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Long before animal life appeared on the Earth, the plants were busy making food for them. By the wonders of a pigment known as chlorophyll, the plants took a simple gas called carbon dioxide and water, then borrowed energy from the sun and made a food called sugar. Sugar is a molecule consisting of carbon, oxygen and hydrogen held together by strong chemical bonds forged in the cell leaf, with ATP generated in the leaf using photons from the sun’s light.
Chloroplasts are organelles found in plant cells that conduct photosynthesis. They capture light energy and convert it to a form of ATP through photosynthesis, which occurs in several steps. Some of the light energy is stored in the form of ATP while most of it is used to remove electrons from water. These electrons are then used in the reactions that turn carbon dioxide into the organic compound know as sugar, actually sucrose. The plant achieves this by a sequence of reactions known as the Calvin cycle.
The Calvin cycle. The fixation or reduction of carbon dioxide is a process in which carbon dioxide combines with a five-carbon sugar, ribulose 1,5-bisphosphate (RuBP), making it a six-carbon sugar that yields two molecules of a three-carbon compound, called glycerate 3-phosphate (GP), also identified as 3-phosphoglycerate (PGA). GP plus ATP and NADPH from the light-dependent stages is reduced to glyceraldehyde 3-phosphate (G3P), a true three-carbon sugar, or triose. Five out of six molecules of the triose produced are used to regenerate RuBP so the process can continue. The other molecules of the triose phosphates that are not recycled condense to form hexose phosphates, namely sucrose, starch and cellulose. The sugars actually yield carbon skeletons that can be incorporated into other metabolic reactions for the production of amino acids and starch.
The leaf consists of an upper and lower epidermis with cells in between called mesophyll cells. Within the mesophyll cells are hundreds of structures called chloroplasts that contain chlorophyll packed into disc-shaped structures called grana. The grana are made up of stacks of single discs called thylakoids. The chlorophylls and other pigments, such as carotenoids, are in the outer layer of the thylakoids. Photons from sunlight hit the pigments and transfer high energy to electrons that are knocked loose from the chlorophyll and fly off to energize the complicated process of photosynthesis. This is the first part of the photosynthesis and the only part that requires light energy, thus it is often called the light reaction or light-dependant reaction. Water enters the process in the stroma where a special enzyme breaks it down to hydrogen and oxygen by photolysis of water. The hydrogen is further broken down into hydrogen ions and electrons. These electrons are returned to chlorophyll to be used again. The oxygen molecule is either used in the formation of other compounds or is returned to the air as molecular oxygen. After the thylakoids, the next process moves out to the stroma. The stroma is where enzymes take the carbon from carbon dioxide and combine it with hydrogen and oxygen to make simple carbohydrate molecules. This part of the process is generally referred to as the light-independent reactions or dark reactions.