. pancreas gland e.g. liver cells. cells.
  • 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?
Golgi Apparatus โ€” The Cell's Post Office Receives Takes in proteins and lipids from the ER. Modifies & Sorts Chemically modifies and sorts the molecules. Packs Packs them into vesicles. Ships parcels to the right place
Image: The Golgi apparatus drawn as a stack of flattened sacs acting like a post office, receiving proteins and lipids, modifying and sorting them, and packing them into vesicles for transport.
  • 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 Lyso breaking down Soma body Membrane-bound sacs of enzymes that break down unwanted material and keep the cell clean.
Image: A lysosome shown as a small round membrane-bound sac filled with enzymes, breaking down waste materials and worn-out cell parts inside a cell, acting as the clean-up system.
  • 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?
Mitochondria โ€” Powerhouse Cells need energy Mitochondria release energy (ATP) Called the powerhouse
Image: A diagram of a mitochondrion showing its smooth outer membrane and inner membrane folded into finger-like projections called cristae, with the intermembrane space, its own DNA and ribosomes labelled inside.
Mitochondria Mito Greek: thread Chondria Greek: granule Thread-like or granular organelles that release energy for 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?
How Mitochondria Release Energy Break down fuel Glucose and other molecules are broken down inside them. Cellular respiration Energy is released during respiration. Store as ATP The energy is stored in a molecule called ATP. ATP = energy currency of the cell
Image: A mitochondrion breaking down glucose during cellular respiration to release energy, which is stored in ATP molecules shown as charged batteries that power the cell's activities.
  • 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 Plastids Organelles in plant cells for food synthesis and storage. Chloroplasts Contain chlorophyll, which absorbs sunlight for photosynthesis. Double Membrane Like mitochondria, chloroplasts are double-membrane-bound.
Image: A diagram of a chloroplast showing its outer and inner membranes, the semi-fluid stroma inside, disc-shaped membrane structures containing chlorophyll, and its own DNA and ribosomes.
Chloroplast Chloro Greek: green Plast Greek: formed body A green-coloured organelle in plant cells that makes food using sunlight.
Photosynthesis Photo light (Greek: phos) Synthesis putting together / making The process of making food using light energy โ€” chloroplasts use sunlight to turn COโ‚‚ and water into glucose.
  • 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?
Colours of Flowers & Fruits Bright colours appear Chromoplasts hold the pigments Attract pollinators
Chromoplast Chromo colour (Greek: chroma) Plast formed body (Greek: plastos) A coloured plastid that gives flowers and fruits their yellow, orange or red colour to attract pollinators.
  • 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?
Mitochondria & Plastids Own DNA and ribosomes Make some of their own proteins Likely evolved from bacteria
Image: A mitochondrion and a chloroplast each shown with their own circular DNA and ribosomes, placed beside a bacterium with similar features, suggesting a shared evolutionary history.
  • 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?
Central Vacuole A mature plant cell usually has one large central vacuole filled with cell sap. It stores water, minerals, sugars and waste, and keeps the cell firm.
Image: A mature plant cell with one large central vacuole filled with cell sap pushing against the cell membrane, keeping the cell firm, beside a wilted cell where the vacuole has lost water.
Vacuole Vacuus Latin: empty -ole Latin: small A small empty sac inside the cell that stores water, food or waste.
  • 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?
Why Plants Wilt Vacuole stores water Water lost โ†’ cells less firm Plant droops and wilts
Image: A healthy upright plant beside a wilted drooping plant, with a zoom-in showing the central vacuole full of water keeping cells firm, and an empty vacuole making cells limp.
  • 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.
๐Ÿงฌ Threads of Curiosity
Image: A red blood cell shown as a round disc with no nucleus, packed full of haemoglobin, beside a note that mature RBCs in humans are enucleate and survive about 120 days.
  • 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.
๐Ÿ”ฌ Meet a Scientist
Image: A portrait of Italian scientist Camillo Golgi, who first observed the Golgi apparatus in 1898 in the nerve cells of a barn owl using special staining techniques.
  • 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.
๐Ÿงฌ Threads of Curiosity
Image: A human sperm cell approaching an egg, with the tip of the sperm releasing lysosomal enzymes that break down the outer layer of the egg to allow fertilisation to take place.
  • 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.
๐Ÿงฌ Threads of Curiosity
Image: Scientist J. Craig Venter and his team in a lab, chemically synthesising a copy of the DNA of the bacterium Mycoplasma mycoides and inserting it into another cell that then grows and divides.
  • 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 โ€” showing DNA controls the cell.
โ“ Test Yourself
  1. Which organelle is the control centre of the cell?
    View Answer Hide Answer
    The nucleus. It holds the DNA.
  2. Where does protein synthesis happen?
    View Answer Hide Answer
    On the ribosomes.
  3. Which organelle is the "post office" of the cell?
    View Answer Hide Answer
    The Golgi apparatus โ€” it packs and ships proteins and lipids.
  4. What is the energy currency of the cell?
    View Answer Hide Answer
    ATP (Adenosine Triphosphate), made in mitochondria.
  5. Which plastid gives fruits their red and orange colour?
    View Answer Hide Answer
    The chromoplast.
Important Definitions
  • Nucleus โ€” the control centre of the cell that holds the DNA.
  • Nucleoid โ€” the region holding the DNA in a prokaryotic cell, without a membrane.
  • Ribosomes โ€” tiny structures that are the sites of protein synthesis.
  • Endoplasmic reticulum โ€” a network that makes and transports proteins and fats.
  • Golgi apparatus โ€” stacked sacs that modify, sort and pack materials into vesicles.
  • Lysosomes โ€” enzyme-filled sacs that break down waste and worn-out parts.
  • Mitochondria โ€” double-membrane organelles that release energy as ATP.
  • Plastids โ€” plant organelles for food synthesis and storage (chloroplast, chromoplast, leucoplast).
  • Vacuole โ€” a sap-filled sac that stores materials and keeps a plant cell firm.

๐Ÿ“‹ NCERT Question 3 โ€” Look at the diagram

Look at the diagram of a cell in Fig. 2.20. Identify the parts labelled from (a) to (g) and correctly match them with their functions given below: (i) Controlling all the activities of a cell. (ii) Site of cellular respiration. (iii) Storage organelle that also provides rigidity to the cell. (iv) Separates the cell contents from surroundings. (v) Provides structural rigidity to the cell. (vi) Packs and stores materials received from ER. (vii) Helps in manufacturing food.
View Answer โ†’

๐Ÿ“‹ NCERT Question 4 โ€” Which of the following option(s)

Which of the following option(s) of the pairs of cell organelles are correctly placed under the given categories? (i) Present in plant cells: Leucoplast | Absent in animal cells: Cell wall. (ii) Mitochondria | Ribosome. (iii) Cell wall | Golgi apparatus. (iv) Lysosome | Endoplasmic reticulum.
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๐Ÿ“‹ NCERT Question 5 โ€” Two students, Renu and Rohit

Two students, Renu and Rohit, were having a discussion on the plastids. Renu emphasised that all parts of the plants, even roots, contain plastids. However, Rohit did not agree with the statement and told her that plastids are absent in plant roots since the roots are underground and do not need to perform photosynthesis. Who is correct? Justify your answer.
View Answer โ†’

๐Ÿ“‹ NCERT Question 6 โ€” Mitochondria and chloroplasts are

Mitochondria and chloroplasts are two important organelles in a plant cell. Discuss how these two organelles are structurally and functionally similar to each other, and different from each other.
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๐Ÿ“‹ NCERT Question 7 โ€” Which of the following pairs

Which of the following pairs of cell organelles contains DNA? (i) Chloroplasts, Ribosomes (ii) Mitochondria, Nucleus (iii) Golgi bodies, Ribosomes (iv) Nucleus, Lysosomes
View Answer โ†’

๐Ÿ“‹ NCERT Question 11 โ€” Identify the pair that

Identify the pair that incorrectly matches the cell organelle with its function. (i) Ribosome โ€” Protein synthesis (ii) SER โ€” Lipid and cellulose synthesis (iii) Lysosome โ€” Digestion of foreign agents
View Answer โ†’

๐Ÿ“‹ NCERT Question 12 โ€” What outcome do you

What outcome do you expect, if all the mitochondria are removed from a eukaryotic cell?
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๐Ÿ“‹ NCERT Question 14 โ€” The cell membrane of

The cell membrane of a cell is made up of proteins and lipids. Which cell organelles help in the synthesis of cell membrane? Write the path of these compounds from their site of synthesis to the cell membrane and show this through a labelled diagram.
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