CBSE Class 11 Biology Notes-Free PDF Download

Class 11 Biology Notes As we know, all organisms are made up of cells and their produces.

Study of structure and composition of cell is known as Cytology.

Cell Biology correlates the structure with the function (life processes) of cells.

All organisms are made up of cells, whether unicellular or multicellular.

Cell is the basic unit of life and structural/functional unit of an organism.

DISCOVERY OF CELL Robert Hooke (1665) : Observed the suberized cell walls with a honey comb like pattern in a thin slice of cork and named them ‘cellulae’ as published in his book: Micrographia Marcello Malpighi (1671) and Grew studied the plant tissues and called the cavities of cellular wall as ‘vesicles’ or utricles’ Leeuwenhoeck (1683) : First to observe living cells like bacteria, protozoa, red blood cells and sperm etc.

Robert Brown (1831) : Found a small sphere within the cells of orchid roots which was later called as ‘nucleus’

CONCEPT OF PROTOPLASM Alfonso Corti (1772) - Observed that all cells contain a living substance.

Felix Dujardin (1836) - Observed living substance in amoeba and called it ‘Sarcode’ Johannes Purkinje (1839) - Called the jelly-like living substance as protoplasm.

Hugo Von Mohl (1846) - Studied the nature of protoplasm present in the embryonic cells of plants and suggested the name ‘protoplast’ for it.

Max Schultze (1861) : Proposed the protoplasm theory. According to it “cell is an accumulation of living substance (or protoplasm) which is limited by an outer membrane, and possesses a nucleus.

Huxley - Stated ‘Protoplasm is physical basis of life’.

Protoplasm - A polyphasic crystalocolloidal solution. It includes nucleus and cytoplasm.

Cytoplasm - Includes cell organelles and ground matrix, term given by Kolliker.

Cytoplasm exits in two stages, Ectoplasm and Endoplasm. Ectoplasm is a viscous stage and present towards the membrane while Endoplasm is non-viscous and present towards inside.

Deutoplasm - is non living substances formed by the protoplasm e.g. lipid droplets, yolk etc.

Hyaloplasm - is cytoplasm minus all cell inclusions and organelles.

Protoplast - includes all living constituents of the protoplasm i.e. vacuoles, nucleus, cytoplasm and membranes, term coined by Hanstein.

According to colloidal theory of Fischer, conversion of sol into gel and vice versa is due to colloidal nature of cytoplasm. Chemically cytoplasm is a crystallocolloidal complex.

Cytoplasm coagulates at temperature above 60°.

Amount of water in cell is usually not more than 3 quarters, i.e. 75%. pH of cytoplasm, nucleoplasm and human blood is 6.9 ± 0.2, 7.4 ± 0.2 and 7.34 ± 0.2 respectively.

Proteins (enzymes) in the cytoplasm are found in colloidal form. This increases their surface area.

Vitamins, amino acids, minerals, sugars and nucleic acids are found in solution form.

Robert Brown (1828) described characteristic dancing of cell particles. It is now known as Brownian movement.

In 1818 Amici reported continuous movement of cytoplasm called Cyclosis or Cytoplsmic Straming. It is streaming of cytoplasm within a living cell without deformation of the external cell membrane. In some plant cells there is a rapid rotatory cytoplasmic movement, which is limited to the peripheral parts of the cell next to the cell wall; chloroplasts and granules move in this stream. This movement may be increased by light, and is dependent on temperature and pH. Auxins, or growth hormones, may also increase the rate of movement. Specialized cell components, microtubules, may direct the flow or may serve as a framework upon which the streaming occurs. Examples of cells in which cyclosis can be seen are the leaf cells of small aquatic plants, such as Eloder, Chara, and root hair cells of many plants. In some protozoans, e.g., the ciliates, slower cyclotic movements transport digestive vacuoles through the cell body.

CELL THEORY In the early 19th century, several facts about plant and animal cells came forth which led the German Botanist Matthias Schleiden and German Zoologist Theodor Schwann (1839) to establish the cell theory.

Schwann coined the word “metabolism” for all chemical processes occuring in the cell and called cells “the unit of life”.

Cell theory was modified by Rudolf Virchow (1855) in his famous aphorism. “Omnis cellulae e cellulae’ i.e. all cells arise from pre-existing cells, which established cell division as the central phenomenon in the reproduction of organisms.

Nageli and Virchow modified the cell theory of Schleiden and Schwann and now this modern version of cell theory states that : (i) Cells are the morphological and physiological units of all living organisms. (ii) The properties of a given organism depend on its individual cells. (iii) Cells originate from other cells and continuity is maintained through genetic material. (iv) Smallest unit of life is the cell.

Objections to Cell Theory Viruses do not have a cellular machinery even then they are considered to be organisms.

In some organisms e.g. Rhizopus (fungi) and Vaucheria (algae), the body is not differentiated into cells though it may have numerous nuclei (coenocytes).

Bacteria and cyanobacteria do not possess nucleus or other cell organelles.

Mature RBCs and sieve tube cells continue to live without nucleus and other vital organelles.