Every energy transfer loses some as heat
- The idea
- Predict
- See it
- Why
- Check
The idea
“Energy is conserved, so all of it can pass to the next step as useful energy.”
It's easy to picture all the energy moving up a food chain. Some leaves at every step as heat, which food web arrows don't show.
The short version
The mistake: Energy is conserved, so all of it can pass to the next step as useful energy.
What's actually true: Energy is never destroyed, but every transfer releases some as heat. Each step passes on less useful energy than it got.
- No transfer destroys energy, but every conversion in a cell, from making ATP to moving a muscle, releases some heat.
- That's the second law of thermodynamics. Each transfer raises total entropy. A cell can't use spread-out heat to make ATP, though the heat can keep a bird or mammal warm.
- In a food chain, much energy is never eaten or digested, and most of the rest leaves as heat from respiration. Usually 5 to 20 percent moves up.
- Efficiencies multiply. Two 10 percent steps pass on 1 percent, so food chains rarely have more than four or five levels.
On the exam: Energy is never destroyed, but every transfer leaks some as heat. Multiply the efficiency at each step.