Short Notes On Biology:CELL

Short Notes On Biology: CELL

Here we are providing the short notes on Biology which will be help to crack science part in may examinations like SSC, Railway and other competitive exams.


Cell: It is the basic structural unit of life.

Cells were first discovered by Robert Hooke.

Note: The smallest cell is 0.1 to 0.5 micrometre in bacteria. The largest cell measuring 170 mm ×130 mm, is the egg of an ostrich.

Amoeba acquires its food through endocytosis.

  1. Prokaryotes cellscells that have no defined nucleus

Eg: Bacteria & Blue-green Algae

  1. Eukaryote – cells which have definite nucleus

Eg: Other than Bacteria & Blue-green Algae

Compounds called proteins and phospholipids make up most of the cell membrane.

Diffusion-It is a process of movements of substance from a region of high concentration to a region where its concentration is low. Water also obeys the law of diffusion.

Eg: Substances like Co2 and O2 can move across the cell membranes by a process called diffusion.


Osmosis: The movement of water molecules is called osmosis. Osmosis is a special case of diffusion through a selectively permeable membrane.


Types of Osmosis:

  1. Hypotonic: more water will come into the cell than will leave. The cell is likely to swell up.
  2. Isotonic: the amount going in is the same as the amount going out of the cell.The cell will stay the same size.
  3. Hypertonic: more water leaves the cell than enters it. Therefore the cell will shrink.

When a living plant cell loses water through osmosis there is shrinkage or contraction of the contents of the cell away from the cell wall. This phenomenon is known as plasmolysis.


Cytoplasm: It is the fluid that fills a cell. Scientists used to call the fluid protoplasm.


Ribosomes: It synthesis protein, and Endoplasmic reticulum sent these protein in various part of the cell. Whereas Smooth Endoplasmic reticulum helps in the manufacture of fats. It a made up of ribonucleic acid.


Functions of these proteins and fats:

  • Protein and fat (lipid) help in building the cell membranes. This process is known as membranes biogenesis.
  • Smooth Endoplasmic reticulum plays a crucial role in detoxifying many poisons and drugs.


Golgi apparatus :  It is another packaging organelle like the endoplasmic reticulum functions:

  • It is the organelle that builds lysosomes (cells digestion machines).


Lysosomes(suicidal bag): It is a kind of waste disposal system of the cell.


Mitochondria(power house):  The energy required for various chemical activities headed for life is released by mitochondria in the form of ATP (adenosine-tri-phosphate) molecules.

  • ATP is known as the energy currency of the cell.
  • Mitochondria are strange organelles in the sense that they have their own DNA and ribosomes, therefore mitochondria are able to make their own protein.
  • Mitochondria is absent in bacteria and the red blood cells of mammals and higher animals.


Centrioles:  centrioles are concerned with cell division. It initiates cell division.


Plastids: These are present only in plant cells.

Types of plastids:-

  • Chromoplast(colour plastides) impart colour to flowers and fruits.
  • Leucoplasts(white or colourless plastids) present in which starch, oils and protein are stored.
  • Plastids are self-replicating. i.e. they have the power to divide, as they contain DNA, RNA and ribosomes.
  • Plastides contains the pigment chlorophyll that is known as chloroplast. It is the site for photo synthesis.

non –living parts with in the cell :-


Vacuoles: it is a fluid filled spaces enclosed by membranes. Its size in animal is small and in plant it is big.

Amino acids and sugars are stored in vacuoles.


Granules: It is not bounded by any membranes. It store fats, proteins and carbohydrates.


Cell nucleus: The cell nucleus acts like the brain of the cell. It helps control eating, movement

and reproduction. Not all cells have a nucleus.

The nucleus contain, the following components :

(a) Nuclear envelope (nuclear membrane)

(b) Chromatin : When the cell is in resting state there is something called chromatin in the nucleus. Chromatin is made up of DNA, RNA and nucleus protein. DNA and RNA are the nucleus acids inside the cell. When the cell is going to divide, the chromatin become very compact. It condenses when the chromatin comes together we can see the chromosomes.

(c) Chromosomes: Chromosomes make organisms what they are. They carry all the information used to help a cell grow, thrive and reproduce.

  • Chromosomes are made up of DNA.
  • Segments of DNA in specific patterns are called genes.
  • In prokaryotes, DNA floats in the cytoplasm in an area called the nucleoid.
  • Chromosomes are not always visible. They usually sit around uncoiled and as loose shards called
  • Chromosomes are usually found in pairs.
  • Human Beings probably have 46 chromosomes (23 pairs).
  • Peas only have 12, a dog has 78 chromosomes.
  • The number of chromosomes is not related to the intelligence or complexity of the creature.

(d) Nucleolus: It is a dense spherical granule contained within the nucleus.It stores proteins.

Cell Division

Organisms grow and reduce through cell  division.

There are two methods of replication mitosis and meiosis.

(a) Mitosis: It duplicates its DNA and the two new cells (daughter cells) have the same pieces and generic code. There are five steps in this process. You should remember the term PMATI. It breaks down to :

  1. Prophase
  2. Metaphase
  3. Anaphase
  4. Telophase
  5. Interphase.

The main theme of meiosis is that there are two cell division. Mitosis has one division


Some important facts regarding cells :

  • Nerve cells in animals are the longest cells.
  • Smallest human cell is red blood cell.
  • Largest human cell is female ovum.
  • The single largest cell in the world is of an ostrich.
  • The smallest cells are those of the mycoplasma.
  • Every minute about 3 million cells in our body die.
  • Sieve tube in plants and the mature mammalian red blood cells do not have a nucleus.
  • The red blood

cell carries respiratory gases.

  • Sieve cells in plants transport nutrients in plants.
  • The lysosomal enzymes of the sperm cells digest the limiting membranes of the ovum (egg). Thus the sperm is able to enter the ovum.
  • During the transformation of tadpole into frog. The embryonic tissues like gills and tail are digested by the lysosome.
  • Mitochondria contain DNA, hence capable of replication.
  • Matrix is a transparent, homogenous semi-fluid substance. In its active state. It remains saturated with water.

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