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damaged parts of the cell, keeping it clean and healthy. The products formed by the breakdown are released into the cytoplasm, where they may be reused in other cellular processes. Threads of Curiosity Human sperm cells contain lysosomal enzymes. When a sperm meets an egg, these enzymes help break down the outer layer of the egg, allowing fertilisation to take place. You will learn about the human sperm cells later in Chapter 11. Ribosomes Mitochondria — The powerhouse of the cell DNA Mitochondria are often called the ‘powerhouses of Intermembrane thecell’becausetheysupplytheenergyneededfor space most cellular activities. Each mitochondrion (Fig. 2.14) is surrounded by two membranes : Grade 7 Cristae y The outer membrane is smooth and Chapter 9 porous. Inner The inner membrane is folded into finger-like membrane y projections called cristae, which increase Outer the surface area for chemical reactions and membrane facilitate energy production. Fig. 2.14: Structure of a mitochondrion In mitochondria, the glucose and other molecules are broken down to release energy during a process called cellular respiration. You have already studied about it. The energy released is stored in the form of a molecule called Adenosine Triphosphate (ATP), which acts as the energy currency and is used for most of the cellular activities. Plastids — Centre for food synthesis in the plant cells and beyond You have learnt that mitochondria provide energy to the cell that comes from food. Animals can obtain food from their surroundings, however, plants synthesise food in the presence of sunlight. But where do plants synthesise their food and obtain energy for cellular activities? Plants use special organelles called Inner plastids for food synthesis and storage. membrane As you have learnt, plants prepare their Curiosity food by the process of photosynthesis in the Chapter 9 Outer presenceofsunlight.Agreenpigmentcalled membrane chlorophyll, which is present in the chloroplast (a type of plastid) absorbs sunlight. Chloroplasts Stroma DNA are double-membrane-bound organelles, like mitochondria. Inside the chloroplast (Fig. 2.15) there is a semi-fluid substance called the stroma. Within Ribosomes Fig. 2.15: Structure of a chloroplasthe stroma are disc-shaped membrane structures that contain chlorophyll. Light energy is absorbed 18 Exploration|Grade 9