(i) Biology Junction. URL: http://www.biologyjunction.com/unit3_cells.htm Visited on 17 January 2013
(ii) cK-12. URL: http://www.ck12.org/workbook/Biology-Workbook/r1/section/3.0/Cellular-Structure-and-Function-Worksheets-%253A%253Aof%253A%253A-Biology-Workbook/