Energy enters the human body in the form of food. Food is first ground by the teeth, which facilitates subsequent digestion. Ptyalin, a digestive enzyme found in saliva, begins hydrolysis – adding radicals from water in between constituent parts of a molecule in order to break it down into simpler, smaller molecules like maltose. Only about 5% of hydrolysis takes place in the mouth before the food is moved by the tongue into the pharynx (throat). The epiglottis ensures that it moves past the tracheal opening and into the esophagus (a process which sometimes functions less well in older people), through which it descends into the stomach. Hydrolysis continues there until the food becomes mixed with enough gastric secretions that the mounting acidity renders the enzyme inactive. After being kneaded in acidic gastric juice, the resulting pasty chyme passes into the small intestine, which secretes mucus, hormones and digestive enzymes. The enzymes break down the food according to its nature, as we shall see shortly in more detail.
Our nutriments are comprised almost entirely of three types of substances – carbohydrates (glucids, in some countries), proteins and fats (or lipids); the rest consists of small amounts of minerals and vitamins (organic nutrients required in small amounts, but which the organism cannot synthesize.).
The big picture is simple. During digestion, the small intestine converts carbohydrates, proteins and fats into glucose, amino acids, fatty acids and glycerol. Glucose is the body’s principal fuel supply and is carried by the blood to all cells. In each cell’s mitochondria, glucose is broken down by cellular respiration, a series of reactions which use various enzymes to convert the glucose into water and CO2 and release the energy needed by metabolism. The overall reaction forming the basis of cellular respiration is
C6H12O6 + 6O2 → 6H2O + 6CO2 + energy
glucose + oxygen →water + carbon dioxide + energy
where the energy is principally stored in molecules of ATP1)Adenosine triphosphate., the body’s “energy currency”. ATP serves as a transport agent, carrying the energy from mitochondria to the cells, such as muscles, which need it in order to function.
Some organs essential to digestion are not part of the digestive tract. The liver produces bile, which is needed for digestion of fats and lipids. Bile can be stored in the gallbladder to be delivered to the duodenum (the first part of the small intestine) when needed. The liver is also essential in energy regulation. The pancreas secretes enzymes needed for the digestion of all three food types, as will be seen shortly when we consider them in more detail.
Digestion in the small intestine is similar for all three principal food types. Carbohydrates, fats and proteins are all hydrolyzed (but by different enzymes) to simpler molecules:
- Carbohydrates are composed of poly- and monosaccharides. These are reduced by digestion to monosaccharides, which are used for energy or stored in cells, especially liver and muscle cells.
- Proteins are polypeptide chains of amino acids, which are broken down into the separate amino acids which will be used to make more proteins.
- Fats are composed mainly of triglycerides, which are separated into fatty acids.
The following sections discuss digestion of each type in more detail.2)Principal source of information, Guyton and Hall (2011), chaps. 67-69. Since we already considered carbohydrates, proteins and lipids in the last chapter, there may be some repetition.
Digestion of carbohydrates
Carbohydrates are composed mainly of sucrose, lactose, long-chain starches and cellulose. Cellulose is not digestible and just passes through the body in the form of fibers that nutritionists are so fond of. Digestion of carbohydrates takes place mainly in the small intestine as an enzyme secreted by the pancreas breaks them down into maltose or small glucose polymers, large molecules composed of repeated subunits. Different enzymes then split these into their constituent monosaccharides, the simplest forms of carbohydrates, which are soluble and so can cross the intestinal wall to enter the blood stream and be carried to the liver.
- Maltose from starches is broken down by maltase and a-dextrinase to glucose.
- Lactose from milk is converted by lactase into glucose and galactose.
- Sucrose (table sugar) is converted by sucrase into fructose.
The products are therefore glucose (about 80%), fructose and galactose. The last two substances are converted by the liver into glucose, which is therefore the common final form of carbohydrates arriving in cells. It is either used right away for energy or converted to glycogen (by the process of glycogenesis) and stored.
Glycogen is a large polymer of glucose molecules and is an efficient form for storage. Storing glycogen instead of the individual glucose molecules allows more storage without substantial modification of the cell’s osmotic pressure.3)Since osmotic pressure depends on the number of molecules in solvent. See the discussion of osmosis and related possible danger for cells in the section on water. All the body’s cells can store small amounts of glycogen, but the champions are the liver (5-8% of its weight) and the muscles (1-3%).
When the body’s glycogen storage capacity is filled, excess glucose is converted to acetyl-CoA and then into triglycerides (lipids), which are stored in adipose tissues. Yes, too much sugar turns to fat. The good news is that fat contains ~2.5 times the energy of the same weight of glycogen. So since much more fat than glycogen can be stored in the body, an average person stores around 150 times as much fat energy as carbohydrate.
Some cells, such as those in the human brain and red blood cells, normally need glucose for energy production. If the body’s supply of carbohydrates becomes too low, the liver goes to work, converting stored glycogen back into glucose (glycogenolysis) or synthesizing glucose from lactate and amino acids (gluconeogenesis). This may happen when eating mostly fats or when fasting, If needed, the kidneys can also synthesize glucose from amino acids and other substances. Fats may also be used as energy source, as discussed several paragraphs further on.
Digestion of proteins
Proteins are partially broken down by the enzyme pepsin. This enzyme prefers a pH of 2.0-3.0, which is maintained by hydrochloric acid secreted at pH 0.8 by gastric glands in the stomach. As in the case of carbohydrates, the pancreas contributes enzymes, proteases, to the small intestine to continue the breakdown of polypeptides into amino acids, although most remain as relatively small di- or tripeptides. Lastly, inside the cytosol of enterocytes1 on the intestinal wall, various peptidases break down what is left into single amino acids which then pass into the blood. These are carried to the cells to be used as raw materials for the construction of new proteins in ribosomes.
Many (~60%) amino acids from proteins in the cells can be converted to fat or glycogen by deamination. If more proteins than can be used as such are consumed, much of this excess is converted and stored as fat. So excess proteins turn to fat, too.
Digestion of lipids
Lipids (fats) consist of triglycerides, phospholipids, cholesterol and small quantities of other substances. Lipids are necessary for the formation of cell membranes and for many functions within cells. Like carbohydrates, they can provide energy. Most ingested fats are neutral fats called triglycerides, consisting of a glycerol nucleus attached to the ends of three fatty-acid side chains. Phospholipids are composed of only two fatty acids, attached to a hydrophilic phosphate “head group”.4)This subject was discussed in some detail in a preceding section. Cholesterol has chemical properties similar to those of fatty acids and is digested similarly,
Fat digestion occurs mainly in the small intestine. The hydrophobic fat globules are relatively large and must be broken up by emulsification. First, fat is mixed with bile (bile salts and, especially lecithin) with polar and non-polar parts. The non-polar parts attach to the fats and leave their hydrophilic (water-loving) polar parts pointing outwards, so the resulting globs are water-soluble. When they are agitated, they break up to form an emulsion, a mixture of one normally non-miscible liquid in another (like well-stirred vinaigrette salad dressing). The smaller units of the fat present a much greater surface area than before and so can be attacked by water-soluble pancreatic lipase and split into free fatty acids (FFAs) and 2-monoglycerides.
Again, the products are surrounded by bile salts with their non-polar ends towards the fats and their polar ends pointing outwards into the water. These form small spheres called micelles. The micelles ferry the products through the aqueous solution to the epithelial cells, where they release the FFAs which then cross the cell membrane. The micelles remain behind to pick up another droplet.
The FFAs are re-synthesized into triglycerides and packaged into tiny phospholipid vesicles called chylomicrons which flow through lymphatic capillaries called lacteals into the lymphatic system. The chylomicrons transport the fats and cholesterol through the aqueous lymph and are then returned to the venous system via the thoracic duct.5)See the chapter on the lymphatic system.
Triglycerides are converted into FFAs and back because triglycerides cannot cross the intestinal cell membranes, but FFAs can. They then enter the lymphatic system. Only small amounts of water-soluble, short-chain fatty acids can pass directly from the small intestine into the blood without making a detour through the lymphatic system.
In short, fats are emulsified by bile salts so that they can be broken up by pancreatic lipase into FFAs. FFAs can cross the intestinal membrane and are reconverted to triglycerides. In order for them to be transported in the aqueous lymphatic system, FFAs and cholesterol are enclosed in phospholipid vesicles, chylomicrons.
The same process and ferrying by micelles takes place for phospholipids and cholesterol using different pancreatic lipases.
In this manner, almost all ingested fats (except short-chain fatty acids) are absorbed from the intestine into the intestinal lymphatic system in chylomicrons. These then are passed through the thoracic duct into venous blood. Several types of tissues, especially skeletal muscles, adipose tissues and heart tissues, synthesize lipoprotein lipase which hydrolyzes the triglycerides from the chylomicrons. The triglycerides are either used for energy or stored in the cells until needed.
After breakup of the chylomicrons, most of the lipids left in blood plasma are in the form of tiny particles called lipoproteins, which are like miniature chylomicrons. The lipoproteins may contain triglycerides, phospholipids, cholesterol or proteins. Like chylomicrons, lipoproteins serve to transport lipids in the blood. Use of a centrifuge to separate their components has led to the definition of four classes of lipoproteins. Each class is analyzed after removal of the more dense classes, so from IDL on, they contain little of triglycerides because these have been removed.
VLDL (very low-density lipoproteins)
IDL (intermediate-density lipoproteins)
LDL (low-density lipoproteins)
HDL (high-density lipoproteins)
It is clear from the table that LDLs contain proportionally more cholesterol, considered to be a risk factor for atheriosclerosis, than HDLs.6)The sequence of events by which LDLs are involved in atheriosclerosis is too long to consider here. But LDL is not exactly cholesterol, although it does transport it.
Use of fats for energy
The fat intake of the world’s peoples is quite variable, being 10-15% in parts of Asia and 35-50% in western countries. Part of this is converted to triglycerides and stored.
Compared to carbohydrates and proteins, triglycerides furnish more than twice the energy per unit mass. So when glucose levels become inadequate, stored triglycerides are hydrolyzed to fatty acids. The rate of conversion and transport of FFAs is great enough that they can be oxidized to fulfill almost all the body’s energy needs without the need of energy from carbohydrates.
In order to be used for energy, fatty acids are carried by carnitine into the mitochondria of the cells. There, they are degraded into acetyl-CoA, which enters the Krebs cycle of cellular respiration (explained shortly).
The liver, which as we have seen stores a large quantity of fatty acids, uses only a small part of these for its own energy requirements. Excess acetyl-CoA is combined in pairs to form acetoacetic acid which is then carried by the blood to other cells. Some of it is converted to β-hydroxybutyrate and acetone, large quantities of which can be transported rapidly to cells. These two substances plus acetoacetic acid are collectively referred to as ketone bodies. On arriving in cells, all three are converted back into acetyl-CoA, which enters the Krebs cycle.
In brief, fats stored as triglycerides may be modified in the liver to ketone bodies and enter cells where they are transformed into acetyl-CoA and enter the Krebs cycle.7)See the figure, “The steps of cellular respiration”, in chapter “Making energy available“.
Glycerol is converted directly into a form which can enter the glycolytic pathway (glycolysis, the introductory step to the Krebs cycle).
glycerol → glycero-3-phosphate → glycolytic pathway → energy
When insufficient carbohydrates are available to furnish needed energy, almost all energy is furnished by fats. However, changing from a carbohydrate-rich to a fat diet may require several weeks for brain cells to switch (mostly) to energy from fats.8)Guyton and Hall, 2011, p. 824.
Regulation of glucose levels
The liver converts glucose into glycogen and stores that; it can be reconverted whenever the level of blood sugar becomes low. This function is mediated by the hormones insulin and glucagon, both produced by the pancreas but having opposite effects.
Glucose concentration in the blood needs to be within a certain limited range in order to distribute enough energy to the body, but not so much as would harm organs and tissues, especially where blood vessels are tiny, as in the retina, the body’s extremities and the kidneys. Most cells have receptors which bind insulin, causing glucose transporters in the cell membranes to allow more glucose to enter the cell for storage, thereby reducing the glucose level of the blood. Abnormally high glucose levels in the blood are a sign of diabetes, which can be due to inadequate insulin production or to the cells’ not reacting correctly to insulin presence.
If the blood glucose level becomes too low, the pancreas secretes glucagon which stimulates production of glucose from the breakdown of glycogen, from amino acids or from stored triglycerides (lipolysis).
Now all this energy must be made available by the process of cellular respiration.
Notes [ + ]
|2.||↑||Principal source of information, Guyton and Hall (2011), chaps. 67-69.|
|3.||↑||Since osmotic pressure depends on the number of molecules in solvent. See the discussion of osmosis and related possible danger for cells in the section on water.|
|4.||↑||This subject was discussed in some detail in a preceding section.|
|5.||↑||See the chapter on the lymphatic system.|
|6.||↑||The sequence of events by which LDLs are involved in atheriosclerosis is too long to consider here.|
|7.||↑||See the figure, “The steps of cellular respiration”, in chapter “Making energy available“.|
|8.||↑||Guyton and Hall, 2011, p. 824.|