bioNRICH offers several mathematical biology problems for solution by students ages 14-19. There are more than 20 problems in the areas of measurement and estimation, graphs and equations, statistics in biology, moles and molarity, dilutions, DNA and genetics, shapes, and evolution. Each problem has notes for the teacher, hints and a detailed solution.

From site:  Although the cell is the smallest unit of life, it is by no means simple. The human body is made up of tens of trillions of cells like this one, which have developed a highly synchronized set of components to carry out the processes that keep the organism alive, allow it to reproduce and adapt to changing environments. New research suggests such complex cells, which scientists call “eukaryotes,” arose from the fusion of two simpler kinds of single-celled organisms in a symbiotic effort to survive. An ostrich egg is the largest known single cell, but most individual cells are too small to see without a microscope.

From site:  Young cells from all life forms on Earth hurry along for another day at the Super Cell Academy.  Click on the door to join the class and see if you will be able to pass the Super Cell Academy examination.  Start with the Organelle Sorting Game.

Here are 38 animations, some with reviews and brief quizzes, on a variety of biology topics.  Among them are mitosis and meosis, organelles and cellular respiration, DNA, enzymes, kidney function, blood flow through the human heart, and inflammatory response to injury.

Practice naming 12 parts of a microscope commonly used in biology labs in this interactive review.