Chapter 3 - Biological Molecules Flashcards
how are atoms bonded in water
with covalent bonds
why is water a polar molecule
electrons are not shared equally between atoms, the atom with a greater share of electrons will be slightly negative and the other slightly positive
what atom in water is SLIGHTLY positive
hydrogen - a smaller share of electrons in covalent bonds
what atom in water is SLIGHTLY negative
Oxygen has the greater share of electrons in covalent
why are many organic molecules polar
molecules contain hydroxyl (OH) group
how are hydrogen bonds formed
o the polar water molecules interact as the + and – regions of the molecule attract each other and from hydrogen bonds
characteristics of hydrogen bonds
- quite weak
-they break and reform between constantly moving water molecules - viscous
characteristics of water (8)
- solvent
- cohesive
- adhesive
- has surface tension
- high SHC
- high latent heat of vaporisation
- density
- colourless/ transparent
why is water a good solvent
o because water is polar, it is attracted to other covalently bonded polar molecules and ions
o The +ve and -ve regions of water molecules are attracted to the -ve and +ve parts of solutes/ions.
o Water molecules cluster around these charged parts and help them separate + stay apart.
example of an ion
sodium chloride
example of a covalently bonded polar molecule
glucose
why is water being a good solvent significant for life
- allows chemical reactions to occur within cells - as the dissolved solutes are more chemically reactive when they are free to move about
- polar molecules (amino acids, nucleic acids) can dissolve in the cytosol
- Metabolites// solutes can be transported efficiently in and out of cells
what is the cytosol
cell liquid if eukaryotic and prokaryotic cells
what is cohesion
o Hydrogen bonds pull molecules towards each other // moves as one mass as molecules are attracted to each other
why is cohesion significant for life
plants can draw water through roots/xylem in transpiration
how is surface tension formed
at air-water surface, the cohesion between water molecules produces surface tension as the hydrogen bonds also pull the molecules inwards
how is surface tension significant for life
surface tension is strong enough for some insects e.g., Pond skaters to inhabit
what is adhesion
water molecules can be attracted to surfaces such as narrow tubes
how is adhesion significant for life
- causes water to exhibit capillary action
- water can rise up narrow tubes = transpiration
define specific heat capacity
the amount of energy needed to raise the temperature by 1 degree Celsius of a 1kg substance, without changing state
why does water have a high SHC
- due to the many hydrogen bonds present in water.
- a relatively large amount of energy is required to raise its temperature
SHC of water
4200 J/kg°C
why is having a high SHC significant for life
- Important in habitats as provides a stable environment eg. for fish
- maintaining temperatures that are optimal for enzyme activity
define the latent heat of vaporisation
the amount of energy needed to change the state (liquid to gas, vice versa) of 1kg a substance without changing its temp
why does water have a high latent heat of vaporisation
o takes a lot of thermal energy to break hydrogen bonds and a lot of energy to build them
o absorbs a large amount of heat before it turns into gas
why is having a high latent heat of vaporisation significant for life
– reduces evaporation from ocean
- provides a cooling effect for living organisms, for example the transpiration from leaves or evaporation of water in sweat on the skin
which is more dense - ice or water
water
why is ice less dense than water
due to the hydrogen bonds
- when water is cooled below 4 degrees Celsius, the hydrogen bonds fix the polar molecules slightly further apart than the average distance in liquid state,
- forming a giant, rigid, open structure which is less dense than liquid water
what type of bonding is hydrogen bonds
intermolecular bonds
at what point when cooling down does water start becoming less dense, instead of more
4 degrees Celsius
structure of ice
a giant, rigid, open structure
- Every oxygen at the centre of a tetrahedral arrangement of hydrogen atoms
why is the anomaly in density of water significant for life
- insulates ponds/lakes
- stops organisms freezing underneath // only surface freezes not whole of ocean
what is a condensation reaction
• Joins two molecules together with the formation of a chemical bond, and involves the elimination of a molecule of water
what is a hydrolysis reaction
• Breaks a chemical bond between two molecules and involves the use of a molecule of water
what type of reactions are condensation and hydrolysis
reversible reaction
What do carbohydrates contain
Carbon, Hydrogen and Oxygen
General formula of a carbohydrate
Cx(H2O)y
Glucose formula
C6H1206
What is a monosaccharides
1 unit
What is a disaccharide
2 units
What is a polysaccharide
Many units
Examples of monosaccharides (3]
glucose, fructose, ribose
Examples of disaccharides
lactose, sucrose, maltose
Examples of polysaccharides
glycogen, starch, cellulose
What is a monosaccharide of 6 carbons called
hexose monosaccharide
Example of a become monosaccharide
Glucose
How are carbons numbered in a structure diagram
carbons are numbered clockwise, beginning with the carbon to the right of oxygen
Is glucose soluble in water
Yes
Why is glucose soluble in water
due to the hydrogen bonds that form between the hydroxyl groups and water molecules.
Why is solubility for glucose important
as it means glucose is dissolved in the cytosol of cell
What are the two isomers of glucose
Alpha (α) glucose and Beta (β) glucose
What is an isomer
molecules with the same molecular formula but with different structural arrangements of atoms.
What’s the difference between alpha and beta glucose
o Alpha has the OH below the C1
o Beta has the OH above the C1
Draw alpha glucose
Draw beta glucose
What is special about monosaccharides
• Same no. of C as O atoms
General formula of monosaccharides
(CH2O)n
General properties of monosaccharides
• White crystalline solids
• Dissolve in water to form sweet tasing solutions
What can monosaccharides be categorised into
Trioses
Tetroses
Pentoses
Hexoses
How many carbons does trioses have
3
How many carbons does tetroses have
4
How many carbons does pentoses have
5
How many carbons does hexoses have
6
Example of a triose
Glyceraldehyde
Properties of trioses
Intermediates in respiration and photosynthesis
Properties of tetroses
Rare
Example of pentoses
RIBOSE or ribulose
Properties of pentoses (ribose)
Used in the synthesis of nucleic acids (RNA and DNA), co-enzymes (NAD, NADP, FAD) and ATP
Example of hexoses
Glucose or Fructose
Properties of hexoses
Used as a store of energy in respiration and as building blocks for larger molecules
How to break down polysaccharides
Hydrolysis
How to synthesise polysaccharides / disaccharides
Condensation reaction
How do two glucose molecules form a disaccharide
Condensation reaction
Describe the process of the condensation reaction of two alpha glucose molecules
• When two alpha glucose molecules (monosaccharides) are side by side, two hydroxyl groups interact, bonds are broken and new bonds reform to produce different molecules (disaccharides)
• 2 hydrogen atoms and 1 oxygen atom are removed to form water
• A bond forms between carbons 1 and 4 and the molecules are now joined
What type of bond is formed between a carbon 1 and a carbon 4 in a condensation reaction
Covalent - 1,4 glycosidic bond
What molecule is made when two alpha glucose react
Maltose
What is requires for a hydrolysis reaction
• Requires one water molecule to supply the H and the OH to the sugars formed
Constituent monomers of sucrose
a- glucose + fructose
Constituent monomers of maltose
a glucose + a glucose
Constituent monomers of lactose
Glucose + galactose
What is starch
• Main storage polysaccharide in plants
Properties of starch
• Pure starch is white, tasteless, and odourless
• It is insoluble in cold water or alcohol
How is starch formed
when alpha glucose monomers are joined together by glycosidic bonds during condensation reactions
What are the two constituents of starch
Amylose + amylopectin
Approx how much starch is made from amylose
o 20-30%
How is amylose formed
o Formed by alpha glucose molecules joined only by 1,4 glycosidic bonds
Properties of amylose
o long unbranched chains
What specific about the angle of the bond in amylose
o angle of bond means that the long chain of glucose twists into a helix, further stabilised by hydrogen bonds
What important about amylose twisting into a helix
o makes it more compact and less soluble in comparison to the glucose molecules
How is amylopectin formed
o Formed when glycosidic bonds form in condensation reactions between carbon 1 and 6.
o Has both 1,4 and 1,6 glycosidic bonds
What types of bonds make up amylopectin
1,4 and 1,6 glycosidic bonds
What does the 1,6 bonds mean in amylopectin
Means it has a branched structure with 1,6 branching points occurring approx. 1 in 25 glucose subunits
How often do branching points occur in amylopectin
approx. 1 in 25 glucose subunits
How is a branching point formed in amylopectin
From 1,6 glycosidic bonds
Why is it important to have branching points
o Branching means there are many free ends where glucose molecules can be added/removed, speeds up processes of storing/releasing glucose molecules required by the cell
Where is glycogen found
in dense granules in liver cells
What percentage of the mass of a liver does glycogen take up
7%
What is glycogen
• Main storage polysaccharide in animals and fungi (animal equivalent of starch)
How is glycogen formed
when alpha glucose monomers are joined together by glycosidic bonds during condensation reactions
What types of bonds are in glycogen
1,4 and 1,6 glycosidic bonds
Which type of bond is responsible for branching
1,6
Which forms more branches glycogen or amylopectin
Glycogen
Why does glycogen form more branches
more compact and less space needed for it to be stored, important as animals are mobile
Properties of glycogen
• Insoluble, branched, compact
What is cellulose
• Main structural constituent of plant cell walls
Why is it important for plants to have cellulose
• As plants do not have a rigid skeleton like the one found in animals, a cell wall needs to be strong enough to enable each cell to support the whole plant
What is cellulose made from
Beta glucose molecules
Why can’t beta glucose molecules join in the same way alpha can
the hydroxyl group on carbon 1 and 4 are too far to react
How do plants solve the problem of beta glucose
alternate beta glucose molecules are turned upside down
Properties of cellulose
• Unable to coil or form branches – a straight chain is produced (cellulose)
What two types of bonds are formed in cellulose
1,4 glycosidic bonds and hydrogen bonds
How are hydrogen bonds formed in cellulose
bonds form between the rotated beta glucose molecules on the same cellulose chain and between the rotated beta glucose on cellulose chains that lie alongside each other
What is a cellulose chain called
Microfibril
What do microfibrils join together to form
Cellulose fibres
Properties of cellulose fibres
strong, insoluble, used to make cell walls
What makes up lipids
Carbon oxygen and hydrogen
Difference between Lipoids and carbohydrates
Lipids have a lot less oxygen
Solubility of lipids
- Insoluble in water = because they are non polar
- soluble in organic solvents
- hydrophobic
What is lipids made from
3 Fatty acids + 1 glycerol
Do lipids form polymers
NO
Examples of lipids
Fats
Oils
Cholesterol
Steroids
Phospholipids
Difference between fats and oils
• Fats = solid at RT // oils = liquid at RT
What type of molecule are lipids
• Macromolecules = large complex molecules
What are the functions of lipids [6]
• Energy storage
• Metabolic water source
• Membrane structure
• Water proofing
• Insulation
• Produces hormones
Types of lipids with fatty acids
o Triglycerides / Phospholipids / Waxes
Types of lipids without fatty acids
o Steroids / cholesterol
How are triglycerides mace
made from combining one glycerol molecule with 3 fatty acids, in a condensation reaction
Functional group of glycerol
Alcohol
Functional group of fatty acids
Carboxylic acid
What is needed when triglycerides are broken down
three water molecules need to be supplied to reverse the reaction that formed the triglyceride = hydrolysis reaction
Why is it called triglycerides
There are three fatty acids
What bond does triglycerides produce when forming from condensation reaction
And ester bond
What else does glycerol + three fatty acids produce in a condensation reaction
Three water molecules
What is a saturated fatty acid chain
the hydrocarbon chain has only single bonds
What is an unsaturated fatty acid
• The hydrocarbon chain consists of at least 1 double bond
What is the name when the lipid has 1 double bond
monosaturated
What is the name when the lipid has 2+ double bonds
polysaturated
What does the presence of a double bond cause in a lipid
causes molecules to bend = therefore not compact = therefore they are liquids/oils rather than fats
Draw a saturated and mono saturated and poly saturated chain
Types of unsaturated fat
• Unsaturated fat can be either cis or trans
What does cis and trans fat refer to
the arrangement of the two hydrogen atoms bonded on to the carbon chain involved in a double bond
What is cis fat
hydrogens are on the same side of the double bond
What is trans fat
hydrogens are on opposite side of the double bond
Draw a cis fatty acid
Draw a trans fatty acid
What are phospholipids
• Modified triglycerides containing phosphorus, carbon, oxygen and hydrogen
How is the phosphate group added into a phospholipid
• One of the fatty acid chains is replaced with a phosphate group
Where are inorganic phosphate ions found
In the cytoplasm of every cell
Are phospholipids soluble in water
Yes
Why are phospholipids soluble in water
o Have extra electrons = soluble in water
Sketch the chemical structure of a phospholipid
The structure of phospholipids
• Have a non-polar fatty acid tail and a charged phosphate head
Property of phospholipid fatty acid tail
Non-polar
Significance of phospholipid non-polar tail
repelled by water = hydrophobic
Significance of phospholipid charges head
interacts/attracted to water = hydrophilic
Significance of BOTH non-polar tail and charged head
• They form a layer on the surface of water with the phosphate heads in the water and the fatty acid tails sticking out of the water
• Can form structured based on 2 layered sheet formations (bilayers), with all the hydrophobic tails pointing towards the centre of the sheet, protected by the hydrophilic heads
Significance of phospholipids forming bilayers
• Able to separate an aqueous environment in which cells usually exist from the aqueous cytosol within
What are peptides
• polymers made up of amino acid molecules (monomers)
What does a protein consist of
• 1+ polypeptides arranged as complex macromolecules and have specific biological functions
What do all proteins contain (biological elements)
• carbon, hydrogen, oxygen and nitrogen
What is the general structure of an amino acid
o An amine group -NH2
o A carboxylic acid group -COOH
o A hydrogen atom
o An R group
Draw the general structure of an amino acid
What do different R groups result in
Different amino acids
How many amino acids are there
20
What is formed when 2 amino acids bond
Dipeptide
What is formed when 3 amino acids bond
Tripeptide
What is formed when many amino acids bond
Polypeptide
How are peptides made - overall reaction
Condensation reaction
How are peptides made - detailed
• Amino acids join when amino and carboxylic acid groups connected to the central carbon atom react.
• The R-groups are not involved at this point.
• The hydroxyl in the carboxylic acid group of one amino acid reacts with the hydrogen in amine group of another amino acid.
• The remaining carbon atom (with the double-bonded oxygen) from the first amino acid bonds to the nitrogen atom of the second amino acid
• Peptide bond is formed between the amino acids and water is produced (condensation reaction).
• Resulting compound: dipeptide
What is specifically not involved in the formation of peptides
R groups
What type of bonds are peptides bonds
Covalent
What is the reaction forming peptides catalysed by
by enzyme peptidyl transferase
Where is peptidyl transferease found
present in ribosomes.
What is the primary structure of a protein
• sequence of amino acids joined by peptide bonds
What determines the primary structure of proteins
DNA
What is the secondary structure of proteins
• occurs when the sequence of amino acids are linked by hydrogen bonds in an alpha helix or beta helix pleated sheet
What interacts to form the secondary structure
• the weak negatively charged nitrogen and oxygen atoms interact with the weak positively charged hydrogen atoms to form hydrogen bonds
What type of bonds make up the secondary structure
Hydrogen bonds
What two types of shapes can be made in the secondary structure
o α-helix
o β-pleated sheet
When does the α-helix shape occur
• when the hydrogen bonds form between every fourth peptide bond (between the oxygen of the carboxyl group and the hydrogen of the amine group)
When does the β-pleated sheet occur
• when the protein folds so that two parts of the polypeptide chain are parallel to each other enabling hydrogen bonds to form between parallel peptide bonds
What type of protein mostly has a secondary structure
• Most fibrous proteins have secondary structures (e.g. collagen and keratin)
What is the secondary structure also known as
The protein backbone
Why is the secondary structure known as the portion backbone
only relates to hydrogen bonds forming between the amino group and the carboxyl group
What can hydrogen bonds be broken by in the secondary structure
• high temperatures and pH changes
What is the tertiary structure in proteins
describes the folding of a polypeptide chain, due to the molecular interactions among the R groups of the amino acids
What are these molecular interactions in the tertiary structure
o Hydrogen (these are between R groups)
o Disulphide (only occurs between cysteine amino acids)
o Ionic (occurs between charged R groups)
o Weak hydrophobic interactions (between non-polar R groups)
What protein is tertiary structure common in
Globular
What type of bond is disulphide bond
Covalent bond
What does disulphide bonds form between
two cysteine R groups
Why do disulphide bonds only form between two cysteine R groups
as this is the only amino acid with a sulphur atom
Which type of R group interaction is the strongest
Disulphide
What can disulphide bonds be broken by
Oxidation
What do ionic bonds in the tertiary structure form between
• form between positively charged (amine group -NH3+) and negatively charged (carboxylic acid -COO-) R groups
What are ionic bonds broken by
Changes in pH
What do hydrogen bonds form between in the tertiary structure
form between strongly polar R groups.
What is the weakest type of R group interaction
Hydrogen bonds
What do hydrophobic interactions form between
• form between the non-polar (hydrophobic) R groups within the interior of proteins
What is the quaternary structure in a protein
• arrangement of two or more polypeptide chains working together as a functional macromolecule
Example of a protein with a quaternary structure
Haemoglobin
What is each polypeptide chain in the quaternary structure referred to as
The subunit of the protein
Summary table of bonds in the different levels of proteins structures
Properties of globular proteins
• compact, roughly spherical (circular) in shape and soluble in water
Why do globular proteins form a spherical shape
o their non-polar hydrophobic R groups are orientated towards the centre of the protein away from the aqueous surroundings and
o their polar hydrophilic R groups orientate themselves on the outside of the protein
What causes globular proteins to be soluble in water
Its arrangement
• the water molecules can surround the polar hydrophilic R groups
Example of a globular protein
Insulin
What is a conjugated protein
Globular proteins which contain a prosthetic group.
What are prosthetic groups
can be lipoproteins, glycoproteins, metal ions or minerals from vitamins.
Examples of conjugated proteins
Haemoglobin + catalase
What prosthetic group does haemoglobin have
Haem
What is haemoglobin
• a globular protein which is an oxygen-carrying pigment found in vast quantities in red blood cells
What type of structure is haemoglobin
Quaternary
How many polypeptide chains does haemoglobin have
4
What types of polypeptide subunits does haemoglobin have
two α–globins and two β–globins
What does each subunit in haemoglobin have
A prosthetic haem group
How are the four subunits held together in haemoglobin
by disulphide bonds and arranged so that their hydrophobic R groups are facing inwards (helping preserve the three-dimensional spherical shape) and the hydrophilic R groups are facing outwards (helping maintain its solubility)
What makes haemoglobin soluble
hydrophilic R groups are facing outwards
What does the prosthetic haem group contain
iron II ion (Fe2+)
What’s the purpose of the iron II ion (Fe2+) in haemoglobin
• able to reversibly combine with an oxygen molecule forming oxyhaemoglobin and results in the haemoglobin appearing bright red
How many oxygen molecules can each haemoglobin molecule carry
• Each haemoglobin with the four haem groups can therefore carry four oxygen molecules (eight oxygen atoms)
Why can oxygen be carried round more efficiently when bound to haemoglobin
• As oxygen is not very soluble in water and haemoglobin is, oxygen can be carried more efficiently around the body when bound to the haemoglobin
What is catalase
Globular conjugated protein
What type of structure does catalyse have
Quatenaryb
How many prosthetic haem groups does catalase have
4
What does the presence of iron II in the haem group in catalase allow
• allow catalase to interact with hydrogen peroxide and speed up its breakdown.
What is hydrogen peroxide
• a common by-product of metabolism but is damaging to cells and cell components if allowed to accumulate.
What does catalase do
Stops hydrogen peroxide accumulating
What is insulin
Globular protein
Where is insulin produced
In the pancreas
Why must insulin be soluble
Hormones transported in bloodstream
Why does insulin need a specific shape
fits into receptors on cell surface membranes to cause more/less glucose production
What does insulin consist of
• Consists of 2 polypeptide chains held together by 3 disulphide bridges
What are fibrous proteins
• long strands of polypeptide chains that have cross-linkages due to hydrogen bonds
Properties of fibrous proteins
• have little or no tertiary structure
Insoluble in water
Suitable,e for structural roles
Why are fibrous proteins insoluble in water
Due to a large number of hydrophobic R groups
Why are fibrous proteins suitable for structural roles
• have a limited number of amino acids with the sequence usually being highly repetitive
• The highly repetitive sequence creates very organised structures that are strong and this along with their insolubility property, makes fibrous proteins very suitable for structural roles
Examples of fibrous proteins
Keratin + elastin + collagen
What does keratin make up
Hair, nail, horns, feathers
Where is elastin found
connective tissue, tendons, skin and bone
Where is collagen found
- skin, tendons and ligaments
Both elastin and collagen are examples of what
Connective tissue
What is the structure of collagen
• formed from three polypeptide chains closely held together by hydrogen bonds to form a triple helix (known as tropocollagen)
• Along with hydrogen bonds forming between the three chains there are also covalent bonds present
• Covalent bonds also form cross-links between R groups of amino acids
• The cross-links hold the collagen molecules together to form fibrils
• The collagen molecules are positioned in the fibrils so that there are staggered ends
• When many fibrils are arranged together they form collagen fibres
• Collagen fibres are positioned so that they are lined up with the forces they are withstanding
How many polypeptide chains in collagen
3
What are the polypeptide chains held together by in collagen
Hydrogen bonds
What is the properties of collagen
- forms connective tissue
- strong + tensile strength
- stable
- insoluble
Why does collagen have great tensile strength
• presence of the many hydrogen bonds within the triple helix structure of collagen results in great tensile strength.
Why is collagen strong
• The staggered ends of the collagen molecules within the fibrils provide strength
Why is collagen stable
• due to the high proportion of proline and hydroxyproline amino acids present. These amino acids increase stability as their R groups repel each other
Why is collagen insuluble
The length of collagen molecules means they take too long to dissolve in water
Properties of keratin
Strong
Flexible
Insoluble
Why is keratin flexible
• Large proportion of sulphur – containing amino acid, cysteine.
• Degree of disulphide bonds determine flexibility – hair contains fewer bonds making it more flexible than nails which contains more bonds.
What determines flexibility
Degree of disulphide bonds
Why is there an unpleasant smell when hair / skin is burnt
Due to larger quantities of sulphur
Where is elastin found specifically
• in elastic fibres (along with small protein fibres).
Where are elastic fibres found
in walls of blood vessels and in the alveoli of the lungs
What do elastic fibres do
• they give these structures flexibility to expand when needed, but also contract to normal size.
What structure does elastic have
Quaternary
Compare globular and fibrous proteins
Mention shape / amino acid sequence / function / examples / solubility
Compare collagen and haemoglobin
Include number of polypeptide chain / shape / type of protein / function / amino acid sequence / prosthetic group / solubility
What test is needed for reducing / non-reducing sugars
Benedict’s test
Difference between reducing + non reducing sugars
Reducing sugars are carbohydrates that can act as reducing agents due to the presence of free aldehyde groups or free ketone groups.
Nonreducing sugars are carbohydrates that cannot act as reducing agents due to the absence of free aldehyde groups or free ketone groups.
Examples of reducing sugars
Galactose
Glucose
Fructose
Maltose
Examples of non - reducing sugars
Sucrose
What colour is Benedict’s Reagent
Blue
What does Benedict’s reagent contain
copper (II) sulphate ions (CuSO4 )
What happens to the copper (II) sulphate ions (CuSO4 ) in Benedict’s solution, when in contact with reducing sugars
• copper (I) oxide forms
Why does copper (I) oxide form a precipitate
Not soluble in water
Method to test for reducing sugars
• Add Benedict’s reagent (which is blue as it contains copper (II) sulfate ions) to a sample solution in a test tube – an equal volume of sample
• Heat the test tube in a water bath or beaker of water that has been brought to a boil for a few minutes
Observe colour
Positive test - reducing sugar
If a reducing sugar is present, a coloured precipitate will form
Why will a coloured precipitate form if a reducing sugar is present in Benedict’s test
• as copper (II) sulfate is reduced to copper (I) oxide which is insoluble in water – reduce copper 2+ ions to Cu+ ions
Why is it important to add an excess of Benedict’s solution
• so that there is more than enough copper (II) sulfate present to react with any sugar present
What is the colour change for reducing sugars
• from blue (no reducing sugar), through green, yellow and orange (low to medium concentration of reducing sugar) to brown/brick-red (a high concentration of reducing sugar)
Why is the reducing sugar test described as semi - quantitative
o as the degree of the colour change can give an indication of how much (the concentration of) reducing sugar present
Is there’s more reducing sugar…
o more precipitate formed = less Cu blue ions left.
o Hence, the actual colour is a mix of the brick-red precipitate and unchanged blue copper ions, so colour will be different depending on reducing sugar concentration, thus test is qualitative.
Method for test for non-reducing sugar
• Add dilute hydrochloric acid to the sample and heat in a water bath that has been brought to the boil
• Neutralise the solution with sodium hydrogencarbonate
o Use a suitable indicator (such as red litmus paper) to identify when the solution has been neutralised, and then add a little more sodium hydrogencarbonate as the conditions need to be slightly alkaline for the Benedict’s test to work
• Then carry out Benedict’s test as normal
Add Benedict’s reagent to the sample and heat in a water bath that has been boiled
Why do we add a little more sodium hydrogen carbonate in the non-reducing sugars test
o as the conditions need to be slightly alkaline for the Benedict’s test to work
Why do we add hydrochloric acid in the non-reducing sugars test
• The addition of acid will hydrolyse any glycosidic bonds present in any carbohydrate molecules
• The resulting monosaccharides left will have an aldehyde or ketone functional group that can donate electrons to copper (II) sulfate (reducing the copper), allowing a precipitate to form
Colour change for Benedict’s test reducing + non-reducing
What test is used to identify starch
Iodine test
Method to test for starch
• add a few drops of orange/brown iodine in potassium iodide solution to the sample
Why do we use potassium iodide when testing for starch
Because iodine by itself is insoluble in water
Positive result - iodine starch test
• If starch is present, iodide ions in the solution interact with the centre of starch molecules, producing a complex with a distinctive blue-black/purple-black colour
What are reagent strips
• can be used to test for the presence of reducing sugars, most commonly glucose.
What type of molecules are lipids
Non-polar
Why do we add ethanol when identifying lipids
Cause ethanol is non-polar, so lipids (being non-polar) will dissolve in it
What test do we use to identify lipids
Emulsion test
Method of emulsion test
• Add ethanol to the sample to be tested
• Shake to mix
• Add the mixture to a test tube of water
Emulsion test - positive result
If lipids are present, a milky emulsion will form (the solution appears ‘cloudy’); = on the top layer
the more lipid present, the more obvious the milky colour of the solution
Emulsion test - negative result
Solution remains clear
Limitations of emulsion test
• This test is qualitative - it does not give a quantitative value as to how much lipid may be present in a sample
What test is used to identify proteins
Biuret test
What does biuret reagent contain
• an alkali and copper (II) sulphate which react in the presence of peptide bonds
Method for biuret test
• Add sodium hydroxide to the food solution sample to make the solution alkaline
• Add a few drops of copper (II) sulfate solution (which is blue) to the sample
o Biuret ‘reagent’ contains an alkali and copper (II) sulfate
• Repeat steps 1 and 2 using the control solution
• Compare the colours of the control solution and the food sample solution
Positive result - biuret test
colour change is observed from blue to lilac/mauve
The colour change for biuret test can be very subtle - how do we help this
o hold the test tubes up against a white tile when making observations
Limitations of biuret test
o For this test to work, there must be at least two peptide bonds present in any protein molecules, so if the sample contains amino acids or dipeptides, the result will be negative
• The Biuret test is qualitative - it does not give a quantitative value as to the amount of protein present in a sample
What is an inorganic ion
• an ion that does not contain carbon
Where are inorganic ions found
• occur in solution in the cytoplasm and body fluids of organisms
What do inorganic ions act as
Co-factors
What are cofactors
o non-protein chemical compounds that are required for a protein to function
How many bonds does carbon form
4
How many bonds does nitrogen form
3
How many bonds does oxygen form
2
How many bonds does hydrogen form
1
What does valency mean
• Number of covalent bonds an atom can make.
Cations we need to know
Hydrogen ions
Calcium ions
Iron ions
Sodium ions
Potassium ions
Ammonium ions
Chemical symbol + function of hydrogen ions
Chemical symbol + function of calcium ions
Chemical symbol + function of iron ions
Chemical symbol + function of sodium ions
Chemical symbol + function of potassium ions
Chemical symbol + function of ammonium ions
Anions we need to know
Nitrate ions
Hydrogen carbonate ions
Chloride ions
Phosphate ions
Hydroxide ions
Chemical symbol + function of nitrate ions
Chemical symbol + function of hydrogen carbonate ions
Chemical symbol + function of chloride ions
Chemical symbol + function of phosphate ions
Chemical symbol + function of hydroxide ions
B
A
D
A
C
A
C
A
A
Two similarities + two differences between this and glycogen
What is special about collagen
Every 3rd amino acid is the same
Benefit of glycogen being insoluble
Has no effect upon water potential
What makes up lactose
Beta glucose + galactose
Which proteins are held together by disulphide bridges = quaternary
Insulin + haemoglobin
B
C
C
C
D
Colour change for iodine test
Orange / brown to black
Is amylose soluble
No
