Phototropism is the process of a plant's reaction to light. The most common and visible example of phototropism is how a plant's stem reacts to light; most stems bend and grow towards the light source, or as a scientist would say, the stems have a positive phototropic reaction to light.
The exceptions to this rule are vines and climbing plants. which require a suitable host to help them reach new heights. Shoots from vines and climbing plants will grow towards a shady spot in search of a tree or some other fixture on which they can attach themselves.
Once securely entwined with their host, vines and climbers will then grow upwards until they reach a light source before flowering. For example, in a tropical rainforest the canopy is often dense, so vines need to grow upwards significantly before they reach the light at the crown of the tree. Upon reaching the light, they no longer grow up, but out--and the focus converts to flowering.
The roots of a plant grow away from the light in search of water. No matter which direction you plant a seed, the primary roots will grow down and the shoot, or stem, will grow up. The roots of all plants react to light the same way; negative phototropism is the term that refers to growing away from the light. The roots reaction to light is not only a way to survive; it's a way to stabilize the plant within the soil.
Photosynthesis is the process that allows plants to use the light from the sun to produce sugar and oxygen. The light reaction during photosynthesis occurs in the thylakoid membrane of the plant which coverts light energy into chemical energy. This chemical reaction must take place in the presence of light. Chlorophyll is built into the thylakoid membranes and aids in the chemical conversion from light to food. Though chlorophyll appears to be green, it is actually absorbing the red and blue colors of the light for photosynthesis.
Humans cannot see the red and blue light that is absorbed; the green light that is not absorbed is the only visible color, therefore chlorophyll appears green. The oxygen that's emitted from the photosynthesis process contributes to the air supply of the plant as well as all living things.