Carbohydrates

What do carbohydrates consist of?

Carbon, hydrogen, oxygen

Cm(H2O)n

What are monosaccharides and what do they consist of?

• Simplest and most basic form of carbohydrate

• Carbonyl group + multiple hydroxyl groups

• -(CH2O)

How are monosaccharides classified?

• Number of carbon atoms

• Location of carbonyl group

• Spatial arrangement of atoms

How are monosaccharides classified based on the number of carbon atoms?

• Triose e.g. glyceraldehyde

• Pentose e.g. ribose

• Hexose e.g. glucose

How are monosaccharides classified based on the location of carbonyl group?

• Aldose / Aldehyde sugar — carbonyl group at the terminal carbon e.g. glucose

• Ketose / Ketone sugar — carbonyl group at a non terminal carbon e.g. fructose

How are monosaccharides classified based on spatial arrangement of atoms?

• Linear forms & ring forms

  • At aqueous states, linear forms of monosaccharides can bend to form the more stable ring form

• α- & β-glucose (-OH group attached to C1)

  • α-glucose — below the plane of the ring

  • β-glucose — above the plane of the ring

What are dissacharides and how are they formed?

Two monosaccharides (that become monomers) joined by a glycosidic bond / linkage

Cn(H2O)n-1

How can the glycosidic bond be formed and broken?

• Formed by condensation, involving the loss of one water molecule

• Broken by hydrolysis, involving the addition of one water molecule

What is the significance of the molecular structure of monosaccharides? (size, linear, rings) (4)

• Small in size and has many hydroxyl groups that can form hydrogen bonds with water

  • Readily soluble in water, transported easily in water in transport systems

• Linear form possesses a free carbonyl group

  • Reducing ability → reducing sugars

• Pentoses and hexoses can exist as rings

  • Stable building blocks for larger molecules

• Ring structures exhibit α- & β-isomerism

  • Increases the diversity of monosaccharides → a variety of molecules

How is maltose formed?

α-glucose + α-glucose in an α(1-4) glycosidic bond

How is sucrose formed?

α-glucose + β-fructose in an α(1-2) glycosidic bond

What are examples of reducing and non-reducing sugars?

• Reducing — Maltose, Lactose (free carbonyl group)

• Non-reducing — Sucrose (both carbonyl groups linked together when forming the glycosidic bond)

How do we test for reducing sugars?

Benedict’s Test

• Copper(II) sulfate [blue alkaline solution] is reduced to insoluble copper(I) oxide [red ppt]

• Presence of reducing sugar is indicated by a brick-red precipitate (Orange → yellow → green → blue)

• Gives ranges

How do we test for non-reducing sugars?

Negative result for Benedict’s test → acid hydrolysis test → Benedict’s test

• Non-reducing sugars have to be hydrolyzed into its reducing components

• Boil with dilute hydrochloric acid, then neutralize acidic content with sodium bicarbonate solution

What are polysaccharides and how are they formed?

Polymerization — Glycosidic bonds between numerous monosaccharides (formed via condensation)

What roles of polysaccharides are there?

Storage — Starch, glycogen

Structural — Cellulose

What is starch (location found, constituents)

• Stored in plant granules in chloroplasts and amyloplasts

• 20% amylose, 80% amylopectin

What is the function of starch?

• Respiratory substrate oxidized during cellular respiration to yield energy in the form of ATP

1. Maltose released ← hydrolysis of glycosidic bonds with amylase

2. Glucose released ← hydrolysis of maltose with maltase

How do you test for starch?

Iodine test

• Iodine dissolved in KI(aq) makes a soluble linear triiodide ion complex which fits into the centre of each amylose helix turn → Starch-iodide complex

• Starch-iodide complex gives a blue-black colouration

What are the differences between amylose and amylopectin? (monomers, bonds, no.of units/shape)

Amylose

• α-glucose monomers forming unbranched polymers

• Monomers linked by α(1-4) glycosidic bonds

• Helix with 200-20000 glucose units

Amylopectin

• α-glucose monomers forming branched polymers

• Monomers linked by α(1-4) glycosidic bonds within a branch and α(1-6) glycosidic bonds at branch points

• Helix with 10^6 glucose monomers and helical side chains attached at branch points

What is the structure of glycogen?

• Like amylopectin (helix) but even more extensively branched

  • Extensiveness allows for more ends for enzymes to work on

• Monomers linked by α(1-4) glycosidic bonds within a branch and α(1-6) glycosidic bonds at branch points

• Each residue is bent in one direction with respect to the previous residue

What is the function of glycogen?

• Animal storage polysaccharide

• Hydrolysis of glycogen mainly in liver and muscle cells give glucose

How do the structures of storage molecules (amylose, amylopectin, glycogen) determine their function? (Glucose residues, helices, branching)

• Many glucose residues

  • Large energy store → hydrolyzed by amylase (commonly available) → numerous monosaccharides, i.e. glucose to obtain ATP

  • Large molecule that is insoluble in water → does not affect WP of cells

• Helices

  • Compact

  • Intramolecular hydrogen bonding (projection of hydroxyl groups into the core of the helix) → fewer H groups available for H bonding with water → insoluble in water → does not affect WP of cells

• Branched (amylopectin, glycogen)

  • Multiple hydrolytic enzymes work on multiple branch ends at a time → increase energy generation per unit time

What is the structure of cellulose?

• β-glucose monomers linked via β(1-4) glycosidic bonds with adjacent monomers rotated 180º with respect to one another

• Long straight chains parallel to each other with hydroxyl groups projecting in both directions, held together by intermolecular hydrogen bonds

• Meshwork of criss-crossing microfibrils of cross-linked cellulose molecules, forming the cellulose cell wal

Describe the cellulose in the cell wall

• Porous structure due to gaps between microfibrils

  • Freely permeable to water and solutes → free movement of substances in and out of cells

• Meshwork distributes stresses in all directions

  • Strong and rigid structure

  • Protection from physical damage and bursting from osmotic stress

How do structures in cellulose contribute to its properties and functions as a structural molecule?

• Adjacent monomers rotated 180º with respect to one another → linear

• Hydroxyl groups of each molecule form H bonds with the OH groups of adjacent chains lying parallel to it → microfibrils

  • Microfibrils have high tensile strength

  • Cellulose is a macromolecule → insoluble in water

• Microfibrils have fewer OH groups available for H bonding ← only the surface of the microfibril is exposed to water + many OH groups are already involved in H bonds with OH groups from parallel cellulose molecules

  • Microfibrils are insoluble in water

Why is cellulose especially suitable for the structural function in plants?

Enzymes that hydrolyze the bonds are rarely found in nature and therefore cellulose is likely to remain intact