Biochemical pathways are the step-by-step, enzyme-controlled reactions that sustain life. For VCE Biology, two pathways are essential:
- Photosynthesis (captures light energy → stores it in glucose)
- Cellular respiration (releases energy from glucose → ATP)
These aren’t just equations—they power everything from leaf growth to muscle movement.
Key Idea 1: Photosynthesis and Respiration Are Interdependent — Not Opposites
Photosynthesis:
6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂ (in chloroplasts)
Cellular Respiration:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP (in mitochondria)
Photosynthesis captures light energy and uses it to build glucose, storing energy in chemical bonds. Cellular respiration breaks down glucose to release that energy as ATP, which powers cellular processes. While the reactants and products of these pathways are connected, they are not simply reverse processes — they occur in different organelles (chloroplasts vs. mitochondria), involve different enzymes and coenzymes, and have distinct purposes.
Plants perform both processes. This connection helps cycle matter and energy through ecosystems — oxygen and glucose from photosynthesis fuel respiration, and carbon dioxide and water from respiration can be reused in photosynthesis.
Want to explore each process in more depth? Check out Photosynthesis and Cellular Respiration.
Key Idea 2: Coenzymes Drive the Reactions
Every step in a biochemical pathway needs an enzyme to speed it up — but many steps also rely on coenzymes to carry energy, electrons, or atoms between reactions. Understanding these helper molecules is key:
- ATP (Adenosine Triphosphate) stores and releases energy. It has three phosphate groups, and when one breaks off, it becomes ADP (Adenosine Diphosphate) — releasing energy in the process. ATP is used in both photosynthesis and respiration as the cell’s rechargeable battery.
- NADPH / NADP⁺ are used in photosynthesis.
- NADP⁺ (unloaded) picks up electrons and hydrogen ions during the light-dependent reactions to become NADPH (loaded).
- Easy way to remember: the P in NADP⁺ stands for Photosynthesis.
- NADH / NAD⁺ and FADH₂ / FAD are used in cellular respiration.
- NAD⁺ and FAD are unloaded. When they collect electrons and hydrogen in glycolysis and the Krebs cycle, they become NADH and FADH₂.
- These carry electrons to the electron transport chain to generate lots of ATP.
- Coenzyme A (CoA) is also part of respiration. It transports acetyl groups to the Krebs cycle — helping link different parts of the process together.
Key Idea 3: Conditions Control the Rate
Biochemical pathways must be tightly regulated so that energy isn't wasted and reactions happen when needed. Enzyme activity is affected by several key factors:
- Temperature: Moderate heat increases reaction speed, but too much heat denatures enzymes and stops the pathway.
- pH: Each enzyme has an optimal pH range. Outside of it, the enzyme may lose its shape and function.
- Substrate concentration: More substrate usually means faster reactions, until enzymes become saturated.
- Inhibitors:
- Competitive inhibitors block the active site.
- Non-competitive inhibitors bind elsewhere, changing the enzyme’s shape.
- Feedback inhibition occurs when the product of a pathway slows down its own production — a way for the cell to regulate itself.
Study Tip 1: Focus on Connections, Not Just Equations
Write out the inputs and outputs of photosynthesis and respiration — and draw arrows showing how glucose, oxygen, carbon dioxide, and water move between them. This reinforces the big picture and how energy flows through ecosystems.
Study Tip 2: Master Graphs to Master Science Skills
In VCE Biology, key science skills like interpreting data and analysing trends are assessable — often through graphs. When studying rates of reaction, don’t just memorise facts; practise reading and explaining graphs.
Take the enzyme activity graph for substrate concentration as an example. Ask yourself:
- What does the curve show about how the rate changes?
- Where does it increase quickly? Where does it level off?
- What biological explanation fits this pattern?
Linking the shape of the graph to what's happening at the molecular level (e.g. enzyme saturation) shows deep understanding — and earns marks. The more you practise reading graphs like this, the better you'll get at identifying trends and communicating your ideas clearly in assessments.
Quick Practice Question
What’s one way a non-competitive inhibitor can reduce the rate of a biochemical pathway? How is that different from a competitive inhibitor?
Want a quick recap and timed challenge to test your knowledge? Head to the Biochemical Pathways Overview for concise notes, a 1-minute quiz, and short answer questions to check your understanding.
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