Polysaccharides are long chains of monosaccharides. Four MCAT-critical polysaccharides: glycogen, starch, cellulose, and chitin. Each is built from a simple sugar but with distinctive linkages that give it a specific role.
The linkage decides everything
Polysaccharides
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Plant storage (starch) Animal storage (glycogen) Structural, and indigestible to us The linkage that decides everything
One bond, two worldsAmylose and cellulose are both unbranched chains of glucose. The only difference is α versus β at carbon 1. α gives a helix you can eat; β gives a flat ribbon that hydrogen bonds into fibers you cannot. Humans have no β-glucosidase, which is why cellulose is dietary fiber rather than food.
Why glycogen is so branchedPhosphorylase can only work from a non-reducing end. Branching every ten residues multiplies those ends, so a whole molecule can be dismantled at once when adrenaline arrives. Amylopectin branches every twenty-five, which is fine for a plant that is in no hurry.
Reducing or notA disaccharide is reducing if at least one anomeric carbon is still free. Maltose and lactose each keep one free. Sucrose joins glucose C1 to fructose C2, locking both, which is why it is the classic non-reducing sugar.
Read the linkage and you have the answer. α means digestible and helical; β means structural and, for us, indigestible. A 1,6 bond means a branch point, which means speed. Everything else on this map follows from those two facts.
Storage Polysaccharides
Glycogen (animals)
Glycogen is the animal storage form of glucose. It is stored mainly in liver (for regulating blood glucose) and muscle (for local energy during exercise). Structure: alpha-1,4 main chain with alpha-1,6 branches every 8-12 residues. The branching is extensive because:
Branches create many non-reducing ends where glycogen phosphorylase can simultaneously remove glucose - rapid mobilization.
Branches pack the polymer compactly and increase water-solubility.
Starch is the plant storage form of glucose. It has two components:
Amylose: unbranched alpha-1,4 chains of glucose. Forms a helix.
Amylopectin: alpha-1,4 main chains with alpha-1,6 branches every 24-30 residues. Less branched than glycogen.
Structural Polysaccharides
Cellulose
Cellulose is the structural polysaccharide of plant cell walls. It is a long chain of glucose units joined by beta-1,4 bonds. The beta linkage makes the chain flat and extended, and adjacent chains hydrogen-bond into rigid sheets and microfibrils.
Humans do not make cellulase and cannot digest cellulose. Cellulose is the main component of dietary fiber, which adds bulk to stool and affects gut transit time.
Chitin
Chitin is the structural polysaccharide of insect exoskeletons and fungal cell walls. It is like cellulose but the monomer is N-acetylglucosamine (GlcNAc, glucose with an -NHCOCH3 on C2) linked by beta-1,4 bonds. The extra N-acetyl groups form additional hydrogen bonds, making chitin even tougher than cellulose.
Why Store Glucose as Glycogen Instead of Free Glucose
Free glucose is osmotically active - every millimolar of dissolved glucose contributes to cellular osmotic pressure. A liver cell storing 100 mM worth of free glucose would draw in so much water that it would burst. Glycogen is one giant polymer: each glycogen particle bundles tens of thousands of glucose residues into a single molecule, so the same 100 mM of glucose residues counts as only a few micromolar glycogen particles. Osmotic pressure drops by roughly the branching factor.
Why is glycogen more extensively branched than starch, and why does that matter?
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Glycogen has alpha-1,6 branches every 8-12 residues; amylopectin every 24-30. More branches mean more non-reducing ends, so more glycogen phosphorylase molecules can chew off glucose at the same time during a rapid energy demand (sprint, fight-or-flight). Animals tend to need glucose faster than plants, so our storage polymer is built for fast mobilization.
What is the structural difference between cellulose and chitin?
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Cellulose is a polymer of glucose connected by beta-1,4 bonds. Chitin is a polymer of N-acetylglucosamine (glucose with an -NH-CO-CH3 group on C2) connected by beta-1,4 bonds. The N-acetyl groups in chitin allow extra hydrogen bonding, making chitin even more rigid than cellulose. Both are structural polysaccharides of beta-1,4 linkages.
Why does the body store glucose as glycogen rather than as free glucose?
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Osmotic pressure. Free glucose is osmotically active - millimolar concentrations pull water into the cell. Polymerizing thousands of glucoses into a single glycogen molecule reduces the particle count by thousands and thereby minimizes osmotic pull. The cell can store large amounts of glucose without drawing in destructive amounts of water.
