A Red Blood Cell Throws Away Its Own Nucleus
Learn to identify every part of a cell and what it does, compare plant and animal cells, see how a cell's shape suits its job, tell unicellular from multicellular life, and order the levels of organisation.
Why does a red blood cell have no nucleus?
To make room. A mature red blood cell gives up its nucleus so it can pack in more haemoglobin and squeeze through the narrowest capillaries — its whole job is carrying oxygen, and a nucleus would only take up space.
The shape and contents of a cell almost always match the work it does. This page covers everything in the CBSE Class 8 Science chapter's first part: the parts of a cell, plant and animal cells, unicellular and multicellular organisms, and levels of organisation.
The shape and contents of a cell almost always match the work it does. This page covers everything in the CBSE Class 8 Science chapter's first part: the parts of a cell, plant and animal cells, unicellular and multicellular organisms, and levels of organisation.
What are the parts of a cell and what does each do?
The cell is the basic structural and functional unit of every living thing. Its main parts are these.
- Cell membrane — the thin, flexible outer covering present in every cell. It gives the cell its shape and controls what enters and leaves, which is why it is called selectively permeable.
- Cytoplasm — the jelly-like substance filling the cell, where most of its chemical activities happen. It holds the other structures in place.
- Nucleus — the dense, usually rounded body that controls all the cell's activities and carries the information passed on to new cells. It is the control centre.
- Cell wall — a rigid outer layer of cellulose found only in plant cells, outside the membrane. It gives strength, protection and a definite shape.
- Vacuole — a fluid-filled space that stores water, food and wastes. Plant cells have one large central vacuole; animal cells have small ones or none.
- Chloroplast — the green body containing chlorophyll, found only in plant cells, where photosynthesis takes place.
The crisp snap of a fresh stick of celery comes from cell walls and full vacuoles; a wilted spinach leaf has lost the water from those vacuoles.
The part most often confused is the cell wall with the cell membrane. Every cell has a membrane; only plant cells add a wall outside it. So a plant cell has both, and an animal cell has the membrane alone.
- Cell membrane — the thin, flexible outer covering present in every cell. It gives the cell its shape and controls what enters and leaves, which is why it is called selectively permeable.
- Cytoplasm — the jelly-like substance filling the cell, where most of its chemical activities happen. It holds the other structures in place.
- Nucleus — the dense, usually rounded body that controls all the cell's activities and carries the information passed on to new cells. It is the control centre.
- Cell wall — a rigid outer layer of cellulose found only in plant cells, outside the membrane. It gives strength, protection and a definite shape.
- Vacuole — a fluid-filled space that stores water, food and wastes. Plant cells have one large central vacuole; animal cells have small ones or none.
- Chloroplast — the green body containing chlorophyll, found only in plant cells, where photosynthesis takes place.
The crisp snap of a fresh stick of celery comes from cell walls and full vacuoles; a wilted spinach leaf has lost the water from those vacuoles.
The part most often confused is the cell wall with the cell membrane. Every cell has a membrane; only plant cells add a wall outside it. So a plant cell has both, and an animal cell has the membrane alone.
How do plant and animal cells differ, and how does shape suit function?
Plant cell
- Has a cell wall outside the membrane
- Has chloroplasts, so it can make food
- Has one large central vacuole
- A fixed, usually rectangular shape
- Nucleus pushed to the side by the vacuole
Animal cell
- No cell wall — only the membrane
- No chloroplasts, so it cannot make food
- Small vacuoles, or none
- A flexible, often rounded shape
- Nucleus usually near the centre
Cells then take shapes that suit their jobs:
- Nerve cell — very long and branched, sometimes a metre or more, so it can carry messages over long distances in one step.
- Muscle cell — long and spindle-shaped, able to contract and relax so bones can be pulled.
- Red blood cell — a small biconcave disc with no nucleus, giving a large surface for absorbing oxygen and enough flexibility to bend through narrow capillaries.
- Root hair cell — drawn out into a long thin projection, increasing the surface for absorbing water from the soil.
- Guard cells — kidney-shaped, so they can open and close the stomata on a leaf.
The usual mistake is assuming all cells look like the round diagram in a textbook. That drawing is a typical cell, and real cells differ enormously — the shape is a clue to the function, so a long branched cell is doing something quite different from a flat tile-like one.
- Has a cell wall outside the membrane
- Has chloroplasts, so it can make food
- Has one large central vacuole
- A fixed, usually rectangular shape
- Nucleus pushed to the side by the vacuole
Animal cell
- No cell wall — only the membrane
- No chloroplasts, so it cannot make food
- Small vacuoles, or none
- A flexible, often rounded shape
- Nucleus usually near the centre
Cells then take shapes that suit their jobs:
- Nerve cell — very long and branched, sometimes a metre or more, so it can carry messages over long distances in one step.
- Muscle cell — long and spindle-shaped, able to contract and relax so bones can be pulled.
- Red blood cell — a small biconcave disc with no nucleus, giving a large surface for absorbing oxygen and enough flexibility to bend through narrow capillaries.
- Root hair cell — drawn out into a long thin projection, increasing the surface for absorbing water from the soil.
- Guard cells — kidney-shaped, so they can open and close the stomata on a leaf.
The usual mistake is assuming all cells look like the round diagram in a textbook. That drawing is a typical cell, and real cells differ enormously — the shape is a clue to the function, so a long branched cell is doing something quite different from a flat tile-like one.
What is the difference between unicellular and multicellular organisms?
A unicellular organism is made of a single cell, which must carry out every life process by itself — nutrition, respiration, movement, excretion and reproduction. A multicellular organism is made of many cells that share the work.
Unicellular: amoeba, paramecium, euglena, bacteria, yeast, chlamydomonas.
Multicellular: human beings, dogs, mango trees, ferns, mushrooms, most animals and plants.
An amoeba shows how much one cell can manage: it moves using finger-like pseudopodia, engulfs its food, and divides in two to reproduce — all within a single cell.
Cells are far too small to be seen with the naked eye, so a microscope is needed. It works with lenses that magnify the image many times, making cells and the parts inside them visible. A drop of pond water under a microscope reveals organisms that are completely invisible in the glass.
So the size difference between an amoeba and an elephant is not a matter of bigger cells. Both are built from cells of broadly similar, tiny size — the elephant simply has an enormous number of them, which is exactly what makes division of labour between tissues possible.
Unicellular: amoeba, paramecium, euglena, bacteria, yeast, chlamydomonas.
Multicellular: human beings, dogs, mango trees, ferns, mushrooms, most animals and plants.
An amoeba shows how much one cell can manage: it moves using finger-like pseudopodia, engulfs its food, and divides in two to reproduce — all within a single cell.
Cells are far too small to be seen with the naked eye, so a microscope is needed. It works with lenses that magnify the image many times, making cells and the parts inside them visible. A drop of pond water under a microscope reveals organisms that are completely invisible in the glass.
So the size difference between an amoeba and an elephant is not a matter of bigger cells. Both are built from cells of broadly similar, tiny size — the elephant simply has an enormous number of them, which is exactly what makes division of labour between tissues possible.
What is the correct order of the levels of organisation?
In a multicellular body the levels build upward, each made of the one below:
cell, then tissue, then organ, then organ system, then organism
- A cell is the basic unit of structure and function.
- A tissue is a group of similar cells doing the same specific job.
- An organ is made of several different tissues working together for one function.
- An organ system is a group of organs carrying out a major function.
- The organism is the complete living body.
Worked through the human body:
- Cell — a muscle cell
- Tissue — muscle tissue
- Organ — the heart
- Organ system — the circulatory system, with heart and blood vessels
- Organism — the human being
And through the digestive route: a lining cell, then epithelial tissue, then the stomach, then the digestive system, then the person.
In a plant: a cell, then xylem tissue, then a leaf, then the shoot system, then the whole plant.
The word that decides the boundary is similar. A tissue is made of similar cells, while an organ contains several different tissues — which is why a leaf and a stomach are organs, not tissues, even though each does one recognisable job.
cell, then tissue, then organ, then organ system, then organism
- A cell is the basic unit of structure and function.
- A tissue is a group of similar cells doing the same specific job.
- An organ is made of several different tissues working together for one function.
- An organ system is a group of organs carrying out a major function.
- The organism is the complete living body.
Worked through the human body:
- Cell — a muscle cell
- Tissue — muscle tissue
- Organ — the heart
- Organ system — the circulatory system, with heart and blood vessels
- Organism — the human being
And through the digestive route: a lining cell, then epithelial tissue, then the stomach, then the digestive system, then the person.
In a plant: a cell, then xylem tissue, then a leaf, then the shoot system, then the whole plant.
The word that decides the boundary is similar. A tissue is made of similar cells, while an organ contains several different tissues — which is why a leaf and a stomach are organs, not tissues, even though each does one recognisable job.
Exam tip
Exam tip: pairing every cell part with its function
Cell questions are marked on named parts and their jobs, so never give one without the other.
Write them as pairs: nucleus — controls the cell's activities, cell membrane — controls what enters and leaves. A labelled diagram with no functions scores only half.
For a plant-versus-animal comparison, give the three big differences — cell wall, chloroplast and large central vacuole — since those are the ones expected.
Keep the cell wall and cell membrane apart: every cell has a membrane, only plant cells add a wall.
When asked how a shape suits a function, state both halves: a red blood cell is a biconcave disc with no nucleus, giving more room for haemoglobin and a large surface for oxygen.
And learn the levels of organisation in order, with an example at each level from the same body system.
Write them as pairs: nucleus — controls the cell's activities, cell membrane — controls what enters and leaves. A labelled diagram with no functions scores only half.
For a plant-versus-animal comparison, give the three big differences — cell wall, chloroplast and large central vacuole — since those are the ones expected.
Keep the cell wall and cell membrane apart: every cell has a membrane, only plant cells add a wall.
When asked how a shape suits a function, state both halves: a red blood cell is a biconcave disc with no nucleus, giving more room for haemoglobin and a large surface for oxygen.
And learn the levels of organisation in order, with an example at each level from the same body system.
Did you know
Why is an elephant not made of bigger cells than an amoeba?
Because a cell has to exchange materials across its surface, and growing larger makes that harder rather than easier.
Everything a cell needs enters through its membrane, and everything it discards leaves the same way. As a cell grows, its volume increases faster than its surface, so a very large cell could not supply its own middle.
So nature builds big bodies from many small cells rather than a few large ones. The elephant's advantage is numbers and specialisation — cells arranged into tissues and organs, each doing one job well, which a single giant cell could never manage.
Everything a cell needs enters through its membrane, and everything it discards leaves the same way. As a cell grows, its volume increases faster than its surface, so a very large cell could not supply its own middle.
So nature builds big bodies from many small cells rather than a few large ones. The elephant's advantage is numbers and specialisation — cells arranged into tissues and organs, each doing one job well, which a single giant cell could never manage.
Key takeaways
Cells and levels of organisation: quick revision
- Every cell has a cell membrane controlling entry and exit, cytoplasm where activities occur, and a nucleus as control centre.
- Only plant cells have a cell wall, chloroplasts for photosynthesis and a large central vacuole — an animal cell has the membrane alone.
- Shape suits function: nerve cells are long and branched, muscle cells spindle-shaped and contractile, red blood cells biconcave with no nucleus to hold more haemoglobin, root hair cells drawn out for absorption.
- Unicellular organisms such as amoeba and yeast do everything in one cell; multicellular ones share the work.
- A microscope magnifies with lenses to make cells visible.
- The levels run cell, tissue, organ, organ system, organism — a tissue is similar cells, while an organ holds several different tissues.
You will remember all of this far better after answering five questions on it than after reading it twice.
- Only plant cells have a cell wall, chloroplasts for photosynthesis and a large central vacuole — an animal cell has the membrane alone.
- Shape suits function: nerve cells are long and branched, muscle cells spindle-shaped and contractile, red blood cells biconcave with no nucleus to hold more haemoglobin, root hair cells drawn out for absorption.
- Unicellular organisms such as amoeba and yeast do everything in one cell; multicellular ones share the work.
- A microscope magnifies with lenses to make cells visible.
- The levels run cell, tissue, organ, organ system, organism — a tissue is similar cells, while an organ holds several different tissues.
You will remember all of this far better after answering five questions on it than after reading it twice.