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Biological molecules (3.1)›Carbohydrates (3.1.2)

Carbohydrates (3.1.2)

An overview of carbohydrates (3.1.2) from AQA A level Biology including: monosaccharides, polysaccharides and how to test for sugars and starch
7 min

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Carbohydrates are biological molecules made from sugars.

Monosaccharides are the simplest form of sugar, consisting of a single monomer. Examples include glucose, fructose, and galactose.

Two monosaccharides join via a condensation reaction to form a disaccharide. Many monosaccharides join via condensation reactions to form a polysaccharide.

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Glucose exists in two isomeric forms: -glucose and -glucose. They have the same chemical formula but different structures and properties.

In -glucose, the (hydroxyl) group on carbon 1 is on the opposite side of the ring from the group, whereas in -glucose it is on the same side.

The image shows the chemical structures of α-glucose and β-glucose. Both structures are cyclic forms of glucose with six carbon atoms. In α-glucose, the hydroxyl group (OH) on carbon 1 is below the plane of the ring, while in β-glucose, the OH group is above the plane. Both structures are labeled with numbers 1 to 6 indicating carbon atoms. In α-glucose, carbon 6 is connected to a CH2OH group above the ring, and the OH group on carbon 1 is labeled 'Below the ring and opposite side to CH2OH.' In β-glucose, the CH2OH group on carbon 6 is also above the ring, and the OH group on carbon 1 is labeled 'Above the ring and same side to CH2OH.' Additional labels highlight the position of the CH2OH group as 'Always above the ring.'
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Monosaccharides are linked by glycosidic bonds, which form in a condensation reaction between hydroxyl () groups on adjacent monosaccharides, releasing a molecule of water.

The name of the glycosidic bond indicates the carbon atoms involved in the linkage. For example, an 1,4 glycosidic bond forms between carbon 1 of one -glucose molecule and carbon 4 of another.

Two -glucose molecules combine to form the disaccharide maltose, joined by an 1,4 glycosidic bond.

The image illustrates the formation of maltose from two alpha-glucose molecules. At the top, two alpha-glucose structures labeled as 'alpha-glucose (monosaccharide)' are shown. Each glucose molecule has carbon atoms numbered 1 through 6, with hydroxyl (OH) and hydrogen (H) groups attached. The two glucose molecules are linked by a blue highlighted area between the OH groups of one glucose's carbon 1 and the other glucose's carbon 4. An arrow labeled 'Condensation' points downward to the formation of maltose, labeled 'Maltose (disaccharide).' In the maltose structure, the glucose units are linked by an 'alpha-1,4 glycosidic bond,' highlighted in green. A blue label 'H2O' indicates the release of water during the reaction. Additional text reads, 'Two alpha-glucose molecules combining to form the disaccharide maltose, featuring an alpha-1,4-glycosidic bond through a condensation reaction.'
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Two monosaccharides bonded together via a glycosidic bond form disaccharides:

  • Maltose = -glucose + -glucose
  • Sucrose = -glucose + -fructose
  • Lactose = -galactose + glucose.
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Starch is the insoluble, primary storage polysaccharide in plants and is formed from a few hundred to a few thousand -glucose monosaccharides bonded together. It is made from a mixture of two polysaccharides.

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The two polysaccharides which form starch are:

  • Amylose constitutes 10–30 of starch and has an unbranched, helix-shaped chain of -glucose molecules linked by 1,4 glycosidic bonds.
  • Amylopectin constitutes 70–90 of starch and has both 1,4 glycosidic bonds and 1,6 glycosidic bonds, creating a branched molecule. Enzymes involved in breaking down these molecules can simultaneously work at multiple branch points, leading to rapid release of glucose for energy. More accessible hydroxyl groups make amylopectin more soluble in water.
The image shows two types of polysaccharides, amylose and amylopectin. The top section illustrates amylose, labeled 'Amylose (1,4 glycosidic bonds)', with a coiled structure on the left and a linear chain of glucose units on the right. Each glucose unit is depicted with hydroxyl (OH) groups and an oxygen bridge indicating 1,4 glycosidic bonds. The bottom section shows amylopectin, labeled 'Amylopectin (1,4 and 1,6 glycosidic bonds)', with a branched structure on the left and a glucose chain on the right. The chain includes glucose units connected by 1,4 glycosidic bonds with occasional 1,6 branching points. Each glucose unit is similarly depicted with hydroxyl groups and connections indicating glycosidic bonds. The illustration uses green spheres to represent the coiled and branched structures of the polysaccharides.
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Glycogen is the primary storage polysaccharide in animals and fungi, characterised by a highly branched and uncoiled structure.

The highly branched structure of glycogen means there are numerous terminal glucose units leading to rapid release of glucose for energy. Branched polysaccharides with more accessible hydroxyl groups are also more soluble in water.

Glycogen’s structure is similar to amylopectin, but its branching is more extensive. This makes it more water-soluble and its terminal glucose molecules more accessible to enzymes.

The image contains three diagrams of polysaccharide structures. The top diagram is labeled 'Amylose' and shows a linear chain of green spheres coiled into a helix. The middle diagram is labeled 'Amylopectin' and depicts a branched structure with a main chain of green spheres and several branching chains. The bottom diagram is labeled 'Glycogen' and illustrates a highly branched structure with green spheres forming multiple short, branching chains. The bottom of the image includes the copyright label '© Medify'.
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Amylose is coiled, making it insoluble and compact which is ideal for storage.

Amylopectin and glycogen are both highly branched molecules, making them compact and allowing more energy to be stored. Their branches provide multiple free ends where glucose molecules can be rapidly added in condensation reactions or removed in hydrolysis reactions, enabling quick adaptation to cellular energy demands.

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Cellulose is a structural polysaccharide in plants. It is composed of long, linear chains of -glucose molecules joined by 1,4 glycosidic bonds.

Every other -glucose molecule in cellulose is inverted, producing a linear structure with extensive hydrogen bonding within and between polysaccharide chains.

Cellulose’s high tensile strength is a combined result of several key structural features:

  • The strong 1,4 glycosidic bonds within a polysaccharide chain.
  • There hydrogen bonding between polysaccharide chains within a microfibril.
  • The alignment of microfibrils into macrofibrils.
  • The alignment of macrofibrils into fibres.
The image shows a structural diagram of cellulose, with multiple rows of β-glucose units linked by 1,4 glycosidic bonds and interconnected by dashed lines labeled 'Hydrogen bond'. The structure consists of hexagonal glucose units with 'OH' groups and oxygen atoms shown. Below the cellulose structure, a labeled illustration of a 'Plant cell wall' features a cross-section of a green wall and an inset diagram of a plant cell with visible organelles. A section of the cell wall is expanded to show a 'Macrofibril', which further expands to a smaller 'Microfibril' structure composed of aligned cellulose chains.
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Individual cellulose chains are held together by hydrogen bonds to form microfibrils.

Microfibrils are organised into larger macrofibrils, further stabilised by additional hydrogen bonds.
The alignment of macrofibrils can vary depending on their role within a plant.

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Cellulose is the main structural component of plant cell walls. The cell wall matrix includes other molecules such as lignin, to further increase cell wall strength.

Its structure gives it the following properties:

  • Its high tensile strength enables cell walls to withstand turgor pressure and maximise leaf surface area for light absorption.
  • Spaces between macrofibrils make cellulose fibres permeable to water and solutes, facilitating diffusion into and out of plant cells.
  • Cellulose is a source of dietary fibre (roughage). Few organisms have the enzyme cellulase, which hydrolyses cellulose, meaning that it is not absorbed during digestion.
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The bonding and structure of the key polysaccharides in plants and animals are summarised below.

A table comparing the characteristics of cellulose, amylose, amylopectin, and glycogen. The columns are labeled: Cellulose, Amylose, Amylopectin, and Glycogen. The rows are labeled: Source, Subunit, Bonds, Branches, Structure, and Shape. - Source: Cellulose (Plant), Amylose (Plant), Amylopectin (Plant), Glycogen (Animal). - Subunit: Cellulose (β-glucose), Amylose (α-glucose), Amylopectin (α-glucose), Glycogen (α-glucose). - Bonds: Cellulose (1,4 glycosidic), Amylose (1,4 glycosidic), Amylopectin (1,4 and 1,6 glycosidic), Glycogen (1,4 and 1,6 glycosidic). - Branches: Cellulose (No), Amylose (No), Amylopectin (Yes, approximately 1 per 20 subunits), Glycogen (Yes, approximately 1 in 10 subunits). - Structure: Depicts linear chains for cellulose and amylose, branched chains for amylopectin and glycogen. - Shape: Cellulose (parallel lines of circles), Amylose (spiral of circles), Amylopectin (branched cluster of circles), Glycogen (highly branched cluster of circles).
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Sugars can be classified as reducing or non-reducing depending on their structure and their functional groups. Reducing sugars can act as reducing agents by donating electrons.

Glucose, fructose, lactose, and maltose are reducing sugars, while sucrose is non-reducing.

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Benedict’s test detects reducing sugars in a sample.

Benedict’s reagent is a blue solution of () that contains copper(II) sulfate ions. When this reacts with reducing sugars, copper(II) is reduced to copper(I). The copper(I) oxide formed is insoluble in water and is detected as a brick-red precipitate.

Method:

  1. Add Benedict’s reagent (blue) to the sample. It is important that an excess of Benedict’s solution is used so that there is more than enough copper(II) sulfate present to fully react with the reducing sugars in the sample.
  2. Heat the test tube in boiling water or in a water bath for a few minutes.

Results:

A colour change from blue to green, then yellow, orange, and finally brick-red, indicates the presence of reducing sugars in increasing concentrations.

The image illustrates the process of testing for reducing sugars using Benedict's reagent. A sample in a test tube is shown with an arrow indicating the addition of Benedict's reagent. The test tube is then heated in a water bath. The results are displayed with five test tubes showing different colors: Blue, Green, Yellow, Orange, and Brick red, labeled as 'Results'. An arrow beneath the test tubes indicates 'Increasing concentration of reducing sugars' from left to right.
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Hydrolysing paired with Benedict’s test can be used to test for non-reducing sugars.

Method:

  1. Follow the protocol for detecting reducing sugars, and if the results are negative, proceed.
  2. Boil the sample with dilute hydrochloric acid. This breaks the glycosidic bonds in the sample, breaking the non-reducing sugars down into reducing sugars.
  3. Neutralise the sample with sodium hydrogencarbonate.
  4. Add an excess of Benedict’s solution.
  5. Heat the test tube in boiling water or in a water bath for a few minutes.

Results:

A colour change from blue to green, then yellow, orange, and finally brick-red indicates the presence of reducing sugars in increasing concentrations — i.e. the non-reducing sugars that have been broken down by hydrolysis into monosaccharides and therefore turned into reducing sugars.

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The iodine test is a qualitative test that detects starch in a sample.

Method:

  1. Add a few drops of iodine (in potassium iodide solution, which is insoluble in water) to the sample.

Results:

A blue-black colour indicates the presence of starch.
The image shows a diagram illustrating a starch test using iodine. On the left, a dropper is adding iodine (as potassium iodide solution) to a test tube labeled 'Sample' containing a light blue liquid. Two arrows point to separate test tubes on the right. The upper test tube is filled with a dark blue liquid, labeled 'Positive test: Blue-black colour, Starch present.' The lower test tube is filled with a yellow-brown liquid, labeled 'Negative test: Yellow-brown colour, Starch absent.'
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