Photosynthesis

What is it?  Photosynthesis is the process of converting light energy to chemical energy and storing it in the bonds of sugar.  Plants need only light energy, CO 2, and H 2 O to make sugar

 Photosynthesis takes place primarily in plant leaves, and little to none occurs in stems.  The process of photosynthesis takes place in the chloroplasts, specifically using chlorophyll, the green pigment involved in photosynthesis.

Look closer at a leaf Then at an individual plant cell. The chloroplast is inside the cell.

Large amounts of chloroplasts Look at the part of a chloroplast

 The thylakoids contain pigments called chlorophylls. Chlorophyll a absorbs more red light than blue and is directly involved in the light reaction. Chlorophyll b assists.

Pigments do 2 things:  Pigments are chemical compounds which reflect only certain wavelengths of visible light. This makes them appear "colorful". Flowers, corals, and even animal skin contain pigments which give them their colors. More important than their reflection of light is the ability of pigments to absorb certain wavelengths.  Note: Carotenoids are an accessory pigment.

Why does a plant look green?  Chlorophyll looks green because it absorbs red and blue light, making these colors unavailable to be seen by our eyes. It is the green light which is NOT absorbed that finally reaches our eyes, making chlorophyll appear green.

Chloroplast (side view)

 After the pigments capture the light energy, how is this converted to chemical energy to make sugar?

1. Photons hit the pigments 2.The electrons are excited! 3. The electrons then go to a “primary electron acceptor”(PEA). 4. Then the electron goes through the ETC (electron transport chain) 5. But note: there are enzymes inside of the thylakoid which causes H 2 0 to split under Photosystem II (keeping H + ) releasing O 2 See next picture

 6. Within the ETC the electrons lose their excitement and are used to help split H 2 O molecules.  7. Light is also absorbed by Photosystem I at the same time and donates electrons to a different PEA and ETC.  8. These electrons make NADPH ( an organic molecule which will be needed later)  9. ATP(energy) is made by a process called “Chemiosmosis” by an enzyme called ATP synthase  See next slide

So what did we get from all this?  NADPH(organic molecule which carries electrons) needed for cellular respiration  H + (hydrogen ions)  ATP (energy)  Now we can go into part II of Photosynthesis The Calvin Cycle (where the sugar is made)  With all of these chains of events, why do you think Photosynthesis is a biochemical pathway???

The Calvin Cycle  This is part II of Photosynthesis. The Calvin cycle is a chemical reaction which produces sugar for the plant for energy and heterotrophs to consume  Where does this reaction take place?  In the stroma of the chloroplast.

How does the Calvin or C 3 cycle work? 1. In the Calvin Cycle, Carbon atoms are “fixed” into organic compounds. 2. Three molecules of CO 2 is incorporated into the cycle to produce sugar in the stroma. 3. Here CO 2 & enzymes combines with RuBP (ribose bisphosphate) to form a six carbon molecule 4. That molecule splits into two three- carbon molecules called 3-PGA (3- Phosphoglycerate).

5. 3-PGA gets converted to another three carbon molecule,G3P (glyceraldehydes 3 phosphate) ATP & H + help to do this. 6. One G3P goes out to make carbohydrates 7. The remaining G3P the rest are used to convert back to RuBP to start the cycle over.