21. Which is NOT a mechanical wave? A. the light from a flashlight
B. the sound heard from the cavity of a shell
C, the echo produced when you shout in a hall
D. the wave produced in a porcelain plate struck with a metal spoon​

the solute can move "downhill," from regions of higher to lower concentration, relying on the specificity of the protein carrier to pass through the membrane. this process is called passive transport or facilitated diffusion, and does not require energy.

only about ten percent of the energy at one trophic level is passed on to organisms at the next trophic level. the remainder of the energy is used in life processes or lost as heat.