0610 · 4.1Biological molecules

Elements and biological molecules

0:00Look at a simple meal: a slice of bread, an egg, some butter and an orange. To you, they are four different foods. To a biologist, they are mostly made of just a handful of kinds of molecule, built from just a handful of chemical elements. Bread is mostly starch, the egg is full of protein, butter is mostly fat, and the orange is rich in sugar and vitamin C.

0:20And with a few simple tests in a test tube, you can find out which of these molecules any food contains. By the end of this video you will be able to list the elements in carbohydrates, fats and proteins, explain how large molecules are built from smaller ones, and describe the five food tests the syllabus asks for. For the extended syllabus, you will also be able to describe the structure of DNA.

0:40You need one idea before we start. Everything is made of chemical elements, such as carbon, hydrogen and oxygen. Atoms of these elements join together to make molecules. The molecules that make up living things are called biological molecules, and many of them are very large. Let us start with the elements. The syllabus asks you to list the chemical elements that make up carbohydrates, fats and proteins. Carbohydrates, such as sugars and starch, are made of carbon, hydrogen and oxygen.

1:08Fats are also made of carbon, hydrogen and oxygen, the same three elements, but with much less oxygen than carbohydrates. Proteins are made of carbon, hydrogen, oxygen and nitrogen. Some proteins also contain a little sulfur. So the key difference to remember is that proteins contain nitrogen, and carbohydrates and fats do not. That is why plants need nitrate ions from the soil, as you will see in a later chapter.

1:30Nitrogen is needed to make proteins. Next, the syllabus wants you to state that large molecules are made from smaller molecules. Think of it like a bead necklace. Each bead is a small molecule, and when many beads are joined together in a chain, they make one large molecule. There are three examples you need. The first is the carbohydrates starch, glycogen and cellulose. All three are made from many glucose molecules joined together.

1:55They are different because the glucose molecules are joined in different ways. Starch is the store of energy in plants, which is why potatoes and bread are full of it. Glycogen is the store of energy in animals and fungi, found in the liver and muscles. And cellulose makes up plant cell walls, as you saw in the cell structure video. Same building block, glucose, but three very different molecules.

2:17The second example is proteins, which are made from amino acids. There are about twenty different kinds of amino acid. Think of them as twenty different kinds of bead. They can be joined together in any order, and different orders make different proteins. That is how living things can make so many different proteins, from the keratin in your hair to the enzymes that digest your food. The third example is fats and oils, which are made from fatty acids and glycerol.

2:41Each fat molecule is usually made of one glycerol molecule joined to three fatty acids. Fats are solid at room temperature, like butter, and oils are liquid, like olive oil, but they are built in the same way. A common mistake is to say that fats are made of glucose, or that proteins are made of fatty acids. Keep the three pairs straight. Starch, glycogen and cellulose come from glucose.

3:03Proteins come from amino acids. Fats and oils come from fatty acids and glycerol.

Starch, sugar and protein tests

3:09Now to the food tests. The syllabus asks you to describe five of them. For each one, you need to know what you add, whether you heat it, and the colour change that means the result is positive. If the food is solid, first crush it and mix it with a little water, so that the test has a liquid to work with. For the ethanol test, crush the food with ethanol instead of water.

3:27The first is the iodine test for starch. Add a few drops of iodine solution to the food. Iodine solution is orange-brown. If starch is present, it turns blue-black. If there is no starch, it stays orange-brown. You can even drop iodine onto a cut potato and watch the surface turn blue-black. The second is Benedict's test for reducing sugars. Reducing sugars include glucose. Add Benedict's solution, which is blue, to the food, and then heat it in a hot water bath at about 80 degrees Celsius for a few minutes.

3:55This test needs heat. Without heating, there will be no colour change, even if sugar is present. If a reducing sugar is present, the blue colour changes to green, then yellow, then orange, and finally brick red. The more reducing sugar there is, the further along that sequence it goes. So green means only a little sugar, and brick red means a lot. If there is no reducing sugar, the solution stays blue.

4:18The third is the biuret test for proteins. Add biuret solution, which is blue, to the food and shake gently. No heating is needed. If protein is present, the solution turns purple, sometimes described as violet or lilac. If there is no protein, it stays blue. Egg white is a good food to try this on.

Fat and vitamin C tests

4:35The fourth is the ethanol emulsion test for fats and oils. First, shake the food with ethanol, so that any fat dissolves in it. Then pour the ethanol, without the solid food, into a test tube of water. If fat is present, a cloudy white emulsion forms. An emulsion is tiny droplets of fat spread through the water. If there is no fat, the mixture stays clear. Ethanol catches fire easily, so this test must be done well away from any flames.

4:59The fifth is the DCPIP test for vitamin C. DCPIP is a blue solution. Add the food, such as a fruit juice, drop by drop to the DCPIP. If vitamin C is present, the blue colour disappears and the DCPIP turns colourless. This test can also compare foods. The fewer drops of juice you need to make the DCPIP colourless, the more vitamin C the juice contains. So if orange juice needs 5 drops and apple juice needs 20 drops, the orange juice has more vitamin C.

5:29Let us put the five tests together. Iodine solution tests for starch, turning orange-brown to blue-black. Benedict's solution, with heating, tests for reducing sugars, turning blue to green, yellow, orange or brick red. Biuret solution tests for protein, turning blue to purple. The ethanol emulsion test shows fat as a cloudy white emulsion. And DCPIP tests for vitamin C, turning blue to colourless.

Interpreting food test results

5:53Let us try an exam-style question. A student tests a food sample. With iodine solution it stays orange-brown. With Benedict's solution, after heating, it turns brick red. With biuret solution it stays blue. What does the food contain? Take each result in turn. Iodine stayed orange-brown, so there is no starch. Benedict's turned brick red, so there is a lot of reducing sugar. Biuret stayed blue, so there is no protein.

6:19So the food contains reducing sugar, but no starch and no protein. Something like a sweet fruit juice would fit. Notice that a negative result is still a result. Saying what is not there is just as important as saying what is. Watch out for these common mistakes. The first is forgetting to heat in the Benedict's test, or heating in the other tests. Only Benedict's needs heating. The second is getting the colours mixed up.

6:41Blue-black is for starch with iodine, and purple is for protein with biuret. The third is being vague about the iodine colour. Write blue-black. Do not just write blue, which could be confused with the starting colour of Benedict's solution or biuret solution. And the fourth is writing that Benedict's tests for all sugars. It tests for reducing sugars, such as glucose.

DNA structure and base pairing

7:02For the extended syllabus, you also need to describe the structure of DNA, the molecule that carries genetic information in the nucleus. DNA is made of two strands coiled around each other, forming a shape called a double helix. Picture a ladder twisted into a spiral. Each strand contains chemicals called bases. There are four kinds of base, known by their letters: A, T, C and G. You do not need their full names.

7:26The bases on one strand pair up with the bases on the other strand, like the rungs of the ladder, and the bonds between each pair of bases hold the two strands together. The most important rule is that the bases always pair up in the same way. A always pairs with T, and C always pairs with G. A never pairs with C or G, and C never pairs with A or T.

7:46So if you know the bases on one strand, you can always work out the bases on the other. Here is a quick example. One strand reads A, T, T, C, G. What is the other strand? Swap each base for its partner. A pairs with T, T pairs with A, T pairs with A, C pairs with G, and G pairs with C. So the other strand reads T, A, A, G, C.

8:10And here is a type of question that often appears. A DNA molecule has 30 percent of its bases as A. What percentage is G? Because A always pairs with T, there must also be 30 percent T. That makes 60 percent together. The remaining 40 percent is split equally between C and G, because they pair with each other. So G is 20 percent. Let us recap. Carbohydrates and fats are made of carbon, hydrogen and oxygen, and proteins also contain nitrogen.

8:38Large molecules are built from smaller ones. Starch, glycogen and cellulose are made from glucose, proteins from amino acids, and fats and oils from fatty acids and glycerol. For the food tests, iodine solution turns blue-black with starch. Benedict's solution, heated, turns from blue through green, yellow and orange to brick red with reducing sugars. Biuret solution turns purple with protein. Ethanol gives a cloudy white emulsion with fats. And DCPIP turns from blue to colourless with vitamin C.

9:07And DNA is two strands coiled into a double helix, held together by bonds between pairs of bases, with A always pairing with T, and C always pairing with G. In the next video, we will look at one very important group of proteins, the enzymes.