1. At the structural level, all living organisms are composed of tiny living units called cells.
2. Organisms consisting of only one cell are called unicellular organisms. e.g. Paramecium, Amoeba, etc.
3. Organisms consisting of more than one (may be millions of cells) cell are known as multicellular organisms, e.g., plants and animals.
4. Though, cells are generally microscopic, some cells can also be seen with the naked eye, e.g., ostrich egg measuring about 170 × 150 mm.
5. Cell is the structural and functional unit of all living organisms.
6. The shape and size of cells are in fact related to the function they perform.
7. Cells are measured in micrometers (mm) sometimes called microns (m).
8. The smallest cell is Mycoplasma having 0.1 micron diameter.
9. The longest cell is nerve cell, measuring about a metre in length.
10. Cells may be spherical, oval, elliptical, spindle shaped, cuboidal, polygonal, columnar or flat.
11. J.E. Purkinje coined the term ‘protoplasm’ – the life-giving substance present in the cell.
12. Robert Hooke discovered the basic unit of life ‘cell’.
13. Schleiden and Schwann proposed the cell theory in 1839.
14. Extremely thin, outer boundary of cytoplasm is cell membrane.
15. Cytoplasm contains different cell organelles like nucleus, mitochondria, endoplasmic reticulum, Golgi body, plastid, lysosome, ribosome, etc.
16. Nucleus is the most important part of a cell having control over all cellular activities.
17. Nuclear membrane, nuclear sap, nucleolus, chromatin, etc., are the parts of nucleus.
18. Animal cells lack cell wall and plastids.
19. Mitochondria are generally called the “Powerhouse of the cell”.
20. Vacuole, surrounded by a single membrane, is called tonoplast.
21. Lysosomes containing powerful enzymes are called digestive bags or suicidal bags.
22. Centrosomes help in cell division.23. Vacuoles provide turgidity and rigidity to the cells.
24. Protoplasm is the physical basis of life.
25. Diffusion is the movement of molecules of a substance (solid, liquid or gas) from a region of their higher concentration to the region of their lower concentration until they are spread out evenly.
26. Osmosis is the diffusion of a solvent, usually water, through a semi-permeable membrane from a dilute or weaker solution into a concentrated or stronger solution.
27. A semi-permeable or partially permeable membrane allows movement of solvent molecules but does not allow the movement of solute molecules.
28. Endosmosis and exosmosis : In cells, water molecules may diffuse into the cell or out of the cell, depending on whether the cells are kept in a weaker solution or a stronger solution respectively.
29. Endosmosis (endo = inward) is the inward diffusion of water when the surrounding solution is less concentrated. This brings about swelling of the cell.
30. Exosmosis (exo = outward) is the outward diffusion of water when the surrounding solution is more concentrated. This brings about shrinkage of the cell.
31. Hypertonic (hyper = more or higher) : The solution outside the cell has more concentration than the cell sap. If a cell is placed in such a solution, water will move out of the cell, i.e., exosmosis will take place. The protoplasm will, therefore, shrink.
32. Hypotonic (hypo = less than or lower) : The solution outside the cell is less concentrated than the cell sap. As a result, water will move into the cell, i.e., endosmosis takes place. The protoplasm will, therefore, increase in volume.
33. Isotonic (iso = same) : The solution outside the cell has same concentration as that of the cell sap. If a cell is placed in such a solution, there is no diffusion of water. There will, thus, be no change in cell size.
34. Turgid or turgidity : When a cell reaches a stage that it cannot accommodate any more water, it is fully distended. The cell at this stage is called turgid and the condition is called turgidity.
35. Plasmolysis : The phenomenon of the loss of water from a plant cell by osmosis when a cell is kept in a hypertonic solution so that the cytoplasm shrinks away from the cell wall is called plasmolysis.
36. Osmoregulation is the control of the quantity of water entering and leaving the cells of an organism.
37. When the food materials engulfed by an organism are solid, the process is called phagocytosis.
38. When the food materials taken in by the organisms are liquid, the process is called pinocytosis