Why do very few trees exist in a grassland ecosystem?

a.
not enough heat

b.
not enough topsoil

c.
not enough sunlight

d.
not enough precipitation

The oxidizing agent our bodies use to obtain energy from food is oxygen (from the air). if you breathe 15 times a minute (at rest), taking in and exhaling 0.5 l of air with each breath, what volume of air do you breathe each day? air is 21% oxygen by volume. what volume of oxygen do you breathe each day?

What percentage of light hits earth's surface directly?

1. which of the following transport mechanisms utilizes energy? a. osmosis b. diffusion c. facilitated diffusion d. endocytosis