Because plant cells have chloroplasts and can do photosynthesis, why do they also need mitochondria?

The concept of keystone species is controversial among ecologists because most organisms are highly interdependent. if each of the trophic levels is dependant on all others how can we say one is most important

If a strand of dna has 35% thymine. what is the percentage of cytosine, adenine, guanine?

If a grape were placed in a hypertonic solution what would happen and why?