IB DP Biology 2025: Form and Function
B1.2 — Proteins
The Building Blocks of Life: Structure, Function, and Diversity
B1.2.1
Amino Acid Structure
Key Understanding Generalized structure of an amino acid and the significance of its functional groups.
Amino acids are the fundamental monomers (building blocks) of proteins. There are 20 different standard amino acids that are encoded directly by the universal genetic code and used by ribosomes to synthesize polypeptides. While diverse, all amino acids share a highly conserved structural foundation.
Generalized Formula
Every amino acid features a central carbon atom, known as the $\alpha$-carbon, which acts as the structural hub.
H₂N—CH(R)—COOH
The central $\alpha$-carbon is covalently bonded to four distinct chemical groups:
- Amine Group (-NH₂): A basic group that can accept a proton to become -NH₃⁺.
- Carboxyl Group (-COOH): An acidic group that can donate a proton to become -COO⁻.
- Hydrogen Atom (-H): A simple hydrogen atom filling the third valence bond.
- Variable R-group (Side Chain): The unique functional group that distinguishes each of the 20 amino acids. The R-group dictates the size, charge, and polarity of the amino acid, ultimately driving protein folding.
Hover: Amphoteric Property
Amino acids can act as both an acid and a base (amphoteric) due to their carboxyl and amine groups. At physiological pH, they exist as zwitterions (dipolar ions).
Skill
Draw the generalized structure of an amino acid, clearly labeling the amine group, carboxyl group, and R-group. Ensure accurate valency for carbon, nitrogen, and oxygen.
B1.2.2
Condensation Reactions & Peptide Bonds
Key Understanding Condensation reactions form dipeptides and longer polypeptide chains by creating peptide bonds.
To build proteins, cells must link amino acids together. This process occurs on the ribosome during translation. The chemical mechanism used to join amino acids is a condensation reaction (also called dehydration synthesis). This is a quintessential anabolic reaction requiring energy, typically in the form of ATP.
Carboxyl group (-COOH) of amino acid 1
+
Amine group (-NH₂) of amino acid 2
↓ (ATP required, catalyzed by rRNA)
Peptide bond (-C-N-) forms + Water (H₂O) released
During this reaction, the hydroxyl (-OH) is lost from the carboxyl group of the first amino acid, and a hydrogen (-H) is lost from the amine group of the second. Together, these form a water molecule (H₂O) which is expelled. The resulting covalent bond between the carbon and nitrogen is a peptide bond.
A chain of two amino acids is a dipeptide. A long, unbranched chain of many amino acids is a polypeptide. The reverse of a condensation reaction is hydrolysis, which breaks peptide bonds by adding water (e.g., during digestion).
B1.2.3 / B1.2.4
Diet, Diversity, and the Proteome
B1.2.3 Dietary requirements for amino acids.
B1.2.4 The infinite variety of possible peptide chains.
Essential vs Non-Essential
Humans require all 20 standard amino acids to build proteins. However, our bodies lack the enzymatic pathways to synthesize 9 of them. These 9 are termed essential amino acids because it is essential they are obtained from the diet. The remaining 11 are non-essential (we can synthesize them from other molecules). A diet lacking essential amino acids leads to protein deficiency conditions like Kwashiorkor.
Infinite Variety
Ribosomes link amino acids in specific sequences dictated by mRNA. Because any of the 20 amino acids can appear at any position, the diversity is staggering. For a polypeptide of length $n$, there are $20^n$ possible sequences. Even a small polypeptide of 400 amino acids has $20^{400}$ potential sequences, a number vastly greater than the atoms in the observable universe!
Hover: Proteome
The proteome is the entire set of proteins expressed by a genome, cell, tissue, or organism at a certain time. It is uniquely larger than the genome.
B1.2.5
Denaturation: Losing the Shape
Key Understanding The destructive effect of pH and temperature extremes on protein structure and function.
A protein's function is exquisitely dependent on its highly specific 3D shape (conformation). Denaturation is a structural change in a protein that results in the loss (usually permanent) of its biological properties. When a protein denatures, it unravels and loses its functional shape, though the primary amino acid sequence remains intact.
- Temperature (Thermal Denaturation): High heat increases kinetic energy, causing the atoms in the protein to vibrate violently. These vibrations break the relatively weak intermolecular bonds (like hydrogen bonds and hydrophobic interactions) that hold the secondary, tertiary, and quaternary structures together. The protein unfolds. This is why cooking an egg solidifies the egg white (albumin).
- pH Extremes: Every protein has an optimal pH. Strong acids (low pH) or alkalis (high pH) alter the charge of the amino acid R-groups. This disrupts the delicate balance of ionic bonds and hydrogen bonds within the folded protein, causing it to unfold and precipitate.
Exam Tip: Denaturation does not break peptide bonds. The primary structure (the linear sequence of amino acids) remains entirely unaffected by denaturation. Only the higher-level folding is destroyed.
B1.2.6 / B1.2.7 HL Only
R-Groups & Primary Structure (HL)
B1.2.6 (HL) Chemical diversity in R-groups as the basis for diversity in protein form/function.
B1.2.7 (HL) Impact of primary structure on conformation.
The primary structure is the specific, linear sequence of amino acids in a polypeptide, determined by the DNA gene sequence. The properties of a protein are ultimately determined by this primary structure because the sequence dictates the order of diverse R-groups.
| R-Group Type | Chemical Property | Structural Impact |
|---|
| Polar (Hydrophilic) |
Contain -OH, -SH, or amide groups. Uncharged but uneven electron distribution. |
Form hydrogen bonds; usually found on the exterior of soluble globular proteins, facing water. |
| Non-Polar (Hydrophobic) |
Contain hydrocarbon chains or rings. |
Cluster in the core of globular proteins, avoiding water, driving the folding process. Found in membrane-spanning regions. |
| Charged (Acidic/Basic) |
Contain carboxyl (-COO⁻) or amine (-NH₃⁺) groups. |
Form strong ionic bonds (salt bridges) with oppositely charged R-groups, stabilizing tertiary structure. |
Because the primary structure defines the exact order of these R-groups, it entirely dictates the intramolecular interactions that cause the protein to spontaneously fold into its unique, functional 3D conformation.
B1.2.8 - B1.2.10 HL Only
Secondary & Tertiary Structure (HL)
Secondary Structure (B1.2.8): Regular folding patterns (coils or pleats). Hydrogen bonds form specifically between the main peptide backbone components (C=O of one amino acid and N-H of another), NOT between R-groups. Common structures are $\alpha$-helices and $\beta$-pleated sheets.
↓
Tertiary Structure (B1.2.9/10): The comprehensive 3D folding of the entire polypeptide chain. This is driven entirely by interactions between the variable R-groups.
Bonds stabilizing Tertiary Structure:
- Hydrogen bonds: Between polar R-groups.
- Ionic bonds: Between acidic and basic (charged) R-groups.
- Disulfide bridges: Strong covalent bonds between the sulfur atoms of two Cysteine amino acids. Highly stabilizes the protein.
- Hydrophobic interactions: Non-polar R-groups clumping together away from water.
B1.2.11 / B1.2.12 HL Only
Quaternary Structure & Protein Types (HL)
B1.2.11 (HL) Quaternary structure of non-conjugated and conjugated proteins.
Quaternary structure exists only in proteins composed of two or more polypeptide chains (subunits). These subunits interact to form a single functional macro-molecule. Furthermore, some proteins incorporate non-polypeptide components called prosthetic groups; these are conjugated proteins (e.g., Hemoglobin contains four polypeptide chains and four iron-containing heme prosthetic groups). Non-conjugated proteins consist exclusively of polypeptides (e.g., Collagen, Insulin).
B1.2.12 (HL) Relationship of form and function in globular and fibrous proteins.
Globular Proteins
Highly folded into compact, roughly spherical shapes. They are generally soluble in water.
- Structure: Hydrophobic R-groups tucked into the core; hydrophilic R-groups exposed on the surface.
- Function: Active, dynamic roles in metabolism. They act as enzymes (Rubisco), antibodies (Immunoglobulins), hormones (Insulin), and transport molecules (Hemoglobin).
Fibrous Proteins
Form long, narrow, extended strands. They are generally insoluble in water.
- Structure: Highly repetitive primary amino acid sequences leading to tough, rope-like secondary/tertiary structures.
- Function: Structural roles providing support, shape, and tensile strength. Examples include Collagen (skin, tendons), Keratin (hair, nails), and Actin/Myosin (muscle fibers).
Self-Assessment
Check Your Understanding
Test your knowledge on the concepts covered in this module.
What is the difference between a polypeptide and a protein?
A polypeptide is a single, linear chain of amino acids linked by peptide bonds. A protein is the final functional molecule, which may consist of a single folded polypeptide (tertiary structure) or multiple interacting polypeptides and prosthetic groups (quaternary structure) properly folded into their functional conformation.
Why does altering the pH of an enzyme's environment cause it to stop working?
Enzymes are globular proteins. Changing the pH alters the charge on the amino acid R-groups. This disrupts the ionic bonds and hydrogen bonds that maintain the enzyme's specific 3D tertiary structure. As the enzyme denatures, its active site changes shape, and it can no longer bind its substrate, ceasing its catalytic function.
HL: Distinguish between the types of bonds that maintain secondary versus tertiary structure.
Secondary structure ($\alpha$-helices, $\beta$-pleated sheets) is maintained exclusively by hydrogen bonds between the C=O and N-H groups of the peptide backbone itself. Tertiary structure is maintained by interactions between the variable R-groups, which include hydrogen bonds, ionic bonds, hydrophobic interactions, and covalent disulfide bridges.
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B1.2 Proteins Complete!
You have successfully reviewed all SL and HL understandings for Biology Theme B1.2.
