How It Was Proved That Deoxyribonucleic Acid (DNA) Is The Heredity Material

Deoxyribonucleic Acid (DNA)

Deoxyribonucleic acid is a molecule that carries most of the genetic instructions used in the development, functioning and reproduction of all known living organisms and many viruses.

DNA as Hereditary Material

Work of Griffith: The first evidence of hereditary nature of DNA was provided by a British microbiologist Frederick Griffith who made some unexpected observations while experimenting with pathogenic bacteria. His observations are:

• When he injected mice with a virulent strain of Streptococus pneumoniae bacteria (then known as Pneumococcus), the mice died of blood poisoning.
• When he injected similar mice with a mutant strain of S. pneumoniae that lacked the virulent polysaccharide coat, the mice showed no ill effects. This shows that coat was apparently necessary for virulence.

S forms: The normal pathogenic form of this bacterium is referred to as the S form because it forms smooth colonies on a culture disk.

R forms: The mutant forms, which lack an enzyme needed to manufacture the polysaccharide coat, is called the R form because it forms rough colonies.

Decisive Experiment

Griffith Experiment of Transformation

Purpose of Experiment: To determine whether the polysaccharide coat that lacked in mice which showed no ill effect, had a toxic effect.

Actual Experiment: Griffith injected heat killed dead bacteria of the virulent S strain into the mice, the mice remained perfectly healthy.

Control Experiment: As a control experiment (a test where the person conducting the test only changes one variable at a time in order to isolate the results. An experiment where all subjects involved in the experiment are treated exactly the same except for one deviation is an example of a control experiment.), he injected mice with a mixture containing dead S bacteria of the virulent strain and live coatless R bacteria, each of which by itself did not harm the mice.

Result: Unexpectedly, the mice developed the disease symptoms and many of them died.

Further Observations: The blood of the dead mice was found to contain high levels of live, virulent streptococcus type S bacteria, which had surface proteins characteristic of the live (previously R) strain. Somehow, the information specifying the polysaccharide coat had passed from the dead, virulent S bacteria to the live, coatless R bacteria in the mixture, transforming permanently the coatless R bacteria into the virulent S variety.

Transformation: Transformation is the transfer of genetic material from one cell to another and can alter the genetic makeup of the recipient cell.

Work of Avery, Macleod AND McCarty

Determining that DNA is Heredity Material

Transforming Principle

In 1944, in a classic series of experiments Oswald Avery along with Colin Macleod and Maclyn McCarty repeated Griffith's experiments and characterized what they referred to as the transforming principle. They first prepared mixture of dead S Streptococcus and live R Streptococcus that Griffith had used. Then they removed as much of the protein as they could from their preparation, eventually achieving 99.98% purity. Despite removal of nearly all the protein, the transforming activity was not reduced. Moreover the properties of transforming principle resembled there of DNA. The protein digesting enzymes or RNA digesting enzymes did not affect the transforming activity, but the DNA digesting enzyme DNase destroyed all the transforming activity.

Hershey and Chase Experiment

Hershey and Chase Experiment To Determine DNA is Heredity Material

In 1952 by Alfred Hershey and Martha Chase experimented with bacteriophages T₂ and provided additional evidence supporting Avery's conclusion.

³²P Labeled Viruses: In some experiments they labeled viruses with radio (radioactive) isotope ³²P, which was incorporated into the newly synthesized DNA of growing phage.

³⁵S Labeled Viruses: In other experiments the viruses were grown on a medium containing ³⁵S an isotope (radioactive) of sulphur which is incorporated into the amino acids of newly synthesized protein coats.

Exposure of Bacteria to Labeled Viruses: Now one bacterial culture was exposed to ³²P labeled viruses and other culture was exposed to ³⁵S labeled viruses.

Removal of Protein Coats: The bacterial cells were agitated violently in a blender to remove the protein coats of the infecting viruses from the surfaces of the bacteria.

Test for the Presence of Label: When bacteria were tested for the presence of label it was found that bacteria had lost nearly all of ³⁵S label. However, 32P label was present in bacteria of the other group.

Reason: It is because ³⁵S was only present in protein coat while ³²P was present in DNA that had transferred to the interior of the bacteria.

Confirmation: It was confirmed when viruses were released from the infected bacteria. The viruses released from ³²P culture were labeled while viruses released from ³⁵S culture were unlabelled.

DNA — the Hereditary Material

Thus it was proved that the hereditary information injected into the bacteria that specified the new generation of viruses was DNA and not protein. 

What Are The Effects Of Low Testosterone Levels In The Body

What is Testosterone?

Crystals of Testosterone

Testosterone is a male sex hormone that is linked with development of male secondary sex characteristics. It is mainly produced in testis, but also in adrenal cortex and ovaries. In actual fact, testosterone not only fuels sex drive and muscle mass, but also controls state of mind and bone strength.

Male Menopause (Aging and Testosterone Levels)

A moderate drop in testosterone is an ordinary piece of maturing, and is called "andropause" or "male menopause." For some men, this doesn't bring about any big problems. Some may see hot flashes and bad-tempered moods.

Low Testosterone Effects in Body

Low testosterone levels in the body can ground noticeable changes in the body like:

• Thinner muscles
• Loss of body hair
• Smaller, softer testicles
• Larger breasts

Testosterone Effects Bones

Bone Density Comparison Between Normal and Osteoporosis Bones

Low testosterone levels in the body are related with long lasting effects like osteoporosis. People usually think that osteoporosis and fragile bones are the women’s ailment, however males can also be its target. You may get symptoms of osteoporosis like thinner, weaker and prone to break bones as testosterone levels fall.

Mood Swings

Low testosterone levels in body can cause mood changes. Poor concentration, irritability, mood swings and low energy can be caused by low testosterone levels but these conditions can also be caused by some other issues like anemia, depression, insomnia (sleep troubles), or a chronic illness. If you have any of these issues, you should immediately visit your physician so that he may diagnose whether these symptoms are by low testosterone levels or by any other issues.

Low Testosterone and Sex Drive

A drop in testosterone levels don't generally meddle with sex, however it can make it more troublesome for your mind and body to get stimulated. A few men may see a drop in drive, while others may lose interest in sex totally. Low testosterone can likewise make it harder to get or keep an erection.

Low Testosterone and Infertility

As testosterone is a male sex hormone and it is associated with making sperms. At the point when levels of the testosterone hormones are low, his sperm count can be low, as well. Without enough sperm, he will be unable to father a kid.

What Causes Low Testosterone

A little drop of testosterone levels is ordinary phase of maturing but if you have a very low testosterone level at young age, you should consult your doctor. In fact, aging is the main cause of low testosterone levels. Some ailments can also be its cause including:

• Type 2 diabetes
• Liver or kidney diseases
• Chronic obstructive pulmonary disease
• Pituitary gland issues
• Testicle wounds

Radiation treatment, chemotherapy, and steroid drugs can influence testosterone levels.

Should You Be Tested

If you have any of the following conditions, then you should consult your doctor so that he may recommend you a testosterone test:

• Erectile dysfunction
• Lower sex drive
• Low sperm count
• A loss of tallness, body hair, or muscle size

Testosterone Levels in Body:

Testosterone levels in the body are measured by a simple blood test done early in the morning, when levels are highest. Normal levels range from 300 to 1,000 ng/DL.

Long Term And Short Term Side Effects of Steroids (Corticosteroids And Anabolic Steroids)

Steroids are produced by Adrenal Glands

What Are Steroids?

Steroids are naturally occurring chemicals (hormones), in your body. They play a role in helping your organs, tissues, and cells to carry out their work efficiently. You require a well healthy equilibrium of them to develop and even to make babies. "Steroids" can likewise refer to man-made meds. They are produced by adrenal glands and primarily sorts into two categories are corticosteroids and anabolic-androgenic steroids (or anabolics for short). Steroid hormones can be grouped into five groups by the receptors to which they bind: glucocorticoids, mineralocorticoids, androgens, estrogens, and progestogens.

Steroid Drugs

What Are Corticosteroids?

They're man-made drugs that closely resemble to cortisol. They rapidly battle irritation in your body. These man-made steroids work like hormones (Cortisol). The function of Cortisol is that it keeps your immune system from making such materials that cause swelling. Corticosteroid medications (prednisone), work in a comparable manner. They moderate or stop the immune system processes that set off inflammation.

What Do Corticosteroids Treat?

Corticosteroids treat:

• Rheumatoid arthritis 
• Asthma 
• Chronic obstructive pulmonary disorder (COPD) 
• Multiple sclerosis 
• Rashes and skin conditions like eczema 
• Lupus and other autoimmune.

Side Effects of Corticosteroids

The side effects usually rely on upon the dosage, to what extent and how much you take the medication. Transient utilization can bring about weight increase, puffy face, sickness, nausea, emotional instability and snooze problem. You may additionally get thinner skin, skin inflammation, and irregular hair development. Side Effects are obvious, as corticosteroids turn down your immune system and their dosage makes you more prone to get infections.

The Long-Term Effects

High dosages of corticosteroids are related with the long lasting effects like osteoporosis. Utilizing them for over 3 months can bring about brittle and fragile bones that can break easily. This condition is called as osteoporosis. Kids who take them may grow slowly. Other symptoms are muscle weakness, eye issues (cataracts), and a higher hazard of diabetes.

What Are Anabolic Steroids?

Like corticosteroids, Anabolic steroids are also man-made drugs which mimic the job of testosterone (naturally occurring male sex hormone) by trigging the muscle growth. 

What Are Anabolic Steroids Used For?

As anabolic steroids can trigger muscle growth, some take their dosages legally while few take it illegally. Legal use is as a doctor can prescribe it to a patient with low testosterone level in the body or doctors can also prescribe to those patients who have lost their muscle mass due to some health conditions like cancer, AIDS. Illegal use of anabolic steroids is usually seen by athletes and sports persons who take this medication to enhance their muscle mass.

Side Effects of Anabolic Steroids

Anabolic steroids can bring about terrible skin inflammation and fluid retention. Long-term utilization can prevent the body from making testosterone. Anabolic steroids can cause little testicles, lower sperm count, sterility and breast development in men while ladies may have pattern baldness, growth of facial hair, periods that change or stop. Youngsters who use them may inhibit their bone development. High dosages can prompt great emotional instability, fury, and aggression called "roid rage."

Long-Term Effects

Long-term utilization of anabolic steroids can harm your liver, kidneys, and heart. Extreme liquid retention can bring cause heart failure. These medications can also raise your bad cholesterol level, which can make you more prone to have heart attacks and strokes at any age.

CLASSIFICATION OF ENZYMES

Enzymes

Enzymes:

Enzymes can be defined as the reaction catalysts of biological systems produced by living cells and are capable of catalyzing chemical reactions. Naturally enzymes are macromolecules having molecular masses ranging into millions. Two exceptional properties of enzymes are their extraordinary specificity — each enzyme catalyzes only one reaction or one group of closely related reactions — and their amazing efficiency — they may speed up reactions by factors of up to 10²⁰. Each enzyme molecule possesses a region known as the active site and the substrate binds itself with this active site. Enzymes are either pure proteins or contain proteins as essential components and in addition require non-protein components which are also essential for their activity. The protein component of the enzyme is called apoenzyme and the non-protein component is called the co-factor or co-enzyme. The co-factors include inorganic ions and complex organic or metallo-organic molecules. Important inorganic co-factors along with their respective enzymes include Fe²⁺ (chrome oxidase) Ze²⁺ (carbonic anhydrase) and Mg²⁺ (glucose 6- phosphatase), etc. Many enzymes contain vitamins as their co-factors, for example; nicotinamide adenine dinucleotide contains nicotinamide vitamin and thiamine pyrophosphatase contains vitamin B₁.

Suffix-"ase" is added to the name of the substrate on which the enzyme acts while naming the enzymes, for example, urease, sucrase, cellulase are the enzymes, which act upon the substrates urea, sucrose and cellulose respectively.

Classification of Enzyme:

Enzymes are classified into six main types by the commission on enzyme, appointed by the International Union of Bio-Chemistry (IUB).

Oxidorednctases:

These are the enzymes which catalyze oxidation-reduction reactions. Common examples are oxidase, dehydrogenase and peroxydase.

Transferases:

These enzymes bring about an exchange of functional group such as phosphate or acyl between two compounds, for example; phospho-transferases, etc.

Hydrolases:

These are the enzymes which catalyze hydrolysis. They include proteases called protelytic enzymes.

Lyases:

These are those enzymes which break and form double bonds. They catalyze the addition of ammonia, water or carbon dioxide to double bonds or removal of these to form double bonds, for example phospho-glyceromutases.

Isomerases:

These enzymes catalyze the transfer of groups within molecules to yield isomeric forms of the substrate. An example is the conversion of fumaric acid to maleic acid in the presence of fumarase enzyme.

Ligases:

These enzymes are those enzymes which link two molecules together by breaking the high-energy bonds, for example; acetyl—S—COH, a carboxylase and succinic thiokinase. 

CLASSIFICATION OF PROTEINS ACCORDING TO THEIR STRUCTURE

Proteins

The majority of proteins are compact, highly convoluted molecules with the position of each atom relative to the others determined with great precision. To describe the structure of a protein in an organism, it is necessary to specify the three-dimensional shape that the polypeptide chain assumes. Every protein has particular properties which are dictated by the number and the particular succession of amino acids in a molecule. There are basically three levels of structural organization in proteins.

• Primary Structure
• Secondary Structure
• Tertiary Structure

Some proteins also possess a fourth structure called Quaternary structure.  

Primary structure:

Primary Structure of Proteins

The sequence of the amino acids combined in a peptide chain is referred to as the primary structure. Peptide bond held together the primary structure which is made during the course of protein biosynthesis. F. Sanger was the first researcher who determined the arrangement of amino acids in a protein atom. Following ten years of cautious work, he finished up, that insulin is made of 51 amino acids in two chains. One chain had 21 amino acids and the second had 30 amino acids in two alpha and two beta chains. Every alpha chain contains 141 amino acids, while every beta chain contains 146 amino acids.

Insulin Structure

There are more than of 10,000 proteins in the human body which are made up of particular sequence of 20 kinds of amino acids. This arrangement depends on the sequence of nucleotides in the DNA. The sequence of amino acids in a protein particle is highly definite for its proper working. On the off chance that any amino acid is not in its typical place, the protein does not carry on its ordinary function. In this case, the sickle cell hemoglobin is the best example. Only one amino acid in every beta chain out of the 574 amino acids don't take up the ordinary place in the proteins (this specific amino acid is displaced by some other amino acid), and the hemoglobin fails to carry sufficient oxygen, thus prompting demise of the patient.

Secondary structure:

Secondary Structure

The secondary structure of protein is a regular coiling or zigzagging of polypeptide chains produced by hydrogen bonding between NH and C=O groups of amino acids near each other in the chains. They typically curl into a helix, or into some other consistent setup. One of the normal secondary structures is the a-helix. It has one winding arrangement of the basic polypeptide chain. The a-helix is an extremely uniform geometric structure with 3.6 amino acids in each one turn of the helix. The helical structure is maintained by the hydrogen bonds between amino acid atoms in consecutive turns of the winding. B-pleated sheet is structured by folding over of the polypeptide.

B-pleated sheet

Tertiary structure:

Tertiary Structure

The three dimensional twisting and folding of the polypeptide chain results in the tertiary structure of proteins. Polypeptide chain twists and folds upon itself shaping a globular shape. The proteins' tertiary conformity is held by three kinds of bonds, i.e. Hydrogen, ionic and disulfide (-S-S-). In the aqueous environment, the steadiest tertiary conformity is that in which hydrophobic amino acids are covered inside while the hydrophilic amino acids are on the surface of the atom.

Quaternary structure:

Quaternary Structure

In complex proteins, polypeptide tertiary sequences are coiled in multi-complex subunits. Quaternary proteins are made up of the arrangement of two or more tertiary chains and are held together by hydrophobic interactions, ionic and hydrogen bonds. Hemoglobin, the oxygen transport protein has such a structure.

Hemoglobin Molecule

Classification of proteins:

Due to the diversity and complexity in proteins’ structure, it is extremely hard to classify proteins in a single general category. So, as per their structure, proteins are categorized as follows:

Fibrous proteins:

Fibrous Proteins, Keratin

They contain atoms having one or more polypeptide chains as fibrils. They are water insoluble and form tendons, muscle fibers, connective tissues and bone matrix. Secondary structure is of great importance in them. They are water insoluble, non-crystalline and elastic. They form tendons, muscle fibers, connective tissues and bone matrix. Myosin, silk fibers, fibrin, and keratin are examples.

Globular proteins:

Globular Protein, Myoglobin

Globular proteins are ellipsoidal and spherical because of numerous wrinkling of polypeptide chains. Tertiary structure is most essential in them. They are soluble in water and can be crystallized. They adjust themselves with changes in the physical environment. Cases are enzymes, hormones, antibiotics and hemoglobin.

Comparison between Fibrous and Globular Proteins:

Fibrous Proteins	Globular Proteins
1. Fibrous Proteins are extended proteins and are insoluble in water.	1. Globular Proteins are compact proteins and are soluble in water.
2. They have long thread like structures.	2. They have folded ball like structures.
3. They have comparatively stronger intermolecular forces.	3. They have weak intermolecular hydrogen bonding.
4. They have helical or sheet like structures.	4. Globular have three dimensional shape.
5. They form structures of body or cells e.g. tendons, ligaments, hairs, etc.	5. They perform metabolism, transport, immune protection, hormones, etc.

CLASSIFICATION OF PROTEINS BASED ON PHYSIOCHEMICAL PROPERTIES

Based on proteins' function, proteins may be classified as regulatory or hormonal proteins, structural proteins, transport proteins, genetic proteins and physiochemical proteins. But here, I am only going to define proteins based on physiochemical behavior. Based on physiochemical properties, proteins are classified into following types:

• Simple Proteins
• Compound or Conjugated Proteins
• Derived Proteins

Simple Proteins:

These proteins on hydrolysis yield only amino acids or occasional carbohydrates and their derivatives. For example, albumins, globulins, lefumin, collagen, etc. Globulins are insoluble in water but soluble in dilute salt solutions. They are found in animals, e.g. lactoglobulin is found in muscles and also in plants. Legumin and collagen proteins are present in the connective tissues throughout the body. They are the most abundant proteins in the animal kingdom forming some 25-35 percent of body protein.

Globulin 

Compound or Conjugated Proteins:

In these molecules the protein is attached or conjugated to some non-protein groups which are called prosthetic groups. For example, phospho-proteins are conjugated with phosphoric acid; lipoproteins are conjugated with lipid substances like lecithin, cholesterol and fatty acids.

Derived Proteins:

As the word “derived” is showing that this class of protein includes substances which are derived from simple and conjugated proteins. For example, proteoses enzymes, peptones, oligopeptides, polypeptides, etc.

Polypeptide chain

BRIEF REVIEW OF LIPIDS

Lipids are actually diverse nature compounds which are related to fatty acids. They are water fearing compounds and are soluble compounds organic compounds like alcohol, ether, chloroform, benzene etc. Lipids are hydrophobic compounds and are components of cellular layers.

Remarkable features of lipids include that they provide wadding against atmospheric stress i.e. heat and cold. As they are hydrophobic so they act as water proof material. Lipids can also store energy. They can almost store double amount of energy as that of carbohydrates. The reason for this double storage of energy in lipids is they have high percentage of C-H bonds and low amount of oxygen. In some plants there is cuticle, a supplementary protective membrane is made up of waxes which is one of the types of lipids. Exoskeleton of insects is also made up of waxes. 

Classification of Lipids:

Lipids are categorized as acylglycerols, waxes, phospholipids, sphingolipids, glycolipids and terpenoids including carotenoids and steroids. Let’s get a succinct review of some of the types of lipids.

So, Lipids are group of naturally occurring molecules.

Acylglycerols:

Acylglycerols are esters of fatty acids. Esters are those compounds which are formed by reaction of alcohols and acids and a water molecule is released. Hydrogen (H) and hydroxyl (OH^-) is released from acid and alcohol respectively and combine to form water (H₂O). Composition of Acylglycerol is fatty acids and glycerol. The most common acylglycerol is triacylglycerol. Triacylglycerol is also called neutral lipid.

Waxes:

cuticle, a waxy layer on leaf

Waxes also fall in lipid’s classification. Waxes are generally found as a protective covering on fruits and leaves. Exoskeleton of insects is also made up of waxes. The reason for waxed coating on plants is they prevent extra water loss and abrasive damage. Chemical composition of waxes includes mixture of long chain alkanes, alcohols, ketones and esters of extended chain fatty acids.

Phospholipids:

Phospholipids are usually found in bacterial, animal and plant’s membrane. These are derived from phosphatidic acid. Choline, ethanolamine and serine are important components of phospholipids.

Terpenoids:

Terpenoids are composed of single repeating units called isoprenoids units. Condensation of isoprenoids units give rise to many useful compounds such as rubber, steroids etc. Terpenoids can also store energy. Applications of terpenoids include insulation and protection from water loss.

Saturated and Unsaturated fatty acids:

It is our common observation that some fatty acids (oils) are liquid at room temperature and some fatty acids (fats) are solid at room temperature. Oils are called unsaturated fatty acids and fats are called saturated fatty acids. Both are lighter than water. Plant fats are liquid, while animal fats are solid at room temperature.

plant fats are liquid

animal fats are solids

CARBOHYDRATES AND ITS TYPES

Carbohydrates occur abundantly in living organisms. They are found in all organisms and in almost all parts of the cell. Cellulose of wood, cotton and paper, starches present in cereals, root tubers, cane sugar and milk sugar are all examples or carbohydrates. Carbohydrates play both structural and functional roles.

Simple carbohydrates are the main source of energy in cells. Some carbohydrates are the main constituents of cell walls in plants and micro-organisms. The word carbohydrate literally means “hydrated carbons”. They are composed of carbon, hydrogen and oxygen and the ratio of hydrogen and oxygen is the same as in water. Chemically, carbohydrates are defined as polyhydroxy aldehydes or kentones, or complex substances which on hydrolysis yield polyhydroxy aldehyde or ketone subunits. (Hydrolysis involves the break down of large molecules into smaller ones utilizing water molecules).

The sources of carbohydrates are green plants. These are the primary products of photosynthesis. Other compounds of plants are produced from carbohydrates by various chemical changes. Carbohydrates in cell combine with proteins and lipids and the resultant compounds are called glycoproteins and glycolipids, respectively. Glycoproteins and glycolipids have structural role in the extracellular matrix of animals and bacterial cell wall. Both these conjugated molecules are components of biological membranes.

CLASSIFICATION OF CARBOHYDRATES:

Carbohydrates are also called 'saccharides' (derived from Greek word 'sakcharon' meaning sugar) and are classified into three groups: (i) Monosaccharides (ii) Oligosacchatides, and (iii) Polysaccharides.

Monosaccharides:

These are simple sugars. They are sweet in taste, are easily soluble in water, and cannot be hydrolyzed into simpler sugars. Chemically they are either polyhydroxy aldehydes or ketones. All carbon atoms in a monosaccharide except one have a hydroxyl group. The remaining carbon atom is either a part of an aldehyde group or a keto group. The sugar with aldehyde group is called aldo-sugar and with the keto group as keto -sugar.

In nature monosaccharides with 3 to 7 carbon atoms are found. They are called trioses (3C), tetroses (4C), pentoses (5C), hexoses - (6C), and heptoses (7C).

Trioses are, intermediates in respiration and photo-synthesis. Tetroses are rare in nature and occur in some bacteria. Pentoses and hexoses are most common. From the biological point of view the most important hexose is glucose. It is an aldose sugar. Structure of ribose and glucose is given below.

structure of Ribose

structure of Glucose

Most of the monosaccharides form a ring structure when in solution. For example ribose will form a five cornered ring known as ribofuranose, whereas glucose will form six cornered ring known as glucopyranose.

In Free State, glucose is present in all fruits, being abundant in grapes, figs, and dates. Our blood normally contains 0.08% glucose. In combined form, it is found in many disaccharides and polysaccharides. Starch, cellulose and glycogen yield glucose on complete hydrolysis. Glucose is naturally produced in green plants which take carbon dioxide from the air and water from the soil to synthesize glucose.

This process is called photosynthesis. 717.6Kcal of solar energy is required for synthesis of 10g of glucose.

Oligosaccharides:

These are comparatively less sweet in taste, and less soluble in water. On hydrolysis, oligosaccharides yield from two to ten monosaccharides. The one yielding two monosaccharides are known as disaccharides, those yielding three a known as trisaccharides and so on. The covalent bond between two monosaccharides called glycosidic bond. Physiologically important disaccharides are maltose, sucrose, and lactose. Most familiar disaccharide is sucrose (cane sugar) which on hydrolysis yields glucose and fructose, both of which are reducing sugars.

Polysaccharides:

Polysaccharides are the most complex and the most abundant carbohydrates in nature. They are usually branched and tasteless. They are formed by several monosaccharide units linked by glycosidic-bonds. Polysaccharides have high molecular weights and are only sparingly soluble in water. Some biologically important polysaccharides are starch, glycogen, cellulose, dextrin, agar, pectin, and chitin.

Starch:

It is found in fruits, grains, seeds, and tubers. It is the main source of carbohydrates for animals. On hydrolysis, it yields glucose molecules. Starches are of two types, Amylose and Amylopectin. Amylose starches have unbranched chains of glucose and are soluble in hot water. Amylopectin starches have branched chains and are insoluble in hot or cold water. Starches give blue colour with iodine.

Glycogen:

It is also called animal starch. It is the chief form of carbohydrate stored in animal body. It is found abundantly in liver and muscles, though found in all animal cells. It is insoluble in water, and gives red colour with iodine. It also yields glucose on- hydrolysis.

Cellulose:

It is the most abundant carbohydrate in nature. Cotton is the pure form of cellulose. It is the main constituent of cell walls of plants and is highly insoluble in water. On hydrolysis it also yields glucose molecules. It is not digested in the human digestive tract. In the herbivores, it is digested because of micro-organisms (bacteria, yeasts, protozoa) in their digestive tract. These micro-organisms secrete an enzyme called cellulase for its digestion. Cellulose gives no colour with iodine.