Why does the cell sometimes have to expend energy to move individual molecules across the cell membrane?

Normally, molecules move naturally down a concentration gradient [From an area of high concentration to an area of low concentration] by a process called diffusion. However, sometimes the concentration of the needed substance is higher in the cell than out of it, so the cell uses a different process called Active Transport. Unlike Diffusion, Active transport requires energy, as it is going against a concentration gradient.

Cells expend energy to move cells across the cell membrane as they use Active transport rather than diffusion.

straight or amphibians are homeothermic to thermal

answer: grassland biomes are closer to the equator and receive more rain.

grasslands are biomes that are dominated by grasses rather shrubs and trees. savanna are biomes that exhibit grasses, shrubs and small tree as vegetation. grasslands are located near the equator of the earth. these are found in north america. savanna biomes are found in africa, south america, australia and india. grassland exhibit moderate climatic conditions, it receives   amount of rainfall higher than savanna.   therefore, savanna exhibit dry and hot climatic conditions.