Why is water so important to life, and how do monosaccharides build the polysaccharides that store and structure cells?
2.1.2 Biological molecules: the properties of water and their importance to living organisms; the structure of monosaccharides, the formation of glycosidic bonds by condensation, and the structure and function of starch, glycogen and cellulose; the biochemical tests for reducing and non-reducing sugars and for starch.
A focused answer to the OCR H420 2.1.2 dot point on water and carbohydrates. Covers the biologically important properties of water, monosaccharides and condensation, the structure and function of starch, glycogen and cellulose, and the Benedict's and iodine tests.
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What this dot point is asking
OCR wants you to explain how the properties of water arise from its structure and why they matter biologically, describe how monosaccharides join by condensation to form di- and polysaccharides, relate the structure of starch, glycogen and cellulose to their functions, and carry out the biochemical tests for sugars and starch.
The answer
Water
Water is a polar molecule: oxygen pulls the shared electrons more strongly than hydrogen, so oxygen carries a slight negative charge and the hydrogens a slight positive charge. This polarity lets water molecules form hydrogen bonds with each other, which explains nearly every property OCR asks about.
- Solvent. Polar water dissolves ions and polar molecules, so it is the medium for metabolic reactions and for transport (in blood and xylem).
- High specific heat capacity. Hydrogen bonds resist temperature change, so cells and aquatic habitats stay thermally stable.
- High latent heat of vaporisation. Evaporating water takes a lot of heat, so sweating and transpiration are efficient cooling mechanisms.
- Cohesion and surface tension. Hydrogen bonds pull water molecules together, supporting the transpiration stream (cohesion-tension) and providing a surface for small organisms.
- Density of ice. Ice is less dense than liquid water and floats, insulating the water below so aquatic life survives in winter.
- Metabolic roles. Water is a reactant in hydrolysis and photosynthesis and a product of condensation and respiration.
Monosaccharides and glycosidic bonds
Monosaccharides are single sugar units (general formula a multiple of ): trioses (3 carbons), pentoses such as ribose (5 carbons) and hexoses such as glucose (6 carbons). Glucose exists as two isomers, alpha-glucose and beta-glucose, differing in the orientation of the hydroxyl group on carbon 1; that small difference drives the very different structures of starch and cellulose.
Two monosaccharides join by a condensation reaction, forming a glycosidic bond and releasing one water molecule. The reverse, hydrolysis, adds water to break the bond. Common disaccharides: maltose (glucose + glucose), sucrose (glucose + fructose) and lactose (glucose + galactose).
Polysaccharides
Starch (the plant energy store) is a mix of amylose and amylopectin, both polymers of alpha-glucose. Amylose has only 1,4 bonds and coils into a compact helix; amylopectin also has 1,6 bonds, making it branched. Starch is insoluble (so it does not affect water potential), compact and easily hydrolysed to release glucose.
Glycogen (the animal and fungal energy store) is also a polymer of alpha-glucose with 1,4 and 1,6 bonds, but it is more highly branched than amylopectin. The many branch ends allow rapid hydrolysis to glucose, suiting animals' higher metabolic rate.
Cellulose (the plant structural polysaccharide) is a polymer of beta-glucose. Alternate molecules are inverted so the chains are straight; many parallel chains hydrogen-bond into microfibrils of high tensile strength, ideal for cell walls.
The biochemical tests
- Reducing sugars (Benedict's test). Add Benedict's reagent and heat in a water bath. A positive result changes blue to green, yellow, orange or brick-red as the amount of reducing sugar increases (a semi-quantitative test).
- Non-reducing sugars. First confirm a negative Benedict's test, then hydrolyse a fresh sample with dilute hydrochloric acid, neutralise with sodium hydrogencarbonate, and repeat Benedict's. A colour change now indicates a non-reducing sugar.
- Starch (iodine test). Add iodine in potassium iodide solution. A colour change from orange-brown to blue-black confirms starch.
Examples in context
Example 1. Sweating and thermoregulation. Because water has a high latent heat of vaporisation, evaporating a small mass of sweat removes a large amount of heat, cooling the body efficiently during exercise. This links directly to homeostasis in Module 5.
Example 2. The transpiration stream. Cohesion between water molecules, caused by hydrogen bonding, lets a continuous column of water be pulled up the xylem under tension as water evaporates from the leaves, connecting this topic to transport in plants in Module 3.
Try this
Q1. Explain why ice floating on a pond is important for organisms living in the water. [2 marks]
- Cue. Ice is less dense than liquid water, so it floats and insulates the water below, which stays liquid so aquatic organisms survive.
Q2. Name the bond formed when two alpha-glucose molecules join, and the type of reaction involved. [2 marks]
- Cue. A 1,4 glycosidic bond, formed by a condensation reaction (releasing water).
Q3. State the colour change for a positive iodine test for starch. [1 mark]
- Cue. Orange-brown to blue-black.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H420/01 20184 marksCellulose and amylose are both polymers of glucose, yet they have very different functions. Explain how the structure of each molecule is related to its function.Show worked answer →
Compare the monomer and the bonding, then link to function (about 2 marks each).
Amylose is made of alpha-glucose joined by 1,4 glycosidic bonds. These bonds make the chain coil into a compact helix that is insoluble, so it is a good energy store: it holds many glucose units without affecting water potential and is easily hydrolysed when glucose is needed.
Cellulose is made of beta-glucose. Alternate molecules are flipped 180 degrees so the chains are straight, and many parallel chains form hydrogen bonds (cross-links) into microfibrils. This gives high tensile strength, ideal for plant cell walls that must resist turgor pressure.
Markers reward naming alpha versus beta glucose, the straight versus helical structure, and the link to either energy storage or mechanical strength.
OCR H420/01 20224 marksDescribe how you would use biochemical tests to show that a solution contains a non-reducing sugar (such as sucrose) but no reducing sugar.Show worked answer →
Two tests in sequence, with the key step of acid hydrolysis.
- Add Benedict's reagent and heat in a water bath. If the solution stays blue (no colour change), no reducing sugar is present.
- To a fresh sample, add dilute hydrochloric acid and heat to hydrolyse the non-reducing sugar into its monosaccharides, then neutralise with sodium hydrogencarbonate.
- Add Benedict's reagent and heat again. A colour change from blue to green, yellow, orange or brick-red now shows a reducing sugar was released, so the original sugar was a non-reducing sugar.
Markers reward the negative first test, the hydrolysis and neutralise step, and the positive second test.
Related dot points
- 2.1.2 Biological molecules: the structure and function of triglycerides and phospholipids; the structure of amino acids, the formation of peptide bonds and the four levels of protein structure; the structure of nucleotides, DNA and RNA; the biochemical tests for lipids (emulsion test) and proteins (biuret test).
A focused answer to the OCR H420 2.1.2 dot point on lipids, proteins and nucleic acids. Covers triglycerides and phospholipids, amino acids and the four levels of protein structure, nucleotide and DNA and RNA structure, and the emulsion and biuret tests.
- 2.1.4 Enzymes: the role of enzymes as biological catalysts in metabolic reactions; the mechanism of enzyme action including the lock-and-key and induced-fit models; the effects of temperature, pH, enzyme and substrate concentration on the rate of reaction; the action of competitive and non-competitive inhibitors; the roles of cofactors, coenzymes and prosthetic groups.
A focused answer to the OCR H420 2.1.4 dot point on enzymes. Covers enzymes as catalysts, the lock-and-key and induced-fit models, activation energy, the effects of temperature, pH and concentration, competitive and non-competitive inhibition, and cofactors.
- 2.1.1 Cell structure: the ultrastructure of eukaryotic and prokaryotic cells, the function of organelles including the role of the rough endoplasmic reticulum and Golgi apparatus in producing and secreting proteins; the use, calibration and resolution of light and electron microscopes.
A focused answer to the OCR H420 2.1.1 dot point on cell structure and microscopy. Covers every required eukaryotic and prokaryotic organelle, the protein secretory pathway, the three microscopes, eyepiece-graticule calibration and the magnification equation.
- 2.1.3 Nucleotides and nucleic acids: the semi-conservative replication of DNA and the roles of DNA helicase, DNA polymerase and the complementary base pairing rule; the nature of the genetic code as a triplet code that is degenerate and non-overlapping; the roles of mRNA and tRNA in protein synthesis.
A focused answer to the OCR H420 2.1.3 dot point on DNA replication and the genetic code. Covers semi-conservative replication, the roles of DNA helicase and DNA polymerase, the Meselson-Stahl evidence, and the triplet, degenerate, non-overlapping code with transcription and translation.
- 3.1.3 Transport in plants: the structure and function of xylem and phloem; the cohesion-tension theory of water transport in the xylem and the factors affecting transpiration; the mass flow hypothesis of translocation in the phloem from source to sink; and the adaptations of xerophytes for reducing water loss.
A focused answer to the OCR H420 3.1.3 dot point on transport in plants. Covers xylem and phloem structure, the cohesion-tension theory of transpiration, the factors affecting transpiration rate, the mass flow hypothesis of translocation, and xerophyte adaptations.
Sources & how we know this
- OCR A Level Biology A (H420) Specification — OCR (2023)