Once food has reached the sea floor, currents carry this food and organisms filter it without having to use their own energy to go and get food. Another way organisms use energy is by coming out at night and rising to feed on upper-level organisms.
Physical Science redirects here. It is not to be confused with Physics. Physical science is a branch of natural science that studies non-living systems, in contrast to life science. It in turn has many branches, each referred to as a physical science, together called the physical sciences.
Some curriculum providers do life science for 7th or 8th, and then physical science for 8th or 9th. The physics in physical science is at a lower level than the physics in a year-long physics high school course. Most physics requires some level of algebra whereas physical science is OK with prealgebra. I'm in Tennessee and physical science can get a high school credit.