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IB DP Biology — C: Interaction and Interdependence | SL | C1.1 Enzymes and Metabolism
📋 What You Will Learn
- Identify the chemical nature of enzymes and describe the relationship between their structure and catalytic function
- Explain how the active site and substrate interact to lower activation energy and accelerate metabolic reactions
- Analyse how enzyme specificity arises from the three-dimensional shape of the active site
- Compare the lock-and-key model and the induced fit model, evaluating which better explains observed enzyme behaviour
- Apply understanding of enzyme structure to predict the effects of temperature, pH, and inhibitors on enzyme activity
🔍 Guiding Question How does the structure of an enzyme determine which reactions it can catalyse, and what does this tell us about the relationship between molecular shape and biological function?
🌍 Why This Matters Every second, your liver cells perform over a billion chemical reactions — breaking down toxins, synthesising proteins, and managing blood glucose — all at body temperature, without the extreme heat or acid a chemist would need in a laboratory. The reason this is possible is enzymes. Without them, the chemistry of life would grind to a halt within milliseconds.
The Problem Cells Had to Solve
Here is the fundamental challenge: the molecules of life — sugars, amino acids, nucleotides — are chemically stable. They do not spontaneously react at the temperatures and pH values compatible with living cells. Yet metabolism requires thousands of specific reactions to occur rapidly, in the right sequence, at the right time. Something had to bridge the gap between chemical stability and biological necessity.
The solution evolution arrived at was the enzyme — a biological catalyst so precisely shaped that it can grab two specific molecules, hold them in exactly the right orientation, and lower the energy barrier to their reaction. The elegance is not just that enzymes speed things up. It is that each enzyme does this for one reaction, and one reaction only. Specificity is the key.
📖 Key Definition — Enzyme An enzyme is a globular protein that acts as a biological catalyst, lowering the activation energy of a specific chemical reaction without being consumed in the process.
📖 Key Definition — Catalyst A catalyst is a substance that increases the rate of a chemical reaction by lowering the activation energy, without itself being permanently altered or consumed by the reaction.
📖 Key Definition — Activation Energy Activation energy is the minimum amount of energy required to initiate a chemical reaction — the energy needed to break existing bonds in the substrate so that new bonds can form in the product.
Enzymes Are Proteins — And That Matters
Enzymes are not just any proteins — they are globular proteins, meaning their polypeptide chains fold into compact, roughly spherical three-dimensional shapes. This folding is not random. It is determined by the sequence of amino acids in the chain, which in turn dictates where hydrophobic regions cluster inward, where ionic bonds form between charged R-groups, and where disulfide bridges lock the structure in place.
The result of all this folding is a protein with a very specific surface topography — hills, valleys, and pockets at the molecular scale. One particular pocket or cleft on the enzyme surface is the active site: the region where the magic happens.
📖 Key Definition — Active Site The