Answer:
Blood Brain Barrier(BBB)
Explanation:
This is a biological interface of special endothelial cells of astrocytes, pericytes , capillaries,  neurons and other  structures, acting as a physical barrier or  gate separating the Central Nervous system from the peripheral  blood circulations. Thus it  formed a tight junction serving as a  physical barrier to  selectively regulate the entrance of substances from the   cerebral blood flow   into the brain,  therefore maintaining homeostasis of the brain.
From above any  substances  absorbed into the blood stream  irrespective of the mode of entry  are transported through  the BBB, and  are selectively  regulated to enter the brain cells by the BBB's   interactions with the blood vessels, hormones etc. 
Basically it allows the passage of nutrients,  a few  drug, hormones and resists certain toxins and pathogens for the brain homeostasis.
 
        
             
        
        
        
They converted carbon dioxide to oxygen. The increased oxygen levels allowed for cellular respiration to occur instead of only glycolysis. The atmosphere became thinner, decreasing temperatures of the planet as well. 
        
             
        
        
        
Answer:
Organisms use energy to survive, grow, respond to stimuli, reproduce, and for every type of biological process. The potential energy stored in molecules can be converted to chemical energy, which can ultimately be converted to kinetic energy, enabling an organism to move.