Carbohydrates

Monomer

= The smaller units from which larger molecules are made

Polymer

= Molecules made from a large number monomers bonded together

Examples of monomers

Monosaccharides, amino acids, nucleotides

Condensation reaction

Joins two molecules together with the formation of a chemical bond and involves the elimination of a molecule of water

Hydrolysis reaction

Breaks a chemical bond between two molecules and involves the use of a water molecule

Condensation reaction diagram

Hydrolysis reaction diagram

What do carbohydrates do?

• Store energy

• Provide structural support to plant cells

What elements to carbohydrates contain?

• Carbon

• Hydrogen

• Oxygen

Monosaccharides

= Monomers of larger carbohydrates

• Sugars that are soluble in water

Disaccharides

= 2 monosaccharides bonded by a glycosidic bond, formed by a condensation reaction

Polysaccharide

= polymers made up of many monosaccharides through many condensation reactions

Groups of carbohydrates

Alpha glucose

Beta glucose

What type of bond forms between two monosaccharides during a condensation reaction?

Glycosidic

What monosaccharides are the 3 disaccharides are made up of?

glucose + glucose --> maltose + water

glucose + fructose --> sucrose + water

glucose + galactose --> lactose + water

How to write glycosidic bond

Type of glucose, between which carbons the bond is between, glycosidic bond.

E.g. α 1-4, glycosidic bond

Isomer

= compounds that have the same formula but the atoms are arranged differently

Chemical formula for glucose

C₆H₁₂O₆

3 things glucose can be polymerised into

• cellulose - structural support

• glycogen - energy stores

• starch - energy stores

About starch

• found in plants

• made from the excess glucose created during photosynthesis

• excess glucose is converted to starch for storage

Structure of starch

• polymer made up of alpha glucose joined together by condensation reactions

• Chains are branched → larger surface area faster → release of glucose by hydrolysis

• held in place by 1,4 glycosidic bonds and 1,6 GB at branching points

• made up of two polymers: amylose and amylopectin

Amylose

• all monomers joined by a 1,4 glycosidic bond so unbranched

• spiral-shaped polymer, helix shape

Amylopectin

• glucose monomers joined by both 1,4 and 1,6 glycosidic bonds

• 1,6 bonds creating branches

Diagram of branch points

Properties of starch

• Insoluble so will not change the water potential of a cell and osmosis will not occur

• Amylopectin is branched →large surface area for enzymes to attach to

About glycogen

• carbohydrate store in animal cells

• glycogen stored in liver and muscle cells

• made from excess glucose taken up by the digestive system that is not used for respiration so is converted to the polymer glycogen

• liver cells remove toxins, muscle cells are for movement so glycogen is stored there so there is ready access to glucose for respiration and release energy

Structure of glycogen

• made up of α glucose joined by condensation reactions

• held in place by 1,4 and 1,6 glycosidic bonds

• more highly branched than starch

Properties of glycogen

• insoluble so can be stored in cells without dissolving, won’t change water potential or cause osmosis which would cause cell lysis

• highly branched → large surface area → quickly hydrolysed to release glucose

• animals have high metabolic rate so need more glucose for respiration

About cellulose

• provide structural strength

• prevents cells bursting if they have excess water due to osmosis

Structure of cellulose

• Unbranched polymer made up of beta glucose joined by 1,4 glycosidic bonds

• Every other beta glucose is inverted 180° relative to the previous one to group the hydroxides (OH) so the bonds can form

• Long straight chains lie parallel to each other held together by hydrogen bonds

Microfibrils

= Chains of beta glucose bonded together

Macrofibrils

= Microfibrils bonded together

Microfibrils →macrofibrils →cell wall

Properties of cellulose

• Insoluble as its a large molecule → won’t change water potential or cause osmosis

• Very strong due to the number of hydrogen bonds between fibrils

• Cellulose is very strong to give plant cell wall strength so when turgid, contents push on cell wall, it does not burst

Test for reducing sugars

1. Add 1 drop each of the sample and Benedict’s reagent into the Eppendorf tube

2. Heat in a water bath for 2 mins

3. Observe colour changes

RS present if the colour changes from blue to green/orange/brown/brick red depending on concentration (green suggests a low concentration).

RS absent if there is no colour change.

Test for non-reducing sugars

1. Proceed if a negative result for RS

2. Add 1 drop each of sample and 2M HCL into Eppendorf tube

3. Place in a hot water bath for 2 mins (stage when acid hydrolysis may occur)

4. When cool, add a small amount of sodium hydrogen carbonate powder with a spatula (about ½ vol of liquid in the tube)

5. Add 1 drop of Benedict’s reagent, heat in hot water bath for 2 mins

6. Observe colour changes

NRS present if the colour changes from blue to green/orange/brown/brick red depending on concentration (green suggests a low concentration) AT THIS STAGE ONLY.

NRS absent if there is no colour change.

A negative result for reducing sugars does not necessarily mean non-reducing sugars are present