Biological molecules: carbohydrates

key molecules required to build structures that enable organisms to function

• carbohydrates

• proteins

• lipids

• nucleic Acids

• water

monomers

are the smaller units from which larger molecules are made

polymers

are molecules made from a large number of monomers joined together in a chain during a process called polymerisation

organic compounds

include carbohydrates, proteins, lipids and nucleic acids as they all contain the elements carbon (C) and hydrogen (H)

macromolecules

• are very large molecules

• They contain 1000 or more atoms, therefore having a high molecular mass

• Polymers can be macromolecules, however, not all macromolecules are polymers, as the subunits of polymers have to be the same repeating units

biological molecules - reactions

• Many biological reactions involve the formation of covalent bonds, which add strength and structure to a molecule

• Two important reactions occur to form covalent bonds in molecules:

  • Condensation reactions

  • Hydrolysis reactions

condensation reaction

A condensation reaction (also known as dehydration synthesis) occurs when monomers combine by covalent bonds to form polymers (during polymerisation) or macromolecules, and water is removed

hydrolysis reaction

• Hydrolysis means ‘lyse’ (to break) and ‘hydro’ (with water)

• In the hydrolysis of polymers, covalent bonds are broken when water is added

monosaccharides - carbohydrate

• Carbohydrates are one of the main carbon-based compounds in living organisms

• All molecules in this group contain carbon, hydrogen and oxygen

examples of carbohydrates

• monosaccharides

• disaccharides

• polysaccharides

monosaccharides

Single reducing sugar monomer

monosaccharides examples

Glucose

Fructose

Deoxyribose

disaccharide

A sugar formed from two monosaccharides joined by a glycosidic bond during a condensation reaction

disaccharide example

Maltose

Sucrose

Lactose

polysaccharide

A polymer formed from many monosaccharides joined by a glycosidic bond during a condensation reaction

polysaccharide examples

Cellulose

Starch

Glycogen

monosaccharides

• Monosaccharides are simple sugars

• These single units of sugars are monomers which join together to form more complex carbohydrates, such as disaccharides and polysaccharides. 

• Sugars can be classified as reducing or non-reducing; this classification is dependent on their ability to donate electrons

reducing sugars

• Reducing sugars can donate electrons (the carbonyl group becomes oxidised), and the sugars become the reducing agent

  • Thus, reducing sugars can be detected using Benedict’s reagent as they reduce the soluble copper sulphate to insoluble brick-red copper oxide

  •  Examples of reducing sugars include: glucose, fructose and galactose

    • Fructose and galactose have the same molecular formula as glucose however, they have a different structural formula

non-reducing sugars

• Non-reducing sugars cannot donate electrons, therefore, they cannot be oxidised

  • To be detected, non-reducing sugars must first be hydrolysed to break the disaccharide into its two monosaccharides before using Benedict’s reagent

  • Example: sucrose

what are the 2 forms of glucose

• In alpha glucose:

  • the hydroxyl (OH) group on carbon 1 is located below the ring

• In beta glucose:

  • the hydroxyl (OH) group on carbon 1 is located above the ring

forming the glycosidic bond

• To make monosaccharides more suitable for transport, storage and to have less influence on a cell’s osmolarity, they are bonded together to form disaccharides and polysaccharides

• Disaccharides and polysaccharides are formed when two hydroxyl (-OH) groups (on different saccharides) interact to create a strong covalent bond called the glycosidic bond

  • Each glycosidic bond is catalysed by enzymes specific to which OH groups are interacting

• Every glycosidic bond results in one water molecule being removed, thus, glycosidic bonds are formed by condensation

• As there are many different monosaccharides, this results in different types of glycosidic bonds forming (e.g. maltose has an α-1,4 glycosidic bond and sucrose has an α-1,2 glycosidic bond)

glycosidic bond in maltose

Maltose is a disaccharide formed by the condensation reaction of two glucose molecules

glycosidic bond in sucrose

Sucrose is a disaccharide formed by the condensation of a glucose molecule and a fructose molecule

glycosidic bond in lactose

Lactose is a disaccharide formed by the condensation of a glucose molecule and a galactose molecule

glycosidic bond in polysaccharides

• Polysaccharides are formed by the condensation of many glucose units

  • E.g. Glycogen and starch are formed by the condensation of α-glucose

  • E.g. Cellulose is formed by the condensation of β-glucose

breaking the glycosidic bond

• The glycosidic bond is broken when water is added in a hydrolysis (meaning ‘hydro’ - with water and ‘lyse’ - to break) reaction

  • Hydrolytic reactions are catalysed by enzymes, these are different to those present in condensation reactions

• Disaccharides and polysaccharides are broken down into smaller molecules in hydrolysis reactions

• Examples of hydrolytic reactions include the digestion of food in the alimentary tract and the breakdown of stored carbohydrates in muscle and liver cells for use in cellular respiration

disaccharides

• Monosaccharides can join together via condensation reactions to form disaccharides

  • The new chemical bond that forms between two monosaccharides is known as a glycosidic bond

common examples of disaccharides

• maltose (the sugar formed in the production and breakdown of starch)

• sucrose (the main sugar produced in plants)

• lactose (a sugar found only in milk)

• All three of the common examples above have the formula C₁₂H₂₂O₁₁, but are comprised of different monomers

starch and glycogen structure

• Starch and glycogen are polysaccharides

• Polysaccharides are macromolecules formed by many monosaccharides joined by glycosidic bonds in a condensation reaction to form long chains. These chains may be:

  • branched or unbranched

  • folded (making the molecule compact, which is ideal for storage, e.g. starch and glycogen)

  • straight (making the molecules suitable to construct cellular structures, e.g. cellulose) or coiled

starch and glycogen functions

• Starch and glycogen are storage polysaccharides because they are:

  • compact (so large quantities can be stored)

  • insoluble (so will have no osmotic effect, unlike glucose, which would lower the water potential of a cell)

starch

• Starch is the storage polysaccharide of plants. It is stored as granules in plastids (e.g. chloroplasts)

• Due to the many monomers in a starch molecule, it takes longer to digest than glucose

• Starch is constructed from two different polysaccharides:

  • Amylose

  • Amylopectin

amylose

• Amyllose comprises 10 - 30% of starch

• It has an unbranched helix-shaped chain with 1,4 glycosidic bonds between α-glucose molecules

• The helix shape enables it to be more compact, and thus it is more resistant to digestion

amylopectin

• Amylopectin is70 - 90% of starch

• It has 1,4 glycosidic bonds between α-glucose molecules, but also 1,6 glycosidic bonds form between glucose molecules, creating a branched molecule

• The branches result in many terminal glucose molecules that can be easily hydrolysed for use during cellular respiration or added to for storage

glycogen

• Glycogen is the storage polysaccharide of animals and fungi; it is highly branched and not coiled

• Liver and muscle cells have a high concentration of glycogen, present as visible granules, as the cellular respiration rate is high in these cells (due to animals being mobile)

• Glycogen is more branched than amylopectin, making it more compact, which helps animals store more

• The branching enables more free ends where glucose molecules can either be added or removed, allowing for condensation and hydrolysis reactions to occur more rapidly, thus the storage or release of glucose can suit the demands of the cell

cellulose structure

• Cellulose consists of long chains of the monomer β-glucose, joined together by 1,4 glycosidic bonds

• As β-glucose is an isomer of α-glucose, to form the 1,4 glycosidic bonds, consecutive β-glucose molecules must be rotated 180° to each other

• Due to the inversion of the β-glucose molecules, many hydrogen bonds form between the long chains, giving cellulose its strength

cellulose function

• Cellulose is the main structural component of cell walls due to its strength, which is a result of the many hydrogen bonds found between the parallel chains of microfibrils

• The high tensile strength of cellulose allows it to be stretched without breaking, which makes it possible for cell walls to withstand turgor pressure

• The cellulose fibres and other molecules (e.g., lignin) found in the cell wall form a matrix which increases the strength of the cell walls

• The strengthened cell walls provide support to the plant

• Cellulose fibres are freely permeable, which allows water and solutes to leave or reach the cell surface membrane

• As few organisms have the enzyme (cellulase) to hydrolyse cellulose, it is a source of fibre