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.

Endocrine System

Basic patterns of simple hormonal control pathways. In each pathway, a receptor/sensor (blue) detects a change in some internal or external variable— the stimulus—and informs the control center (gold). The control center sends out an efferent signal, either a hormone (red circles) or neurohormone (red squares). An endocrine cell carries out both the receptor and control center functions.

Graves′ disease, the most common form of hyperthyroidism in humans. Tissue behind the eyes can become swollen and fibrous, causing the characteristic symptom of bulging eyes.

Good Teaching??

All students have had hundreds of teachers in their lifetimes. A very few of these teachers they remember as being exceptionally good. What are the qualities that combine to create an excellent, memorable teacher? Why do some teachers inspire students to work three times harder than they normally would, while others inspire students to skip class? Why do students learn more from some teachers than others?

If you are trying to become a better teacher, these are important questions. These four essential qualities are important: knowledge, communication skills, interest, and respect for students.

An Experiment

Here's an experiment I have done in a number of my classes. The results may surprise you. Go into one of the classes you are teaching and have your students take out a sheet of paper. Ask them to list for you the qualities they feel are important in a good teacher. Ask them to identify the qualities they admire in the best teachers they have had. Then give the students enough time to think about it and write something down. Five minutes is good, but ten might be better. Let them answer the questions anonymously if they desire.

What you will get if you combine all of the responses is a fascinating collage of ideas. I have found that most of the responses fall into two specific categories: 1) a set of "core qualities" that students recognise in good teachers, and 2) a set of specific skills that are developed by good teachers.

"Core qualities" are the essential characteristics needed to be a good teacher. I would like to concentrate on the core qualities.

Knowledge

In every survey I have given, students consistently and clearly target as the number one quality of a good teacher exactly what you would expect: knowledge of the subject. You must be an expert in your field if you are going to be a good teacher. This is a prerequisite.

Communication

The second core quality that good teachers possess is the ability to communicate their knowledge and expertise to their students. You may be the greatest expert ever in your field, but what would happen if you taught in Latin? How much would your students learn?

It is a common misconception at the school level that knowledge of a subject is all that's required to be a good teacher; that the students should be willing and able to extract the meat from what you say regardless of how it is delivered (even if it is delivered in Latin). This might be true at the upper level, but elsewhere it is definitely untrue. It is especially untrue at the school level. The teacher's job is to take advanced knowledge and make it accessible to the students. A good teacher allows students to understand the material, and to understand what it means (because it is one thing to understand how nuclear bombs work, but quite another to understand what nuclear bombs mean).

A good teacher can take a subject and help make it crystal clear to the students. A bad teacher can take that same material and make it impenetrable. Or a bad teacher can devote so little time and effort to preparation that the material presented is intrinsically confusing and disorganised. A good teacher is willing to expend the effort needed to find innovative and creative ways to make complicated ideas understandable to their students, and to fit new ideas into the context available to the student. A good teacher can explain complicated material in a way that students can understand and use.

There is a saying, "Give me a fish and I eat for a day, teach me to fish and I eat for a lifetime." This is the philosophy of a good teacher. Give your students an answer and they can solve one problem, but show students the techniques needed to find the answer for themselves and they can become self-sufficient in the field. Students need to be shown how to apply the new techniques you teach to problem solving.

Interest

A good teacher starts with a firm knowledge of the subject, and builds on that with a clarity and understanding designed to help students master the material. The best teachers then go one step further. Because good teachers are interested in the material being taught, they make the class interesting and relevant to the students. Knowledge is worthless unless it is delivered to the students in a form they can understand. But the effort expended making the material understandable is wasted if the students are asleep when it is delivered, or if the students can see no point in learning the material.

Good teachers recognise this, and work hard to make their material relevant. They show students how the material will apply to their lives and their careers. Bad teachers make material "relevant" by threatening students with failure on a test. Good teachers go far beyond this: they make students want to learn the material by making it interesting.

This is one of the things that makes research so important and vital: research makes the ideas discussed in class exciting and important to the teacher, as well as to the students. If the teacher isn't interested in what's being taught, then why should the students be?

Respect

Good teachers always possess these three core qualities: knowledge, the ability to convey to students an understanding of that knowledge, and the ability to make the material interesting and relevant to students. Complementing these three is a fourth quality: good teachers have a deep-seated concern and respect for the students in the classroom. Why else would a teacher put in the time and effort needed to create a high quality class?

The creation of a good class requires an immense amount of work. You don't simply come up with clear explanations and examples and experiments for class off the top of your head. You don't create fair, consistent, high quality tests and homework assignments (read "learning experiences") five minutes before you hand them out. You don't figure out ways to integrate new materials and research into a class in an understandable way on the drive in one morning. You work at this sort of quality all the time. You spend time with your students so you can learn about holes in their understanding. You read and write and create to build an exciting and interesting class every day. The only thing that would drive you to do that is a concern and respect for the adults in your classroom.

Conclusion

When you strive and work to become a good teacher and to create a good class, the four core qualities are essential: knowledge, the skills to convey that knowledge, the ability to make the material you are teaching interesting and relevant, and a deep-seated respect for the student. Without these four qualities, good teaching will not exist.

Tata titi tutu..........

Sensor

A bat using sonar to locate its prey.

An insect ear. The tympanic membrane, visible in this SEM of a cricket′s front leg, vibrates in response to sound waves. The vibrations stimulate mechanoreceptors attached to the inside of the tympanic membrane.

The lateral line system in a fish. Water flowing through the system bends hair cells. The hair cells transduce the energy into receptor potentials, triggering action potentials that are conveyed to the brain. The lateral line system enables a fish to monitor water currents, pressure waves produced by moving objects, and low–frequency sounds conducted through the water.