2.1.2 Biological Molecules

What is the structure of water?

Water consists of 2 hydrogen atoms covalently bonded to one oxygen atom

How does hydrogen bonding occur in water?

Each hydrogen shares a pair of electrons with the oxygen (covalent bonding).

Oxygen has a greater affinity for electrons than hydrogen, so it ‘pulls’ the electrons closer

What are the properties of water?

• High specific heat capacity

• High latent heat of vaporisation

• Surface tension

• A solvent

• An insulator

• Higher density than ice

• A transport medium

What is surface tension and how could it benefit organisms?

The hydrogen bonds mean the water is ‘sticky’ by forces of adhesion & cohesion.

Molecules in the bulk of liquid are not affected by molecules above, therefore ‘pull’ more strongly to form a stretched membrane → a habitat can exist on water (pond skaters)

How can being a solvent be beneficial for organisms?

• dissolves most organic & inorganic substances

• needed for biochemical reactions

• removes excretory products e.g. urea

• in plants, root hairs absorb mineral salts present in soil

How can being an insulator be beneficial for organisms?

• ice is less dense than water, therefore floats above it → insulates the water below the surface of ponds & lakes

• water won’t freeze, so aquatic organisms can survive

How can a high latent heat of vaporisation be beneficial for organisms?

• loss of heat through vaporisation → sweating, which helps us cool down & prevents overheating

How does being a transport medium be beneficial for organisms?

• human blood plasma consists mainly of water (90%)

• carries many dissolved substances like excretory waste, hormones & gases around the body

• in plants, sugar & mineral salts are transported in solution in vascular bundles

What are carbohydrates?

A group of substances used as both energy sources and structural materials in organisms

• contains C, H & O

What are monosaccharides?

• simple sugar molecules (monomers)

• all are reducing sugars

• general formula = (CH2O)n

What is the difference between a hexose monosaccharide and a pentose monosaccharide?

• hexose contains 6 carbon atoms

• pentose contains 5 carbon atoms

What are the properties of glucose?

• hexose sugar

• highly soluble

What is the structure of alpha glucose?

What is the structure of beta glucose?

What is the difference between alpha and beta glucose?

The position of the -OH group attached to carbon 1:

• in alpha it is below

• in beta it is above

What is an isomer?

Molecules having the same chemical formula but a different structural one

e.g. alpha & beta glucose

What is the structure of ribose?

What are dissacharides?

• formed from 2 monosaccharides

• joined by glycosidic bond → condensation reaction

How are disaccharides broken down?

Through a hydrolysis reaction → glycosidic (covalent) bond is broken

Give examples of some dissacharides

• Maltose (malt sugar) 

• Sucrose (table sugar)

• Lactose (milk sugar)

How is maltose formed?

2 glucose molecules are joined by an alpha 1-4 glycosidic bond

How is sucrose formed?

Glucose and fructose are joined by an alpha 1-2 glycosidic bond

How is lactose formed?

Galactose and glucose are joined by a beta 1-4 glycosidic bond

How do you test for a reducing sugar?

Using Benedict’s solution:

• Cu2+ ions are reduced to Cu+ when solution is boiled → brick red precipitate

• sucrose doesn’t react

Why does sucrose not react with Benedict’s soultion?

It is a non-reducing sugar so it doesn’t react

• Part of the molecule that needs to react is already in the glycosidic bond

How can sucrose react with Benedict’s solution?

Sucrose needs to be hydrolysed first:

1. boil sucrose with acid

2. neutralise it

3. boil it with Benedict’s

What are polysaccharides?

Polymers containing many monosaccharides & linked together by glycosidic bonds (condensation reaction)

e.g. starch, cellulose & glycogen

What are the two types of starch?

• Amylose (α helix) - 20% of starch

• Amylopectin (branched starch) - 80%

What is the structure of starch?

What is the structure of amylose?

• Unbranched helix-shaped chain with 1-4 glycosidic bonds between α-glucose molecules

• The helix shape enables it to be more compact and thus it is more resistant to digestion

What is the structure of amylopectin?

• Highly branched

• α glucose molecules joined by α1-4 glycosidic bonds with α1-6 branches every 20-30 monomers

What are the properties of starch?

• Compact

• Insoluble

• Many monomers → longer to digest than glucose

What is the use of starch?

It is a storage molecule for plants

→ They are stored as granules in plastids

What is cellulose?

It is a polysaccharide and the main part of cell walls

What is the structure of cellulose?

• long chains of b-glucose molecules joined by b1-4 glycosidic bonds

• glucose chains form rope-like microfibrils

What are the properties of cellulose?

• Very strong → provides support

• Freely permeable → allows water & solutes to leave cell

What is glycogen?

It is the storage sugar found in animals

What is the structure of glycogen?

• Similar structure to amylopectin but more branches

• 1-4 glycosidic bonds between α-glucose molecules & also 1-6 glycosidic bonds between glucose molecules creating a branched molecule

What are the properties of glycogen?

• Can quickly be hydrolysed when energy supply needed

• Made mostly by livers & muscles

Where is glycogen stored?

Stored as granules in the cytoplasm of cells

What is the structure of triglycerides?

• Made up of glycerol & 3 fatty acids

• Joined by ester bonds formed in condensation reactions

What is the structure of saturated fats?

• Fatty acid chain contains max no. of H atoms

→ no carbon to carbon double bonds & no more hydrogen can be added

• Only single bonds between carbons

What is the structure of unsaturated fats?

• Have at least one carbon to carbon double bond

What is the structure of phospholipids?

• 2 fatty acids bonded to glycerol

• 3rd fatty acid is replaced with a phosphate group

How are triglycerides formed?

Through condensation reactions

• Bond is formed between each fatty acid & the glycerol (ester bond)

• → Hydroxyl group (OH) on the fatty acid molecule bonds to a H atom on the glycerol by condensation, releasing a molecule of water

How are triglycerides broken down?

Through hydrolysis reactions (e.g. digestion)

• A molecule of water is added to each ester bond to break it apart

• Triglyceride splits up into glycerol & 3 fatty acids

What is the general formula of amino acids?

• Contains an amino group (NH2)

• Contains carboxylic acid group

• Contains an R group

How are dipeptides formed?

• A condensation reaction forms a peptide bond, making a new dipeptide molecule

• (2 amino acids)

How are polypeptides formed?

When three or more amino acids are joined

Describe the primary structure of proteins

Peptide bonds formed between amino acids to form a polypeptide chain

Describe the secondary structure of proteins

The shape that the chain of amino acids takes - either alpha-helices or beta-pleated sheets

Describe the tertiary structure of proteins

3D shape of a protein

What bonds are present in proteins?

• Hydrogen bonds

• Hydrophobic interactions

• Disulfide bonds

What are hydrogen bonds?

• Non-covalent & weak

• Involved in all levels of structure

Where are hydrophobic interactions found?

• Between non-polar sections of the protein

What are disulfide bonds?

• One of the strongest & most important bond in proteins

• Occurs between two cysteine amino acids

Describe the quaternary structure of proteins

Exists in proteins containing more than one polypeptide chain

What is the structure of globular proteins?

• Spherical shape by tightly folded polypeptide chains

What is the function of globular proteins?

• Transport proteins e.g. haemoglobin, myoglobin etc.

• Catalyse reactions in enzymes e.g. lipase

• Transmit signals in hormones e.g. insulin

What is a conjugated protein? 

A globular protein with a prosthetic group

What is an example of a conjugated protein? Describe its structure and function

• Haemoglobin

→ Water soluble, contains 2 alpha & 2 beta polypeptide chains

→ Carries oxygen in the blood & releases it when necessary

What are the properties of fibrous proteins?

• Long, rope-like fibres

• High tensile strength

• Insoluble in water

→ e.g. collagen, keratin, elastin, silk

What is collagen?

The main component of connective tissue e.g. ligaments, tendons & cartilage

What is keratin?

The main component of hard structures e.g. hair, nails, claws & hooves

What are the key inorganic ions involved in biological processes (Cations)?

• Calcium ions (Ca2+)

• Sodium ions (Na+)

• Potassium ions (K+)

• Hydrogen ions (H+)

• Ammonium ions (NH4+)

What are the key inorganic ions involved in biological processes (Anions)?

• Nitrate (NO3 –)

• Hydrogencarbonate (HCO3 –)

• Chloride (Cl –)

• Phosphate (PO4 3–)

• Hydroxide (OH–)