Ch7. Photosynthesis 7.10 – 7.12

How the ETC helps create ATP & NADPH

Calvin cycle: Converting CO 2 to Sugar Where does the Calvin cycle happen? The stroma What is needed to start the Calvin cycle? ATP = usable energy NADPH = High energy electrons CO2 = used to make sugar Where did this chemical energy come from? The Light Reaction

Four steps, Step 1 What happens in step 1, carbon fixation? The enzyme rubisco combines 3 molecules CO2 with 3 molecules RuBP (ribulose bisphosphate) Making 6 molecules of 3 PGA a 3carbon organic-acid,(3- phosphoglyceric acid)

Step 2 What are the two chemical reactions in Step 2? 1. Energy consumption of 6 ATP’s 2. Oxidation of 6 NADPH What does this step yield? 6 molecules of 3PGA are reduced producing 6 G3P’s. G3P’s have the high energy electrons from NADPH.

Step 3 How many CO2 or carbon molecules does the Calvin cycle use at once? It uses only one carbon at a time Where do the other 5 G3P’s go? They are recycled in the cycle. How many cycles does it take to produce glucose? Since the cycle uses only one carbon at a time, Six full turns are needed to make 2 G3P molecules (3 carbons each) that make up 1 Glucose molecule (6 carbons each)

Step 4 What happens to start the cycle over? ATP supplies the energy to make 3 RuBP from 5 G3P which start the cycle over again.

Summary of Photosynthesis Where does the Light reaction take place? Occurs in the thylakoids, the photosystems capture solar energy & use it to energize electrons. What is Oxidized? Water is Oxidized to O 2 How is chemical energy made? PS1 & PS2 transfer electrons to ETC. ATP & NADPH contain energy from the excited electrons.

Summary of Photosynthesis What happens to all the food made by plants? They stock up extra sugar as starch, storing it in chloroplasts, roots, tubers and fruits. They can also use it to make proteins, fats & nucleic acids. How is even a carne asada burrito or hamburger the product of photosynthesis? The cow ate grass which contained the carbs, proteins, fats and nucleic acids ALL built as a result of photosynthesis.

3 Adaptations to fix carbon & save water What is photorespiration? (Left picture) (C3 plants like rice, wheat, soybeans & oats) C3 (3 carbon) plants close stomata due when it is hot to reduce water loss. This prevents CO 2 from entering or O 2 from leaving. Rubisco binds O2 and can’t do Calvin cycle. Decreasing photosynthesis What do C4 plants do differently? (corn & sugarcane) Store CO2 in 4 carbon sugar so that there is always enough CO2 even when stomata are closed. How have CAM plants adapted? (cactus, pineapple, & succulents) Close stoma during the day when hot, but store CO2 (at night) like C4 plants so they can prevent photorespiration.