IB Biology HLEnzymes & MetabolismPaper 1 & 2~11 min read
Metabolic Pathways
Cells almost never turn a starting molecule into a finished one in a single step. They do it in a series of small steps, each with its own enzyme — and that is what makes metabolism controllable.
📚 What you need to know
A metabolic pathway is a series of linked, enzyme-catalysed reactions. Each step causes one chemical change and has its own enzyme.
The molecules in a pathway are metabolites, also called intermediates.
Pathways are either linear (chain) or cyclical.
Glycolysis, part of respiration, is a linear pathway. The Calvin cycle (photosynthesis) and the Krebs cycle (aerobic respiration) are cyclical.
In a cycle, the end product of one turn starts the next turn. Cycles are less common than chains.
Intracellular enzymes work inside the cell; extracellular enzymes are packaged into vesicles and secreted to work outside it.
Exergonic reactions release free energy; endergonic reactions absorb it.
Energy transfer is not 100% efficient, so some is released as heat. Endotherms use that heat to keep a constant body temperature; ectotherms cannot.
ATP is the intermediate that links energy-releasing reactions to energy-absorbing ones.
Where enzymes work
Most enzymes never leave the cell that made them. A few are built specifically to be exported.
Type
Where it acts
Examples
Intracellular
Produced and used inside the cell — the large majority of enzymes
The enzymes of glycolysis and the Krebs cycle
Extracellular
Made inside the cell, packaged into vesicles, then secreted to catalyse reactions outside
Digestive enzymes released into the gut
The reason digestive enzymes have to be extracellular is worth a moment’s thought. Starch and protein are far too big to cross a cell membrane. They have to be broken down before they can be absorbed, so the enzyme goes out to meet them.
Chains and cycles
A linear or chain pathway has a clear beginning and a clear end. Substance A is converted to B, B to C, C to D, each by a different enzyme. Glycolysis works like this.
A cyclical pathway loops back on itself. The final product of the sequence is the molecule that the next turn starts with, so the cycle can keep running as long as new material is fed in. The Calvin cycle and the Krebs cycle both work like this.
Each arrow is a separate enzyme. That is the point of a pathway: the cell can slow the whole thing down by controlling just one of them.
Why split a reaction into steps at all? Small steps release energy in manageable amounts instead of one destructive burst, and they give the cell many separate points where it can intervene. Control is much easier over ten small taps than one big one.
Energy, heat and ATP
During aerobic respiration, glucose is oxidised to carbon dioxide and water. Some of the chemical potential energy stored in glucose bonds is released as free energy, which the cell can spend. Reactions that release free energy like this are exergonic.
The transfer is never perfectly efficient, so some energy always escapes as heat. That is not entirely a waste:
Endotherms — birds and mammals — rely on the heat from metabolic reactions to hold their body temperature constant.
Ectotherms cannot regulate their temperature this way and depend on their surroundings instead.
Reactions that absorb energy are endergonic, and their products store more energy than the reactants did. Protein synthesis from amino acids is a standard example.
Look at where each curve starts and ends. In the left diagram the products sit lower, so energy has left the system.
Why ATP is needed
Endergonic reactions need an energy input, so in metabolism they are coupled to exergonic ones. Adenosine triphosphate (ATP) is the intermediate that makes the coupling possible: energy-yielding reactions make ATP, and energy-absorbing reactions spend it.
The link in one line
Exergonic reactions make ATP → ATP moves through the cell → endergonic reactions break down ATP to get the energy they need.
🧠
ATP is a currency, not a battery
Cells do not store much ATP. They earn it and spend it almost immediately, the way a shop takes cash at the till and pays a supplier the same afternoon.
Worked examples
WE 1
Compare linear and cyclical pathways
Distinguish between a linear and a cyclical metabolic pathway, giving one example of each. (4 marks)
Linear
A sequence with a distinct beginning and end: the starting substrate is converted step by step into a final product, for example glycolysis.
Cyclical
The end product of the sequence becomes the starting molecule of the next turn, so the pathway loops, for example the Krebs cycle or the Calvin cycle.
What they share
Both are made of small steps, and each step is catalysed by its own enzyme.
Chains finish; cycles feed back into themselvesname a real example for each — one mark is usually reserved for it
WE 2
Explain where body heat comes from
Explain how the heat that keeps a mammal warm is produced. (3 marks)
Point 1: the source reaction
Aerobic respiration is exergonic: glucose is oxidised and free energy stored in its bonds is released.
Point 2: why heat appears
Energy transfer in these reactions is not 100% efficient, so some energy is released as heat rather than being captured.
Point 3: what the animal does with it
Mammals are endotherms and use this heat to keep body temperature constant, unlike ectotherms which rely on the environment.
Inefficient exergonic reactions leak heat — endotherms use it“not 100% efficient” is the phrase that earns the second mark
💡 Exam tips
Remember that each step of a pathway has its own enzyme. It comes up constantly.
Learn the three named pathways: glycolysis (linear), Krebs cycle and Calvin cycle (both cyclical).
Use the word metabolite or intermediate for the molecules in between.
For extracellular enzymes, say they are packaged into vesicles and secreted — that detail is often credited.
Say coupled when describing how ATP links exergonic and endergonic reactions.
Endotherm and endergonic are unrelated words. Read the question carefully.
⚠ Common mistakes
Saying one enzyme runs a whole pathway. Every step has its own.
Calling the Krebs cycle linear. It is a cycle; glycolysis is the linear one.
Saying extracellular enzymes are made outside the cell. They are made inside and then secreted.
Describing heat as the purpose of respiration. Heat is a by-product of inefficiency; ATP is the point.
Writing that ATP stores energy long term. It is made and used continuously.
Mixing up endotherm and ectotherm. Endotherms generate their own heat internally.
Up next: Enzyme Inhibition. Pathways need brakes as well as accelerators — the last page of this topic covers how molecules switch enzymes off, including how a pathway shuts itself down.
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