Photosynthesis

Warm-Up Why do leaves change colors in the fall? What does this have to do with photosynthesis?

What is Photosynthesis? Photosynthesis converts light energy into chemical energy through complex series reactions known as biochemical pathways Autotrophs use photosynthesis to make organic compounds from carbon dioxide and water 6CO2 + 6H20 = 6O2 + C6H12O6

Where does Photosynthesis occur? In plants and algae, photosynthesis occurs inside the chloroplasts

Chloroplasts

Roy G. Biv White light from the sun is composed of an array of colors called the visible spectrum Different colors in the visible spectrum have different wavelengths Pigments absorb certain colors of light and reflect or transmit the other colors

Stages of Photosynthesis STAGE 1 - LIGHT DEPENDENT REACTIONS Energy is Capture from Sunlight. Water is Split into Hydrogen Ions, Electrons, and Oxygen (O2). The O2 Diffuses out of the Chloroplasts (Byproduct). The Light Energy is Converted to Chemical Energy, which is Temporarily Stored in ATP and NADPH. STAGE 2 - CALVIN CYCLE The Chemical Energy Stored in ATP and NADPH powers the formation of Organic Compounds (Sugars), Using Carbon Dioxide, CO2.

Stages of Photosynthesis

Stages of Photosynthesis

Photosystems I & II Photosystem II - photons split water molecules (producing O2), electrons produced from the split travel down electron transport chain. Energy provided in the transport chain is used to make ATP from ADP Photosystem I - photons boost electrons to a higher energy state, electrons travel down electron transport chain, energy is used to make NADPH from NADP. The ATP and the NADPH are used for the next main step, the Calvin cycle,or "Light independent reactions" - also called the Dark Reaction

Photosystems

Light Reactions The light reactions of photosynthesis begin with the absorption of light by chlorophyll a and accessory pigments in the thylakoids Accessory pigments absorb colors of light that aren’t absorbed by chlorophyll a, and they transfer some of the energy in this light to chlorophyll a

Electron Transport Excited electrons that leave chlorophyll a travel along two electron transport chains, resulting in the production of NADPH The electrons are replaced when water is split into electrons, protons, and oxygen in the thylakoid Oxygen is released as a byproduct of photosynthesis

Electron Transport continued… As electrons travel along the electron transport chains, a concentration gradient of protons builds up across the thylakoid membrane The movement of protons down this gradient results in the synthesis of ATP through chemiosmosis

Calvin Cycle The ATP and NADPH produced in the light reactions drive the second part of photosynthesis, the Calvin cycle In the Calvin cycle, CO2 is incorporated into organic compounds, a process referred to as carbon fixation

Calvin Cycle continued… The Calvin cycle produces a compound called PGAL Three turns of the Calvin cycle are needed to produce one PGAL molecule

Calvin Cycle continued… Most PGAL molecules are converted into another molecule that keeps the Calvin cycle operating However, some PGAL molecules are used to make other organic compounds, including amino acids, lipids, and carbohydrates

During this phase, carbon dioxide is "fixed" or attached to a 5-carbon sugar called RUBP, producing a 6-carbon sugar similar to the sugar glucose. Each (unstable) 6 carbon molecules splits into 2 (3 Carbon compounds) known as phosphoglyceric acid (PGA) ATP and NADPH used to convert PGA to PGAL PGAL can be removed from the cycle and used for the formation of larger carbohydrates such as glucose, sucrose and starch.

Photosynthesis Reaction Summary In the overall equation for photosynthesis, CO2 and water are the reactants, and carbohydrate and O2 are the products 6CO2 + 6H20 = 6O2 + C6H12O6

Alternate Pathways Most plants fall into the C3 category Some plants living in hot, dry climates supplement the Calvin cycle with the C4 or CAM pathways These plants carry out carbon fixation and the Calvin cycle either in different cells or at different times

Alternate Pathways CAM Cactus, pineapples have different adaptations to Hot, Dry Climates. They Fix Carbon through a pathway called CAM. Plants that use the CAM Pathway Open their Stomata at NIGHT and Close during the DAY, the opposite of what other plants do. At NIGHT, CAM Plants take in CO2 and fix into Organic Compounds. During the DAY, CO2 is released from these Compounds and enters the Calvin Cycle. Because CAM Plants have their Stomata open at night, they grow very Slowly, But they lose LESS Water than C3 or C4 Plants.

Alternate Pathways C4 Allows certain plants to fix CO2 into FOUR-Carbon Compounds. During the hottest part of the day, C4 plants have their Stomata Partially Closed. C4 plants include corn, sugar cane and crabgrass. Such plants lose only about half as much water as C3 plants when producing the same amount of Carbohydrate.

Rate of Photosynthesis The rate of photosynthesis increases and then reaches a plateau as light intensity or CO2 concentration increases Below a certain temperature, the rate of photosynthesis increases as temperature increases Above that temperature, the rate of photosynthesis decreases as temperature increases