Complete Notes of The Fundamental Unit of Life

Fundamental Unit of Life

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The fundamental unit of life : CELL

The cell is the smallest structural and functional unit of life having ability. It is the building block of all living organisms.


  • Discovered by Robert Hooke in cork cells, in year 1665.
  • He used his self-designed primitive microscope to examine a thin slice of cork (tree bark). Through his observations he saw that it resembled the structure of honeycomb i.e., consists of many small compartments.
  • He named those tiny compartments as “Cell”, which is a Latin word for ‘a little room’.
  • The first live cell was discovered by Anton van Leeuwenhoek in algae spirogyra found in pond water.
  • Robert Brown discovered nucleus in 1831.
  • In 1839, Purkinje named the ‘fluid substance of cell’ as protoplasm.

Cell Theory : ‘All plants and animals are composed of cells and the cell is the basic unit of life.’

  • Given by Schleiden(1838) & Schwann(1839).
  • Expansion of cell theory à “All cells arise from pre-existing cells.” (Virchow in 1855)
  • Discovery of electron microscope à 1940
  • On the basis of number of cells making up the body of the organisms, the organisms are divided into two class:
  • Unicellular organisms à Organism composed of single cell. Ex- Amoeba, Paramoecium, Bactria etc.
  • Multicellular organisms à Organisms composed of many cells grouping together in a single body to form various body parts. Ex- fungi, plants & animals
  • The shape and size of the cells are related to their specific functions.
  • Each living cells perform certain basic function. There is division of labour. Similarly, there’s a division of labour inside a single cell. Each components of the cell, i.e., cell organelle, performs a specific function.
  • All the functions performed by the cell is the collective of the functions performed by its organelles altogether.

Components of cells:
Plasma membrane or cell membrane or plasma lemma

Outermost covering of the cell separating the cellular contents from the external environment.

It is called a selective permeable membrane as it allows the entry and exit of selected materials in and out of the cell and prevents the others.

Movement of materials across the plasma membrane takes place by diffusion (movement of substances from a region of high concentration to the region of low concentration) or osmosis (a special type of diffusion in which movement of water occurs from the region of high concentration to the region of low concentration through a selectively permeable membrane).

When the concentration of the particular substance is low inside the cell and higher in the outside environment, they move inwards and when the cellular concentration increases as compared to the environment, the movement is outwards.

Hypotonic solution   à Medium surrounding the cell has higher water concentration and low solute concentration than the cell (Water moves inside the cell)

Isotonic solution       à Medium surrounding the cell has the same water and solute concentration as the cell (No net water movement)

Hypertonic solution àMedium has lower water concentration as compared to the cell (cell will lose water).


Rigid outer covering lying outside the cell membrane providing external support.

Provides extra protection to the plants, fungi & bacteria to withstand extreme changes in environment.

Protect against plasmolysis (the phenomena of shrinking of living cells after losing its water content).

Absent in animal cells.

Plant cell wall is made up of cellulose.


A double membrane bound organelle containing the genetic materials (DNA).

Found in eukaryotic cells only and absent in prokaryotes.

Bound by nuclear membrane (2 layers), which contains pores allowing the transfer of materials across it.

It contains chromosomes (visible as rod-shaped structures when cell is about to divide.

Chromosomes contain genetic information in the form of DNA (Deoxyribo Nucleic acid), which is inherited from one generation to another.

Chromosomes are made up of DNA & proteins.

DNA contains information necessary for the organisation and formation of cell.

Functional segments of DNAs are known as genes.

In non-dividing cells DNA is present as a part of chromatin, visible as entangled mass of thread like structure, which get organised when the cells are about to divide.

a. Plays a central role in cellular reproduction (single cell dividing into 2 daughter cells)
b. Controls the development of cell and its functions by directing the chemical activities of the cell.

Nucleoidà Poorly developed or undefined nucleus containing only nucleic acids without the membrane found in bacteria.


  • Prokaryotes (pro=primitive; karyon=nucleus) : organisms lacking nuclear membrane
  • Eukaryotes: Organisms having well defined nucleus in their cells (i.e., nuclear membrane present)
  • The large region of the cell, containing fluid content enclosed by the plasma membrane
  • It contains all the cell organelles necessary for performing the normal functions of a living cell.
  • There are a lot of chemical activities needed by a cell to support its structure and functions.
  • The different chemical activities occurring inside a cell is separated inside the cell enclosed in small membrane bound compartments i.e., organelles.
  • EX- Nucleus, Endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, plastids and vacuoles.
  • These are very crucial for the proper functioning of the cell.
  • A large network of membrane bound tubes and sheets, looking like long tubules or round vesicles.
  • Two types of ER depending upon presence or absence of ribosomes on their surface
  • Looks rough under the microscope due to the ribosomes attached to its surface.
  • Ribosomes are the protein manufacturing sites and the proteins are transported inside the cell as per the need via ER
  • Have smooth appearance under the microscope due to absence of ribosomes on its surface.
  • Helps in fat or lipid manufacturing for cellular needs
  • Aids in membrane biogenesis (building the cell membrane)
  • There might be variations in the appearance of ER in different cells but they always form network system.
  • Serves as channel for protein transportation throughout the cytoplasm
  • Doubles as cytoplasmic network by providing a site for biochemical activities of the cell
  • Poison & drugs detoxification in vertebrates (SER)
  • Discovered by Camillo Golgi.
  • A system of membrane bound vesicles stacked parallel to each other, called cisternae.
  • Membrane of Golgi are usually connected to the ER membrane as the materials synthesized in the ER are packaged and dispatched to the target site through the Golgi apparatus.
  • Storage, modification and packaging of the products of ER
  • Synthesis of complex sugars from simple sugars.
  • Involved in the formation of Lysosomes.
  • Lysosomes are membrane bound sacs filled with very powerful digestive enzymes synthesized by RER, capable of breaking down all the organic materials.
  • It is known as waste disposal system of the cell, as it cleans the cell by digesting foreign materials and worn-out cell organelles.
  • Lysosomes are also known as ‘suicidal bags’ of a cell because when the cell gets damaged the lysosomes burst open and all the digestive enzymes come out and digest their own cells.
  • It is known as the powerhouse of the cell as the energy required for life processes is released by mitochondria in the form of ATP (Adenosine Triphosphate).
  • ATP is known as energy currency of the cell because the energy stored in the ATP is used for mechanical work and making new chemical compounds.
  • Double membrane bound organelle
  • Outer membrane à porous
  • Inner membrane à deeply folded to create a large surface area for ATP generating chemical reactions.
  • Mitochondria is able to make some of its proteins as it contains its own DNA and ribosomes.
  • Found only in plant cells.
  • Mainly of three types:
  • Chromoplast à coloured plastids i.e., contains coloured pigments
  • Leucoplast à colourless plastid i.e., contains white or colourless pigments. Starch, oils and protein granules are stored in leucoplast.
  • Chloroplast à contains green coloured pigment known as chlorophyll as well as various other yellow and orange pigments.
  • Plastids consists of numerous membrane layers embedded in stroma.
  • It is structurally similar to mitochondria and also possess its own DNA and ribosomes.
  • They work as the storage sac of the cell for solid or liquid contents.
  • Animal cells possess a small vacuole whereas plant cells do have very large contractile vacuole occupying 50-90% of the cytoplasm.
  • Plant vacuoles are full of cell sap and provide turgidity and rigidity to the cell.
  • It stores amino acids, sugars, organic acids and proteins.
  • In unicellular organisms like Amoeba, food vacuoles are present which stores the food consumed by the organism.
  • In some single-celled organisms there are specialized vacuoles helping in excretion of wate materials.
  • The structure and the functional ability of the cell is due to the organization of its membrane and organelles, collectively helping to perform normal cellular functions.


Q1. Make a comparison and write down ways in which plant cells are different from animal cells.

Answer :

                             Animal Cell                             Plant Cell
Vacuoles are smaller in size.Vacuoles are larger in size.
They have centrosome.They do not have centrosome.
Lysosomes are larger in number.Lysosomes are absent or very few in number.
The do not have cell wall.They have cell wall made up of cellulose.
They do not have chloroplast.They have chloroplast.

Q2. How is a prokaryotic cell different from a eukaryotic cell?

Answer :

                    Prokaryotic cell                          Eukaryotic cell
Prokaryotic cells are found in bacteria and blue-green algae.Eukaryotic cells are found in fungi, plants, and animal cells.
Membrane-bound cell organelles such as plastids, mitochondria, endoplasmic reticulum, Golgi apparatus, etc. are absent.Cell organelles such as mitochondria, plastids, endoplasmic reticulum, Golgi apparatus, lysosomes, etc. are present.
Cell division occurs through binary fission.Cell division occurs by mitosis.
Nucleolus is absent.Nucleolus is present.
Nuclear region is poorly defined due to the absence of a nuclear membrane or the cell lacks true nucleus.Nuclear region is well-defined and is surrounded by a nuclear membrane, or true nucleus bound by a nuclear membrane is present in the cell.
It contains a single chromosome.It contains more than one chromosome.
Size of the cell is generally small (0.5- 5 µm).Size of the cell is generally large (50- 100 µm).
Most prokaryotes are unicellular.Most eukaryotes are multicellular.

Q3. What would happen if the plasma membrane ruptures or breaks down?

Answer :

Plasma membrane is the permeable membrane of the cell. If the plasma membrane ruptures or breaks down then materials present in it will start to move freely. Therefore, if there is any rupture in the plasma membrane then the cell might leak out its contents and it will not be able to exchange material from its surrounding by diffusion or osmosis. Thereafter the cell will die.

Q4. What would happen to the life of a cell if there was no Golgi apparatus?

Answer :

Golgi apparatus has the function of storage modification and packaging of the products.

(i) Golgi bodies helps in the formation of complex sugars from simple sugar with help of enzymes. Therefore, if the Golgi apparatus is absent then the above process will not happen.

(ii) Lysosomes or peroxisomes are formed by the involvement of Golgi apparatus. Hence, if the Golgi body is absent in a cell, the synthesis of lysosomes or peroxisomes will not be possible in the cell.

(iii) Membranes of the Golgi apparatus are connected to ER membranes to collect simpler molecules and combines them to make more complex molecules. These are then packaged in small vesicles and are either stored in the cell or sent out as per the requirement. Thus, if the Golgi apparatus is absent in the cell, then the above process of storage, modification, and packaging of products will not be possible.

Q5. Which organelle is known as the powerhouse of the cell? Why?

Answer :

Mitochondria are known as the powerhouse of cell. They contain enzymes that are needed for stepwise oxidation of food present in the cells to carbon dioxide and water. Mitochondria create energy for the cell, and this process of creating energy for the cell is known as cellular respiration. The energy required for various chemical activities needed for life is released by the mitochondria in the form of Adenosine triphosphate (ATP) molecules. Hence, mitochondria are known as the powerhouse of cells.

Q6. Where do the lipids and proteins constituting the cell membrane get synthesized?

Answer :

Lipids are synthesized in Smooth endoplasmic reticulum (SER) and the proteins are synthesized in rough endoplasmic reticulum (RER).

Q7. How does an Amoeba obtain its food?

Answer :

Amoeba has flexible cell membrane. Amoeba obtains its food through the process of endocytosis. It takes in food using temporary finger-like extensions of the cell surface which fuse over the food particle forming a food-vacuole. Inside the food vacuole, complex substances are broken down into simpler ones which then diffuse into the cytoplasm. The remaining undigested material is moved to the surface of the cell and thrown out.

Q8. What is osmosis?

Answer :

The movement of water molecules from a region of high concentration to a region of low concentration through a selectively permeable membrane is called osmosis. It is a special case of diffusion, where the medium is water.

Q9. Carry out the following osmosis experiment:
Take four peeled potato halves and scoop each one out to make potato cups. One of these potato cups should be made from a boiled potato. Put each potato cup in a trough containing water. Now,
(a) Keep cup A empty
(b) Put one teaspoon sugar in cup B
(c) Put one teaspoon salt in cup C
(d) Put one teaspoon sugar in the boiled potato cup D.
Keep these for two hours. Then observe the four potato cups and answer the following:
(i) Explain why water gathers in the hollowed portion of B and C.
(ii) Why is potato A necessary for this experiment?
(iii) Explain why water does not gather in the hollowed out portions of A and D.

Answer :

(i) Water gets gathered in the hollowed portions of set-up B and C because water enters the potato as a result of osmosis. Since the medium surrounding the cell has a higher water concentration than the cell, the water moves inside by osmosis. Hence, water gathers in the hollowed portions of the potato cup.
(ii) Potato A in the experiment acts as a control set-up. No water gathers in the hollowed portions of potato A.
(iii) Water does not gather in the hollowed portions of potato A because potato cup A is empty. It is a control set-up in the experiment.
Water is not able to enter potato D because the potato used here is boiled. Boiling denatures the proteins present in the cell membrane and thus, disrupts the cell membrane. For osmosis, a semi-permeable membrane is required, which is disrupted in this case. Therefore, osmosis will not occur. Hence, water does not enter the boiled potato cup.


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