- Cytology: 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.Cell Biology
- A cell is called the structural unit of living being because the body of all living organisms is made up of one or more cells.
- Cell is called functional unit of all organisms, because all the vital activities or physiological activities, i.e. respiration, digestion, excretion, circulation, etc are performed at the level of cell.
➢ Discovery of Cell
1. Robert Hooke (1665): An English man and first curator of Royal Society of London.
- Observed a thin transverse section of the bark of a tree under a self-designed microscope.
- He noticed honey - comb like compartments.
- He coined the term cell.
- He wrote a book - Micrographia.
- He actually observed dead cells.
Image drawn by Hooke and Cork Tissue
2. Antony Van Leeuwenhoek (1674): was first to observe living cells like Bacteria (from tartar of teeth), Erythrocytes (fish), Sperms and Protozoans (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.
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:
- All living beings are composed of minute units, the cells.
- 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.
- All cells are basically alike in structure and metabolic activities.
- The functions of an organism as a whole is the result of the activities and interactions of the constituent cells.
- All living cells arise from the pre-existing cells.
Question 1:Cell theory was given by
(a) Size of Cell
Normal size in human 20 µm to 30 µm in diameter.
- Largest Cell: In animals - Ostrich egg (15 cm is diameter), In plants - Acetabularia (6 - 10 cm).
- Longest Cell: In animals - Nerve cell (upto 1m), In plants - Hemp fibre.
- 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.
- Elongated - Nerve cell
- Discoidal/saucer - RBC
- Spindal - Muscle cell
- Spherical - Eggs
- Branched - Pigment cell of the skin
- Slipper shaped - Paramecium
- Cuboidal - Germ cells of gonads
- Polygonal - Liver cells
(c) 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.
1 mm (millimetre) = 10-3 metre
- One-thousandth of a millimetre is called a micron, or micrometer. Its symbol is mm and it is equal to 10-6 m.
1 mm (micrometre) = 10-3 mm
- One thousandth of a micron is termed millimicron or nanometre. Its symbol is nm.
1 nm or 1 mm (millimicrometre) = 10-3 mm or 10-6 mm
- One-tenth of a millimicron is known as Angstrom. Its symbol is Å.
1 Å (Angstrom) = 10-1 nm or 10-1 mm or 10-7 mm
Prokaryotes and Eukaryotes
- 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. Structure of 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.
(a) Differences Between Prokaryotic & Eukaryotic Cells
- Cell Size
Prokaryotic Cell: Average diameter 0.5 - 5 μm.
Eukaryotic Cell: Diameter varies between 1μm - 40 μm.
Prokaryotic Cell: Relatively rigid, resistant to desiccation (drying) and can withstand wide changes in pressure and temperature.
Eukaryotic Cell: More fluid and sensitive to drying and to changes in temperature and pressure.
Prokaryotic Cell: Lacks true nucleus; circular DNA lies naked in the cytoplasm; no chromosomes, nucleolus or nuclear membrane; nucleoplasm undifferentiated from cytoplasm.
Eukaryotic Cell: True nucleus bound by nuclear membrane contains linear DNA associated with proteins and RNA (forming chromosomes); nucleolus and nuclear
membrane present; nucleoplasm distinct.
Prokaryotic Cell: Membrane-bound organelles like Golgi bodies, plastids, mitochondria and endoplasmic reticulum (ER) are absent.
Eukaryotic Cell: Membrane-bound organelles present.
Prokaryotic Cell: Smaller and randomly scattered in the cytoplasm.
Eukaryotic Cell: Bigger, can be free or attached to the ER.
- Cell division
Prokaryotic Cell: Divides by simple fission; spindle is not formed; no mitosis and meiosis.
Eukaryotic Cell: Divides by mitosis or by meiosis.
Prokaryotic Cell: Respiratory enzymes are located on the plasma membrane.
Eukaryotic Cell: Mitochondria are the seat of aerobic respiration.
Prokaryotic Cell: No organized chloroplast; photosynthesis takes place on photosynthetic membranes which lie freely in the cytoplasm.
Eukaryotic Cell: Organized chloroplasts (containing stacked membranes called grana) take part in photosynthesis.
Prokaryotic Cell: Bacteria and cyanobacteria (blue-green algae).
Eukaryotic Cell: All other organisms.
Question 3:The only cell organelle seen in prokaryotic cell is
Types of Organism on Basis of Number of Cells
On the basis of number of cells the organisms can be categorised as:
(i) Unicellular: Single celled
Example: Amoeba, Paramecium, Euglena, Plasmodium- Malarial parasite, Chlamydomonas.
(ii) Multicellular: Numerous cells
Example: Higher Plants & Animals.
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.
Example: A human being weighing about 60 kg has about 60 × 1015 cells, and the blood (vascular tissue) 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.