Source is from ncert 8 th standard text book

2. • You know , that all organisms are made up of small units, called cells .
• As are the bricks to building , so are the cells to the body.
• You have already learnt that organisms may be unicellular or multicellular.
• The number of cells in multicellular organisms may be from a few hundreds to
billions’
• Cells perform activities such as nutrition respiration and cell division , which
contribute to overall growth of an organism. hence, cells are called structural and
functional unit of an organism.
INTRODUCTION

3. • Cells are microscopic structures and are not visible to our naked eyes.
• Hence we need a microscope to study the biology of cells.
• As cells are very small , ordinary units are not convenient to express the size of
the cells.
• Micron is the most commonly used unit to express the size of the cell.
• One micron is equal to 1/1000 of a millimetre.
• The cell and its components can be observed clearly with the help of electron
microscope.
• Electron microscope uses the beam of electrons to get the image of object.
• Its magnification is about 200000 times as against the compound microscope,
which magnifies objects up to 2000 times.

4. NUMBER OF CELLS
• The number of cells varies from tall tree to huge animal.
• The number runs into billions and trillions.
• Human body has trillion of cells which vary in shapes and sizes.
• Organisms made of more than one cell are called multicellular organisms.
• Organisms made of only one cell where all the necessary functions are carried out only
• Through single cell is called unicellular organisms.
• Example :amoeba , paramecium

6. SHAPE OF CELLS
• The shape may vary from organism to organism.
• The shape of amoeba is irregular, it has no definite shape.
• The change in shape is due to formation of pseudopodia which facilitates movement
• And help in capturing food.
• A white blood cell in human is another example of a single cell which can change its shape.
• Generally cells are round , spherical and elongated.
• Some cells are long and pointed at both ends.
• Some are branched like nerve cell or a neuron
• Cell membrane gives a definite shape and rigidity to the cell.

7. SIZE OF CELLS
The size of cells in living organisms may be as small as millionth of a metre or may be as large as a few centimetres.
Most of the cells are microscopic in size and are not visible to the unaided eye.
The smallest cell is 0.1 to 0.5 mm in bacteria.
The largest cell measuring 170mm x 130mm, is the egg of the ostrich.

8. CELL STRUCTURE AND FUNCTION
• Each organ in the system performs different functions such as digestion , assimilation and absorption.
• Each organ is further made up of tissues . A tissue is a group of similar cells performing a specific function.
• Organ is made up of tissues which in turn , are made up of cells.
• Hence the cell is the structural and functional unit of an living organism.

9. ACTIVITY
Peel off a thin layer from a piece of an onion. Put this on a watch glass containing water. Take a
glass slide, put a drop of water. Transfer a small piece of the onion layer, on the water drop on the
slide. Put a drop of stain mostly iodine solution on this piece. Observe the slide under compound
Microscope.
We observe,
1. Cell membrane
2. Cytoplasm
3. nucleus

11. •
• A cell is surrounded by a cell membrane . also called plasma membrane.
• It separates the interior of the cell from outside environment.
• Cell membrane forms the boundary of the cell.
• It helps to maintain the shape of the cell.
• It controls the movement of selected substances into and out of it, hence the cell membrane is
• described as semi permeable membrane.
• The movement of the molecules across the cell membrane occurs by two process
• Diffusion
• osmosis
CELL MEMBRANE

12. Activity
Diffusion experiment

13. Activity
Osmosis experiment

14. • It is a jelly like substance found within the cell membrane.
• Nearly 80% of cytoplasm is water.
• It is the seat for most cellular activities.
• Various other components or cell organelles are present in
• The cytoplasm.
• There are mitochondria and golgicomplex ,ribosomes, lysomes and
other or
• Organelles are present in cytoplasm.
CYTOPLASM

15. NUCLEUS
Nucleus is a spherical organelle found in all eukaryotes
Nucleus is covered by a double membrane called nuclear membrane.
The membrane encloses a fluid called nucleoplasm.
nucleoplasm has a network of thread like structures called chromatin.
During cell division chromatin undergoes coiling and super coiling and becomes short and
Thick to for chromosomes.
Think
Mammalian rbc cells do not have nucleus at maturity?