Stem are the cells that travel around the body to heal and part injured.
brain is the organ which does play a role in mechanical or chemical digestion
The plasma membrane needs lipids, which make a semi-permeable barrier between the cell and its environment. It also needs proteins, which are involved in cross-membrane transport and cell communication, and carbohydrates, which decorate both the proteins and lipids and help cells recognize each other. Hope this helps :)
They are changed because of the environments changing
Through electrochemical processes
