Biology and Earth Science (ជីវវិទ្យា និងផែនដីវិទ្យា)
What the paper tests
This domain combines cell biology, photosynthesis and respiration, human body systems, ecology, and Earth processes such as the rock cycle and water cycle. Questions often ask you to interpret a diagram, compare processes, or explain a cause–effect relationship.
Core rules and concepts
- Plant cells have a cell wall, chloroplasts, and a large vacuole; animal cells do not.
- Photosynthesis: carbon dioxide + water → glucose + oxygen, using light energy and chlorophyll.
- Respiration releases energy from glucose; it occurs in all living cells.
- In a food chain, energy decreases at each level; roughly 10% is transferred to the next trophic level.
- The water cycle includes evaporation, condensation, precipitation, and runoff. The rock cycle links igneous, sedimentary, and metamorphic rocks.
Worked example
A food chain has 10,000 J of energy in producers. How much reaches the secondary consumer? Reasoning: about 10% passes to herbivores, so 1,000 J; about 10% of that passes to carnivores, so 100 J. The other energy is lost mainly as heat through respiration, plus waste and uneaten parts. This is why food chains rarely have more than four or five levels.
Traps that cost marks
Do not say plants respire only at night; they respire all the time. Do not confuse chloroplasts (photosynthesis) with mitochondria (respiration). In ecology, energy and biomass are not the same, and a food web is not the same as a food chain.
Exam focus
When explaining a process, name the input, the output, and the organ or organelle involved.