Preview
IB DP Biology — C: Interaction and Interdependence | SL | C1.1 Enzymes and Metabolism
📋 What You Will Learn
- Identify the structural features of enzymes that allow them to catalyse specific reactions, including the active site and induced fit model
- Explain how substrate concentration, enzyme concentration, temperature, and pH affect the rate of enzyme-catalysed reactions
- Analyse rate-of-reaction graphs and interpret data on enzyme activity under varying conditions
- Explain the mechanisms of competitive and non-competitive inhibition and their effects on enzyme kinetics
- Apply understanding of allosteric regulation and end-product inhibition to explain how metabolic pathways are controlled
🔍 Guiding Question How do organisms control the rate of enzyme-catalysed reactions to maintain the dynamic equilibrium required for life?
🌍 Why This Matters Every second, your liver cells are making split-second decisions about which metabolic pathways to accelerate and which to slow — not through conscious thought, but through the exquisite sensitivity of enzymes to their chemical environment. Understanding enzyme kinetics is the key to understanding how drugs work, how poisons kill, and how life regulates itself from within.
The Problem the Cell Had to Solve
Imagine a factory that runs thousands of chemical reactions simultaneously, each one needing to happen at precisely the right speed, in the right order, at body temperature. Without a catalyst, most biochemical reactions would take years. With the wrong catalyst, they would happen chaotically, producing toxic by-products. The cell's solution was elegant: protein molecules folded into precise three-dimensional shapes, each one perfectly matched to a single type of reaction.
These molecules are enzymes — and they do not merely speed reactions up. They control when reactions happen, how fast they run, and when they stop. Enzyme kinetics is the study of that control. It is, in essence, the study of how life manages its own chemistry.
📖 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 — Metabolism Metabolism is the sum of all enzyme-catalysed reactions occurring within a cell or organism, including both anabolic (synthesis) and catabolic (breakdown) reactions.
📖 Key Definition — Activation Energy Activation energy is the minimum energy required to initiate a chemical reaction. Enzymes lower this energy barrier, allowing reactions to proceed at the temperatures found in living organisms.
How Enzymes Work: The Active Site and Induced Fit
Every enzyme has a region of unique three-dimensional shape called the active site. This is not a rigid socket — it is a dynamic, flexible pocket formed by specific amino acid residues whose R-groups create a precise chemical environment. The substrate molecule that fits this site is held in exactly the right orientation for the reaction to proceed.
The Induced Fit Model
The older "lock and key" model described the active site as a rigid structure perfectly complementary to the substrate. The induced fit model, which is the accepted model in IB Biology, refines this. When a substrate approaches the active site, the enzyme undergoes a conformational change — it flexes and reshapes slightly to mould around the substrate. This induced fit optimises the interaction between enzyme and substrate, straining chemical bonds in the substrate and lowering the activation energy even further.
📖 Key Definition — Active Site The active site is the region of an enzyme, formed by specific amino acid