aqa a level biology

What is covalent bonding?

When atoms share a pair of electrons in their outer shells

What is ionic bonding?

When ions with opposite charges attract one another

Describe hydrogen bonding

When a polarised molecule negatively charged area attracts a positively charged area of another molecule forming a weak electrostatic bond

What type of sub-unit makes up a polymer?

A monomer

What is the process of how monomers are joined?

Polymerisation

The monomers of a polymer are usually based on which element?

Carbon

What is the basic sub unit for a polysaccharide?

A monosaccharide

What are the three monosaccharides you need to know?

Glucose, galactose and fructose

What reaction joins molecules together?

Condensation reactions

What is released during a condensation reaction?

Water

Hydrolysis happens because of the addition of what molecule?

Water

What is hydrolysis?

The splitting of molecules via the addition of water

Define metabolism

ALL the chemical processes that take place in a living organism

What feature of carbon atoms allows for life?

Carbon can easily form bonds with other carbon molecules, forming a backbone

Molecules containing carbon are know as what?

Organic molecules

What is the general formula for a monosaccharide?

(CH2O)n

Draw the structure of alpha glucose

Draw the structure for beta glucose

Glucose is a hexose sugar. What does this mean?

It has 6 carbon atoms in its structure

What is the general formula for glucose?

C6H12O6

All monosaccharides and some disaccharides are reducing sugars. What does this mean?

A sugar that can donate electrons/reduce another chemical

What is the used to test for a reducing sugar?

Benedict’s Reagent

The test for reducing sugars is called what?

The Benedict’s test

What is Benedict’s reagent?

An alkaline solution of copper(II) sulphate

What colour is copper (II) sulphate?

Blue

When a reducing sugar is present what is formed during the Benedict’s test?

The insoluble, red precipitate of copper (I) sulphate

Describe the steps to the Benedict’s test

1. Add 2cm³ of the food sample, if this is not in liquids form grind it in water

2. Add an equal volume of Benedict’s reagent

3. Heat the mixture gently for 5 minutes

How is the Benedict’s test semi-quantitative?

The closer the colour of the final precipitate is to red the more reducing sugars are present

When combined in pairs, monosaccharides form what?

Disaccharides

Name three disaccharides and the monosaccharides that make them

Maltose = glucose + glucose

Sucrose = glucose + fructose

Lactose = glucose + galactose

Which disaccharide is a reducing sugar?

Maltose

What type of bond joins monosaccharides together?

A glycosidic bond

List the steps required in the test for non-reducing sugars

1. If the sample is not already in water it must be ground up into water

2. Add 2cm³ of the food sample being tested to 2cm³ of Benedict’s reagents in a test tube and filter

3. Place the test tube in gently boiling water for 5 minutes, if the colour of the Benedict’s reagent stays the same then a reducing sugar is not present

4. Add another 2cm³ of the food sample to 2cm³ of dilute hydrochloric acid in a test tube and place in the gently boiling water for another five minutes

5. The hydrochloric acid will hydrolyse the disaccharides leaving monosaccharides behind

6. Slowly add sodium hydrogen carbonate solution to the test tube to neutralise the hydrochloric acid and then test with pH paper to make sure the solution is alkaline

7. Re-test the resulting solution by heating gently with 2cm³ of Benedict’s solution for 5 minutes

8. If a non-reducing sugar was present the original sample will now turn orange-brown due to the reducing sugars that were produced by the hydrolysis of non-reducing sugars

Polysaccharides are large, insoluble molecules this makes them good for what?

Storage

What type of glucose is starch made out of?

Alpha glucose

What is cellulose used for in plants?

Structural support

Describe the test for starch?

1. Place 2cm³ of the sample being tested into a test tube

2. Add two drops of iodine solution and shake

3. The presence of starch is indicated when the iodine turns from an orangey yellow to a blue black

Where is starch found?

In many parts of a plant such as the seeds and storage organs

Starch acts as what for plants?

A major energy source

An unbranched chain of alpha glucose molecules that form starch can be wound up into a tight coil, why is this useful?

It makes the molecule very compact and therefore take up less space

Are starch chains branched or unbanched?

They can be both

Why is starch being branched an advantage for the plant?

It has many ends that enzymes can act on simultaneously meaning that glucose is readily available for respiration

Why is starch being insoluble good for the plant?

• It does not affect water potential so too much water isn’t drawn into the plant

• It cannot diffuse out of cells

Where is starch never found?

In animal cells

What is the animal equivalent of starch?

Glycogen

Why is the mass off carbohydrates stored in glycogen low?

Because fats are the main storage molecule in animals

What three features of glycogen make it suited to its role?

It’s insoluble, compact and highly branched

Why is glycogen more highly branched than starch?

Because animals are more metabolically active and therefore need energy faster than plants

What is the main difference between cellulose and starch?

Cellulose is made of beta glucose and starch is made of alpha glucose

Rather than forming a coiled chain what structure does cellulose form?

It forms a straight, unbranched chain

What does the straight structure of cellulose allow it to do?

It allows it to run parallel to each other and form hydrogen bonds which create cross linkages between adjacent chains.

Cellulose molecules are grouped together to form what?

Microfibrils

Microfibrils are arranged are arranged in parallel groups called what?

Fibres

What is cellulose used for in plants?

It provides structure and rigidity to the plants cell wall

What does cellulose prevent the cell from doing during osmosis?

Bursting

Why do adjacent cellulose molecules have so many cross linkages?

To provide more strength to the molecule?

What four characteristics do lipids share with one another?

• they all contain carbon, hydrogen and oxygen

• the proportion of oxygen to carbon and hydrogen is smaller than in carbohydrates

• they are insoluble in water

• they are soluble in organic solvents such as alcohols and acetones

What are the two main groups of lipids?

Triglycerides and phospholipids

What are the main roles of lipids?

• a source of energy

• waterproofing

• Insulation

• Protection

Why are lipids a source of energy?

When oxidised lipids provide more than twice the energy as the same mass of carbohydrates

How are lipids waterproofing?

Lipids are insoluble in water, for example plants have a waxy cuticle layer that conserves water.

How do fats insulate the body?

They are slow conductors of heat so help retain body heat. They also act as electrical insulators in the myelin sheath around nerve cells

How do fats offer protection to the body?

It is often stored around delicate organs such as the kidneys, this protects them

Describe the structure of a triglyceride

They have three fatty acids and a glycerol molecule

What type of bond forms between each of the fatty acids and the glycerol in triglycerides

An ester bond

If a fatty acid has a double carbon-carbon bond what is it?

Polyunsaturated

If a fatty acid has a single carbon-carbon bond what is it?

Mono-unsaturated

If a fatty aid has no carbon-carbon bonds what is it?

Saturated

Why are triglycerides a good source of energy?

Carbon-hydrogen bonds store a lot of energy and they have a low mass to energy ratio so lots of energy is stored in a small volume.

Why are triglycerides non-polar?

So that they are insoluble in water and do not affect the water potential of the cell

What is the benefit of triglycerides releasing water when oxidised?

They provide a source of water to organisms such as camels that live in the desert where water is sparse

What is the difference in structure between triglycerides and phospholipids?

Phospholipids have a phosphate molecule in place of one of the fatty acids

Which part of a phospholipid is hydrophilic?

The head

What does hydrophilic mean?

To be attracted to water

Which area of a phospholipid is hydrophobic?

The tail

What does hydrophobic mean?

To repel water

Because one part of a phospholipid is hydrophilic and one is hydrophobic what can a phospholipid be described as?

A polar molecule

Because phospholipids are polar what do they do in the presence of water?

They position themselves so that the hydrophilic heads are as close to the water as possible and the hydrophobic tails are as far away as possible

What are the advantages of phospholipids being polar?

In an aqueous environment they form a bilayer such as in a cell surface membrane. As a result a hydrophobic barrier is created between the inside and outside of the cell

What can phospholipids in the cell-surface membrane form when combined with carbohydrates and what do these structures do?

They form glycolipids which are important in cell recognition

Describe the steps in the emulsion test

1. Take a completely dry and grease free test tube

2. Take 2cm³ of the sample and add 5cm³ of ethanol

3. Shake the tube thoroughly to dissolve any lipid in the sample

4. Add 5cm³ of water and shake gently

5. A milky white emulsion forming indicates the presence of a lipid

6. As a control repeat the process using just water and the final solution should remain clear

How many amino acids occur naturally?

20

Every amino acid has a central what?

Carbon atom

What do is a chain of amino acids called?

A polypeptide

Polypeptides can be combined to form what?

Proteins

What is the amino group of an amino acid?

-NH2

What group makes an amino acid an acid?

The carboxyl group

What is the structure of the carboxyl group?

-COOH

What five things does every amino acid have?

• a central carbon atom

• an amino group

• a carboxyl group

• a hydrogen atom

• a R group

How are peptide bonds formed?

A condensation reaction between the -OH from a carboxyl group of one amino acid and the -H from the amino group of another amino acid

What is a proteins primary structure?

The sequence of amino acids that make up a polypeptide chain

What determines the primary structure of a protein?

The DNA sequence

What is the secondary structure of a protein?

The hydrogen bonds between the amine and carboxyl groups of the amino acids that determine the folded shape of the polypeptide chain

What are the two possible secondary structures of proteins?

Alpha sheets and beta pleats

What is the tertiary structure of a protein?

The 3D structure of the protein

What three bonds create a proteins tertiary structure?

• disulphide bridges

• ionic bonds

• hydrogen bonds

What is the quaternary structure of a protein?

If a protein contains more than one polypeptide chain it has a quaternary structure

What is the test for proteins?

The biurets test

Describe the steps in the biurets test

1. Place a sample of the solution in a test tube.

2. Add an equal volume of sodium hydroxide solution

3. Add a few drops of very dilute copper (II) sulphate solution and mix gently

4. If the solution turns from blue to purple a protein is present