Lipids

Lipids are the one class of large biological molecules that does not consist of polymers. The compounds called lipids are grouped together because they share one important trait: They have little or no affinity for water. The hydrophobic behavior of lipids is based on their molecular structure. Although they may have some polar bonds associated with oxygen, lipids consist mostly of hydrocarbons. Smaller than true (polymeric) macromolecules, lipids are a highly varied group in both form and function. Lipids include waxes and certain pigments, but we will focus on the most biologically important types of lipids: fats, phospholipids, and steroids.

Fats

Although fats are not polymers, they are large molecules, and they are assembled from smaller molecules by dehydration reactions. A fat is constructed from two kinds of smaller molecules: glycerol and fatty acids.

The synthesis and structure of a fat, or triacylglycerol. The molecular building blocks of a fat are one molecule of glycerol and three molecules of fatty acids. (a) One water molecule is removed for each fatty acid joined to the glycerol. (b) A fat molecule with three identical fatty acid units. The carbons of the fatty acids are arranged zig–zag to suggest the actual orientations of the four single bonds extending from each carbon.

Glycerol is an alcohol with three carbons, each bearing a hydroxyl group. A fatty acid has a long carbon skeleton, usually 16 or 18 carbon atoms in length. At one end of the fatty acid is a carboxyl group, the functional group that gives these molecules the name fatty acid. Attached to the carboxyl group is a long hydrocarbon chain. The nonpolar C–H bonds in the hydrocarbon chains of fatty acids are the reason fats are hydrophobic. Fats separate from water because the water molecules hydrogen–bond to one another and exclude the fats. A common example of this phenomenon is the separation of vegetable oil (a liquid fat) from the aqueous vinegar solution in a bottle of salad dressing.

In making a fat, three fatty acid molecules each join to glycerol by an ester linkage, a bond between a hydroxyl group and a carboxyl group. The resulting fat, also called a triacylglycerol , thus consists of three fatty acids linked to one glycerol molecule. (Still another name for a fat is triglyceride, a word often found in the list of ingredients on packaged foods.) The fatty acids in a fat can be the same, as in Figure 5.11b, or they can be of two or three different kinds.

These terms refer to the structure of the hydrocarbon chains of the fatty acids. If there are no double bonds between carbon atoms composing the chain, then as many hydrogen atoms as possible are bonded to the carbon skeleton. Such a structure is described as being saturated with hydrogen, so the resulting fatty acid is called a saturated fatty acid. An unsaturated fatty acid has one or more double bonds, formed by the removal of hydrogen atoms from the carbon skeleton. The fatty acid will have a kink in its hydrocarbon chain wherever a cis double bond occurs.

A fat made from saturated fatty acids is called a saturated fat. Most animal fats are saturated: The hydrocarbon chains of their fatty acids—the “tails” of the fat molecules—lack double bonds, and the molecules can pack tightly, side by side. Saturated animal fats—such as lard and butter—are solid at room temperature. In contrast, the fats of plants and fishes are generally unsaturated, meaning that they are built of one or more types of unsaturated fatty acids. Usually liquid at room temperature, plant and fish fats are referred to as oils—olive oil and cod liver oil are examples. The kinks where the cis double bonds are located prevent the molecules from packing together closely enough to solidify at room temperature. The phrase “hydrogenated vegetable oils” on food labels means that unsaturated fats have been synthetically converted to saturated fats by adding hydrogen. Peanut butter, margarine, and many other products are hydrogenated to prevent lipids from separating out in liquid (oil) form.

A diet rich in saturated fats is one of several factors that may contribute to the cardiovascular disease known as atherosclerosis. In this condition, deposits called plaques develop within the walls of blood vessels, causing inward bulges that impede blood flow and reduce the resilience of the vessels. Recent studies have shown that the process of hydrogenating vegetable oils produces not only saturated fats but also unsaturated fats with trans double bonds. These trans fat molecules may contribute more than saturated fats to atherosclerosis and other problems.

Fat has come to have such a negative connotation in our culture that you might wonder whether fats serve any useful purpose. The major function of fats is energy storage. The hydrocarbon chains of fats are similar to gasoline molecules and just as rich in energy. A gram of fat stores more than twice as much energy as a gram of a polysaccharide, such as starch. Because plants are relatively immobile, they can function with bulky energy storage in the form of starch. (Vegetable oils are generally obtained from seeds, where more compact storage is an asset to the plant.) Animals, however, must carry their energy stores with them, so there is an advantage to having a more compact reservoir of fuel—fat. Humans and other mammals stock their long–term food reserves in adipose cells (see Figure 4.6b), which swell and shrink as fat is deposited and withdrawn from storage. In addition to storing energy, adipose tissue also cushions such vital organs as the kidneys, and a layer of fat beneath the skin insulates the body. This subcutaneous layer is especially thick in whales, seals, and most other marine mammals, protecting them from cold ocean water.

The structure of a phospholipid. A phospholipid has a hydrophilic (polar) head and two hydrophobic (nonpolar) tails. Phospholipid diversity is based on differences in the two fatty acids and in the groups attached to the phosphate group of the head. This particular phospholipid, called a phosphatidylcholine, has an attached choline group. The kink in one of its tails is due to a cis double bond. (a) The structural formula follows a common chemical convention of omitting the carbons and attached hydrogens of the hydrocarbon tails. (b) In the space–filling model, black = carbon, gray = hydrogen, red = oxygen, yellow = phosphorus, and blue = nitrogen.

A phospholipid is similar to a fat, but has only two fatty acids attached to glycerol rather than three. The third hydroxyl group of glycerol is joined to a phosphate group, which has a negative electrical charge. Additional small molecules, usually charged or polar, can be linked to the phosphate group to form a variety of phospholipids.

Phospholipids show ambivalent behavior toward water. Their hydrocarbon tails are hydrophobic and are excluded from water. However, the phosphate group and its attachments form a hydrophilic head that has an affinity for water. When phospholipids are added to water, they self–assemble into double–layered aggregates—bilayers—that shield their hydrophobic portions from water.

Steroids
Steroids are lipids characterized by a carbon skeleton consisting of four fused rings.

Different steroids vary in the functional groups attached to this ensemble of rings. One steroid, cholesterol , is a common component of animal cell membranes and is also the precursor from which other steroids are synthesized. Many hormones, including vertebrate sex hormones, are steroids produced from cholesterol. Thus, cholesterol is a crucial molecule in animals, although a high level of it in the blood may contribute to atherosclerosis. Both saturated fats and trans fats exert their negative impact on health by affecting cholesterol levels.

Gallstones

What are gallstones?

Gallstones are small, pebble-like substances that develop in the gallbladder. The gallbladder is a small, pear-shaped sac located below your liver in the right upper abdomen. Gallstones form when liquid stored in the gallbladder hardens into pieces of stone-like material. The liquid—called bile—helps the body digest fats. Bile is made in the liver, then stored in the gallbladder until the body needs it. The gallbladder contracts and pushes the bile into a tube—called the common bile duct—that carries it to the small intestine, where it helps with digestion.

Bile contains water, cholesterol, fats, bile salts, proteins, and bilirubin—a waste product. Bile salts break up fat, and bilirubin gives bile and stool a yellowish-brown color. If the liquid bile contains too much cholesterol, bile salts, or bilirubin, it can harden into gallstones.

The two types of gallstones are cholesterol stones and pigment stones. Cholesterol stones are usually yellow-green and are made primarily of hardened cholesterol. They account for about 80 percent of gallstones. Pigment stones are small, dark stones made of bilirubin. Gallstones can be as small as a grain of sand or as large as a golf ball. The gallbladder can develop just one large stone, hundreds of tiny stones, or a combination of the two.

Gallstones can block the normal flow of bile if they move from the gallbladder and lodge in any of the ducts that carry bile from the liver to the small intestine. The ducts include the hepatic ducts, which carry bile out of the liver cystic duct, which takes bile to and from the gallbladder common bile duct, which takes bile from the cystic and hepatic ducts to the small intestine.

Bile trapped in these ducts can cause inflammation in the gallbladder, the ducts, or in rare cases, the liver. Other ducts open into the common bile duct, including the pancreatic duct, which carries digestive enzymes out of the pancreas. Sometimes gallstones passing through the common bile duct provoke inflammation in the pancreas—called gallstone pancreatitis—an extremely painful and potentially dangerous condition.

If any of the bile ducts remain blocked for a significant period of time, severe damage or infection can occur in the gallbladder, liver, or pancreas. Left untreated, the condition can be fatal. Warning signs of a serious problem are fever, jaundice, and persistent pain.

What causes gallstones?

Scientists believe cholesterol stones form when bile contains too much cholesterol, too much bilirubin, or not enough bile salts, or when the gallbladder does not empty completely or often enough. The reason these imbalances occur is not known.

The cause of pigment stones is not fully understood. The stones tend to develop in people who have liver cirrhosis, biliary tract infections, or hereditary blood disorders—such as sickle cell anaemia—in which the liver makes too much bilirubin.

The mere presence of gallstones may cause more gallstones to develop. Other factors that contribute to the formation of gallstones, particularly cholesterol stones, include

Sex. Women are twice as likely as men to develop gallstones. Excess estrogen from pregnancy, hormone replacement therapy, and birth control pills appears to increase cholesterol levels in bile and decrease gallbladder movement, which can lead to gallstones.

Family history. Gallstones often run in families, pointing to a possible genetic link.

Weight. A large clinical study showed that being even moderately overweight increases the risk for developing gallstones. The most likely reason is that the amount of bile salts in bile is reduced, resulting in more cholesterol. Increased cholesterol reduces gallbladder emptying. Obesity is a major risk factor for gallstones, especially in women.

Diet. Diets high in fat and cholesterol and low in fiber increase the risk of gallstones due to increased cholesterol in the bile and reduced gallbladder emptying.

Rapid weight loss. As the body metabolizes fat during prolonged fasting and rapid weight loss—such as “crash diets”—the liver secretes extra cholesterol into bile, which can cause gallstones. In addition, the gallbladder does not empty properly.

Age. People older than age 60 are more likely to develop gallstones than younger people. As people age, the body tends to secrete more cholesterol into bile.

Ethnicity. American Indians have a genetic predisposition to secrete high levels of cholesterol in bile. In fact, they have the highest rate of gallstones in the United States. The majority of American Indian men have gallstones by age 60.

Cholesterol-lowering drugs. Drugs that lower cholesterol levels in the blood actually increase the amount of cholesterol secreted into bile. In turn, the risk of gallstones increases.

Diabetes. People with diabetes generally have high levels of fatty acids called triglycerides. These fatty acids may increase the risk of gallstones.

Who is at risk for gallstones?

People at risk for gallstones include :

women—especially women who are pregnant, use hormone replacement therapy, or take birth control pills

people over age 60

overweight or obese men and women

people who fast or lose a lot of weight quickly

people with a family history of gallstones

people with diabetes

people who take cholesterol-lowering drugs

What are the symptoms of gallstones?

As gallstones move into the bile ducts and create blockage, pressure increases in the gallbladder and one or more symptoms may occur. Symptoms of blocked bile ducts are often called a gallbladder “attack” because they occur suddenly. Gallbladder attacks often follow fatty meals, and they may occur during the night. A typical attack can causesteady pain in the right upper abdomen that increases rapidly and lasts from 30 minutes to several hours pain in the back between the shoulder blades pain under the right shoulder

Notify your doctor if you think you have experienced a gallbladder attack. Although these attacks often pass as gallstones move, your gallbladder can become infected and rupture if a blockage remains.

People with any of the following symptoms should see a doctor immediately:

prolonged pain—more than 5 hours

nausea and vomiting

fever—even low-grade—or chills

yellowish color of the skin or whites of the eyes

clay-coloured stools

Many people with gallstones have no symptoms; these gallstones are called “silent stones.” They do not interfere with gallbladder, liver, or pancreas function and do not need treatment.

How are gallstones diagnosed?

Frequently, gallstones are discovered during tests for other health conditions. When gallstones are suspected to be the cause of symptoms, the doctor is likely to do an ultrasound exam—the most sensitive and specific test for gallstones. A handheld device, which a technician glides over the abdomen, sends sound waves toward the gallbladder. The sound waves bounce off the gallbladder, liver, and other organs, and their echoes make electrical impulses that create a picture of the gallbladder on a video monitor. If gallstones are present, the sound waves will bounce off them, too, showing their location. Other tests may also be performed.

Computerized tomography (CT) scan. The CT scan is a noninvasive x ray that produces cross-section images of the body. The test may show the gallstones or complications, such as infection and rupture of the gallbladder or bile ducts.

Cholescintigraphy (HIDA scan). The patient is injected with a small amount of nonharmful radioactive material that is absorbed by the gallbladder, which is then stimulated to contract. The test is used to diagnose abnormal contraction of the gallbladder or obstruction of the bile ducts.

Endoscopic retrograde cholangiopancreatography (ERCP). ERCP is used to locate and remove stones in the bile ducts. After lightly sedating you, the doctor inserts an endoscope—a long, flexible, lighted tube with a camera—down the throat and through the stomach and into the small intestine. The endoscope is connected to a computer and video monitor. The doctor guides the endoscope and injects a special dye that helps the bile ducts appear better on the monitor. The endoscope helps the doctor locate the affected bile duct and the gallstone. The stone is captured in a tiny basket and removed with the endoscope.

Blood tests. Blood tests may be performed to look for signs of infection, obstruction, pancreatitis, or jaundice.

Because gallstone symptoms may be similar to those of a heart attack, appendicitis, ulcers, irritable bowel syndrome, hiatal hernia, pancreatitis, and hepatitis, an accurate diagnosis is important.

How are gallstones treated?

Surgery. If you have gallstones without symptoms, you do not require treatment. If you are having frequent gallbladder attacks, your doctor will likely recommend you have your gallbladder removed—an operation called a cholecystectomy. Surgery to remove the gallbladder—a nonessential organ—is one of the most common surgeries performed on adults in the United States.

Nearly all cholecystectomies are performed with laparoscopy. After giving you medication to sedate you, the surgeon makes several tiny incisions in the abdomen and inserts a laparoscope and a miniature video camera. The camera sends a magnified image from inside the body to a video monitor, giving the surgeon a close-up view of the organs and tissues. While watching the monitor, the surgeon uses the instruments to carefully separate the gallbladder from the liver, bile ducts, and other structures. Then the surgeon cuts the cystic duct and removes the gallbladder through one of the small incisions.

Recovery after laparoscopic surgery usually involves only one night in the hospital, and normal activity can be resumed after a few days at home. Because the abdominal muscles are not cut during laparoscopic surgery, patients have less pain and fewer complications than after “open” surgery, which requires a 5- to 8-inch incision across the abdomen.

If tests show the gallbladder has severe inflammation, infection, or scarring from other operations, the surgeon may perform open surgery to remove the gallbladder. In some cases, open surgery is planned; however, sometimes these problems are discovered during the laparoscopy and the surgeon must make a larger incision. Recovery from open surgery usually requires 3 to 5 days in the hospital and several weeks at home. Open surgery is necessary in about 5 percent of gallbladder operations.

The most common complication in gallbladder surgery is injury to the bile ducts. An injured common bile duct can leak bile and cause a painful and potentially dangerous infection. Mild injuries can sometimes be treated nonsurgically. Major injury, however, is more serious and requires additional surgery.

If gallstones are present in the bile ducts, the physician—usually a gastroenterologist—may use ERCP to locate and remove them before or during gallbladder surgery. Occasionally, a person who has had a cholecystectomy is diagnosed with a gallstone in the bile ducts weeks, months, or even years after the surgery. The ERCP procedure is usually successful in removing the stone in these cases.

Nonsurgical Treatment.Nonsurgical approaches are used only in special situations—such as when a patient has a serious medical condition preventing surgery—and only for cholesterol stones. Stones commonly recur within 5 years in patients treated nonsurgically.

Oral dissolution therapy. Drugs made from bile acid are used to dissolve gallstones. The drugs ursodiol (Actigall) and chenodiol (Chenix) work best for small cholesterol stones. Months or years of treatment may be necessary before all the stones dissolve. Both drugs may cause mild diarrhea, and chenodiol may temporarily raise levels of blood cholesterol and the liver enzyme transaminase.

Contact dissolution therapy. This experimental procedure involves injecting a drug directly into the gallbladder to dissolve cholesterol stones. The drug—methyl tert-butyl ether—can dissolve some stones in 1 to 3 days, but it causes irritation and some complications have been reported. The procedure is being tested in symptomatic patients with small stones.

Do people need their gallbladder?

Fortunately, the gallbladder is an organ people can live without. Your liver produces enough bile to digest a normal diet. Once the gallbladder is removed, bile flows out of the liver through the hepatic ducts into the common bile duct and directly into the small intestine, instead of being stored in the gallbladder. Because now the bile flows into the small intestine more often, softer and more frequent stools can occur in about 1 percent of people. These changes are usually temporary, but talk with your health care provider if they persist.

Points to Remember

Gallstones form when bile hardens in the gallbladder.

Gallstones are more common among older adults; women;  people with diabetes; those with a family history of gallstones; people who are overweight, obese, or undergo rapid weight loss; and those taking cholesterol-lowering drugs.

Gallbladder attacks often occur after eating a meal, especially one high in fat.

Symptoms can mimic those of other problems, including a heart attack, so an accurate diagnosis is important.

Gallstones can cause serious problems if they become trapped in the bile ducts.

Laparoscopic surgery to remove the gallbladder is the most common treatment.

Proteins

Proteins have many structures, resulting in a wide range of functions.

The importance of proteins is implied by their name, which comes from the Greek word proteios, meaning “first place.” Proteins account for more than 50% of the dry mass of most cells, and they are instrumental in almost everything organisms do. Some proteins speed up chemical reactions, while others play a role in structural support, storage, transport, cellular communications, movement, and defense against foreign substances.

The most important type of protein may be enzymes. Enzymatic proteins regulate metabolism by acting as catalysts , chemical agents that selectively speed up chemical reactions in the cell without being consumed by the reaction.

Proteins have many structures, resulting in a wide range of functions.

Table 5.1 An Overview of Protein Functions

The most important type of protein may be enzymes. Enzymatic proteins regulate metabolism by acting as catalysts , chemical agents that selectively speed up chemical reactions in the cell without being consumed by the reaction.

Figure 5.16 The catalytic cycle of an enzyme. The enzyme sucrase accelerates hydrolysis of sucrose into glucose and fructose. Acting as a catalyst, the sucrase protein is not consumed during the cycle, but is available for further catalysis.

Because an enzyme can perform its function over and over again, these molecules can be thought of as workhorses that keep cells running by carrying out the processes of life.

A human has tens of thousands of different proteins, each with a specific structure and function; proteins, in fact, are the most structurally sophisticated molecules known. Consistent with their diverse functions, they vary extensively in structure, each type of protein having a unique three–dimensional shape, or conformation.

Polypeptides

Diverse as proteins are, they are all polymers constructed from the same set of 20 amino acids. Polymers of amino acids are called polypeptides. A protein consists of one or more polypeptides folded and coiled into specific conformations.

Amino Acid Monomers

Amino acids are organic molecules possessing both carboxyl and amino groups. The illustration at the right shows the general formula for an amino acid. At the center of the amino acid is an asymmetric carbon atom called the alpha (α ) carbon. Its four different partners are an amino group, a carboxyl group, a hydrogen atom, and a variable group symbolized by R. The R group, also called the side chain, differs with each amino acid.

The figure above shows the 20 amino acids that cells use to build their thousands of proteins. Here the amino and carboxyl groups are all depicted in ionized form, the way they usually exist at the pH in a cell. The R group may be as simple as a hydrogen atom, as in the amino acid glycine (the one amino acid lacking an asymmetric carbon, since two of its α carbon’s partners are hydrogen atoms), or it may be a carbon skeleton with various functional groups attached, as in glutamine. (Organisms do have other amino acids, some of which are occasionally found in proteins.

The physical and chemical properties of the side chain determine the unique characteristics of a particular amino acid. In Figure 5.17, the amino acids are grouped according to the properties of their side chains. One group consists of amino acids with nonpolar side chains, which are hydrophobic. Another group consists of amino acids with polar side chains, which are hydrophilic. Acidic amino acids are those with side chains that are generally negative in charge owing to the presence of a carboxyl group, which is usually dissociated (ionized) at cellular pH. Basic amino acids have amino groups in their side chains that are generally positive in charge. (Notice that all amino acids have carboxyl groups and amino groups; the terms acidic and basic in this context refer only to groups on the side chains.) Because they are charged, acidic and basic side chains are also hydrophilic.

Amino Acid Polymers

Now that we have examined amino acids, let’s see how they are linked to form polymers .

When two amino acids are positioned so that the carboxyl group of one is adjacent to the amino group of the other, an enzyme can cause them to join by catalyzing a dehydration reaction, with the removal of a water molecule. The resulting covalent bond is called a peptide bond . Repeated over and over, this process yields a polypeptide, a polymer of many amino acids linked by peptide bonds. At one end of the polypeptide chain is a free amino group; at the opposite end is a free carboxyl group. Thus, the chain has an amino end (N–terminus) and a carboxyl end (C–terminus). The repeating sequence of atoms highlighted in purple in Figure 5.18b is called the polypeptide backbone. Attached to this backbone are different kinds of appendages, the side chains of the amino acids. Polypeptides range in length from a few monomers to a thousand or more. Each specific polypeptide has a unique linear sequence of amino acids. The immense variety of polypeptides in nature illustrates an important concept introduced earlier—that cells can make many different polymers by linking a limited set of monomers into diverse sequences.

Determining the Amino Acid Sequence of a Polypeptide

The pioneer in determining the amino acid sequence of proteins was Frederick Sanger, who, with his colleagues at Cambridge University in England, worked on the hormone insulin in the late 1940s and early 1950s. His approach was to use protein–digesting enzymes and other catalysts that break polypeptides at specific places rather than completely hydrolyzing the chains to amino acids. Treatment with one of these agents cleaves a polypeptide into fragments (each consisting of multiple amino acid subunits) that can be separated by a technique called chromatography. Hydrolysis with a different agent breaks the polypeptide at different sites, yielding a second group of fragments. Sanger used chemical methods to determine the sequence of amino acids in these small fragments. Then he searched for overlapping regions among the pieces obtained by hydrolyzing with the different agents. Consider, for instance, two fragments with the following sequences:

We can deduce from the overlapping regions that the intact polypeptide contains in its primary structure the following segment: Cys–Ser–Leu–Tyr–Gln–Leu–Glu–Asn

Just as we could reconstruct this sentence from a collection of fragments with overlapping sequences of letters, Sanger and his co–workers were able, after years of effort, to reconstruct the complete primary structure of insulin. Since then, most of the steps involved in sequencing a polypeptide have been automated.

Protein Conformation and Function

Once we have learned the amino acid sequence of a polypeptide, what can it tell us about protein conformation and function? The term polypeptide is not quite synonymous with the term protein . Even for a protein consisting of a single polypeptide, the relationship is somewhat analogous to that between a long strand of yarn and a sweater of particular size and shape that one can knit from the yarn. A functional protein is not just a polypeptide chain, but one or more polypeptides precisely twisted, folded, and coiled into a molecule of unique shape

Four Levels of Protein Structure

In the complex architecture of a protein, we can recognize three superimposed levels of structure, known as primary, secondary, and tertiary structure. A fourth level, quaternary structure, arises when a protein consists of two or more polypeptide chains

Carbohydrates

Carbohydrates include both sugars and the polymers of sugars. The simplest carbohydrates are the monosaccharides, or single sugars, also known as simple sugars. Disaccharides are double sugars, consisting of two monosaccharides joined by a condensation reaction. The carbohydrates that are macromolecules are polysaccharides, polymers composed of many sugar building blocks.

Sugars

Monosaccharides (from the Greek monos, single, and sacchar, sugar) generally have molecular formulas that are some multiple of the unit CH2O

The structure and classification of some monosaccharides. Sugars may be aldoses (aldehyde sugars, top row) or ketoses (ketone sugars, bottom row), depending on the location of the carbonyl group (dark orange). Sugars are also classified according to the length of their carbon skeletons. A third point of variation is the spatial arrangement around asymmetric carbons (compare, for example, the purple portions of glucose and galactose).

Glucose, the most common monosaccharide, is of central importance in the chemistry of life. In the structure of glucose, we can see the trademarks of a sugar: The molecule has a carbonyl group and multiple hydroxyl groups (–OH). Depending on the location of the carbonyl group, a sugar is either an aldose (aldehyde sugar) or a ketose (ketone sugar). Glucose, for example, is an aldose; fructose, a structural isomer of glucose, is a ketose. (Most names for sugars end in –ose.) Another criterion for classifying sugars is the size of the carbon skeleton, which ranges from three to seven carbons long. Glucose, fructose, and other sugars that have six carbons are called hexoses. Trioses (three–carbon sugars) and pentoses (five–carbon sugars) are also common.

Still another source of diversity for simple sugars is in the spatial arrangement of their parts around asymmetric carbons. Glucose and galactose, for example, differ only in the placement of parts around one asymmetric carbon. What seems like a small difference is significant enough to give the two sugars distinctive shapes and behaviours.

Although it is convenient to draw glucose with a linear carbon skeleton, this representation is not completely accurate. In aqueous solutions, glucose molecules, as well as most other sugars, form rings.

Monosaccharides, particularly glucose, are major nutrients for cells. In the process known as cellular respiration, cells extract the energy stored in glucose molecules. Not only are simple sugar molecules a major fuel for cellular work, but their carbon skeletons serve as raw material for the synthesis of other types of small organic molecules, such as amino acids and fatty acids. Sugar molecules that are not immediately used in these ways are generally incorporated as monomers into disaccharides or polysaccharides.

A disaccharide consists of two monosaccharides joined by a glycosidic linkage , a covalent bond formed between two monosaccharides by a dehydration reaction. For example, maltose is a disaccharide formed by the linking of two molecules of glucose.

 

Also known as malt sugar, maltose is an ingredient used in brewing beer. The most prevalent disaccharide is sucrose, which is table sugar. Its two monomers are glucose and fructose (Figure 5.5b). Plants generally transport carbohydrates from leaves to roots and other nonphotosynthetic organs in the form of sucrose. Lactose, the sugar present in milk, is another disaccharide, in this case a glucose molecule joined to a galactose molecule.

Polysaccharides

Polysaccharides are macromolecules, polymers with a few hundred to a few thousand monosaccharides joined by glycosidic linkages. Some polysaccharides serve as storage material, hydrolyzed as needed to provide sugar for cells. Other polysaccharides serve as building material for structures that protect the cell or the whole organism. The architecture and function of a polysaccharide are determined by its sugar monomers and by the positions of its glycosidic linkages.

Storage Polysaccharides

Starch , a storage polysaccharide of plants, is a polymer consisting entirely of glucose monomers. Most of these monomers are joined by 1–4 linkages (number 1 carbon to number 4 carbon), like the glucose units in maltose (see Figure 5.5a). The angle of these bonds makes the polymer helical. The simplest form of starch, amylose, is unbranched. Amylopectin, a more complex form of starch, is a branched polymer with 1–6 linkages at the branch points.

Plants store starch as granules within cellular structures called plastids, which include chloroplasts.

Synthesizing starch enables the plant to stockpile surplus glucose. Because glucose is a major cellular fuel, starch represents stored energy. The sugar can later be withdrawn from this carbohydrate “bank” by hydrolysis, which breaks the bonds between the glucose monomers. Most animals, including humans, also have enzymes that can hydrolyze plant starch, making glucose available as a nutrient for cells. Potato tubers and grains—the fruits of wheat, corn, rice, and other grasses—are the major sources of starch in the human diet.

Animals store a polysaccharide called glycogen , a polymer of glucose that is like amylopectin but more extensively branched (Figure 5.6b). Humans and other vertebrates store glycogen mainly in liver and muscle cells. Hydrolysis of glycogen in these cells releases glucose when the demand for sugar increases. This stored fuel cannot sustain an animal for long, however. In humans, for example, glycogen stores are depleted in about a day unless they are replenished by consumption of food.

Structural Polysaccharides

Organisms build strong materials from structural polysaccharides. For example, the polysaccharide called cellulose is a major component of the tough walls that enclose plant cells. On a global scale, plants produce almost 1011 (100 billion) tons of cellulose per year; it is the most abundant organic compound on Earth. Like starch, cellulose is a polymer of glucose, but the glycosidic linkages in these two polymers differ. The difference is based on the fact that there are actually two slightly different ring structures for glucose .

When glucose forms a ring, the hydroxyl group attached to the number 1 carbon is positioned either below or above the plane of the ring. These two ring forms for glucose are called alpha (α) and beta (β), respectively. In starch, all the glucose monomers are in the α configuration (Figure 5.7b), the arrangement we saw in Figures 5.4 and 5.5. In contrast, the glucose monomers of cellulose are all in the β configuration, making every other glucose monomer upside down with respect to its neighbours (Figure 5.7c).

The differing glycosidic links in starch and cellulose give the two molecules distinct three–dimensional shapes. Whereas a starch molecule is mostly helical, a cellulose molecule is straight (and never branched), and its hydroxyl groups are free to hydrogen–bond with the hydroxyls of other cellulose molecules lying parallel to it. In plant cell walls, parallel cellulose molecules held together in this way are grouped into units called microfibrils

These cable–like microfibrils are a strong building material for plants as well as for humans, who use wood, which is rich in cellulose, for lumber.

Enzymes that digest starch by hydrolyzing its α linkages are unable to hydrolyze the β linkages of cellulose because of the distinctly different shapes of these two molecules. In fact, few organisms possess enzymes that can digest cellulose. Humans do not; the cellulose in our food passes through the digestive tract and is eliminated with the feces. Along the way, the cellulose abrades the wall of the digestive tract and stimulates the lining to secrete mucus, which aids in the smooth passage of food through the tract. Thus, although cellulose is not a nutrient for humans, it is an important part of a healthful diet. Most fresh fruits, vegetables, and whole grains are rich in cellulose. On food packages, “insoluble fiber” refers mainly to cellulose.

Some microbes can digest cellulose, breaking it down to glucose monomers. A cow harbors cellulose–digesting bacteria in the rumen, the first compartment in its stomach.

The bacteria hydrolyze the cellulose of hay and grass and convert the glucose to other nutrients that nourish the cow. Similarly, a termite, which is unable to digest cellulose by itself, has microbes living in its gut that can make a meal of wood. Some fungi can also digest cellulose, thereby helping recycle chemical elements within Earth’s ecosystems.

Another important structural polysaccharide is chitin , the carbohydrate used by arthropods (insects, spiders, crustaceans, and related animals) to build their exoskeletons.

An exoskeleton is a hard case that surrounds the soft parts of an animal. Pure chitin is leathery, but it becomes hardened when encrusted with calcium carbonate, a salt. Chitin is also found in many fungi, which use this polysaccharide rather than cellulose as the building material for their cell walls. Chitin is similar to cellulose, except that the glucose monomer of chitin has a nitrogen–containing appendage.

The True Teacher Accepts All Students

A teacher says: "I can accept my good students, those who behave and do good work, but I can't accept those who do not work, who have the wrong attitude and who cause me trouble." They forget that it's the acceptance of all that gives power to the teacher. In fact, it is in relation to students who are difficult that the teacher's true qualities are demonstrated. We all find it easy to accept those who lend themselves to our designs. It is in their relationship to those who cause them trouble, who are dirty and poorly dressed, and who fail to achieve that teachers prove their beliefs.

It is the essence of the point of view here presented that only a complete gift of oneself makes the teacher an artist. Teaching is a jealous profession; it is not a sideline. This is not only because of the problem of time, nor because of the impact of lesser efforts on pupils: it is because of the effect on the teacher himself. It is only as we give fully of ourselves that we can become our best selves. Thus halfway measures and attitudes of whatever kind reduce our effectiveness.

When we ask the teacher to give himself fully to his students, to his colleagues, to his community, and to humanity, we are thus only asking him to be maximally effective. Moreover, it is only as he gives himself that he can experience completely the joys and satisfactions of being a teacher. In this situation he is in the same position as any artist. Frustrated artists are often those who for one reason or another are unable or unwilling to make a complete gift of themselves to their art. Similarly, the unhappiest teachers are those who bemoan the weaknesses of their pupils and the conditions under which they work and who fail to sense that it is their own half-hearted efforts that defraud them.

One measure of the teacher's willingness to give of himself is his accessibility to his students, his willingness to spend time with them. One difficulty here is the narrow conception that often prevails about what it means to teach. To teach means more than to lecture or explain before a group of students. The best teachers influence their students more in their personal, individual contacts with them than in strict classroom situations. If teaching and learning are complementary processes, if the teacher is to teach by learning and if his teaching is to be directed toward an individual, he must know that individual. And how is he to know that individual if he spends little or no time with him alone?

Another illusion defeats us. It is that there is some magic in lecturing and in the hearing of recitations. We want as much time for this as possible. We begrudge taking time to work with individual pupils. Yet we know very little about the actual effectiveness of what we do. Is it not at least possible that our classroom work would be greatly increased in effectiveness if only we spent more time with our pupils as individuals? We seem to be obsessed with teaching. We know that no one can educate another person, that all of us must educate ourselves. The teacher's role is that of a helper in this process. The question is: How can we best help?

Purine, Uric Acid and Gout

Ingestion of foods high in purines can raise uric acid levels in the blood, which leads to painful gout attacks in some people. The excess can be due to either an over-production of uric acid by the body, or the under-elimination of uric acid by the kidneys. It is important to seek medical advice from your doctor.

Purines are important components of DNA and RNA. They are the genetic material of all living cells. They are part of the human tissue and are found in many foods.

Foods with a high purine content that should be avoided are: organ meat (liver, kidneys and pancreas), anchovies, sardines, herring (especially the head and entrails), fish roe, legumes (dried beans, dhal), yeast, meat extract, gravies, beer and other alcoholic beverages.

It is important to remember that purines are found in all foods, especially foods with high protein content. But we need protein to maintain good health. Lean meat (chicken, turkey and pork), tofu and beans (such as black, kidney and lima beans) are good sources of protein which should be consumed in moderation.

Although asparagus, cauliflower, mushrooms, peas, spinach, Chinese cabbage, whole grain breads and cereals contain purine, they do not increase the risk of gout if consumed in moderation.

A diet that is beneficial to people suffering from gout should be low in sugar and fat, but rich in fibre (or complex carbohydrates). In other words, have a balanced diet that includes fish, lean meat, whole grains, high-fibre rice, cereal, and fruits and vegetables, as well as a handful of nuts and seeds. Popular plant foods are: pineapple, banana, papaya, oranges, pears, apple, (blue, purple or red) berries, nangka, dragon fruit, watermelon and other melons, celery, kalian, sawi, cabbage, parsley, hot chilli, bell pepper, kunyit, loofah, lady’s finger, sweet potato leaves, ferns, and other green, leafy vegetables.

Avoiding purine-rich foods is only one aspect of treatment. Drink six to eight glasses of water per day, exercise, maintain a healthy body weight and follow medical advice.

The Nephron

Nephron is the basic structural and functional unit of the kidney. Its chief function is to regulate the concentration of water and soluble substances like sodium salts by filtering the blood, reabsorbing what is needed and excreting the rest as urine. A nephron eliminates wastes from the body, regulates blood volume and blood pressure, controls levels of electrolytes and metabolites, and regulates blood pH. Its functions are vital to life and are regulated by the endocrine system by hormones such as antidiuretic hormone, aldosterone, and parathyroid hormone. In humans, a normal kidney contains 800,000 to one million nephrons.

Ultrafiltration

In biological terms, ultrafiltration occurs at the barrier between the blood and the filtrate in the renal corpuscle or Bowman's capsule in the kidneys. The Bowman's capsule contains a dense capillary network called the glomerulus. Blood flows into these capillaries through a wide afferent arteriole and leaves through a narrower efferent arteriole. The blood pressure inside these capillaries is high because:

- The renal artery contains blood at very high pressure which enters the glomerulus via the short afferent arteriole.

- The efferent arteriole has a smaller diameter than the afferent arteriole.

The high pressure forces small molecules such as water, glucose, amino acids, sodium chloride and urea through the filter, from the blood in the glomerular capsule across the basement membrane of the Bowman's capsule and into the nephron. This type of high pressure filtration is ultrafiltration. The fluid formed in this way is called glomerular filtrate.

Once inside the lumen of the nephron, small molecules, such as ions, glucose and amino acids, get reabsorbed from the filtrate:

Specialized proteins called transporters are located on the membranes of the various cells of the nephron.

These transporters grab the small molecules from the filtrate as it flows by them.

Each transporter grabs only one or two types of molecules. For example, glucose is reabsorbed by a transporter that also grabs sodium.

Transporters are concentrated in different parts of the nephron. For example, most of the Na transporters are located in the proximal tubule, while fewer ones are spread out through other segments.

Some transporters require energy, usually in the form of adenosine triphosphate (active transport), while others don't (passive transport).

Water gets reabsorbed passively by osmosis in response to the buildup of reabsorbed Na in spaces between the cells that form the walls of the nephron.

Other molecules get reabsorbed passively when they are caught up in the flow of water (solvent drag).

Reabsorption

Reabsorption of most substances is related to the reabsorption of Na, either directly, via sharing a transporter, or indirectly via solvent drag, which is set up by the reabsorption of Na.

The reabsorption process is similar to the "fish pond" game that you see in some amusement parks or state fairs. In these games, there is a stream that contains different colored plastic fish with magnets. The children playing the game each have a fishing pole with an attached magnet to catch the fish as they move by. Different coloured fish have different prize values associated with them, so some children will be selective and try to grab the colored fish with the highest prize value. Now suppose our nephron is the stream, the filtered molecules are the various colored fish, and our children are the transporters. Furthermore, each child is fishing for a specific colored fish. Most children start at the beginning of the stream and some spread out further downstream. By the end of the stream, most of the fish have been caught. This is what happens as the filtrate travels through the nephron.

Two major factors affect the reabsorption process:

-Concentration of small molecules in the filtrate - the higher the concentration, the more molecules can be reabsorbed. Like our children in the fish pond game, if you increase the number of fish in the stream, the children will have an easier time catching them.

In the kidney, this is true only to a certain extent because:

There is only a fixed number of transporters for a given molecule present in the nephron.

There is a limit to how many molecules the transporters can grab in a given period of time.

- Rate of flow of the filtrate - flow rate affects the time available for the transporters to reabsorb molecules. As with our fish pond, if the stream moves by slowly, the children will have more time to catch fish than if the stream were moving faster.

To give you an idea of the quantity of reabsorption across the nephron, let's look at the sodium ion (Na) as an example:

Proximal convulated tubule - reabsorbs 65 percent of filtered Na. In addition, the proximal tubule passively reabsorbs about 2/3 of water and most other substances.

Loop of Henle - reabsorbs 25 percent of filtered Na.

Distal convulated tubule - reabsorbs 8 percent of filtered Na.

Collecting duct - reabsorbs the remaining 2 percent only if the hormone aldosterone is present.

Secretion

Secretion is a process in which waste and excess substances that were not initially filtered are secreted in renal tubule. Secretion takes place at the renal tubule and collecting ducts but is active at distal convulated tubule. Secretion occurs by passive diffusions and active transport.

Secreted substances include hydrogen ions, potassium ions, ammonia, urea, creatinine, toxins and certain drugs.