Nutrition

The desire for food is apparently controlled in part by an area of the lateral hypothalamus called the [1]. Damage to this area causes [2], a profound loss of appetite. The feeling of having eaten enough is mediated through a/an [3] in the ventromedial hypothalamus, and damage here causes grossly excessive eating, called [4]. Neurons called [5] monitor the level of blood glucose and perhaps suppress the appetite when glucose level is high. As glucose level falls, their inhibitory signals to 1 decline and one begins to feel hungry. Hunger is also stimulated by strong gastric peristalsis, called [6], but even when these are very painful, they do not cause us to consume greater quantities of food than usual once it becomes available.

What we call a Calorie in dietetics is usually called a [7] in biochemistry and physiology. Foods are said to provide [8] if they provide calories but almost nothing else of nutritional value. Most dietary calories come from three classes of nutrients: [9], [10], and [11]. These are called [12] because they must be consumed in relatively large quantities. Vitamins and minerals, required in smaller quantities, are called [13]. [14] nutrients are molecules the body needs but cannot synthesize for itself; thus they must be included in the diet.

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Carbohydrates form a part of many important biological molecules, although most of the carbohydrate in the diet serves as [15]–that is, it is oxidized to get the energy from it. A deficiency of blood glucose, called [16], can cause nervous system disturbances and other problems. Blood glucose is therefore carefully regulated by hormones such as insulin and glucagon, and we have reserves of glucose, especially in the polymer [17], that we can draw from when dietary intake is low (for example, overnight). [18] is the only nutritionally significant polysaccharide. Only trivial amounts of 17 are present in the diet, and while the diet normally does and should contain large amounts of [19], this is not considered a nutrient because it is not digested and never enters the body's tissues. This substance is important because it swells with water and increases the bulk of the [20]. This stimulates intestinal motility and prevents residue from stagnating in the intestine and potentially causing colon cancer. Pectin, a [21] fiber found in beans, oats, brown rice, etc., helps to reduce blood cholesterol and LDL levels, whereas 19 does not.

Fats have more than twice as many calories per gram as carbohydrates do, and account for most of the body's stored energy. Fat can store more energy per volume of tissue than carbohydrates can because it is [22] and therefore has less tendency to swell with water. Fat has glucose- and protein-[23] effects because, as long as it is available to meet the energy needs of many of our cells, these other energy substrates are not consumed or are left available for cells that cannot use fats. The four fat-soluble vitamins, [24] (list the four letters), depend on dietary fat for their absorption; they would be wasted if ingested with an otherwise fat-free meal. Foods can be truthfully described as cholesterol-free, yet raise our blood cholesterol anyway if they are high in [25]. Being hydrophobic, lipids require special provisions to be transported in the body fluids. In the blood, we find four major classes of [26]–droplets of lipid coated with protein. Of these four, [27] are produced by the small intestine as a way of getting dietary fats into the lymph and blood. [28] are produced by the liver as a way of transporting fats and cholesterol to other cells that need them. When the fats are removed, these become [29]. A high level of 28 and 29 in the blood may indicate excessive deposition of fats and cholesterol in the tissues, and these are often nicknamed "bad cholesterol" in the popular media. [30] are produced by the liver as empty protein shells that pick up cholesterol and phospholipids from other tissues and return them to the liver for disposal. Since a high level of 30s indicates a high rate of cholesterol removal from the circulation, these are nicknamed "good cholesterol." These nicknames are misleading, however, because there is only one form of cholesterol and cholesterol is essential to many physiological functions and cellular structures; whether it is good or bad depends on the amount present.

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The nutritional quality of a protein depends on the types and relative amounts of [31] in its structure. It is especially important that dietary proteins provide the [32] 31s that the body cannot synthesize. Proteins that provide all of these in the proportions needed by the body are called [33] proteins. One should ideally be in a state of [34] in which the rate of nitrogen intake in proteins equals the rate of nitrogen loss in the urine and feces. Pregnant women, children, and athletes in training tend to be in a state of [35], in which nitrogen intake exceeds output.

The mineral that is most overabundant a typical diet is [36], since it is so widely and heavily used as a food additive. Vegetables are a relatively [37] (good or poor?) source of this mineral. Hemoglobin synthesis requires the mineral [38] and thyroid hormone synthesis requires the mineral [39]. Anemia or goiter result from deficiencies of these minerals, respectively. Most water-soluble vitamins function as [40], which transfer electrons from one metabolic pathway to another. Visual pigments are made from vitamin [41], calcium absorption is promoted by vitamin [42], and blood clotting is promoted by vitamin [43].

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Carbohydrate Metabolism

The net reaction for carbohydrate oxidation is C₆H₁₂O₆ + 6 O₂ -> [44]. The process begins with a sugar-splitting pathway called [45], which produces two molecules of [46] as its major end product. When oxygen is lacking, 46 is further metabolized through a pathway called [47], but when oxygen is available, it is metabolized through a much more efficient pathway, [48]. Pathway 45 gives a net yield of 2 ATP molecules and is the only source of ATP under anaerobic conditions. If not for pathway 47, 45 would soon cease to function under such conditions because it would run out of a coenzyme abbreviated [49]. The primary purpose of 47 is to replenish this coenzyme, but a disadvantage of it is that its end product, [50], is toxic to cells. This chemical contributes to muscle fatigue and, in most cells, sets a limit on how long 47 can continue.

Aerobic respiration is carried out in the [51], where there is a cycle of reactions in the matrix called the [52] cycle followed by a series of reactions involving a/an [53] chain of enzymes on the inner membrane. The 52 cycle is where most of the CO₂ in your breath is generated, along with [54] (how many?) ATP molecules for every glucose molecule we started with. But the most important products of this cycle are two reduced coenzymes, [55] and [56], which now carry most of the energy that was originally in the glucose. These coenzymes release this energy when they shuttle electrons to the 53 chain.

Enzymes of the 53 chain are grouped into three enzyme complexes. Each enzyme becomes [57] when it accepts electrons from a coenzyme or from the preceding member of the chain, and [58] when it passes these electrons along to the next member. The final electron acceptor is [59]; its reduction produces the [60] in the net reaction at 44 above. But the most important thing to happen in this reaction chain is that each enzyme complex acts as a [61] that drives H⁺ from the matrix into the space between the inner and outer membranes. This creates a high concentration and electrical gradient between the matrix and the intermembrane space, but the only way H⁺ can diffuse down the gradient back into the matrix is to pass through a channel protein called [62]. This creates an electrical current passing through that protein, and the protein taps that energy source to synthesize ATP. This is called the [63] mechanism of ATP synthesis.

As a net effect of all these processes, one mole of glucose provides enough energy to synthesize up to [64] moles of ATP. However, all that ATP has only [65]% of the energy that was in the single glucose molecule. The rest of the energy escapes from the system as [66].

For long-term energy storage, the body uses not ATP but fat and [67]. The synthesis of 67 is called [68], and when the energy in it is needed elsewhere, the glycogen is hydrolyzed in a process called [69]. If not enough glycogen is available for the body's energy needs, glucose can be synthesized from fats and amino acids. This process is called [70].

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Lipid and Protein Metabolism

Triglyceride synthesis, called [71], requires glycerol and fatty acids. Glycerol can be made from one of the intermediates of glycolysis, [72]. Fatty acids can be made from [73] diverted from the citric acid cycle. The catabolism of triglycerides is called [74]. After fatty acids are hydrolyzed from the glycerol backbone, they are broken down two carbons at a time in a process called [75], and then fed into the citric acid cycle. If there are excess C₂ compounds (acetyl groups), the liver converts them to ketone bodies in a process called [76]. Ketones can be used as fuel by other organs, but if present in excess, they can cause a pH imbalance called [77], seen in diabetes mellitus and starvation.

Free amino acids in the body, called the [78], provide a source of raw material for protein synthesis, but they can also be converted to fats or burned as fuel. When used as fuel, they must first be [79]—that is, have the –NH₂ group removed. This generates ammonia (NH₃), which must be detoxified. A metabolic pathway in the liver called the [80] cycle converts NH₃ and CO₂ to the nitrogenous waste, [81].

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Metabolic States and Metabolic Rate

For the first four hours after a meal, food or chyme is present in the stomach and small intestine and a person is in an [82] state. Later, in the [83] state, the stomach and small intestine are empty and the body draws on its stored energy reserves. The levels of insulin and chylomicrons in the blood are highest in the [84] state (which of the preceding?), whereas the pancreatic hormone [85] is secreted in the other. In prolonged fasting, [86] and [87] reserves are consumed first as fuel, and then the body begins to draw on its [88] for gluconeogenesis. The last of these can result in a wasting away of the muscles. Adipose tissue is well innervated by the [89] nervous system, which stimulates fat lipolysis under conditions of stress or injury.

The amount of energy released from organic compounds of the body in a given time period is called the [90]. It can be expressed in such terms as kcal/day, and can be indirectly determined from a person's rate of [91] consumption, measured with a spirometer. The [92], measured when a person is awake, relaxed, comfortable, and hasn't eaten for 12 to 14 hours, serves as a basis of comparison but is not the minimum needed to keep a person alive. A typical adult male value for 90 is [93] kcal/day, but even a low level of physical activity requires another 500 kcal/day, and total metabolic rate may be as high as [94] for someone who does hard physical labor.

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Body Heat and Thermoregulation

The function of [95] is to maintain the body temperature within a safe and optimal range, and avoid dangerously high or low body temperatures. The easiest temperature to measure, typically with an oral thermometer, is called the body's [96] temperature; however, the [97] temperature, better estimated from a rectal thermometer, is around 0.6°C warmer and is a better measure of the temperature most relevant to a person's metabolism.

At rest, most body heat is generated by [98] chemical reactions such as aerobic metabolism, but the [99] can produce 30 to 40 times as much heat during exercise. There are three mechanisms of heat loss from the body: [100], the production of infrared rays; [101], the transfer of energy from the body to its surroundings by molecular collisions; and [102], the cooling of the body by sweat. The movement of unevenly heated air or water around the body, called [103], aids in heat loss, especially by mechanism 99, but has no effect on 98. At a comfortable air temperature, we lose the most heat by [104] (which of the three foregoing mechanisms?), but if the air temperature is higher than body temperature, [105] becomes the only mechanism of heat loss.

Body heat is monitored by a nucleus of neurons in the brain called the [106]. If the temperature rises too high, [107] of arterioles in the skin helps to get rid of heat, and if this does not suffice, [108] occurs. If temperature drops too low, cutaneous blood circulation is reduced to keep the warm blood deeper in the body, and if that does not suffice, [109] occurs. Over the long term, however–for example, when the weather turns colder in the fall–the body increases its heat production by raising its metabolic rate. This is called [110].

Excessively high body temperatures, or [111], can lead to enzyme denaturation, neurological damage, and death. Heavy sweating can deplete electrolytes from the body and cause painful muscle spasms called [112]. In [113], this progresses to hypotension, dizziness, fainting, or vomiting. In [114], the body gains so much heat from the environment that the brain malfunctions. Convulsions, coma, and death may occur. Excessively low body temperature, called [115], can also be lethal. The metabolic rate slows down so much the body cannot produce enough heat to make up for its loss. Thus the temperature and metabolic rate fall still more, and a [116] loop sets in that may lead to death.

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