photo) and for putting together (synthesis). in the 18th century, the scientist Joseph Priestley was one of the first people to realise that plants gave off oxygen, after performing experiments with mice and candles in bell jars; he found that placing either of these in a bell jar eventually led to their expiration, while placing a plant in the bell jar served to "revivify" the air. The full chemical process of photosynthesis, however, was not fully observed and understood until the early 20th century. Photosynthesis has two main reactions: light-dependent reactions, which need light to work; and light-independent reactions, which do not need light to work. In the first of these, light energy from the sun is used to split water (called photolysis) which has been absorbed by a plants through its roots. The sunlight hits chloroplasts in the plant's cells, causing an enzyme to break apart the water. Water, when broken into its constituent parts, makes oxygen, hydrogen, and electrons. The hydrogen converts to NADPH (nicotinamide adenine dinucleotide phosphate) which is then used in the light-independent reactions. Oxygen diffuses out of the plant as a waste product of photosynthesis and ATP (adenosine triphosphate) is synthesised from ADP (adenosine diphosphate) and inorganic phosphate. The second, light-independent, reaction, occurs after the first has taken place, and during it glucose is built up using carbon dioxide and the products of the light-dependent reactions (ATP and NADPH), as well as various other chemicals found in the plant. This glucose is used by the plant as energy to grow. Temperature is also an important factor in photosynthesis. If it is too low, this will inhibit the process. This is why plants don't grow in well in cold conditions (such as at the earth's poles), and also why they thrive in warm conditions (greenhouses, for example). In fact, if any of the four major factors in plant photosynthesis - water, sunlight, carbon dioxide and moderate heat- are absent, a plant's growth will be significantly inhibited." />

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