Forms of Carbohydrates

Five- and six-membered ring structures are common in carbohydrates and are often in equilibrium with each other. The five-membered ring structures are called furanose forms the six-membered ring structures are pyranose forms. D-Ribose, especially in its P-furanose form, is a famihar carbohydrate. 

What is the orientation of the OH groups at C-2 and C-3 in the P-pyranose form of D-ribose  

All of the OH groups of the P-pyranose form of D-xylose are equatorial. Which of the following is the P-furanose form of D-xylose  

The carbohydrate shown here is a component of a drug used in veterinary medicine. Which is its most stable pyranose conformation  

6 Physical Properties of Alcohols and Alkyl Halides Intermolecular Forces 137 

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In spite of their easy interconversion in solution a and p forms of carbohydrates are capable of independent existence and many have been isolated m pure form as crys talline solids When crystallized from ethanol d glucose yields a d glucopyranose mp 146°C [a]o +112 2° Crystallization from a water-ethanol mixture produces p d glucopyranose mp 148-155°C [aj +18 7° In the solid state the two forms do not mterconvert and are stable indefinitely Their structures have been unambiguously con firmed by X ray crystallography... 

Sfrucfurally O glycosides are mixed acefals fhaf involve fhe anomeric posifion of furanose and pyranose forms of carbohydrates Recall fhe sequence of mfermediafes m acefal formalion (Secfion 17 8)... 

Haworth formulas (Section 25.6) Planar representations of furanose and pyranose forms of carbohydrates. 

Those with a normal diet take in food in the forms of carbohydrates, fats and proteins. Because it has a low water content and produces so many ATP molecules, fat yields 9.3 Calories per gram while carbohydrates and proteins yield less than half as much (4.1 and 4.3 calorics per gi-am respectively). Thus, we get a huge number of calories from a small quantity of fat eaten. The average person in the United States has a diet with 50 percent of the calories in the form of carbohydrates, 35 percent in the form of fat and 15 percent in the form of protein. We need about 1 gram of protein per kilogram of body weight per day to replace body proteins that are broken down. A 70 kg person... 

Glycogen represents the principal storage form of carbohydrate in the mammalian body, mainly in the liver and muscle. 

The energy in food is in the form of carbohydrate, fat and protein, and the oxidation of these compounds, in vitro, transfers the chemical energy into heat which can then be measured. This is done in what is known as a bomb calorimeter (Figure 2.1). The heat released in the calorimeter when 1 g of carbohydrate, fat, protein or other fuel is fully oxidised (i.e. Af/) is given in Table 2.3. 

The diet provided is high in protein (about 114 g per day) and 60% of the energy is in the form of carbohydrate with 15% in fat. Some foods are fresh (e.g. fruit) but most are dehydrated. 

It is recommended that about half of the energy intake should be in the form of carbohydrates, a third at most in the form of fat, and the rest as protein. The fact that alcoholic beverages can make a major contribution to daily energy intake is often overlooked. Ethanol has a caloric value of about 30 kj g (see p. 320). 

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