Molecules, Transport and Health

Water and Carbohydrates

Pearson Edexcel International A Level Biology

Water

A water molecule is two hydrogens joined to an oxygen by covalent bonds.
There is an uneven distribution of electrons: the oxygen pulls the electrons towards it.
The oxygen atom is slightly negative and the hydrogen atoms are slightly positive: δ− on the O, δ+ on each H. Water is dipolar.
Write oxygen atom and hydrogen atoms. Never write oxygen and hydrogen molecules in a water molecule.
Water is a solvent, so substances are transported dissolved in it.
Around a positive ion: water molecules clustered around the Na⁺ with the O facing the Na⁺ and the Hs facing away.
A hydrogen bond forms between the O of one water molecule and an H of another.

The oxygen of water carries a small negative charge and each hydrogen a small positive charge; around a positive ion, the oxygen of each water molecule faces the ion.

Monosaccharides, disaccharides and polysaccharides

Carbohydrates contain carbon, hydrogen and oxygen only.
Glucose is a monosaccharide and glycogen is a polysaccharide. Glucose, fructose and galactose are monosaccharides.
Starch and glycogen are both polysaccharides formed from many alpha glucose monomers.

Disaccharide	Monosaccharides
Maltose	α glucose and α glucose
Sucrose	Glucose and fructose
Lactose	Glucose and galactose

Sucrose is C₁₂H₂₂O₁₁: glucose and fructose, each C₆H₁₂O₆, join and H₂O is formed. The O joins carbon 1 of glucose to carbon 2 of fructose.
The disaccharide in milk is lactose. Never write lactase as the disaccharide.

Condensation and hydrolysis

Condensation joins two molecules and forms water. Hydrolysis splits a bond by adding water.
Two monosaccharides join by condensation, forming a glycosidic bond, and one molecule of water is formed.
Disaccharides and polysaccharides are broken down by hydrolysis of glycosidic bonds.
Different enzymes needed to break each type of glycosidic bond.
Write hydrolysis. Never write hydration for splitting a glycosidic bond.

Two α-glucose molecules join by condensation between the OH groups on carbon 1 and carbon 4, forming maltose with a 1,4 glycosidic bond and a molecule of water.

Starch and glycogen

Starch is a mixture of amylopectin and amylose: starch is composed of two polysaccharides, glycogen only one.
Amylose, amylopectin and glycogen are all composed of α glucose.
Starch is the store in plants; glycogen is the store in animals.

	Amylose	Amylopectin	Glycogen
Bonds	1,4 only	1,4 and 1,6	1,4 and 1,6
Shape	helical, unbranched	branched	branched

Write amylopectin has 1,4 and 1,6 bonds. Never write amylopectin has only 1,6 bonds.
As stores: large molecule therefore insoluble, so no osmotic effect; compact therefore has a high energy density; branched structure therefore hydrolysis is faster.

Amylopectin and glycogen are branched chains of α-glucose: 1,4 glycosidic bonds along each chain and 1,6 glycosidic bonds at the branches.

Core practical 1: Use a semi-quantitative method with Benedict's reagent to estimate the concentrations of reducing sugars and with iodine solution to estimate the concentrations of starch, using colour standards

Make known concentrations of reducing sugar or starch by serial dilution of stock solution.
Put the same volume of each into test tubes of the same diameter.
Add the same volume of Benedict's reagent or of iodine solution to each tube.
Benedict's only: heat in a water bath for the same time.
Record the colour of each tube: these are the known standards.
Treat the sample the same way: use same mass for each food and same volume of distilled water.
Sample compared with the known standards.
Estimate the concentration: for iodine, the darker the colour the more starch; for Benedict's, from the colours below.

Colour with Benedict's reagent	Reducing sugar
Blue	None
Green	Trace
Green with precipitate	Very low
Yellow with precipitate	Low
Orange with precipitate	Moderate
Red with precipitate	High

Semi-quantitative result: an estimate, subjective. Quantitative result: objective, exact.
Quantitative method: filter the solution, then weigh the precipitate remaining in the filter paper.
The palest blue filtrate had the most reducing sugar, therefore leaving least copper ions in the solution.

Estimating from the standards

The estimate comes from the colours of the known concentrations, made and tested the same way as the sample, so write sample compared with the known standards. Never write just colour chart used or compare with a calibration curve for the iodine estimate.

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