2. Biological Molecules

Question 1

How to test for reducing and non-reducing sugars?

Answer 1

• Heat a sample with Benedicts reagent in a water bath (if the solution remains blue there is no reducing sugar present)
• Heat a fresh sample in a water bath for 5 minutes with dilute HCI acid to hydrolyse the non-reducing sugar, then neutralise with sodium hydrogencarbonate and allow to cool
• Re-test the resulting solution by heating in a water bath with Benedicts reagent which will turn yellow/brown/red due to the reducing sugars made from the hydrolysis of the non-reducing sugar

Question 2

How to perform a semi-quantative Benedicts test on a reducing sugar?

(Can be used to estimate the approximate concentration of reducing sugars in a sample)

Answer 2

• A range of colour standards is produced by preparing a series of glucose solutions of a known concentration
• An excess of Benedicts reagent is added to the test tubes containing an equal volume of each
• They are then heated for the same length of time before being cooled to room temperature
• An equal volume of an unknown sample is then treated in the same way and the colour is compared with that of the colour standards
• Use a piece of white card placed behind the tubes to make the colours easier to see
• The test isn’t fully quantitative as you cannot be sure of the actual concentration of the unknown sample

Question 3

Further extension (carry out the reducing sugar test and then filter the suspensions)

Answer 3

• The precipitate is then dried and weighed (the greater the mass of precipitate, the more reducing sugar is present)
• Alternatively, the filtrate could be placed in a colorimeter (the more intense the blue colour, the less concentrated the reducing sugar)

Question 4

Monomer

Answer 4

a small molecule that can chemically bond with other monomers to form a larger molecule known as a polymer

Question 5

Polymer

Answer 5

a large molecule composed of similar repeating subunits called monomers. A polymer is a macromolecule, but not all macromolecules are polymers (lipids are polymers as they aren’t made up of repeating subunits). Formed by polymerization.

Question 6

Polymerization

Answer 6

monomers are chemically bonded together to form a large molecule, known as a polymer. This process occurs through repeated chemical reactions, linking the monomers together in a chain-like structure or in a network.

Question 7

Macromolecule

Answer 7

a large molecule, typically composed of thousands or even millions of atoms, that forms when smaller molecules (monomers) chemically bond together. Macromolecules are often polymers, meaning they consist of repeating units of monomers linked together through covalent bonds.

Question 8

Monosaccharide

Answer 8

monomers of disaccharides and polysaccharides

Question 9

Disaccharide

Answer 9

composed of 2 monosaccharides joined by a single glycosidic bond

Question 10

Polysaccharide

Answer 10

composed of more than 2 monosaccharides, with glycosidic bonds connecting adjacent monosaccharides. They are macromolecules, with some composed of many thousands of monosaccharides.

Question 11

Glycosidic bond

Answer 11

the bond formed between the anomeric carbon atom of one sugar molecule and a hydroxyl group of another molecule (often another sugar). This bond forms through a condensation reaction, where a water molecule is eliminated as the bond forms between the two molecules.

Question 12

Peptide bond

Answer 12

Covalent bond between amino acids. primary structure.

Question 13

Covalent bonding

Answer 13

atoms share a pair of electrons in their outer shells and as a result the outer shell of both atoms is filled and a more stable compound called a molecule is formed.

Question 14

Ionic bonding

Answer 14

ions with opposite charges attract one another and this is due to an electrostatic attraction. Ionic bonds are weaker than covalent bonds.

Question 15

Hydrogen bonding

Answer 15

occurs when a weak attractive force occurs between an electronegative atom of one molecule and a hydrogen of another molecule that is bonded to an electronegative atom. The electronegative ion has a tendency to attract electrons therefore giving the hydrogen a slightly positive charge. Hydrogen bonding causes water molecules to stick together (cohesion)

Question 16

Polar

Answer 16

in a covalent bond when one atom slightly attracts the shared electrons towards its nucleus so that even though the molecule has no overall charge, one atom has a slightly negative charge (delta negative) and the other a slightly positive charge (delta positive). Water is an example. They are hydrophilic.

Question 17

Non-polar

Answer 17

where there is an equal sharing of electrons in a covalent bond and is hydrophobic.

Question 18

Condensation reaction

Answer 18

reaction that produces water by removing it (formation of a polypeptide from amino acids and starch from glucose)

Question 19

Hydrolysis reaction

Answer 19

reaction that takes in water by adding it to break down a molecule into its constituent parts (polypeptides can be hydrolysed into amino acids and starch into glucose)

Question 20

Anabolism

Answer 20

an energy requiring process in which small molecules are combined to make larger ones.

Question 21

Catabolism

Answer 21

chemical reactions involving the release of energy in the breakdown of larger molecules into smaller ones.

Question 22

What is the role of covalent bonds in joining smaller molecules together to form polymers?

Answer 22

• Glycosidic bonds that form in carbohydrates
• Ester bonds in lipids
• Peptide and disulfide bonds in proteins
• Phosphodiester bonds in nucleic acids

Question 23

What are reducing sugars and what aren’t ?

Answer 23

Reducing sugars:
* Glucose
* Fructose
* Maltose
Non-reducing
* Sucrose
* Lactose

Question 24

How are glycosidic bonds formed by condensation?

Answer 24

• In disaccharides
• When 2 monosaccharides join, a water molecule is removed (condensation)
• The disaccharide maltose is produced in a condensation reaction between 2 a-glucose molecules and has reactive groups for the reduction reaction with Benedicts solution and so is therefore a reducing sugar
• In the formation of sucrose, the glycosidic bond is between carbon atom 1 of a-glucose and carbon atom 2 of B-fructose (known as 1,2 glycosidic bond)
• In polysaccharides
• The monosaccharides are joined by glycosidic bonds that are formed by condensation reactions
• The resulting chain may vary in length and be branched and folded in various ways

Question 25

Condensation

Answer 25

formation of a glycosidic bond by the removal of water

Question 26

How is a glycosidic bond broken by hydrolysis (refer to non-reducing sugar test)?

Answer 26

• In disaccharides
• When water is added to a disaccharide under suitable conditions, it breaks the glycosidic bond into its constituent monosaccharides (hydrolysis)
• Sucrose cannot react with Benedicts, so it is a non-reducing sugar. The hydrolysis of sucrose occurs rapidly in the presence of sucrase and without it the breakdown would be very slow.
• During hydrolysis by boiling with acid, the glycosidic bond is broken to release fructose and glucose, which are both reducing sugars
• In polysaccharides
• When they are hydrolysed they break down into monosaccharides or disaccharides

Question 27

Hydrolysis

Answer 27

breaking of glycosidic bond by addition of water

Question 28

What is the molecular structure of the polysaccharide starch (amylose and amylopectin) and how do their structures relate to their functions?

Answer 28

• Starch
• A polysaccharide found in many parts of a plant in the form of small granules or grains
• Major energy source in most diets
• As it is a polysaccharide it is a macromolecule which makes it insoluble (suits them for storage)
• As it is a polysaccharide it is formed by combining many monosaccharide units which are joined by glycosidic bonds that are formed by condensation reactions
• When they are hydrolysed they break down into monosaccharides or disaccharides
• It is a mixture of 2 polymers of a-glucose: amylose and amylopectin
• Amylose
• Composed of 200-5000 glucose units which are joined in a straight chain by a-1, 4 glycosidic bonds
• This chain is then wound into a tight helix which makes it more compact and therefore it can be stored more efficiently as it takes up less space
• Amylopectin
• Composed of 5000-100 000 a-glucose units which are joined to each other by a-1,4 and a-1,6 glycosidic bonds
• Branched and so has many free ends that amylase (the enzyme that catalyses the hydrolysis of starch) can work on simultaneously (meaning that glucose monomers are rapidly released)
• When hydrolyzed it forms glucose which is easily transported and readily used in respiration to provide ATP
• Amylose
• Composed of 200-5000 glucose units which are joined in a straight chain by a-1, 4 glycosidic bonds
• This chain is then wound into a tight helix which makes it more compact and therefore it can be stored more efficiently as it takes up less space
• Amylopectin
• Composed of 5000-100 000 a-glucose units which are joined to each other by a-1,4 and a-1,6 glycosidic bonds
• Branched and so has many free ends that amylase (the enzyme that catalyses the hydrolysis of starch) can work on simultaneously (meaning that glucose monomers are rapidly released)
• When hydrolyzed it forms glucose which is easily transported and readily used in respiration to provide ATP

Question 29

Molecular structure and function of glycogen?

Answer 29

• Glycogen
• Similar to amylopectin in structure, but has shorter chains and is more highly branched
• Major carbohydrate storage product of animals and it is stored as small granules (mainly in muscle and liver cells)
• Has a-1,4 and a-1,6- glycosidic bonds between monomers
• Its structure suits it for storage for the same reasons as starch, but it is more highly branched and so has more ends that can be simultaneously acted on by enzymes and is therefore more rapidly broken down to release energy

Question 30

What makes starch suited for its main role of energy storage?

Answer 30

• It is insoluble and therefore does not have any osmotic effects within cells and does not diffuse out of cells
• Starch molecules can be compactly stored within plant cells in structures like plastids (e.g., chloroplasts) and this compact storage allows plants to store a large amount of energy in a relatively small space
• it can be broken down into glucose molecules relatively easily when needed

Question 31

What is the molecular structure of the polysaccharide cellulose and how does its arrangement of cellulose molecules contribute to the function of plant cell walls?

Answer 31

• Makes up 20-50% of plant cell walls
• Made from monomers of b-glucose rather than a-glucose like starch and glycogen
• Rather than forming a coiled chain like starch, cellulose has straight, unbranched chains and these run parallel to one another, allowing hydrogen bonds to form cross-linkages between adjacent chains
• While each individual hydrogen bond adds very little strength to the molecule, the overall number of them makes a considerable contribution to strengthening cellulose
• The cellulose molecules are grouped to form microfibrils arranged in parallel groups called fibers.
• Within the microfibril, the individual linear cellulose molecules start and end in different places so that they overlap, and this contributes to the strength of the microfibril as well as the cellulose fibrils laid down at different angles.
• With many –OH groups, cellulose can form hydrogen bonds with water and so the molecule is hydrophilic, however, due to the large size of the molecules cellulose is insoluble

Question 32

What is function of cellulose?

Answer 32

• The cellulose cell wall is freely permeable, allowing materials to access the cell surface membrane and allowing the movement of water along the cell walls of adjacent cells
• Performs a mainly structural role by providing rigidity to the plant cell wall, which prevents the cell from bursting as water enters via osmosis
• The cellulose cell wall exerts an inward pressure that stops further influx of water and as a result living plant cells are turgid and push against one another, making herbaceous parts of the plant semi-rigid (important in maintaining stems and leaves in a turgid state so that they can provide the maximum surface area for photosynthesis)

Question 33

What is the molecular structure of triglycerides (refer to fatty acids, glycerol and the formation of ester bonds) and relate their structures to their function?

Answer 33

Triglycerides (fats and oils)
- Fats are solid at room temperature (10-20 degrees Celsius), whereas oils are liquid
- They have 3 fatty acids combined with a glycerol
- Each fatty acid forms an ester bond with glycerol in a condensation reaction and in a hydrolysis reaction a triglyceride produces glycerol and 3 fatty acids

The structure of triglycerides related to their functions
* High ratio of energy-storing carbon-hydrogen bonds to carbon atoms- excellent source of energy and they therefore supply many hydrogens for the reduction of NAD (molecule involved in the production of ATP)
* Low mass to volume ratio- makes them good storage molecules, because much energy can be stored in a small volume (beneficial for animals)
* Being large, non-polar molecules- they are insoluble in water and as a result their storage does not affect the water potential of cells
* High ratio of hydrogen to oxygen atoms- triglycerides therefore release water when oxidized and therefore provide an important source of water (important for organisms in dry deserts)