chapter 3: biological molecules

define carbohydrates

organic molecules made up of the elements carbon, hydrogen, and oxygen. Ratio of H:O = 2:1

C_nH_2mO_m

describe the 3 common single sugars

monosaccharides	occurrence	chem formula
glucose	found in plants and animals	C6H12O6 but different arrangement of atoms
fructose	common in plants, rare in animals	C6H12O6 but different arrangement of atoms
galactose	present in milk sugar in mammals	C6H12O6 but different arrangement of atoms

describe the 3 common double sugars

disaccharides	made up of	chem formula
maltose	glucose + glucose	C12H11O6
sucrose	glucose + fructose	C12H11O6
lactose	glucose + galactose	C12H11O6

condensation of monosaccharides

condensation: reaction in which two simple molecules are joined together to form a larger molecule with the removal of one molecule of water

glucose + glucose —> maltose + water

glucose + fructose —> sucrose + water

glucose + galactose —> lactose + water

hydrolysis of disaccharides

hydrolysis: reaction in which water is used to break up a complex molecule into smaller molecules

maltose + water —> glucose + glucose

sucrose + water —> glucose + fructose

lactose + water —> glucose + galactose

describe the test for reducing sugars (not sucrose)

1) Place 2cm³ of Benedict’s solution to 2cm³ of food sample into a test tube. (1:1 ratio)

2) Shake the mixture and place the test-tube in a boiling water bath (the flame should be turned off) for 2-3 mins.

3) Record the colour of the solution.

observation	amount present
solution remained blue	no reducing sugars
solution turned from blue to green	traces of reducing sugars
solution turned from blue to green to yellow / orange	moderate amount of reducing sugars
solution turned from blue to green to yellow / orange to brick red // red precipitate formed	large amount of reducing sugars

what are complex carbohydrates?

a complex carbohydrate (polysaccharide) is made up of many similar molecules of single sugar joined together to form a large molecule

starch, glycogen, and cellulose have different chemical and biological properties as the glucose molecules are linked in different ways, giving rise to different structures

describe the structure, role, and occurrence of the 3 common polysaccharides

polysaccharide	structure	role	occurrence
starch	several thousand glucose molecules joined together	storage form of carbohydrates in plants	in storage organs of plants
cellulose	many glucose molecules joined together (bonds between glucose units are different from starch)	structural support in plant - protects plant cells from bursting / damage serve as dietary fibre to prevent constipation	present in cell walls of plants
glycogen	a branched molecule made up of many glucose molecules joined together	storage form of carbohydrstes in mammals	stored in the liver and muscles of mammals

why are glycogen and starch suitable as storage materials in cells?

• insoluble in water → do not change water potential in cells

• large molecules that cannot diffuse through cell membranes → not be lost from the cell

• easily be broken down into glucose when needed, e.g. tissue respiration

• compact shapes → occupy less space compared to all the individual glucose molecules that make up a glycogen / starch molecule

describe the test for starch

1) Add a few drops of iodine in potassium iodide solution to a food sample.

2) Record your observations.

observation	conclusion
iodine turns from yellowish-brown to blue-black	starch is present
iodine remains yellowish-brown	starch is absent

what are the functions of carbohydrates?

• a substrate for respiration to provide energy for cell activities

• to be converted into other organic compounds, such as amino acids and fats

• for the formation of nucleic acids e.g. DNA

• to synthesise lubricants, e.g. mucus (carbohydrate + protein)

• to synthesise nectar in some flowers

define fats (lipids)

organic molecules made up of the elements carbon, hydrogen, and oxygen (CHO)

contain much less oxygen in proportion to hydrogen (H>>>>>>>O)

no fixed ratio

3 fatty acid molecules + glycerol + water → triglyceride

describe the test for fats

ethanol emulsion test

If it is liquid:

1) Add 2cm³ of ethanol to 2cm³ of liquid food sample in a test tube. (1:1 ratio)

2) Add 2cm³ of water to the mixture and shake vigorously.

3) Record your observation.

If it is solid:

1) Crush the solid sample into small pieces using a mortar and pestle and place the pieces in a test-tube.

2) Add 2cm³ of ethanol into the test-tube and shake thoroughly.

3) Allow the solid particles to settle. Carefully decant the ethanol into another test-tube containing 2cm³ of water and shake the mixture.

4) Record your observation

A cloudy white emulsion is produced → Fats are present

what are the functions of fats

• a source and long-term storage of energy (have a higher energy value compared to carbohydrates)

• an insulating material to prevent excessive heat loss

• solvent for fat-soluble vitamins and many other vital substances such as hormones

• essential part of cells, especially in cell membranes

• a way to reduce water loss from the skin surface - glands in the skin secrete an oily substance that forms a thin layer over the skin, reducing the rate of evaporation of water and the rate of heat loss from the skin

define proteins

organic molecules made up of the elements carbon, hydrogen, oxygen, and nitrogen. sulfur may also be present. (CHONS)

describe amino acids

• made up from an amino group (-NH₂), an an acidic group (-COOH) and a side chain (R), which may contain sulfur

• amino acids differ by their R group

• amino acids (around 20 different naturally occurring ones) are combined in various ways to form millions of different protein molecules

formation of proteins

• many amino acid molecules are joined in a linear manner to form a polypeptide

• polypeptides in turn may be linked up to form an even longer amino acid chain

• a protein molecule is made up of one or more such polypeptide chains folded together into a more complex, 3D shape

describe the test for proteins

Biuret test: Biuret solution is a blue solution made up of sodium hydroxide and copper(II) sulfate

1) Add an equal volume of biuret solution to 2cm³ of liquid food sample

2) Shake well and allow the mixture to stand for 5 mins.

3) Record your observation

Alternative:

1) Add 2cm³ of sodium hydroxide solution to 2cm³ of food sample in a test tube. (1:1 ratio)

2) Shake the mixture thoroughly.

3) Add 1% copper(II) sulfate solution drop by drop, shaking the mixture after each drop.

observation	conclusion
biuret solution remains blue	proteins are absent
biuret solution turns from blue to violet	proteins are present

what are the functions of proteins?

• synthesis of new protoplasm, for growth and repair of worn-out body cells

• synthesis of enzymes and some hormones

• formation of antibodies to combat diseases