Polysaccharides, lipids and water
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The teaching part (2 min) is free to watch. A free account opens the whole lecture: 4 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
- Store it, build with it, dissolve in it
- Starch
- Glycogen and cellulose
Read the transcript of the teaching part
Last time
Before we start: last time, in Monomers, sugars and Benedict's test, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Store it, build with it, dissolve in it
A plant needs to store energy, build a strong wall, keep cell membranes together and move materials around. It uses polysaccharides for storing and building, lipids for membranes and long term fuel, and water for almost everything else. Each molecule's shape explains its job.
Starch
Starch is the energy store of plants. It is a mixture of two polymers of alpha glucose: amylose and amylopectin. Amylose is an unbranched chain with alpha one four bonds. The chain coils into a helix, which makes it compact. Amylopectin has alpha one four bonds in its chains and alpha one six bonds at the branch points. Branching gives many ends for enzymes to work on. Starch is insoluble, so it does not change the water potential of the cell, and it is compact, so a lot of glucose fits in a small space.
Glycogen and cellulose
Glycogen is the energy store of animals and fungi. It is similar to amylopectin but more highly branched, with alpha one four and alpha one six bonds. Many branches mean many ends, so enzymes can hydrolyse glucose off quickly. That suits animals, which need energy fast. Cellulose is built from beta glucose. Every other glucose is turned upside down, giving long straight unbranched chains with beta one four bonds. Hydrogen bonds between neighbouring chains make microfibrils, which give the plant cell wall great tensile strength.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Lipids: triglycerides and phospholipids · Polysaccharides: starch, glycogen and cellulose · Properties of water · Polysaccharides: starch, glycogen and cellulose.
The whole chapter: Biological Molecules, Biological Molecules.
Chapter 2: Biological molecules
- Monomers, sugars and Benedict's test 6 min
- Polysaccharides, lipids and water 6 min
- Amino acids, protein structure, haemoglobin and collagen 6 min
Also for A Level Biology 9700
Which topics come up most in A Level Biology 9700 past papers, counted from every paper since 2004: Paper 1 topic frequency, Paper 2 topic frequency, Paper 4 topic frequency.