The answer is down the molecule's concentration gradient. To simplify, the movement of molecules through a plasma membrane channel protein is always down the molecule's concentration gradient. In addition, carrier proteins and channel proteins are the two main classes of membrane transport proteins. The carrier proteins bind the specific solute to be transported and undergo a series of conformational changes to transfer the bound solute across the membrane.
The origin of man took place about 200,000 years ago and looking at the records that are stored in the ice cores before the existence of man which were found by the deep drilling of the antarctic and Greenland shows that how the temperatures have changed and as the example of the carbon in the air.
As in the 800,000 years go the Carbon dioxide levels were on average of 170 to 280 ppm and now they have crossed the 400 ppm mark also at that time the sea levels were high as 80 feet and now are more than 100 feet.
