Class 9 Science Chapter 5 MCQ

In the year 1670, Anton Van Leuwenhoek, a Dutch biologist discovered the living cell when he looked at the pond water under the microscope, he made of lenses.

Mitochondria are absent in bacteria and the red blood cells of mammals.

Schleiden (1836) and Schwann (1834) gave the cell theory, which was further refined by R. Virchow (1855). The main postulates of cell theory are (i)All organisms are composed of cells and cell products. (ii)All metabolic reactions take place in cells. Thus, cell are structural and functional units of life. (iii)All cell arise from pre-existing cells only. No cell can originate spontaneously but they comes into being only by division of pre-existing cells. (iv) Every organism starts its life as a single cell.

Generally, inside the cell there exist a membranous network enclosing a fluid-filled lumen which almost fills up the intracellular cavity, called endoplasmic reticulum (ER). Endoplasmic reticulum is absent in the red blood cells of mammals.

If the medium surrounding the cell has a higher water concentration, than the cell will gain water by osmosis, such a dilute solution is called hypotonic solution.

Endocytosis is the ingestion of material by the cells through their plasma membrane. It is a collective term that describes three similar processes “phagocytosis” (cell eating), “potocytosis” (cell drinking) and receptor-mediated endocytosis. These processes are pathways to specifically internalise solid particles, small molecules and ion, and macromolecules, respectively. All of them require energy, so they may be regarded as different forms of active transport. Out of these phagocytosis is the common method of feeding among the Protozoa (e.g., Amoeba) and lower “Metazoa” (e.g., sponges). In pseudopodial movement the phagocyte enguls the particle lysosomes, join with the vacoule containing ingested particle and pour their contents into the vacoule to destroy the particle.

Golgi body was first discovered by Camillo Golgi. Golgi bodies, or Golgi complex are formed by stacks of flattened (saucer-shaped) membranes or flattened sacs called cisternae. Golgi bodies are usually called dictyosomes in plants. Golgi apparatus is absent in bacteria, blue-green algae, mature sperms and red blood cells of mammals and other animals. The Golgi apparatus arises from the membrane of the smooth endoplasmic reticulum, which in turn originates from the rough endoplasmic reticulum. The proximal Golgi saccules (cisternae at cis face) are formed by fusion of ER-derived vesicles, while distal saccules (cisternae at trans face) “give their all” to vesicle formation and disappear. Thus, Golgi saccules are constantly and rapidly renewed. The main functions of Golgi apparatus are (i)They store, modify, package and condense the proteins synthesised in the ribosomes. (ii)They form the cell plate during the cell division. (iii)They set aside digestive enzymes in tiny membrane bound vesicles which later become lysosomes. (iv)They synthesise some polysaccharides for the cell membrane.

In eukaryotic cell membrane bound cell organelles such as mitochondria and plastids endoplasmic reticulum, Golgi apparatus etc. are present. In prokaryotic cell membrane bound cell organelles are absent.

Lysosomes are simple, tiny spherical sac-like structures evenly distributed in the cytoplasm each lysosome is surrounded by a double membrane and contains powerful enzymes capable of digesting or breaking down all organic material. These enzymes are made by rough endoplasmic reticulum. They perform following functions in our body: i)They help in breaking down or digesting large molecules of the cell. ii)They help in a defence against bacteria and viruses. iii)During starvation, lysosomes act on their own cellular organelles and digest them. This results in cell death. The lysosomes are also called suicide bags or demolition squads of the cell.

Ribosomes are dense, spherical and granular particles which remains freely in the matrix or remain attached to endoplasmic reticulum. They are not bound by membrane and are present in both prokaryotic and eukaryotic cells. They play important role in synthesis of proteins. All enzymes are proteins but all hormones are not proteins.