Concentration Gradient- The concentration gradient is a difference in molecules outside of the cell versus inside of the cell. Some molecules are small enough to slip through the membrane, while others cannot pass because of their charge, and still others are too large to just slip through. The diagram below shows two ways that molecules can move across the membrane.
Membrane transport proteins- They are specific and selective for the molecules they move, and they often use energy to catalyze passage. Also, these proteins transport some nutrients against the concentration gradient, which requires additional energy. The ability to maintain concentration gradients and sometimes move materials against them is vital to cell health and maintenance. Thanks to membrane barriers and transport proteins, the cell can accumulate nutrients in higher concentrations than exist in the environment and, conversely, dispose of waste products.