When a plant is carrying out photosynthesis carbon dioxide needs to move from the air into the leaf. It does this by diffusing through small pores called stomata. At the same time oxygen moves out of the leaf through the stomata. This movement of gases in opposite directions is called gas exchange.
A Punnett Square is used to predict the chances of an offspring to have its parents' traits. These squares are most commonly divided into four parts, with each part equaling a 25% chance of the offspring receiving that set of genes.