carbohydrates (s)

8/17/2019 Carbohydrates (s)

1/11

Carbohydrates (hydrates of carbon)

1. All carbohydrates are organic compounds that contain carbon, hydrogen and oxygen. (C:H:O isabout 1:2:1)

2. The general formula for carbohydrates is Cx (H2O) y (x and y are variable numbers, that may besimilar or different).

Carbohydrates

Sugars (-ose) PolysaccharidesPhysical properties

Small moleculesSweet

Soluble in waterCrystalline

Macromolecules Not sweet

Slightly/insoluble in H2OAmorphous

Monosaccharides Disaccharides

Synthesis Simple sugars Made by joining 2

mono-saccharides

Made by joining many mono-

saccharides

Diagrammatic

representation

glycosidic bond

General formula (CH2O)n , where3


2/11

3. Another important characteristic of monosaccharides is isomerism. Isomerism occurs when

compounds have the same molecular formula but different structures and hence different physical properties.

Fig 2.1 Classification of some monosaccharides into aldoses and ketoses depending on the locationof the carbonyl group (in pink).

Quick check: Which of the above are isomers?

4. Sugars are important energy-storage molecules in organisms, especially the hexoses. The

carbon-hydrogen (C-H) bonds release energy when they are broken.

5. In aqueous solutions, most sugars form rings. In the case of glucose, carbon atom number 1 may bond with the oxygen atom on carbon 5 to form a six-sided ring known as a pyranose ring (Fig

2.5a). In the case of fructose, carbon atom number 2 links with the oxygen atom on carbon atom

5 to form a five-sided ring known as a furanose ring (Fig 2.5b).

Fig 2.5a Fig 2.5b

Carbonylgroup inaldehyde

Carbonyl

group in

ketone


3/11

6. In the glucose ring (pyranose) structure, carbon atom number 1 becomes asymmetric and givesrise to two isomers, the !-form and the "-form. These two forms are important in the formation

of starch and cellulose (Fig 2.6).

D-glucose

Fig 2.7a Formation of pyranose glucose molecules.

Fig 2.7b Formation of furanose fructose molecules.


4/11

7. All monosaccharides are reducing sugars.They are also sweet, soluble and crystalline.

Disaccharides (C12H22O11)

1. Disaccharides are formed when two monosaccharides, usually hexoses, are joined by a

glycosidic bond in a condensation reaction with the loss of a single water molecule.

2 (C6H12O6 )! C12H22O11 + H2O

2. The bond is normally formed between carbon atoms 1 and 4 of neighbouring units, resulting in a1- 4 glycosidic bond.

3. The most common disaccharides are maltose, sucrose and lactose (Fig 2.7):

maltose (malt sugar) = glucose + glucosesucrose (cane sugar) = glucose + fructose

lactose (milk sugar) = glucose + galactose

4. All disaccharides are sweet, soluble and crystalline. They can be hydrolysed into simple sugarswith acid or enzymatic hydrolysis (with the addition of water).

5. Maltose and lactose are reducing sugars, whereas sucrose is a non-reducing sugar because it has

no free carbonyl group. Two common tests for reducing sugars are Benedict’s test and Fehling’s

test. A brick-red precipitate is formed when the available carbonyl group reduces copper (II) inCuSO4 to copper (I) in Cu2O.

2 Cu(OH)2 + RCHO ! Cu2O + R.COOH + 2H2Ocopper (II) reducing copper(I) sugar water

hydroxide sugar oxide acid


5/11


6/11

Polysaccharides ( C x [H2O] y )

1. Polysaccharides are polymers of monosaccharides - formed when a few hundred to a fewthousand monosaccharides are joined by glycosidic bonds in condensation reactions (Fig 2.8).

Fig 2.8 Formation of disaccharides and polysaccharides by condensation, and their breakdown tomonosaccharides by hydrolysis.

2. They function mainly as food & energy storage compounds (starch and glycogen) and structural

materials (cellulose).

3. They are suitable as storage compounds because they

(i) are large molecules, therefore are quite insoluble & have no osmotic or chemicalinfluence in the cell

(ii) have compact molecules & do not take up much space(iii) are easily hydrolysed into sugars when required.

4. Plants store starch mainly in tubers and grains. Starch is a mixture of two substances - amyloseand amylopectin.

a. Amylosei. a simpler, straight chain polymer of several thousand !-glucose units

ii. joined by 1-4 linkagesiii. chain coils helically because of the angle of the 1-4 glycosidic linkage.

iv. has a compact shape for storage.


7/11

b. Amylopectin

i. a more complex form, with many branchesii. 1-6 glycosidic linkages at the branch points

iii. has up to twice as many !-glucose units as amylose (see Fig 2.9 a, b & c below).iv. has a compact shape for storage

Fig 2.9(a) Part of an amylose molecule showing many alpha glucose units linked by 1-4 glycosidic

bonds

Fig 2.9(b) Part of an

amylopectin moleculeshowing 1-4 glycosidic bonds

in straight chains and a 1-6glycosidic bond at a branch

point in the chain.

Fig 2.9(c) Two forms of starch are amylose (unbranched) and amylopectin (branched) found as

granules within a chloroplast of a plant cell.


8/11

5. Some differences between amylose & amylopectin

Amylose Amylopectin

Stains deep blue with iodine Stains red to purple with iodine

Relative molecular mass up to 50 000 Relative molecular mass up to 500 000

Up to 300 glucose units per molecule 1300 – 1500 glucose units per molecule

Unbranched helical chain Branched helical chain

6. Animals store glycogen mainly in liver and muscle cells. Glycogen is similar to amylopectin instructure but shows more branching and forms tiny granules inside cells (Fig 2.10).

Fig 2.10 Glycogen is a storage polysaccharide in animals which is more extensively branchedthan amylopectin. It is stored as dense clusters of granules within liver and muscle cells

3. Cellulose is a major structural component of all plant cell walls (20% - 40%). It consists of

long chains of "-glucose units (about 10,000 units per chain) which run parallel to each other.

Hydroxyl groups (-OH) project outwards from each chain in all directions to form hydrogen bonds with neighbouring chains, thus establishing a rigid cross-linking between chains. Thechains associate in groups to form microfibrils, which are arranged in larger bundles to form

macrofibrils. Thus, the cell wall has great tensile strength and yet is fully permeable to waterand solutes (fig 2.11).


9/11

Fig 2.11 Structure of a cellulose molecule formed from beta glucose units linked by 1-4

glycosidic linkages. The adjacent straight chains are linked by hydrogen bonds making it astrong building material in plant cell walls.

Fig.2.12 Electron micrograph of cell wall showing cellulosemicrofibrils about 20 nm in diameter.


10/11


11/11

Other compounds closely related to polysaccharides:

Chitin (kitin)- similar to cellulose, but has acetyl-amino groups replacing some hydroxyl (OH) groups.

- a major component in insect & crustacean exoskeleton- also found in fungal cell walls.

Inulin

- a fructose polymer- found as a storage carbohydrate in the root tubers of some plants

Pectin

- compound of galactose & galacturonic acid- helps to “stick” plant cell walls together.

Glycoproteins

- compounds of carbohydrates and proteins- found in blood plasma and saliva

Glycolipids- compounds of carbohydrates and lipids.- found on surfaces of plasma membranes

Exercise 1

1a) The diagram above shows part of a polymer. Name the polymer shown in diagram A

and B.

b) (i) Name a place where polymer A can be found.

(ii)What is the role of polymer A? Give 2 reasons to support this role.

c) (i) Name the bond found between the 2 units of polymer B.

(ii) What is the role of polymer B and name a place where it can be found.

d) Give two structural differences between A and B.

carbohydrates (s)

Documents