Reproductive system and it's functions
        
             
        
        
        
Answer:
Cells are considered the basic units of life in part because they come in discrete and easily recognizable packages. That's because all cells are surrounded by a structure called the cell membrane — which, much like the walls of a house, serves as a clear boundary between the cell's internal and external environments.
 
        
                    
             
        
        
        
Answer:
Population.
Explanation:
Population may be defined as the smallest unit of ecology and the evolution might occur at the population level. 
Population is the group of the organisms that belong to the particular species and lives in a specific particular habitat. The individual of the population breeds among themselves. Sometimes the individual of two different population can also breed among themselves in extreme cases.
Thus, the answer is population.
 
        
                    
             
        
        
        
In FILTRATION, some type of pressure, such as gravity or blood pressure, forces substances across a membrane that acts like a filter.
Filtration
<span>Urea is the cellular waste product that the kidneys remove from the blood.Kidneys are the two bean shaped organs that has several functions to keep the body healthy. It not only removes urea from the blood, but also helps in the formation of urine and maintaining the fluid level of the body. The two kidneys are placed on the two sides of the spinal cord. The blood enters the kidney through the renal artery. The nephron within the kidneys mainly take out the waste products and the excess water from the blood and purifies it.If the kidneys of a person fail to work properly then it becomes important to perform dialysis for taking out the waste materials from the blood.</span>