Cells are the basic unit of life and come in two main types - prokaryotic and eukaryotic. Prokaryotic cells lack a nucleus and are smaller, while eukaryotic cells have a nucleus enclosed in a nuclear envelope and are generally larger. All cells contain a cell membrane, cytoplasm, and various organelles that carry out specific functions necessary for life. Key organelles include mitochondria that generate energy, ribosomes for protein synthesis, the endoplasmic reticulum for transport, Golgi bodies for processing and packaging molecules, lysosomes for digestion, and a cytoskeleton for structural support and movement within the cell.

1. THE CELL
➢ Cell biology (also cellular biology or cytology) is a branch of biology that studies the
structure, function, and behaviour of cells. All living organisms are made of cells.
➢ A cell is the basic unit of life that is responsible for the living and functioning
organism.
DEFINITION OF CELL
❖ Cells are the structural, functional, and biological units of all living beings.
❖ A cell can replicate itself independently. Hence, they are known as the building blocks
of life.
DISCOVERY OF CELL
• Robert Hooke discovered the cell in 1665. Robert Hooke observed a piece of bottle
cork under a compound microscope and noticed minuscule structures that reminded
him of small rooms. Consequently, he named these “rooms” as cells. However, his
compound microscope had limited magnification, and hence, he could not see any
details in the structure. Owing to this limitation, Hooke concluded that these were
non-living entities.
• Later Anton Van Leeuwenhoek observed cells under another compound microscope
with higher magnification. This time, he had noted that the cells exhibited some form
of movement (motility). As a result, Leeuwenhoek concluded that these microscopic
entities were “alive.” Eventually, after a host of other observations, these entities were
named as animalcules.

2. • In 1883, Robert Brown, a Scottish botanist, provided the very first insights into the
cell structure. He was able to describe the nucleus present in the cells of orchid.
TYPES OF CELLS
Based on cellular structure, there are two types of cells:
• Prokaryotes
• Eukaryotes

3. Prokaryotic Cells
1. Prokaryotic cells have no nucleus. Instead, some prokaryotes such as bacteria have a
region within the cell where the genetic material is freely suspended. This region is
called the nucleoid.
2. They all are single-celled microorganisms. Examples include archaea, bacteria, and
cyanobacteria.
3. The cell size ranges from 0.1 to 0.5 µm in diameter.
4. The hereditary material can either be DNA or RNA.
5. Prokaryotes generally reproduce by binary fission, a form of asexual reproduction.
Eukaryotic Cells
1. Eukaryotic cells are characterised by a true nucleus.
2. The size of the cells ranges between 10–100 µm in diameter.
3. This broad category involves plants, fungi, protozoans, and animals.
4. The plasma membrane is responsible for monitoring the transport of nutrients and
electrolytes in and out of the cells. It is also responsible for cell-to-cell
communication.
5. They reproduce sexually as well as asexually.

4. s
Cell Structure
The cell structure comprises individual components with specific functions essential to carry
out life’s processes. These components include- cell wall, cell membrane, cytoplasm,
nucleus, and cell organelles.
Cell Membranes
➢ The cell membrane controls the movement of substances in and out of the cells. It
separates the cell from the external environment.
➢ It consists of two layers of phospholipids with protein and sugar molecules embedded
in them. The phospholipid molecules have a head, which is electrically charged &
hydrophilic, and a tail has no charge & is hydrophobic.
➢ The cell membrane is the outer covering of a cell within which, all other organelles
are present, such as the cytoplasm and nucleus, are enclosed. It is also referred to as
the plasma membrane.
➢ Plasma membrane is a semi permeable/ selectable membrane. It acts as a barrier.
➢ Besides this, the cell membrane also protects the cellular component from damage &
leakage.

5. Cytoplasm
➢ The cytoplasm is a thick, clear, jelly-like substance present inside the cell membrane.
➢ The cytoplasm contains all cellular constituents present between the plasma
membrane & nucleus. It can be divided in to cytosol & organelles. The cytosol is the
fluid portion of the cytoplasm & is the site for all cellular reaction.
➢ The cell organelles such as endoplasmic reticulum, vacuoles, mitochondria,
ribosomes, are suspended in this cytoplasm.
➢ A fluid filled within the cytoplasm called matrix. It helps maintain pressure inside the
cells and ensures the cell organelles are doesn’t shrinkage & burst.
Nucleus
➢ Nucleus is the largest organelle in the cells
➢ The nucleus contains the genetic material of the cell, the DNA.
➢ It sends signals to the cells to grow, mature, divide and die.
➢ The nucleus is surrounded by the nuclear envelope that separates the DNA from the
rest of the cell. The spherical body present at center of nucleus called nucleolus.
➢ It consists of 46 chromosomes, which are made from DNA
➢ Pores present in the nuclear envelope are called nuclear pores. It helps in transport of
substance between the nucleus & cytoplasm.

6. CELL ORGANELLES
Cells are composed of various cell organelles that perform certain specific functions to carry
out life’s processes. The different cell organelles, along with its principal functions, are as
follows:
MITOCHONDRIA
➢ Mitochondria are the 'power houses' of the cell. They are involved in the aerobic
respiration, the process by which chemical energy is made available in the cell. The
mitochondria generate ATP through chemical reactions. Their number varies
depending on how active the cell is.
➢ The mitochondrial wall consists of two membranes such as the outer mitochondrial
membrane and the inner mitochondrial membrane.
➢ The outer mitochondrial membrane covers the inner mitochondrial membrane from
outside. The Inner mitochondrial membrane has many infolding called cristae.
➢ Cristae increase the surface area for chemical reactions. The large area present at the
center of the mitochondria is called matrix. Matrix is the fluid filled cavity present
inner to the inner mitochondrial Membrane.
Ribosomes
➢ Ribosome is the site for protein synthesis where the translation of the RNA takes place
➢ As protein synthesis is very important to the cell, ribosomes are found in large number
in all cells
➢ Ribosomes are found freely suspended in the cytoplasm and also are attached to the
endoplasmic reticulum.
➢ Ribosomes are also present in mitochondrial & they synthesize mitochondrial protein.

7. Endoplasmic Reticulum
➢ Within cytoplasm of the cell is an extensive network of membrane arranged in plates
and tubules, collectively called as endoplasmic reticulum (ER)
➢ ER is the transport system of the cell. It transports molecules that need certain changes
and also molecules to their destination.
➢ ER is of two types, rough and smooth ER.
➢ ER bound to the ribosomes appears rough and is the rough endoplasmic reticulum.
These are the sites for the protein synthesis, while the smooth ER does not have the
ribosomes & they help in the synthesis of lipids & steroid hormones and it also helps
in the detoxification of some drugs.
Golgi Bodies
➢ Golgi bodies are compact and consist of parallel membrane plates and tubule
➢ It is the site for enzyme secretion. It participates in formation of lysosomes
➢ The Golgi bodies modify the molecules from the rough ER by dividing them into
smaller units with membrane known as vesicles
➢ It is involved with processing and packaging of the molecules that are synthesized by
the cells.
Lysosomes
➢ It is the digestive system of the cell
➢ These are the small secretory vesicle formed by Golgi apparatus
➢ They have digestive enzymes helps in breakdown the waste molecules & also help in
detoxification of the cell
➢ If the lysosomes were not membrane bound the cell could not have used the destructive
enzymes
➢ They are referred to as the suicide bags of M the cell. They have digestive enzymes and
are involved in clearing the unwanted waste materials from the cell. They also engulf

8. damaged materials like the damaged cells, and invading microorganisms and digest
food particles.
Cytoskeleton
➢ These are small protein strands or Fibers. It includes: microfilaments, microtubules,
centrosomes and cell extensions.
➢ Microfilaments are smallest protein Fibers. They provide structural support and
maintain shape of the cell. It also helps in contraction of the cell as in muscle cell.
➢ Microtubules are larger contractile protein Fibers. They are involved in the movement
of organelles within the cell.
Vacuoles
➢ They are bound by single membrane and small organelles
➢ In many organisms ‘vacuoles are storage organelles. They store excess food or water
➢ Vesicles are smaller vacuoles which function for transport in/out of the cell.