The correct answer is an ecosystem.
An entire group of muscles that work together is called a synergist.
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Pseudo vacuoles are not true organelles since they are just air bubbles which do not possess tonoplast and cell sap like real vacuoles. But they do help by increasing buoyancy of cells.
A lake that freeze over is helpful to the organisms inside the water because, the solid ice serves as a barrier between the water and the cold air above the water. The ice prevents the chilled air surrounding the water body from getting to the organisms inside the water. <span />
