A Bag of Jelly That Runs Every Process Keeping You Alive
State the cell theory and what protoplasm is, learn the structure and job of every organelle from nucleus to vacuole, compare the cell wall with the cell membrane, and separate prokaryotic from eukaryotic cells.
What makes a cell the smallest thing that can be called alive?
Break a machine into parts and none of the parts works. Break a living body into cells and each cell keeps living. It takes in food, releases energy, gets rid of waste, grows and divides.
Go one step smaller — break a cell open — and nothing survives. The pieces are chemicals, and they do nothing on their own.
So the cell is the smallest unit that still does everything living things do, and that is a strong enough claim to be worth stating as a theory.
Inside a cell the work is divided. A nucleus keeps the instructions and issues the orders; mitochondria release the energy; ribosomes build the proteins; a Golgi body packages them for despatch; lysosomes dispose of the rubbish. Each of those is an organelle — a little organ, doing one job inside one cell.
The reason to learn the organelles one by one is that a cell is a division of labour, not a bag of soup, and almost every question on this chapter is really asking which part does which job.
This page covers the first part of the ICSE Class 9 Biology chapter on basic biology: the cell theory and protoplasm, the structure and function of every organelle, the two cell boundaries, and the comparisons between prokaryotic and eukaryotic cells and between plant and animal cells.
Go one step smaller — break a cell open — and nothing survives. The pieces are chemicals, and they do nothing on their own.
So the cell is the smallest unit that still does everything living things do, and that is a strong enough claim to be worth stating as a theory.
Inside a cell the work is divided. A nucleus keeps the instructions and issues the orders; mitochondria release the energy; ribosomes build the proteins; a Golgi body packages them for despatch; lysosomes dispose of the rubbish. Each of those is an organelle — a little organ, doing one job inside one cell.
The reason to learn the organelles one by one is that a cell is a division of labour, not a bag of soup, and almost every question on this chapter is really asking which part does which job.
This page covers the first part of the ICSE Class 9 Biology chapter on basic biology: the cell theory and protoplasm, the structure and function of every organelle, the two cell boundaries, and the comparisons between prokaryotic and eukaryotic cells and between plant and animal cells.
What does the cell theory state, and what exactly is protoplasm?
The cell theory states three things.
- All living organisms are made of cells and the products of cells
- The cell is the basic structural and functional unit of life — the smallest unit that both builds the body and carries out its activities
- All cells arise from pre-existing cells by division; no cell forms from non-living material
Protoplasm is the living substance of the cell — the entire living content inside the cell membrane.
It has two parts:
- Cytoplasm — the protoplasm outside the nucleus, in which the organelles lie
- Nucleoplasm — the protoplasm inside the nucleus
What it is like. Protoplasm is a semi-fluid, jelly-like, granular and transparent substance. It is not a simple solution but a complex mixture — partly a colloid, partly a true solution.
What it is made of. Chiefly water, which makes up by far the largest part; and then proteins, carbohydrates, fats, nucleic acids and dissolved mineral salts.
What it does. Every life process happens in it — nutrition, respiration, excretion, growth, movement, sensitivity and reproduction. For that reason protoplasm is described as the physical basis of life.
Now a genuine exception to the first statement of the theory, which is worth knowing. A virus is not made of cells at all. It is a piece of nucleic acid in a protein coat, it has no protoplasm, no organelles and no membrane of its own, and it cannot carry out a single life process on its own. It reproduces only inside a host cell.
So a virus sits on the boundary of the theory rather than inside it — alive when it is in a host, inert when it is not. A question asking whether the cell theory has exceptions wants the virus, and it wants the reason: no cells, and no independent life processes.
One more point about the third statement. "All cells arise from pre-existing cells" rules out living things appearing from non-living matter, and it also means that every cell in your body traces back through an unbroken line of divisions to a single fertilised egg. The statement is a claim about continuity, and it is the reason cell division is the next topic in every syllabus that teaches this one.
- All living organisms are made of cells and the products of cells
- The cell is the basic structural and functional unit of life — the smallest unit that both builds the body and carries out its activities
- All cells arise from pre-existing cells by division; no cell forms from non-living material
Protoplasm is the living substance of the cell — the entire living content inside the cell membrane.
It has two parts:
- Cytoplasm — the protoplasm outside the nucleus, in which the organelles lie
- Nucleoplasm — the protoplasm inside the nucleus
What it is like. Protoplasm is a semi-fluid, jelly-like, granular and transparent substance. It is not a simple solution but a complex mixture — partly a colloid, partly a true solution.
What it is made of. Chiefly water, which makes up by far the largest part; and then proteins, carbohydrates, fats, nucleic acids and dissolved mineral salts.
What it does. Every life process happens in it — nutrition, respiration, excretion, growth, movement, sensitivity and reproduction. For that reason protoplasm is described as the physical basis of life.
Now a genuine exception to the first statement of the theory, which is worth knowing. A virus is not made of cells at all. It is a piece of nucleic acid in a protein coat, it has no protoplasm, no organelles and no membrane of its own, and it cannot carry out a single life process on its own. It reproduces only inside a host cell.
So a virus sits on the boundary of the theory rather than inside it — alive when it is in a host, inert when it is not. A question asking whether the cell theory has exceptions wants the virus, and it wants the reason: no cells, and no independent life processes.
One more point about the third statement. "All cells arise from pre-existing cells" rules out living things appearing from non-living matter, and it also means that every cell in your body traces back through an unbroken line of divisions to a single fertilised egg. The statement is a claim about continuity, and it is the reason cell division is the next topic in every syllabus that teaches this one.
Which organelle does which job inside the cell?
Each organelle has a structure suited to one specific job, and the commonest exam question is to match the two.
Nucleus — a large spherical body bounded by a double nuclear membrane with nuclear pores through which materials pass. Inside is the nucleoplasm, a network of chromatin made of DNA and protein, and the nucleolus. During cell division the chromatin condenses into chromosomes. Function: it controls all the activities of the cell and carries the hereditary material that is passed to the next generation.
Nucleolus — a dense, darkly staining spherical body inside the nucleus, with no membrane of its own. Function: it makes ribosomes.
Mitochondria — rod-shaped or oval bodies with a double membrane; the inner membrane is folded inwards into finger-like cristae, and the space within is the matrix. The cristae carry the enzymes of respiration. Function: the site of aerobic respiration, releasing energy and storing it as ATP — which is why mitochondria are called the powerhouse of the cell. They have their own DNA and can divide.
Endoplasmic reticulum — a network of membranous tubes, sacs and sheets running through the cytoplasm and continuous with the nuclear membrane. Rough ER is studded with ribosomes and handles protein transport; smooth ER has none and makes lipids and steroids. Function: the internal transport system of the cell, and mechanical support for the cytoplasm.
Ribosomes — extremely small granular bodies with no membrane, made of RNA and protein. They lie free in the cytoplasm or attached to the rough ER. Function: the site of protein synthesis.
Golgi bodies — a stack of flattened membranous sacs with small vesicles at the edges, called dictyosomes in plant cells. Function: they modify, package and despatch the substances made in the endoplasmic reticulum, secrete enzymes and hormones, and form lysosomes.
Plastids — found only in plant cells. Three kinds: chloroplasts, green with chlorophyll, which carry out photosynthesis; chromoplasts, of other colours, which give flowers and fruits their colour and so help attract pollinators and dispersal agents; and leucoplasts, colourless, which store starch, oil and protein.
Lysosomes — small sacs bounded by a single membrane and filled with powerful digestive enzymes. Function: they digest worn-out organelles and any foreign particle that enters. If the cell is injured they can burst and digest the entire cell, which is why they are called suicidal bags.
Centrosome — found only in animal cells; it contains two rod-like centrioles at right angles. Function: it forms the spindle fibres that separate the chromosomes during cell division.
Vacuole — a fluid-filled sac bounded by a membrane called the tonoplast. In a plant cell there is usually one large central vacuole occupying most of the cell, filled with sap. In an animal cell the vacuoles are small and numerous. Function: it stores water, food and waste, and in plants it gives the cell turgidity, keeping the cell firm and the plant upright.
Notice the two organelles with their own DNA. Mitochondria and chloroplasts both have a double membrane and their own genetic material, and both can divide independently of the cell. Both are also the cell's energy organelles — one releases energy from food, the other captures it from light. That shared set of features is not a coincidence, and a question asking which organelles are semi-autonomous wants exactly those two.
Nucleus — a large spherical body bounded by a double nuclear membrane with nuclear pores through which materials pass. Inside is the nucleoplasm, a network of chromatin made of DNA and protein, and the nucleolus. During cell division the chromatin condenses into chromosomes. Function: it controls all the activities of the cell and carries the hereditary material that is passed to the next generation.
Nucleolus — a dense, darkly staining spherical body inside the nucleus, with no membrane of its own. Function: it makes ribosomes.
Mitochondria — rod-shaped or oval bodies with a double membrane; the inner membrane is folded inwards into finger-like cristae, and the space within is the matrix. The cristae carry the enzymes of respiration. Function: the site of aerobic respiration, releasing energy and storing it as ATP — which is why mitochondria are called the powerhouse of the cell. They have their own DNA and can divide.
Endoplasmic reticulum — a network of membranous tubes, sacs and sheets running through the cytoplasm and continuous with the nuclear membrane. Rough ER is studded with ribosomes and handles protein transport; smooth ER has none and makes lipids and steroids. Function: the internal transport system of the cell, and mechanical support for the cytoplasm.
Ribosomes — extremely small granular bodies with no membrane, made of RNA and protein. They lie free in the cytoplasm or attached to the rough ER. Function: the site of protein synthesis.
Golgi bodies — a stack of flattened membranous sacs with small vesicles at the edges, called dictyosomes in plant cells. Function: they modify, package and despatch the substances made in the endoplasmic reticulum, secrete enzymes and hormones, and form lysosomes.
Plastids — found only in plant cells. Three kinds: chloroplasts, green with chlorophyll, which carry out photosynthesis; chromoplasts, of other colours, which give flowers and fruits their colour and so help attract pollinators and dispersal agents; and leucoplasts, colourless, which store starch, oil and protein.
Lysosomes — small sacs bounded by a single membrane and filled with powerful digestive enzymes. Function: they digest worn-out organelles and any foreign particle that enters. If the cell is injured they can burst and digest the entire cell, which is why they are called suicidal bags.
Centrosome — found only in animal cells; it contains two rod-like centrioles at right angles. Function: it forms the spindle fibres that separate the chromosomes during cell division.
Vacuole — a fluid-filled sac bounded by a membrane called the tonoplast. In a plant cell there is usually one large central vacuole occupying most of the cell, filled with sap. In an animal cell the vacuoles are small and numerous. Function: it stores water, food and waste, and in plants it gives the cell turgidity, keeping the cell firm and the plant upright.
Notice the two organelles with their own DNA. Mitochondria and chloroplasts both have a double membrane and their own genetic material, and both can divide independently of the cell. Both are also the cell's energy organelles — one releases energy from food, the other captures it from light. That shared set of features is not a coincidence, and a question asking which organelles are semi-autonomous wants exactly those two.
How does the cell wall differ from the cell membrane?
The cell wall is non-living, rigid and freely permeable; the cell membrane is living, flexible and selectively permeable.
Cell wall.
- Present only in plant cells, and in bacteria and fungi. Animal cells have none
- It is the outermost layer, lying outside the cell membrane
- Non-living and rigid, made chiefly of cellulose
- Freely permeable — water and dissolved substances pass through it almost unhindered
- It has pits and fine strands connecting neighbouring cells
- Functions: gives the cell its definite shape and rigidity, protects the contents mechanically, gives the plant support, and prevents the cell bursting when it absorbs a great deal of water
Cell membrane — also called the plasma membrane or plasmalemma.
- Present in all cells, plant and animal alike
- It lies just inside the cell wall in a plant cell, and is the outermost boundary in an animal cell
- Living, extremely thin, elastic and flexible, made of lipids and proteins
- Selectively permeable — it allows some substances through and holds others back
- Functions: it encloses the protoplasm, and it controls what enters and leaves the cell. Its flexibility also allows an animal cell to change shape and to engulf particles
The difference in permeability is the examinable point, and it decides which structure can explain osmosis. Osmosis is the movement of water through a selectively permeable membrane. The cell wall lets almost everything through, so it cannot select anything and cannot control anything.
So the cell wall is a fence and the cell membrane is the gate. A question about what a cell takes in or keeps out must be answered on the membrane, and answering it on the cell wall is the standard error in this topic.
Why the wall matters all the same. Put a plant cell in pure water and it absorbs water until it is swollen and firm — turgid — and the rigid wall stops it going further. Put an animal cell in the same water and it swells and bursts, because nothing holds it. A red blood cell placed in distilled water is destroyed, and a plant cell in distilled water simply becomes firm, and the whole difference is the presence of a cell wall.
That is also why plants can stand up without a skeleton. Millions of turgid cells, each held in shape by its own rigid wall, together support a stem — which is why a plant deprived of water wilts, its cells losing turgidity, and recovers when watered.
Cell wall.
- Present only in plant cells, and in bacteria and fungi. Animal cells have none
- It is the outermost layer, lying outside the cell membrane
- Non-living and rigid, made chiefly of cellulose
- Freely permeable — water and dissolved substances pass through it almost unhindered
- It has pits and fine strands connecting neighbouring cells
- Functions: gives the cell its definite shape and rigidity, protects the contents mechanically, gives the plant support, and prevents the cell bursting when it absorbs a great deal of water
Cell membrane — also called the plasma membrane or plasmalemma.
- Present in all cells, plant and animal alike
- It lies just inside the cell wall in a plant cell, and is the outermost boundary in an animal cell
- Living, extremely thin, elastic and flexible, made of lipids and proteins
- Selectively permeable — it allows some substances through and holds others back
- Functions: it encloses the protoplasm, and it controls what enters and leaves the cell. Its flexibility also allows an animal cell to change shape and to engulf particles
The difference in permeability is the examinable point, and it decides which structure can explain osmosis. Osmosis is the movement of water through a selectively permeable membrane. The cell wall lets almost everything through, so it cannot select anything and cannot control anything.
So the cell wall is a fence and the cell membrane is the gate. A question about what a cell takes in or keeps out must be answered on the membrane, and answering it on the cell wall is the standard error in this topic.
Why the wall matters all the same. Put a plant cell in pure water and it absorbs water until it is swollen and firm — turgid — and the rigid wall stops it going further. Put an animal cell in the same water and it swells and bursts, because nothing holds it. A red blood cell placed in distilled water is destroyed, and a plant cell in distilled water simply becomes firm, and the whole difference is the presence of a cell wall.
That is also why plants can stand up without a skeleton. Millions of turgid cells, each held in shape by its own rigid wall, together support a stem — which is why a plant deprived of water wilts, its cells losing turgidity, and recovers when watered.
How do you tell a prokaryotic cell from a eukaryotic one, and a plant cell from an animal cell?
A prokaryotic cell has no true nucleus and no membrane-bound organelles; a eukaryotic cell has both.
Prokaryotic cell.
- No true nucleus — the DNA lies free in the cytoplasm as a nucleoid, with no nuclear membrane around it
- No membrane-bound organelles — no mitochondria, no endoplasmic reticulum, no Golgi bodies, no plastids, no lysosomes
- A single circular DNA molecule, not associated with histone proteins
- Ribosomes are present, of the smaller kind
- Smaller in size, generally a few micrometres
- Divides by simple binary fission — no mitosis or meiosis
- Examples: bacteria and blue-green algae
Eukaryotic cell.
- A true nucleus enclosed in a nuclear membrane
- Membrane-bound organelles present
- DNA associated with proteins and organised into chromosomes
- Ribosomes of the larger kind
- Larger in size, generally tens of micrometres
- Divides by mitosis and meiosis
- Examples: all plants, animals, fungi and protists
So the name is the definition. Karyon means nucleus — a prokaryote is a cell from before the nucleus, a eukaryote a cell with a proper one. Everything else on the list follows from that one difference, because the organelles are all membrane-bound and a cell that cannot build a nuclear membrane builds none of the others either.
Now the plant cell against the animal cell. Both are eukaryotic, and four structures separate them.
- Cell wall — present in a plant cell, absent in an animal cell
- Centrosome — absent in a plant cell, present in an animal cell
- Vacuole — one large central vacuole in a plant cell, small and numerous in an animal cell
- Plastids — present in a plant cell, absent in an animal cell
Two further differences are worth adding: a plant cell has a fixed shape while an animal cell is more flexible, and stored carbohydrate is starch in a plant cell and glycogen in an animal cell.
Every one of the four differences is explained by how the two live. A plant is fixed in one place and makes its own food, so it needs plastids to make it, a rigid wall to hold it up without a skeleton, and a large vacuole to give the cell bulk cheaply with water instead of expensive protoplasm. An animal moves, changes shape and hunts for food, so a rigid wall would be a liability and plastids would be useless.
So the list is not four unrelated facts but one way of life against another, and reasoning from that is far more reliable under exam pressure than memorising the list. A cell with a wall and chloroplasts is a plant cell; a cell with a centrosome and no wall is an animal cell — and the vacuole settles the doubtful cases.
Prokaryotic cell.
- No true nucleus — the DNA lies free in the cytoplasm as a nucleoid, with no nuclear membrane around it
- No membrane-bound organelles — no mitochondria, no endoplasmic reticulum, no Golgi bodies, no plastids, no lysosomes
- A single circular DNA molecule, not associated with histone proteins
- Ribosomes are present, of the smaller kind
- Smaller in size, generally a few micrometres
- Divides by simple binary fission — no mitosis or meiosis
- Examples: bacteria and blue-green algae
Eukaryotic cell.
- A true nucleus enclosed in a nuclear membrane
- Membrane-bound organelles present
- DNA associated with proteins and organised into chromosomes
- Ribosomes of the larger kind
- Larger in size, generally tens of micrometres
- Divides by mitosis and meiosis
- Examples: all plants, animals, fungi and protists
So the name is the definition. Karyon means nucleus — a prokaryote is a cell from before the nucleus, a eukaryote a cell with a proper one. Everything else on the list follows from that one difference, because the organelles are all membrane-bound and a cell that cannot build a nuclear membrane builds none of the others either.
Now the plant cell against the animal cell. Both are eukaryotic, and four structures separate them.
- Cell wall — present in a plant cell, absent in an animal cell
- Centrosome — absent in a plant cell, present in an animal cell
- Vacuole — one large central vacuole in a plant cell, small and numerous in an animal cell
- Plastids — present in a plant cell, absent in an animal cell
Two further differences are worth adding: a plant cell has a fixed shape while an animal cell is more flexible, and stored carbohydrate is starch in a plant cell and glycogen in an animal cell.
Every one of the four differences is explained by how the two live. A plant is fixed in one place and makes its own food, so it needs plastids to make it, a rigid wall to hold it up without a skeleton, and a large vacuole to give the cell bulk cheaply with water instead of expensive protoplasm. An animal moves, changes shape and hunts for food, so a rigid wall would be a liability and plastids would be useless.
So the list is not four unrelated facts but one way of life against another, and reasoning from that is far more reliable under exam pressure than memorising the list. A cell with a wall and chloroplasts is a plant cell; a cell with a centrosome and no wall is an animal cell — and the vacuole settles the doubtful cases.
Exam tip
Exam tip: match the organelle to the job, and give both halves of every comparison
State the cell theory as three numbered points, and add the virus if asked for an exception — not made of cells, and cannot live independently.
Define protoplasm as cytoplasm PLUS nucleoplasm, and call it the physical basis of life.
For every organelle give structure AND function. A function alone rarely earns full marks.
Learn the one-line identifiers: nucleus controls and carries heredity; nucleolus makes ribosomes; mitochondria are the powerhouse and make ATP; ER is the transport system; ribosomes make protein; Golgi bodies package and despatch and form lysosomes; chloroplasts do photosynthesis; lysosomes are the suicidal bags; centrosome forms the spindle; vacuole gives turgidity.
Mitochondria and chloroplasts both have a double membrane and their own DNA.
Plastids and the centrosome are the exclusive ones — plastids only in plant cells, centrosome only in animal cells.
The cell wall is non-living, cellulose and FREELY permeable; the cell membrane is living, lipid and protein, and SELECTIVELY permeable. Osmosis must be explained on the membrane.
Use the bursting comparison: a plant cell in pure water becomes turgid, an animal cell bursts — because only one has a wall.
Prokaryote means no true nucleus and no membrane-bound organelles. Name bacteria and blue-green algae.
Learn the plant-animal differences as four pairs — cell wall, centrosome, vacuole, plastids — and always give both sides of each.
And in a comparison question, name the feature and then both organisms — "cell wall: present in plant, absent in animal" scores where "plants have a cell wall" does not.
Define protoplasm as cytoplasm PLUS nucleoplasm, and call it the physical basis of life.
For every organelle give structure AND function. A function alone rarely earns full marks.
Learn the one-line identifiers: nucleus controls and carries heredity; nucleolus makes ribosomes; mitochondria are the powerhouse and make ATP; ER is the transport system; ribosomes make protein; Golgi bodies package and despatch and form lysosomes; chloroplasts do photosynthesis; lysosomes are the suicidal bags; centrosome forms the spindle; vacuole gives turgidity.
Mitochondria and chloroplasts both have a double membrane and their own DNA.
Plastids and the centrosome are the exclusive ones — plastids only in plant cells, centrosome only in animal cells.
The cell wall is non-living, cellulose and FREELY permeable; the cell membrane is living, lipid and protein, and SELECTIVELY permeable. Osmosis must be explained on the membrane.
Use the bursting comparison: a plant cell in pure water becomes turgid, an animal cell bursts — because only one has a wall.
Prokaryote means no true nucleus and no membrane-bound organelles. Name bacteria and blue-green algae.
Learn the plant-animal differences as four pairs — cell wall, centrosome, vacuole, plastids — and always give both sides of each.
And in a comparison question, name the feature and then both organisms — "cell wall: present in plant, absent in animal" scores where "plants have a cell wall" does not.
Did you know
Why a plant cell is mostly water and not mostly protoplasm
Look at a diagram of a plant cell and something odd stands out. The nucleus is small, the organelles are crowded into a thin layer around the edge, and the middle of the cell is one enormous blank space — the vacuole.
That space is most of the cell's volume, and it is essentially water with some dissolved salts and sugars. It is not protoplasm and it does not run any process.
So the largest part of a plant cell is doing nothing metabolically at all. Why build it?
Because a plant has to hold itself up, and protoplasm is expensive. Every bit of it is proteins, nucleic acids and lipids, and each of those costs the plant energy and nitrogen and carbon to make. Filling a whole cell with protoplasm just to make it big would be an enormous waste.
Water is free. It is pumped in from the soil, it fills the vacuole, and it presses outwards against the rigid cellulose wall. The wall cannot stretch, so the cell becomes firm — turgid — and a stack of turgid cells is stiff enough to hold up a stem and spread a leaf into the light.
So the plant has found a way to be large and rigid at a very low cost: build a thin shell of living material, wrap a strong wall around it, and inflate the middle with water.
You can watch the arrangement fail and recover. A plant left unwatered wilts — the vacuoles lose water, the cells stop pressing on their walls, and the whole plant goes limp although not a single cell has died. Water it and within hours it stands up again. Nothing was repaired; the cells were simply re-inflated.
A wilted plant is a hydraulic structure with the pressure let out, and the reason an animal cannot do the same thing is the reason it needs bones instead.
That space is most of the cell's volume, and it is essentially water with some dissolved salts and sugars. It is not protoplasm and it does not run any process.
So the largest part of a plant cell is doing nothing metabolically at all. Why build it?
Because a plant has to hold itself up, and protoplasm is expensive. Every bit of it is proteins, nucleic acids and lipids, and each of those costs the plant energy and nitrogen and carbon to make. Filling a whole cell with protoplasm just to make it big would be an enormous waste.
Water is free. It is pumped in from the soil, it fills the vacuole, and it presses outwards against the rigid cellulose wall. The wall cannot stretch, so the cell becomes firm — turgid — and a stack of turgid cells is stiff enough to hold up a stem and spread a leaf into the light.
So the plant has found a way to be large and rigid at a very low cost: build a thin shell of living material, wrap a strong wall around it, and inflate the middle with water.
You can watch the arrangement fail and recover. A plant left unwatered wilts — the vacuoles lose water, the cells stop pressing on their walls, and the whole plant goes limp although not a single cell has died. Water it and within hours it stands up again. Nothing was repaired; the cells were simply re-inflated.
A wilted plant is a hydraulic structure with the pressure let out, and the reason an animal cannot do the same thing is the reason it needs bones instead.
Exam relevance
Why does NEET keep returning to cell organelles?
Because Cell: The Unit of Life is a Class 11 chapter examined almost entirely as recall, and the organelle list is the recall.
This is the foundation for Class 11 Biology Cell: The Unit of Life and Biomolecules, examined in NEET. That chapter repeats every organelle on this page in greater detail and adds the reasoning: the fluid mosaic model of the cell membrane, the structure of the nuclear pore complex, the difference between the two subunits of a ribosome, and the cis and trans faces of the Golgi apparatus. Match-the-column questions pairing an organelle with its function are one of the most predictable NEET types, and the list here is exactly what they draw on.
The membrane becomes the whole of transport. Class 11 Transport in Plants and Class 11 Biology's cell chapter build diffusion, osmosis, facilitated diffusion and active transport on the selective permeability described here. Water potential and plasmolysis are examined as numericals and as diagram questions, and every one of them depends on the membrane being selective while the cell wall is not — the distinction drawn on this page.
Mitochondria and chloroplasts are examined together. Class 11 Respiration in Plants and Photosynthesis in Higher Plants treat them as the sites of ATP synthesis, with the cristae and the thylakoids as the membranes that carry the electron transport chains. Their double membrane, their own DNA and their ability to divide are asked directly as the evidence for their semi-autonomous nature, so the pairing noted above is itself examinable.
The prokaryote-eukaryote comparison is a standing NEET item. Class 11 adds the bacterial cell envelope, mesosome, plasmid and flagellum, and the ribosome sizes are given as 70S against 80S. Assertion-reason questions on prokaryotic features are common, and so are questions asking which organelle a bacterium lacks.
The cell theory and its exception are examined as short recall. Class 11 states the theory and discusses the virus and the viroid as exceptions. The reason a virus is excluded — no cells and no independent metabolism — is the expected answer.
Biomolecules supplies what protoplasm is made of. Class 11 Biomolecules treats the water, proteins, carbohydrates, lipids and nucleic acids listed here as separate topics, with structures and classifications. So "what is protoplasm made of" becomes five chapters, and lysosomal enzymes reappear there as hydrolases.
For Class 12, the nucleus and its chromatin lead directly into Molecular Basis of Inheritance — DNA packaging, histones and the nucleosome — and the centrosome's spindle leads into Cell Cycle and Cell Division. Both are heavily examined in NEET.
What the questions look like. For board work, expect state the cell theory, define protoplasm, give the structure and function of a named organelle, distinguish the cell wall from the cell membrane, distinguish prokaryotic from eukaryotic cells, and give four differences between a plant and an animal cell. Diagrams of the cell with labels are standard. For NEET, expect organelle-function matching, prokaryotic feature identification, membrane transport and assertion-reason items.
How board and competitive emphasis differ. A board paper rewards a labelled diagram and a structure-plus-function answer. A competitive paper assumes both and asks which organelle is absent from a stated cell, or which is single-membraned rather than double.
The single trap that costs the most marks. Explaining what a cell absorbs or excludes by the cell wall. The wall is freely permeable and selects nothing — the cell membrane is the selective barrier, and every question about entry, exit, osmosis or plasmolysis is a question about the membrane. The defence is to ask which structure could possibly refuse something entry, because only one of the two can, and naming the wall gives away that you have not grasped the difference.
This is the foundation for Class 11 Biology Cell: The Unit of Life and Biomolecules, examined in NEET. That chapter repeats every organelle on this page in greater detail and adds the reasoning: the fluid mosaic model of the cell membrane, the structure of the nuclear pore complex, the difference between the two subunits of a ribosome, and the cis and trans faces of the Golgi apparatus. Match-the-column questions pairing an organelle with its function are one of the most predictable NEET types, and the list here is exactly what they draw on.
The membrane becomes the whole of transport. Class 11 Transport in Plants and Class 11 Biology's cell chapter build diffusion, osmosis, facilitated diffusion and active transport on the selective permeability described here. Water potential and plasmolysis are examined as numericals and as diagram questions, and every one of them depends on the membrane being selective while the cell wall is not — the distinction drawn on this page.
Mitochondria and chloroplasts are examined together. Class 11 Respiration in Plants and Photosynthesis in Higher Plants treat them as the sites of ATP synthesis, with the cristae and the thylakoids as the membranes that carry the electron transport chains. Their double membrane, their own DNA and their ability to divide are asked directly as the evidence for their semi-autonomous nature, so the pairing noted above is itself examinable.
The prokaryote-eukaryote comparison is a standing NEET item. Class 11 adds the bacterial cell envelope, mesosome, plasmid and flagellum, and the ribosome sizes are given as 70S against 80S. Assertion-reason questions on prokaryotic features are common, and so are questions asking which organelle a bacterium lacks.
The cell theory and its exception are examined as short recall. Class 11 states the theory and discusses the virus and the viroid as exceptions. The reason a virus is excluded — no cells and no independent metabolism — is the expected answer.
Biomolecules supplies what protoplasm is made of. Class 11 Biomolecules treats the water, proteins, carbohydrates, lipids and nucleic acids listed here as separate topics, with structures and classifications. So "what is protoplasm made of" becomes five chapters, and lysosomal enzymes reappear there as hydrolases.
For Class 12, the nucleus and its chromatin lead directly into Molecular Basis of Inheritance — DNA packaging, histones and the nucleosome — and the centrosome's spindle leads into Cell Cycle and Cell Division. Both are heavily examined in NEET.
What the questions look like. For board work, expect state the cell theory, define protoplasm, give the structure and function of a named organelle, distinguish the cell wall from the cell membrane, distinguish prokaryotic from eukaryotic cells, and give four differences between a plant and an animal cell. Diagrams of the cell with labels are standard. For NEET, expect organelle-function matching, prokaryotic feature identification, membrane transport and assertion-reason items.
How board and competitive emphasis differ. A board paper rewards a labelled diagram and a structure-plus-function answer. A competitive paper assumes both and asks which organelle is absent from a stated cell, or which is single-membraned rather than double.
The single trap that costs the most marks. Explaining what a cell absorbs or excludes by the cell wall. The wall is freely permeable and selects nothing — the cell membrane is the selective barrier, and every question about entry, exit, osmosis or plasmolysis is a question about the membrane. The defence is to ask which structure could possibly refuse something entry, because only one of the two can, and naming the wall gives away that you have not grasped the difference.
Key takeaways
The cell, its organelles and its boundaries: quick revision
- Cell theory: all organisms are made of cells and their products; the cell is the basic structural and functional unit of life; all cells arise from pre-existing cells.
- Exception: a virus — not made of cells, no protoplasm, and no independent life processes.
- Protoplasm = cytoplasm (outside the nucleus) + nucleoplasm (inside it). Semi-fluid, jelly-like, mostly water, with proteins, carbohydrates, fats, nucleic acids and mineral salts. Called the physical basis of life.
- Nucleus — double nuclear membrane with pores, nucleoplasm, chromatin (becomes chromosomes) and the nucleolus. Controls the cell and carries heredity.
- Nucleolus — dense body in the nucleus; makes ribosomes.
- Mitochondria — double membrane, inner folded into cristae, inner space the matrix. Site of aerobic respiration, makes ATP — the powerhouse. Has its own DNA.
- Endoplasmic reticulum — network continuous with the nuclear membrane. Rough (with ribosomes) transports protein; smooth makes lipids. The cell's transport system and support.
- Ribosomes — tiny, no membrane, RNA and protein; site of protein synthesis.
- Golgi bodies (dictyosomes in plants) — stacked sacs; modify, package and despatch, secrete enzymes and hormones, and form lysosomes.
- Plastids — plant cells only. Chloroplasts (chlorophyll, photosynthesis), chromoplasts (colour), leucoplasts (storage).
- Lysosomes — enzyme sacs; digest worn-out organelles and foreign matter; the suicidal bags.
- Centrosome — animal cells only; two centrioles; forms the spindle fibres.
- Vacuole — bounded by the tonoplast. One large central one in a plant cell giving turgidity; small and many in an animal cell.
- Mitochondria and chloroplasts both have a double membrane, their own DNA, and can divide — the two semi-autonomous organelles.
- Cell wall — plant cells only, outermost, non-living, cellulose, freely permeable; gives shape and rigidity and prevents bursting.
- Cell membrane — in all cells, living, lipids and proteins, selectively permeable; encloses the protoplasm and controls entry and exit.
- Osmosis must be explained on the MEMBRANE — the wall selects nothing.
- A plant cell in pure water becomes turgid; an animal cell bursts — the difference is the wall.
- Prokaryotic — no true nucleus (a free nucleoid), no membrane-bound organelles, circular DNA, smaller ribosomes, binary fission. Bacteria, blue-green algae.
- Eukaryotic — true nucleus, membrane-bound organelles, chromosomes, larger ribosomes, mitosis and meiosis. Plants, animals, fungi, protists.
- Plant against animal cell: cell wall present / absent; centrosome absent / present; vacuole one large central / small and many; plastids present / absent. Plus fixed / flexible shape, and starch / glycogen.
- The four differences follow from how each lives — a fixed, food-making organism needs plastids, a rigid wall and a cheap water-filled bulk; a moving one does not.
Cover the organelle list and name the job of each in one word, then say which two share a double membrane and their own DNA — if both come out, this chapter is secure.
- Exception: a virus — not made of cells, no protoplasm, and no independent life processes.
- Protoplasm = cytoplasm (outside the nucleus) + nucleoplasm (inside it). Semi-fluid, jelly-like, mostly water, with proteins, carbohydrates, fats, nucleic acids and mineral salts. Called the physical basis of life.
- Nucleus — double nuclear membrane with pores, nucleoplasm, chromatin (becomes chromosomes) and the nucleolus. Controls the cell and carries heredity.
- Nucleolus — dense body in the nucleus; makes ribosomes.
- Mitochondria — double membrane, inner folded into cristae, inner space the matrix. Site of aerobic respiration, makes ATP — the powerhouse. Has its own DNA.
- Endoplasmic reticulum — network continuous with the nuclear membrane. Rough (with ribosomes) transports protein; smooth makes lipids. The cell's transport system and support.
- Ribosomes — tiny, no membrane, RNA and protein; site of protein synthesis.
- Golgi bodies (dictyosomes in plants) — stacked sacs; modify, package and despatch, secrete enzymes and hormones, and form lysosomes.
- Plastids — plant cells only. Chloroplasts (chlorophyll, photosynthesis), chromoplasts (colour), leucoplasts (storage).
- Lysosomes — enzyme sacs; digest worn-out organelles and foreign matter; the suicidal bags.
- Centrosome — animal cells only; two centrioles; forms the spindle fibres.
- Vacuole — bounded by the tonoplast. One large central one in a plant cell giving turgidity; small and many in an animal cell.
- Mitochondria and chloroplasts both have a double membrane, their own DNA, and can divide — the two semi-autonomous organelles.
- Cell wall — plant cells only, outermost, non-living, cellulose, freely permeable; gives shape and rigidity and prevents bursting.
- Cell membrane — in all cells, living, lipids and proteins, selectively permeable; encloses the protoplasm and controls entry and exit.
- Osmosis must be explained on the MEMBRANE — the wall selects nothing.
- A plant cell in pure water becomes turgid; an animal cell bursts — the difference is the wall.
- Prokaryotic — no true nucleus (a free nucleoid), no membrane-bound organelles, circular DNA, smaller ribosomes, binary fission. Bacteria, blue-green algae.
- Eukaryotic — true nucleus, membrane-bound organelles, chromosomes, larger ribosomes, mitosis and meiosis. Plants, animals, fungi, protists.
- Plant against animal cell: cell wall present / absent; centrosome absent / present; vacuole one large central / small and many; plastids present / absent. Plus fixed / flexible shape, and starch / glycogen.
- The four differences follow from how each lives — a fixed, food-making organism needs plastids, a rigid wall and a cheap water-filled bulk; a moving one does not.
Cover the organelle list and name the job of each in one word, then say which two share a double membrane and their own DNA — if both come out, this chapter is secure.