Biological Molecules Flashcards
Define the structure of water
- Two hydrogen atoms covalently bonded to one oxygen atom.
- Polar molecule
- Hydrogen bonds form between electronegative oxygen atoms and electropositive hydrogen
atoms on adjacent water molecules
Water has important solvent properties. Explain these properties using an example to illustrate your answer
- Water is polar
- Substances that dissolve in water are hydrophilic
- Water forms hydrogen bonds with polar substances
- Positive pole of water attracted to negative pole of molecules
- e.g. Glucose dissolves because it is polar
Which properties explain the ability of water to dissolve solutes?
- Polarity of water molecules
- Hydrogen bonding
Which property of water accounts for its moderating effects on the Earth’s atmosphere?
Thermal properties
Explain the reasons for the unique thermal properties of water
- Water is polar and exhibits dipolarity
- Causes strong hydrogen bonds to form between the molecules
- Requires more energy to overcome hydrogen bonds
- Increases the melting point
Define specific heat capacity
Heat energy required to raise the temperature of 1Kg of a substance by 1°C
Define latent heat of vaporisation
Heat energy required to change the state of a substance from liquid to gas
Why does water act as an adhesive?
Water is a polar molecule allowing it to ‘stick’ to other substances
Why is water cohesive?
Hydrogen bonds between molecules allow them to ‘stick’ together
Why does water act as a coolant?
Hydrogen bonds between water molecules must be broken when water evaporates - removes heat energy from body
Outline the properties of water molecules that permit them to move upwards in plants
- Water molecules are polar and can form hydrogen bonds.
- Cohesion between water molecules allows transpiration stream to form in xylem.
- Adhesion of water to the walls of xylem vessel helps water rise.
- Water evaporates at environmental temperatures allowing transpiration pull.
Why is water’s/ice’s density a benefit to living organisms?
Ice is less dense than water and floats on top of it, creating an insulating layer and preventing animals in large bodies of water from freezing
Why is water being a solvent a benefit to living organisms?
Allows many metabolic reactions occur
How does water having a high specific heat capacity benefit living organisms?
Lots of energy required to warm water up, minimising temperature fluctuations
Define condensation reaction
- Reaction which involves the elimination of a small molecule e.g. water
- Occurs when two molecules chemically bond togeth
Define polymer
- Very large molecule formed by the joining together of smaller subunits (monomers)
- Formed by condensation reactions
Define hydrolysis reaction
- Reaction that breaks a chemical bond between two molecules
- Involves the use of a water molecule
Define anabolism
- Synthesis of complex molecules from simpler molecules
- Includes the formation of polymers from monomers by condensation reactions
Define catabolism
- Breakdown of complex molecules into simpler molecules
- Includes the hydrolysis of polymers into monomers
- e.g. amylose (polymer) into maltose (monomer) by amylase (enzyme)
List the elements found in carbohydrates
Carbon, hydrogen, oxygen
List the elements found in lipids
Carbon, hydrogen, oxygen
List the elements found in proteins
Carbon, oxygen, hydrogen, nitrogen (+ sometimes sulfur)
List the elements found in nucleic acids
Carbon, oxygen, hydrogen, nitrogen, phosphorous
Define monosaccharide
imple carbohydrates consisting of one subunit (monomer)
- e.g alpha (α)-glucose, beta (β)-glucose
What type of bond forms between monosaccharides?
Glycosidic bonds
Define disaccharide
- Carbohydrate consisting of two subunits (monomers)
- Glycosidic bonds formed by condensation reactions
What is maltose made of?
Maltose formed from two glucose monomers
What is lactose made of?
Lactose formed from one glucose and one galactose monomer
What is sucrose made of?
Sucrose formed from one glucose and one fructose monomer
Define polysaccharide
Complex carbohydrates consisting of more than two subunits (monomers)
- e.g. glycogen, cellulose, starch
Describe the structure of starch
- Polymer
- Formed from α-glucose
- Two forms - amylose and amylopectin
Describe the structure of cellulose
- Unbranched polymer
- Formed from β-glucose monomers
- Long straight chains linked together by many hydrogen bonds to form fibrils
- Provides high tensile strength to plant cell walls
Describe the structure of amylose
- Polysaccharide
- Formed from α-glucose
- 1,4 linkages → unbranched (linear)
Describe the structure of amylopectin
- Polysaccharide
- Formed from α-glucose
- 1,4 and 1,6 linkages → branched
What is the function of starch?
Used by plants as a storage of glucose/energy
How are starch molecules adapted for their function in plant cells?
- Insoluble - do not affect water potential
- Helical/spirals - compact
- Large molecule - cannot leave cell
- Branched amylopectin can load and unload glucose more quickly than unbranched
amylose - more exposed subunits for enzymes to act upon
Which type of reaction is the breakdown of starch into sugars?
Hydrolysis
Describe the role of glycogen
- Used by animals and fungi as a storage of glucose
- Stored in animal liver cells and muscle
Describe the structure of glycogen
- Formed from α-glucose
- Highly branched
- Branched structure allows fast loading and unloading of glucose
- Insoluble - does not affect water potential of cell
Why is glycogen more branched than starch?
- Animals require more energy than plants (for movement, digestion etc.)
Which sugars are reducing sugars?
- All monosaccharides
- Some disaccharides (e.g. maltose and lactose)
- All others are non-reducing sugars
What is the test for reducing sugars?
- Benedict’s reagent (copper(II) sulfate)
- Place sample in water
- Add equal amount of Benedict’s reagent
- Heat gently in water bath for five minutes
- Positive result: Brick red precipitate forms
What is the test for non-reducing sugars?
- Perform test for reducing sugar
- If solution remains blue:
- Boil sample with hydrochloric acid
- Neutralise with sodium hydrogen carbonate
- Add equal amount of Benedict’s reagent
- Heat gently in water bath for five minutes
- Positive result: Brick red precipitate forms
Why is hydrochloric acid added when testing for non-reducing sugars?
Hydrolyses glycosidic bonds
Describe how to measure the change in concentration of the reducing sugar maltose as the enzyme amylase breaks down starch into maltose
- Take samples at a range of times
- Same volumes of solutions removed each time
- Heat samples with Benedict’s solution
- Colour changes from blue → green → yellow → orange → brick red
- Colour change depends on concentration of maltose in solution
- Calibrate a colorimeter using unreacted Benedict’s solution
- Record the absorbance of each sample
- Less absorbance = more maltose present
- Using known concentrations of maltose, plot a calibration curve
- Plot absorbance against reducing sugar concentration
- Use graph to read off concentration of maltose
What are reagent strips?
Paper strips that can detect the presence of reducing sugars
What is the advantage of reagent strips?
Concentration of the sugar can be easily determined
What is the test for starch?
- Iodine solution added to sample
- Positive result: Brown solution turns blue/black
What are the three major types of lipid?
- Phospholipids
- Triglycerides
- Steroids
Describe the structure of a triglyceride
Three fatty acids, one unit of glycerol
What type of bond forms between glycerol and a fatty acid?
Ester
Describe the structure of a phospholipid
- Two fatty acids, phosphate, glycerol
- Heads are hydrophilic
- Tails are hydrophobic
Describe the structure of a steroid
- Four fused rings
- e.g. cholesterol, progesterone, estrogen, testosterone
Define saturated
All carbon bonds are single
Define unsaturated
Contains one or more carbon-carbon double bond
Define monounsaturated
Contains only one carbon-carbon double bond
Define polyunsaturated
Contains two or more carbon-carbon double bonds
Describe the difference in appearance between saturated and unsaturated lipids at room temperature
- Saturated - solid (e.g. animal fats)
- Unsaturated - liquid (e.g. plant oils)
What is the role of fats and oils?
Concentrated, long-term energy store
What is the advantage of using lipids as an energy store over carbohydrates?
- Amount of energy released per gram of lipids is double that of carbohydrates
- Insoluble - do not affect water potential of the cell
What is the role of phospholipids?
- Form phospholipid bilayer cell membranes
- Hydrophilic head faces outwards
- Hydrophobic tail faces inwards
What are the roles of steroids?
- Hormones
- Vitamins
- Cholesterol adds stability to cell membranes
- Keeps membranes fluid at low temperatures
- Prevents membranes becoming too fluid at high temperatures
What medical conditions are associated with increased blood cholesterol levels?
- Coronary heart disease
- Atherosclerosis
- Stroke
- Type 2 diabetes
What is the test for lipids?
- Emulsion test
- Mix sample with ethanol, then water
- Positive test: Milky white emulsion forms
What is the basic building block of a protein called?
Amino acid
How many different amino acids exist?
20
Describe the structure of an amino acid
- Amine group (NH2)
- Carboxyl group (COOH)
- Central carbon atom
- R group (variable depending on the amino acid)
Define polypeptide
Long unbranched chain formed of many amino acids
What type of bond forms between amino acids?
Peptide bonds
How are amino acids joined together to form a polypeptide?
- Condensation reactions
- Peptide bond forms between amine group of one amino acid and carboxyl group of another
Outline the general structure of a protein
Primary structure
- Order of amino acids
Secondary structure
- Folding of polypeptide into alpha-helix or beta-sheet
- Held in position by hydrogen bonds
Tertiary structure
- Folding of polypeptide chain in 3D space
- Held in position by hydrogen bonds, ionic bonds, hydrophobic and
hydrophilic interactions and disulfide bridges between amino acids
Quaternary structure
- Joining of two or more polypeptides to form a functioning protein
Explain how the secondary structure is held in position
- Hydrogen bonds
- Between amino (N-H) and carboxyl (C=O) groups on different amino acids
Explain how alpha helices/beta pleated sheets are formed
- Hydrogen bonds form between N-H and C=O groups
What type of interactions form between R-groups?
- Ionic interactions between positively charged and negatively charged R-groups
- Hydrogen bonds between polar R-groups
- Covalent disulphide bridges between cysteine R-groups
- Hydrophobic (non-polar) amino acids orientate themselves towards centre of polypeptide
- Hydrophilic (polar) amino acids orientate themselves towards outside of polypeptide
Outline the importance of polar and non-polar amino acids in proteins
- Polar and non-polar amino acids help determine protein structure
- Polar amino acids on the outside of proteins make them soluble in water
- Polar amino acids in channels in membranes allow passage of polar substances
- Polarity or non-polarity of surface amino acids on proteins determines their interaction with
other molecules (e.g substrates, hormones, signalling molecules)
Provide examples of different roles and types of proteins
Enzymes
- Globular proteins
- e.g. rubisco which catalyses photosynthesis
Hormones
- Globular proteins
- e.g. insulin triggers conversion of glucose to glycogen to lower blood glucose levels
Structural
- Fibrous proteins
- e.g. keratin found in hair, collagen found in skin
Distinguish between fibrous and globular proteins
Fibrous - Linear / long chains
Globular - Spherical
Fibrous - Usually insoluble
Globular - Usually soluble
Fibrous - Provide strength
Globular - Forms hydrogen bonds with water
Fibrous - Structural roles
Globular - Catalysis/transport roles
What is a conjugated protein?
Globular protein with a prosthetic group
- e.g. haemoglobin - protein combined with iron
Describe the structure of collagen
- Peptide bonds between amino acids
- Every 3rd amino acids is the same (glycine)
- Forms left-handed helix
- Glycine allows polypeptide chains to pack close together
- Forms fibrils
- Hydrogen bonds between three polypeptide chains
- Few hydrophilic R groups on outside of molecule
Describe the quarternary structure of haemoglobin
- 4 polypeptide subunits
- 1 haem group (a prosthetic group, contains Fe2+) per polypeptide
Describe the ways in which the structure of collagen is similar to the structure of haemoglobin
- Both made from amino acid chains
- Peptide bonds between amino acids
- Disulfide bridges, ionic bonds, hydrogen bonds and hydrophobic/hydrophilic interactions
form tertiary structure - Both contain more than one polypeptide in quaternary structure
Define denaturation
- A deformation of 3D structure of proteins (tertiary structure)
- In enzymes can cause change in shape of active site
- Substrate can no longer bind
What causes denaturation?
- High temperature (usually irreversible)
- Changes in pH (usually reversible)
How does high temperature alter the protein’s structure?
- Intermolecular bonds break altering the protein’s 3D structure
- Usually irreversible
What is the test for protein?
- Biuret test
- Place liquid sample in equal volume of 10% sodium hydroxide solution
- Add 1% copper sulphate solution
- Mix and leave for 5 minutes
- Positive result: blue solution turns purple/lilac
Describe how thin layer chromatography can be used to separate amino acids
- Amino acids added to one end of layer of silica gel (stationary phase)
- This end is submerged in organic solvent
- Solvent moves through silica gel (mobile phase)
- Different components of the mixture travel at different speeds, causing them to separate
- The more soluble the amino acid, the further it will travel through the gel
- An Rf value can be calculated and compared to data tables
