The Cell Keeps Its Digestive Enzymes in a Bag for Good Reason
Learn what each cell organelle does, why eukaryotic cells need compartments, how mitosis and meiosis differ in chromosome number, and what cell theory unifies.
Why does a cell keep its own digestive enzymes locked away?
Because if they were loose in the cytoplasm they would digest the cell itself.
A cell needs powerful enzymes to break down worn-out organelles and unwanted material. Those same enzymes cannot tell the difference between waste and working machinery, so the cell keeps them sealed inside lysosomes — small membrane-bound sacs.
When a lysosome membrane is damaged, the enzymes do escape and the cell is destroyed, which is why lysosomes are described as the suicide bags of the cell.
That is compartmentalisation in one example: a job that must be done, done safely by being kept apart. The same logic explains almost every organelle in a eukaryotic cell, and it is why a large cell needs internal walls at all. This page covers the second part of the CBSE Class 9 Science chapter on the cell.
A cell needs powerful enzymes to break down worn-out organelles and unwanted material. Those same enzymes cannot tell the difference between waste and working machinery, so the cell keeps them sealed inside lysosomes — small membrane-bound sacs.
When a lysosome membrane is damaged, the enzymes do escape and the cell is destroyed, which is why lysosomes are described as the suicide bags of the cell.
That is compartmentalisation in one example: a job that must be done, done safely by being kept apart. The same logic explains almost every organelle in a eukaryotic cell, and it is why a large cell needs internal walls at all. This page covers the second part of the CBSE Class 9 Science chapter on the cell.
What does each cell organelle actually do?
Each organelle is a compartment with one main job, and the names are easiest to hold if you learn them by function.
- Nucleus — contains the chromosomes, which carry DNA, and directs the cell's activities. The nucleolus inside it makes ribosomes. Enclosed by a nuclear membrane with pores.
- Mitochondria — the site of aerobic respiration, releasing energy stored as ATP. Double membrane, the inner one folded into cristae to give more surface for reactions. Has its own DNA and ribosomes.
- Chloroplasts — found in green plant cells, containing chlorophyll, the site of photosynthesis. Also have their own DNA.
- Endoplasmic reticulum — a network of membranes running through the cytoplasm. Rough ER, studded with ribosomes, makes proteins; smooth ER makes lipids and helps detoxify. Both act as a transport channel.
- Ribosomes — the site of protein synthesis. Found on the rough ER and free in the cytoplasm, and not membrane-bound.
- Golgi apparatus — modifies, packages and dispatches materials made by the ER, and forms lysosomes.
- Lysosomes — digestive enzymes in a sealed sac, breaking down waste and worn-out parts.
- Vacuoles — storage sacs. The large central vacuole of a plant cell holds sap and keeps the cell turgid.
- Plastids — chloroplasts (green, photosynthetic), leucoplasts (colourless, storing starch, oil or protein) and chromoplasts (coloured, giving flowers and fruits their colour).
- Cytoplasm — the jelly-like fluid in which the organelles sit, and where many reactions happen.
- Centrosome — in animal cells, forms the spindle during cell division.
Everyday parallel. A plant cell's leucoplasts are why a potato is full of starch, and its chromoplasts are why a ripe tomato is red and a carrot orange. The large vacuole is why a fresh vegetable is crisp and a stale one limp.
Two organelles are semi-autonomous. Mitochondria and chloroplasts have their own DNA and their own ribosomes, so they can make some of their own proteins and can divide independently of the cell. No other organelle can. That fact is examined repeatedly, and it is worth learning as a pair rather than separately.
Ribosomes are the exception in this list. Everything else named here is enclosed by a membrane; ribosomes are not, which is exactly why prokaryotes have ribosomes but no other organelles. Calling a ribosome a membrane-bound organelle contradicts the prokaryote definition from the first part of this chapter.
- Nucleus — contains the chromosomes, which carry DNA, and directs the cell's activities. The nucleolus inside it makes ribosomes. Enclosed by a nuclear membrane with pores.
- Mitochondria — the site of aerobic respiration, releasing energy stored as ATP. Double membrane, the inner one folded into cristae to give more surface for reactions. Has its own DNA and ribosomes.
- Chloroplasts — found in green plant cells, containing chlorophyll, the site of photosynthesis. Also have their own DNA.
- Endoplasmic reticulum — a network of membranes running through the cytoplasm. Rough ER, studded with ribosomes, makes proteins; smooth ER makes lipids and helps detoxify. Both act as a transport channel.
- Ribosomes — the site of protein synthesis. Found on the rough ER and free in the cytoplasm, and not membrane-bound.
- Golgi apparatus — modifies, packages and dispatches materials made by the ER, and forms lysosomes.
- Lysosomes — digestive enzymes in a sealed sac, breaking down waste and worn-out parts.
- Vacuoles — storage sacs. The large central vacuole of a plant cell holds sap and keeps the cell turgid.
- Plastids — chloroplasts (green, photosynthetic), leucoplasts (colourless, storing starch, oil or protein) and chromoplasts (coloured, giving flowers and fruits their colour).
- Cytoplasm — the jelly-like fluid in which the organelles sit, and where many reactions happen.
- Centrosome — in animal cells, forms the spindle during cell division.
Everyday parallel. A plant cell's leucoplasts are why a potato is full of starch, and its chromoplasts are why a ripe tomato is red and a carrot orange. The large vacuole is why a fresh vegetable is crisp and a stale one limp.
Two organelles are semi-autonomous. Mitochondria and chloroplasts have their own DNA and their own ribosomes, so they can make some of their own proteins and can divide independently of the cell. No other organelle can. That fact is examined repeatedly, and it is worth learning as a pair rather than separately.
Ribosomes are the exception in this list. Everything else named here is enclosed by a membrane; ribosomes are not, which is exactly why prokaryotes have ribosomes but no other organelles. Calling a ribosome a membrane-bound organelle contradicts the prokaryote definition from the first part of this chapter.
Why do eukaryotic cells need compartments when bacteria manage without?
Because they are much bigger, and diffusion alone is too slow to run a large cell.
Three separate advantages come from dividing the interior up.
Keeping incompatible processes apart. Digestive enzymes in lysosomes, as the opening described. Respiration inside mitochondria, so its reactions do not interfere with the rest of the cytoplasm.
Concentrating reactants with their enzymes. A reaction runs faster when the substances and the enzyme that acts on them are held close together rather than spread through the whole cell. A small compartment does that automatically.
Division of labour. Each organelle specialises, so each can be built for one job — folded membranes in mitochondria for respiration, a sealed sac for lysosomes, a network of channels for the ER.
Why size forces the issue — with numbers. Take a cube-shaped cell of side , close to a bacterium. Its surface area is and its volume is , so
Now take a cell of side , close to an animal cell. Its surface is and its volume :
Ten times less membrane for each unit of interior, and ten times further for anything to diffuse across. A bacterium can supply its whole interior through its outer membrane; a eukaryotic cell cannot, and the internal membranes of the ER, mitochondria and Golgi restore the surface area it needs.
Everyday parallel. A one-room stall can cook, store and serve in a single space. A large restaurant has a kitchen, a cold store and a serving counter — not because the work changed, but because the scale did. Trying to run a restaurant as one undivided room is exactly the problem a large cell would face.
Prokaryotes are not primitive failures. They are efficient at their size, and their high surface-to-volume ratio lets them grow and divide very quickly. Compartmentalisation is a solution to the problems of being large, not an upgrade every cell needs.
Three separate advantages come from dividing the interior up.
Keeping incompatible processes apart. Digestive enzymes in lysosomes, as the opening described. Respiration inside mitochondria, so its reactions do not interfere with the rest of the cytoplasm.
Concentrating reactants with their enzymes. A reaction runs faster when the substances and the enzyme that acts on them are held close together rather than spread through the whole cell. A small compartment does that automatically.
Division of labour. Each organelle specialises, so each can be built for one job — folded membranes in mitochondria for respiration, a sealed sac for lysosomes, a network of channels for the ER.
Why size forces the issue — with numbers. Take a cube-shaped cell of side , close to a bacterium. Its surface area is and its volume is , so
Now take a cell of side , close to an animal cell. Its surface is and its volume :
Ten times less membrane for each unit of interior, and ten times further for anything to diffuse across. A bacterium can supply its whole interior through its outer membrane; a eukaryotic cell cannot, and the internal membranes of the ER, mitochondria and Golgi restore the surface area it needs.
Everyday parallel. A one-room stall can cook, store and serve in a single space. A large restaurant has a kitchen, a cold store and a serving counter — not because the work changed, but because the scale did. Trying to run a restaurant as one undivided room is exactly the problem a large cell would face.
Prokaryotes are not primitive failures. They are efficient at their size, and their high surface-to-volume ratio lets them grow and divide very quickly. Compartmentalisation is a solution to the problems of being large, not an upgrade every cell needs.
How do mitosis and meiosis differ in chromosome number?
Mitosis keeps the number the same; meiosis halves it.
Before either can happen, the cell passes through interphase, during which it grows and its DNA replicates so that every chromosome is copied.
Mitosis then proceeds through four stages:
- Prophase — chromosomes condense and become visible; the nuclear membrane starts to break down
- Metaphase — chromosomes line up at the centre of the cell
- Anaphase — the copies separate and move to opposite poles
- Telophase — two new nuclei form and the chromosomes uncoil
Cytokinesis then divides the cytoplasm, giving two daughter cells.
Meiosis consists of two successive divisions with no DNA replication in between, giving four daughter cells, each with half the parent's chromosome number.
Worked example 1 — human cells. A human body cell has **** chromosomes, in pairs.
- Mitosis gives cells of chromosomes each
- Meiosis gives cells of chromosomes each
At fertilisation, a sperm with and an egg with combine:
restoring the body-cell number. Halving in meiosis is what makes fertilisation possible — without it, each generation would double its chromosome number.
Worked example 2 — a general count. A cell with chromosomes divides.
- By mitosis: cells with each, so chromosomes in total
- By meiosis: cells with each, so also in total
The totals match because the same replicated DNA is being shared out either way — only the number of cells it is divided among differs.
What each is for.
- Mitosis: growth, repair of tissues, replacement of worn-out cells. A cut healing over is mitosis.
- Meiosis: formation of gametes, and it shuffles the chromosomes so that offspring differ from their parents and from one another.
Meiosis does not discard chromosomes at random. Each daughter cell receives one chromosome of each pair, so it gets a complete set of every kind — just one copy instead of two. A cell that lost chromosomes arbitrarily would be missing information and could not function.
A boundary case. If a haploid cell divides by mitosis, the daughters are also haploid. Mitosis conserves the number, whatever it was; it does not make cells diploid.
Before either can happen, the cell passes through interphase, during which it grows and its DNA replicates so that every chromosome is copied.
Mitosis then proceeds through four stages:
- Prophase — chromosomes condense and become visible; the nuclear membrane starts to break down
- Metaphase — chromosomes line up at the centre of the cell
- Anaphase — the copies separate and move to opposite poles
- Telophase — two new nuclei form and the chromosomes uncoil
Cytokinesis then divides the cytoplasm, giving two daughter cells.
Meiosis consists of two successive divisions with no DNA replication in between, giving four daughter cells, each with half the parent's chromosome number.
Worked example 1 — human cells. A human body cell has **** chromosomes, in pairs.
- Mitosis gives cells of chromosomes each
- Meiosis gives cells of chromosomes each
At fertilisation, a sperm with and an egg with combine:
restoring the body-cell number. Halving in meiosis is what makes fertilisation possible — without it, each generation would double its chromosome number.
Worked example 2 — a general count. A cell with chromosomes divides.
- By mitosis: cells with each, so chromosomes in total
- By meiosis: cells with each, so also in total
The totals match because the same replicated DNA is being shared out either way — only the number of cells it is divided among differs.
What each is for.
- Mitosis: growth, repair of tissues, replacement of worn-out cells. A cut healing over is mitosis.
- Meiosis: formation of gametes, and it shuffles the chromosomes so that offspring differ from their parents and from one another.
Meiosis does not discard chromosomes at random. Each daughter cell receives one chromosome of each pair, so it gets a complete set of every kind — just one copy instead of two. A cell that lost chromosomes arbitrarily would be missing information and could not function.
A boundary case. If a haploid cell divides by mitosis, the daughters are also haploid. Mitosis conserves the number, whatever it was; it does not make cells diploid.
What does cell theory say, and what does it leave out?
Three postulates, and one clear exception.
Cell theory states:
- All living organisms are composed of cells and the products of cells
- The cell is the basic structural and functional unit of life
- All cells arise from pre-existing cells
How it unifies biology. An elephant, a mango tree, a mushroom and a bacterium are built from the same kind of unit, and that unit works on the same principles in all of them — a membrane boundary, DNA carrying instructions, ribosomes making proteins.
That is why something learned in one organism so often applies to another. Studying respiration in yeast tells you about respiration in your own muscle cells, and studying inheritance in pea plants tells you about inheritance in people. Without cell theory there would be no reason to expect any of that to transfer.
The third postulate is the sharpest of the three. All cells arise from pre-existing cells rules out living things appearing spontaneously from non-living material. Every cell in your body traces back through an unbroken chain of cell divisions to a single fertilised egg — and every cell in that chain was made by the division described in the previous section.
Everyday consequence. Food does not spoil because microorganisms arise from it; it spoils because microorganisms already present divide and multiply. That is exactly why boiling, sealing and refrigerating work — they remove existing cells or slow their division, rather than preventing new ones appearing from nothing.
The exception worth knowing: viruses. A virus is not made of cells. It has genetic material and a protein coat, but no cell membrane, no cytoplasm and no ribosomes, and it cannot divide by itself — it can only reproduce inside a host cell using that cell's machinery.
So viruses sit outside cell theory, and whether they count as living at all is a genuinely open question rather than a gap in your knowledge. A theory with a known boundary is not a weak theory; knowing where it stops applying is part of understanding it, exactly as it was for the models in the opening chapter of this course.
Cell theory states:
- All living organisms are composed of cells and the products of cells
- The cell is the basic structural and functional unit of life
- All cells arise from pre-existing cells
How it unifies biology. An elephant, a mango tree, a mushroom and a bacterium are built from the same kind of unit, and that unit works on the same principles in all of them — a membrane boundary, DNA carrying instructions, ribosomes making proteins.
That is why something learned in one organism so often applies to another. Studying respiration in yeast tells you about respiration in your own muscle cells, and studying inheritance in pea plants tells you about inheritance in people. Without cell theory there would be no reason to expect any of that to transfer.
The third postulate is the sharpest of the three. All cells arise from pre-existing cells rules out living things appearing spontaneously from non-living material. Every cell in your body traces back through an unbroken chain of cell divisions to a single fertilised egg — and every cell in that chain was made by the division described in the previous section.
Everyday consequence. Food does not spoil because microorganisms arise from it; it spoils because microorganisms already present divide and multiply. That is exactly why boiling, sealing and refrigerating work — they remove existing cells or slow their division, rather than preventing new ones appearing from nothing.
The exception worth knowing: viruses. A virus is not made of cells. It has genetic material and a protein coat, but no cell membrane, no cytoplasm and no ribosomes, and it cannot divide by itself — it can only reproduce inside a host cell using that cell's machinery.
So viruses sit outside cell theory, and whether they count as living at all is a genuinely open question rather than a gap in your knowledge. A theory with a known boundary is not a weak theory; knowing where it stops applying is part of understanding it, exactly as it was for the models in the opening chapter of this course.
Exam tip
Exam tip: learn organelles by function, and halve only in meiosis
Learn each organelle with its one main job — nucleus controls, mitochondria respire, ribosomes make proteins, Golgi packages, lysosomes digest, chloroplasts photosynthesise.
Ribosomes are not membrane-bound, which is why prokaryotes have them.
Mitochondria and chloroplasts have their own DNA and ribosomes — learn this pair together, since it is asked as a pair.
Name the three plastids: chloroplast, leucoplast, chromoplast.
**Mitosis gives cells with the same number; meiosis gives cells with half.** For humans: or , and fertilisation restores .
Mitosis conserves the number — a haploid cell dividing by mitosis gives haploid daughters.
Learn the four stages in order: prophase, metaphase, anaphase, telophase, then cytokinesis. Interphase comes before and is where DNA replicates.
For cell theory, state all three postulates — the third, all cells arise from pre-existing cells, is the one most often left out.
Name viruses as the exception if asked about the limits of cell theory.
And when a question asks why eukaryotes need organelles, answer with compartmentalisation and division of labour, not just a list of organelles.
Ribosomes are not membrane-bound, which is why prokaryotes have them.
Mitochondria and chloroplasts have their own DNA and ribosomes — learn this pair together, since it is asked as a pair.
Name the three plastids: chloroplast, leucoplast, chromoplast.
**Mitosis gives cells with the same number; meiosis gives cells with half.** For humans: or , and fertilisation restores .
Mitosis conserves the number — a haploid cell dividing by mitosis gives haploid daughters.
Learn the four stages in order: prophase, metaphase, anaphase, telophase, then cytokinesis. Interphase comes before and is where DNA replicates.
For cell theory, state all three postulates — the third, all cells arise from pre-existing cells, is the one most often left out.
Name viruses as the exception if asked about the limits of cell theory.
And when a question asks why eukaryotes need organelles, answer with compartmentalisation and division of labour, not just a list of organelles.
Did you know
The organelles that behave like guests
Mitochondria and chloroplasts are unlike every other organelle in a cell, and the differences all point the same way.
Each has a double membrane, as though something with its own boundary had been wrapped in a second one. Each has its own DNA, separate from the DNA in the nucleus. Each has its own ribosomes, and can make some of its own proteins. And each divides on its own schedule, rather than being built by the cell when needed.
Every one of those features is what you would expect of a small independent cell living inside a larger one, and that is the explanation biologists give: these organelles descend from free-living cells that came to live inside a host, each side gaining from the arrangement. The host got a supply of energy; the guest got shelter and raw materials.
The evidence is not just structural. Their DNA is circular, like a bacterium's rather than like the linear chromosomes in the nucleus. Their ribosomes resemble bacterial ribosomes more closely than the cell's own. And their inner membrane, the one that would have been the guest's own, is the one carrying the machinery of respiration or photosynthesis.
There is a neat consequence for how inheritance works. Because mitochondria come with the egg cell and not with the sperm, mitochondrial DNA is passed down through the mother's line alone — a completely separate inheritance from the chromosomes in the nucleus.
So the two semi-autonomous organelles are not an odd pair of exceptions to memorise. They are the clearest surviving evidence of how the eukaryotic cell, with all its compartments, came to be built.
Each has a double membrane, as though something with its own boundary had been wrapped in a second one. Each has its own DNA, separate from the DNA in the nucleus. Each has its own ribosomes, and can make some of its own proteins. And each divides on its own schedule, rather than being built by the cell when needed.
Every one of those features is what you would expect of a small independent cell living inside a larger one, and that is the explanation biologists give: these organelles descend from free-living cells that came to live inside a host, each side gaining from the arrangement. The host got a supply of energy; the guest got shelter and raw materials.
The evidence is not just structural. Their DNA is circular, like a bacterium's rather than like the linear chromosomes in the nucleus. Their ribosomes resemble bacterial ribosomes more closely than the cell's own. And their inner membrane, the one that would have been the guest's own, is the one carrying the machinery of respiration or photosynthesis.
There is a neat consequence for how inheritance works. Because mitochondria come with the egg cell and not with the sperm, mitochondrial DNA is passed down through the mother's line alone — a completely separate inheritance from the chromosomes in the nucleus.
So the two semi-autonomous organelles are not an odd pair of exceptions to memorise. They are the clearest surviving evidence of how the eukaryotic cell, with all its compartments, came to be built.
Exam relevance
Why do NEET questions keep coming back to the cell organelles?
Because organelle structure and function is the vocabulary the rest of biology is written in, and this page is where it is first assembled.
This is the foundation for the Class 11 Biology chapter Cell: The Unit of Life, a standing part of the NEET syllabus, which takes every organelle named here and adds detail — the cristae and matrix of mitochondria, the grana and stroma of chloroplasts, cisternae in the Golgi, the S against S ribosome. Every one of those additions assumes you already know the organelle's job.
Cell division feeds into Class 11 Cell Cycle and Cell Division, which treats mitosis and meiosis stage by stage, including the sub-stages of prophase I. The Class 9 point that matters later is the chromosome number outcome — the arithmetic of to against to — because NEET items very often give a chromosome number and ask for the count in the daughter cells.
Meiosis and variation feed into Class 12 Principles of Inheritance and Variation, where the shuffling of chromosomes becomes the basis of genetics. NEET draws heavily on that chapter, and the idea that meiosis produces variation starts here.
Lysosomes and the Golgi reappear in Class 11 when protein trafficking is described, and the semi-autonomous nature of mitochondria and chloroplasts becomes a favourite fact once Biological Classification and Photosynthesis are covered.
What the questions look like. Match-the-column pairing organelle with function is the most common form. Assertion-reason items are frequent, and a classic pairs lysosomes are called suicide bags with a reason about their enzymes. Numerical items on chromosome number after one or two divisions appear regularly, and diagram-based questions ask you to identify a labelled organelle from an electron-micrograph sketch.
How board and competitive emphasis differ. A board paper asks you to write the functions of three organelles, or to state the postulates of cell theory. A NEET item is more likely to give a property — *has circular DNA and S ribosomes* — and ask which organelle it is, which rewards knowing the semi-autonomous pair rather than a memorised list.
The single trap that costs the most marks. Writing that meiosis produces two cells, or that mitosis halves the chromosome number. Fix the pair by their purpose: mitosis is for growth and repair, so the daughters must match the parent; meiosis is for gametes, so the number must halve for fertilisation to restore it.
A second trap worth naming. Listing ribosomes among the membrane-bound organelles. They are not, and that single fact is what allows prokaryotes to make proteins without having any organelles at all.
This is the foundation for the Class 11 Biology chapter Cell: The Unit of Life, a standing part of the NEET syllabus, which takes every organelle named here and adds detail — the cristae and matrix of mitochondria, the grana and stroma of chloroplasts, cisternae in the Golgi, the S against S ribosome. Every one of those additions assumes you already know the organelle's job.
Cell division feeds into Class 11 Cell Cycle and Cell Division, which treats mitosis and meiosis stage by stage, including the sub-stages of prophase I. The Class 9 point that matters later is the chromosome number outcome — the arithmetic of to against to — because NEET items very often give a chromosome number and ask for the count in the daughter cells.
Meiosis and variation feed into Class 12 Principles of Inheritance and Variation, where the shuffling of chromosomes becomes the basis of genetics. NEET draws heavily on that chapter, and the idea that meiosis produces variation starts here.
Lysosomes and the Golgi reappear in Class 11 when protein trafficking is described, and the semi-autonomous nature of mitochondria and chloroplasts becomes a favourite fact once Biological Classification and Photosynthesis are covered.
What the questions look like. Match-the-column pairing organelle with function is the most common form. Assertion-reason items are frequent, and a classic pairs lysosomes are called suicide bags with a reason about their enzymes. Numerical items on chromosome number after one or two divisions appear regularly, and diagram-based questions ask you to identify a labelled organelle from an electron-micrograph sketch.
How board and competitive emphasis differ. A board paper asks you to write the functions of three organelles, or to state the postulates of cell theory. A NEET item is more likely to give a property — *has circular DNA and S ribosomes* — and ask which organelle it is, which rewards knowing the semi-autonomous pair rather than a memorised list.
The single trap that costs the most marks. Writing that meiosis produces two cells, or that mitosis halves the chromosome number. Fix the pair by their purpose: mitosis is for growth and repair, so the daughters must match the parent; meiosis is for gametes, so the number must halve for fertilisation to restore it.
A second trap worth naming. Listing ribosomes among the membrane-bound organelles. They are not, and that single fact is what allows prokaryotes to make proteins without having any organelles at all.
Key takeaways
Organelles, compartments, cell division and cell theory: quick revision
- Nucleus controls the cell and holds the chromosomes; the nucleolus makes ribosomes.
- Mitochondria carry out aerobic respiration, releasing ATP, with a double membrane folded into cristae.
- Chloroplasts hold chlorophyll and carry out photosynthesis.
- Rough ER makes proteins, smooth ER makes lipids, and both transport materials.
- Ribosomes make proteins and are not membrane-bound — which is why prokaryotes have them.
- Golgi apparatus modifies, packages and dispatches materials, and forms lysosomes.
- Lysosomes hold digestive enzymes and are called suicide bags because their escape destroys the cell.
- Vacuoles store; the large central vacuole keeps a plant cell turgid. Plastids are chloroplasts, leucoplasts and chromoplasts.
- Mitochondria and chloroplasts are semi-autonomous — own circular DNA, own ribosomes, own division.
- Compartmentalisation keeps incompatible processes apart, concentrates reactants with enzymes, and allows division of labour.
- Size forces it: a cube has surface-to-volume per ; a cube has — ten times less membrane per unit of interior.
- Interphase comes first, with growth and DNA replication. Then prophase, metaphase, anaphase, telophase, and cytokinesis.
- Mitosis: daughter cells with the same chromosome number — for growth, repair and replacement.
- Meiosis: two divisions giving daughter cells with half the number — for gametes and variation.
- Human cells: mitosis gives ; meiosis gives ; fertilisation restores .
- A cell with chromosomes gives cells of by mitosis, or cells of by meiosis — in total either way.
- Meiosis gives each daughter one of each pair, not a random selection. Mitosis conserves the number, so a haploid cell gives haploid daughters.
- Cell theory: all organisms are made of cells and cell products; the cell is the basic structural and functional unit; all cells arise from pre-existing cells.
- It unifies biology by making findings transfer between organisms — and food spoils because microorganisms already present multiply.
- Viruses are the exception: no membrane, cytoplasm or ribosomes, and unable to divide alone.
Take a chromosome number of your own choosing and write the outcome of mitosis and of meiosis side by side, including the total chromosome count — if both totals match, you have understood what each division actually does.
- Mitochondria carry out aerobic respiration, releasing ATP, with a double membrane folded into cristae.
- Chloroplasts hold chlorophyll and carry out photosynthesis.
- Rough ER makes proteins, smooth ER makes lipids, and both transport materials.
- Ribosomes make proteins and are not membrane-bound — which is why prokaryotes have them.
- Golgi apparatus modifies, packages and dispatches materials, and forms lysosomes.
- Lysosomes hold digestive enzymes and are called suicide bags because their escape destroys the cell.
- Vacuoles store; the large central vacuole keeps a plant cell turgid. Plastids are chloroplasts, leucoplasts and chromoplasts.
- Mitochondria and chloroplasts are semi-autonomous — own circular DNA, own ribosomes, own division.
- Compartmentalisation keeps incompatible processes apart, concentrates reactants with enzymes, and allows division of labour.
- Size forces it: a cube has surface-to-volume per ; a cube has — ten times less membrane per unit of interior.
- Interphase comes first, with growth and DNA replication. Then prophase, metaphase, anaphase, telophase, and cytokinesis.
- Mitosis: daughter cells with the same chromosome number — for growth, repair and replacement.
- Meiosis: two divisions giving daughter cells with half the number — for gametes and variation.
- Human cells: mitosis gives ; meiosis gives ; fertilisation restores .
- A cell with chromosomes gives cells of by mitosis, or cells of by meiosis — in total either way.
- Meiosis gives each daughter one of each pair, not a random selection. Mitosis conserves the number, so a haploid cell gives haploid daughters.
- Cell theory: all organisms are made of cells and cell products; the cell is the basic structural and functional unit; all cells arise from pre-existing cells.
- It unifies biology by making findings transfer between organisms — and food spoils because microorganisms already present multiply.
- Viruses are the exception: no membrane, cytoplasm or ribosomes, and unable to divide alone.
Take a chromosome number of your own choosing and write the outcome of mitosis and of meiosis side by side, including the total chromosome count — if both totals match, you have understood what each division actually does.