سؤال قراءة ستيب محلول — المفردات في السياق (تدريب 5319)

(1) All living organisms need food. They need it as a source of raw materials to build new cells and tissues as they grow. They also need food as a source of energy. Food is a kind of 'fuel' that drives essential living processes and brings about chemical changes. Animals take in food, digest it and use the digested products to build their tissues or to produce energy. (2) Plants also need energy and raw materials but, apart from a few insect eating species, plants do not appear to take in food. The most likely source of their raw materials would appear to be the soil. However, experiments show that the weight gained by a growing plant is far greater than the weight lost by the soil it is growing in. So, there must be additional sources of raw materials. (3) Jean-Baptiste van Helmont was a Dutch scientist working in the 17th century. At that time very little was known about the process of photosynthesis. He carried out an experiment using a willow shoot. He planted the shoot in a container with 90.8 kg of dry soil and placed a metal grill over the soil to prevent any accidental gain or loss of mass. He left the shoot for 5 years in an open yard, providing it with only rainwater and distilled water for growth. After 5 years he reweighed the tree and the soil and came to the conclusion that the increase in mass of the tree (74.7 kg) was due entirely to the water it had received. However, he was unaware that plants also take in mineral salts and carbon dioxide, or that they use light as a source of energy.