Plants, unlike mammals, do not require the consumption of large amounts of food to create energy to live. Instead, plants use a process called photosynthesis to create sugars that are then converted to fuel that plants use to grow.
During photosynthesis, a plant absorbs sunlight, carbon dioxide from air and water as well as the nutrients it contains. The plant splits the carbon dioxide into carbon and oxygen, expelling the oxygen into the air as a byproduct, and forms starches and sugars from the rest. So carbon dioxide+water+sunlight=sugars and starches+oxygen. This makes the food that the plant requires to live.
After producing sugars and starches, the plant uses them as energy, stores them or may build them into oils or proteins. It uses the stores of energy to stay alive when light is limited, such as during winter.
In plant leaves, and sometimes in plant stems, are structures called mesophyll cells. These cells contain further structures called chloroplasts, which contain the pigment chlorophyll. Chlorophyll is what makes a plant green and creates the act of photosynthesis.
With a fully functioning system of photosynthesis, a plant will grow as intended. If any of the elements required to carry out photosynthesis are missing from the equation--light, water or carbon dioxide--the plant will not be able to feed itself and will wilt or die. Lack of a photosynthesis element during growth periods causes a plant to suffer, making the production of new leaves, fruit or new height impossible.
Certain diseases, such as fungal growth, make a plant unable to perform photosynthesis. Diseases that attack a plant's root system make water absorption impossible, which affects photosynthesis, too. Photosynthesis also is affected when a plant's leaves or stem are attacked, impeding its ability to absorb water and sunlight.