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Variety in Shape of Cells
- A body has many cell types.
- Each type has its own shape.
- Shape is related to function.
- Humans have about 20 cell types.
- A nerve cell is long and branched.
- Its long shape carries messages over long distances.
- So its shape helps it pass messages quickly.
- A muscle cell is pointed at both ends, like a spindle.
- It can contract and relax.
- So its shape helps bring about movement in the body.
- The nerve cell is the longest cell in the human body.
- White blood cells change shape to fight germs.
- Trap: xylem carries water up, phloem carries food both ways.
Variety in Number of Cells
- Cell number differs between organisms.
- Amoeba has one; humans have trillions.
- Small organisms have fewer cells.
- Fewer cells still work perfectly well.
- Amoeba has just one cell.
- That one cell does all its life jobs.
- So a small cell count is enough for it.
- A human body is made of trillions of cells.
- Different cells do different jobs.
- So a large body needs a huge number of cells.
- Unicellular = one cell; multicellular = many cells.
- An Amoeba's single cell does the work a whole body does.
- Trap: more cells does not mean a "better" organism.
Variety in Size of Cells
- Cells come in many sizes.
- Most are tiny and need a microscope.
- A few are visible to the eye.
- Bacteria measure 0.1 to 0.5 micrometre.
- A bacterial cell is extremely tiny.
- It can be seen only under a microscope.
- So most cells are microscopic in size.
- A bird's egg is a single cell.
- The ostrich egg is the biggest cell, about 17 cm long.
- So a few cells are big enough to see easily.
- One micrometre is one-millionth of a metre.
- A bird's egg yolk is a single cell, however large.
- Trap: largest cell = ostrich egg; longest cell = nerve cell.
Name the spindle-shaped cells present in the human body.
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Explanation
- Muscle cells are shaped like a spindle.
- This shape helps them contract and relax.
Name a single cell which behaves like a complete organism.
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Explanation
- Amoeba is made of just one cell.
- That single cell does all life functions.
Fill in the blank: The shape of a cell is related to its ______.
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Explanation
- A cell's shape suits the job it does.
- So shape is related to function.
State whether true or false: Muscle cells are branched.
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Explanation
- Nerve cells are branched, not muscle cells.
- Muscle cells are spindle-shaped.
State whether true or false: Amoeba has irregular shape.
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Explanation
- Amoeba keeps changing its shape.
- So it has an irregular shape.
State whether true or false: All the cells in our body are alike.
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Explanation
- Body cells are of many shapes and sizes.
- Each shape suits a different function.
State whether true or false: A hen's egg is a group of cells.
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Explanation
- A bird's egg is a single cell.
- So a hen's egg is one cell, not a group.
(a) Why are nerve cells long and have branches? (b) Make a sketch of a human nerve cell.
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Explanation
- The long shape lets one cell reach far across the body.
- The branches connect to many other body parts.
- So messages pass fast from one place to another.
- The sketch shows a cell body with nucleus, a long thread and branches.
A multicellular organism: (a) cat (b) bacteria (c) amoeba (d) paramecium
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Explanation
- A cat is made of many cells.
- The others are single-celled.
The largest cell: (a) amoeba (b) hen's egg (c) nerve cell (d) ostrich's egg
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Explanation
- The yolk of an ostrich egg is the largest cell.
