Biological molecules and the structure of DNA
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The teaching part (2 min) is free to watch. A free account opens the whole lecture: 3 worked examples, 5 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- What are you made of?
- The elements
- Carbohydrates
- Fats and proteins
- Big molecules, small units
- Food tests
- DNA
- Base pairing
Read the transcript of the teaching part
Last time
Before we start: last time, in Osmosis and plant cells in solutions, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
What are you made of?
Your body is mostly water, and almost everything else is made of three kinds of large molecules: carbohydrates, fats and proteins. Each one is built from smaller units, like a wall from bricks. And the instructions for building you are stored in one more molecule: DNA.
The elements
Carbohydrates contain carbon, hydrogen and oxygen. Fats and oils contain carbon, hydrogen and oxygen too, but much less oxygen. Proteins contain carbon, hydrogen, oxygen and nitrogen, and some contain sulfur. So nitrogen is the element that tells you it is a protein.
Carbohydrates
Glucose is a simple sugar. Many glucose molecules join to make large carbohydrates. Starch is the energy store in plants. Glycogen is the energy store in animals and fungi. Cellulose makes plant cell walls. All three are made from glucose.
Fats and proteins
Fats and oils are made from fatty acids and glycerol. Proteins are made from amino acids joined in long chains. There are about twenty different amino acids, and their order decides the shape of the protein.
Big molecules, small units
Here is the table to learn. Each large molecule and the small units it is made from. Notice that starch, glycogen and cellulose all come from glucose.
Food tests
Iodine solution turns from orange-brown to blue-black with starch. Benedict's solution, heated, turns from blue to orange-red with reducing sugars such as glucose. Biuret solution turns from blue to purple with protein. Ethanol gives a cloudy white emulsion with fats and oils.
DNA
DNA is made of two strands coiled together to form a double helix. Each strand contains chemicals called bases. There are four: A, T, C and G. The two strands are held together by bonds between pairs of bases.
Base pairing
The key rule. A always pairs with T, and C always pairs with G. So if you know one strand, you can write the other.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Chemical elements in biological molecules · Large molecules from smaller molecules · The structure of DNA · DNA · Elements and large molecules.
The whole chapter: Biological Molecules, Inheritance, Biological Molecules.
Chapter 4: Biological molecules
- Biological molecules and the structure of DNA 4 min
Also for IGCSE Biology 0610
Which topics come up most in IGCSE Biology 0610 past papers, counted from every paper since 2004.