Cell

A cell is the basic structural and functional unit of all living organisms. It is the smallest
independently functioning unit that can carry out the processes essential for life. From simple
single-celled organisms like bacteria to complex multicellular organisms like plants and animals, all
living things are made up of cells.

Discovery of the Cell

a) In 1665, an English scientist named Robert Hooke was examining thin slices of cork under a
microscope.
b) Cork is a part of the bark of a tree, and Hooke wanted to see what it looked like under
magnification.
c) To his surprise, he observed tiny box-like structures within the cork that looked like a honeycomb.
d) These structures were actually compartments in the cork, separated by walls or partitions.
e) Inspired by their resemblance to small rooms or cells in a monastery or convent, Hooke called
these structures "cells."
f) However, it's essential to note that these were not living cells; they were the empty spaces left
behind by dead plant cells, particularly the cell walls.
g) Despite this, Hooke's discovery was groundbreaking because it was the first time anyone had
seen such structures under a microscope.
h) This discovery sparked a new field of study called cell biology and led to further investigations
into the microscopic world.
i) Scientists continued to explore using more advanced microscopes and eventually observed living
cells, the real building blocks of life.
j) Today, we know that all living things are made up of cells, and cells are like tiny building blocks that
make up every living organism.
k) The discovery of the cell revolutionized biology and changed how we understand life at its
smallest level.

Characteristics of Cells
Structure and Function: Cells have a specific structure that
allows them to carry out various functions necessary for
the survival and functioning of the organism. Different
types of cells are specialized to perform specific tasks,
such as muscle cells for movement, nerve cells for
communication, and red blood cells for oxygen transport.

Basic Unit of Life: The cell is considered the fundamental

unit of life because all living organisms are made up of
one or more cells. Whether it's a single-celled organism
like a bacterium or a complex multicellular organism like a
human, all life starts and functions at the cellular level.

Reproduction and Growth: Cells have the ability to reproduce and grow. They can divide into two
daughter cells through cell division, allowing the organism to develop and repair tissues.

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Homeostasis: Cells maintain a stable internal environment, known as homeostasis, by regulating the
balance of nutrients, water, and other substances within the cell.

Interactions: Cells communicate and interact with each other to coordinate the functions of different
tissues and organs within an organism. These interactions are crucial for proper functioning and
survival.

Organisms Show Variety in Cell Number, Shape and Size

1. Cell Number

a) Cells can vary significantly in number among different organisms. Some organisms, like
unicellular bacteria, consist of a single cell that performs all essential functions for survival.
b) On the other hand, multicellular organisms, like humans and animals, are made up of numerous
cells, ranging from billions to trillions. The number of cells in an organism depends on its complexity
and specialised functions.

2. Cell Shape

a) Cells come in various shapes, and their shape is

often related to their specific function. For example,
nerve cells (neurons) are long and branched to
facilitate the transmission of electrical signals over
long distances.
b) Muscle cells are elongated and cylindrical to
generate force and movement. Red blood cells are
biconcave discs to increase their surface area for
efficient oxygen exchange.

3. Cell Size

a) Cells can differ significantly in size, ranging from

the tiniest bacterial cells, which may be only a few
micrometres in length, to the much larger plant and
animal cells, which can be tens to hundreds of
micrometres in size.

b) The size of the cell is usually proportional to the overall size of the organism, but not always.
Some single-celled organisms can be relatively large, while some multicellular organisms have small
cells.
c) Some cells, like amoeba, have an irregular shape. These cells can constantly change their shape
and form temporary extensions called pseudopodia. Pseudopodia are temporary projections that aid
in cell movement and capturing food particles.

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4. Cell Specialisation

a) In multicellular organisms, cells show specialisation based on their roles in the body. Specialised
cells form tissues, which then combine to form organs.
b) Each specialised cell type performs specific functions crucial for the overall functioning of the
organism. For example, muscle cells are specialised for contraction, and nerve cells are specialised
for transmitting electrical signals.

5. Cell Complexity

Cells can vary in their complexity based on the type of organism they belong to. Prokaryotic cells,
found in bacteria and archaea, are relatively simple and lack membrane-bound organelles. Eukaryotic
cells, present in plants, animals, fungi, and protists, are more complex and contain membrane-bound
organelles, such as the nucleus, mitochondria, and endoplasmic reticulum.

Prokaryotic and Eukaryotic Cells

Prokaryotic Cell

a) Prokaryotic cells are simple and smaller in size compared to eukaryotic cells.
b) They lack a distinct nucleus and other membrane-bound organelles.
c) The genetic material in prokaryotic cells is organized as a single circular DNA molecule located in
the nucleoid region.
d) They do not have a true nucleus, and the DNA is not enclosed within a membrane.
e) Prokaryotic cells lack membrane-bound organelles like mitochondria, endoplasmic reticulum, and
Golgi apparatus.
f) They have a cell wall made of peptidoglycan, which provides structural support and protection.
g) Prokaryotic cells are found in unicellular organisms like bacteria and archaea.

Eukaryotic Cell

a) Eukaryotic cells are more complex and larger in size compared to prokaryotic cells.
b) They have a true nucleus, where the genetic material is enclosed within a double membrane.
c) The nucleus houses the cell's DNA in the form of multiple linear chromosomes.
d) Eukaryotic cells have various membrane-bound organelles, such as mitochondria, endoplasmic
reticulum, Golgi apparatus, lysosomes, and more.
e) These organelles compartmentalize the cell's functions and allow for specialised processes to
occur.
f) Eukaryotic cells may have a cell wall (e.g., in plant cells) or lack one (e.g., in animal cells).
g) They are found in multicellular organisms like plants, animals, fungi, and protists.

6. Cell Arrangement

In multicellular organisms, cells are organized into tissues, organs, and organ systems. The
arrangement and coordination of cells in these structures allow for the efficient functioning of the
entire organism. Different organs and tissues are composed of various types of cells working
together to perform specific functions.

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Parts of the Cell
A cell is a complex and organised structure with various components called organelles, each having
specific functions that contribute to the cell's overall functioning. Here are the main parts of the cell:

Cell Membrane (Plasma Membrane)

a) The cell membrane is a thin, flexible barrier that
surrounds the cell and separates its internal contents
from the external environment.
b) It is composed of a lipid bilayer with embedded
proteins.
c) The cell membrane regulates the movement of
substances in and out of the cell, maintaining its
internal environment and allowing essential materials to
enter while removing waste products.

Cytoplasm
a) The cytoplasm is a gel-like substance that fills the cell's interior, surrounding the organelles.
b) It contains various organelles suspended in a fluid called cytosol.
c) Many cellular processes, including protein synthesis and metabolism, occur in the cytoplasm.
d) The cell membrane separates the cell's internal contents (cytoplasm and nucleus) from the
external environment.
e) It forms a boundary that defines the cell's shape and structure.
f) The cell membrane protects the cell from harmful substances and controls the entry and exit of
molecules and ions.
g) It is selectively permeable, allowing only specific substances to pass through while restricting
others.
h) The cell membrane facilitates the movement of essential molecules in and out of the cell through
specialized transport proteins and channels.
i) It plays a crucial role in maintaining the internal balance of the cell's environment.
j) The cell membrane contains receptors and signalling molecules for communication with the
surrounding environment and neighbouring cells.

Nucleus
a) The nucleus is the control centre of the cell and houses the cell's genetic material in the form of
chromosomes made of DNA.
b) It is surrounded by a double-layered membrane called the nuclear envelope, which has pores that
allow the movement of molecules between the nucleus and cytoplasm.
c) The nucleus regulates gene expression and controls cellular activities, including growth, division,
and reproduction.

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Endoplasmic Reticulum (ER)
a) The endoplasmic reticulum is a complex network of membranes throughout the cell.
b) There are two types of ER: rough ER, studded with ribosomes on its surface, and smooth ER,
lacking ribosomes.
c) Rough ER is involved in protein synthesis and modification, while smooth ER plays a role in lipid
metabolism and detoxification.

Golgi Apparatus (Golgi Body)

a) The Golgi apparatus consists of stacks of flattened membranous sacs.
b) It receives proteins and lipids from the ER, modifies them, and sorts them for transport to specific
destinations within or outside the cell.
c) The Golgi apparatus is responsible for packaging and distributing cellular products.

Mitochondria
a) Mitochondria are double-membraned organelles known as the powerhouse of the cell.
b) They generate adenosine triphosphate (ATP), the cell's primary energy currency, through cellular
respiration.
c) Mitochondria contain their own DNA and replicate independently within the cell.

Lysosomes
a) Lysosomes are small sac-like organelles containing digestive enzymes.
b) They break down cellular waste, damaged organelles, and foreign materials, aiding in cell
recycling and maintaining cellular health.

Vacuoles
a) Vacuoles are membrane-bound sacs that store water, nutrients, waste products, and other cellular
materials.
b) In plant cells, a large central vacuole helps maintain turgor pressure and stores nutrients.

Ribosomes
a) Ribosomes are tiny structures responsible for protein synthesis.
b) They can be found free in the cytoplasm or attached to the rough ER.
c) Ribosomes read the genetic information from the nucleus and assemble amino acids into
proteins.
d) Each part of the cell has its specific function, and together, they work harmoniously to ensure the
cell's survival and proper functioning within the organism. The coordinated activities of these
organelles allow cells to carry out essential processes and maintain the life of the organism.

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Comparison of Plant and Animal Cells

Characteristic Plant Cell Animal Cell

Present in plant cells, providing

Absent in animal cells, allowing for more
Cell Wall structural support and protection.
flexibility in shape.
Made of cellulose.

Found in plant cells, including

Absent in animal cells; they do not have
Plastids chloroplasts that contain chlorophyll
chloroplasts.
for photosynthesis.

Large central vacuole present in plant Smaller and multiple vacuoles may be
Vacuole cells, storing water and other present in animal cells, but not as
substances. prominent as in plant cells.

Plant cells are typically more rigid

Animal cells are more flexible in shape
Shape and have a fixed shape due to the
and can change shape as needed.
presence of cell walls.

Abundant in animal cells, containing

Lysosomes Rarely found in plant cells. digestive enzymes for breaking down
waste materials.

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Similarities Between Plant and Animal Cells
Cell Membrane: Present in both plant and animal cells, surrounds the cell and controls the
movement of substances.
Nucleus: Both plant and animal cells have a nucleus, housing the genetic material.
Cytoplasm: Both cells have cytoplasm, where various cell organelles are located.
Mitochondria: Both cells have mitochondria, which are responsible for producing energy through
cellular respiration.
Endoplasmic Reticulum: Both cells have endoplasmic reticulum, involved in protein synthesis and
lipid metabolism.
Golgi Apparatus: Present in both cells, involved in the processing and packaging of proteins.