Describe glucose and where it is produced.

Glucose is a simple (single molecule) sugar. Sugars are organic compounds chemically referred to as carbohydrates. Like most simple sugars, glucose follows the typical carbon and hydrate pattern of a certain number of carbons plus a certain number of H2O groups (hence the name), often written as:

From an energy standpoint, glucose is the carbohydrate most readily used in the series of chemical reactions that facilitate the production of ATP, the energy currency of all multicellular animals. Animals and other non-photosynthetic organisms, including fungi and bacteria, cannot produce glucose and must take it up from their environment; animals do this by eating other animals or plants. Plants can make glucose via photosynthesis, which, in a nutshell, results in the following chemical reaction:

Plants absorb the reactants, carbon dioxide and water, from their environment. Photons from light provide the energy that powers the steps that produce this chemical reaction. Plant cells in all green parts of plants, including leaves and stems, contain organelles called chloroplasts, which is where photosynthesis takes place. Most chloroplasts reside deep within leaves in cells called mesophylls.

Each mesophyll contains an average of 30 chloroplasts. The chloroplast is a double-membraned envelope filled with stroma, a viscous fluid within which reside membranous sacs called thylakoids. These thylakoids contain chlorophyll, the chromophore which gives plants their green pigment. The wavelengths which are of most use to plants are outside the green area of the visual spectrum; the chlorophyll thereby reflects the green wavelengths and absorbs only the light that is most useful to the chloroplast.

There are two reactions that comprise photosynthesis, the light reaction and the Calvin cycle. In the light reaction, a photon provides the energy to split a water molecule and produce oxygen, a compound called nicotinamide adenine dinucleotide phosphate hydrogen (NADPH), and ATP. In the Calvin cycle, carbon dioxide from the air is complexed into organic compounds in a process known as carbon fixation. NADPH and ATP are used to add electrons to the compounds (termed reduction) and build a three-carbon sugar called glyceraldehyde-3-phosphate, or G3P.  

Some of the G3P produced is used to make sugars, including sucrose and glucose. Some goes back into the Calvin cycle to allow the cycle to repeat, and the rest the plant uses for its own structures and energy stores.

REFERENCES:

Alberts B., Johnson A. Lewis J. et al. (2008) Molecular Biology of the Cell. 5th ed. Garland Science: New York, NY

Loudon, M. (2009) Organic Chemistry. 5th ed. Roberts & Co Publishers: Greenwood, CO

Reece, J.B., Urry, L.A., Cain, M.L. et al. (2011) Campbell Biology. 9th ed. Benjamin Cummings: San Francisco, CA