سؤال قراءة ستيب محلول — فهم التفاصيل (تدريب 5779)

(1) Our food is primarily made of carbohydrates, proteins, fats, minerals, vitamins and water. We need to regularly ingest food to sustain ourselves. (2) Living things do not follow the pattern of decay seen in the rest of the natural world. Unlike non-living things (such as rocks), living things maintain themselves. They build new cells, tissues and organs in the process of development. Once mature, organisms can sustain existing biological structures by cellular division. This process, also called mitosis, allows cells to use molecules such as water, amino acids (building blocks of proteins), carbohydrates and lipids to create new cells. This process requires the transfer of energy among chemical bonds. (3) Our cells are regularly being replaced and recycled and this requires a regular influx of energy and matter. We obtain energy in the form of food calories: new matter in the form of the molecules contained in our food. Not all of these molecules are digestible by all animals. These unneeded molecules may be eaten along with more desirable molecules. This is part of the reason why animals must have an opening in their digestive system (4) A balanced diet for humans includes several servings of breads, cereals, fruits and vegetables every day. Also essential are smaller servings of milk, dairy, meat and eggs. Fats, oils and sweets should make up a very small portion of a balanced diet. (5) Human beings eat several hundred pounds of food a year. From this food we obtain all the molecules we need so our bodies can operate, grow and replace the cells lost to wear, old age, or damage. (6) Animals, such as humans, have evolved intricate strategies of intercellular communication. Feedback, feed-forward, and inhibition allow us to maintain steady-state, or homeostasis. We get hungry because we want to eat. This seemingly simple cause and effect is the result of the empty stomach secreting the peptide hormone ghrelin into the bloodstream. When this hormone is received by the hypothalamus, it stimulates the release of growth hormones, which makes us want to eat. After eating to satiation, the release of the peptide hormone leptin causes the hypothalamus to inhibit hunger sensations. Leptin is released by a variety of tissues including adipose (fat cells), stomach, bone, pituitary, and the liver. Metabolism (7) The word metabolism comes from the Greek root “change” and involves the building up or breaking down of molecules to be used by the body. These molecules will be used for cell growth, fuel and other processes. If the process involves breaking down molecules into simpler ones, usually to release energy, it is called catabolic. If building up complex molecules, for structures or energy storage, it is called anabolic. (8) Developed and less active bodies do not need as many energy calories as developing and active bodies. Developing bodies are adding cells at a rapid rate, and this requires an infusion of both new material (molecules) and energy. Active bodies, likewise, need more calories to sustain the energetic demands of more motion Proteins (9) Proteins are molecules made of nitrogen, carbon, oxygen and hydrogen and sometimes sulfur. These molecules are found in animal tissue such as meat (including fish), eggs, cheese and legumes, and many other plant foods. (10) Proteins are large molecules assembled from smaller units called amino acids and serve many important roles in living things. Amino acids are linked by covalent bonds called peptide bonds. Proteins can serve as enzymes, hormones as well as the building blocks of complex physical features. Proteins comprise teeth, bone, muscle, tendon, cartilage, skin - most of what makes up an organism. (11) As enzymes, proteins facilitate chemical reactions that would not otherwise happen in the short time, or lower temperatures required by living things. As hormones (such as the previously mentioned appetite inhibitor leptin) they are protein messengers that deliver information around the body Carbohydrates (12) Carbohydrates are made of hydrogen, oxygen, and carbon that combine to form monosaccharides. These simple sugars are made of five (pentose) or six (hexose) carbon rings. Additional hydrogen and oxygen atoms allow these rings to form disaccharides (“two sugars”) such as sucrose and lactose or polysaccharides (“many sugars”) such as starch. (13) Sugars are essential to living things. They provide a way to store chemical energy to operate living things. They are also the structural backbone to the information storage molecules DNA and RNA, as well as many of the other essential molecules in living things. Sugar is transported through the bloodstream in the form of the simple sugar glucose. They have 4 calories of energy per gram.