IP Biology Notes: Organisms and Their Environment (Upper Sec 11)

Study guide26 Nov 2025, 00:00 Z

Free IP Biology notes on energy flow, food webs, the carbon cycle, human impacts, and conservation for Sec 3 to Sec 4.

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Use this as a free IP Biology notes chapter on organisms and their environment for Year 3 to Year 4. It keeps the IP pacing while reinforcing the 6093 biology foundations most schools test through DBQs, diagrams, and practical explanations.

If ecology questions keep losing marks because the causal chain is incomplete, use the IP Upper Secondary Biology tuition page for Year 3-4 support that connects food webs, sampling, conservation, DBQs, and practical planning.

Status: SEAB O-Level Biology 6093 syllabus (exams from 2026) checked 2025-11-30 - scope unchanged; remains the reference for this note.

The core idea is simple: Energy flows through ecosystems, while nutrients cycle.

Use it as a working check: Food-chain arrows show energy moving from food to eater. Energy is lost at each trophic level, so biomass usually decreases up the chain.

Then go one layer deeper: Example: fertiliser runoff can cause algal bloom, blocked light, plant death, bacterial decay, oxygen loss, and fish death. Give the sequence in order.

What you must know

Energy flow: sunlight → chemical energy in producers → consumers → decomposers; energy lost as heat/respiration, not recycled.
Food chains/webs: producers, primary/secondary/tertiary consumers, decomposers; energy decreases up trophic levels; pyramids of biomass reflect energy content better than numbers.
Carbon cycle: photosynthesis removes $\ce{CO2}$

Reviewed by

Ezekiel Tan·Academic Advisor (Biology)

Sources

SEAB GCE O-Level Biology (6093) syllabus (examinations from 2026)

Step	What happens	Why it matters
1	Fertiliser or sewage adds excess nitrate or phosphate to water.	Nutrients stop being the limiting factor for algal growth.
2	Algae multiply rapidly and form a bloom.	The surface bloom blocks light from reaching submerged plants.
3	Submerged plants die and more dead organic matter enters the water.	Bacteria have more material to decompose.
4	Bacterial respiration increases.	Dissolved oxygen is used up faster.
5	Fish and other aerobic organisms die.	They cannot obtain enough oxygen for respiration.

Starting change	First direct effect	Next effect to check	Answer wording
Predator population decreases	Its prey are eaten less often.	The prey may eat more of their food source.	"The prey population increases, so the plants or smaller organisms it feeds on may decrease."
Prey population decreases	Its predator has less food.	The predator may compete more strongly or decline.	"The predator population may decrease because less energy is available from that prey."
Producer population decreases	Primary consumers have less food.	Higher trophic levels receive less energy.	"Primary consumers may decrease first, then secondary consumers may also decrease."
New competitor enters	Two populations use the same resource.	The better competitor may reduce the other's food supply.	"The weaker competitor may decrease if the shared food, space, or nesting sites become limiting."

Question clue	Best method to name	What to make explicit	Common trap
Count plants in a field	Random quadrats	Use random coordinates, count the species in each quadrat, then calculate mean density.	Placing quadrats only where the species is easy to see.
Compare plants along a shore, path, or light gradient	Transect with quadrats at intervals	Keep the line direction fixed and record how abundance changes with distance.	Calling it random sampling when the point is to follow a gradient.
Estimate a mobile animal population	Mark-release-recapture	Capture, mark harmlessly, release, allow mixing, recapture, then compare marked and unmarked animals.	Assuming the method works if marks fall off or animals do not mix.
Compare two habitats	Same method and effort in both habitats	Keep quadrat size, number of samples, time of day, and counting rules consistent.	Changing two variables and then blaming the result on the habitat alone.

Mark-loss pattern	What is missing	Fix to practise
Food-web answer names the right organism but gives no direction	Increase or decrease plus reason	Write "X increases because..." or "X decreases because..." for every affected population.
Eutrophication answer jumps from fertiliser to fish death	Sequential causal chain	Write nutrients, algal bloom, blocked light, plant death, bacterial respiration, oxygen loss, fish death.
Sampling answer says "random" but no method	Bias control	State how random coordinates, transect intervals, or equal sampling effort are chosen.
Conservation answer is generic	Threat-to-method match	Name the threat first, then pick the conservation method that targets it.

IP Biology Notes: Organisms and Their Environment (Upper Sec 11)

What you must know

Sources

Detailed notes

Eutrophication Answer Chain

Food Web Change Checkpoint

Sampling Method Checkpoint

Ecology tuition checkpoint

Worked walkthroughs

Pitfalls and fixes

Practice drills

Quick applications

Exam cues

Next steps

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