The only way for gases to diffuse in and out of the leaf is through small openings on the bottom of the leaf, the stomata.
Parietal cells within the stomach lining secrete hydrochloric acid that lowers the pH of the stomach. A low pH (1.5 to 2) activates pepsin. Acetylcholine, gastrin, and histamine stimulate the proton pump in parietal cells to release hydrogen ions and decrease pH.
The Type O blood group is commonly called as universal donor because any blood group can receive the O blood group. The blood group O has no antigen. Due to the absence of antigen it doesn't agglutinate when come in contact with other blood groups in the plasma.
The agglutination reaction happens when opposite antigen and antibody reacts with each other. e.g.- Blood group A contains antigen A and antibody b and the blood group B contains antigen B and antibody a. Agglutination reaction occurs when the antigen A reacts with antibody a. As O blood group has no antigen, agglutination reaction doesn't occur.