Photosynthesis Rajni Govindjee and Govindjee

Sun is the source of almost all energy that sustains Life on Earth. Each minute the Sun converts 120 million tons of its mass into electromagnetic radiation and dumps it out into space. One billionth of that reaches the Earth. It takes only 8 minutes for this radiation to travel 93 million miles to reach us. The visible portion of this electromagnetic radiation (VIBGYOR, violet, indigo, blue, green, yellow, orange and red, the colors in the rainbow) is captured by plants, algae and cyanobacteria. The green color of the leaves is due to the presence of a pigment called chlorophyll. Chlorophyll absorbs blue and red light efficiently, but not green, the transmitted green light gives the leaves their green color. The process by which plants utilize sunlight (absorbed by chlorophyll) to make food (that we need for our lives) and oxygen (that we need to breathe) from carbon dioxide (CO₂) and water is called Photosynthesis. In addition, past photosynthesis provides the fossil fuels needed to power the industry and automobiles etc.

Carbon Dioxide + Water + Light = Carbohydrate + Oxygen         6 CO2 + 6 H2O + Light = (CH2O)6 + 6 O2

Almost all life (except those in the hot vents on the ocean floor) depends on photosynthesis. Plants make leaves, roots, stems and fruits that herbivores (such as caterpillars) eat; carnivores eat plants and the herbivores. Photosynthesis is the basis of this food web that links almost all living beings on Earth. In the oceans, small fishes eat the phytoplankton (algae) and the bigger fishes eat the small fishes, and the "web of life" goes on. Plants capture only one thousandth of the sunlight that falls on the Earth. Yet, without this process all life on Earth would come to a halt.

Photosynthesis also links plants and animals (such as Dog and Dino) through the air since oxygen released by plants is used not only by plants themselves (for their respiration) but also by animals for their respiration. Respiration by both plants and animals, in turn, releases carbon dioxide that plants use to perform photosynthesis. Photosynthesis occurs in tiny organelle called chloroplast. The chloroplast has membranes, called the "thylakoid membranes" (thylakoid in greek means sacs); they are arranged as stacks of flattened vesicles (like stacks of chapatis, or pitabread) called grana (granum is singular). The outside space is called the stroma matrix. The space inside the vesicle is called "Lumen".

The process of photosynthesis can be divided into two major phases: a light-dependent phase (the "Light Reactions") and a light-independent phase (the "Dark Reactions"). In the light reactions, two things happen: (1) electrons are transferred from water to NADP+ (nicotinamide adenine nucleotide phosphate, the oxidized form) by a scheme that is called the "Z Scheme", producing oxygen and NADPH (the reduced form of NADP) and (2) the ADP (adenosine diphosphate) is converted to energy rich compound ATP (adenosine tri phosphate) . In the dark reactions, these two compounds (NADPH and ATP) are used to convert carbon dioxide to sugars, and the ADP and NADP are made available to carry on the process.