The plasma membrane, because it's a selective permeable which means it only allows certain things in like sucrose.
        
             
        
        
        
The seasons are caused as the Earth, tilted on its axis, travels in a loop around the Sun each year. Summer happens in the hemisphere tilted towards the Sun, and winter happens in the hemisphere tilted away from the Sun. As the Earth travels around the Sun, the hemisphere that is tilted towards or away from the Sun changes.
 
        
             
        
        
        
6) humans
7) bacteria
8) caterpillars & rabbits
9) trout, and predators
10) It can be overfeeded , and everybody will die sooner or later
        
                    
             
        
        
        
Answer:
You could use one to see if that $210934879207344329 dollar "living" wood table is living or not. It could also be used at hospitals...
Sorry never heard of that "key" in my life...
 
        
             
        
        
        
Starfish have a unique form of digestion in the animal kingdom and a water-vascular system that allows the animal to breathe.
All your answers should be in this article: 
http://animals.mom.me/feeding-respiration-starfish-6596.html