The cell is the structural and functional unit of all living
organisms, and is sometimes called the “building block of
life.” Some organisms, such as bacteria are unicellular,
consisting of a single cell. Other organisms such as humans
are multicellular. Humans have an estimated 100 trillion or
1014 cells. The largest known cell is an Ostrich egg.
Anton van Leeuwenhoek was the first person to build
a microscope and draw protozoa, such as Vorticella from
rain water, and bacteria from his own mouth.
In 1665 Robert Hooke discovered cells in cork, then in
living plant tissue using an early microscope.
First of all in 1839 Schleiden and Schwann, states that
all organisms are composed of one or more cells. All
cells come from preexisting cells. Vital functions of an
organism occur within cells, and all cells contain the
hereditary information necessary for regulating cell
functions and for transmitting information to the next
generation of cells.
In 1931 Ernst Ruska built first transmission electron
microscope (TEM) at the University of Berlin and by
1935, he built an EM with twice the resolution of a light
microscope, revealing previously-unresolvable
organelles.
In 1953 Watson and Crick made their first announcement
on the double-helix structure for DNA
Each cell is at least somewhat self-contained and selfmaintaining:
It can take in nutrients, convert these nutrients
into energy, carry out specialized functions, and reproduce
as necessary. Each cell stores its own set of instructions for
carrying out each of these activities.
Anatomy of Cell
There are two types of cells, eukaryotic and prokaryotic.
Prokaryotic cells are usually singletons, while eukaryotic
cells are usually found in multicellular organisms.
1. Prokaryotic Cells: Prokaryotes are distinguished
from eukaryotes on the basis of nuclear organization,
specifically their lack of a nuclear membrane.
Prokaryotes also lack most of the intracellular
organelles and structures that are characteristic of
eukaryotic cells (an important exception is the
ribosomes, which are present in both prokaryotic and
eukaryotic cells). Most of the functions of organelles,
such as mitochondria, chloroplasts, and the Golgi
apparatus, are taken over by the prokaryotic plasma
membrane.
The plasma membrane separates the interior of the cell
from its environment and serves as a filter and
communications beacon.
Most prokaryotes have a cell wall which acts as an
additional barrier against exterior forces. It also prevents
the cell from exploding (cytolysis) from osmotic pressure
against a hypotonic environment. A cell wall is also
present in some eukaryotes like fungi, but has a different
chemical composition.
A prokaryotic chromosome is usually a circular molecule
without a real nucleus and the DNA is condensed in a
nucleoid.
2. Eukaryotic Cells: Eukaryotic cells are about 10 times
the size of a typical prokaryote and can be as much as
1000 times greater in volume. The major difference
between prokaryotes and eukaryotes is that eukaryotic
cells contain membrane-bound compartments in which
specific metabolic activities take place. Most important
among these is the presence of a cell nucleus, a
membrane-delineated compartment that houses the
eukaryotic cell’s DNA. It is this nucleus that gives the
eukaryote its name, which means “true nucleus”.
The plasma membrane resembles that of prokaryotes in
function, with minor differences in the setup. Cell walls
may or may not be present.
The eukaryotic DNA is organized in one or more linear
molecules, called chromosomes, which are associated
with histone proteins. All chromosomal DNA are stored
in the cell nucleus, separated from the cytoplasm by a
membrane. Some eukaryotic organelles also contain
some DNA.
Eukaryotes can move using cilia or flagella. Their
flagella are more complex than those of
prokaryotes.
All cells, whether prokaryotic or eukaryotic, have a
membrane, which envelopes the cell, separates its interior
from its environment, regulates what moves in and out
(selectively permeable), and maintains the electric potential
of the cell. Inside the membrane, a salty cytoplasm takes up
most of the cell volume. All cells possess DNA, the
hereditary material of genes, and RNA, containing the
information necessary to build various proteins such as
enzymes, the cell’s primary machinery. There are also other
kinds of biomolecules in cells. The primary components of
the cell and their functions are as follows:
Cell Membrane (A cell’s defining boundary): The
cytoplasm of a cell is surrounded by a plasma
membrane. The plasma membrane in plants and
prokaryotes is usually covered by a cell wall. This
membrane serves to separate and protect a cell from its
surrounding environment and is made mostly from a
double layer of lipids and hydrophilic phosphorus
molecules. Hence, the layer is called a phospholipid
bilayer. This membrane has a variety of protein
molecules that act as channels and pumps that move
different molecules into and out of the cell. The
membrane is said to be ‘semi-permeable’. Cell surface
membranes also contain receptor proteins that allow cells
to detect external signalling molecules such as hormones.
Cytoskeleton (A cell’s scaffold): The cytoskeleton acts
to organize and maintain the cell’s shape; anchors
organelles in place; helps during endocytosis, the uptake
of external materials by a cell, and cytokinesis, the
separation of daughter cells after cell division; and moves
parts of the cell in processes of growth and mobility.
Eukaryotic cytoskeleton is composed of microfilaments,
intermediate filaments and microtubules. There is a great
number of proteins associated with them, each
controlling a cell’s structure by directing, bundling, and
aligning filaments.
Genetic Material: Within a cell two different kinds of
genetic material exist: deoxyribonucleic acid (DNA) and
ribonucleic acid (RNA). Most organisms use DNA for
their long-term information storage, but some viruses
(e.g., retroviruses) have RNA as their genetic material.
The biological information contained in an organism is
encoded in its DNA or RNA sequence. RNA is also
used for information transport (e.g., mRNA) and
enzymatic functions (e.g., ribosomal RNA) in organisms
that use DNA for the genetic code itself.
Prokaryotic genetic material is organized in a simple
circular DNA molecule (the bacterial chromosome) in the
nucleoid region of the cytoplasm. Eukaryotic genetic material
is divided into different, linear molecules called
chromosomes inside a discrete nucleus, usually with
additional genetic material in some organelles like
mitochondria and chloroplasts.
A human cell has genetic material in the nucleus (the
nuclear genome) and in the mitochondria (the mitochondrial
genome). In humans the nuclear genome is divided into 46
linear DNA molecules called chromosomes. The
mitochondrial genome is a circular DNA molecule separate
from the nuclear DNA. Although the mitochondrial genome
is very small, it codes for some important proteins.
Organelles: As the human body contains many different
organs, such as the heart, lung, and kidney with different
functions cells also have a set of little organs, called
organelles, that are specialized for carrying out one or
more vital functions. Membrane-bound organelles are
found only in eukaryotes.
Cell Nucleus (a cell’s information center): The cell
nucleus, found in a eukaryotic cell, is the house of
chromosomes, and is the place where almost all DNA
replication and RNA synthesis occur. The nucleus is
spheroid in shape and separated from the cytoplasm by
a double membrane called the nuclear envelope. During
processing, DNA is transcribed, or copied into a special
RNA, called mRNA. This mRNA is then transported
out of the nucleus, where it is translated into a specific
protein molecule. In prokaryotes, DNA processing takes
place in the cytoplasm.
Mitochondria and Chloroplasts (the power generators):
Mitochondria are self-replicating organelles that occur
in various numbers, shapes, and sizes in the cytoplasm
of all eukaryotic cells. As mitochondria contain their
own genome that is separate and distinct from the
nuclear genome of a cell, they play a critical role in
generating energy in the eukaryotic cell. Chloroplasts,
broadly called plastids, are often involved in storage.
Endoplasmic Reticulum and Golgi Apparatus
(macromolecule managers): The endoplasmic reticulum
(ER) is the transport network for molecules targeted for
certain modifications and specific destinations, as
compared to molecules that will float freely in the
cytoplasm. The ER has two forms: the rough ER, which
has ribosomes on its surface, and the smooth ER, which
lacks them. The ER contains many Ribosomes, the
protein production machine. The ribosome is a large
complex composed of many molecules, only exist
floating freely in the cytosol, whereas in eukaryotes they
can be either free or bound to membranes.
Lysosomes and Peroxisomes: The eukaryotic cell could
not house such destructive enzymes if they were not
contained in a membrane-bound system.
Centrosome (the cytoskeleton organiser): The
centrosome produces the microtubules of a cell, a key
component of the cytoskeleton. It directs the transport
through the ER and the Golgi apparatus. Centrosomes
are composed of two centrioles, which separate during
cell division and help in the formation of the mitotic
spindle. A single centrosome is present in the animal
cells. They are also found in some fungi and algae cells.
Vacuoles: They store food and waste. Some vacuoles
store extra water. They are often described as liquid
filled space and are surrounded by a membrane. Some
cells, most notably Amoeba have contractile vacuoles,
which are able to pump water out of the cell if there is
too much water.
organisms, and is sometimes called the “building block of
life.” Some organisms, such as bacteria are unicellular,
consisting of a single cell. Other organisms such as humans
are multicellular. Humans have an estimated 100 trillion or
1014 cells. The largest known cell is an Ostrich egg.
Anton van Leeuwenhoek was the first person to build
a microscope and draw protozoa, such as Vorticella from
rain water, and bacteria from his own mouth.
In 1665 Robert Hooke discovered cells in cork, then in
living plant tissue using an early microscope.
First of all in 1839 Schleiden and Schwann, states that
all organisms are composed of one or more cells. All
cells come from preexisting cells. Vital functions of an
organism occur within cells, and all cells contain the
hereditary information necessary for regulating cell
functions and for transmitting information to the next
generation of cells.
In 1931 Ernst Ruska built first transmission electron
microscope (TEM) at the University of Berlin and by
1935, he built an EM with twice the resolution of a light
microscope, revealing previously-unresolvable
organelles.
In 1953 Watson and Crick made their first announcement
on the double-helix structure for DNA
Each cell is at least somewhat self-contained and selfmaintaining:
It can take in nutrients, convert these nutrients
into energy, carry out specialized functions, and reproduce
as necessary. Each cell stores its own set of instructions for
carrying out each of these activities.
Anatomy of Cell
There are two types of cells, eukaryotic and prokaryotic.
Prokaryotic cells are usually singletons, while eukaryotic
cells are usually found in multicellular organisms.
1. Prokaryotic Cells: Prokaryotes are distinguished
from eukaryotes on the basis of nuclear organization,
specifically their lack of a nuclear membrane.
Prokaryotes also lack most of the intracellular
organelles and structures that are characteristic of
eukaryotic cells (an important exception is the
ribosomes, which are present in both prokaryotic and
eukaryotic cells). Most of the functions of organelles,
such as mitochondria, chloroplasts, and the Golgi
apparatus, are taken over by the prokaryotic plasma
membrane.
The plasma membrane separates the interior of the cell
from its environment and serves as a filter and
communications beacon.
Most prokaryotes have a cell wall which acts as an
additional barrier against exterior forces. It also prevents
the cell from exploding (cytolysis) from osmotic pressure
against a hypotonic environment. A cell wall is also
present in some eukaryotes like fungi, but has a different
chemical composition.
A prokaryotic chromosome is usually a circular molecule
without a real nucleus and the DNA is condensed in a
nucleoid.
2. Eukaryotic Cells: Eukaryotic cells are about 10 times
the size of a typical prokaryote and can be as much as
1000 times greater in volume. The major difference
between prokaryotes and eukaryotes is that eukaryotic
cells contain membrane-bound compartments in which
specific metabolic activities take place. Most important
among these is the presence of a cell nucleus, a
membrane-delineated compartment that houses the
eukaryotic cell’s DNA. It is this nucleus that gives the
eukaryote its name, which means “true nucleus”.
The plasma membrane resembles that of prokaryotes in
function, with minor differences in the setup. Cell walls
may or may not be present.
The eukaryotic DNA is organized in one or more linear
molecules, called chromosomes, which are associated
with histone proteins. All chromosomal DNA are stored
in the cell nucleus, separated from the cytoplasm by a
membrane. Some eukaryotic organelles also contain
some DNA.
Eukaryotes can move using cilia or flagella. Their
flagella are more complex than those of
prokaryotes.
All cells, whether prokaryotic or eukaryotic, have a
membrane, which envelopes the cell, separates its interior
from its environment, regulates what moves in and out
(selectively permeable), and maintains the electric potential
of the cell. Inside the membrane, a salty cytoplasm takes up
most of the cell volume. All cells possess DNA, the
hereditary material of genes, and RNA, containing the
information necessary to build various proteins such as
enzymes, the cell’s primary machinery. There are also other
kinds of biomolecules in cells. The primary components of
the cell and their functions are as follows:
Cell Membrane (A cell’s defining boundary): The
cytoplasm of a cell is surrounded by a plasma
membrane. The plasma membrane in plants and
prokaryotes is usually covered by a cell wall. This
membrane serves to separate and protect a cell from its
surrounding environment and is made mostly from a
double layer of lipids and hydrophilic phosphorus
molecules. Hence, the layer is called a phospholipid
bilayer. This membrane has a variety of protein
molecules that act as channels and pumps that move
different molecules into and out of the cell. The
membrane is said to be ‘semi-permeable’. Cell surface
membranes also contain receptor proteins that allow cells
to detect external signalling molecules such as hormones.
Cytoskeleton (A cell’s scaffold): The cytoskeleton acts
to organize and maintain the cell’s shape; anchors
organelles in place; helps during endocytosis, the uptake
of external materials by a cell, and cytokinesis, the
separation of daughter cells after cell division; and moves
parts of the cell in processes of growth and mobility.
Eukaryotic cytoskeleton is composed of microfilaments,
intermediate filaments and microtubules. There is a great
number of proteins associated with them, each
controlling a cell’s structure by directing, bundling, and
aligning filaments.
Genetic Material: Within a cell two different kinds of
genetic material exist: deoxyribonucleic acid (DNA) and
ribonucleic acid (RNA). Most organisms use DNA for
their long-term information storage, but some viruses
(e.g., retroviruses) have RNA as their genetic material.
The biological information contained in an organism is
encoded in its DNA or RNA sequence. RNA is also
used for information transport (e.g., mRNA) and
enzymatic functions (e.g., ribosomal RNA) in organisms
that use DNA for the genetic code itself.
Prokaryotic genetic material is organized in a simple
circular DNA molecule (the bacterial chromosome) in the
nucleoid region of the cytoplasm. Eukaryotic genetic material
is divided into different, linear molecules called
chromosomes inside a discrete nucleus, usually with
additional genetic material in some organelles like
mitochondria and chloroplasts.
A human cell has genetic material in the nucleus (the
nuclear genome) and in the mitochondria (the mitochondrial
genome). In humans the nuclear genome is divided into 46
linear DNA molecules called chromosomes. The
mitochondrial genome is a circular DNA molecule separate
from the nuclear DNA. Although the mitochondrial genome
is very small, it codes for some important proteins.
Organelles: As the human body contains many different
organs, such as the heart, lung, and kidney with different
functions cells also have a set of little organs, called
organelles, that are specialized for carrying out one or
more vital functions. Membrane-bound organelles are
found only in eukaryotes.
Cell Nucleus (a cell’s information center): The cell
nucleus, found in a eukaryotic cell, is the house of
chromosomes, and is the place where almost all DNA
replication and RNA synthesis occur. The nucleus is
spheroid in shape and separated from the cytoplasm by
a double membrane called the nuclear envelope. During
processing, DNA is transcribed, or copied into a special
RNA, called mRNA. This mRNA is then transported
out of the nucleus, where it is translated into a specific
protein molecule. In prokaryotes, DNA processing takes
place in the cytoplasm.
Mitochondria and Chloroplasts (the power generators):
Mitochondria are self-replicating organelles that occur
in various numbers, shapes, and sizes in the cytoplasm
of all eukaryotic cells. As mitochondria contain their
own genome that is separate and distinct from the
nuclear genome of a cell, they play a critical role in
generating energy in the eukaryotic cell. Chloroplasts,
broadly called plastids, are often involved in storage.
Endoplasmic Reticulum and Golgi Apparatus
(macromolecule managers): The endoplasmic reticulum
(ER) is the transport network for molecules targeted for
certain modifications and specific destinations, as
compared to molecules that will float freely in the
cytoplasm. The ER has two forms: the rough ER, which
has ribosomes on its surface, and the smooth ER, which
lacks them. The ER contains many Ribosomes, the
protein production machine. The ribosome is a large
complex composed of many molecules, only exist
floating freely in the cytosol, whereas in eukaryotes they
can be either free or bound to membranes.
Lysosomes and Peroxisomes: The eukaryotic cell could
not house such destructive enzymes if they were not
contained in a membrane-bound system.
Centrosome (the cytoskeleton organiser): The
centrosome produces the microtubules of a cell, a key
component of the cytoskeleton. It directs the transport
through the ER and the Golgi apparatus. Centrosomes
are composed of two centrioles, which separate during
cell division and help in the formation of the mitotic
spindle. A single centrosome is present in the animal
cells. They are also found in some fungi and algae cells.
Vacuoles: They store food and waste. Some vacuoles
store extra water. They are often described as liquid
filled space and are surrounded by a membrane. Some
cells, most notably Amoeba have contractile vacuoles,
which are able to pump water out of the cell if there is
too much water.
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