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Other types of lipids found in the body are fatty acids and membrane lipids. Lipid metabolism is often considered the digestion and absorption process of dietary fat; however, there are two sources of fats that organisms can use to obtain energy: from consumed dietary fats and from stored fat. [ 5 ]
Fat has a food energy content of 38 kilojoules per gram (9 kilocalories per gram) proteins and carbohydrates 17 kJ/g (4 kcal/g). [ 2 ] Water makes up a large proportion of the total mass ingested as part of a normal diet but it does not provide any nutritional value.
The energy yield from a gram of fatty acids is approximately 9 kcal (37 kJ), much higher than the 4 kcal (17 kJ) for carbohydrates. Since the hydrocarbon portion of fatty acids is hydrophobic , these molecules can be stored in a relatively anhydrous (water-free) environment.
These chloroplast membranes contain a high quantity of monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG). They probably also assume a direct role in photosynthesis , as they have been found in the X-ray structures of photosynthetic complexes.
The unit is most commonly used to express food energy, namely the specific energy (energy per mass) of metabolizing different types of food. For example, fat (triglyceride lipids) contains 9 kilocalories per gram (kcal/g), while carbohydrates (sugar and starch) and protein contain approximately 4 kcal/g. [29] Alcohol in food contains 7 kcal/g. [30]
[56] [2]: 329–331 In plants and algae, the galactosyldiacylglycerols, [57] and sulfoquinovosyldiacylglycerol, [32] which lack a phosphate group, are important components of membranes of chloroplasts and related organelles and are among the most abundant lipids in photosynthetic tissues, including those of higher plants, algae and certain ...
Typically, the complete breakdown of one molecule of glucose by aerobic respiration (i.e. involving glycolysis, the citric-acid cycle and oxidative phosphorylation, the last providing the most energy) is usually about 30–32 molecules of ATP. [16] Oxidation of one gram of carbohydrate yields approximately 4 kcal of energy. [3]
For example, the values for glucose, sucrose, and starch are 15.57, 16.48 and 17.48 kilojoules per gram (3.72, 3.94 and 4.18 kcal/g) respectively. The differing energy density of foods (fat, alcohols, carbohydrates and proteins) lies mainly in their varying proportions of carbon, hydrogen, and oxygen atoms.