Photosynthesis Ch 6

ATP Remember… ATP is cellular energy Energy from the chemical bonds of ATP is released when the bond between the 2nd and 3rd phosphate groups is broken. It then becomes adenosine DIphosphate – ADP ATP is then recharged! It uses energy to gain an extra P and become ATP again

Metabolism Anabolism: metabolic reactions that build molecules [endergonic reaction] Catabolism: metabolic reactions that break down molecules [exergonic reaction] Life is sustained by inputs of energy, however not all forms of energy can sustain life. The sun is abundant, but cannot directly power protein synthesis or energy-requiring reactions, it must first be converted to chemical bond energy

Photosynthesis Energy flow through ecosystems begins when photosynthesizers [plants] capture sunlight and convert it to chemical energy This chemical energy can power the reactions of life, and can be stored for use at a later time

Photosynthesis Autotrophs are producers Heterotrophs are consumers Make food using energy from environment and carbon from inorganic molecules Heterotrophs are consumers Obtain carbon from organic compounds assembled by other organisms

Photosynthesis Photosynthesis: Metabolic pathway that uses light energy to turn carbon dioxide (CO2) and water (H2O) into carbohydrates (sugar) Also creates OXYGEN! The process is cyclical, the products from one reaction are the reactants for the next reaction

Photosynthesis Photosynthesis has 2 parts Light-dependent: converts light energy into chemical energy; produces oxygen and ATP to be used in light- independent reaction Light-independent: does not require light energy, uses CO2 and H2O to build sugars; powered by ATP, also called the CALVIN CYCLE

Chloroplasts Photosynthesis occurs in the chloroplast The chloroplast is a plastid that carries out photosynthesis Plastids are double membrane organelles that contain pigment Chloroplasts resemble the photosynthetic bacteria they evolved from, so photosynthesis in eukaryotic cells is similar to bacterial photosynthesis

Chloroplasts Thylakoid membrane: Highly folded into stacks of interconnected thylakoids [Light-dependent reactions] Folds in this membrane form disks called thylakoids The membrane encloses a single, continuous internal space Stroma: Cytoplasm-like fluid inside the chloroplast [Light-independent reactions] thylakoid membrane, ribosomes, and chloroplast’s DNA are suspended in the stroma

Photosynthesis Pigment: organic molecule that selectively absorbs certain wavelengths of light If a wavelength is not absorbed, it is reflected, and that gives the pigment its color Chlorophyll a is the most common pigment in plants It absorbs violet, red, and orange light, but reflects green!

Photosynthesis Most photosynthetic cells have accessory pigments that capture other wavelengths of light Other functions (attract pollinators; antioxidants) Disassembly of chlorophylls during autumn reveals accessory pigments

Phase 1: Light Reaction 1. Light hits a leaf, causing an electron in chlorophyll to enter an excited state (higher energy level) within the chloroplast’s thylakoid membrane. 2. The excited electrons leaves the chlorophyll and an enzyme splits a water molecule to replace the electron lost by the chlorophyll. Oxygen gas is formed and exits through the membrane and into the atmosphere. H+ ions accumulate inside the thylakoid space. Ferredoxin LIGHT O2 H2O H+

3. The released electron goes through electron transport chain 3. The released electron goes through electron transport chain. The electron is bounced from between proteins in the thylakoid membrane. Note: Energy lost along electron transport chain 4. The electron is eventually passed to another chlorophyll molecule and then on to the final electron acceptor (ferredoxin NADP+ reductase) where the electron transport chain ends. 5. Ferredoxin NADP+ reductase then gives the electron to an electron carrier NADP which turns it into NADPH. NADPH carriers the electron to the next phase. O2 NADP NADPH Ferredoxin H+ H+ H+

6. As more copies of steps 1 through 5 occur, more and more H+ ions from step 2 begin to build up in the thylakoid space. H+ ions begin to pass from an area of higher concertation (thylakoid space) to an area of lower concentration (stroma) through an enzyme called ATP synthase. This process called, chemiosmosis, creates ATP. H+ H+ ADP ATP Ferredoxin H+ H+ H+ H+ H+ H+ H+

Phase 1: Light Reaction Summary Lost energy used to recharge ATP from ADP NADPH produced from e- transport chain Stores energy until transfer to stroma Plays important role in light-independent reaction (LIR) Total byproducts: ATP (Used in LIR), NADPH (Used in LIR), O2 (Exits plant)

Phase 2: Dark Reaction (Light-Independent) In the second phase, also called the Calvin-Benson Cycle, carbon dioxide, ATP and NADPH are used to make sugar (glucose) in the stroma. The “synthesis” part Plants must make glucose to store energy long term. ATP is only short term energy!

Phase 2: Dark Reaction In the stroma, an enzyme called RUBISCO takes a few molecules of carbon dioxide and connects it to a molecule called RuBP and turns it into a 3 carbon molecule called PGA Then more enzymes use ATP and NADPH from the light reactions to rearrange the molecules in PGA to store energy and create a few molecules called G3P. NOTE: When ATP and NADPH are used as energy it they become ADP and NADP respectively. The NADP and ADP are sent back to the light reactions to get recharged. Some G3P’s are sent into the cytoplasm to make glucose. Some G3P molecules are used to make RuBP that are needed to restart the dark reactions.

Chloroplast H2O CO2 Light Light Reactions Calvin Cycle ATP Sugar (Glucose) O2

Alternative Pathways Plants in some climates have alternative ways of doing photosynthesis to help them survive better.

Alternative Pathways Alternatives

Rate of Photosynthesis Light intensity, carbon dioxide levels, and temperature effect the rate of photosynthesis As light increases, rate of photosynthesis increases As CO2 increases, rate of photosynthesis increases Temperature Low = Rate of photosynthesis low Temperature Increases = Rate of photosynthesis increases If temperature too hot, rate drops

Chemosynthesis The synthesis of organic compounds by bacteria or other living organisms using energy from reactions involving inorganic chemicals, typically in the absence of sunlight. They use chemicals instead of water as an electron donor before the electron transport chain. They generally don’t produce oxygen. Example: Hydrogen sulfide eating bacteria in deep ocean vents.

Light-Independent Reactions The Calvin–Benson cycle Builds sugars in the stroma of chloroplasts Not powered by light energy Driving force is ATP and NADPH formed by the light- dependent reactions Uses carbon atoms from CO2 to make sugars