What are the three ways in which substances can pass through the cell membrane? All cells must expend energy in order to maintain which two substances in correct proportion within the cytoplasm?
First, if a favorable concentration gradient is present, substances such as water or those soluble in lipids can pass through the membrane via simple diffusion. Second, substances, such as simple sugars or amino acids, that are too big to move through the membrane's pores and are non-lipid soluble, can pass through the membrane through a process called facilitated diffusion or passive transport. If a favorable concentration gradient is present, proteins, called permeases, can bind to substances and transport them across the membrane without the expenditure of energy. Third, substances can even pass through the membrane against the concentration gradient via a process called active transport. In active transport, the cell must expend energy in order for the permeases to bind with the substance and pump it through the membrane. Cells must use active transport to maintain the proper concentration of sodium and potassium ions in their cytoplasm, since the concentration gradients favor too much sodium and too little potassium entering the cell.
