Photosynthesis Chapter 8, Section #2 SC.912.L.18.7 Identify the reactants, products, and basic functions of photosynthesis.
Essential Questions 1.What are the two phases of photosynthesis? 2.What is the function of a chloroplast during the light reactions? 3.How can electron transport be described and diagramed?
Overview of Photosynthesis u Autotrophs (organisms that create their own food), make organic compounds (sugars/glucose) using a process called photosynthesis. u Recall that photosynthesis is the process by which light energy is converted into chemical energy, as shown by the equation below:
Photosynthesis occurs inside the chloroplast! Plant cells contain a pigmented organelle called a chloroplast that captures light energy. Chloroplasts are the site of photosynthesis and they have a specialized structure. Recall: Where are chloroplasts located in a plant?
Structure of a Chloroplasts u Chloroplasts contain specialized structures to help them carry out the two part process of photosynthesis. Thylakoid: flattened, sac-like membranes that are arranged in stacks Grana (granum): stacks of thylakoids Stroma: the fluid-filled space that is outside the grana
Phase One: Light Reactions u The absorption of light is the first step of photosynthesis. u Light is captured in the chloroplast by the thylakoids. u Once light is captured, two energy storage molecules (NADPH and ATP) are produced. These are used in phase 2 of photosynthesis.
Phase One: Light Reactions What are the outputs of the light dependent reactions?
Pigments u Thylakoids are able to absorb light energy because they contain light absorbing colored molecules called pigments. u Different pigments absorb different wavelengths of light.
Pigments u Plants contain several different kinds of pigments: uchlorophyll a & Chlorophyll b the two main types of chlorophyll found in plants, chlorophyll a and b absorb light most strongly in the violet-blue region of the visible light spectrum and reflect light in the green region of the spectrum uthis is why plant parts that contain chlorophyll appear green to the human eye!
Pigments u Plants also contain other light-absorbing pigments: ucarotenoids: absorb light mainly in the blue and green region of the spectrum and reflect light in they yellow, orange, and red regions. uthis is why vegetables such as carrots and sweet potatoes have their distinct colors! uleaves change colors in the fall when chlorophyll molecules break down, revealing the colors of the other pigments.
Electron Transport u Electron transport occurs in the thylakoid membranes, which have a large enough surface area to hold large numbers of electron-transporting molecules and two types of protein complexes called photosystems. u Photosystem I and Photosystem II contain light-absorbing pigments and proteins that play important roles in the light dependent reactions.
Electron Transport u The electron transport chain occurs in steps: 1.Light energy excites electrons in photosystem II. The light energy also causes a water molecule to split, releasing an electron into the electron transport chain, a hydrogen ion (H+) into the thylakoid space, and oxygen as a waste product. uthis breakdown of water is essential for photosynthesis to occur!
Electron Transport 2. The excited electrons move from photosystem II to an electron-acceptor molecule in the thylakoid membrane. 3. The electron-acceptor molecule transfers the electrons along a series of electroncarriers to photosystem I.
Electron Transport 4. In the presence of light, photosystem I transfers the electrons to a protein called ferrodoxin. The electrons lost by photosystem I are then replaced by electrons shuttled in from photosystem II. 5. Finally, ferrodoxin transfers the electrons to the electron carrier NADP+, forming the energy storing molecule NADPH.
Electron Transport
Chemiosmosis u Chemiosmosis: the mechanism by which ATP is produced as a result of the flow of electrons down a concentration gradient u When the water molecule is split during the electron transport chain, H+ accumulates in the interior of the thylakoid. u As it accumulates it creates a high concentration of H+ in the thylakoid membrane as opposed to the low concentration of H+ in the stroma. u Because of the concentration gradient (diffusion) the H+ protons diffuse into the stroma creating ATP in the stroma.
Phase 2: The Calvin Cycle (Light- Independent Reactions) u NADPH & ATP produced during the Light Dependent Reactions provide the cells with energy, but are not stable enough to store energy for long periods of time. u The second phase of photosynthesis (The Calvin Cycle) creates a storage place for energy glucose!
The Calvin Cycle u The Calvin Cycle does not require light energy and occurs in steps: 1. The first step is called carbon fixation carbon dioxide joins with other organic molecules. 2. The chemical energy stored in ATP and NADPH from the light reactions form special molecules called G3P that will be used to form glucose and other organic compounds. 3. In the final step, an enzyme called rubisco remaining G3P molecules back into carbon dioxide molecules to continue the cycle.
The Calvin Cycle
Alternative Pathways u Some autotrophs cannot carry out photosynthesis due to limited light availability in their environments. They may use: u C4 Plants: plants like sugarcane and corn fix carbon dioxide into four-carbon compounds instead of three-carbon compounds during the Calvin Cycle uallows for sufficient carbon dioxide uptake while simultaneously minimizing water loss u CAM Plants: plants that live in deserts fix carbon dioxide into organic compounds at night uallows for sufficient carbon dioxide uptake while minimizing water loss