Biology Past Paper PDF 2023-24

Activity ______________ 5.5 Q Find out about electron microscopes from resources in the school library or through the internet. Discuss it with your teacher. uestions 1. How do substances like CO2 and water move in and out of the cell? Discuss. 2. Why is the plasma membrane called a selectively perm...

Activity ______________ 5.5 Q Find out about electron microscopes from resources in the school library or through the internet. Discuss it with your teacher. uestions 1. How do substances like CO2 and water move in and out of the cell? Discuss. 2. Why is the plasma membrane called a selectively permeable membrane? 5.2.3 NUCLEUS 5.2.2 CELL WALL Plant cells, in addition to the plasma membrane, have another rigid outer covering called the cell wall. The cell wall lies outside the plasma membrane. The plant cell wall is mainly composed of cellulose. Cellulose is a complex substance and provides structural strength to plants. When a living plant cell loses water through osmosis there is shrinkage or contraction of the contents of the cell away from the cell wall. This phenomenon is known as plasmolysis. We can observe this phenomenon by performing the following activity: Activity ______________ 5.6 Mount the peel of a Rhoeo leaf in water on a slide and examine cells under the high power of a microscope. Note the small green granules, called chloroplasts. They contain a green substance called chlorophyll. Put a strong solution of sugar or salt on the mounted leaf on the slide. Wait for a minute and observe under a microscope. What do we see? Now place some Rhoeo leaves in boiling water for a few minutes. This kills the cells. Then mount one leaf on a slide and observe it under a microscope. Put a strong solution of sugar or salt on the mounted leaf on the slide. Wait for a minute and observe it again. What do we find? Did plasmolysis occur now? THE FUNDAMENTAL UNIT OF What do we infer from this activity? It appears that only living cells, and not dead cells, are able to absorb water by osmosis. Cell walls permit the cells of plants, fungi and bacteria to withstand very dilute (hypotonic) external media without bursting. In such media the cells tend to take up water by osmosis. The cell swells, building up pressure against the cell wall. The wall exerts an equal pressure against the swollen cell. Because of their walls, such cells can withstand much greater changes in the surrounding medium than animal cells. Remember the temporary mount of onion peel we prepared? We had put iodine solution on the peel. Why? What would we see if we tried observing the peel without putting the iodine solution? Try it and see what the difference is. Further, when we put iodine solution on the peel, did each cell get evenly coloured? According to their chemical composition dif ferent regions of cells get coloured differentially. Some regions appear darker than other regions. Apart from iodine solution we could also use safranin solution or methylene blue solution to stain the cells. We have observed cells from an onion; let us now observe cells from our own body. Activity ______________ 5.7 LIFE Let us take a glass slide with a drop of water on it. Using an ice-cream spoon gently scrape the inside surface of the cheek. Does any material get stuck on the spoon? With the help of a needle we can transfer this material and spread it evenly on the glass slide kept ready for this. To colour the material we can put a drop of methylene blue solution on it. Now the material is ready for observation under microscope. Do not forget to put a cover-slip on it! What do we observe? What is the shape of the cells we see? Draw it on the observation sheet. 53 Rationalised 2023-24