Cell BIOLOGYFig: CellCytology:- The cell and its structures are studied under a branch of biology called cytology.
Cell- The structural & functional unit of living beings is called a cell.
A mass of protoplasm bounded by a plasma membrane.
A unit of biological activity, delimited by a differentially permeable membrane and capable of self-reproduction.
Discovery of cell
1. Robert Hooke (1665):- An English man and first curator of
Royal Society of London.
2. Antony Van Leeuwenhoek (1674) was first to observe living cells like bacteria [from tartar of teeth] erythrocytes [fish], sperms and protozoans [eg. Vorticella]
3. N. Grew (1682):- Proposed cell concept which states that a cell is the unit of structure of organisms.
4. Rudolf Virchow (1858):- Proposed that new cells formed from the pre-existing cells.
5. Knoll & Ruska (1932) :- Designed the electron microscope which was employed to study the ultra structure of a cell and various cell organelles.
Fig: microscope designed by knoll and ruska
Basic criteria for defining the cell-
(i) Presence of genetic material
(ii) Presence of limiting plasma membrane.
(iii) Presence of a metabolic machinery.
Cell is called structural & functional unit of life because-
(i) All the living organisms are composed of one or more cells.
(ii) All the cells have similar basic structure.
(iii) Similar cell organelles of different cells perform similar functions.
Cell theory was presented by Schleiden (Botanist) (1838) and Schwann (Zoologist) (1839). Accordingly, all plants and animals are composed of cells and the cell is the basic unit of life.
Later discoveries led to the modification of cell theory. Modified cell theory is termed as cell principle or modern cell theory.
Modern cell theory postulates that :-
1. All living beings are composed of minute units, the cells ;
2. A cell is a mass of protoplasm containing a nucleus and bounded by a cell membrane, and in many cases by a cell wall also;
3. All cells are basically alike in structure and metabolic activities;
4. The functions of an organism as a whole is the result of the activities and interactions of the constituent cells; and
5. All living cells arise from the pre-existing cells.
(A) Size of cell- Normal size in human 20 µm to 30 µm in diametre.
(i) Largest cell- In animals - Ostrich egg [15 cm is diametre]
In plants - Acetabularia [6-10 cm]
(ii) Longest cell- In animals - Nerve cell [upto 1mt]
In plants - Hemp fibre.
(iii) Smallest cell- PPLO - Pleuro Pneumonia Like Organism [Mycoplasma - 0.1 to 0.5 µm.]
(B) Shape of cell- Shape of cell mainly depends upon the specific function it performs.
(i) Elongated - Nerve cell
(ii) Discoidal/saucer - RBC
(iii) Spindal - Muscle cell
(iv) Spherical - Eggs.
(v) Branched - Pigment cell of the skin.
(vi) Slipper shaped - Paramecium
(vii) Cuboidal - Germ cells of gonads.
(viii) Polygonal - Liver cells.
Units of Measurement Used in Cell Biology:-
The cells and their organelles are measured by fractions or multiples of a millimetre. A millimetre is one-thousandth of a metre. Its symbol is mm. One-thousandth of a millimetre is called a micron, or micrometer. Its symbol is mm and it is equal to 10-6 mm. One thousandth of a micron is termed millimicron or nanometre. Its symbol is nm. One-tenth of a millimicron is known as Angstrom. Its symbol is Å.
1 mm (millimetre) = 10-3 metre
1 mm (micrometre) = 10-3 mm
1 nm or 1 mm (millimicrometre) = 10-3 mm or 10-6 mm
1 Å (Angstrom) = 10-1 nm or 10-1 mm or 10-7 mm.
Prokaryotes and Eukaryotes:Fig: Eukaryote and prokaryote respectively
All living organisms are made of cells. And based on the kind of cells they are made up of, they are divided into two groups -prokaryotes and eukaryotes. Prokaryotes, which include bacteria and blue-green algae (cyanobacteria), appeared (on the earth) about 3, 500 million years ago. All other organisms are eukaryotes, and they probably evolved from prokaryotes.
Differences between prokaryotic & Eukaryotic cells
|Feature||Prokaryotic cell||Eukaryotic cell|
|Cell size||Average diameter 0.5-5μm||Diameter varies between. 1μm-40 μm|
|Relatively rigid, resistant to desiccation (drying) and can withstand wide changes in pressure and temperature||More fluid and sensitive to drying and to changes in temperature and pressure.|
|Nucleus||Lacks true nucleus; circular DNA lies|
naked in the cytoplasm; no
chromosomes, nucleolus or nuclear
undifferentiated from cytoplasm
|True nucleus bound by nuclear membrane|
contains linear DNA associated with
proteins and RNA (forming
chromosomes); nucleolus and nuclear
membrane present; nucleoplasm distinct
|Organelles||Membrane-bound organelles like Golgi|
bodies, plastids, mitochondria and
endoplasmic reticulum (ER) are absent.
|Membrane-bound organelles present.|
|Ribosomes||Smaller and randomly scattered in the|
|Bigger, can be free or attached to the ER|
|Cell division||Divides by simple fission; spindle is not|
formed; no mitosis and meiosis
|Divides by mitosis or by meiosis|
|Respiration||Respiratory enzymes are located on the|
|Mitochondria are the seat of aerobic|
|Photosynthesis||No organized chloroplast; photosynthesis|
takes place on photosynthetic membranes
which lie freely in the cytoplasm
|Organized chloroplasts (containing stacked|
membranes called grana) take part in
|Examples||Bacteria and cyanobacteria (blue-green|
|All other organisms.|
Number of cells - On the basis of number of cells the organisms can be categorised as -
(i) Unicellular- Single celled - eg - Amoeba, Paramecium, Euglena, Plasmodium- Malarial parasite, Chlamydomonas.
(ii) Multi cellular- Numerous cells eg - Higher Plants & Animals.
Fig: Diversity of cells
Majority of the multicellular organisms have a large body composed of numerous cells. The number of cells in such organisms is correlated to the size of the body. For instance, a human being weighing about 60 kg has about 60 × 1015 cells, and the blood (vascular tisue) in human contains about 30 × 1015 corpuscles. There are also instances wherein the number of cells are fixed in multicellular organisms, e.g., rotifers and nematodes. In such organisms, mitosis stops during embryonic development. The phenomenon of having a fixed, genetically constant number of cells is called eutely.