Photosynthesis is the process by which your plants convert hydrogen, oxygen and carbon molecules into basic sugar structures and later, by breaking these down, create a source of energy and food for themselves. Chlorophyll is a combination of two pigments which make the entire process possible.
Chlorophyll is a green pigment, which is actually a combination of two individual pigments; chlorophyll and chlorophyll B. Other aquarium plant pigments exists, carotene for example is an orange pigment whereas xanthophyll is a yellow pigment. These kinds of pigments are not actually involved in photosynthesis and are normally disguised by chlorophyll, although it is possible to see them in variegated plants. In plants which do not receive light, the resultant sickly yellow appearance is a reduction in the green pigment of chlorophyll, meaning that the yellow pigment is displayed more prominently. As Iron is essential to chlorophyll, Iron deficiencies commonly result in sickly, yellow-looking aquatic plants.
When chlorophyll is exposed to light, it makes the process of photosynthesis possible. After photosynthesis has taken place, respiration means your plants will take in oxygen and produce heat energy and carbon dioxide. It is important that you aquarium has a dark period long enough for your plants to respire and levels of Co2 in the aquarium will rise during periods where there are no lights. Higher concentrations of Co2 and light will increase the activity of chlorophyll, which for many plants (including algae), results in more growth in shorter spaces of time.
Research into the various different species of plants which could be kept in the aquarium is essential to be able to understand how much light and darkness they will need. Depending on the origin of the plants, they may have different needs relating to how much time they should spend under lights and the time that they should be given afterwards to respire. If not enough light is given, or not enough time is given for the sugar structures to be broken down, the plants will become sickly or growth will be stunted.
The need for a specific amount of incoming light and time to respire is another reason why plants with similar needs should be kept together. In a biotope aquarium, due to the plants originating from the same geographical location, ensuring that they do is likely to be less of a problem. Attempting to mimic the natural environment of what you intend to keep in the aquarium is largely always advisable.