7.5 Proteins – summary of mark schemes
7.5.1
Explain the four levels of protein structure, indicating the significance of each level.
Mark Scheme
A.
B.
C.
primary structure / level: order / sequence of amino acids;
linked by peptide bonds;
determines the type / function of protein / 2º and 3º structures;
D.
E.
secondary structure / level: regular folding / beta-pleated sheets / spiralling / alpha-helices;
held through hydrogen bonding;
F. tertiary structure / level: 3-dimensional conformation of a polypeptide / protein;
G. held with ionic bonds, hydrogen bonds, disulfide bonds / bridges and hydrophobic bonds; (must give at least two
bonds)
H. determines overall shape / a named example eg: active sites on enzymes;
I.
J.
K.
L.
7.5.2
quaternary structure linking together of polypeptides to form a single protein;
using the same bonding as for tertiary structure;
linking of a non-polypeptide structure / prosthetic group;
named example of quaternary structure eg hemoglobin (has four polypeptides);
Outline the difference between fibrous and globular proteins, with reference to two examples of each protein type.
Mark Scheme
Fibrous vs globular
A. repetitive amino-acid sequences
B. long and narrow / long strands
C. support / structural functions
D. (mostly) insoluble in water vs.
vs.
irregular amino acid sequences;
vs.
rounded / spherical / ball shaped;
vs.
metabolic / other functions;
(mostly) soluble in water;
Fibrous examples
E. collagen;
F. myosin;
G. keratin;
H. fibroin / elastin / silk protein in insects and spiders;
Globular examples.
I. catalyses / other named enzyme;
J. hemoglobin / myoglobin;
K. insulin / other peptide hormone;
L. immunoglobulin / other globular protein;
7.5.4
State four functions of proteins, giving a named example of each.
Mark Scheme
A.
B.
C.
D.
E.
F.
G.
H.
I.
J.
K.
L.
storage – zeatin (in corn seeds) / casein (in milk);
transport – hemoglobin / lipoproteins (in blood);
hormones – insulin / growth hormone / TSH / FSH / LH;
receptors – hormone receptor / neurotransmitter receptor / receptor in chemoreceptor cell;
movement – actin / myosin;
defense – antibodies / immunoglobin;
enzymes – catalase / RuBP carboxylase;
structure – collagen / keratin / tubulin / fibroin;
electron carriers – cytochromes;
pigments – opsin
active transport – sodium pumps / calcium pumps;
facilitated diffusion – sodium channels / aquaporins;