One Structure Every Living Cell Has, and One It Can Do Without
Learn how magnification gives a cell's real size, why the cell membrane is universal, how the cell wall separates plant from animal cells, and how to tell a prokaryote from a eukaryote.
What does every living cell have, whatever else it lacks?
A cell membrane. Nothing else is universal.
Plant cells have a cell wall; animal cells do not. Plant cells have chloroplasts; animal cells do not. Bacteria have no nucleus at all. Mature red blood cells lose their nucleus, and mature plant sieve tube cells lose most of their contents.
But every one of them, from a bacterium in pond water to a nerve cell running down your leg, is wrapped in a membrane. Without it there is no inside and no outside, and therefore nothing that could be called a cell.
That is why the membrane, rather than the nucleus, is the honest starting point for studying cells. This page covers the first part of the CBSE Class 9 Science chapter on the cell as the building block of life.
Plant cells have a cell wall; animal cells do not. Plant cells have chloroplasts; animal cells do not. Bacteria have no nucleus at all. Mature red blood cells lose their nucleus, and mature plant sieve tube cells lose most of their contents.
But every one of them, from a bacterium in pond water to a nerve cell running down your leg, is wrapped in a membrane. Without it there is no inside and no outside, and therefore nothing that could be called a cell.
That is why the membrane, rather than the nucleus, is the honest starting point for studying cells. This page covers the first part of the CBSE Class 9 Science chapter on the cell as the building block of life.
How do you work out a cell's real size from a microscope image?
Divide the size you measure by the magnification.
In a compound microscope the total magnification is the eyepiece multiplied by the objective. A eyepiece with a objective gives
Worked example 1. A cell measures mm across in the image, under magnification.
Since , that is — a typical animal cell.
Worked example 2. An onion epidermal cell appears mm long at .
Worked example 3 — an organelle under an electron microscope. A mitochondrion appears mm long at .
The units to keep straight: and nm. Cell sizes are quoted in micrometres, organelles in micrometres or nanometres.
Rough sizes worth carrying: a bacterial cell is a few micrometres, a typical plant or animal cell tens of micrometres, and a human egg cell is large enough to be seen without a microscope. Some nerve cells run more than a metre from end to end, so cells are microscopic is a generalisation with real exceptions.
Magnification is not the same as resolution. Magnification makes the image bigger; resolution is the ability to show two close points as separate. A light microscope cannot separate points closer than roughly a fifth of a micrometre, because of the wavelength of light — so magnifying a light-microscope image a thousand times more would give a bigger blur, not more detail. That limit is exactly why electron microscopes are needed to see organelles, and it is the distinction questions on this topic are built around.
In a compound microscope the total magnification is the eyepiece multiplied by the objective. A eyepiece with a objective gives
Worked example 1. A cell measures mm across in the image, under magnification.
Since , that is — a typical animal cell.
Worked example 2. An onion epidermal cell appears mm long at .
Worked example 3 — an organelle under an electron microscope. A mitochondrion appears mm long at .
The units to keep straight: and nm. Cell sizes are quoted in micrometres, organelles in micrometres or nanometres.
Rough sizes worth carrying: a bacterial cell is a few micrometres, a typical plant or animal cell tens of micrometres, and a human egg cell is large enough to be seen without a microscope. Some nerve cells run more than a metre from end to end, so cells are microscopic is a generalisation with real exceptions.
Magnification is not the same as resolution. Magnification makes the image bigger; resolution is the ability to show two close points as separate. A light microscope cannot separate points closer than roughly a fifth of a micrometre, because of the wavelength of light — so magnifying a light-microscope image a thousand times more would give a bigger blur, not more detail. That limit is exactly why electron microscopes are needed to see organelles, and it is the distinction questions on this topic are built around.
Why is the cell membrane called selectively permeable?
Because it lets some substances through and blocks others — it chooses, rather than simply holding everything in.
The membrane is a thin flexible sheet of lipids and proteins. Small molecules such as oxygen, carbon dioxide and water cross it readily; larger molecules and many ions do not cross freely, and their entry is controlled.
Two kinds of movement to know.
- Diffusion — movement of a substance from a region of higher concentration to lower concentration, without any energy being spent. This is how oxygen enters a cell and carbon dioxide leaves it.
- Osmosis — movement of water across a selectively permeable membrane, from the side with more water to the side with less.
What osmosis does to a cell, in three cases.
- In a hypotonic surrounding (more dilute than the cell), water enters and the cell swells. Raisins soaked in water plump up for this reason.
- In a hypertonic surrounding (more concentrated), water leaves and the cell shrinks. In a plant cell the contents pull away from the wall, which is called plasmolysis.
- In an isotonic surrounding, water moves both ways equally and the cell keeps its size.
Everyday examples. Salt sprinkled on chopped cucumber or onion draws water out within minutes, which is why a salad left standing turns watery — the surroundings became hypertonic. Dried grapes swell in water. A wilted spinach leaf placed in water becomes firm again.
The membrane is flexible, and that matters. Because it can bend and fold, an animal cell can change shape, and some cells can engulf food particles by wrapping the membrane around them. A rigid boundary could not do that.
Diffusion and osmosis are not two names for one thing. Diffusion describes any substance moving down its own concentration gradient; osmosis is specifically water moving across a membrane. A question asking why a cell swells in pure water is asking about osmosis, and answering diffusion of water misses the role of the membrane entirely.
The membrane is a thin flexible sheet of lipids and proteins. Small molecules such as oxygen, carbon dioxide and water cross it readily; larger molecules and many ions do not cross freely, and their entry is controlled.
Two kinds of movement to know.
- Diffusion — movement of a substance from a region of higher concentration to lower concentration, without any energy being spent. This is how oxygen enters a cell and carbon dioxide leaves it.
- Osmosis — movement of water across a selectively permeable membrane, from the side with more water to the side with less.
What osmosis does to a cell, in three cases.
- In a hypotonic surrounding (more dilute than the cell), water enters and the cell swells. Raisins soaked in water plump up for this reason.
- In a hypertonic surrounding (more concentrated), water leaves and the cell shrinks. In a plant cell the contents pull away from the wall, which is called plasmolysis.
- In an isotonic surrounding, water moves both ways equally and the cell keeps its size.
Everyday examples. Salt sprinkled on chopped cucumber or onion draws water out within minutes, which is why a salad left standing turns watery — the surroundings became hypertonic. Dried grapes swell in water. A wilted spinach leaf placed in water becomes firm again.
The membrane is flexible, and that matters. Because it can bend and fold, an animal cell can change shape, and some cells can engulf food particles by wrapping the membrane around them. A rigid boundary could not do that.
Diffusion and osmosis are not two names for one thing. Diffusion describes any substance moving down its own concentration gradient; osmosis is specifically water moving across a membrane. A question asking why a cell swells in pure water is asking about osmosis, and answering diffusion of water misses the role of the membrane entirely.
What does the cell wall do that the cell membrane cannot?
It gives rigidity and stops the cell bursting — jobs a flexible membrane cannot perform.
A plant cell has a cell wall made mainly of cellulose, lying outside the cell membrane. An animal cell has no cell wall at all.
What the wall provides:
- Shape and rigidity, so a plant can stand upright without a skeleton
- Protection from mechanical injury
- Resistance to bursting, by pushing back when water enters
The wall is freely permeable. Water and dissolved substances pass straight through it. So the wall does not control what enters the cell — that remains the membrane's job. Saying the cell wall is selectively permeable is the commonest error in this topic, and it confuses the two structures completely.
A boundary case that shows why the wall matters. Place a red blood cell in pure water and water rushes in by osmosis until the cell bursts. Place a plant cell in pure water and water also rushes in, but the wall resists the swelling and the cell becomes firm — turgid — without breaking. The two cells face the same osmotic situation and survive it differently, because of one structure.
Everyday evidence. The crunch of a fresh cucumber or carrot is the sound of cellulose walls resisting your teeth. A limp, wilted leaf has lost water, so its cells are no longer turgid and the plant droops. Water the plant and the cells take water back in, and the leaves stiffen again within hours.
Walls are not unique to plants, but cellulose walls are. Bacteria and fungi have cell walls too, of different materials — so has a cell wall does not mean is a plant cell, and it certainly does not mean is not a eukaryote, which is the next section's trap.
Other plant-animal differences worth listing:
- Plant cells usually have one large central vacuole; animal cells have small ones or none
- Plant cells have plastids, including chloroplasts in green parts; animal cells have none
- Animal cells have a centrosome; plant cells generally do not
A plant cell has a cell wall made mainly of cellulose, lying outside the cell membrane. An animal cell has no cell wall at all.
What the wall provides:
- Shape and rigidity, so a plant can stand upright without a skeleton
- Protection from mechanical injury
- Resistance to bursting, by pushing back when water enters
The wall is freely permeable. Water and dissolved substances pass straight through it. So the wall does not control what enters the cell — that remains the membrane's job. Saying the cell wall is selectively permeable is the commonest error in this topic, and it confuses the two structures completely.
A boundary case that shows why the wall matters. Place a red blood cell in pure water and water rushes in by osmosis until the cell bursts. Place a plant cell in pure water and water also rushes in, but the wall resists the swelling and the cell becomes firm — turgid — without breaking. The two cells face the same osmotic situation and survive it differently, because of one structure.
Everyday evidence. The crunch of a fresh cucumber or carrot is the sound of cellulose walls resisting your teeth. A limp, wilted leaf has lost water, so its cells are no longer turgid and the plant droops. Water the plant and the cells take water back in, and the leaves stiffen again within hours.
Walls are not unique to plants, but cellulose walls are. Bacteria and fungi have cell walls too, of different materials — so has a cell wall does not mean is a plant cell, and it certainly does not mean is not a eukaryote, which is the next section's trap.
Other plant-animal differences worth listing:
- Plant cells usually have one large central vacuole; animal cells have small ones or none
- Plant cells have plastids, including chloroplasts in green parts; animal cells have none
- Animal cells have a centrosome; plant cells generally do not
How do you tell a prokaryotic cell from a eukaryotic one?
Look for a true nucleus and membrane-bound organelles. If both are absent, the cell is prokaryotic.
A prokaryotic cell:
- Has no nuclear membrane, so no true nucleus — the DNA lies in an undefined region called the nucleoid
- Has no membrane-bound organelles — no mitochondria, no plastids, no endoplasmic reticulum, no Golgi apparatus
- Has a single circular strand of DNA
- Is small, usually a few micrometres across
- Examples: bacteria and blue-green algae
A eukaryotic cell:
- Has a true nucleus enclosed in a nuclear membrane
- Has membrane-bound organelles
- Has DNA on several linear chromosomes
- Is larger, usually tens of micrometres
- Examples: plants, animals, fungi and protozoa
What both share: a cell membrane, cytoplasm, ribosomes and DNA. Ribosomes are present in prokaryotes precisely because they are not membrane-bound.
Worked classification 1. A cell has a nucleus, mitochondria and a cell wall of cellulose. Nucleus and mitochondria present, so eukaryotic — and the cellulose wall makes it a plant cell.
Worked classification 2. A cell has a cell wall and ribosomes but no nuclear membrane and no mitochondria. No nucleus and no membrane-bound organelles, so prokaryotic, despite having a wall.
Worked classification 3. *A cell measures about across and its DNA is a single circular strand.* Both features point to prokaryotic.
**Prokaryotic does not mean no DNA. Every living cell has DNA. The difference is whether that DNA is enclosed in a nuclear membrane**, and a student who answers bacteria have no genetic material has lost the whole distinction.
**Nor does has a wall mean prokaryotic. A plant cell has a wall and is firmly eukaryotic, as worked example 1 showed. The test is the nucleus and the organelles, and nothing else.
How prokaryotes manage without organelles.** They are small, so any substance can diffuse right across the cell quickly, and reactions can proceed in a shared cytoplasm. A eukaryotic cell is many times larger, and that is exactly why it needs internal compartments — which is where the second part of this chapter begins.
A prokaryotic cell:
- Has no nuclear membrane, so no true nucleus — the DNA lies in an undefined region called the nucleoid
- Has no membrane-bound organelles — no mitochondria, no plastids, no endoplasmic reticulum, no Golgi apparatus
- Has a single circular strand of DNA
- Is small, usually a few micrometres across
- Examples: bacteria and blue-green algae
A eukaryotic cell:
- Has a true nucleus enclosed in a nuclear membrane
- Has membrane-bound organelles
- Has DNA on several linear chromosomes
- Is larger, usually tens of micrometres
- Examples: plants, animals, fungi and protozoa
What both share: a cell membrane, cytoplasm, ribosomes and DNA. Ribosomes are present in prokaryotes precisely because they are not membrane-bound.
Worked classification 1. A cell has a nucleus, mitochondria and a cell wall of cellulose. Nucleus and mitochondria present, so eukaryotic — and the cellulose wall makes it a plant cell.
Worked classification 2. A cell has a cell wall and ribosomes but no nuclear membrane and no mitochondria. No nucleus and no membrane-bound organelles, so prokaryotic, despite having a wall.
Worked classification 3. *A cell measures about across and its DNA is a single circular strand.* Both features point to prokaryotic.
**Prokaryotic does not mean no DNA. Every living cell has DNA. The difference is whether that DNA is enclosed in a nuclear membrane**, and a student who answers bacteria have no genetic material has lost the whole distinction.
**Nor does has a wall mean prokaryotic. A plant cell has a wall and is firmly eukaryotic, as worked example 1 showed. The test is the nucleus and the organelles, and nothing else.
How prokaryotes manage without organelles.** They are small, so any substance can diffuse right across the cell quickly, and reactions can proceed in a shared cytoplasm. A eukaryotic cell is many times larger, and that is exactly why it needs internal compartments — which is where the second part of this chapter begins.
Exam tip
Exam tip: divide by the magnification, and name the right membrane
**Actual size **, and total magnification is eyepiece objective. A eyepiece with a objective gives .
Convert units carefully: mm and nm. A mm image at is a cell of .
Magnification is not resolution. Enlarging a blurred image gives a bigger blur.
The cell membrane is selectively permeable; the cell wall is freely permeable. Never swap those two words — it is the single most penalised confusion in the chapter.
Osmosis is about water crossing a membrane; diffusion is any substance moving down its concentration gradient.
Plasmolysis is the plant cell's contents pulling away from the wall in a hypertonic solution.
A plant cell in pure water becomes turgid; an animal cell bursts. The wall is the difference.
For prokaryote against eukaryote, test for a nuclear membrane and membrane-bound organelles. A cell wall proves nothing either way, and prokaryotes do have DNA and ribosomes.
When a question gives features, quote the feature you used to classify — no nuclear membrane, therefore prokaryotic earns the mark that prokaryotic alone does not.
Convert units carefully: mm and nm. A mm image at is a cell of .
Magnification is not resolution. Enlarging a blurred image gives a bigger blur.
The cell membrane is selectively permeable; the cell wall is freely permeable. Never swap those two words — it is the single most penalised confusion in the chapter.
Osmosis is about water crossing a membrane; diffusion is any substance moving down its concentration gradient.
Plasmolysis is the plant cell's contents pulling away from the wall in a hypertonic solution.
A plant cell in pure water becomes turgid; an animal cell bursts. The wall is the difference.
For prokaryote against eukaryote, test for a nuclear membrane and membrane-bound organelles. A cell wall proves nothing either way, and prokaryotes do have DNA and ribosomes.
When a question gives features, quote the feature you used to classify — no nuclear membrane, therefore prokaryotic earns the mark that prokaryotic alone does not.
Did you know
Why the biggest single cell is one you have seen in a shop
Cells are usually described as microscopic, and most are. But an unfertilised bird's egg is a single cell, and an ostrich egg is the largest one there is — held in the hand, not on a slide.
What makes it so large is that it is packed with stored food. The working part of the cell is tiny; almost all the volume is yolk, laid down as a food supply for the embryo that will develop from it. A cell can grow enormous if most of it is store rather than active machinery.
At the other extreme are nerve cells, which are large in a completely different way. A single nerve cell running from the base of the spine to a toe is over a metre long and yet far too thin to see. It is not bulky; it is stretched, because its job is to carry a signal a long distance without handing it over.
Between those extremes sit the ordinary cells, and their size is limited by something specific: diffusion. Substances have to reach the middle of a cell by moving through it, and the bigger the cell, the further that is. Double a cell's width and its volume grows eight times while its surface area grows only four — so each unit of interior has less membrane to supply it.
That is the real reason most cells stay a few tens of micrometres wide, and it is the reason a large cell has to do something unusual: fill itself with inert store like an egg, stretch thin like a nerve cell, or divide its interior into compartments, which is the solution the next part of the chapter describes.
What makes it so large is that it is packed with stored food. The working part of the cell is tiny; almost all the volume is yolk, laid down as a food supply for the embryo that will develop from it. A cell can grow enormous if most of it is store rather than active machinery.
At the other extreme are nerve cells, which are large in a completely different way. A single nerve cell running from the base of the spine to a toe is over a metre long and yet far too thin to see. It is not bulky; it is stretched, because its job is to carry a signal a long distance without handing it over.
Between those extremes sit the ordinary cells, and their size is limited by something specific: diffusion. Substances have to reach the middle of a cell by moving through it, and the bigger the cell, the further that is. Double a cell's width and its volume grows eight times while its surface area grows only four — so each unit of interior has less membrane to supply it.
That is the real reason most cells stay a few tens of micrometres wide, and it is the reason a large cell has to do something unusual: fill itself with inert store like an egg, stretch thin like a nerve cell, or divide its interior into compartments, which is the solution the next part of the chapter describes.
Exam relevance
Why does NEET keep returning to the prokaryote and eukaryote distinction?
Because it is the first branching point in the classification of all life, and almost every later biology chapter assumes it.
This page is the foundation for the Class 11 Biology chapter Cell: The Unit of Life, which is a standing part of the NEET syllabus. That chapter revisits the prokaryotic cell in much more detail — the nucleoid, the cell envelope, the mesosome and the S ribosome — and it takes the Class 9 distinction as already understood. A student still unsure whether bacteria have ribosomes will find the whole chapter harder than it is.
Osmosis and plasmolysis feed directly into Class 11 Transport in Plants, where water potential, turgor pressure and plasmolysis are treated quantitatively, and into Class 12 for membrane transport. This is common ground for NEET Biology.
The cell wall and membrane distinction returns in Class 11 Biological Classification, where bacterial cell wall composition separates major groups.
What the questions look like. In NEET, this material appears most often as assertion-reason items — a statement about prokaryotes paired with a reason about their organelles — and as match-the-column questions pairing a structure with its function or its composition. Diagram-based questions asking you to identify a labelled cell as plant, animal or bacterial are also standard. Magnification calculations of the kind worked above appear more in school and olympiad papers than in NEET itself.
How board and competitive emphasis differ. A board paper is likely to ask you to state three differences between plant and animal cells, or to define osmosis. A NEET item rarely asks for a definition outright — it embeds it, for instance by describing a cell's features and asking which group it belongs to, or by pairing a true statement with a false reason.
The single trap that costs the most marks. Treating the presence of a cell wall as evidence that a cell is prokaryotic. Plant cells and fungal cells both have walls and both are eukaryotic; the wall's composition differs between groups but its presence classifies nothing. The test is always the nuclear membrane and the membrane-bound organelles.
One more worth guarding against. Calling the cell wall selectively permeable. It is freely permeable, and the selectivity belongs to the membrane — a distinction NEET has good reason to test, because the whole of plant water transport depends on it.
This page is the foundation for the Class 11 Biology chapter Cell: The Unit of Life, which is a standing part of the NEET syllabus. That chapter revisits the prokaryotic cell in much more detail — the nucleoid, the cell envelope, the mesosome and the S ribosome — and it takes the Class 9 distinction as already understood. A student still unsure whether bacteria have ribosomes will find the whole chapter harder than it is.
Osmosis and plasmolysis feed directly into Class 11 Transport in Plants, where water potential, turgor pressure and plasmolysis are treated quantitatively, and into Class 12 for membrane transport. This is common ground for NEET Biology.
The cell wall and membrane distinction returns in Class 11 Biological Classification, where bacterial cell wall composition separates major groups.
What the questions look like. In NEET, this material appears most often as assertion-reason items — a statement about prokaryotes paired with a reason about their organelles — and as match-the-column questions pairing a structure with its function or its composition. Diagram-based questions asking you to identify a labelled cell as plant, animal or bacterial are also standard. Magnification calculations of the kind worked above appear more in school and olympiad papers than in NEET itself.
How board and competitive emphasis differ. A board paper is likely to ask you to state three differences between plant and animal cells, or to define osmosis. A NEET item rarely asks for a definition outright — it embeds it, for instance by describing a cell's features and asking which group it belongs to, or by pairing a true statement with a false reason.
The single trap that costs the most marks. Treating the presence of a cell wall as evidence that a cell is prokaryotic. Plant cells and fungal cells both have walls and both are eukaryotic; the wall's composition differs between groups but its presence classifies nothing. The test is always the nuclear membrane and the membrane-bound organelles.
One more worth guarding against. Calling the cell wall selectively permeable. It is freely permeable, and the selectivity belongs to the membrane — a distinction NEET has good reason to test, because the whole of plant water transport depends on it.
Key takeaways
The cell membrane, wall and cell types: quick revision
- The cell membrane is the only structure every living cell has. Cell walls, nuclei, chloroplasts and vacuoles are all optional.
- **Actual size **, with total magnification eyepiece objective, so and give .
- A mm image at is mm ; a mm image at is ; a mm mitochondrion at is .
- mm and nm.
- Magnification is not resolution — a light microscope cannot separate points closer than about a fifth of a micrometre, so bigger is not clearer.
- The membrane is made of lipids and proteins and is selectively permeable.
- Diffusion moves any substance down its concentration gradient; osmosis moves water across a membrane.
- Hypotonic surroundings make a cell swell, hypertonic make it shrink (plasmolysis in a plant cell), isotonic leave it unchanged. Salted cucumber turns watery; raisins in water swell.
- The membrane is flexible, letting cells change shape and engulf particles.
- The cell wall is made mainly of cellulose, lies outside the membrane, and gives shape, protection and resistance to bursting.
- The wall is freely permeable — it controls nothing. In pure water a plant cell becomes turgid while a red blood cell bursts.
- Bacteria and fungi have walls of other materials, so has a wall does not mean is a plant.
- Plant cells also have a large central vacuole and plastids; animal cells have a centrosome.
- Prokaryotic: no nuclear membrane, no membrane-bound organelles, single circular DNA, a few micrometres — bacteria and blue-green algae.
- Eukaryotic: true nucleus, membrane-bound organelles, linear chromosomes, tens of micrometres — plants, animals, fungi, protozoa.
- Both have a membrane, cytoplasm, ribosomes and DNA. Prokaryotes are not without genetic material.
- Classify by the nuclear membrane and organelles — a cell wall is no evidence either way.
- Prokaryotes manage without compartments because they are small enough for diffusion to reach everywhere.
Take five cells described only by their features and classify each, writing down the one feature that decided it — if you can name the deciding feature every time, the distinction is genuinely yours.
- **Actual size **, with total magnification eyepiece objective, so and give .
- A mm image at is mm ; a mm image at is ; a mm mitochondrion at is .
- mm and nm.
- Magnification is not resolution — a light microscope cannot separate points closer than about a fifth of a micrometre, so bigger is not clearer.
- The membrane is made of lipids and proteins and is selectively permeable.
- Diffusion moves any substance down its concentration gradient; osmosis moves water across a membrane.
- Hypotonic surroundings make a cell swell, hypertonic make it shrink (plasmolysis in a plant cell), isotonic leave it unchanged. Salted cucumber turns watery; raisins in water swell.
- The membrane is flexible, letting cells change shape and engulf particles.
- The cell wall is made mainly of cellulose, lies outside the membrane, and gives shape, protection and resistance to bursting.
- The wall is freely permeable — it controls nothing. In pure water a plant cell becomes turgid while a red blood cell bursts.
- Bacteria and fungi have walls of other materials, so has a wall does not mean is a plant.
- Plant cells also have a large central vacuole and plastids; animal cells have a centrosome.
- Prokaryotic: no nuclear membrane, no membrane-bound organelles, single circular DNA, a few micrometres — bacteria and blue-green algae.
- Eukaryotic: true nucleus, membrane-bound organelles, linear chromosomes, tens of micrometres — plants, animals, fungi, protozoa.
- Both have a membrane, cytoplasm, ribosomes and DNA. Prokaryotes are not without genetic material.
- Classify by the nuclear membrane and organelles — a cell wall is no evidence either way.
- Prokaryotes manage without compartments because they are small enough for diffusion to reach everywhere.
Take five cells described only by their features and classify each, writing down the one feature that decided it — if you can name the deciding feature every time, the distinction is genuinely yours.