Provided that there's no diagram, most molecules have particular areas on the molecules, called the "active area" that act as a lock to a particular substrate's molecular structure, providing the key. 
An enzyme's active area is only able to be "unlocked" by a certain substrate's "key". 
        
                    
             
        
        
        
Answer:
causes alteration
Explanation:
when one particular species that should perform a particular function is removed it will change the ecosystem
 
        
             
        
        
        
Answer:
Enzymes bind with chemical reactants called substrates. ... In others, two substrates may come together to create one larger molecule. Two reactants might also enter a reaction, both become modified, and leave the reaction as two products. The enzyme's active site binds to the substrate.
 
        
             
        
        
        
The question has incorrect option, however the question can be answered by the general explanation. On the basis of the general explanation the answer is as follows:
Answer:
The correct answer is - Serine has a small side chain that can fit in any relatively polar environment.
Explanation:
Serine is an amino acid that have a small side chain in comparison of other amino acid and can be replaced by another amino acid with very least effect on the structure and function of a particular protein as they can fit in any relatively polar environment.
Serine have hydroxyl group in their side chain. It is polar amino acid that can easily establish itself in any polar condition and cause least changes in protein structure and function.
Thus, the correct answer is - Serine has a small side chain that can fit in any relatively polar environment.