Biology unit guide
Interaction and interdependence
Enzymes, respiration, photosynthesis, ecology, carbon cycling and population dynamics.
Unit 3 carries the heaviest data-handling load in the course. Respiration and photosynthesis supply the graph questions, and ecology supplies the ones where you interpret an unfamiliar study. Both reward the same skill: describing what the data shows before explaining why, in that order. Examiners report that students routinely explain a trend they never described, and lose the description marks that were the easier half.
Topics in this unit
What gets examined
- Effects of temperature, pH and substrate concentration on enzyme activity, and explaining each in terms of active site and collisions.
- The stages of aerobic respiration, where each occurs, and the net ATP yield.
- Light-dependent and light-independent reactions, and what limits the rate of each.
- Interpreting limiting-factor graphs for photosynthesis, including where the plateau comes from.
- Food chains, energy transfer between trophic levels, and why the pyramid narrows.
- Carbon cycling between reservoirs, and how human activity shifts the balance.
How to revise it
Describe before you explain
State what the graph does - rises, plateaus, falls beyond a point - then say why. The description marks are the easy ones and they are the ones most often skipped.
Keep the two photosynthesis stages apart
Where each happens, what goes in, what comes out. Building one table for both and one for respiration covers most of what this unit asks.
Explain denaturation properly
Bonds holding tertiary structure break, the active site changes shape, the substrate no longer fits. All three steps, every time.
Give a number for energy transfer
Roughly ten percent between trophic levels, with the rest lost to respiration, heat and undigested material. The figure and the reasons both earn marks.
Where marks get lost
- Saying an enzyme is "killed" rather than denatured.
- Claiming the active site is destroyed when it changes shape.
- Placing the Krebs cycle in the wrong part of the mitochondrion.
- Explaining a plateau as "the enzyme is full" without naming the limiting factor.
- Describing energy as recycled in an ecosystem - it flows through and is lost; matter cycles.
Practise this unit
Reading about a unit only goes so far. Work through real IB-style questions on it, with mark schemes.