The horse and donkey, though closely related by sharing a common ancestor, are different species. They have different number of chromosomes hence pairing of homologous chromosomes during fusion of gametes becomes complicated. The mule will have an extra chromosome from the horse hence will have abnormalities such as sterility. A Mule is unable to reproduce due to this same phenomenon. Homologous chromosomes are not well paired for meiosis I.
 
        
             
        
        
        
Answer:
They are semi-permeable, which means that some molecules can diffuse across the lipid bilayer but others cannot. Small hydrophobic molecules and gases like oxygen and carbon dioxide cross membranes rapidly. 
Explanation:
 
        
             
        
        
        
Except: Cells need energy to survive.
That's the Answer.
 
        
             
        
        
        
Answer: Net Energy
Explanation:
In energy partitioning, metabolizable energy less less heat increment (energy lost as heat) gives the net energy. The net energy is further broken down to energy used for production and energy used for maintenance. 
 
        
             
        
        
        
Answer:
As rust or iron oxide absorbs moisture and becomes a poor conductor, its electrical conductivity decreases.