2011年六月英语六级考试模拟练习题答案(3)_第2页

    
    Vitamins D
Vitamin D, or calciferol, is another fat soluble steroid vitamin which functions to stimulate calcium uptake from the gut and its deposition in bone. vitamin D acts as a hormone when converted by enzymes in the gut and liver into an active form of "active vitamin D", which stimulates epithelial cells in the intestine to absorb calcium. vitamin D is therefore essential in growing children's diets to enable the growth of strong bones. Without adequate amounts of vitamin D children can develop rickets, which is the deformation of the legs caused when they lack calcium to strengthen the bones. In adults a lack of vitamin D in the diet can lead to osteomalacia, a progressive softening of the bones which can make them highly susceptible to fracture.
    
    Vitamin D is made by the body when exposed to sunlight and is stored in the muscles, however, if the skin is rarely exposed to the sunlight or is dark little vitamin D is produced. Foods such as eggs and oily fish are all rich in vitamin D.
    
    Vitamins K
    
    Vitamin K, phylloquinone, is found in dark green leafy vegetables such as spinach and kale. It is a fat soluble vitamin which is involved in the clotting process of blood. In the intestines bacteria synthesize a number of important clotting factors which need vitamin K. Without vitamin K cuts can fail to heal and internal bleeding can occur.
    
    Vitamins C
    
    Vitamin C is a water soluble vitamin, known chemically as ascorbic acid. It is found in citrus fruits such as oranges and lemons, and also in potatoes and tomatoes. The main function of vitamin C is the formation of connective tissues such as collagen. It is also known to be an antioxidant which helps to remove toxins and aids the immune system. A lack of vitamin C leads to Scurvy, a condition experienced by sailors on long journeys when they did not have fruit in their diets. Scurvy causes painful, bleeding gums. As vitamin C is water soluble, it is not toxic in high doses as it can be excreted in the urine, very high doses can however cause diarrhea.
    
    Vitamins B
    
    B group vitamins have a wide range of roles acting as co-enzymes in metabolic pathways. They are found in most plant and animal tissues involved in metabolism, therefore foods such as liver, yeast and dairy products are all rich in B group vitamins. Deficiency of B group vitamins include dermatitis, fatigue and malformation of red blood cells.
1. An adult needs about 12,000kJ of energy a day from ________.
    
    A. the cell
    
    B. the respiring process of carbohydrates
    
    C. fats in the cell
    
    D. a balanced diet
    
    2. Carbohydrates are ultimately absorbed into cells in the process of _______.
    
    A. digestion
    
    B. respiration
    
    C. oxidization
    
    D. mobilization
    
    3. The Essential Amino Acids which build part of proteins can be obtained from______.
    
    A. stomach
    
    B. body tissues
    
    C. the body
    
    D. the diet
    
    4. The ultimate cause of kwashiorkor is lack of ________.
    
    A. protein
    
    B. carbohydrates
    
    C. vitamins
    
    D. diet
    
    5. Vitamins are called “micronutrients” in that _________.
    
    A. excessive fat soluble vitamins can be excreted in the urine
    
    B. the body only requires small amount of vitamins
    
    C. a dose of 3300mg of vitamins can be considered toxic
    
    D. the high concentrations of water soluble vitamins are toxic