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  • Cell Structure and Functions
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Carbohydrates and Lipids


Table of Content



(1) Definition : These are molecules of low molecular weight and have higher solubility. These include minerals, water, amino acid, sugars and nucleotides. All molecules or chemicals functional in life activity are called biomolecules.

(2) Elements : They are naturally occurring and they are classified on the basis of their property into metals and non-metals. Again on the basis of presence and requirement in plants and animals, they are grouped into major and minor bioelements. Which are required in large amount are major bioelements e.g. Ca, P, Na, Mg, S, K, N, etc., while those are required in small amount are called minor bioelements e.g. Fe, Cu, Co, Mn, Mo, Zn, I, etc.

On the basis of function, they may be of following types :–

(i) Framework elements : Carbon, oxygen and hydrogen.

(ii) Protoplasmic elements : Protein, nucleic acid, lipids, chlorophyll, enzymes, etc.

(iii) Balancing elements : Ca, Mg and K. counteract the toxic effect of other minerals by ion-balancing. There are 17 essential elements in plants and 24 in animals. 14 elements are non-essential.          

(3) Biological compounds : These involve two kinds of compounds.

(i) Inorganic compounds : Characterised by absence of carbon, simple structure with low molecular weights e.g. water, minerals, ions and gases etc. Water 80% and inorganic salts 1-3%.

(ii) Organic compounds : Characterised by presence of carbon bonded to form a straight chain or ring structure.

(4) Cellular pool : Aggregated and interlinked  various kinds of biomolecules in a living system. So cell is called cellular pool. It includes over 5000 chemicals.

(5) Water : Liquid of life, major constituent of cell (about 60-90%) and exists in intracellular, intercellular and in vacuoles. In cells it occurs in free state or bound state (KOH, CaOH etc.).

(i) Properties of water : It is colourless, transparent, tastless and odourless, neutral (pH-7) liquid.  It is universal solvent, as it can dissolve both polar and non-polar solutes. High boiling point due to hydrogen bonding. Shows high degree of cohesion and adhesion. It can undergo three states of matter i.e. solid, liquid, gas. 


e.g. sugars, glycogen (animal starch), plant starch and cellulose.

(i) Source of carbohydrate : Mainly photosynthesis. It exists only in 1% but constitutes 80% of the dry weight of plants.

(ii) Composition : It consists of carbon, hydrogen and oxygen in the ratio CnH2nOn. It is also called saccharide and sugars are their basic components. Classification of carbohydrates can be summarised as :–


These are single sugar units which cannot be hydrolysed furthur into smaller carbohydrates. General formula is CnH2nOn, e.g. Triose-3C, glyceraldehyde, dihydroxyacetone, etc., tetrose, pentose, hexose, etc. About 70 monosaccharides are known, out of which only 20 are present in plants and animals.

(i) Important Hexoses

(a) Glucose : C6H12O6 Grape sugar is dextrose. Grape is sour due to presence of tartaric acid. Fructose is called fruit sugar (sweetest among natural sugars) and glucose is called " sugar of body". Normal level of blood glucose is 80-120mg/100ml. If it exceeds then condition is called "glucosuria".Glucose is a monosaccharide

(b) Fructose : Occurs naturally in fruit juices and honey. Hydrolysis of cane sugar in body also yields  fructose.

(c) Galactose : It is called as brain sugar. It's an important constituent of glycolipids and glycoproteins.

(d) Mannose : It is obtained on hydrolysis of plant mannans and gums. It is constituent of albumins, globulins and mucoproteins.

 (iii) Properties of monosaccharide  Fructose is a monosaccharide

(a) Monosaccharides are colourless, sweet tasting, solids.

(b) Due to asymmetric carbon, they exist in different isomeric forms. They can rotate polarized light hence they are dextrorotatory and leavorotatory.

(c) D-glucose after reduction gives rise to a mixture of polyhydroxy alcohol, sorbitol or mannitol.

(d) The sugars with a free aldehyde or ketone group reduce Cu++to Cu+ (cupric to cuprous)Mannose is a monosaccharide

(e) Sugars show oxidation, esterification and fermentation.

(f) The aldehyde or ketone group of a simple sugar can join an alcoholic group of another organic compound bond C-O-C the process involves loss of water and is called condensation (H-O-H) or H+OH ->H2O.

(iv) Functions of monosaccharides

(a) Glucose is the ultimate source of ATP in the cell respiration.

(b) It is used in formation of vitamin C.

(c) The intermediate compounds for the formation of glucose in photosynthesis are triose, tetrose, pentose and heptose, etc.

(d) Galactose is a constituent of agar-agar.

(e) Glucose is a blood sugar and xylose is a non-nutritive sweetener.

(f) Polymerisation of these molecules forms macromolecules.

(g) Ribose and deoxyribose are constituent of nucleic acids and nucleotides

(h) Glyceraldehyde and dihydroxyacetone are trioses.

(i) Sugars have free aldehyde or ketone group which can reduce Cu++ to Cu+ and are called reducing sugars. Benedicts or fehling's test are used to confirm the presence of reducing sugars.


Formed due to condensation of 2-10 monosaccharide units, the Oxygen bridge is known as "glycoside linkage" and water molecule is eliminated. The bond may be a and b.

(i) Disaccharides : Composed of two molecules of same or different monosaccharide units. Also called "double sugars". Molecular formula is C12H22O11.

(a) Maltose : Also called "malt sugar" stored in germinating seeds of barley, oat, etc. It is formed by enzymatic (enzyme amylase) action on starch. It is a reducing sugar.

(b) Sucrose : "Cane sugar" or " table-sugar". Obtained from sugarcane and beet root and on hydrolysis splits into glucose and fructose.

(c) Lactose : Milk sugar or 5% in mammalian milk. On hydrolysis yields glucose and galactose. Streptococus lacti converts lactose in to lactic acid and causes souring of milk.

(ii) Trisaccharides : Composed of three molecules of sugars. Molecular formula is C18H32O16.

(a) Raffinose : Found in sugar beet, cotton and in some fungi. It is made up of glucose, fructose and galactose.

(b) Gentianose : Found in rhizomes of gentian species, made up of glucose and fructose.

(iii) Tetrasaccharides : Composed of four molecules of same or different sugars. Stachyose is found in Stachys tubefera. It is made up of two unit of galactose, one unit of glucose and one unit of fructose.

(iv) Polysaccharides : General formula is (C6H10O5)n formed by condensation of several molecules (300-1000) of monosaccharides, (Described under " Macromolecules").

(v) Reducing and Non-reducing carbohydrates : Those which reduce Tollen's reagent or fehling solution are called reducing sugars and those do not reduce are called non-reducing sugars. All monosaccharides and disaccharides except sucrose are reducing. While all polysaccharides are non-reducing sugars.


Term lipid was coined by Bloor. These are esters of fatty acids and alcohol. They are hydrophobic insoluble in water but soluble in benzene, ether and chloroform. Lipids are classified into three groups:–

(i) Simple lipids : These are the esters of fatty acids and glycerol. Again they are typed as :–

(a) Fats and Oils : (Natural lipids or true fats). These triglycerides of fatty acid and glycerol. Fats which are liquid at room temperature are called oils. Oils with polyunsaturated fatty acids are called polyunsaturated e.g. sunflower oil, lower blood cholesterol.

(b) Fatty acids : Obtained by hydrolysis of fats. Formic acid is simplest fatty acid (HCOOH). These are of 2 types :–

Saturated fatty acids : The fatty acids which do not have double bond in between carbon atoms.e.g. butyric acid, palmitic acid,hexanoic acid, etc. They have high melting points, solid at room temperature and increase blood cholesterol.

Unsaturated fatty acids : The fatty acids which have double bonds in carbon atoms. e.g. 8 hexadecanoic acid, 9 octadecanoic acid etc. They have lower melting points mostly found in plant fats, liquid at room temperature and lower the blood cholesterol.

(c) Waxes : These are simple lipids composed of one molecule of long chain fatty acid and long chain monohydric alcohol. Waxes have high melting point, insoluble in water, resistant to atmospheric oxidation, chemically inert and not digested by enzymes. They reduce rate of transpiration by making plant tissue water proof and work as excellent lubricant.

Types of waxes

  • Plant wax : Forms coating.
  • Bee's wax : It is secretion of abdominal glands of worker honeybee. It consists of palmitic acid and myricyl alcohol.
  • Lanolin or Wool fat :  It is secreted by cutaneous glands,  also obtained from wool of sheeps. It consists of palmitic acid, oleic or stearic acid and cholesterol.
  • Sebum : It is secretion of sebaceous gland of skin.
  • Paraffin wax : Obtained from petrolium.

(ii) Compound lipids : They contain some additional or element. Group with fatty acid and alcohol on the basis of group they may be of following types:

(a) Phospholipids : These contain phosphoric acid. It helps in transport, metabolism, blood clotting and permeability of cell membrane. It is a bipolar molecule i.e. phosphate containing end is hydrophilic whereas fatty acid molecules represent hydrophobic (non-polar tail). Phospholipids again comprises.     

Lecithin : These are yellowish grey solids, soluble in ether and alcohol but insoluble in acetone. On hydrolysis they yield glycerol, fatty acid, phosphoric acid and choline. Lecithins are broken down by enzyme lecithinase to lysolecithin. The enzyme is found in venom of bee and cobra.

Cephalins : Found in animal tissue and soyabean oil. Cephalin contains choline or serine sometimes and stearic acid, oleic acid, linoelic and arachidonic acid.

(b) Glycolipids : These contain nitrogen and carbohydrate beside fatty acids. Generally found in white matter of nervous system. e.g. sesocine frenocin.

(c) Chromolipids : It includes pigmented lipids e.g. carotene.

(d) Aminolipids : Also known as sulpholipids. It contains sulphur and amino acids with fatty acid and glycerol. Cutin and suberin are also compound lipids resistant to water and also provide mechanical support in plants.

(iii) Derived lipids : These are obtained by hydrolysis of simple and compound lipids. Derived lipids include following components :–

(a) Sterols : Lipids without straight chains are called sterols. They are composed of fused hydrocarbon rings and a long hydrocarbon side chain. Best known sterol is cholesterol, present in high concentration in nervous tissue and in bile. Cholesterol is also the precursor of hormones like progesterone, testosterone, estradiol and cortisol and vitamin D. Diosgenin is obtained from yam plant (Dioscorea) used in making anti- infertility pills.

(b) Digitalin : It is prepared from leaves of Foxglove (Digitalis lantana) is a heart stimulant.

(c) Ergosterol : Present in food, found in ergot and yeast. It is precursor of another form of vitamin D, ergocalciferol (D2).

(d) Coprosterol : It is found in faeces. It is formed as a result of the reduction by bacteria in intestine from the double bond of cholesterol between C5 and C6.

(e) Tarpens : It is essential oil and present mostly in oils of camphor, eucalyptus, lemon and mint. Phytol is a terpenoid alcohol present in Vitamin A, K, E and in pigments like chlorophyll carotenoid. Other forms are licopene, gibberellins and natural rubber.

(f) Prostaglandin : It is hormone like compound derived  from arachidonic acid. Mostly present in secretion of seminal  vesicles in males and menstrual cycle fluid in females.

(g) Blubber : A very thick layer of subcutaneous fat in whale.

(iv) Functions of lipids

(a) Oxidation of lipids yields comparatively more energy in the cell than protein and carbohydrates. 1gm of lipids account for 39.1 KJ.

(b) The oil seeds such as groundnut, mustard, coconut store fats to provide nourishment to embryo during germination.

(c) They function as structural constituent i.e. all the membrane system of the cell are made up of lipoproteins.

(d) Amphipathic lipids are emulsifier.

(e) It works as heat insulator.

(f) Used in synthesis of hormones.

(g) Fats provide solubility to vitamins A, D, E, and K.

Q.1 Some of the enzymes, which are associated in converting fats into carbohydrates, are present in                                [CBSE PMT 1999]

(a)       Liposomes     (b)       Golgi bodies   (c)       Microsome     (d)       Glyoxysomes

Q.2 Largest physical and chemical molecules are [CBSE PMT 1996]    

(a)       Carbohydrates          (b)       Lipids  (c)       Proteins          (d)       Nucleic acids

Q.3 No cell could live without          [MP PMT 1997]        

(a)       Phytochrome (b)       Enzyme           (c)       Chloroplasts  (d)       Protein

Q.4 Which of the following is the characteristics of plants          [MP PMT 2003]        

(a)       Glucose and cellulose           (b)       Pyruvic acid and glucose     

(c)       Cellulose and starch              (d)       Starch and pyruvic acid

Q.5 Starch and cellulose are the compounds made of many units of     [CPMT 1988, 89]      

(a)       Simple sugar  (b)       Fatty acid       (c)       Glycerol          (d)       Amino acid

Q.6 Oval shaped and eccntric starch particles are found in         [RPMT 1995]

(a)       Wheat     (b)       Maize             (c)       Potato             (d)       Rice

Q.7 What are the most diversed molecules in the cell      [MP PMT 2000]        

(a)       Lipids      (b)       Proteins          (c)       Carbohydrates          (d)       Mineral salts

Q.8 The form in which sugar is present in sugarcane      [Manipal 2001]        

(a)       Maltose           (b)       Sucrose          (c)       Fructose         (d)       Glucose

















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