Key Idea: Topic 2.5 explains how energy flows through ecosystems, why energy transfer is inefficient, how energy enters via photosynthesis and is released by respiration, why food chains are short, and how energy is represented using biomass, pyramids, and productivity.
⚛️ Laws of thermodynamics
- First law: energy cannot be created or destroyed, only transformed
- Second law: energy transfers are inefficient and release heat
- Heat energy cannot be reused by organisms
🌞 Energy input and release
- Photosynthesis converts light energy into chemical energy stored as biomass
- Respiration releases chemical energy for life processes
- Energy is lost as heat during respiration
📉 Energy loss in food chains
- Not all biomass is eaten
- Not all food is digested
- Energy is lost as heat during respiration
- Only a small fraction becomes new biomass
Only about 10% of energy is transferred to the next trophic level.
🏔️ Biomass and pyramids
- Biomass is measured as dry mass
- Pyramids of energy are always upright
- Biomass pyramids may be inverted in aquatic systems
⚡ Productivity
Net productivity = Gross productivity − Respiration
1️⃣ Why is energy transfer inefficient? Energy is lost as heat during respiration and not all biomass is eaten or digested. 2️⃣ Why are food chains short? Too much energy is lost at each trophic level. 3️⃣ Which process brings energy in, and which releases it? Photosynthesis brings energy in; respiration releases it. 4️⃣ Why are pyramids of energy always upright? Energy decreases at each trophic level. 5️⃣ Net productivity formula? NP = GP − respiration.