The troposphere has the highest density.
The advantage of sexual reproduction over asexual reproduction is that the organisms produced are each diverse. For example, with bacteria, if each is genetically the same and there is a way to "kill" them, all of them will die. However with sexual reproduction, each organism will be different, and have natural selection, they will build up immunities and grow and change.
The vacuole is used to store waste products, water, or toxins. It is like a large holding tank for plant cells.
Hope this helps!