PSLE Adaptations — Life Science Cheatsheet

TEACH Lesson Summary — Teach This First

Suggested flow: walk through these 8 points with your student first (this is the lesson), then let them attempt the question bank, and finish by going back over the topic cards below as a conclusion. Each point below is a generalisation of what the questions in this document actually test.

1Plants make their own food — and you can prove it. Photosynthesis needs light, carbon dioxide and water to produce glucose (food) and oxygen. The classic proofs: a leaf in a sealed bottle releases gas that relights a splint = oxygen; the iodine starch test turns blue-black when starch is present. If an experiment changes one condition (light on/off, CO₂ with/without), you are testing whether that factor is needed.

2Water leaves plants through stomata — tiny pores mostly on the leaf underside. Transpiration increases when it is warmer, drier, windier or brighter. The wet-paper-bag experiment: a bag over a leaf collects water droplets = the plant released water vapour. Oil paint blocks stomata; pollutants shrink them — both reduce transpiration.

3Seeds germinate without light, but light changes how they grow. Germination needs water, oxygen and a suitable temperature. In the dark, seedlings grow faster but taller and thinner (they stretch to find light); in the light, shorter and sturdier with green leaves. When measuring growth, always measure at the same time of day so conditions are comparable.

4The plant reproduction chain is fixed: flower parts → pollination → fertilisation → fruit & seed. Pollen lands on the stigma (pollination); it grows down to the ovule, where male and female cells fuse (fertilisation). After that, the ovary becomes the fruit and the ovule becomes the seed. Pollination is NOT fertilisation — students lose marks by mixing these up.

5Seeds travel in four ways, and each matches a feature of the plant's home zone. Hooks/spines → carried by animals (low forest zone); wings/parachutes → wind (canopy/tall trees); buoyant or waterproof → water; splitting/bursting → flung from the pod. When a question describes a seed, name its dispersal method AND link it to where that plant grows.

6Insect life cycles and food webs are about chains — follow the arrows. Mosquitoes: egg → larva → pupa (all in water) → adult; warmer temperatures speed up the cycle, so more adults. In a food web, arrows point from what is eaten to the eater = energy flow. Remove one organism and trace who loses their food source — that population drops first.

7Vocabulary traps: population vs community; structural vs behavioural adaptation. A population is ONE species in an area; a community is ALL the species there. An adaptation is structural if it is something the animal HAS (leaf insect's leaf-like body, roots gripping soil) and behavioural if it is something it DOES (migrating, building a nest). Always finish with the purpose: to survive.

8Every experiment question rewards fair-test language. The control setup is the one WITHOUT the test factor. Change ONE variable at a time; keep everything else the same. When reading bar charts: compare heights, state which condition gave more/less, and link it back to the factor being tested — "more water droplets formed in the warm setup because transpiration increases with temperature."

The mark-earning habit for this whole paper: give cause → effect, not just a label. Name the process AND explain what happens. One complete sentence with both parts usually earns full marks where a bare keyword does not.

1 Photosynthesis — Experiments You Must Be Able to Read

What photosynthesis does (recall answers)

Uses carbon dioxide + water + light energy → makes food (glucose) and releases oxygen

Exam tip: if asked for 'two other functions' besides removing CO₂ — say it produces OXYGEN and FOOD (glucose)

Light intensity graph pattern (Q10, 2023 Booklet A)

Curve rises as light increases, then FLATTENS off at high light
Below the flat part: 'As amount of light increases, rate of photosynthesis increases'
Above the flat part: 'More light has NO effect on the rate' — another factor (CO₂ / temperature) is now limiting

CO₂ jar experiment across sunset (Q16, 2025 Booklet B)

Set-up: plant in glass jar with CO₂ sensor; soil pot wrapped in plastic bag
Daytime (e.g. 1 pm – 6 pm): photosynthesis is FASTER than respiration → CO₂ inside the jar DECREASES
After sunset: no light, so only respiration happens → CO₂ INCREASES again
Why bag the soil? To stop CO₂ from soil microbes / roots mixing in — controls the variable so the sensor reads ONLY the plant's gas exchange

2 Transpiration & Stomata

Stomata facts (Q11, 2024 Booklet B)

Tiny openings on leaves — MOSTLY on the BOTTOM surface of the leaf
Functions: gas exchange (CO₂ in, O₂ out) AND water vapour escapes through them (transpiration)
Their size changes with light and air conditions — this is what experiments measure

Oil-on-leaves mass experiment (Q12, 2023 Booklet A)

Oil BLOCKS stomata on the side it is applied to → no water loss from that side
Leaves have MORE stomata on the bottom → sealing the bottom stops more water loss than sealing the top
Water loss order: A (no oil) > C (oil on top only) > B (oil on bottom only) > D (both sides)
Final mass is the REVERSE of water loss: least mass = most water lost. Increasing final mass: A < C < B < D

Pollutant table pattern (Q11, 2024 Booklet B)

Table: % pollutant in air vs average size of stomata — higher pollutant → SMALLER openings
Answer format: 'As the percentage of pollutant S increases, the average size of the tiny openings decreases'
Why measure all towns at the SAME time? Light also affects opening size — same time keeps light constant (fair test)

3 Germination & Growth

Light vs dark containers (Q13, 2023 Booklet A)

Seeds germinate in BOTH the well-lit room and the dark room → seeds do NOT need light to germinate
Seedlings in the DARK are taller and thinner → they grow FASTER (longer) in the dark, but weakly

The classic trap: 'seeds need warmth' CANNOT be concluded — temperature was kept the same in both rooms, so it was not tested

Germination needs to state (recall)

Water + oxygen + suitable (warm) temperature — light is NOT required for germination itself

4 Reproduction in Plants & Seed Dispersal

Flower parts → what they become (Q6, Q18)

Ovary develops into the FRUIT; ovule develops into the SEED — this exact pair is asked directly
Process after pollination that produces seeds = FERTILISATION (male cell fuses with female cell in the ovule)
Pollination ≠ fertilisation: pollination = pollen transferred anther → stigma; fertilisation happens later, inside the ovule

Animal & bird pollination (Q8, 2024 Booklet B)

Bird/insect feeds on nectar → pollen sticks to its body → carried to another flower's stigma = POLLINATION
Name-the-process questions: 'The bird helps in a process of reproduction' → answer is pollination, not fertilisation

Mutualism pattern (Q6, 2023 Booklet B)

Insect lays egg inside the flower's ovary AND rubs pollen on the stigma while doing so
Benefit to PLANT: gets pollinated → fertilisation → seeds and fruit form
Benefit to INSECT: its young hatches right where the seeds are — a ready food supply in a safe place

Seed dispersal + forest zones (Q18, 2025 Booklet B)

Why disperse? To reduce competition for light/water/space with the parent plant and siblings; reach new areas
Fruit with HOOKS/SPINES → animal dispersal → grows in the LOW zone (understorey): passing animals catch the hooks on fur and carry seeds away
Fruit with WINGS/PARACHUTES → wind dispersal → grows HIGH in the canopy: open air lets wind lift and scatter them far
Other methods to know: water (buoyant, e.g. coconut) and splitting/explosive (pod shoots seeds out)

Plant life cycle fill-in (Q30, 2024 Booklet B)

Cycle: adult plant → seed → young plant → adult plant
'Dispersal followed by process R' — process R = GERMINATION (seed grows into a young plant)

5 Mosquito Life Cycle & Insect Facts

Complete metamorphosis — memorise the order (Q1, Q17)

EGG → LARVA → PUPA → ADULT (drawn as a loop with arrows; fill-in-the-blank boxes follow arrow direction)
Larva and pupa live in WATER; the adult lives on land/air

'State one characteristic of an insect' (Q1b, 2023 Booklet B)

Three body regions: head, thorax, abdomen
Six legs (three pairs) — all on the thorax
One pair of antennae; hard outer covering (exoskeleton); two pairs of wings in most insects

Temperature → life cycle speed graph (Q1c, 2023 Booklet B)

Graph: as temperature RISES, days to complete one life cycle FALL (line slopes down)
Global warming chain for the 2-mark answer: warmer → each generation takes FEWER days → MORE generations per year → number of adult mosquitoes INCREASES

Repellent experiment (Q17, 2025 Booklet B)

Count = mosquitoes staying on the hand > 1 minute. FEWER mosquitoes = MORE effective repellent (F: 6 < G: 11 < E: 15 → F wins)
Control experiment: same set-up, hand with NO repellent applied — gives a baseline to compare against
Net's other function: keeps OUT other insects/contaminants while still letting air in (any sensible controlled-environment reason works)

6 Food Webs & Ecosystems

Drawing food webs from words (Q4a, 2023 Booklet B)

Arrows point FROM the organism being eaten TO the one that eats it ('energy flows to')
'B and C feed on F' → arrows F→B and F→C. 'E eats B and C; G feeds on E' → B→E, C→E, E→G
Producers (plants) sit at the base with arrows going AWAY from them

Nest advantages — link diagram to food web (Q4b)

Pattern: advantage + explanation. 'Underwater entrance → land predator G cannot reach E inside'
'Sturdy twig walls → shelter from predators / safe place to rest and raise young'

Pond disturbance — fertiliser runoff (Q10, 2024 Booklet B)

Fertiliser flows in → surface plant F BOOMS → blocks sunlight → submerged plant G cannot photosynthesise and dies
More food (plant F) → bird A's population INCREASES — state the link: 'more of its direct food source'
Why fish C declines when G disappears (bird B unchanged, so NOT predation): less dissolved OXYGEN from photosynthesis; loss of HABITAT/shelter and the small organisms that live on G

Insect + fungus nest (Q31, 2024 Booklet B)

Nest in MOIST soil underground → fungi need moisture + shelter to grow; stored leaves are their food source
Eggs laid among the stored leaves: hatching young has FOOD (fungus L) right there, and the nest protects eggs from predators/drying out

7 Population vs Community — Vocabulary Traps

The two classic diagrams (Q11 2023, Q25 2024)

Several grasshoppers in a garden / tadpoles + frogs around a pond → all ONE species = POPULATION
Tadpole is just a young frog — different life stages of the SAME species still count as one population

Definitions to keep straight

POPULATION = all individuals of ONE species in an area
COMMUNITY = ALL the different populations (many species) living together in an area
HABITAT = the place where an organism lives; ECOSYSTEM = community + its non-living surroundings

Habitat preference table (Q26, 2024 Booklet A)

Insect P: bright+dry 1, bright+damp 1, dark+dry 18, dark+damp 25 → conclusion: PREFERS DARK PLACES
Why not 'prefers damp'? In the bright row, dry = damp (1 = 1) — no clear difference. Compare ONE factor at a time while holding the other constant

8 Adaptations — Structural vs Behavioural

Leaf insect question pattern (Q39, 2025 Booklet A)

STRUCTURAL adaptation: body shape/colour physically resembles a leaf (built-in feature)
BEHAVIOURAL adaptation: stays motionless or sways like a leaf in the wind (something it DOES)
Purpose = to AVOID being spotted by predators — NOT 'to catch prey' (it eats leaves, so any prey option is wrong)
Answer row: behavioural AND structural → avoid being spotted by predators

Root systems as adaptation (Q7, 2024 Booklet B)

Two functions of roots (2-mark recall): absorb water + dissolved minerals; anchor the plant in the soil
Plant E: one main DEEP taproot → advantage where rain is little — reaches water stored deep underground
Plant H: many SHALLOW fibrous roots near the surface — absorbs rainwater quickly, holds soil together

Eggshell + temperature patterns (Q12, 2024 Booklet B)

Substance X from decomposing dead plants THINS the eggshell → thinner shell helps the young hatch more easily
Bar graph: cooler temperatures → only females; warmer → only males. For the species to continue, BOTH sexes are needed → pick the temperature that gives both (e.g. 32°C)

9 Fair Test & Data — Answer Patterns for Every Question

Control experiment (asked in Q17c and similar)

Same set-up, but WITHOUT the thing being tested: repellent test → hand with NO repellent; fruit ripening → box with only unripe bananas
Purpose: baseline to compare results against

Fair test / controlling variables (Q40, Q11c, Q16c)

Keep EVERYTHING the same except ONE variable. Fruit ripening: same fruit type + number; only difference = presence of ripe fruit → box B = one unripe banana + one ripe banana
'Why measure at the same time?' → because light also affects stomata — same time keeps it constant
'Why bag the soil pot?' → stops CO₂ from soil microbes/roots contaminating the reading

Reading data — sentence templates that earn marks

Trend: 'As X increases, Y decreases/increases' (pollutant vs stomata size; temperature vs days per life cycle)
Comparison: state which is highest/lowest AND why it matters ('F had the fewest mosquitoes → most effective')
Conclusion limits: only conclude what was actually tested — if a factor was kept constant, you CANNOT draw conclusions about it (Q13 warmth trap)

🧠 Chapter Concept Map