Why Every Cell Runs Like a Tiny Factory With Its Own Power Stations
Explore the nucleus, mitochondria and plastids, the endomembrane system, ribosomes and microbodies, the cytoskeleton, cilia, flagella and centrioles, and the key differences between prokaryotic and eukaryotic cells and between plant and animal cells.
How does a single cell divide up its work?
Inside every eukaryotic cell, different jobs happen in different compartments: instructions are stored in the nucleus, energy is released in mitochondria, proteins are built on ribosomes and packaged in the Golgi, and a protein scaffold holds everything in shape.
This lesson covers the main organelles, the cytoskeleton, and how cell types differ.
This lesson covers the main organelles, the cytoskeleton, and how cell types differ.
What are the structures and functions of the nucleus, mitochondria and plastids?
The nucleus stores genetic information and controls the cell, mitochondria carry out aerobic respiration to make ATP, and plastids in plant cells carry out photosynthesis, give colour or store food.
Nucleus:
- A nuclear envelope of two membranes, with nuclear pores for exchange with the cytoplasm
- Nucleoplasm containing the nucleolus, where ribosomal RNA is made, and chromatin, which condenses into chromosomes during division
Mitochondria:
- A double membrane, with the inner membrane folded into cristae that increase its surface area
- The matrix holds circular DNA, 70S ribosomes and the enzymes of the Krebs cycle
- The site of aerobic respiration, producing ATP
Plastids:
- Chloroplasts — contain chlorophyll; stacks of thylakoids called grana lie in a fluid stroma with circular DNA and 70S ribosomes
- Chromoplasts — hold fat-soluble carotenoid pigments that give yellow, orange or red colours
- Leucoplasts — colourless stores: amyloplasts for starch, elaioplasts for oils and fats, aleuroplasts for proteins
An everyday example. A potato slice turning blue-black with iodine reveals starch packed into amyloplasts, while a ripe tomato owes its red colour to chromoplasts.
The substance. Mitochondria and chloroplasts are semi-autonomous — they carry their own DNA and ribosomes and can make some of their own proteins.
Nucleus:
- A nuclear envelope of two membranes, with nuclear pores for exchange with the cytoplasm
- Nucleoplasm containing the nucleolus, where ribosomal RNA is made, and chromatin, which condenses into chromosomes during division
Mitochondria:
- A double membrane, with the inner membrane folded into cristae that increase its surface area
- The matrix holds circular DNA, 70S ribosomes and the enzymes of the Krebs cycle
- The site of aerobic respiration, producing ATP
Plastids:
- Chloroplasts — contain chlorophyll; stacks of thylakoids called grana lie in a fluid stroma with circular DNA and 70S ribosomes
- Chromoplasts — hold fat-soluble carotenoid pigments that give yellow, orange or red colours
- Leucoplasts — colourless stores: amyloplasts for starch, elaioplasts for oils and fats, aleuroplasts for proteins
An everyday example. A potato slice turning blue-black with iodine reveals starch packed into amyloplasts, while a ripe tomato owes its red colour to chromoplasts.
The substance. Mitochondria and chloroplasts are semi-autonomous — they carry their own DNA and ribosomes and can make some of their own proteins.
What makes up the endomembrane system, and what do ribosomes and microbodies do?
The endomembrane system — endoplasmic reticulum, Golgi apparatus, lysosomes and vacuoles — works together to make, modify, package and break down materials, while ribosomes build proteins and microbodies hold enzymes for special reactions.
Endoplasmic reticulum:
- Rough ER carries ribosomes and makes proteins, many of them for secretion
- Smooth ER lacks ribosomes and makes lipids, including steroid hormones in animal cells
Golgi apparatus:
- Stacks of flat, disc-shaped cisternae near the nucleus, with a receiving cis face and a releasing trans face
- Packages materials, and is the main site where glycoproteins and glycolipids are formed
Lysosomes:
- Membrane-bound vesicles formed from the Golgi, filled with hydrolytic enzymes that work best in acidic conditions
Vacuoles:
- Bounded by a membrane called the tonoplast; in plant cells one vacuole can fill most of the cell
Ribosomes:
- Granules of RNA and protein with no membrane
- 80S in the eukaryotic cytoplasm, made of 60S and 40S subunits; 70S in prokaryotes, mitochondria and chloroplasts, made of 50S and 30S subunits
Microbodies. Small membrane-bound vesicles holding enzymes, such as peroxisomes and glyoxysomes, found in both plant and animal cells.
An everyday example. Digesting a plate of dal and rice depends on enzymes from pancreatic cells, which are packed with rough ER and Golgi stacks that make and ship those proteins.
The substance. Lysosomes can digest the cell itself if their membrane bursts — which is why they are sometimes called suicidal bags.
Endoplasmic reticulum:
- Rough ER carries ribosomes and makes proteins, many of them for secretion
- Smooth ER lacks ribosomes and makes lipids, including steroid hormones in animal cells
Golgi apparatus:
- Stacks of flat, disc-shaped cisternae near the nucleus, with a receiving cis face and a releasing trans face
- Packages materials, and is the main site where glycoproteins and glycolipids are formed
Lysosomes:
- Membrane-bound vesicles formed from the Golgi, filled with hydrolytic enzymes that work best in acidic conditions
Vacuoles:
- Bounded by a membrane called the tonoplast; in plant cells one vacuole can fill most of the cell
Ribosomes:
- Granules of RNA and protein with no membrane
- 80S in the eukaryotic cytoplasm, made of 60S and 40S subunits; 70S in prokaryotes, mitochondria and chloroplasts, made of 50S and 30S subunits
Microbodies. Small membrane-bound vesicles holding enzymes, such as peroxisomes and glyoxysomes, found in both plant and animal cells.
An everyday example. Digesting a plate of dal and rice depends on enzymes from pancreatic cells, which are packed with rough ER and Golgi stacks that make and ship those proteins.
The substance. Lysosomes can digest the cell itself if their membrane bursts — which is why they are sometimes called suicidal bags.
What are the cytoskeleton, cilia, flagella and centrioles?
The cytoskeleton is a network of protein filaments — microtubules, microfilaments and intermediate filaments — that gives the cell shape and movement; cilia and flagella are hair-like projections for movement with a 9+2 arrangement of microtubules; and centrioles are cylinders of nine triplet microtubules that help form the spindle in animal cells.
Cytoskeleton:
- Provides mechanical support and maintains cell shape
- Allows motility and moves organelles and chromosomes within the cell
Cilia and flagella:
- Cilia are short and beat like oars; flagella are longer and move the cell with a whip-like action
- Both are covered by the plasma membrane, and their core, the axoneme, has nine doublets of microtubules around two central single microtubules — the 9+2 arrangement
- Both grow from a centriole-like basal body
Centrioles and centrosome:
- A centrosome holds two cylindrical centrioles lying at right angles to each other
- Each centriole has nine evenly spaced triplets of microtubules in a cartwheel pattern
- Centrioles form the basal bodies of cilia and flagella, and the spindle apparatus during animal cell division
An everyday example. Dust breathed in on a dusty road is swept up and out of the windpipe by countless beating cilia lining the airway.
The substance. Most higher plant cells have no centrioles yet still divide — they form a spindle without them, which shows centrioles are not essential for every spindle.
Cytoskeleton:
- Provides mechanical support and maintains cell shape
- Allows motility and moves organelles and chromosomes within the cell
Cilia and flagella:
- Cilia are short and beat like oars; flagella are longer and move the cell with a whip-like action
- Both are covered by the plasma membrane, and their core, the axoneme, has nine doublets of microtubules around two central single microtubules — the 9+2 arrangement
- Both grow from a centriole-like basal body
Centrioles and centrosome:
- A centrosome holds two cylindrical centrioles lying at right angles to each other
- Each centriole has nine evenly spaced triplets of microtubules in a cartwheel pattern
- Centrioles form the basal bodies of cilia and flagella, and the spindle apparatus during animal cell division
An everyday example. Dust breathed in on a dusty road is swept up and out of the windpipe by countless beating cilia lining the airway.
The substance. Most higher plant cells have no centrioles yet still divide — they form a spindle without them, which shows centrioles are not essential for every spindle.
How do prokaryotic and eukaryotic cells differ, and how are plant cells different from animal cells?
Prokaryotic cells lack a nuclear envelope and membrane-bound organelles and have 70S ribosomes, while eukaryotic cells have a true nucleus, membrane-bound organelles and 80S ribosomes; plant cells differ from animal cells in having a cell wall, plastids and a large central vacuole, but usually no centrioles.
Prokaryotic versus eukaryotic:
- Nucleus — a nucleoid without a membrane versus a true nucleus with an envelope
- DNA — one circular chromosome, often with plasmids, versus several linear chromosomes with histone proteins
- Organelles — no membrane-bound organelles versus mitochondria, ER, Golgi and others
- Ribosomes — 70S versus 80S in the cytoplasm
Plant versus animal cells:
- Cell wall — present, made of cellulose, versus absent
- Plastids — present versus absent
- Vacuole — one large central vacuole versus small, temporary vacuoles
- Centrioles — usually absent versus present
- Stored food — starch versus glycogen
An everyday example. The bacteria that set curd are prokaryotes, while the cells scraped from the inside of your cheek are eukaryotic, though both need a microscope to be seen.
The substance. Cyanobacteria photosynthesise without chloroplasts — their pigments sit on membrane infoldings called chromatophores, so a prokaryote can do the job of a eukaryote's chloroplast.
Prokaryotic versus eukaryotic:
- Nucleus — a nucleoid without a membrane versus a true nucleus with an envelope
- DNA — one circular chromosome, often with plasmids, versus several linear chromosomes with histone proteins
- Organelles — no membrane-bound organelles versus mitochondria, ER, Golgi and others
- Ribosomes — 70S versus 80S in the cytoplasm
Plant versus animal cells:
- Cell wall — present, made of cellulose, versus absent
- Plastids — present versus absent
- Vacuole — one large central vacuole versus small, temporary vacuoles
- Centrioles — usually absent versus present
- Stored food — starch versus glycogen
An everyday example. The bacteria that set curd are prokaryotes, while the cells scraped from the inside of your cheek are eukaryotic, though both need a microscope to be seen.
The substance. Cyanobacteria photosynthesise without chloroplasts — their pigments sit on membrane infoldings called chromatophores, so a prokaryote can do the job of a eukaryote's chloroplast.
Exam tip
What earns full marks on cell organelles?
Pair each organelle with one structure word and one function word — cristae with ATP, grana with photosynthesis, cisternae with packaging — and draw the 9+2 and cartwheel patterns carefully.
- Semi-autonomous: mitochondria and chloroplasts, with their own DNA and 70S ribosomes
- Cilia and flagella: 9+2; centrioles: nine triplets
The trap. Adding subunit values to get the ribosome value. 60S and 40S make 80S, not 100S, because S measures how fast a particle settles, not its mass.
- Semi-autonomous: mitochondria and chloroplasts, with their own DNA and 70S ribosomes
- Cilia and flagella: 9+2; centrioles: nine triplets
The trap. Adding subunit values to get the ribosome value. 60S and 40S make 80S, not 100S, because S measures how fast a particle settles, not its mass.
Did you know
Why does the DNA in your mitochondria come from your mother?
When a sperm fertilises an egg, it contributes its nucleus, but the mitochondria of the embryo come almost entirely from the egg. Any mitochondria from the sperm are usually broken down.
So the DNA inside every mitochondrion of a person is inherited through the mother's line, passed on unchanged except for occasional mutations.
Because this DNA carries its own genes and multiplies inside the cell, it also supports the idea that mitochondria descend from free-living bacteria that came to live inside larger cells.
So the DNA inside every mitochondrion of a person is inherited through the mother's line, passed on unchanged except for occasional mutations.
Because this DNA carries its own genes and multiplies inside the cell, it also supports the idea that mitochondria descend from free-living bacteria that came to live inside larger cells.
Exam relevance
How does NEET test cell organelles and cell types?
Cell: The Unit of Life is a recurring NEET chapter, and its questions on organelles are highly detail-based.
What gets asked. Structure and function of mitochondria, plastids and the Golgi, the endomembrane system and what it leaves out, ribosome types and subunits, andprokaryotic versus eukaryotic features.
Question types. Mostly statement-based and match-the-column questions, often mixing several organelles in one list.
Why it matters later. Mitochondria and chloroplasts return in Respiration in Plants and Photosynthesis in Higher Plants, and ribosomes in Molecular Basis of Inheritance.
The trap that costs marks. Including mitochondria, chloroplasts or peroxisomes in the endomembrane system — their functions are not coordinated with the ER, Golgi and lysosomes.
What gets asked. Structure and function of mitochondria, plastids and the Golgi, the endomembrane system and what it leaves out, ribosome types and subunits, andprokaryotic versus eukaryotic features.
Question types. Mostly statement-based and match-the-column questions, often mixing several organelles in one list.
Why it matters later. Mitochondria and chloroplasts return in Respiration in Plants and Photosynthesis in Higher Plants, and ribosomes in Molecular Basis of Inheritance.
The trap that costs marks. Including mitochondria, chloroplasts or peroxisomes in the endomembrane system — their functions are not coordinated with the ER, Golgi and lysosomes.
Key takeaways
What must you be able to do from this lesson?
- Nucleus, mitochondria and plastids: a nuclear envelope with pores; cristae and ATP; chloroplasts, chromoplasts and leucoplasts
- Endomembrane system, ribosomes and microbodies: ER, Golgi, lysosomes and vacuoles; 80S and 70S ribosomes; peroxisomes and glyoxysomes
- Cytoskeleton and movement: filaments and microtubules; 9+2 cilia and flagella; centrioles of nine triplets
- Cell types: prokaryotic versus eukaryotic, and plant versus animal
Which two organelles have their own DNA and 70S ribosomes — and what does that suggest about their origin?
- Endomembrane system, ribosomes and microbodies: ER, Golgi, lysosomes and vacuoles; 80S and 70S ribosomes; peroxisomes and glyoxysomes
- Cytoskeleton and movement: filaments and microtubules; 9+2 cilia and flagella; centrioles of nine triplets
- Cell types: prokaryotic versus eukaryotic, and plant versus animal
Which two organelles have their own DNA and 70S ribosomes — and what does that suggest about their origin?