9th Class Science The Fundamental Unit of Life

The Fundamental Unit of Life

• At the structural level, all living organisms are composed of tiny living units called, cells.
• Organisms consisting of only one cell are called unicellular organisms, e.g. Paramedum, Amoeba, etc.
• Organisms consisting of more than one (may be millions of cells) cell are known as multicellular organisms/ e.g., plants and animals.
• Though, cells are generally microscopic, some cells can also be seen with the naked eye, e.g., ostrich egg measuring about \[170\times 150\] mm.
• Cell is the structural and functional unit of all living organisms.
• The shape and size of cells are in fact related to the function they perform.
• Cells are measured in micrometers (mm) sometimes called microns (m).
• The smallest cell is Mycoplasma having 0.1 micron diameter.
• The longest cell is nerve cell, measuring about a metre in length.
• Cells may be spherical, oval, elliptical, spindle shaped, cuboidal, polygonal, columnar or flat.
• E. Purkinje coined the term 'protoplasm' - the life-giving substance present in the cell.
• Robert Hooke discovered the basic unit of life 'cell',
• Schleiden and Schwann proposed the cell theory in 1839.
• Extremely thin, outer boundary of cytoplasm is cell membrane.
• Cytoplasm contains different cell organelles like nucleus, mitochondria, endoplasmic reticulum, Golgi body, plastid, lysosome, ribosome, etc.
• Nucleus is the most important part of a cell having control over all cellular activities.
• Nuclear membrane, nuclear sap, nucleolus, chromatin, etc., are the parts of nucleus.
• Animal cells lack cell wall and plastids.
• Mitochondria are generally called the "Powerhouse of the cell".
• Vacuole, surrounded by a single membrane, is called tonoplast.
• Lysosomes containing powerful enzymes are called digestive bags or suicidal bags.
• Centrosomes help in cell division.
• Vacuoles provide turgidity and rigidity to the cells.
• Protoplasm is the physical basis of life.
• 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.
• 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.
• A semi-permeable or partially permeable membrane allows movement of solvent molecules but does not allow the movement of solute molecules.
• 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.
• Endosmosis (endo = inward) is the inward diffusion of water when the surrounding solution is less concentrated. This brings about swelling of the cell.
• Exosmosis (exo = outward) is the outward diffusion of water when the surrounding solution is more concentrated. This brings about shrinkage of the cell.
• 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.
• 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.