1 Biological molecules

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  • Created on: 02-12-19 10:37

Starch - link between structure and function

Polysaccharide formed by the condensation reaction between alpha-glucose monomers, joined by glycosidic bonds.

Main energy storage material in plants, and is found in seeds and storage organs (e.g. starch granules and roots).

Relationship between stucture and function:

- insoluble: does not affect water potential; water not drawn into cells by osmosis. Does not intefere with metabolic reactions.

- forms alpha-glucose monomers when hydrolysed: easily transported and readily used in respiration.

- branched form has many ends: can be acted on simultaneously by enzymes, rapidly releasing glucose monomers.

- compact: a lot can be stored in a small space.

- large: does not diffuse out of cells.

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Starch - amylose and amylopectin.


- polymer of alpha-glucose joined by 1,4- glycosidic bonds.

- long and unbranched chain with helical structure.

- OH groups point inwards forming hydrogen bonds strengthening helix.

- compact so good for storgage


- polymer of alpha-glucose joined by 1,4- and 1,6- glycosidic bonds.

- long and branched chain with helical structure

- branching reduces the tendency for spiralling

- branching provides easy access to glycosidic bonds for enzymes; rapidly hydrolysed.

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Glycogen - link between structure and function

Polysaccharide formed by the condensation reaction between alpha-glucose monomers joined by 1,4- and 1,6- glycosidc bonds.

Similar to starch but has shorter, more highly branched chains owing to more 1,6- bonds; it is the main energy storage material in animals.

Found in solid grains in liver and skeletal muscle cells.

Relationship between structure and function:

- insoluble: does not interfere with metabolic reactions or affect water potential in cells.

- large: does not diffuse out of cells.

- compact: lots can be stored in a small space

- more highly branched than starch: acted on simulataneously by enzymes, more rapidly broken down to form alpha-glucose monomers (used in respiration); important for animals as they have higher metabolic and respiratory rates than plants, as they are more active.

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Cellulose - structure and function.

Polysaccharide formed by the condensation reaction of beta-glucose monomers joined by 1,4-glycosidic bonds.

Straight unbranched chains that run in parallel to one another, with glucose molecules rotated by 180 degrees, allowing hydrogen bonds (found between OH groups in parallel chains)  to form cross-linkages between adjacent chains. The sheer number of hydrogen bonds makes a considerable contribution to strengthening cellulose and giving it its structural stability

The cellulose molecules are grouped together to form microfibrils, which, in turn, are arranged in parallel groups called fibres.

Cellulose is a major component of cell walls and provides rigidity to them. The cellulose cell wall prevents the cell from bursting as water enters it by osmosis by exerting an inward pressure that stops any further influx of water. Plant cells are therefore turgid and push against one another, making these parts semi-rigid. Especially important in stems and leaves to provide maximum surface area for photosynthesis.

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Cellulose - link between structure and function

- cellulose molecules are made up of beta-glucose monomers and so form long, straight, unbranched chains.

- these cellulose molecualr chains run parallel to each other and are cross-linked by hydrogen bonds, which add collective strength.

- these molecules are grouped to form microfibrils, which in turn aare grouped to form fibres, all of which provides yet more strength.

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Test for reducing sugars.

Benedict's test:

A reducing sugar is a sugar which can donate electrons to (or reduce) another chemical.

Benedict's reagent is a blue alkaline solution of copper(II) sulfate.

- add 2cm^3 of food sample (in liquid form) to a test tube.

- add an equal volume of Benedict's reagent.

- heat the mixture in a gently boiling water bath for 5 minutes.

Red ppt of copper(I) oxide indicates a positive result.

Semi-quantitaive test.

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Test for non-reducing sugars.

A non-reducing sugar is a sugar that cannot donate electrons to (or reduce) another chemical.

e.g. some disaccharides like sucrose.

To detect a non-reducing sugar it must first be hydrolysed into its monosaccharide components by hydrolysis.

- add 2cm^3 hydrochloric acid to 2cm^3 of food sample in a test tube and heat in a gently boiling water bath for 5 minutes. The HCL will hydrolyse any disaccharide present.

- slowly add some sodium hydrogencarbonate (NaHCO3) to test tube to neutralise the hydrochloric acid.

- test with pH paper to make sure solution is alkaline.

- re-test with Benedict's reagent; if non-reducing sugar was present in original sample the solution will now turn orange-brown.

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Test for starch.

Starch is easily detected by its ability to change the colour of the iodine in potassium iodide solution from yellow to blue-black.

Carried out a room temp.

- add 2cm^3 of sample to a test tube.

- add 2 drops of potassium iodide solution.

- shake or stir.

The presence of starch is indicated by a blue-black colouration.

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Roles of lipids.

  • Source of energy: when oxidised, lipids provide more than twice the energy as the same mass of carbohydrate and release valuable water.
  • Waterproofing: lipids are insoluble in water. Plants and insects have a lipid cuticles that conserve water, while mammals produce an oily secretion from the sebaceous glands in the skin.
  • Insulation: fats are slow conductors of heat and when stored beneath the body surface help to retain body heat. They also act as electrical insulators in the myelin sheath around nerve cells.
  • Protection: fat is stored around delicate organs, e.g. the kidney.
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Describe the characteristics of lipids.

- contain carbon, hydrogen and oxygen.

- the proportion of oxygen to carbon and hydrogen is smaller than in carbohydrates.

- insoluble in water.

- soluble in organic solvents like alcohols and acetone.

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Triglycerides - linking structure and properties.

  • high ratio of energy-storing carbon-hydrogen bonds to carbon atoms: an excellent source of energy.
  • low mass to energy ratio:  good storage molecules as lots of energy can be stored in a small volume. Beneficial to animals as it reduces the mass they have to carry.
  • insoluble as they are large and non-polar: does not affect the water potential of cells.
  • high ratio of hydrogen to oxygen atoms: release a lot of water when oxidised and so an important source of water, especially to those living in dry deserts.
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Phospholipid - linking structure and properties

- phospholipids are polar molecules, having a hydrophilic phosphate head and a hydrophobic tail of two fatty acids. Meaning in aqueous environments they form a bilayer within cell-surface membranes. A hydrophobic barrier is formed between the inside and the outside of the cell.

- the hydrophilic phosphate heads help to hold at the surface of the cell-surface membrane.

- allows them to form glycolipids by combining with carbohydrates within the cell-surface membrane. These glycolipids are important in cell recognition.

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Test for lipids.

Emulsion test:

- take a completely dry and grease-free test tube.

- add 2cm^3 of sample and 5cm^3 of ethanol.

- shake the test tune thoroughly to dissolve any lipid.

- add 5cm^3 of water and shake gently.

- a cloudy-white colour indicates the presence of a lipid.

- as a control, repeat the experiment using water instead of the sample; the final solution should remain clear.

The cloudy colour is due to any lipid in the sample being finely dispersed in the water to form an emulsion. Light passing through this emulsion is refracted as it passes from oil droplets to water droplets, making it appear cloudy.

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The secondary structure of proteins.

The linked amino acids that make up a polypeptide possess both -NH and -C=O groups on either side of every peptide bond. The hydrogen of the -NH group has an overall positive charge while the oxygen of the -C=O group has an overall negative charge. 

These two groups readily form weak hydrogen bonds which cause the polypeptide chain to be twisted into a 3D shape, such as the coil known as an alpha-helix.

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The tertiary structure of proteins.

The alpha-helices of the secondary protein structure can be twisted and folded even more to give the complex, and often specific, 3D structure of each protein.

This structure is maintained by a number of different bonds, including:

- disulfide bridges (fairly strong and not easily broken).

- ionic bonds; formed between any carboxyl group and amino groups that are not involved in forming peptide bonds (weaker than disulfide bonds and are easily broken by changes in pH).

- hydrogen bonds (numerous but easily broken).

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Test for proteins.

Biuret test:

- place a sample of solution to be tested in a test tube and add an equal volume of sodium hydroxide solution at room temperature.

- add a few drops of very dilute (0.05%) copper(II) sulfate solution and mix gently.

- a purple colouration indicates the presence of peptide bonds and hence a protein. If no protein is present, the solution remains blue.

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Conditions for chemical reactions.

A number of conditions must be satisified for a reaction to take place:

- the substrates must collide with sufficient energy to alter the arrangement of their atoms to form the products.

- the free energy (the energy of a system that is available to perform work) of the products must be less than that of the substrates.

- must meet the activation energy.

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Effect of pH on enzyme action.

- a change in pH alters the charges on the amino acids that make up the active site of the enzyme; the substrate can no longer bind to the active site, and so the enzyme-substrate complex cannot be formed.

- a significant change in pH may cause the bonds maintaining the enzyme's tertiary structure to break; the active site therefore changes shape.

The arrangement of the active site is partly determined by the hydrogen and ionic bonds between -NH2 and -COOH groups of the polypeptides that make up the enzyme. The change in H+ ions affects this bonding, causing the active site to change shape.

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Effect of enzyme concentration on rate.

- low enzyme concentration; too few enzyme molecules to allow all substrate molecules to find an active site at one time.

- intermediate enzyme concentration; all the substrate molecules can occupy an active site at the same time. The rate of reaction is at its maximum.

- high enzyme concentration (excess); the addition of further enzyme molecules has no effect as there is already enough active sites to accomodate all the available substrate molecules. There is no increase in the rate of reaction.

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Effect of substrate concentration on rate.

- low substrate concentration; there are too few substrate molecules to occupy all the available active sites.

- intermediate substrate concentration; all the active sites are occupied at one time. The rate of reaction is at its maximum.

- high substrate concentration (excess); the addition of further substrate molecules has no effect as all the active sites are occupied at one time. There is no increase in the rate of reaction.

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Competitive inhibitors.

Have a molecular shape similar to that of the substrate. 

They compete with the substrate for the available active sites.

It is the difference between the concentration of the substrate and that of the inhibitor that determines the effect that this has on enzyme activity.

The inhibitor is not permenantly bound.


Malonate can inhibit the respiratory enzyme that acts on succinate. It combines with the enzyme and blocks succinate from combining with the enzyme's active site.

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Non-competitive inhibitors

Attach themselves to the enzyme at a binding site which is not the active site. 

Upon attaching to the enzyme the inhibitor alters the shape of the enzyme and thus its active site in such a way that substrate molecules can no longer occupy it, and so the enzyme cannot function.

As the substrate and inhibitor are not competiting for the same site, an increase in substrate concentration does not decrease the effect of the inhibitor.

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