Section 2: Biological Molecules

Question 1

What are the functions of water?

Answer 1

Water is vital to living organisms. It makes up about 80% of a cell’s contents and has loads of important functions, inside and outside cells, such as:
>Water is a reactant in loads of important chemical reactions, including hydrolysis reactions.
>Water is a solvent, which means some substances dissolve in it. Most biological reactions take place in solution (e.g. in the cytoplasm of eukaryotic and prokaryotic cells) so water’s pretty essential.
>Water transports substances. The fact that it’s a liquid and a solvent means it can easily transport all sorts of materials, like glucose and oxygen, around plants and animals.
>Water helps with temperature control because it has a high specific heat capacity and a high latent heat of evaporation.
>Water is a habitat. The fact that it helps with temperature control, is a solvent and becomes less dense when it freezes means many organisms can survive and reproduce in it.

Question 2

Describe the polarity of water.

Answer 2

A molecule of water (H20) is one atom of oxygen (O) joined to two atoms of hydrogen (H2) by shared electrons.
Because the shared negative hydrogen electrons are pulled towards the oxygen atom, the other side of each hydrogen atom is left with a slight positive charge. The unshared negative electrons on the oxygen atom give it a slight negative charge. This makes the water a polar molecule - it has a partial negative charge (delta-) on one side and a partial positive charge (delta+) on the other.

Question 3

Describe the hydrogen bonding in water.

Answer 3

The slightly negatively-charged oxygen atoms attract the slightly positively-charged hydrogen atoms of other water molecules. This attraction is called hydrogen bonding and it gives water some of its useful properties.

Question 4

What does it mean by water has a high specific heat capacity?

Answer 4

Hydrogen bonds give water a high specific heat capacity - this is the energy needed to raise the temperature of 1 gram of a substance by 1 degreeC. The hydrogen bonds between water molecules can absorb a lot of energy. So water has a high specific heat capacity - it takes a lot of energy to heat it up. This means water doesn’t experience rapid temperature changes, which is one of the properties that makes it a good habitat - the temperature under water is likely to be more stable that it is on land

Question 5

What does it mean by water has a high latent heat of evaporation?

Answer 5

It takes a lot of energy (heat) to break the hydrogen bonds between water molecules. So water has a high latent heat of evaporation - a lot of energy is used up when water evaporates (changes from a liquid to a gas). This is useful for living organisms because it means water’s great for cooling things. This is why some mammals, like us, sweat when they’re too hot. When sweat evaporates, it cools the surface of the skin.

Question 6

What does it mean by water is very cohesive?

Answer 6

Cohesion is the attraction between molecules of the same type (e.g. two water molecules). Water molecules are very cohesive (they tend to stick together) because they’re polar. This helps water to flow, making it great for transporting substances. It also helps water to be transported up plant stems in the transpiration stream.

Question 7

What does it mean by water has a lower density when solid?

Answer 7

At low temperatures water freezes - it turns from a liquid to a solid. Water molecules are held further apart in ice than they are in liquid water because each water molecule forms four hydrogen bonds to other water molecules, making lattice shapes. This makes ice less dense than liquid water - which is why ice floats. This is useful for living organisms because, in cold temperatures, ice forms an insulating layer on top of water - the water below doesn’t freeze. So organisms that live in water, like fish, don’t freeze and can still move around.

Question 8

What does it mean by water is a good solvent?

Answer 8

A lot of important substances in biological reactions are ionic (like salt, for example). This means they’re made form one positively-charged atom or molecule and one negatively-charged atom or molecule (e.g. salt is made from a positive sodium ion and a negative chloride ion). Because water is polar, the slightly positive end of a water molecule be attracted to the negative ion, and the slightly negative end of a water molecule will be attracted to the positive ions. This means ions will get totally surrounded by water molecules – in other words they will dissolve. Water’s polarity makes it useful as a solvent in living organisms, e.g. in humans, important ions can dissolve in the water in blood and then be transported around the body.

Question 9

What are macromolecules?

Answer 9

Macromolecules are complex molecules with a relatively large molecular mass. Examples of biological macromolecules include proteins, some carbohydrates and lipids. Polymers are a group of macromolecules.

Question 10

What are polymers?

Answer 10

Most carbohydrates and all proteins are polymers. Polymers are large, complex molecules composed of long chains of monomers joined together. Monomers are small, basic molecular units. Examples of monomers include monosaccharides and amino acids.

Question 11

Describe the process of making polymers.

Answer 11

Most biological polymers are formedfrom the monomers by condensation reactions. A condensation reaction forms a chemical bond between monomers, releasing a molecule of water.

Question 12

Describe the process of breaking down polymers

Answer 12

Biological polymers can be broken down into monomers by hydrolysis reactions. The hydrolysis reaction breaks the chemical bonds between monomers using a water molecule. It’s basically the opposite of a condensation reaction.

Question 13

What are carbohydrates are made from?

Answer 13

Most carbohydrates are polymers. All carbohydrates are made up of the same three chemical elements – carbon (C), hydrogen(H) and oxygen (O). For every carbon atom in the carbohydrate there are usually two hydrogen atoms and one oxygen atom. The monomers that make up carbohydrates are called monosaccharides.

Question 14

Describe the structure of the monosaccharide glucose.

Answer 14

Glucose is a monosaccharide with six carbon atoms. This means it’s a hexose monosaccharide. There are two forms of glucose – alpha and beta. They both have a ring structure. The difference between alpha and beta glucose is that their OH groups are reversed. Glucose’s structure is related to its function as the main energy source in animals and plants. Its structure makes it soluble, so it can easily be transported. It’s chemical bonds contain lots of energy.

Question 15

Describe the structure of the monosaccharide ribose.

Answer 15

U ribose is a monosaccharide with five carbon atoms – this means it’s a pentose monosaccharide. Ribose is the sugar component of RNA nucleotides.

Question 16

Describe the polysaccharide formation.What are the bonds holding together monosaccharides?

Answer 16

Monosaccharides are joined together by glycosidic bonds. During synthesis, a hydrogen atom on one monosaccharide bonds to the hydroxyl (OH) group on the other, releasing a molecule of water – this is a condensation reaction. The reverse of this synthesis reaction is hydrolysis – a molecule of water reacts with the glycosidic bond, breaking it apart.

Question 17

How is a disaccharide formed? And give an example.

Answer 17

Disaccharide is formed when two monosaccharides joined together.
For example two alpha glucose molecules are joined together by glycosidic bond to form maltose. Other disaccharides are formed in a similar way. Sucrose is a disaccharide formed when an alpha glucose and a fructose join together. Lactose is a disaccharide formed by the joining together of a galactose with either alpha glucose or beta glucose.

Question 18

How is a polysaccharide formed? And give an example.

Answer 18

A polysaccharide is formed with more than two monosaccharides joined together.
For example lots of alpha glucose molecules are joined together by glycosidic bonds to form amylose.

Question 19

What is starch and what is its function?

Answer 19

Starch is the main energy storage material implants. Cells get energy from glucose and plants store excess glucose as starch (when a plant needs more glucose for energy it breaks down starch to release the glucose). Starch is insoluble in water so it doesn’t cause water to enter cells by osmosis which would make them swell. This makes it good for storage. Starch is a mixture of two polysaccharides of alpha glucose – amylose and amylopectin.

Question 20

Describe the structure of amylose.

Answer 20

Amylose in a long, unbranched chain of alpha-glucose. The angles of the glycosidic bonds give it a coiled structure, almost like a cylinder. This makes it compact, so it’s really good for storage because you can fit more into a small space.

Question 21

Describe the structure of amylopectin.

Answer 21

Amylopectin is a long, branched-chain of alpha glucose. It’s side branches allow the enzymes that break down the molecule to get at the Glycosidic bonds easily. This means that the glucose can be released quickly.

Question 22

What is glycogen?

Answer 22

Glycogen is the main energy storage material in animals. Animal cells get energy from glucose too, but animals store excess glucose as glycogen - another polysaccharide of alpha-glucose. It’s structure is very similar to amylopectin, except that it has loads more side branches coming off it. Loads of branches means that stored glucose can be released quickly, which is important for energy release in animals. It’s also a very compact molecule, so it’s good for storage.

Question 23

What is cellulose?

Answer 23

Cellulose is the major component of cell walls in plants. It’s made of long, unbranched chains of beta-glucose. When beta-glucose molecules bond, they form straight cellulose chains. The cellulose chains are linked together by hydrogen bonds to form strong fibres called microfibrils. The strong fibres mean cellulose provides structural support for cells (e.g. in plant cell walls).

Question 24

What are lipids?

Answer 24

Lipids are macromolecules. They all contain the chemical elements carbon, hydrogen and oxygen. There are three types of lipids you need to know about - triglycerides, phospholipids and cholesterol.

Question 25

Describe the structure of triglycerides.

Answer 25

Triglycerides have one molecule of glycerol with three fatty acids attached to it. They’re synthesised by the formation of an ester bond between each fatty acid and the glycerol molecule.

Question 26

What is an ester bond?

Answer 26

One triglyceride molecule has three ester bonds. Each ester bond is formed by a condensation reaction (in which a water molecule is released). The process in which triglycerides are synthesised is called esterification. Triglycerides break down when the ester bonds are broken. Each ester bond is broken in a hydrolysis reaction (in which a water molecule is used up).

Question 27

What are fatty acids?

Answer 27

Fatty acid molecules have long ‘tails’ made of hydrocarbons (compounds that contain only carbon and hydrogen atoms). The tails are ‘hydrophobic’ (they repel water molecules). These tails make lipids insoluble in water. All fatty acids have the same basic structure, but the hydrocarbon tail varies.
There are two kinds of fatty acids - saturated and unsaturated. The difference is in their hydrocarbon tails.

Question 28

What are saturated fatty acids?

Answer 28

Saturated fatty acids don’t have any double bonds between their carbon atoms in their hydrocarbon tails. The fatty acid is ‘saturated’ with hydrogen.

Question 29

What are unsaturated fatty acids?

Answer 29

Unsaturated fatty acids have at least once double bond between carbon atoms, which causes the chain to kink.

Question 30

What are phospholipids?

Answer 30

Phospholipids are pretty similar to triglycerides, except one of the fatty acid molecules is replaced by a phosphate group. The phosphate group is hydrophilic (it attracts water molecules) and the fatty acid tails are hydrophobic.

Question 31

What is cholesterol?

Answer 31

Cholesterol is another type of lipid - it has a hydrocarbon ring structure attached to a hydrocarbon tail. The ring structure has a polar hydroxyl (OH) group attached to it.

Question 32

How are the functions of triglycerides related to their function?

Answer 32

In animals and plants, triglycerides are mainly used as energy storage molecules. Some bacteria (e.g. mycobacterium tuberculosis) use triglycerides to store both energy and carbon. Triglycerides are good for storage because the long hydrocarbon tails of the fatty acids contain lots of chemical energy - a load of energy is released when they’re broken down. Because of thee tails, lipids contain about twice as much energy per gram as carbohydrates.
Triglycerides are also insoluble, so they don’t cause water to enter the cells by osmosis which would make them swell. The triglycerides bundle together as insoluble droplets in cells because the fatty acid tails are hydrophobic (water-repelling) - the tails face inwards, shielding themselves from water with their glycerol heads.

Question 33

How are the functions of phospholipids related to their function?

Answer 33

Phospholipids are found in the cell membranes of all eukaryotes and prokaryotes. They make up what’s known as the phospholipid bilayer. Cell membranes control what enters and leaves a cell. Phospholipid heads are hydrophilic and their tails are hydrophobic, so they form a double layer with their heads facing out towards the water on either side. The centre of the bilayer is hydrophobic, so water-soluble substances can’t easily pass through it - the membrane acts as a barrier to those substances.

Question 34

How is the function of cholesterol related to its function?

Answer 34

In eukaryotic cells, cholesterol molecules help strengthen the cell membrane by interacting with the phospholipid bilayer. Cholesterol has a small size and flattened shape - this allows cholesterol to fit in between the phospholipid molecules in the membrane. They bind to the hydrophobic tails of the phospholipids, causing them to pack more closely together. This helps to make the membrane less fluid and more rigid.

Question 35

What are proteins made from?

Answer 35

Proteins are polymers. The monomers of proteins are amino acids. A dipeptide is formed when two amino acids join together. Proteins are made up of one or more polypeptides.

Question 36

Describe amino acid structure.

Answer 36

All amino acids have the same general structure - a carboxyl group (-COOH) and an amino acid group (-NH2) attached to a carbon atom. The difference between different amino acids is the variable group they contain.
All amino acids contain the chemical elements carbon, oxygen, hydrogen and nitrogen. Some also contain sulphur.

Question 37

Describe dipeptide and polypeptide formation.

Answer 37

Amino acids are linked together by peptide bonds to form dipeptides and polypeptides. A molecule of water is released during the reaction - it’s a condensation reaction. The reverse of this reaction adds a molecule of water to break the peptide bond - it’s a hydrolysis reaction.

Question 38

What are the four structural levels of a protein?

Answer 38

Primary, secondary, tertiary and quaternary.

Question 39

Describe the primary structure of a protein.

Answer 39

This is the sequence of amino acids in the polypeptide chain. Different proteins have different sequences of amino acids in their primary structure. A change in just one amino acid may change the structure of the whole protein. It is held together by the peptide bonds between the amino acids.

Question 40

Describe the secondary structure of a protein.

Answer 40

The polypeptide chain doesn’t remain flat and straight. Hydrogen bonds from between the -NH and -CO groups of the amino acids in the chain. This makes it automatically coil into an alpha helix or fold into a beta pleated sheet - this is the secondary structure.

Question 41

Describe the tertiary structure of a protein.

Answer 41

The coiled or folded chain of amino acids is often coiled and folded further. More bonds form between different parts of the polypeptide chain such as:
IONIC BONDS - these are the attractions between negatively-charged R groups and positively-charged R groups on different parts of the molecule.
DISULFIDE BONDS - whenever two molecules of the amino acid cysteine come close together, the sulfur in the other cysteine, forming a disulfide bond
HYDROPHOBIC AND HYDROPHILIC INTERATIONS - when hydrophobic (water-repelling) R groups are close together in the protein, they tend to clump together. This means that hydrophilic (water-attracting) R groups are more likely to be pushed to the outside, which affects how he protein folds up into its final structure
HYDROGEN BONDS - these weak bonds from between slightly-positively charged hydrogen atoms in some R groups and slightly-negatively charged atoms in other R groups on he polypeptide chain.

For proteins made from a single polypeptide chain, the tertiary structure forms their final 3D structure.

Question 42

Describe the quaternary structure of a protein and give an example.

Answer 42

Some proteins are made of several different polypeptide chains held together by bonds. The quaternary structure is the way these polypeptide chains are assembled together.
The quaternary structure tends to be determined by the tertiary structure of the individual polypeptide chains being bonded together. Because of this, it can be influenced by all the bonds in the tertiary structure (ionic, disulfide, hydrophobic and hydrophilic interations and hydrogen bonds). For proteins made from more than one polypeptide chain, the quaternary structure is the protein’ s final 3D structure.

Question 43

How can you investigate protein structure?

Answer 43

Computer modelling can create 3D interactive images if proteins. This is really handy for investigating the different levels of structure in a protein molecule.

Question 44

What are globular proteins and describe their structure?

Answer 44

Globular proteins are round and compact. In a globular protein, the hydrophilic R groups on the amino acids tend to be pushed to the outside of the molecule. This is caused by the hydrophobic and hydrophilic interactions in the protein’s tertiary structure. This makes the globular proteins soluble, so they’re easily transported in fluids.
Globular proteins have a range of functions in living organisms, some including haemoglobin, insulin and amylase.

Question 45

Describe how haemoglobin is a globular protein.

Answer 45

Haemoglobin is a globular protein that carries oxygen around the body in red blood cells. It’s known as a conjugated protein - this means it’s a protein with a non-protein group attached. The non-protein part is called a prosthetic group. Each of the four polypeptide chains in haemoglobin has a prosthetic group called haem. A haem group contains iron, which oxygen binds to.

Question 46

Describe how insulin is a globular protein.

Answer 46

Insulin is a hormone secreted by the pancreas. It helps to regulate the blood glucose level. It’s solubility is important - it means it can be transported in the blood to the tissues where is acts. An insulin molecule consists of two polypeptide chains, which are held together by disulfide bonds. When they’re in the pancreas, six of these molecules bing together to form a large, globular structure.

Question 47

Describe how amylase is a globular protein.

Answer 47

Amylase is a enzyme that catalyses the breakdown of starch in the digestive system. It is made of a single chain of amino acids. It’s secondary structure contains both alpha-helix and beta-pleated sheet sections. Most enzymes are globular proteins.

Question 48

What are fibrous proteins and describe their structure?

Answer 48

Fibrous proteins are tough and rope-shaped. They’re also insoluble and strong. They’re structural proteins and are fairly unreactive (unlike many globular proteins).

Question 49

Describe how collagen is a fibrous protein.

Answer 49

Collagen is found in animal connective tissues, such as bone, skin and muscle. It is a very strong molecule. Minerals can bind to the protein to increase its rigidity, e.g. in the bone.

Question 50

Describe how keratin is a fibrous protein.

Answer 50

Keratin is found in many of the external structures of animals, such as skin, hair, nails, feathers and horns. It can either be flexible (as it is in skin) or hard and tough (as it is in nails).

Question 51

Describe how elastin is a fibrous protein.

Answer 51

Elastin is found in elastic connective tissue, such as skin, large blood vessels and some ligaments. It is elastic, so it allows tissues to return to their original shape after they have been stretched.

Question 52

What is an atom and what are inorganic ions?

Answer 52

An ion is an atom (or group of atoms) that has an electric charge. An inorganic ion is one which doesn’t contain carbon (although there are a few exceptions to this rule). Inorganic ions are really important in biological processes.

Question 53

What is a cation?

Answer 53

An ion with a positive charge is called a cation.

Question 54

Describe 5 cations, including their chemical symbol and an example of their roles in biological processes.

Answer 54

Calcium, Ca 2+, is involved in the transmission of nerve impulses and the release of insulin from the pancreas. Acts as a cofactor for many enzymes, e.g. those involved in blood clotting. Is important for bone formation
Sodium, NA +, is important for generating nerve impulses, for muscle contraction and for regulating fluid balance in the body.
Potassium, K+, is important for generating nerve impulses, for muscle contraction and for regulating fluid balance in the body. It also activates essential enzymes needed for photosynthesis in plant cells.
Hydrogen, H+, affects the pH of substances (more H+ ions than OH- ions in a solution creates an acid). Also important for photosynthesis reactions that occur in the thylakoid membranes inside chloroplasts and respiration reactions that occur in the inner membrane of mitochondria.
Ammonium, NH 4+, is absorbed from the soil by plants and is an important source of nitrogen (which is the used to make, e.g. amino acids, nucleic acids).

Question 55

What is an anion?

Answer 55

An ion with a negative charge is called an anion.

Question 56

Describe 5 anions, including their chemical symbol and example of roles in biological processes.

Answer 56

Nitrate, NO 3-, is absorbed from the soil by plants and is an important source of nitrogen (which is then used to make, e.g. amino acids, nucleic acids).
Hydrogencarbonate, HCO 3-, acts as a buffer, which helps to maintain the pH of the blood.
Chloride, CL-, is involved in the ‘chloride shift’ which helps to maintain the pH of the blood during gas exchange. Acts as a cofactor for the enzyme amylase. Also involved in some nerve impulses.
Phosphate, PO 4 3-, involved in photosynthesis and respiration reactions. Needed for the synthesis of many biological molecules, such as nucleotides (including ATP), phospholipids, and calcium phosphate (which strengthens bones).
Hydroxide, OH-, affects the pH of substances (more OH- ions than H+ ions in a solution creates an alkali).

Question 57

What is qualitative testing?

Answer 57

Qualitative testing is how you determine whether a substance is present in a sample or not. There are different qualitative tests for different biological molecules.

Question 58

Why would you use the biuret test and how do you carry out this test?

Answer 58

If you needed to find out if a substance contained protein, you’d use the biuret test. There are two stages to the test.
1. The test solution needs to be alkaline, so first you add a few drops of sodium hydroxide solution.
2. Then you add some copper(II) surface solution.
If protein is present, the solution turns purple. If there’s no protein, the solution will stay blue. The colours can be fairly pale so you might need to look carefully.

Question 59

Why would use the iodine test and how do you carry out this test?

Answer 59

The iodine test is a test for starch.
If you want test for the presence of starch in a sample, you’ll need to do the iodine test. Just add iodine dissolved in potassium iodide solution to the test sample. If starch is present, the sample change from browny-orange to a dark, blue-black colour. If there is no starch, it stays browny-orange.

Question 60

Why would you use the emulsion test and how do you carry out the test?

Answer 60

The emulsion test is a test for lipids.
If you want to test for the presence of lipids in a sample, you’ll need to do the emulsion test. To do this you shake the test substance with ethanol for about a minute, then pour the solution into water. If lipid is present, the solution will turn milky. The more lipid there is, the more noticeable the milky colour will be. If there’s no lipid, the solution will stay clear.

Question 61

Why would you use the Benedict’s test and how do you carry out the test?

Answer 61

Sugar is a general term for monosaccharides and disaccharides. All sugars can be classified as reducing or non-reducing. To test for sugars, you use the Benedict’s test. The test differs depending on the type of sugar you are testing for.
Reducing sugars include all monosaccharides (e.g. glucose) and some disaccharides (e.g. maltose and lactose). You add Benedict’s reagent (which is blue) to a sample and heat it in a water bath that’s been brought to the boil. If the test’s positive it will form a coloured precipitate - solid particles suspended in the solution. The colour of the precipitate changes so that;
If the sample stays blue, a non-reducing sugar is present and if the sample forms green-yellow-orange-brick red precipitate, a reducing sugar is present.
The higher the concentration of reducing sugar, the further the colour change goes - you can use this to compare the amount of reducing sugar in different solutions. (A more accurate way of doing this is to filter the solution and weigh the precipitate).
If the result of the reducing sugars test is negative, there could still be a no reducing sugar present. To test for non-reducing sugars, like sucrose, first you have to break them down into monosaccharides. You do this by getting a new sample of the test solution (i.e. not the same one you’ve already added Benedict’s reagent to) adding dilute hydrochloric acid and carefully heating it in a water bath that’s been brought to a boil. You then neutralise it with sodium hydrocarbonate. Then just carry out the Benedict’s test as you would for a reducing sugar.

Question 62

How do you test for glucose?

Answer 62

Glucose can also be tested for using test strips coated in a reagent. The strips are dipped in a test solution and change colour if glucose is present . The colour change can be compared to a chart to give an indication of the concentration of glucose present. The strips are useful for testing a person’s urine for glucose, which may indicate they have diabetes.

Question 63

What are quantitative tests?

Answer 63

Quantitative tests tell you the amount (i.e. concentration) of a substance that is present in a sample

Question 64

How do you get a quantitative estimate of how much glucose (or other reducing sugar) there is in a solution?

Answer 64

You can use Benedict’s reagent and a colorimeter to get a quantitative estimate of how much glucose (or other reducing sugar) there is in a solution.

Question 65

What is a colorimeter, what does it measure and how do you carry out this test?

Answer 65

A colorimeter is a device that measures the strength of a coloured solution by seeing how much light passes through it. A colorimeter measures absorbance (the amount of light absorbed by the solution). The more concentrated the colour of the solution, the higher the absorbance is.
To find out the glucose concentration of an unknown solution, you first need to make up several solutions of known glucose concentrations, then measure the absorbance of these solutions, and finally plot these absorbances on a graph to make a calibration curve. You can then use the calibration curve to estimate the concentration of glucose in the unknown solution
It’s easiest to measure the concentration of the blue Benedict’s solution that’s left after the test (the paler the solution, the more glucose there was). So, the higher the glucose concentration, the lower the absorbance of the solution.

Question 66

How do you make known concentrations of glucose? (Serial dilution)

Answer 66

1. Line up five test tubes in a rack
2. Add 10cm3 of the initial 40mM sucrose solution to the fist test tube and 5cm3 of distilled water to the other four test tubes.
3. Then, using a pipette, draw 5cm3 of the solution from the first test tube, add it to the distilled water in the second test tube and mix the solution thoroughly. You now have 10cm3 of solution that’s half as concentrated as the solution in the first tube (it’s 20mM).
4. Repeat the process three more times to create solutions of 10mM, 5mM and 2.5mM.

Question 67

How do you measure the absorbance of known solutions?

Answer 67

• do a Benedict’s test on each solution (plus a negative control of pure water). Use the America’s amount of Benedict’s solution in each case.
• emote any precipitate - either leave for 24 hours or centrifuge them.
• use a colorimeter to measure the absorbance of the Benedict’s solution remaining in each tube. The method is outlines:
  1. Switch the colorimeter on and allow five minutes for it to stabilise. Then set up the colorimeter so you’re using a red filter (or a wavelength of 630nm).
  2. Add distilled water to a cuvette so it is three quarters full (a cuvette is a small container that fits inside a colorimeter). Put the cuvette into the colorimeter. Two of the cuvette‘s sides may be ridged or frosted - you need to make sure you put the cuvette into the colorimeter the correct way, so that the light will be passing through the clear sides. Calibrate the machine to zero.
  3. Next, use a pipette to transfer a sample of the solution from the first test tube to a clean cuvette - again it should be about three quarter full.
  4. Put the cuvette in the colorimeter and read and record the absorbance of the solution.
  5. Repeat steps 1-4 for the remaining solutions, using a clean pipette and cuvette each time.

Question 68

What is a biosensor and give an example.

Answer 68

A biosensor is a device that uses a biological molecule, such as an enzyme to detect a chemical. The biological molecule produces a signal (e.g. a chemical signal), which is converted to an electrical signal by a transducer (another part of the biosensor). The electrical signal is then processed and can be used to work out other information.
A glucose biosensor is used to determine the concentration of glucose in a solution. It does this by using the enzyme glucose oxidase and electrodes. The enzyme catalyses the oxidation of glucose at the electrodes - this creates a charge, which is converted into an electrical signal by the electrodes (the transducer). The electrical signal is then processed to work out the initial glucose concentration.

Question 69

What is chromatography and what is it used for?

Answer 69

Chromatography is used to separate stuff in a mixture - once it’s separated out, you can often identify the components. For example, chromatography can be used to separate out and identify biological molecules such as amino acids, carbohydrates, vitamins and nucleic acids. There are quite a few different types of chromatography.

Question 70

How does chromatography work?

Answer 70

All types of chromatography (including paper and thin-layer) have the same basic set up:

• a mobile phase - where the molecules can move. In both paper and thin-layer chromatography, the mobile phase is a liquid solvent, such as ethanol or water.
• a stationary phase - where the molecules can’t move. In paper chromatography, the stationary phase is a piece chromatography paper. In thin-layer chromatography the stationary phase is a thin layer of solid, e.g. silica gel, on a glass or plastic plate.

They all use the same basic principle:

• the mobile phase moves through or over the stationary phase.
• the components in the mixture spend different amounts of time in the mobile phase and the stationary phase.
• the components that spend longer in the mobile phase travel faster or further.
• the time spent in the different phases is what separates out the components of the mixture.

Question 71

What are the steps in separating amino acids?

Answer 71

1. Draw a pencil line near the bottom of a piece of chromatography paper and put a concentrated spot of the mixture of amino acids on it. It’s best to carefully roll the paper into a cylinder with the spot on the outside so it’ll stand up.
2. Add a small amount of prepared solvent to a beaker and dip the bottom of the paper into it (not the spot). This should be done in a fume cupboard. Cover with a lid to stop the solvent evaporating.
3. As the solvent spreads up the paper, the different amino acids (solutes) move with it, but at different rates, so they separate out.
4. When the solvent’s nearly reached the top, take the paper out and mark the solvent front with pencil. Then you can leave the paper to dry out before you analyse it.
5. Amino acids aren’t coloured, which means you won’t be able to see them on paper. So before you can analyse them, you have to spray the paper with ninhydrin solution to turn the amino acids purple. This should also be done in a fume cupboard.

Question 72

What are rf values?

Answer 72

You can use Rf values to identify the separated molecules. An Rf value is the ratio of the distance travelled by a solute to the distance travelled by the solvent. You can calculate it using this formula:
Rf value = distance moved by the solute DIVIDED BY distance moved by the solvent.