AS Biology Unit F212: Molecules, Biodiversity,
Food and Health
Module 1: Biological Membranes
Topic 1: Biological Membranes
● Biological molecules grouped according to chemical properties, made of C, H, O, N ( some
have P and S)
● Single small molecules (monomers) -> join to make polymers
● Carbohydrates: monosaccharide – polysaccharide
● Proteins: amino acids – polypeptides/proteins
● Nucleic acids: nucleotides – DNA/RNA
● Lipids: don’t form monomers, are in components but have largest unit i.e. triglyceride
Carbon:
● Framework atom, forms basis of all biological molecules
● Forms long chains and rings – forms dif molecules w/ dif structures and properties
● Forms covalent bonds – electrons shared between atoms to form new molecules, v strong as
full outer shells
Condensation - Chemical reaction that links biological monomers together to form polymers
by the formation of new bonds and releasing water (anabolic)
● Water molecule released
● Covalent bond formed
● Larger molecule formed by bonding of smaller molecule
Hydrolysis - Chemical reaction that splits larger molecules into smaller molecules by splitting
bonds w/ the addition of water (catabolic)
● Water molecule added
● Covalent bond broken
● Smaller molecule formed by splitting of larger molecule
Carbohydrates:
● 10% organic matter in a cell
● Group of molecules containing C, H, O in ratio C n(H2O)n
● Energy source – released from glucose in resp.
● Energy store – starch (plants) glycogen (animals)
● Structural molecule – cellulose
● Some form larger molecules – nucleotides/glycolipids
● 3 main groups of carbohydrates: monosaccharides/single sugars (glucose, fructose,
galactose), disaccharides/double sugars (sucrose, maltose, lactose), polysaccharides/many
sugars (starch, glycogen, cellulose)
● Monosaccharides = sweet, soluble, form crystals (properties of all mono/di)
● Cn(H2O)n
● Grouped according to no. of carbons in molecule: Triose (3-membered ring), Pentose (5-
membered ring), Hexose (6-membered ring)
● Differences: α-glucose OH group on C1 BELOW, β-glucose OH group on C1 ABOVE = this
changes overall shape = dif properties
● Isomers = dif forms of same molecule
,Formation and breaking down a disaccharide
α-glucose + α-glucose -> maltose +water maltose +water -> α-glucose + α-glucose
C6H12O6 + C6H12O6 -> C12H22O11 + H2O C12H22O11 + H2O -> C6H12O6 + C6H12O6
Glucose - Energy and structure:
● resp. = breaking down glucose = release energy needed to make ATP
● Glucose + Oxygen -> Carbon Dioxide + Water + Energy
- Breaking down glucose and using it in resp:
● series of steps catalysed by specific enzymes
● carbohydrate molecules = dif shapes
● active site and substrate must be complimentary
● so that substrate will fit/lock and key hypothesis
● Animals and plants – only break down α-glucose
● Enzyme function based on shape
● Don’t have enzyme to break down β-glucose which has dif arrangement of –H and –OH
● Bacteria does have enzyme to break down β-glucose
Relating structure to function
● Soluble: easily transported around organism
● Small: transported/diffuse across membrane quick
● Easily/quickly respired: to form ATP
● Molecules can join: to make di/polysaccharides
Carbohydrate polymers – energy storage
● Amylose – condensation reaction of thousands of
glucose
-molecules formed by α-1, 4-glycosidic bonds
-long chains of α-glucose molecules coil into helical structure = compact for storage
,● Amylopectin – α-1,4-glycosidic bonds w/ branches of α-1,6-glycosidic
bonds
-more branched than helical = release energy quicker
● Glycogen – similar to amylopectin
-shorter α-1,4-glycosidic bonds
-more branches of α-1,6-glycosidic bonds
- Features of starch and glycogen – relating structure and functions
● Large and insoluble: doesn’t affect WP of the cell
● Held in chains: hydrolysed quickly from the ends to provide energy for resp
● Compact: high energy content for mass
● Branched: more SA for enzyme attachment
-Starch – few branches for enzyme attachment resulting in slow energy
release; makes for great long-term storage molecule
-Glycogen – highly branched so more surface for enzyme attachment to
release glucose quickly
Carbohydrate polymers – structural units
● Β-glucose monomers w/ alternate molecules inverted
● Joined by β-1,4-glycosidic bonds = forms long straight chains
● Cellulose, plant cell wall – most abundant structural
polysaccharide in nature
● Cellulose chains = stronger than amylose chains
Cellulose - relation structure and function
● Long, straight parallel chains w/ high tensile strength = gives
wall strength to prevent cell bursting
● Linked by H-bonds (monomers contain many –OH groups)
● 60-70 molecules cross linked by H-bonds = microfibrils
● Microfibrils held together by more H-bonds = macrofibrils
● Resistant to hydrolysis
● Macrofibril arrangement = fully permeable, allows water
movement along cell walls and in and out of cell
Proteins:
● 50% organic matter in cell
● large molecules
● made of C, H, N (some have S)
● Functions –
- structural components e.g. muscle/bones
- membrane carriers and pores e.g. active transport
Amino acid monomer:
● 20 types
● involved in protein syn
● same basic structure but w/ dif. R-groups bonded to the central C
● simplest a.a = glycine
● some R-groups: larger than N-C-C backbone, positively/negatively charged, hydrophobic/phallic
● PLANTS: make amino acids needed from nitrates in soil – nitrates converted into amino group –
amino group bonded to organic groups that are made from the products of photosyn
● ANIMALS: take amino acids from diet – 10/20 a.a can’t be made from materials taken into the
body – ‘essential amino acids’ = essential part of diet (found in red meat)
● Animals can’t store excess amino acids as it’s toxic
● Amino group removed in liver (deamination) –> amino group –> urea (removed)
,Formation and breakage of peptide bonds:
● Between amine group of an amino acid and carboxyl of another
● H from amine combines w/ OH from carboxyl
● Condensation reaction occurs = water is lost
● Covalent bond formed
Dipeptides, polypeptides and proteins
Making polypeptides and proteins:
● Protein syn = ribosomes
● mRNA = determines amino acid sequence
● mRNA passes through ribosome and amino acids are joined together by condensation
reaction forming new peptide bonds which make a specific polypeptide chain
Forming dif. Proteins
● 20 e.g. 4 amino acids = 20 = 20 x 20 x 20 x 20 = 1600
n 4
-Breaking down proteins and polypeptides:
● Formation and breakage of peptide bonds catalysed by protease enzymes
● Hormone regulation – hormones broken down so effects aren’t permanent, any cells targeted
by hormones contain enzymes to break them down
● Ageing – older skin can’t rebuild protein collagen and other proteins that make skin smooth
and elastic
● Enzymes for breaking down located in lysosome
- Levels of protein structure
Primary structure: sequence of amino acids that are determined by DNA
● Organisms have over 10000 dif proteins w/ specific functions
● DNA determines amino acid sequence -> amino acid sequence determines structure ->
structure determines function
Secondary structure: chain stabilised by coiled α-helix or folded β-pleated sheets; held in
place by H-bonds, depends on primary structure
● Chain of amino acids coils up to form either α-helix or β-pleated
● Stabilised by H-bonds
Tertiary structure: final 3D structure held by disulfide, ionic, hydrogen bonds or
hydrophilic/phobic interactions; either fibrous or globular
● Overall 3D structure of final protein/polypeptide molecule
● Coils/pleats fold into globular or fibrous protein shape
● Held by one of the four bonds/interactions
● Vital to function
● E.g. hormone – specific shape to fit into the receptor of a target cell
● E.g. collagen – structural protein, shaped to be strong w/ chains wound in certain way
● E.g. enzyme – active site must be complimentary to substrate
, Feature Globular Fibrous
3D Structure Spherical Forms fibres
Structure Hydrophobic R-groups face Regular repetitive sequence
inwards of amino acids
Hydrophilic R-groups face
outwards
Solubility Soluble Insoluble
Role Metabolic Structural
Examples Haemoglobin, antibodies, Collagen, keratin
hormones, enzymes
● Disulfide bond: amino acid contains sulfur, two cysteines close together form covalent bond,
can only be broken down by reducing agent
● Ionic bond: oppositely charged R-groups close together form bond, a lot of heat required to
break
● Hydrogen bond: electronegative group close to electropositive group can form H-bond,
relatively weak
● Hydrophilic/phobic interaction: hydrophobic amino in centre hydrophilic outside, weakest
not even a bond
● Heat on tertiary structure:
-Increase in kinetic energy = molecule vibrates
-Some bonds/interactions break e.g. H-bonds
-Change in 3D shape of protein
-No longer functions and is denatured
● Quaternary structure: more than 1 polypeptide subunit joined together or a polypeptide and
inorganic compound; all subunits come together for protein function e.g. haemoglobin
● Some proteins made of more than one polypeptide subunit or a polypeptide and inorganic
compound
● All subunits must be present
● Polypeptide subunit may be either identical or different
● E.g. haemoglobin, insulin
Haemoglobin:
● Globular transport protein
● Function – binds O2 at lungs takes to tissues
● 4 polypeptide subunits, 2 α and β chains
● Water soluble
●
2+
1 haem group (prosthetic group) per polypeptide – has Fe
● Hb + O2 (purple red) -> HbO8 (bright red)
Collagen:
● Fibrous structural protein
● 3 polypeptide chains wound around each other w/ H-bonds for strength (each chain 1000
a.a.) -> Collagen molecules (form covalent bonds w/ each other) -> Collagen fibril -> Collagen
fibre
● Functions:
-Artery walls – prevents bursting when blood is pumped at high pressure
-Tendons – connect bones to muscle for movement
-Bones – reinforced with Calcium phosphate to make them hard
-Cartilage – connective tissue
-Cosmetic treatment – lip fillers
● Properties of collagen: flexible, doesn’t stretch, high tensile strength, insoluble
