7. A cell is capable of independent existence and
can carry out all the functions which are
necessary for a living being. A cell carries out
nutrition, respiration, excretion, transportation
and reproduction; the way an individual
organism does. Unicellular organisms are
capable of independent existence which shows
a cell’s capability to exist independently. Due to
this, a cell is called the fundamental and
structural unit of life. All living beings are
composed of the basic unit of life, i.e. cell.
8. Shape and Size of Cells:- Cells come in all shapes and sizes.
While most of the cells are spherical in shape, cells of various
other shapes are also found. Most of the cells are microscopic in
size, i.e. it is impossible to see them with naked eyes. Some cells
are fairly large, e.g. a neuron in human body can be as long as 1
meter. The egg of an ostrich is the largest known cell of a living
animal and an average egg is 15 cm long and 13 cm wide.
A cell is enclosed in a membranous casing and is filled with a
liquid substance which is called the cytoplasm. There are many
cell organelles in a typical cell. Some of the main structures of a
cell are as follows:
9.
10. Cell wall is made of cellulose. It is
somewhat hard but permeable to most
of the substances. Cell wall is available
in plant cells and in cells of bacteria and
fungi.
11. Plasma membrane is a semi-permeable membrane. It
is composed of bilayer of lipid and protein.
Plasma membrane provides a container to the
cytoplasm. It facilitates passage of various substances
in and out of the cell.
12. Nucleus is covered by double membrane; called
nuclear membrane. The fluid which is inside the
nucleus is called nucleoplasm. Nucleus contains
chromosomes which are important for the functioning
of a cell. Chromosomes contain genes which are the
carriers of genetic information. Nucleus plays an
important role during cell division. Nucleus controls all
the functions of the cell.
13.
14.
15.
16. The cell was first discovered by Robert Hooke in 1665,
which can be found to be described in his
book Micrographia. In this book, he gave 60 ‘observations’
in detail of various objects under a coarse, compound
microscope.One observation was from very thin slices of
bottle cork. Hooke discovered a multitude of tiny pores
that he named "cells". This came from the Latin word
Cella, meaning ‘a small room’ like monks lived in and also
Cellulae, which meant the six sided cell of a honeycomb.
However, Hooke did not know their real structure or
function. What Hooke had thought were cells, were
actually empty cell walls of plant tissues.
17. Drawing of the structure of cork byRobert Hooke that
appeared in Micrographia
18. With microscopes during this time having a low
magnification, Hooke was unable to see that there were
other internal components to the cells he was observing.
Therefore, he did not think the "cellulae" were alive.[5] His cell
observations gave no indication of the nucleus and
other organelles found in most living cells. In Micrographia,
Hooke also observed mould, bluish in color, found on leather.
After studying it under his microscope, he was unable to
observe “seeds” that would have indicated how the mould
was multiplying in quantity. This led to Hooke suggesting that
spontaneous generation, from either natural or artificial heat,
was the cause. Since this was an old Aristotelian theory still
accepted at the time, others did not reject it and was not
disproved until Leeuwenhoek later discovers generation is
achieved otherwise.
19. Anton van Leeuwenhoek is another scientist who saw these cells soon
after Hooke did. He made use of a microscope containing improved
lenses that could magnify objects almost 300-fold, or 270x.[5] Under
these microscopes, Leeuwenhoek found motile objects. In a letter toThe
Royal Society on October 9, 1676, he states that motility is a quality of
life therefore these were living organisms. Over time, he wrote many
more papers in which described many specific forms of microorganisms.
Leeuwenhoek named these “animalcules,” which includedprotozoa and
other unicellular organisms, like bacteria.[3] Though he had did not have
much formal education, was able to identify the first accurate description
of red blood cells and discovered bacteria after gaining interest in the
sense of taste that resulted in Leeuwenhoek to observe the tongue of an
ox, then leading him to study "pepper water" in 1676. He also found for
the first time the sperm cells of animals and humans. Once discovering
these types of cells, Leeuwenhoek saw that the fertilization process
requires the sperm cell to enter theegg cell. This put an end to the
previous theory of spontaneous generation. After reading letters by
Leeuwenhoek, Hooke was the first to confirm his observations that were
thought to be unlikely by other contemporaries.
20. The cells in animal tissues were observed after plants were
because the tissues were so fragile and susceptible to tearing, it
was difficult for such thin slices to be prepared for studying.
Biologists believed that there was a fundamental unit to life, but
were unsure what this was. It would not be until over a hundred
years later that this fundamental unit was connected to cellular
structure and existence of cells in animals or plants. This
conclusion was not made until Henri Dutrochet. Besides stating
“the cell is the fundamental element of organization”, Dutrochet
also claimed that cells were not just a structural unit, but also a
physiological unit.
In 1804, Karl Rudolphi and J.H.F. Link were awarded the prize for
"solving the problem of the nature of cells", meaning they were the
first to prove that cells had independent cell walls by
the Königliche Societät der Wissenschaft (Royal Society of
Science), Göttingen. Before, it had been thought that cells shared
walls and the fluid passed between them this way.
21. Based on the level of organization of nuclear material, a cell
can be categorized as prokaryote or eukaryote. In some
organisms, the true nucleus is absent. This means that the
DNA and RNA are not bound by a membrane. Bacteria are
examples of prokaryotes. When genetic materials are bound
by a membrane, it is termed as true nucleus. In this case, the
cell is called eukaryotic. Organisms other than bacteria are
eukaryotes. Eukaryotes are considered as more advanced
than prokaryotes.
25. Credit for developing cell theory is usually given to two scientists: Theodor
Schwann and Matthias Jakob Schleiden. While Rudolf Virchowcontributed
to the theory, he is not as credited for his attributions toward it. In 1838,
Schleiden suggested that every structural part of a plant was made up of
cells or the result of cells. He also suggested that cells were made by a
crystallization process either within other cells or from the outside.
However, this was not an original idea of Schlieden. He claimed this theory
as his own, though Barthelemy Dumortierhad stated it years before him.
This crystallization process is no longer accepted with modern cell
theory. In 1839, Theodor Schwannstates that along with plants, animals are
composed of cells or the product of cells in their structures. This was a
major advancement in the field of biology since little was known about
animal structure up to this point compared to plants. From these
conclusions about plants and animals, two of the three tenets of cell theory
were postulated.
All living organisms are composed of one or more cells
The cell is the most basic unit of life
26. Mitochondrion is a capsule-like structure. It is a double
membrane structure. Its inner membrane is projected into
numerous finger-like structures; called cristae. Mitochondria
are the sites of cellular respiration. After cellular respiration,
energy is stored in the form of ATP (Adenosine triphosphate);
in mitochondria. Mitochondria have their own DNA and
ribosomes and hence mitochondria can produce their own
protein.
Cellular respiration; due to this, mitochondria are also known
as the ‘powerhouse of the cell’.
27.
28. Endoplasmic reticulum is a mesh-like
structure which is composed of numerous tubes. It
extends from the plasma membrane to the nuclear
membrane. There are two kinds of endoplasmic
reticulum, viz. smooth ER and rough ER. Rough
ER has ribosomes on its surface which give it the
rough appearance.
29. It serves as the transport channel in the cell.
Substances are transported from cell membrane to
cytoplasm and to nucleus and vice-versa. ER also
serves the role of packaging many substances in
the cell.
30. Golgi Complex was discovered by
Camillo Golgi. It is composed of many
sac-like structures which are stacked
one above another.
31. Golgi complex is responsible for packaging of
various substances in the cell .
Lysosome are small sac-like structures and they are
derived from Golgi complex. Lysosome contains
digestive enzymes.
32. The enzymes in the lysosome digest foreign
particles and thus destroy them. Sometimes,
the lysosome may burst open and its content
ends up digesting the contents of the cell. The
cell gets killed in the process. Due to this,
lysosome is also called the ‘suicide bag of the
cell’.
33. These are tiny dot like structures interspersed in
the cytoplasm and also on the surface of Rough
ER. Ribosome is responsible for protein synthesis.
These are somewhat similar to mitochondria; in
appearance. Plastids are found in plant cells.
They are of two types, chromoplast and
leucoplast. Colourful plastids are called
chromomplast and colourless plastids are called
leucoplast. Chloroplast is green in colour and is
found in green parts of plants. Plastids too have
their own DNA and ribosome.
34. Leucoplasts are responsible for storing
food; such as carbohydrates, protein and
lipid. Chromoplasts impart various colours
to the plant parts. A leaf of a plant is green
in colour because of chloroplast.
Chloroplast is the site of photosynthesis.
35. These are fluid filled chambers and are often
seen in many cells. Vacuoles are very large in
plant cells. A plant cell usually has single but
large vacuole. Such a vacuole fills almost the
entire space inside the cell. Vacuoles are
much smaller and very few in animal cells.
36.
37. • Robert Hooke was the first to discover cell (1665).
• Leeuwenhoek was the first to discover free living
cells in pond water (1674).
• Robert Brown discovered the nucleus (1831).
• Purkinje coined the term ‘protoplasm (1839).
• Schleiden (1838) and Schwann (1839) proposed the
Cell Theory. Virchow (1855) made further addition to
the cell theory.
• The discovery of electron microscope (1940) made it
possible to study the structures of cell organelles.
