Biological Molecules: Carbohydrates (AQA A Level Biology): Flashcards

Covalent bonds add strength and structure to biological molecules, making them stable for the functions of life.

A condensation reaction is a chemical process where two monomers join together by forming a covalent bond and releasing water.

During condensation reactions, monomers are joined together and water is removed.

False.

Hydrolysis reactions involve the addition of water to break covalent bonds.

A hydrolysis reaction is a chemical process where covalent bonds in a polymer are broken by the addition of water.

A hydrolysis reaction breaks a covalent bond by adding water.

Condensation reactions form biological molecules such as carbohydrates, proteins, lipids and nucleic acids.

In hydrolysis, a polymer plus water forms monomers.

A monosaccharide is a simple sugar that acts as a single monomer for more complex carbohydrates.

All carbohydrates contain carbon, hydrogen and oxygen.

The three main types of carbohydrates are monosaccharides, disaccharides and polysaccharides.

A disaccharide is a sugar formed when two monosaccharides join through a glycosidic bond in a condensation reaction.

A polysaccharide is a polymer made from many monosaccharides joined by glycosidic bonds during condensation reactions.

A reducing sugar is a sugar that can donate electrons, causing the carbonyl group to become oxidised. This property allows it to reduce Benedict’s reagent.

False.

Sucrose is a non-reducing sugar. It cannot donate electrons and must be hydrolysed before it can be detected by Benedict’s reagent.

Glucose, fructose and galactose are all examples of reducing sugars.

Monosaccharides join together to form disaccharides and polysaccharides.

Glucose is the most common monosaccharide and an important simple sugar monomer with the molecular formula C₆H₁₂O₆.

The molecular formula of glucose is C₆H₁₂O₆.

Glucose exists as two isomers: α-glucose and β-glucose.

In α-glucose, the hydroxyl (OH) group on carbon 1 is located below the ring.

In β-glucose, the hydroxyl (OH) group on carbon 1 is located above the ring.

True.

Both starch and glycogen are polysaccharides formed from α-glucose.

Cellulose is a polysaccharide formed from β-glucose.

The difference is the position of the hydroxyl (OH) group on carbon 1: in α-glucose it is below the ring, while in β-glucose it is above the ring.

Cellulose is formed from β-glucose and not from α-glucose.

A glycosidic bond is a strong covalent bond formed between two monosaccharides during a condensation reaction, joining them into a disaccharide or polysaccharide.

A condensation reaction forms a glycosidic bond between monosaccharides by removing a water molecule.

Disaccharides and polysaccharides are formed when monosaccharides are joined by a glycosidic bond.

A condensation reaction is a chemical process in which two molecules are joined together with the removal of a water molecule.

Maltose is formed by the condensation of two glucose molecules.

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

Lactose is formed by the condensation of glucose and galactose molecules.

Cellulose is a polysaccharide formed by the condensation of many β-glucose units.

Hydrolysis is a reaction in which a water molecule is added to break a glycosidic bond between saccharide units.

False.

Polysaccharides must undergo many hydrolytic reactions and require multiple water molecules to be broken down into monosaccharides.

The breakdown of a glycosidic bond by adding water is known as hydrolysis.

During hydrolysis, a disaccharide is split into two monosaccharides by the addition of a water molecule.

Condensation reactions form glycosidic bonds between monosaccharides by removing water, while hydrolysis reactions break glycosidic bonds by adding water.

Chromatography is a technique used to separate a mixture into its components based on their differing solubilities.

Chromatography separates components based on their differences in solubility in the mobile phase.

All chromatography techniques use two phases: the mobile phase and the stationary phase.

True.

Components with higher solubility in the mobile phase spend more time in it and are carried further, resulting in greater travel distance.

Paper chromatography is a form of chromatography where the stationary phase is chromatography paper and the mobile phase is a liquid solvent.

In paper chromatography, the stationary phase is the chromatography paper, and the mobile phase is the solvent.

Larger molecules move more slowly because they interact more with the stationary phase, so they do not travel as far as smaller molecules.

You can identify unknown monosaccharides by comparing the distance travelled by their spots to those of known standard solutions on the same chromatogram.

A disaccharide is a sugar molecule formed when two monosaccharides join together by a glycosidic bond.

A glycosidic bond is the chemical bond formed between two monosaccharide molecules during a condensation reaction.

The three common disaccharides are maltose, sucrose and lactose.

Maltose is formed from two α-glucose molecules joined together.

Sucrose is formed from α-glucose and fructose monosaccharides.

Lactose is the disaccharide found in milk.

True.

Maltose, sucrose, and lactose all have the formula C₁₂H₂₂O₁₁, but are made from different monosaccharide units.

Glucose, galactose, and fructose all have the same molecular formula but their atoms are arranged differently, giving them different structures and properties.

A polysaccharide is a macromolecule formed by many monosaccharides joined by glycosidic bonds in a condensation reaction to form long chains.

Starch and glycogen are compact and insoluble, making them ideal for storage as they can be stored in large quantities and do not have an osmotic effect.

False.

Amylose and amylopectin are the two components of starch, while glycogen is a separate polysaccharide found in animals and fungi.

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

Glycogen is the storage polysaccharide found in animals and fungi, characterised by being highly branched and compact for efficient storage and rapid glucose release.

Glycogen contains 1,4 and 1,6 glycosidic bonds between α-glucose molecules, making it more branched than amylopectin.

Starch is made up of the polysaccharides amylose and amylopectin.

Starch is stored as granules in plastids (e.g. chloroplasts) in plant cells.

Cellulose is a polysaccharide found in plant cells, consisting of long chains of β-glucose joined by 1,4 glycosidic bonds.

Cellulose is formed from the monomer β-glucose.

In cellulose, long chains of β-glucose are joined by 1,4 glycosidic bonds to form strong fibres.

Every alternate β-glucose molecule is rotated 180° to allow the formation of 1,4 glycosidic bonds between them.

The strength of cellulose comes from many hydrogen bonds between parallel chains.

The main function of cellulose is to provide structural support to plant cell walls due to its high strength.

Cellulose fibres are freely permeable, allowing water and solutes to pass through the cell wall.

False.

Few organisms can digest cellulose because most lack the enzyme cellulase, so cellulose acts as a source of fibre.

A reducing sugar is a sugar that can donate electrons to another chemical, such as copper (II) ions in Benedict’s reagent, and cause reduction reactions.

A brown or brick-red precipitate indicates a high concentration of reducing sugar in Benedict’s test.

Benedict's reagent contains copper (II) sulfate, which is reduced by reducing sugars to form a coloured precipitate.

A non-reducing sugar is a sugar that cannot donate electrons and therefore does not react directly with Benedict’s reagent.

The sample must be hydrolysed with dilute hydrochloric acid and then neutralised with sodium hydrogencarbonate before adding Benedict’s reagent.

The addition of dilute hydrochloric acid to a sample will hydrolyse any glycosidic bonds present in carbohydrates.

A blue-black colour appears when starch is present after adding iodine in potassium iodide solution.

Iodine is dissolved in potassium iodide solution because it is insoluble in water.

Benedict's reagent is a chemical solution used to identify the presence of reducing sugars such as glucose via a semi-quantitative colour change reaction.

The colour changes from blue → green → yellow → orange → brick-red as the concentration of reducing sugar increases.

The Benedict’s test works because reducing sugars reduce blue copper(II) sulfate to brick-red copper(I) oxide.

A semi-quantitative method is a procedure that provides an approximate measurement of a substance, such as glucose, rather than an exact value, often by comparing colour intensity.

To make serial dilutions, you add equal volumes of distilled water to each test tube, then transfer a known volume of the stock solution to the first tube and mix.

Consistent dilution steps ensure that each solution's concentration is known and can be used accurately for calibration curves or comparisons.

A calibration curve is a graph plotting absorbance or transmission against known concentrations of a substance, used to estimate unknown concentrations.

True.

A colorimeter measures absorbance or transmission of light, reducing subjective interpretation and increasing reliability.