The graphic here shows the SDRAM memory cell structure. Each memory cell consists of one capacitor and one transistor. The transistor works like a switch and the capacitor is used to store the charges or data. Random access is achieved using a combination of row plus column addresses. The green arrow means data is written into the cell and the red arrow means the data is being read from the cell. Since the capacitor will leak charge (in other words, stored electrons) over time, SDRAM memory cell needs to be refreshed regularly.