Unit notes and practice questions
Everything here is for unit 6 of the Grade 10 Biology textbook: what the chapter covers, the short notes, practice questions with their answers, and the lesson video where one exists.
The cards filed under this chapter on the Grade 10 Biology page. Open one at a time.
When you use it
Use this to contrast the movement of energy with the movement of chemical elements through an ecosystem.
Watch out
Energy flows in a unidirectional path from the sun through the food chain and cannot be recycled. Nutrients like carbon, nitrogen, and phosphorus are recycled repeatedly between living and non-living components.
Drafted from Grade 10 Biology, page 153 onwards, then checked twice before it went up
| Process | Chemical Change | Key Microorganisms |
|---|---|---|
| Nitrogen Fixation (Symbiotic) | Rhizobium | |
| Nitrogen Fixation (Free-living) | Azotobacter, Klebsiella | |
| Nitrification (Step 1) | Nitrosomonas | |
| Nitrification (Step 2) | Nitrobacter | |
| Ammonification | Organic N | Decomposing bacteria, Fungi |
| Denitrification | Pseudomonas, Thiobacillus |
When you use it
Use this to identify which specific bacteria carry out each chemical transformation in the nitrogen cycle.
Watch out
Denitrification is an anaerobic process that converts nitrates back to nitrogen gas, while nitrification requires oxygen to oxidize ammonia into nitrate.
Drafted from Grade 10 Biology, page 153 onwards, then checked twice before it went up
When you use it
Use this to calculate energy transfer across trophic levels and explain why food chains rarely exceed 4 or 5 levels.
Watch out
Only about 10 percent of energy entering a trophic level becomes biomass in the next level. The other 90 percent is lost as metabolic heat, work, or undigested waste.
Drafted from Grade 10 Biology, page 153 onwards, then checked twice before it went up
When you use it
Use this when numbering the exact feeding position of an organism in a food chain.
Watch out
Producers are at trophic level 1 but have no feeding level. Herbivores (primary consumers) are at trophic level 2, which corresponds to feeding level 1.
Drafted from Grade 10 Biology, page 153 onwards, then checked twice before it went up
10 multiple-choice questions written against this chapter, each with its answer and the reason. Try one before you open the answer.
Question 1
What is the ultimate source of energy for living organisms in an ecosystem?
Answer: B The sun
Why
The textbook states that the ultimate source of energy in living organisms is the sun, which plants capture during photosynthesis.
Question 2
In a food chain consisting of Grass → Grasshopper → Lizard → Hawk, what are the trophic level and feeding level of the grasshopper?
Answer: B Second trophic level, first feeding level
Why
Plants stand at the first trophic level. Herbivores like grasshoppers stand at the second trophic level, but since they are the first organisms to eat another organism, they are at the first feeding level (primary consumers).
Question 3
Why is a pyramid of energy always upright and narrowing towards the top?
Answer: C There is roughly a 90% loss of energy at each trophic level
Why
As energy is transferred from one trophic level to the next, about 90% is lost as heat, metabolic work, or undigested waste, so only about 10% is passed on.
Question 4
In the water cycle, what is the term for water vapor cooling in the atmosphere to form liquid droplets?
Answer: C Condensation
Why
Condensation occurs when warm water vapor rises, cools, and turns into liquid water droplets that form clouds.
Question 5
If the teff plants (producers) in a field contain 50,000 kJ of energy, how much energy will be transferred to the biomass of secondary consumers (carnivores)?
Answer: A 500 kJ
Why
With an average trophic efficiency of 10%, primary consumers receive 10% of 50,000 kJ (5,000 kJ), and secondary consumers receive 10% of that (500 kJ).
Question 6
Which genus of soil bacteria is responsible for oxidizing nitrite (NO2-) into nitrate (NO3-) during nitrification?
Answer: B Nitrobacter
Why
Nitrosomonas converts ammonia into nitrite, while Nitrobacter performs the second step of nitrification by converting nitrite into nitrate.
Question 7
Why do plant roots preferentially take up nitrogen in the form of ammonium (NH4+) rather than nitrate (NO3-)?
Answer: B Ammonium requires a much lower energy expenditure to assimilate
Why
Plants prefer reduced ammonium because the energy cost for assimilating NH4+ into organic compounds is lower than for NO3-.
Question 8
What environmental problem is caused when excess chemical fertilizers containing phosphorus and nitrogen wash from farmlands into nearby lakes, leading to massive algal blooms?
Answer: B Eutrophication
Why
Eutrophication is the excessive growth of aquatic plants and algae caused by nutrient-rich runoff (nitrogen and phosphorus) entering water bodies.
Question 9
Under which soil condition does bacterial denitrification occur, and what is its main gaseous end product returned to the atmosphere?
Answer: B Waterlogged, anoxic soil; produces nitrogen gas (N2)
Why
Denitrification is an anaerobic process occurring mainly in waterlogged, oxygen-deficient (anoxic) soils, where denitrifying bacteria convert nitrates back into nitrogen gas (N2).
Question 10
Why do Ethiopian farmers practice crop rotation by planting legumes (such as beans or peas) before growing non-legume cereals?
Answer: B Symbiotic Rhizobia in legume root nodules enrich the soil with fixed nitrogen
Why
Legumes host Rhizobium bacteria in their root nodules, which fix atmospheric nitrogen. After harvest, the accumulated nitrogen in the soil benefits the following non-legume crop.
The app holds 12 questions on this unit, marked as you go. These are the first 10.
The whole Grade 10 Biology textbook, free as a PDF.
Ministry of Education edition, English, 177 pages
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