Ex7PhotosynthesisFall07 - K101 LAB EXERCISE 7...

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K101 LAB EXERCISE 7 PHOTOSYNTHESIS: SPINNING SUGAR OUT OF SUNLIGHT During the process of photosynthesis, light energy from the sun is used to generate ATP through electron transport reactions, called the “ light reactions ”, on the surface of chlorophyll - containing membranes, such as those found in the chloroplast. During this process water is “split” by a process called “ photolysis ”, to obtain the electrons ( and hydrogens ) needed to start photosynthesis, and releasing oxygen gas (O 2 ) as a byproduct. The ATP made during the light reactions is then used, during a series of enzymatic steps called the Calvin Cycle , to convert inorganic carbon dioxide (CO 2 ) to an organic form rich in free energy: sugar (C 6 H 12 O 6 )! The first photosynthetic organisms (single- celled, prokaryotic cyanobacteria ) appeared on the Earth somewhere around 2 billion years before the present (YBP), and over the course of time, have raised the levels of atmospheric oxygen to today’s current levels, about 21% of the gas in the atmosphere. Without photosynthesis, levels of food (in the form of plant and plankton biomass) and atmospheric oxygen would become depleted to a level that would be incapable of supporting animal life, either on land or in the oceans. Photosynthesis can be summarized by the equation: 12H 2 O + 6CO 2 C 6 H 12 O 6 + 6O 2 (gas) + 6H 2 O Glucose (C 6 H 12 O 6 ) is one of the primary products of photosynthesis. However, most plants convert a fair amount of glucose into the disaccharide sucrose (glu-fru) as a liquid ‘transport’ form of sugar (like maple syrup sap). The majority of excess glucose in photosynthetic organisms, though, is converted into the storage carbohydrate starch , consisting of numerous glucose units linked together with the chemical formula being (C 6 H 12 O 6 ) n , where n is the number of subunits. Starch can be stored as starch grains inside chloroplasts, in separate plastids within the cell just for starch storage, called amyloplasts , or in entire organs of the plant that store large quantities of starch, like the potato tuber . A plant can mobilize starch quickly if it has used up its cellular supplies of glucose or sucrose, such as during a cold, rainy spell when very little photosynthesis is taking place. Plants produce a variety of pigments for a variety of purposes. A pigment is simply a substance that absorbs light. Plants make several green chlorophyll pigments involved in photosynthesis. Chlorophylls absorb light strongly in the red and blue regions of the visible spectrum, and reflect or transmit green wavelengths of light, giving us the green appearance we see when we look at a plant. As a result, red and blue light are the most effective wavelengths of light for stimulating photosynthesis. Plants ‘boost’ their ability to capture other wavelengths of light by also having accessory pigments such as carotenoids and xanthophylls , which absorb a different range of wavelengths of light, and reflect orange and yellow wavelengths, respectively. In today’s lab, we will do four exercises relating to photosynthesis.
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Ex7PhotosynthesisFall07 - K101 LAB EXERCISE 7...

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