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5.15: Introduction to Carbohydrates - Biology

5.15: Introduction to Carbohydrates - Biology


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Summarize the roles carbohydrates play in biological systems

Are carbohydrates good for you? People who wish to lose weight are often told that carbohydrates are bad for them and should be avoided. However, carbohydrates have been an important part of the human diet for thousands of years; artifacts from ancient civilizations show the presence of wheat, rice, and corn in our ancestors’ storage areas.

Carbohydrates should be supplemented with proteins, vitamins, and fats to be parts of a well-balanced diet. Calorie-wise, a gram of carbohydrate provides 4.3 Kcal. For comparison, fats provide 9 Kcal/g, a less desirable ratio. Carbohydrates contain soluble and insoluble elements; the insoluble part is known as fiber, which is mostly cellulose. Fiber has many uses; it promotes regular bowel movement by adding bulk, and it regulates the rate of consumption of blood glucose. Fiber also helps to remove excess cholesterol from the body. In addition, a meal containing whole grains and vegetables gives a feeling of fullness. As an immediate source of energy, glucose is broken down during the process of cellular respiration, which produces ATP, the energy currency of the cell. Without the consumption of carbohydrates, the availability of “instant energy” would be reduced.

Eliminating carbohydrates from the diet is not the best way to lose weight. A low-calorie diet that is rich in whole grains, fruits, vegetables, and lean meat, together with plenty of exercise and plenty of water, is the more sensible way to lose weight.

What You’ll Learn to Do

  • Distinguish between monosaccharides, disaccharides, and polysaccharides
  • Identify several major functions of carbohydrates

Learning Activities

The learning activities for this section include the following:

  • Structure and Function of Carbohydrates
  • Self Check: Carbohydrates

Introduction

The formulas of many carbohydrates can be written as carbon hydrates, Cn(H2O)n, hence their name. The carbohydrates are a major source of metabolic energy, both for plants and for animals that depend on plants for food. Aside from the sugars and starches that meet this vital nutritional role, carbohydrates also serve as a structural material (cellulose), a component of the energy transport compound ATP, recognition sites on cell surfaces, and one of three essential components of DNA and RNA.

Carbohydrates are called saccharides or, if they are relatively small, sugars. Several classifications of carbohydrates have proven useful, and are outlined in the following table.


Biology Essay on Carbohydrates | Organic Molecules | Biology

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Essay # 1. Introduction to Carbohydrates:

Chemically, carbohydrates are organic molecules in which carbon, hydrogen, and oxy­gen bond together in the ratio Cx (H2O)y, where x and y are whole numbers that differ de­pending on the specific carbohydrate. They are reduced compounds having large quantities of hydroxyl groups. The presence of the hydroxyl groups allows carbohydrates to interact with the aqueous environment and to participate in hydrogen bond formation, both within and between chains.

The simplest carbohydrates also contain either an aldehyde moiety (termed polyhydroxyaldehydes) or a ketone moiety (polyhydroxyketones). Derivatives of the carbohydrates can contain nitrogen/s, phosphates and sulfur compounds. Carbohydrates can also combine with lipid to form glycolipids or with protein to form glycoproteins.

The aldehyde and ketone moieties of the carbohydrates with five and six carbons will spontaneously react with alcohol groups present in neighboring carbons to produce in­tramolecular hemiacetals or hemiketals, respectively. This results in the formation of five- or six-membered rings.

As the five-membered ring structure resembles the organic molecule furan, the derivatives with this structure are termed as furanoses. Those with six-membered rings, resemble the organic molecule pyran are termed pyranoses and are depicted by either Fischer or Haworth style diagrams. The numbering of the carbons in carbohydrates proceeds from the carbonyl carbon, for aldoses, or the carbon nearest the carbonyl, for ketoses.

The rings can open and re-close, allowing rotation to occur about the carbon bearing the reactive carbonyl, yielding two distinct configurations (α and β) of the hemiacetals and hemiketals. The carbon about which this rotation occurs is the anomeric carbon and these two forms are termed anomers. Carbohydrates can change spontaneously between α and β configurations- a process known as mutarotation. In the Fischer projection, α configuration places the hydroxyl attached to the anomeric carbon to the right, towards the ring, while in the Haworth projection, α configuration places the hydroxyl downward.

Carbohydrates can exist in either of two conformations, as determined by the orientation of the hydroxyl group about the asymmetric carbon farthest from the carbonyl. With a few exceptions, those carbohydrates that are of physiological significance exist in the D- conformation. Carbohydrates are the main energy source for the human body. Animals (including humans) break down carbohydrates during the process of metabolism to release energy.

For example, the chemical metabolism of the sugar (glucose) is shown below:

Carbohydrates are manufactured by plants during the process of photosynthesis. Plants har­vest energy from sunlight and stores in carbohydrate moieties.

All carbohydrates can be classified as monosaccharides, oligosaccharides or polysaccha­rides. Two to ten monosaccharide units, linked by glycosidic bonds, make up an oligosac­charide. Polysaccharides are much larger and contain hundreds of monosaccharide units.

Essay # 2. Classification of Carbohydrates:

Monosaccharides are simple sugars, having 3 to 7 carbon atoms. They can be bonded together to form polysaccharides. Cells also use simple sugars to store energy and construct other kinds of organic molecules. The names of most sugars end with the letters ‘ose’. Glucose and other kinds of sugars (fructose, and galactose) may be linear molecules (C6H12O6) but in aqueous solution they take ring form.

There are two isomers of the ring form of glucose. They differ in the location of the OH group on the number 1 carbon atom. The number 1 carbon atom of the linear form of glu­cose is attached to the oxygen on the number 5 carbon atom.

Disaccharides are composed of 2 monosaccharides joined together by a condensation reaction.

There are three common disaccharides:

i. Maltose (or malt sugar) consists of glucose monomers. Amylase enzyme digests starch molecules to produce maltose.

ii. Sucrose (or cane sugar) composed of glucose and fructose. Plants synthesize sucrose to transport to non-photosynthetic parts of the plant, because it is less reactive than glucose.

iii. Lactose (or milk sugar) is made up of galactose and glucose. It is found only in mam­malian milk.

(c) Polysaccharides:

Monosaccharides may be bonded together to form long chain compounds called polysaccha­rides. The monomeric building blocks used to generate polysaccharides can be varied in all cases, however, the predominant monosaccharide found in polysaccharides is D-glucose. Polysaccharides that are composed of a single monosaccharide building block are termed as homopolysaccharides, while polysaccharides composed of more than one type of monosac­charide, they are termed as heteropolysaccharides.

For examples, starch and glycogen are composed of glucose monomers bonded together, producing long chains. They serve the function as stored food, starch in plants and glycogen in animal, in the liver and muscles. Glycogen is poly (1-4) glucose with 9% (1-6) branches (Fig. 3.5).

Starch is a long (100s) polymer of glucose molecules, where all the sugars are oriented in the same direction. Unbranched starch is called amylose, while branched starch is known as amylopectin. Amylose is simply poly-(1-4) glucose units in a straight chain. In fact the chain is floppy, and it tends to coil up into a helix. Amylopectin is poly (1-4) glucose with about 4% (1-6) branches. This gives it a more open molecular structure than amylose.

As it has more ends, it can be broken more quickly than amylose by amylase enzymes. Amy­lopectin is a form of starch that is very similar to glycogen except for a much lower degree of branching (about every 20-30 residues). Another example of polysaccharide is cellulose. Cellulose is a long (100’s) polymer of glucose molecules. However, the orientation of the sugars is little different. In Cellulose, every other sugar molecule is “upside-down”. Glyco­gen is different from both, starch and cellulose in that the glucose chain is branched or “forked” (Fig. 3.6).


○ CO 2 + H 2 O + sunlight → (CH 2 O)n + O 2

Energy Stored in Glucose Is Used to Make ATP ● When a cell needs energy, it breaks down glucose ● Captured energy is used to make ATP: ○ (CH2O)n +O2 +ADP+Pi CO2 +H2O+ATP• The energy in ATP is used to drive endergonic reactions and perform cell work

Starch: A Storage Polysaccharide in Plants 1. Plants store sugar as starch ○ Composed of α-glucose monomers ○ Forms a helix 2. Animals store sugar as glycogen ○ Stored in liver and muscle cells ○ Can be broken down into glucose monomers for energy ○ Highly branched -glucose polymer, nearly identical to starch Starch: some branches, energy storage in plants Glycogen: highly branched, highly soluble, short term energy storage in animals

Cellulose: A Structural Polysaccharide in Plants 3. Cellulose is a structural polymer ○ Forms a protective layer around plant cells called the cell wall ○ Every other glucose is flipped, so it generates a linear molecule rather than a


5.15: Introduction to Carbohydrates - Biology

Are carbohydrates good for you? People who wish to lose weight are often told that carbohydrates are bad for them and should be avoided. Some diets completely forbid carbohydrate consumption, claiming that a low-carbohydrate diet helps people to lose weight faster. However, carbohydrates have been an important part of the human diet for thousands of years artifacts from ancient civilizations show the presence of wheat, rice, and corn in our ancestors’ storage areas.

Carbohydrates should be supplemented with proteins, vitamins, and fats to be parts of a well-balanced diet. Calorie-wise, a gram of carbohydrate provides 4.3 Kcal [1] . For comparison, fats provide 9 Kcal/g, a less desirable ratio. Carbohydrates contain soluble and insoluble elements the insoluble part is known as fiber, which is mostly cellulose. Fiber has many uses it promotes regular bowel movement by adding bulk, and it regulates the rate of consumption of blood glucose. Fiber also helps to remove excess cholesterol from the body. In addition, a meal containing whole grains and vegetables gives a feeling of fullness. As an immediate source of energy, glucose is broken down during the process of cellular respiration, which produces ATP, the energy currency of the cell. Without the consumption of carbohydrates, the availability of “instant energy” would be reduced.

Eliminating carbohydrates from the diet is not the best way to lose weight. A low-calorie diet that is rich in whole grains, fruits, vegetables, and lean meat, together with plenty of exercise and plenty of water, is the more sensible way to lose weight.


4.4.2 Carbohydrate Benefits

Are carbohydrates good for you? People who wish to lose weight are often told that carbohydrates are bad for them and should be avoided. Some diets completely forbid carbohydrate consumption, claiming that a low-carbohydrate diet helps people to lose weight faster. However, carbohydrates have been an important part of the human diet for thousands of years artifacts from ancient civilizations show the presence of wheat, rice, and corn in our ancestors’ storage areas.

Carbohydrates should be supplemented with proteins, vitamins, and fats for a well-balanced diet. Carbohydrates contain soluble and insoluble elements. The insoluble part, known as fiber, is mostly cellulose. Fiber has many uses it promotes regular bowel movement by adding bulk, and it regulates the rate of consumption of blood glucose. Fiber also helps to remove excess cholesterol from the body by binding cholesterol in the small intestine, preventing it from entering the bloodstream, and causing it to exit the body via the feces. Fiber-rich diets also reduce the occurrence of colon cancer. In addition, eating a meal containing whole grains and vegetables gives a feeling of fullness. As an immediate source of energy, glucose is broken down during the process of cellular respiration, which produces ATP, the energy currency of the cell. Without the consumption of carbohydrates, the availability of “instant energy” is reduced. A low-calorie diet that is rich in whole grains, fruits, vegetables, and lean meat, together with plenty of exercise and plenty of water, is the most sensible way to lose weight.


Classification of Carbohydrates

1. Monosaccharides

Monosaccharides are colorless, crystalline solids that are freely soluble in water but insoluble in nonpolar solvents. Most have a sweet taste. The backbones of common monosaccharides are unbranched carbon chains in which all the carbon atoms are linked by single bonds. In this open-chain form, one of the carbon atoms is double-bonded to an oxygen atom to form a carbonyl group each of the other carbon atoms has a hydroxyl group. If the carbonyl group is at an end of the carbon chain (that is, in an aldehyde group) the monosaccharide is an aldose if the carbonyl group is at any other position (in a ketone group) the monosaccharide is a ketose. The simplest monosaccharides are the two three-carbon trioses: glyceraldehyde, an aldotriose, and dihydroxyacetone, a ketotriose.

Monosaccharides with four, five, six, and seven carbon atoms in their backbones are called, respectively, tetroses, pentoses, hexoses, and heptoses. There are aldoses and ketoses of each of these chain lengths: aldotetroses and ketotetroses, aldopentoses and ketopentoses,
and so on. The hexoses, which include the aldohexose D-glucose and the ketohexose D-fructose, are the most common monosaccharides in nature—the products of photosynthesis, and key intermediates in the central energy-yielding reaction sequence in most organisms. The aldopentoses D-ribose and 2-deoxy-D-ribose are components of nucleotides and nucleic acids. (Figure Below)

2. Disaccharides

Disaccharides (such as maltose, lactose, and sucrose) consist of two monosaccharides joined covalently by an O-glycosidic bond, which is formed when a hydroxyl group of one sugar molecule, typically cyclic, reacts with the anomeric carbon of the other. This reaction represents the formation of an acetal from a hemiacetal (such as glucopyranose) and an alcohol (a hydroxyl group of the second sugar molecule), and the resulting compound is called a glycoside.

Glycosidic bonds are readily hydrolyzed by acid but resist cleavage by base. Thus disaccharides can be hydrolyzed to yield their free monosaccharide components by boiling with dilute acid. N-glycosyl bonds join the anomeric carbon of a sugar to a nitrogen atom in glycoproteins and nucleotides.

Formation of Maltose: A disaccharide is formed from two monosaccharides (Glucose + Glucose = Maltose)

3. Oligosaccharides

These are compound sugars that yield 2 to 10 molecules of the same or different monosaccharides on hydrolysis. Accordingly, an oligosaccharide yielding 2 molecules of monosaccharide on hydrolysis is designated as a dissaccharide, and the one yielding 3 molecules of monosaccharide as a trisaccharide and so on. The general formula of disaccharides is:

Cn(H2O)n – 1 and that of trisaccharides is Cn(H2O)n – 2 and so on.

A few examples are :
Disaccharides – Sucrose, Lactose, Maltose, Cellobiose, Trehalose, Gentiobiose, Melibiose
Trisaccharides – Rhamninose, Gentianose, Raffinose (= Melitose), Rabinose, Melezitose
Tetrasaccharides – Stachyose, Scorodose
Pentasaccharide – Verbascose

The molecular composition of the 3 legume oligosaccharides (viz., raffinose, stachyose and verbascose) is shown below :
α-Galactose (1–6) + α-Glucose (1–2) + β-Fructose = Raffinose
α-Galactose (1–6) + α-Galactose (1–6) + α-Glucose (1–2) + β-Fructose = Stachyose
α-Galactose (1–6) + α-Galactose (1–6) + α-Galactose (1–6) + α-Glucose (1–2) + β-Fructose = Verbascose

4. Polysaccharides

These are also compound sugars and yield more than 10 molecules of monosaccharides on hydrolysis. These may be further classified depending on whether the monosaccharide molecules produced as a result of the hydrolysis of polysaccharides are of the same type (homopolysaccharides) or of different types (heteropolysaccharides).

Their general formula is (C6H10O5)x.

Some common examples are :
Homopolysaccharides –Starch, Glycogen, Inulin, Cellulose, Pectin, Chitin
Heteropolysaccharides – “Specific soluble sugar” of pneumococcus type III, Hyaluronic acid, Chondrotin


Watch the video: Carbohydrates - cyclic structures and anomers. Chemical processes. MCAT. Khan Academy (May 2022).