Biological Molecules: Carbohydrates - Comprehensive Notes

Biological Molecules: Carbohydrates

Key Terms

  • The biochemical basis of life is similar for all living things.
  • Key molecules required to build structures for organisms:
    • Carbohydrates
    • Proteins
    • Lipids
    • Nucleic Acids
    • Water
  • Monomers: Smaller units that make up larger molecules.
  • Polymers: Molecules made of a large number of monomers joined together.

Organic Compounds

  • Key biological molecules (carbohydrates, proteins, lipids, nucleic acids) contain carbon (C) and hydrogen (H).
  • Carbon atoms are essential due to:
    • Each carbon atom forming four covalent bonds, providing high stability as they require significant energy to break.
    • Ability to form covalent bonds with oxygen, nitrogen, and sulfur.
    • Ability to bond in straight chains, branched chains, or rings.
    • Ability to form small single subunits (monomers) that bond with repeating subunits to form large molecules (polymers) through polymerization.

Macromolecules and Polymers

  • Macromolecules: Very large molecules containing 1000 or more atoms, thus having a high molecular mass.
  • Polymers can be macromolecules, but not all macromolecules are polymers as subunits must be repeating units.

Carbohydrates

  • One of the main carbon-based compounds in living organisms.
  • Contain C, H, and O.
  • H and O atoms are always present in a 2:1 ratio, represented by the formula Cx(H2O)_y.
  • Three types: monosaccharides, disaccharides, and polysaccharides.

Lipids

  • Lipid molecules are not made from monomers or polymers as each fatty acid joins to a glycerol molecule, rather than to each other.
  • Separate lipid molecules, such as triglycerides, are not held together by covalent bonds and therefore lipids cannot be classed as polymers.

Biological Molecules: Reactions

Covalent Bonds

  • Covalent bond: Sharing of two or more electrons between two atoms.
  • Electrons can be shared equally (nonpolar covalent bond) or unequally (polar covalent bond).
  • Atoms form a certain number of covalent bonds based on free electrons in the outer orbital (e.g., H = 1 bond, C = 4 bonds).
  • Covalent bonds are very stable due to high energy requirements to break them.
  • Multiple pairs of electrons can be shared forming double bonds (e.g. unsaturated fats C=C) or triple bonds

Polymerization

  • When monomers are close enough, their outer orbitals overlap, resulting in electron sharing and covalent bond formation.
  • Addition of more monomers leads to polymerization and/or macromolecule formation.

Condensation (Dehydration Synthesis)

  • Monomers combine via covalent bonds to form polymers (polymerization) or macromolecules (lipids), and water is removed.

Hydrolysis

  • ‘Lyse’ (to break) and ‘hydro’ (with water).
  • Covalent bonds are broken when water is added.

Monosaccharides

Reducing vs. Non-Reducing Sugars

  • Sugars are classified based on their ability to donate electrons.
  • Reducing sugars: Can donate electrons (carbonyl group becomes oxidized), acting as reducing agents.
  • Non-reducing sugars: Cannot donate electrons unless hydrolyzed.

Benedict’s Test

  • Used to detect reducing sugars; they reduce soluble copper sulphate to insoluble brick-red copper oxide.

Examples of Reducing Sugars

  • Glucose, fructose, and galactose.
  • Fructose and galactose share the same molecular formula as glucose but have different structural formulas, leading to slightly different properties.

Non-Reducing Sugars

  • Must be hydrolyzed into monosaccharides before Benedict’s test.
  • Example: Sucrose.

Glucose

Molecular Formula & Importance

  • Molecular formula: C6H{12}O_6
  • Most common monosaccharide, central to most life forms.

Types of Monosaccharides

  • Varying numbers of carbon atoms:
    • Trioses (3C): glyceraldehyde
    • Pentoses (5C): ribose
    • Hexoses (6C): glucose

Isomers of Glucose

  • Exists in two structurally different forms: alpha (α) glucose and beta (β) glucose.
  • This structural variety results in different functions between carbohydrates
  • Different polysaccharides are formed from the two isomers of glucose.

The Glycosidic Bond

Formation

  • Monosaccharides bond together to form disaccharides and polysaccharides to make them more suitable for transport, storage and to have less influence on a cell’s osmolarity.
  • Formed when two hydroxyl (-OH) groups (on different saccharides) interact to form a strong covalent bond.
  • Oxygen link that holds molecules together.
  • Every glycosidic bond formation results in the removal of one water molecule (condensation).

Types of Glycosidic Bonds

  • Catalyzed by enzymes specific to interacting OH groups.
  • Different types form due to many different monosaccharides (e.g., maltose has α-1,4 glycosidic bond, sucrose has α-1,2 glycosidic bond).

Breaking Glycosidic Bonds (Hydrolysis)

  • Occurs when water is added in a hydrolysis reaction (hydro - with water, lyse - to break).
  • Disaccharides and polysaccharides are broken down in hydrolysis reactions.
  • Hydrolytic reactions are catalyzed by enzymes different from those in condensation reactions.
  • Examples: digestion of food in the alimentary tract, breakdown of stored carbohydrates in muscle and liver cells.
  • Sucrose, a non-reducing sugar, gives a negative result in Benedict's test, but when heated with hydrochloric acid, hydrolysis occurs, yielding monosaccharides that produce a positive Benedict's test.

Chromatography: Monosaccharides

Basics of Chromatography

  • Technique to separate a mixture into individual components based on differences in solubility.
  • Involves two phases:
    • Mobile phase
    • Stationary phase
  • Components separate as the mobile phase travels over the stationary phase.
  • Differences in solubility affect how far each component travels; higher solubility components travel further.

Paper Chromatography

  • Specific form of chromatography.
    • Mobile Phase: solvent (liquid) in which sample molecules move (e.g., water or ethanol).
    • Stationary Phase: chromatography paper.

Method of Paper Chromatography

  • A spot of the mixture is placed on chromatography paper and left to dry.
  • The chromatography paper is then suspended in a solvent
  • As the solvent travels up through the chromatography paper, the different components move up the paper at different speeds. Larger molecules move slower than smaller ones
  • This causes the original mixture to separate out into different spots or bands on the chromatography paper

Separating Monosaccharides

  • Paper chromatography can separate a mixture of monosaccharides.
  • Colored molecules don't need staining, but colorless molecules (like monosaccharides) do.
  • Spots of known standard solutions and the sample mixture are placed on the chromatography paper.
  • The solvent travels up the paper, separating the monosaccharides.
  • Unknown monosaccharides are identified by comparing their distances from the line with those of known standards.

Terminology

  • Paper chromatography: the name given to the overall separation technique
  • Chromatogram: the name given to the visual output of a chromatography run

Disaccharides

Formation

  • Monosaccharides join via condensation reactions to form disaccharides.
  • A condensation reaction is one in which two molecules join together via the formation of a new chemical bond, with a molecule of water being released in the process
  • The new chemical bond thatforms between two monosaccharides is known as a glycosidic bond
  • To calculate the chemicalformula of a disaccharide, you add allthe carbons, hydrogens and oxygens in both monomers then subtract 2x H and 1x O (forthe water molecule lost)

Common Examples

  • Maltose (sugar formed in starch production and breakdown)
  • Sucrose (main sugar produced in plants)
  • Lactose (sugar found only in milk)
  • All have the formula C{12}H{22}O_{11}

Monosaccharide Subunits

  • The disaccharide maltose is formed from two α-glucose monomers (sub-units)
  • The disaccharide sucrose is formed from α-glucose and fructose monomers (sub-units)
  • Like glucose, galactose and fructose are monosaccharides and actually have the same molecularformula as glucose. However, the atoms that make up these three monosaccharides are arranged in di erent ways, meaning they each have slightly di erent molecular structures, giving them slightly di erent properties.

Starch & Glycogen

Polysaccharides

  • Macromolecules formed by many monosaccharides linked by glycosidic bonds in a condensation reaction.
  • Chains can be branched or unbranched, folded or straight/coiled.

Storage Polysaccharides

  • Starch and glycogen are storage polysaccharides because they are:
    • Compact (so large quantities can be stored)
    • Insoluble (so will have no osmotic e ect, unlike glucose which would lowerthe water potential of a cell causing waterto move into cells, plant cells would then have to have thicker cell walls, and animal cells would burst underthe increased pressure)

Starch

  • Storage polysaccharide in plants, stored as granules in plastids (e.g., chloroplasts).
  • Takes longer to digest than glucose.
  • Composed of two different polysaccharides:
    • Amylose (10-30% of starch):
      • Unbranched helix-shaped chain with 1,4 glycosidic bonds between α-glucose molecules.
      • The helix shape enables itto be more compact and thus itis more resistantto digestion
    • Amylopectin (70-90% of starch):
      • 1,4 glycosidic bonds between α-glucose molecules, with 1,6 glycosidic bonds forming branches.
      • Branches provide many terminal glucose molecules for easy hydrolysis during cellular respiration or storage.

Glycogen

  • Storage polysaccharide in animals and fungi, highly branched and not coiled.
  • High concentration in liver and muscle cells as visible granules.
  • More branched than amylopectin, making it more compact.
  • Branching allows rapid glucose addition or removal to meet cellular demands.

Cellulose

Structure

  • Polysaccharide; polymer of long chains of β-glucose joined by 1,4 glycosidic bonds.
  • Consecutive β-glucose molecules are rotated 180° to each other.
  • Many hydrogen bonds form between long chains, providing strength.

Function

  • Main structural component of cell walls due to its strength from many hydrogen bonds.
  • High tensile strength allows it to be stretched without breaking, enabling cell walls to withstand turgor pressure.
  • Cellulose fibers and other molecules (e.g., lignin) form a matrix that increases cell wall strength.
  • Cellulose bres are freely permeable which allows water and solutes to leave or reach the cell surface membrane
  • Few organisms have the enzyme (cellulase)to hydrolyse cellulose itis a source of bre

Biochemical Tests: Sugars & Starch

Qualitative Tests

  • Determine the presence of a certain type of sugar, but do not quantify the amount.

Reducing vs. Non-Reducing Sugars

  • Sugars are classi ed as reducing or non-reducing;this classi cation is dependent on their ability to donate electrons (a reducing sugarthatis able to donate electrons is itself oxidised)
  • Reducing sugar donates electrons (is oxidized).
  • Non-reducing sugar cannot donate electrons unless hydrolyzed.

Benedict’s Test for Reducing Sugars

  • Benedict’s reagent (blue solution with copper (II) sulfate ions) reacts with reducing sugars to form copper (I) oxide.
  • Copper (I) oxide is insoluble and forms a precipitate.
  • Method:
    • AddBenedict's reagent (which is blue as it contains copper (II) sulfate ions)to a sample solution in a testtube
    • Heat the testtube in a water bath or beaker of waterthat has been broughtto a boilfor a few minutes
    • If a reducing sugaris present, a coloured precipitate willform as copper (II) sulfate is reduced to copper (I) oxide which is insoluble in water
    • A positive testresultis a colour change somewhere along a colour scale from blue (no reducing sugar), through green, yellow and orange (low to medium concentration of reducing sugar)to brown/brick-red (a high concentration ofreducing sugar)
  • Itis importantthat an excess ofBenedict’s solution is used so thatthere is more than enough copper (II) sulfate presentto react with any sugar present
  • Semi-quantitative test; degree of color change indicates the concentration of reducing sugar.
  • Reducing sugars include galactose, glucose, fructose, and maltose. Sucrose is a non-reducing sugar.

Test for Non-Reducing Sugars

  • Add dilute hydrochloric acid to the sample and heatin a water bath that has been broughtto the boil
  • Neutralise the solution with sodium hydrogencarbonate
  • Use a suitable indicator (such as red litmus paper)to identify when the solution has been neutralised, and then add a little more sodium hydrogencarbonate as the conditions need to be slightly alkaline fortheBenedict’s testto work
  • Then carry outtheBenedict’s test as normal; addBenedict’s reagentto the sample and heatin a water bath that has been boiled – if a colour change occurs, a reducing sugar is present.
  • Explanation:
    • The addition of acid will hydrolyse any glycosidic bonds presentin any carbohydrate molecules
    • The resulting monosaccharides left will have an aldehyde or ketone functional group that can donate electrons to copper (II) sulfate (reducing the copper), allowing a precipitate to form

Test for Starch

  • Add iodine in potassium iodide solution to the sample.
  • If starch is present, a blue-black color develops as iodide ions interact with starch molecules.

Finding the Concentration of Glucose

Semi-Quantitative Test

  • Benedict’s solution is used to estimate reducing sugar concentration.
  • Intensity of color change correlates with reducing sugar concentration.
  • A positive testis indicated along a spectrum of colourfrom green (low concentration)to brick-red (high concentration ofreducing sugar present)
  • Method:
    • Set up standard solutions with known glucose concentrations using serial dilutions of an existing stock solution
    • Treat each solution identically: add the same volume ofBenedict’s solution to each sample and heatin a water bath that has been boiled (ideally atthe same temperature each time)for a settime (5 minutes or so)to allow colour changes to occur
    • The same procedure is carried out on a sample with an unknown concentration of reducing sugar which is then compared to the stock solution colours to estimate the concentration ofreducing sugar present

Alterations

  • Time how long ittakes forthe rst colour change to occur (blue to green)
  • A colorimeter could be used to measure the absorbance ortransmission oflightthrough the sugar solutions of known concentration to establish a range of values that an unknown sample can be compared against a calibration curve

Serial Dilutions

  • Created by taking a series of dilutions of a stock solution
  • The concentration decreases by the same quantity between each testtube
  • They can either be ‘doubling dilutions’ (where the concentration is halved between each testtube) or a desired range (e.g. 0, 2, 4, 6, 8, 10 mmol dm )
  • Serial dilutions are completed to create a standard to compare unknown concentrations against
  • They can be used when:
    • Counting bacteria or yeast populations
    • Determining unknown glucose, starch, protein concentrations

Using a Colorimeter

  • Instrument that beams a specific wavelength (color) of light through a sample and measures light absorbance.
  • Colour lters are used to controlthe light wavelength emitted
  • A solution thatis orange/green will absorb less blue lightthan a solution thatis brick red
  • Absorbance value provides a quantitative measure of the orange colour.
  • Calibration is essential before measurements; a blank should read 0 absorbance.
  • A calibration curve (absorbance vs. known concentrations) is plotted.
  • Unknown concentrations are determined from the calibration curve.