Protein channels are needed to transport substances because it accelerates the transport process. Very few substances enter or leave cells without being attached to a transport protein. Different types of substances require different types of proteins to transport them. Protein channels transport water or ions downwards and ATP-powered pumps transport molecules or ions upwards. ATP generates energy to transport the molecules upwards. Gravity is used to transport the water molecules downwards. So, in a nutshell, if substances need to be transported from the top of the body downward, the force of gravity is used to transport the substances down. If substances need to be transported upwards, ATP, a coenzyme, is used to generate energy to transport the substance upwards
