Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

1.4Unit 1: Lifestyle, Transport, Genes and Health

SNAB Proteins — IAL Edexcel Linear Revision Notes & Topical Questions

10 min read 13 parts Lesson 5 of 29 1 quiz

📚 IAL Edexcel Linear Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
0%read
Lesson summary

1.4 Proteins — IAL EDEXCEL LINEAR notes: Describe how genes code for proteins by transcription and translation, relate protein structure to function, and explain enzyme action and the effect of enzyme and substrate concentration.Gene = a sequence of nucleotide bases in DNA that c…

  • Triplet code
  • Non-overlapping
  • Degenerate
  • Protein functions
  • Amino acid
  • Peptide bond

Part 1 of 11The genetic code and genes

Gene
Gene a sequence of nucleotide bases in DNA that codes for a specific polypeptide.
  • The sequence of amino acids is the primary structure, which determines the protein's shape and function.
Triplet code
  • Triplet code each sequence of three bases codes for one amino acid, and some triplets are start or stop signals.
    • (AUG, methionine)
  • 20 amino acids are commonly used in proteins.
Non-overlapping
  • Non-overlapping each base is read once and belongs to only one codon.
Degenerate
  • 4³ = 64 codons exist for only 20 amino acids, so several codons code for the same amino acid.
  • This limits the effect of some mutations.
Universal
  • Universal almost all organisms use the same code, so genes can be transferred between species in genetic engineering.

Part 2 of 11Transcription

Location
  • Location it occurs in the nucleus.
mRNA
mRNA a single-stranded copy of a gene that carries the code from the nucleus to ribosomes.
  • RNA has ribose instead of deoxyribose, is usually single-stranded and has uracil instead of thymine.
1
Part of the DNA unwinds and the hydrogen bonds between complementary bases break, exposing the gene.
2
Free activated RNA nucleotides pair with the complementary bases on the template strand by hydrogen bonds.
3
RNA polymerase joins the sugar-phosphate groups of the RNA nucleotides to form the mRNA backbone.
4
The hydrogen bonds between mRNA and DNA break and the DNA double helix reforms.
5
The mRNA leaves the nucleus through a nuclear pore.
  • Strands
    • only the antisense strand is copied, and the sense (coding) strand has the same base sequence as mRNA except T is replaced by U.
      • (template, transcribed)
    • RNA polymerase moves along the template strand 3′ to 5′, so mRNA grows 5′ to 3′.
    • DNA polymerase is used in replication, not transcription.

Part 3 of 11Translation

Location
  • Location it occurs on ribosomes in the cytoplasm.
tRNA
tRNA a single-stranded RNA folded into a clover shape with an anticodon at one end and an amino acid binding site at the other.
  • About 20 types of tRNA exist, each with a specific anticodon and amino acid.
1
mRNA leaves the nucleus and attaches to a ribosome.
2
The start codon AUG signals the start, and a tRNA with the complementary anticodon brings methionine.
3
Two tRNA molecules sit on the ribosome at once, with their anticodons paired to adjacent codons.
4
A peptide bond forms by condensation between the two amino acids.
5
The ribosome moves along the mRNA and the chain grows until a stop codon ends translation.
6
The finished amino acid chain forms the polypeptide.

Part 4 of 11Amino acids and peptide bonds

Amino acids and peptide bonds
Protein functions
  • Protein functions enzymes, membrane proteins, hormones, immunoglobulins, transport proteins (haemoglobin), structural proteins and contractile proteins (myosin).
    • (keratin, collagen)
Amino acid
Amino acid a monomer with a central carbon bonded to an amine group (–NH2), a carboxyl group (–COOH), a hydrogen atom and an R group.
  • The R group makes each of the 20 amino acids different, for example acidic or basic, polar or non-polar.
Peptide bond
Peptide bond covalent bond between the carbon of one carboxyl group and the nitrogen of the next amine group.
  • It forms by condensation, losing –OH from one acid and H from the other, which releases water.
  • The R group is not involved.
Dipeptide and polypeptide
  • Dipeptide and polypeptide a dipeptide has two amino acids and a polypeptide has three or more.
  • Hydrolysis adds water and breaks peptide bonds.

Part 5 of 11Levels of protein structure

LevelDescriptionBonds
PrimarySequence of amino acids in the polypeptidePeptide bonds (covalent)
SecondaryAlpha-helix or beta-pleated sheetHydrogen bonds between the amine and carboxyl groups of the backbone
TertiaryFurther folding into a 3D shapeHydrogen, ionic, disulphide (between cysteines) and hydrophobic interactions between R groups
QuaternaryTwo or more polypeptide subunits working togetherSame bonds as tertiary, as in haemoglobin
Alpha-helix
  • Alpha-helix hydrogen bonds form between every fourth peptide bond.
Beta-pleated sheet
  • Beta-pleated sheet parts of the chain run parallel and hydrogen bond between peptide bonds.
Denaturation
  • Denaturation heat and pH change break hydrogen bonds and alter the shape.

Part 6 of 11Globular proteins and haemoglobin

Globular proteins
  • Globular proteins they are compact and spherical with hydrophobic R groups inside and hydrophilic R groups outside.
  • They are generally soluble, so they are easily transported and take part in metabolism, for example enzymes and immunoglobulins.
  • Some are conjugated proteins with a prosthetic group.
Haemoglobin
  • Haemoglobin an oxygen-carrying globular protein in red blood cells with quaternary structure.
  • It has four subunits (two alpha-globins and two beta-globins), each with a haem group.
  • Each haem group contains an Fe2+ ion that reversibly binds one oxygen molecule, forming bright red oxyhaemoglobin.
  • One haemoglobin carries four oxygen molecules, and binding the first changes the shape so later molecules bind more easily.
  • Binding by haemoglobin lets oxygen, which is poorly soluble, be carried efficiently.
Sickle cell anaemia
  • Sickle cell anaemia base substitution replaces polar glutamic acid with non-polar valine, which makes haemoglobin less soluble.

Part 7 of 11Fibrous proteins and collagen

Fibrous proteins
  • Fibrous proteins they are long strands with cross-links, little or no tertiary structure and a repetitive sequence of few amino acids.
  • Many hydrophobic R groups make them insoluble, and with their strength this suits structural roles.
  • Keratin forms hair, nails, horns and feathers; elastin stretches and recoils in connective tissue.
Collagen
  • Collagen the most common vertebrate structural protein, found in tendons, cartilage, ligaments, bone, teeth, skin, blood vessel walls and the cornea.
  • Three polypeptide chains form a triple helix (tropocollagen) held by hydrogen bonds and covalent bonds.
  • Almost every third amino acid is glycine, the smallest, which lies inside so the chains pack tightly.
  • Proline and hydroxyproline give stability because their R groups repel each other.
  • Covalent cross-links join triple helices into fibrils with staggered ends, and fibrils form fibres aligned with the forces they resist.
  • Many hydrogen bonds give high tensile strength, and the long molecules make it insoluble.

Part 8 of 11Enzymes as catalysts

Enzyme
Enzyme a biological catalyst, a globular protein that speeds up a reaction without being used up.
  • Enzymes may be a single polypeptide or have quaternary structure.
  • Nearly every metabolic reaction is enzyme-catalysed in controlled pathways.
Intracellular enzymes
  • Intracellular enzymes they are made and work inside cells.
Extracellular enzymes
  • Extracellular enzymes they are secreted to work outside cells, such as digestive enzymes.
Activation energy
Activation energy the energy the substrate needs to become unstable enough to react.
  • Enzymes provide an alternative pathway with lower activation energy by destabilising bonds in the substrate.
  • This avoids the extreme temperatures or pressures that would kill cells.
  • Enzymes are proteins, so anything that denatures proteins also denatures them.

Part 9 of 11Mode of enzyme action

Mode of enzyme action
Active site
  • Active site a specific region whose shape is determined by the tertiary structure, where the substrate binds to form an enzyme–substrate complex.
Specificity
  • Specificity only a substrate with a complementary shape fits.
  • Changing the amino acid order changes the tertiary structure and active site, so no complex forms.
  • Heat or pH extremes denature the enzyme by changing the active site shape.
Lock-and-key hypothesis (Fischer, 1890s)
  • Lock-and-key hypothesis (Fischer, 1890s) enzyme and substrate are rigid and fit precisely.
Induced-fit hypothesis (1959)
  • Induced-fit hypothesis (1959) the active site, and sometimes the substrate, changes shape slightly on binding.
  • These conformational changes give an ideal fit and maximise catalysis.
Collisions
  • Collisions substrates must collide with the active site at the correct orientation and speed.

Part 10 of 11Enzyme and substrate concentration

Enzyme concentration
  • Enzyme concentration more active sites give a higher initial rate, which rises linearly if substrate is in excess.
  • If substrate is limited, extra enzyme does not increase the rate.
Substrate concentration
  • Substrate concentration the rate rises as substrate increases until all active sites are occupied at the saturation point.
  • The rate falls over time as substrate is used up, so the initial rate is the fastest.
Initial rate
Initial rate gradient of the tangent drawn at the origin of a graph of product (or substrate) against time.
Method
  • Method measure either how fast product forms or how fast substrate is used up.
    • (catalase, amylase and protease are common)
1
Add set volumes of hydrogen peroxide and buffer to a boiling tube.
2
Invert a full measuring cylinder in a water trough and place the delivery tube end inside it.
3
Add a set volume of one catalase concentration and quickly insert the bung.
4
Record oxygen collected every 10 seconds for 60 seconds.
5
Repeat twice and find the mean volume at each interval.
6
Repeat for each catalase concentration, plot volume against time and compare initial rates.
Result
  • Result more catalase gives more oxygen at first, then the volume plateaus as hydrogen peroxide is used up.
Substrate variation
  • Substrate variation starch and amylase with iodine can be followed in a colorimeter, with absorbance falling quickly then levelling.
Controls
  • Controls use a control with water in place of enzyme and keep pH and temperature constant.

Part 11 of 11More on this topic

Temperature buffering
  • Temperature buffering water has a high specific heat capacity and a high latent heat of vaporisation because many hydrogen bonds must be broken.
Haemoglobin
  • Haemoglobin it has four haem groups, each binding one oxygen molecule to form oxyhaemoglobin.
  • Binding the first oxygen changes shape and makes later binding easier (cooperative binding), giving an S-shaped dissociation curve.
  • At high pO2 in the lungs haemoglobin loads oxygen, and at lower pO2 in tissues it unloads readily.
Carbon dioxide transport
  • Carbon dioxide transport a little dissolves in plasma, some binds haemoglobin as carbaminohaemoglobin, and most forms hydrogen carbonate ions.
  • Carbonic anhydrase in red cells speeds CO2 + H2O → H2CO3, which dissociates into H+ and HCO3−.
  • Haemoglobin binds H+ and acts as a buffer.

Key terms

TermMeaning
GeneA sequence of nucleotide bases that codes for a specific polypeptide.
CodonA triplet of bases on mRNA coding for one amino acid.
AnticodonA triplet of bases on tRNA complementary to an mRNA codon.
Peptide bondCovalent bond between amino acids formed by condensation.
Tertiary structureThe 3D folding of a polypeptide held by bonds between R groups.
Active siteThe region of an enzyme where the substrate binds.
Activation energyThe energy needed for a reaction to start.
DenaturationLoss of protein shape caused by heat or pH change.

Avoid these mistakes

1
Confusing DNA polymerase and RNA polymerase — DNA polymerase acts in replication and RNA polymerase in transcription.
2
Saying transcription happens in the cytoplasm — transcription occurs in the nucleus and translation at ribosomes in the cytoplasm.
3
Saying tertiary hydrogen bonds link backbone groups — in tertiary structure they join R groups, whereas secondary hydrogen bonds link amine and carboxyl groups.
4
Saying heat kills enzymes — heat denatures them by altering the shape of the active site.

Last updated:

\n