Biological molecules A-level

What is a polar molecule?

Electrons unevenly distributed creating positive and negative poles in the molecule

How is a hydrogen bond formed?

Negative pole of one polar molecule and positive pole of another polar molecule form a weak electrostatic bond.

What is the formula for monosaccharides?

(CH2O)n (N is any number from 3-7)

Features of monosaccharides

Sweet tasting and soluble

Is the OH on the top or the bottom of Alpha glucose?

Bottom

Is the OH on the top or the bottom of Beta glucose?

Top

Define a reducing sugar

A sugar that can donate electrons to another chemical

What is Benedict's reagent?

An alkaline solution of copper (II) sulfate

How do you test for reducing sugars?

-Add 2cm^3 of sample to a test tube as a liquid (or grind it up)
-Add an equal amount of benedict's reagent
-Heat the water gently for 5 mins
-If a reducing sugar present, will change from blue -> red

What makes the Benedicts test semi-quantitative?

Differences in color of results can be used to estimate the amount of reducing sugar present in the solution.

Equation for maltose

glucose + glucose

Equation for sucrose

glucose + fructose

Equation for lactose

glucose + galactose

What is the bond between monosaccharides?

glycosidic bond

What happens when water is added to a disaccharide?

Breaks the glyosidic bonds to create monosaccharides (hydrolysis)

What is the name of the test used to test for non reducing sugars?

Benedict's test

How can you identify a non reducing sugar using benedict's reagent?

When added to reagent doesn't change colour of the reagent when heated.

What must you do to a non reducing sugar before trying to detect it?

Hydrolyse it into into monosaccharides

How do you hydrolyse a non reducing sugar into it's monosaccharides?

1. equal amount of sample and benedict's reagent into a test tube and filter

2. gently heat the test in a water bath for 5mins ~ if the solution doesn't change colour than no reducing sugars present.

3. equal amount of sample and hydrochloric acid to new test tube and heat for 5mins

4. slowly add sodium hydrogencarbonate solution to neutralise the acid until it's alkaline (use pH paper)

5. repeat steps 1 and 2 and reagent should turn red/brown.

Why must a non reducing sugar be added to acid before being added to Benedict's Reagent?

The dilute hydrochloric acid will hydrolyse the disaccharide

Why must the solution be neutralised before adding Benedict's Reagent?

Benedict's reagent doesn't work in acidic conditions

Why is it useful that polysaccharides are large molecules?

makes them insoluble and so are suitable for storage.

What is cellulose used for?

structural support in plants

How is starch stored?

granules

What is an isomer?

Organic molecules that have the same chemical formulas but different structural formulas

What type of bond is between the alpha glucose molecules in starch?

glyosidic bonds formed by condensation reactions

How do you test for starch?

add 2cm^3 of sample into test tube with 2 drops of iodine solution and shake. If colour changes from yellow to blue or black then starch is present

Structure of starch

branched or unbranched
unbranched is wound into a tight coil which makes it compact.
insoluble
contains alpha glucose

Why is it important that starch is insoluble?

doesn't effect water potential so water not drawn into cells by osmosis
doesn't diffuse out of cells (also caused by it being large)

Why is it important that starch is made of alpha glucose?

When hydrolysed forms alpha glucose which is easily transported and readily used in respiration

Why is it important that starch can be branched?

Has many ends that enzymes can work on simultaneously so glucose monomers can be released quickly.

Where is starch never found?

animal cells

Where is glycogen found?

animal and bacteria cells

How is glycogen stored?

As small granules mainly in the muscles and liver

Why is there not many carbohydrate stored in animals?

because fat is the main storage molecule in animals.

Why does glycogen's structure make it suitable for storage?

-insoluble ~ doesn't draw water into cells or diffuse out of cells

• compact

• more highly branched than starch so has more space for enzymes so rapidly broken down into glucose molecules for respiration

What type of glucose is found in glycogen?

alpha glucose

What type of glucose is found in cellulose?

beta glucose

Structure of cellulose

straight, parallel unbranched chains
hydrogen bond cross-links

Where are the hydrogen cross-links formed in cellulose?

between the -OH group of the beta glucose molecules.

What is formed when cellulose molecules group together?

microfibrils which then form fibres

Why is cellulose important in plants?

-provides rigidity
-prevents cell from bursting due to water intake from osmosis

How does cellulose stop the cell from bursting due to osmosis?

exerts an inward pressure that stops any further influx of water

Why is it important that stems and leaves are in a turgid state?

to provide the maximum surface area for photosynthesis

Why is it important that cellulose groups to form microfibrils?

provides strength

Features of all lipids

-contain carbon, oxygen and hydrogen
-insoluble in water
-soluble in organic substances like alcohol and acetone

What are the different roles of lipids?

-phospholipids make membrane flexible and transport lipid soluble substances across the membrane
-energy source
-waterproofing
-insulation
-protection

Why are lipids a good energy source?

When oxidised lipids provided twice as much energy as carbohydrates due to a higher ratio of H atoms

Why are lipids good for waterproofing?

insoluble in water

Why are lipids good for insulating?

good conductors of heat and when stored beneath the body surface, help to retain body heat. Also are electrical insulators in the myelin sheath.

How are lipids used for protection?

Fat is often stored around delicate organs, such as the kidney

What are triglycerides made up of?

3 fatty acids and glycerol

What type of bond is formed between the fatty acids and glycerol in triglycerides?

ester bonds formed by condensation

What is the difference between saturated and unsaturated fatty acids?

unsaturated has double carbon bonds

Why are triglycerides a good energy source?

a high ratio of hydrogen-carbon bonds to carbon atoms which contain a lot of energy.

Why are triglycerides good storage molecules?

Low mass to energy ratio so a lot of energy can be stored in a small volume.

Why is it useful that triglycerides are large and non-polar?

Makes them insoluble so doesn't effect osmosis or water potential.

Why are triglycerides an important source of water?

Have a high ratio of hydrogen to oxygen atoms so release water when oxidised.

How do phospholipids differ from triglycerides?

1 of the fatty acids is replaced by a phosphate molecule

Which part of a phospholipid is hydrophilic?

phosphate head

key features of phospholipids

-polar (hydrophilic and hydrophobic ends)
-form a bilayer which creates a hydrophobic barrier in membrane

Phospholipids + carbohydrates (found in the membrane) --> ?

glycolipids

How do you test for lipids?

1. 2cm^3 of sample to 5cm^3 of ethanol in a dry and grease free tube

2. shake to dissolve lipids

3. Add 5cm^3 of water and shake gently

4. will turn milky-white if a lipid is present

Why does the solution turn cloudy when testing for lipids?

lipid is finely dispersed to form an emulsion. As light passes through the emulsion it is refracted as it passes from oil droplets to water droplets, making it appear cloudy.

What should you do as a control for the lipids test?

replace the sample with water and the solution should stay clear.

What do amino acids combine to make (polymer of amino acids)?

Polypeptide

What are amino acids made up of?

-Central carbon atom
-hydrogen
-R group
-Carboxyl group
-amino group

What do 2 amino acids combine to make?

dipeptides

What type of reaction creates polypeptides?

condensation reaction
Water comes from OH of carboxyl group and H of amino group

How are amino acids bonded together?

peptide bonds between carbon atom of 1 amino acid and nitrogen atom of another.

What is polymerisation?

Joining of amino acids into a polypeptide

What is the primary structure of a protein?

sequence of amino acids in a polypeptide chain determined by DNA

What is the secondary structure of a protein?

weak hydrogen bonds between H of NH and O of C=O making the protein twist into a 3D shape (alpha helix)

What is the tertiary structure of a protein?

Further folding to create a complex 3D structure. Maintained by various bonds.

What bonds occur in the tertiary structure of proteins?

Disulfide bridges
Hydrogen bonds
Ionic bonds

Features of disulfide bridges?

strong and not easily broken

Features of ionic bonds in proteins?

Form between carboxyl and amino groups not involved in forming peptide bonds
Weaker than disulfate bridges
easily broken by changes in pH

Why is the 3D structure of a protein important?

determines the function of the protein and allows them to be distinctive and to be recognized by, and to recognize, other molecules.

What is the quaternary structure of a protein?

Polypeptide chains bonded in various ways with non-protein parts associated with the molecule.

What is the test for proteins called

Biuret test

How do you do test for proteins?

1.Add equal volume of sample and sodium hydroxide to test tube at room temp
2.Add a few drops of very dilute copper (II) sulfate solution and mix gently
3.If solution changes from blue to purple then peptide bonds present and so proteins present.