Photosynthesis

The Light Reactions Photosynthesis converts light energy into chemical energy through a series of reactions known as biochemical pathways. Almost al life depends on photosynthesis.

Obtaining Energy Organisms can be classified according to how they get energy Autotrophs use energy from sunlight or chemical bonds to make organic compounds. Most autotrophs use the process of photosynthesis to convert light energy into chemical energy. Heterotrophs must get energy from food instead of directly from sunlight or inorganic substances.

Photosynthesis is a complex series of chemical reactions in which the product of one reaction is consumed by the next reaction. A series of chemical reaction linked in this way is referred to as a biochemical pathway.

Overview of Photosynthesis. Two stages Light Reaction: light energy is converted to chemical energy and stored temporarily. Calvin Cycle: organic compounds are formed using CO2 and the chemical energy stored in ATP and NADPH Equation: 6CO2 + 6H2O - C6H12O6 + 6O2

Capturing Light Energy Structure of chloroplasts: Double membrane organelle Inside inner membrane are stacks (grana) of flattened membrane disks called thylakoids The grana are surrounded by a solution called stroma.

Chloroplast pigments. In the thylakoids are light sensitive pigments. The most important of these are the chlorophylls. Chlorophyll a is directly involved in the light reactions. Chlorophyll b and other pigments such as the yellow, orange and brown carotenoids are accessory pigments.

Absorption Spectra of Photosynthetic Pigments.

Converting Light Energy to Chemical Energy Light energy must be converted to chemical energy. The chemical energy is temporarily stored as ATP and NADPH. O2 is given off. These beginning reactions take place in the thylakoid membrane where the pigment molecules are grouped into photosystems (I and II) The light reactions begin with the accessory pigments absorbing light energy and passing that energy onto chlorophyll a.

Capturing Light Energy http://highered.mcgraw- hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/00 72437316/120072/bio13.swf::Photosynthetic Electron Transport and ATP Synthesis Together ATP and NADPH provide energy for the second set of reaction in photosynthesis which will be described next

The Calvin Cycle Plants use the energy from the ATP and NADPH generated in the Light Reactions to produce organic compounds in the form of sugars. The Calvin Cycle is a series of enzyme-assisted reaction that produces a three-carbon sugar. The carbon atoms from CO2 in the atmosphere are bonded or “fixed” into organic compounds. This incorporation of CO2 into organic compounds is called carbon fixation. A total of three CO2 molecules from the atmosphere are needed to produce one three-carbon sugar.

The Calvin Cycle http://highered.mcgraw- hill.com/sites/0070960526/student_view0/chapter5/animation_quiz _1.html

Alternative Pathways Not all plants use the same pathways to “fix” carbon In hot, dry climates plants loose water through small pores called stomata that are located on the underside of leaves. Stomata are important for allowing CO2 into the plant and O2 to leave.

C4 Pathway: plants leave stomata partially closed C4 Pathway: plants leave stomata partially closed. Special cells fix CO2 into four-carbon compounds which are transported to the cells carrying on the Calvin Cycle where the carbon is released as CO2, Corn, sugar cane, and crab grass are examples.

CAM Pathway: Plants open stomata at night CAM Pathway: Plants open stomata at night. Fix CO2 into a variety of organic compounds. During the day, the CO2 is released for the Calvin Cycle. Examples: cactus, pineapple.