For a guide from class 9 to 12

1. What is a cell ?
Cell is the structural and functional unit of all
living beings.
Cell is often referred as “structural and functional unit of living organisms”. The reason
is that the cell is the smallest unit of body and directly or indirectly, it is responsible for
functions also. If we study about the hierarchy then we will notice that the level of
organization is :

3. The smallest unit of body is cell.
Microscope is an instrument that enables us to see those things that otherwise, can’t be
seen with the help of naked eye. It has the property to magnify the objects.
Microscope was discovered by

4. Cell membrane: It is the outer covering of a cell. It is a porous membrane through which selected
substances can enter or leave the cell. The cell membrane is also called plasma membrane.
Robert Brown, a Scottish
botanist best known for his
descriptions of cell nuclei and of
the continuous motion of minute
particles in solution, which came
to be called Brownian motion.
Cytoplasm: It is a jelly-like fluid that fills up the part of the cell
between the cell membrane and the nucleus. Several organelles are
embedded in the cytoplasm.
Nucleus: It is considered to be the brain of the cell. Nucleus is
surrounded by a membrane called the nuclear membrane and is filled
with a semi-solid substance called the nucleoplasm. Thread-like
structures called chromosomes are present in the nucleoplasm.
Chromosomes carry genes, which transfer the characteristics of a cell to
the new cells that are formed during cell division. Also present inside the
nucleus is a round granule called nucleolus, which contains a network of
fibrous material called chromatin fibres.

5. What is Compound Microscope?
Compound microscopes use three lenses are used to accomplish the viewing:-
 the eyepiece lens
 condenser lens
 objective lens.
• It is said that the invention of the
compound microscope was done
by the Dutch spectacle-maker
Zacharias Janssen in early 1590.

7. Characteristic Compound Microscope Electron Microscope
Mechanism Uses glass lenses. Uses electromagnets.
Illumination Uses a beam of light to
illuminate the object.
Uses a beam of electrons to
illuminate the object.
Internal Vacuum Is not required. Is essential.
Magnification Magnifies the object about
2000 times
Magnifies the object about
2,00,000 times.

8.  Ernst Ruska and Max Knoll, a physicist
and an electrical engineer, respectively, from
the University of Berlin, created the first
electron microscope in 1931.
 This prototype was able to produce a
magnification of four-hundred-power and
was the first device to show what was
possible with electron microscopy.

9. In 1838, a German botanist, Matthias Schleiden was the first to state that cells are the
building blocks of all plants. In the following year, another German physician and
physiologist, Theodor Schwann stated that cells are the fundamental units of animals
too. These statements ended the notion that plants and animals have fundamental
differences in structure.
Their discoveries led to the formulation of the
‘Cell Theory’.
Cell Theory states that cells are the basic units of
all living organisms. But, the cell theory failed to
explain how new cells arise. In 1855, Rudolf
Virchow, a German embryologist stated in
German ‘Omnis cellula e cellula’ which means
that new cells come from already existing cells.
The three important points of the
modified cell theory are as follows:
 The cell is the basic functional and
structural unit of all living organisms.
 All living organisms are made up of
cells.
 All cells arise from pre-existing cells.

10. Virus is an exception to cell theory as it lacks cell and protoplasm,
the living substance in which all life processes take place. But it can reproduce
in a living host cell, by using raw materials (nucleotides) and biosynthetic
machinery (ribosomes, /RNA, enzymes) of the host cell.

11. Characters Prokaryotic Cell Eukaryotic Cell
Prokaryotic Cell

12. Cell shape and Cell size
It is seen that cells can be of different shapes and sizes and even their number that make
up a body can be different in different living forms as it depends upon the function the
cell is going to perform. Accordingly the structures, sizes, shapes are assigned to cells .
The shape of a cell may change or fixed.
Shape of cells
• The shape can be fixed or can be
variable.
• The cells with Variable shape are WBC
(white blood cell) and Amoeba.
• Please do remember that “the shape of
the cell depends upon the function it
performs”.
Different Shapes that the cell can have:
 Spherical => eggs of many animals
 Spindle shaped => smooth muscle fiber
 Elongated => nerve cell
 Branched => cells of skin
 Discoid => RBC.

13. Size of cell
Sizes also show a lot of variations. We have cell that is too tiny like PPLO(pleuropneumonia-
like organisms ) to largest cell in the form of an ostrich egg.
 Smallest cell – Mycoplasma
 Largest cell – Ostrich egg
In Human body the smallest cell is: – RBC and longest /largest is Nerve cell .Nerve cell is
designed accordingly to function it performs. As we cleared above also shapes are in
accordance to function. Like, nerve cell help in transmission of impulse. So, it has to be
long and branched.

14. Cell number
The number of cells also varies from species to species. There can be organism made up
of one cell and there can also be organism like us whose body is made up of many cells.
Depending upon the number of cells present, cells are of two types:
 Unicellular
 Multicellular
Unicellular organisms – are single celled body (example Amoeba).
Multicellular – are multi-celled body that is many cells (example Man)

15. • Robertson proposed unit membrane concept for biological membranes.
• According to this concept, the biological membrane is a lipid bilayer
surrounded on either side by proteins with a difference in their type for
the outer and inner side.
• Danielli and Davson, proposed a model, called sandwich model, for membrane
structure in which a lipid bilayer was coated on its either side with
hydrated proteins (globular proteins)

16.  All cells have three things in common:

17. The plasma membrane forms the cell’s flexible outer surface, separating the cell’s
internal environment from the external environment. The side that faces the cytoplasm
is the intracellular face of the membrane, and the side that faces outward is the
extracellular face.
• It is a selective barrier that regulates the flow of materials into and out of a cell.
• This selectivity helps establish and maintain the appropriate environment for normal
cellular activities.
• The plasma membrane also plays a key role in communication among cells and
between cells and their external environment.
• The plasma membrane is an oily, two-layered lipid film with proteins embedded in it.

18. • Membranes are made of a double layer of lipids, mainly phospholipids, containing embedded
proteins.
• About 75% of the membrane lipid molecules are phospholipids.
• A phospholipid consists of a head (hydrophilic) and two fatty acid tails
(hydrophobic).
• The embedded proteins are important as facilitators in moving molecules through the
membrane.
• The membrane itself is organized into a bimolecular layer, meaning that the non-polar region is
organized in the middle (away from water as it is hydrophobic) and the polar regions are
oriented toward the outside: the extracellular fluid and the cytosol.
• Another way to think of it is two rows of pins with their heads to the outside and the needle part
to the inside. Heads, needles, needles, heads. Like a sandwich.
• As the phospholipid molecules are not chemically bound to each other and thus each molecule is
free to move independently, the overall bi-layer structure has a flexible fluidity.
• Cholesterol (20%) molecules are also embedded in the plasma membrane and serve to deliver
substances to cell organelles by forming vesicles.
• Various glycolipids (about 5%), lipids with attached carbohydrate groups.

20. Functions of the Plasma Membrane
1. Acts as a barrier separating inside
and outside of the cell.
2. Controls the flow of substances into
and out of the cell.
3. Helps identify the cell to other cells
(e.g., immune cells).
4. Participates in intercellular
signalling.

21. Cell transport is movement of materials across cell membranes.
It includes : PASSIVE TRANSPORT and ACTIVE TRANSPORT.
• Passive transport does not require energy whereas active transport requires energy to proceed.
Passive transport proceeds through diffusion, facilitated diffusion and osmosis.
• Active Transport can involve either a pump or a vesicle.
Pump Transport can be Primary/Secondary.
Vesicle Transport can involve.  Exocytosis.
 Endocytosis- which includes:- Pinocytosis & Phagocytosis.

22. PASSIVE TRANSPORT
 Simple diffusion is the process by which solutes
are moved along a concentration gradient in a
solution or across a semipermeable membrane
 Osmosis is the net movement of solvent molecules
through a semipermeable membrane.
It is similar to diffusion as the movement is downhill,
meaning from higher to lower concentration.
In osmosis though, the movement has to occur across a
semipermeable membrane. Without this element, it cannot
be called osmosis.
 Facilitated diffusion is a form of facilitated transport
involving the passive movement of molecules along
their concentration gradient, guided by the presence of
another molecule– usually an integral membrane protein
forming a pore or channel.

23. Active Transport
Primary active transport
Secondary Active Transport

24. Exocytosis is the process of moving materials from
within a cell to the exterior of the cell. This process
requires energy and is therefore a type of active transport.

25. The cytoplasm contains all of the sugars, amino acids, and proteins the cell uses to carry
out its everyday activities. Although it is an aqueous medium, cytoplasm is more like
Jell-O than water due to the high concentration of proteins and other macromolecules.
The part of the cytoplasm that contains organic molecules and ions in solution is called
the cytosol to distinguish it from the larger organelles suspended in this fluid.
The cytosol is also called the intracellular fluid (ICF).
All body fluids not contained in the cells are collectively called the extracellular fluid
(ECF). The ECF located between the cells is also called tissue (interstitial) fluid.

26. A cell wall is a structural layer surrounding some types of cells, just outside
the cell membrane.
It is a semi-rigid thick protective structure ,provides protection and defines the
shape of the cell.

29. Thank you