6CO₂ + 6H₂O + light energy------> C₆H₁₂O₆ + 6O₂

Where in the plant does photosynthesis occur? Of course, the chloroplast is the cellular structure which contains all of the necessary molecules to carry out photosynthesis. From that you can deduce that any part of the plant that has chloroplasts will be photosynthetic. However, some plant parts contain relatively higher numbers of chloroplasts, particularly the mesophyllic cells in leaf cells. Notice that there are stomata (stoma, singular) on the underside of the leaf. These are openings through which CO₂ can enter the leaf and O₂ exits.

Pigments. Chlorophyll is the green pigment involved in harvesting light energy. Chlorophyll a absorbs light with wavelengths around 680-700 nanometers in length. Chlorophyll b broadens the range of light wavelengths that a plant can use. Carotenoids are pigments that appear red/orange and they absorb light from the blue/green range of the visible spectrum. Phycocyanins absorb light in the green/yellow range and appear blue/purple.

Electromagnetic Spectrum.

gamma rays X rays UV rays visible light Infrared Microwaves Radiowaves

violet/indigo/blue/green/yellow/orange/red

Chemical reactions of photosynthesis. The chemical reactions involved in photosynthesis can be divided into the Light Dependent Reactions which are made up of two Photosystems and the Light Independent (or dark) Reactions which comprise the Calvin-Benson Cycle.

The major goal of the light dependent reactions is to generate ATP and NADPH (electron carrier) which are needed in the glucose synthesis stage of photosynthesis. Each photosystem consists of a group of chlorophyll molecules which harvest light energy (in photons) which excites electrons in the chlorophyll and they get passed to the reaction center. The reaction center chlorophyll molecule then passes those excited electrons to its electron transport system. Each electron transport system can then make ATP and in Photosystem I the terminal electron acceptor in the electron transport chain is NADP+ which becomes reduced (NADPH) and becomes useful for the Calvin Benson cycle.

Light Independent Reactions. It is during the Calvin Benson cycle that CO₂ comes into play and is ‘fixed’ into a 3 carbon molecule (C3 plants). Through a cyclical series of reactions, carbon is captured and becomes part of a G3P molecule—the precursor of a glucose molecule. One G3P leaves the cycle each time around and it takes 2 G3P molecules to form one glucose. The remaining G3P is ‘recycled’ and becomes RuBP which is the molecule that binds with CO₂.

The enzyme that catalyzes the linking between RuBP and CO₂ is called rubisco. Many plants like soybeans and wheat are C3 plants. The rubisco found in C3 plants is also able to bind to O₂. This will happen in hot, arid conditions because the stomata in plant leaves close to prevent water loss. As a result, CO₂ cannot get into the leaf and O₂ can’t escape so there is a higher concentration of O₂ in the cells and a lower concentration of CO₂. Rubisco begins binding to O₂, resulting in photorespiration, a process which results in no ATP or glucose synthesis.

Some plants (e.g. corn, sugarcane), called C4 plants, have special adaptations to surviving drought conditions. While their stomata stay closed most of the time, they avoid photorespiration because their "rubisco-like" enzyme has a special modification. It will only bind to CO₂ and not O₂, so even when the relative quantity of CO₂ is low, the chloroplasts will continue to ‘fix’ carbon. Carbon is fixed in a 4-carbon molecule which then releases CO₂ into the Calvin cycle.

CAM plants (e.g. jade plants and aloe vera) conserves water by opening stomata only at night. Like C4 plants, CO₂ is fixed in a 4-carbon molecule which then releases CO₂ into the Calvin cycle.

Greenhouse Effect

Ozone Depletion