Biological Molecules: Carbohydrates

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Biological Molecules: Key Terms

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 containing 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.
A condensation reaction (also known as dehydration synthesis) occurs when monomers combine by covalent bonds to form polymers or macromolecules, and water is removed.
Hydrolysis means ‘lyse’ (to break) and ‘hydro’ (with water); in the hydrolysis of polymers, covalent bonds are broken when water is added.
Covalent bonds in organic molecules are formed during condensation reactions and broken during hydrolysis reactions.

Monosaccharides

Carbohydrates are one of the main carbon-based compounds in living organisms; all molecules in this group contain carbon, hydrogen and oxygen.
The three types of carbohydrates are monosaccharides, disaccharides and polysaccharides.
A monosaccharide is a single reducing sugar monomer; examples include glucose, fructose and deoxyribose.
A disaccharide is a sugar formed from two monosaccharides joined by a glycosidic bond during a condensation reaction; examples include maltose, sucrose and lactose.
A polysaccharide is a polymer formed from many monosaccharides joined by glycosidic bonds during condensation reactions; examples include cellulose, starch and glycogen.
Reducing sugars can donate electrons (the carbonyl group becomes oxidised), and the sugars become the reducing agent; they can be detected using Benedict’s reagent as they reduce 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 but a different structural formula.
Non-reducing sugars cannot donate electrons, therefore they cannot be oxidised; to be detected, they must first be hydrolysed to break the disaccharide into its two monosaccharides before using Benedict’s reagent. Example: sucrose.

Glucose

Glucose has the molecular formula C₆H₁₂O₆ and is the most common monosaccharide, of central importance to most forms of life.
Glucose exists in two structurally different forms – α (α) glucose and β (β) glucose; these structural forms are known as isomers of 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.
Different polysaccharides are formed from the two isomers: starch and glycogen are formed from α-glucose, while cellulose is formed from β-glucose.

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.
Different types of glycosidic bonds form (e.g. maltose has an α-1,4 glycosidic bond and sucrose has an α-1,2 glycosidic bond).
Maltose is a disaccharide formed by the condensation reaction of two glucose molecules.
Sucrose is a disaccharide formed by the condensation of a glucose molecule and a fructose molecule.
Lactose is a disaccharide formed by the condensation of a glucose molecule and a galactose molecule.
Polysaccharides are formed by the condensation of many glucose units: glycogen and starch from α-glucose, cellulose from β-glucose.
The glycosidic bond is broken when water is added in a hydrolysis reaction; hydrolytic reactions are catalysed by enzymes different to those in condensation reactions.
Disaccharides and polysaccharides are broken down into smaller molecules in hydrolysis reactions; examples include digestion of food in the alimentary tract and breakdown of stored carbohydrates in muscle and liver cells for use in cellular respiration.

Chromatography: Monosaccharides

Chromatography is a technique that can be used to separate a mixture into its components, relying on differences in the solubility of the different chemicals (solutes) within a mixture.
All chromatography techniques use two phases: the mobile phase and the stationary phase; components separate as the mobile phase travels over the stationary phase.
Components with higher solubility travel further because they spend more time in the mobile phase and are carried further up the paper than less soluble components.
In paper chromatography, the mobile phase is the solvent (e.g. water or ethanol) and the stationary phase is the chromatography paper.
Method: a spot of the mixture is placed on chromatography paper and left to dry; the paper is suspended in a solvent; as the solvent travels up, components move at different speeds; larger molecules move more slowly than smaller ones; this produces a chromatogram.
Paper chromatography can be used to separate a mixture of monosaccharides; mixtures of colourless molecules, such as monosaccharides, have to be stained first.
Spots of known standard solutions of different monosaccharides are placed beside the sample spot; the unknown monosaccharides can then be identified by comparing and matching them with the chromatograms of the known standard solutions.
If a spot from the monosaccharide sample mixture is at the same distance from the line as a spot from one of the known standard solutions, then the mixture must contain this monosaccharide.

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 include maltose (the sugar formed in the production and breakdown of starch), sucrose (the main sugar produced in plants) and lactose (a sugar found only in milk).
All three common disaccharides have the formula C₁₂H₂₂O₁₁, but are comprised of different monomers.
Maltose is made from α-glucose and α-glucose.
Sucrose is made from α-glucose and fructose.
Lactose is made from α-glucose and galactose.

Starch & Glycogen

Starch and glycogen are polysaccharides – macromolecules formed by many monosaccharides joined by glycosidic bonds in a condensation reaction to form long chains that may be branched or unbranched, folded, straight or coiled.
Starch and glycogen are storage polysaccharides because they are compact (so large quantities can be stored) and insoluble (so will have no osmotic effect, unlike glucose, which would lower the water potential of a cell).
Starch is the storage polysaccharide of plants; it is stored as granules in plastids (e.g. chloroplasts) and takes longer to digest than glucose due to the many monomers.
Starch is constructed from two different polysaccharides: amylose and amylopectin.
Amylose 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 more resistant to digestion.
Amylopectin is 70–90% 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 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.
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.
Comparison: Amylose – monomer α-glucose, unbranched, helix shape, 1,4 glycosidic bonds, source plant. Amylopectin – monomer α-glucose, branched, no helix, 1,4 and 1,6 glycosidic bonds, source plant. Glycogen – monomer α-glucose, branched, no helix, 1,4 and 1,6 glycosidic bonds, source animal.

Cellulose

Cellulose is a polysaccharide found in plant cells; it 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 is used as a structural component due to the strength it has from the many hydrogen bonds that form between the long chains of β-glucose molecules.
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.

Biochemical Tests: Sugars & Starch

Several tests can be carried out to determine if a sample contains a certain type of sugar; the following tests are qualitative – they do not give a quantitative value as to how much of each type of molecule may be present in a sample.
Sugars can be classified as reducing or non-reducing; this classification is dependent on their ability to donate electrons.
Test for reducing sugars: Benedict’s reagent is a blue solution that contains copper (II) sulfate ions (CuSO₄); in the presence of a reducing sugar, copper (I) oxide forms, which is not soluble in water, so it forms a precipitate.
Benedict's test method: add Benedict's reagent to a sample solution in a test tube (an excess must be used); heat the test tube in a water bath or a beaker of water that has been brought to a boil for a few minutes; if a reducing sugar is present, a coloured precipitate will form.
A positive test result is 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 of reducing sugar).
This test is semi-quantitative as the degree of the colour change can indicate the concentration of reducing sugar present.
Test for non-reducing sugars: non-reducing sugars are tested differently because they lack the chemical groups necessary for reduction reactions; they cannot directly react with oxidising agents like those in Benedict's test.
The addition of an acid will hydrolyse any glycosidic bonds present in 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.
Method for non-reducing sugars: add dilute hydrochloric acid to the sample and heat in a water bath that has been brought to 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 for Benedict’s test to work); then carry out the Benedict’s test as normal.
Test for starch: add a few drops of orange/brown iodine in potassium iodide solution to the sample; if starch is present, iodide ions in the solution interact with the centre of starch molecules, producing a complex with a distinctive blue-black colour.
The iodine test for starch is useful for determining the digestion of starch.

Finding the Concentration of Glucose

Benedict’s reagent is used to identify the presence of reducing sugars, such as glucose, through a semi-quantitative method.
The test relies on a colour change that occurs when reducing sugars reduce blue copper(II) sulfate in the reagent to a brick-red precipitate of copper(I) oxide.
The intensity of the colour change correlates with the concentration of reducing sugar present: Blue → Green → Yellow → Orange → Brick-red.
A semi-quantitative method can be carried out by preparing a range of glucose solutions of known concentrations via serial dilution from a stock solution.
Serial dilution method: label a series of test tubes; add equal volumes of distilled water to each test tube; add a known volume of the stock solution to the first tube and mix thoroughly; transfer the same volume from the first tube to the second, and mix; repeat this process for each remaining tube using the same volume, creating a series of decreasing concentrations; ensure each dilution step is consistent, either by equal volume subtraction or halving (doubling dilutions); use each diluted solution for testing or plotting a calibration curve.
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; an estimate of the concentration of reducing sugar present can then be made.
To avoid issues with human interpretation of colour, a colorimeter could be used to obtain a quantitative measure of the colour intensity.
Colorimeters pass light of a specific wavelength (e.g. blue) through a solution of known concentration to establish a range of values of the absorbance or transmission of light; an unknown sample can then be compared using a calibration curve.
Colorimeter method: select an appropriate filter (e.g. blue light for orange/red solutions); calibrate the colorimeter using a blank sample (e.g. water), which should give a reading of zero absorbance/100% transmission; measure the absorbance/transmission of each standard and unknown sample; plot a calibration curve of absorbance/transmission against known glucose concentrations.
Use the absorbance of the unknown sample to estimate the concentration of glucose in an unknown sample by locating its absorbance value on the calibration curve and reading off the corresponding concentration; this is called interpolation.
This method minimises subjective interpretation of colour changes and provides more reliable data.
Applications of serial dilutions: prepare standard solutions for concentration comparison; count microbial populations (e.g. bacteria or yeast); determine unknown concentrations of glucose, protein, or starch.

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1.Which of the following is the monomer present in cellulose?
Easy
- Aα glucose
- Bβ glucose
- CFructose
- DGalactose
2.Which of the following is the monomer present in starch?
Easy
- Aα glucose
- Bβ glucose
- CFructose
- DGalactose
3.A disaccharide is formed by a condensation reaction between two monosaccharides. In the equation: α glucose + A → sucrose + B, what are molecules A and B?
Easy
- AA = fructose, B = water
- BA = galactose, B = water
- CA = fructose, B = hydrogen
- DA = water, B = fructose
4.The equation below shows the reaction catalysed by the enzyme maltase: maltose + A → glucose + B. What is the name of molecule A and the type of chemical reaction shown?
Medium
- AA = water, hydrolysis
- BA = water, condensation
- CA = oxygen, hydrolysis
- DA = hydrogen, condensation
5.A laboratory assistant added Benedict’s reagent to a solution of sucrose in a test tube and applied heat. What is the expected result and explanation?
Medium
- AA brick-red precipitate forms because sucrose is a reducing sugar.
- BThe solution remains blue because sucrose is a non-reducing sugar.
- CA green precipitate forms because sucrose is partially reducing.
- DThe solution turns blue-black because sucrose reacts with copper ions.
6.A student tested a muscle cell sample for starch using iodine in potassium iodide solution. What is the expected result and reason?
Medium
- ABlue-black colour because muscle cells store starch.
- BNo blue-black colour because muscle cells store glycogen, not starch.
- CBrick-red precipitate because muscle cells contain reducing sugars.
- DBlue colour because muscle cells contain sucrose.
7.Which of the following correctly defines a disaccharide?
Medium
- AA single reducing sugar monomer.
- BA sugar formed from two monosaccharides joined by a glycosidic bond during a condensation reaction.
- CA polymer formed from many monosaccharides joined by glycosidic bonds.
- DA sugar formed from two monosaccharides joined by a peptide bond.
8.In the reaction catalysed by maltase: maltose + water → glucose + glucose, what is the formula for maltose?
Medium
- AC6H12O6
- BC12H22O11
- CC12H24O12
- DC6H12O11

더 많은 문제

9.Which of the following statements about glycogen are correct? (select all that apply)
Medium
- AIt is a storage polysaccharide in animals and fungi.
- BIt is formed from β glucose monomers.
- CIt is more branched than amylopectin.
- DIt is found in plant cells as granules.
- EIt contains 1,4 and 1,6 glycosidic bonds.
10.Which of the following are features of cellulose? (select all that apply)
Hard
- AIt is formed from β glucose monomers.
- BIt has 1,4 glycosidic bonds.
- CIt is highly branched.
- DIt forms hydrogen bonds between parallel chains.
- EIt is a storage polysaccharide in plants.
11.In β glucose, the hydroxyl (OH) group on carbon 1 is located above the ring.
Easy
True or false?
12.Sucrose is a reducing sugar.
Easy
True or false?
13.The glycosidic bond is broken by a hydrolysis reaction.
Easy
True or false?
14.Starch is a highly branched molecule, more branched than glycogen.
Medium
True or false?
15.Match each disaccharide with its monosaccharide components.
Medium
- Maltose
- Sucrose
- Lactose
- α-glucose and α-glucose
- α-glucose and fructose
- α-glucose and galactose
16.Place the steps of the Benedict's test for reducing sugars in the correct order.
Medium
- Add Benedict's reagent to the sample solution in a test tube.
- Heat the test tube in a water bath that has been brought to a boil.
- Observe the colour change from blue to brick-red.
- If a reducing sugar is present, a coloured precipitate forms.
17.Which of the following statements about the structure of cellulose is correct?
Medium
- ACellulose is formed from α-glucose monomers joined by 1,4 glycosidic bonds.
- BCellulose is formed from β-glucose monomers joined by 1,4 glycosidic bonds.
- CCellulose is formed from β-glucose monomers joined by 1,6 glycosidic bonds.
- DCellulose is formed from α-glucose monomers joined by 1,6 glycosidic bonds.
18.Which of the following is a non-reducing sugar?
Medium
- AGlucose
- BFructose
- CGalactose
- DSucrose
19.A student tested an unknown solution with Benedict's reagent and obtained a brick-red precipitate. What does this indicate about the solution?
Medium
- AIt contains a high concentration of reducing sugar.
- BIt contains a high concentration of non-reducing sugar.
- CIt contains starch.
- DIt contains a low concentration of reducing sugar.
20.Amylose is a branched molecule found in starch.
Easy
True or false?
21.Glycogen is more branched than amylopectin.
Medium
True or false?
22.Match each carbohydrate to the category it belongs to.
Medium
- Fructose
- Maltose
- Glycogen
- Cellulose
- Monosaccharide
- Disaccharide
- Polysaccharide
23.Place the steps of paper chromatography to separate a mixture of monosaccharides in the correct order.
Medium
- A spot of the stained monosaccharide mixture is placed on a line at the bottom of the chromatography paper and left to dry.
- Spots of known standard solutions of different monosaccharides are placed on the line beside the sample spot.
- The chromatography paper is suspended in a solvent.
- As the solvent travels up the paper, the different monosaccharides separate at different distances from the line.
- The unknown monosaccharides are identified by comparing their positions with those of the known standard solutions.
24.Which of the following statements about amylose and amylopectin are correct? (select all that apply)
Medium
- AAmylose has an unbranched helix-shaped chain with 1,4 glycosidic bonds.
- BAmylopectin has both 1,4 and 1,6 glycosidic bonds, creating a branched molecule.
- CAmylose makes up 70–90% of starch.
- DAmylopectin has many terminal glucose molecules that can be easily hydrolysed.
- EAmylose is more branched than amylopectin.
25.Which of the following is the correct description of the mobile phase in paper chromatography?
Medium
- AThe chromatography paper
- BThe solvent in which the sample molecules can move
- CThe spot of the mixture placed on the paper
- DThe coloured precipitate that forms during the test
26.In paper chromatography, components with higher solubility in the mobile phase travel further up the paper.
Easy
True or false?
27.Which of the following are reasons why starch and glycogen are suitable storage polysaccharides? (select all that apply)
Medium
- AThey are compact, so large quantities can be stored.
- BThey are insoluble, so they have no osmotic effect on the cell.
- CThey are highly soluble, so they can be easily transported in solution.
- DThey are made of glucose monomers, which can be hydrolysed for use in respiration.
- EThey contain many hydrogen bonds between parallel chains, giving high tensile strength.
28.A student carries out a Benedict's test on a solution and obtains a brick-red precipitate. The student then repeats the test on the same solution after boiling it with dilute hydrochloric acid and neutralising with sodium hydrogencarbonate. The result is the same. What can be concluded about the original solution?
Hard
- AThe original solution contained a non-reducing sugar.
- BThe original solution contained a reducing sugar.
- CThe original solution contained starch.
- DThe original solution contained neither reducing nor non-reducing sugar.
29.A student set up a serial dilution from a 10 mmol dm⁻³ glucose stock solution. They placed 4 cm³ of distilled water in a test tube and added 1 cm³ of stock solution. What is the glucose concentration in this tube?
Medium
- A2 mmol dm⁻³
- B2.5 mmol dm⁻³
- C4 mmol dm⁻³
- D8 mmol dm⁻³
30.In a paper chromatography experiment to separate monosaccharides, a spot from an unknown mixture travelled 6.0 cm from the baseline. A spot from a known glucose standard travelled 6.0 cm. What can be concluded?
Medium
- AThe unknown mixture contains glucose.
- BThe unknown mixture contains fructose.
- CThe unknown mixture contains sucrose.
- DThe unknown mixture does not contain any monosaccharides.
31.A student carried out a Benedict's test on a series of glucose solutions and recorded the colour change. Which sequence correctly shows increasing glucose concentration?
Medium
- ABlue → green → yellow → orange → brick-red
- BBrick-red → orange → yellow → green → blue
- CBlue → yellow → green → orange → brick-red
- DGreen → blue → orange → yellow → brick-red
32.Which of the following statements about the use of a colorimeter to estimate glucose concentration are correct? (select all that apply)
Hard
- AA blue filter is suitable for orange/red solutions.
- BThe colorimeter should be calibrated with a blank sample, such as water.
- CA calibration curve is plotted of absorbance against known glucose concentrations.
- DThe unknown concentration is found by reading from the calibration curve (interpolation).
- EA colorimeter gives qualitative results only.
33.A student tested an unknown solution for reducing sugars and obtained a brick-red precipitate. Which of the following could be present in the solution? (select all that apply)
Medium
- AGlucose
- BFructose
- CGalactose
- DSucrose
- EStarch
34.In paper chromatography, monosaccharides with higher solubility in the mobile phase travel further up the paper.
Easy
True or false?
35.A colorimeter gives a qualitative result that cannot be used to estimate glucose concentration.
Medium
True or false?
36.Match each carbohydrate with its correct description.
Medium
- Amylose
- Amylopectin
- Glycogen
- Unbranched helix with 1,4 glycosidic bonds
- Branched with 1,4 and 1,6 glycosidic bonds; found in plants
- Highly branched with 1,4 and 1,6 glycosidic bonds; found in animals
37.Place the steps of using a colorimeter to estimate glucose concentration in the correct order.
Medium
- Select an appropriate filter (e.g. blue light for orange/red solutions).
- Calibrate the colorimeter using a blank sample (e.g. water).
- Measure the absorbance of each standard and unknown sample.
- Plot a calibration curve of absorbance against known glucose concentrations.
- Use the calibration curve to estimate the concentration of the unknown sample.
38.A student set up a serial dilution using a stock glucose solution of 10 mmol dm⁻³. They prepared five tubes with concentrations 0, 2, 4, 6, 8 and 10 mmol dm⁻³. What volume of stock solution is needed to make 5 cm³ of a 6 mmol dm⁻³ solution?
Medium
- A3 cm³
- B2 cm³
- C4 cm³
- D1 cm³
39.In the test for non-reducing sugars, dilute hydrochloric acid is added to the sample and it is heated in a water bath. Why is the solution then neutralised with sodium hydrogencarbonate before Benedict's test is carried out?
Medium
- ABecause Benedict's reagent only works in slightly alkaline conditions, and the acid would prevent the copper(II) sulfate from being reduced
- BBecause the acid would hydrolyse the Benedict's reagent and destroy the copper(II) sulfate
- CBecause sodium hydrogencarbonate hydrolyses the glycosidic bonds that the acid could not break
- DBecause the acid makes the solution too hot, and neutralising it cools the solution to the correct temperature
40.A student set up a column containing inert beads to which the enzyme sucrase was fixed. A sugar solution containing sucrose, lactose and glucose was poured into the top of the column. Which compounds are present in the solution collected from the bottom of the column?
Hard
- AGlucose, fructose, lactose and glucose
- BSucrose, lactose and glucose only
- CGlucose and lactose only
- DFructose and lactose only
41.Name the type of reaction that forms polysaccharides from their monomers.
Easy
42.Name the bond that forms between two monosaccharides in a condensation reaction.
Medium
43.Name the two monosaccharides that make up a lactose molecule.
Medium
44.A student set up a serial dilution with a stock glucose solution of 10 mmol dm⁻³. In one test tube, they mixed 3 cm³ of distilled water with 2 cm³ of stock solution. What is the glucose concentration of this solution in mmol dm⁻³?
Medium
45.In a colorimeter calibration, a glucose solution of 0.3 mol dm⁻³ gave an absorbance of 0.25 a.u. and a solution of 0.4 mol dm⁻³ gave an absorbance of 0.48 a.u. Estimate the concentration of an unknown glucose solution that has an absorbance of 0.4 a.u. Give your answer to 2 decimal places in mol dm⁻³.
Medium
46.Complete the sentence about the structure of starch.
Easy
Starch is made up of two polysaccharides: ____ and ____. The first is unbranched and helical, while the second is branched.
47.Complete the sentence about the test for starch.
Medium
To test for starch, add a few drops of ____ in ____ solution to the sample. A positive result is a ____ colour.
48.Complete the table comparing starch and glycogen by filling in the missing words.
Medium
Starch is found in ____ cells, while glycogen is found in ____ cells.
49.Complete the sentences about the structure of amylose and amylopectin.
Medium
Amylose has an unbranched ____ shaped chain with 1,4 glycosidic bonds between α-glucose molecules. Amylopectin has 1,4 glycosidic bonds but also 1,6 glycosidic bonds, creating a ____ molecule.
50.A stock glucose solution has a concentration of 10 mmol dm⁻³. A student mixes 2 cm³ of stock solution with 3 cm³ of distilled water. What is the glucose concentration of the resulting solution in mmol dm⁻³?
Medium
51.Complete the sentence about the colorimeter calibration curve.
Medium
In a colorimeter, a calibration curve is plotted with absorbance on the y-axis and ____ on the x-axis.
52.What is the name of the technique used to separate a mixture of monosaccharides?
Medium
53.Explain how cellulose gives cotton its strength.
Hard
54.Compare the structures of starch and glycogen molecules.
Medium
55.Describe how the student could use chromatography to separate a mixture of monosaccharides in solution.
Medium
56.Explain why pea cells use starch as a storage molecule rather than glucose.
Hard
57.Explain how the structure of glycogen is related to its function in animal cells.
Hard
58.A student set up a serial dilution with a stock glucose solution of 10 mmol dm⁻³. In one test tube, they mixed 3 cm³ of distilled water with 2 cm³ of stock solution. What is the glucose concentration of this solution in mmol dm⁻³?
Medium
- A6 mmol dm⁻³
- B3 mmol dm⁻³
- C4 mmol dm⁻³
- D5 mmol dm⁻³
59.In a colorimeter calibration, a glucose solution of 0.3 mol dm⁻³ gave an absorbance of 0.25 a.u. and a solution of 0.4 mol dm⁻³ gave an absorbance of 0.48 a.u. Estimate the concentration of an unknown glucose solution that has an absorbance of 0.4 a.u. Give your answer to 2 decimal places in mol dm⁻³.
Medium
- A2.36 mol dm⁻³
- B-0.64 mol dm⁻³
- C1.36 mol dm⁻³
- D0.36 mol dm⁻³
60.A stock glucose solution has a concentration of 10 mmol dm⁻³. A student mixes 2 cm³ of stock solution with 3 cm³ of distilled water. What is the glucose concentration of the resulting solution in mmol dm⁻³?
Medium
- A4 mmol dm⁻³
- B5 mmol dm⁻³
- C6 mmol dm⁻³
- D3 mmol dm⁻³

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## Biological Molecules: Carbohydrates **Carbohydrates** contain C, H, O. Three types: monosaccharides, disaccharides, polysaccharides. **Monomers** are small units; **polymers** are many monomers joined in polymerisation. **Macromolecules** are very large (1000+ atoms). ### Reactions - **Condensation** (dehydration synthesis): monomers join by covalent bonds, water removed. - **Hydrolysis**: water added, covalent bonds broken ('hydro' = water, 'lyse' = break). ### Monosaccharides Simple sugars; single monomers. **Reducing sugars** donate electrons (carbonyl group oxidised) and reduce soluble copper(II) sulfate to insoluble brick-red copper(I) oxide. Examples: glucose, fructose, galactose. **Non-reducing** (e.g. sucrose) cannot donate electrons; must be hydrolysed first. ### Glucose Formula: **C₆H₁₂O₆**. Two isomers: - **α-glucose**: OH on carbon 1 is **below** the ring. - **β-glucose**: OH on carbon 1 is **above** the ring. Starch and glycogen are made from α-glucose; cellulose from β-glucose. ### Glycosidic Bond Formed when two hydroxyl (-OH) groups interact, creating a strong covalent bond. Each bond removes one water molecule (condensation). Examples: maltose has α-1,4; sucrose has α-1,2. Broken by hydrolysis. ### Disaccharides (all C₁₂H₂₂O₁₁) - **Maltose** = α-glucose + α-glucose - **Sucrose** = α-glucose + fructose - **Lactose** = α-glucose + galactose ### Starch & Glycogen (storage) Compact and insoluble (no osmotic effect). - **Starch** (plants, granules in plastids): **amylose** (10–30%, unbranched helix, 1,4 bonds) and **amylopectin** (70–90%, branched, 1,4 and 1,6 bonds). - **Glycogen** (animals/fungi): highly branched, not coiled; more branched than amylopectin → more free ends for rapid condensation/hydrolysis. ### Cellulose (structure) Long chains of **β-glucose** joined by **1,4 glycosidic bonds**; consecutive molecules rotated **180°**. Many **hydrogen bonds** between chains give strength. Main structural component of cell walls; high tensile strength withstands turgor pressure; freely permeable. ### Chromatography Separates mixtures by solubility. **Mobile phase** = solvent; **stationary phase** = paper. More soluble components travel further. Colourless monosaccharides must be stained; identify by comparing with known standard solutions. ### Biochemical Tests - **Reducing sugars**: add Benedict's reagent (blue, CuSO₄), heat in water bath. Colour: blue → green → yellow → orange → brick-red (semi-quantitative). - **Non-reducing sugars**: add dilute HCl, heat, neutralise with sodium hydrogencarbonate, then Benedict's test. - **Starch**: add iodine in potassium iodide solution → blue-black. ### Finding Glucose Concentration **Serial dilution** from stock solution creates known concentrations. **Colorimeter** gives quantitative absorbance; plot **calibration curve** and use **interpolation** to find unknown concentration.

Monomer: The smaller units from which larger molecules are made.
Polymer: Molecules made from a large number of monomers joined together in a chain during polymerisation.
Condensation reaction: Monomers combine by covalent bonds to form polymers or macromolecules, and water is removed.
Hydrolysis reaction: Covalent bonds are broken when water is added.
Monosaccharide: A single reducing sugar monomer.
Disaccharide: A sugar formed from two monosaccharides joined by a glycosidic bond during a condensation reaction.
Polysaccharide: A polymer formed from many monosaccharides joined by a glycosidic bond during a condensation reaction.
Reducing sugar: A sugar that can donate electrons (carbonyl group becomes oxidised) and reduce soluble copper(II) sulfate to insoluble brick-red copper(I) oxide.
Glycosidic bond: A strong covalent bond formed when two hydroxyl (-OH) groups on different saccharides interact; each bond removes one water molecule.
α-glucose: Isomer of glucose with the hydroxyl (OH) group on carbon 1 located below the ring.
β-glucose: Isomer of glucose with the hydroxyl (OH) group on carbon 1 located above the ring.
Amylose: 10–30% of starch; unbranched helix-shaped chain with 1,4 glycosidic bonds between α-glucose molecules.
Amylopectin: 70–90% of starch; has 1,4 and 1,6 glycosidic bonds between α-glucose molecules, creating a branched molecule.
Glycogen: Storage polysaccharide of animals and fungi; highly branched and not coiled.
Cellulose: Polysaccharide of β-glucose joined by 1,4 glycosidic bonds; chains rotated 180° with many hydrogen bonds giving strength.
Benedict's reagent: Blue solution containing copper(II) sulfate ions (CuSO₄); forms a coloured precipitate with reducing sugars.
Colorimeter: Instrument that passes light of a specific wavelength through a solution to measure absorbance or transmission quantitatively.
Calibration curve: A graph of absorbance/transmission against known glucose concentrations, used to estimate unknown concentrations by interpolation.

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