Carbohydrates
Introduced| English | Meaning |
|---|---|
| monosaccharide/ˈmɒnəʊsækɑːraɪd/ | A single sugar unit that cannot be hydrolysed into smaller carbohydrates. |
| disaccharide/ˌdɪsəkˈhɑːraɪd/ | A carbohydrate made of two monosaccharide units joined by a glycosidic bond. |
| polysaccharide/ˌpɒlɪˈsækɑːraɪd/ | A carbohydrate polymer composed of many sugar units linked together. |
| cellulose/ˈseljuːləʊs/ | A structural polysaccharide made of glucose units, forming a major component of plant cell walls. |
| condensation/kɒndenˈseɪʃn/ | A chemical reaction in which molecules join and a small molecule, commonly water, is eliminated. |
| glycosidic bond/ˌɡlaɪkəˈsɪdɪk bɒnd/ | A covalent linkage joining a sugar to another group, including the links between sugar units in carbohydrates. |
| hydrolysis/haɪˈdrɒləsɪs/ | Breaking chemical bonds by reaction with water. |
The body's quick fuel
- Carbohydrates are the cell's quick energy molecules — and its building material.
- They come in three sizes, all built from sugar units.
- A tiny difference in one glucose shape decides everything that follows.
Three sizes of sugar
- A monomer is a single small unit; a polymer is many monomers joined into a long chain.
- A monosaccharide 单糖 is one sugar unit — glucose, fructose.
- A disaccharide 二糖 is two units joined — maltose, sucrose.
- A polysaccharide 多糖 is many units joined into a polymer — starch, glycogen, cellulose 纤维素.
- Glucose, fructose and maltose are reducing sugars; sucrose is non-reducing.
Alpha- and beta-glucose differ only at carbon 1
Which is a monosaccharide?
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Glucose is a single sugar unit (monosaccharide); sucrose is a disaccharide; starch and glycogen are polysaccharides.
Match each carbohydrate to what it is.
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Mono = one unit, di = two, poly = many. Cellulose is β-glucose; glycogen is α-glucose.
Sucrose is a non-reducing sugar, so it gives a negative Benedict's test unless it is first hydrolysed.
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Glucose and maltose are reducing sugars (positive Benedict's); sucrose is non-reducing, so you must hydrolyse it (acid + heat) first.
Joining and breaking: α and β glucose
- Glucose is a six-carbon sugar that folds into a six-membered ring (five carbons and one oxygen), with two forms: in α-glucose the –OH on carbon 1 points down; in β-glucose it points up.
- Two sugars join by condensation 缩合: a glycosidic bond 糖苷键 forms and one water molecule is removed.
- Hydrolysis 水解 is the reverse: adding water breaks the bond. (That's why the sucrose test needs acid + heat.)
- Step through it below to see one water leave as the bond forms.
Condensation and hydrolysis
Step through how two sugars join. Condensation removes one water to make the bond; hydrolysis is the reverse — adding water splits it again.
Two monosaccharides join by a ______ bond, removing one water molecule.
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A glycosidic bond forms by condensation; one water molecule is removed for each bond.
Put the steps of condensation in order.
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Condensation removes one water to form the bond; hydrolysis (adding water) is the reverse.
α-glucose and β-glucose differ in:
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They are the same atoms — only the –OH on carbon 1 differs: down in α, up in β. This decides which polysaccharide forms.
Storage vs structure
- Starch (plants) and glycogen (animals) are made of α-glucose — coiled/branched, compact and insoluble, so they store energy without pulling in water by osmosis. Glycogen is the most branched, so it breaks down fastest.
- Cellulose is made of β-glucose: every other unit flips, giving long straight chains that bundle into strong microfibrils — the plant cell wall.
Amylose coils, amylopectin and glycogen branch, and cellulose forms straight bundled chains
Cellulose is strong and structural because it is made of:
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β-glucose chains are straight; many lie side by side and hydrogen-bond into strong microfibrils — the cell wall.
Why is glycogen a good energy store in animals?
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Storage polysaccharides are compact and insoluble (no osmotic effect); glycogen's many branches give many ends for fast glucose release.
You've got it
- mono → di → polysaccharide; reducing sugars (glucose, maltose) vs non-reducing (sucrose)
- α-glucose –OH down → starch/glycogen (storage); β-glucose –OH up → cellulose (structure)
- condensation joins monomers (glycosidic bond, water removed); hydrolysis splits them (water added)
- storage polysaccharides are compact + insoluble; glycogen is most branched (fast release)