Overview All organisms need a constant supply of energy to survive. For most life on earth, the ultimate source of energy is the sun. Converting that energy source into something usable is accomplished by photosynthesis.

Overview Photosynthesis: the overall process by which sunlight (solar/light energy) chemically converts water and carbon dioxide into chemical energy stored in glucose (a sugar/carbohydrate.) Water is absorbed in roots CO2 is absorbed through stomata It can be represented by the following chemical equation: 6CO2 + 6H2O C6H12O6 + 6O2 Image Credits: © Zappys Technology Solutions’s 2013 https://flic.kr/p/gwd2Wh Solar energy

Overview 6CO2 + 6H2O C6H12O6 + 6O2 Reactants = ingredients Solar energy 6CO2 + 6H2O C6H12O6 + 6O2 Reactants = ingredients CO2 (carbon dioxide) and H2O (water) Products = results C6H12O6 (glucose) and O2 (oxygen) Note: Solar energy from the sun is necessary for photosynthesis to happen (as well as some enzymes) but isn’t considered a reactant or product.

Structure of Chloroplast Photosynthesis takes place in the chloroplast which has 2 main parts: Grana: pancake-like stacks of thylakoid membrane Stroma: fluid-like substance that fills the space between the grana grana thylakoid membrane

Why are plants green? The presence of the pigment chlorophyll Chlorophyll a, chlorophyll b, and other pigments called carotenoids absorb every color of light in sunlight except green Therefore, green is leftover and is reflected and is what we see

Two Stages of Photosynthesis Photosynthesis can be divided into two sets of reactions: 1. Light-dependent (“photo”) Reaction Requires solar energy. AKA the Electron Transport Chain (or light rxn) 2. Light-independent (“synthesis”) Reaction Does not require any solar energy. AKA the Calvin Cycle (or dark rxn)

Light-Dependent Reaction Purpose = Capture energy from the sun and store energy in “energy-carrying molecules” (ATP and NADPH) Location = occurs in the grana (specifically the thylakoid membrane) where the chlorophyll is stored.

Light-Dependent Reaction Summary: Water molecules are split into hydrogen and oxygen. Oxygen is released as a waste product. ATP and NADPH are charged up by the sun.

Light-Dependent Reaction Details: Energy from sun is passed down the Electron Transport Chain and is stored in the bonds of ATP and NADPH Light energy excites e- (electrons) e- move down ETC At end they combine with “final electron acceptors/carriers” of NADP+ and ADP, making NADPH and ATP Chemiosmotic process b/c H+ ions move down the gradient to make ATP ATP, NADPH, and H+ leave the grana and go into the stroma for the next stage!

How is light absorbed? Photosystems absorb light They are clusters of chlorophyll and proteins that trap energy from the sun Chlorophyll is a pigment that can absorb sunlight Energy is transferred to electrons  makes “excited” electrons

What are electron carriers? Molecules that carry electrons in order to pass on their energy Ex. Compound (NADP+) that can accept a pair of high-energy electrons and transfer them to another molecule NADP+ grabs/carries 2 electrons and a H+  becomes NADPH ATP and NADPH carry energy from the light-dependent rxn to the light-independent rxn.

Light-Dependent Reaction Image Credits: © BlueRidgeKitties 2010 https://flic.kr/p/83eJuy I like to show them this picture so they can appreciate the complexity of the process and understand that I am not making them memorize every detail but want them to have a solid foundation in the process in case they are to ever take AP bio.

Light-Independent Reaction Purpose = use the energy from the “energy-carrying molecules” from the light-dependent reaction to make sugar (glucose) Location = occurs in the stroma

Light-Independent Reaction Summary = Calvin Cycle Series of enzyme-assisted chemical reactions powered by ATP and NADPH that produce three-carbon (3-C) sugars from CO2 and the H+ from water. The cycle happens twice and then these 3-C sugars combine to make glucose = C6H12O6

Light-Independent Reaction Details Grab CO2 diffuses into stroma Enzyme attaches CO2 to 5-C RuBP Produce unstable 6-C molecules Split Energy from ATP and NADPH and an enzyme break the 6-C molecule into 2 3-C molecules (PGA) RuBP = 5-carbon ribulose bisphosphate PGA = phosphoglycerate G3P = glyceraldehyde 3-phosphate

Light-Independent Reaction Details Leave Each 3-C molecule (PGA) is converted to a different 3-C molecule (G3P) One G3P leaves the cycle to become glucose The other G3P moves on to next step Switch Remaining G3P converts back to 5-C RuBP by using a phosphate from ATP and the cycle starts again! RuBP = 5-carbon ribulose bisphosphate PGA = phosphoglycerate G3P = glyceraldehyde 3-phosphate

Light-Independent Reaction Image Credits: © Allen Gathman 2010 https://flic.kr/p/7FHg1a I like to show them this picture so they can appreciate the complexity of the process and understand that I am not making them memorize every detail but want them to have a solid foundation in the process in case they are to ever take AP bio.

Rate of Photosynthesis Speed is affected by 3 factors: Light intensity Excites more e- causing light reactions to happen faster Amount of CO2 More ingredients to work with and process through cycle Temperature Increased temperature accelerates chemical reactions to a degree

Why do root cells in a plant not need chloroplasts? Chloroplasts catch sunlight! Since roots are underground, they are not exposed to the sun! So they can’t do photosynthesis.

Alternate Pathways Stomata = pores on underside of leaf. Where… Plants lose water CO2 enters O2 exits If it is too hot or dry out, the plant will close its stomata so that it doesn’t lose too much water and become dehydrated However this eliminates the gas exchange!! SO  the levels of CO2 drop and the levels of O2 increase This results in…. PHOTORESPIRATION Photorespiration adds oxygen to the Calvin Cycle instead of carbon dioxide - This makes NO sugar or ATP  - This wastes all of the plants resources! Two types of alternative pathways in plants to avoid this: 1. CAM 2. C4

Alternate Pathways CAM Done by cacti and pineapples Open stomata at night and close during day Opposite of normal plants Causes them to grow slowly

Alternate Pathways C4 Done by corn and sugarcane Partially close stomata during hottest part of day Allows them to only need ½ as much water as normal plants!