Photosynthesis Photosynthesis is the process of harnessing the energy of sunlight to make carbohydrates (sugars). Plants do photosynthesis to make their own food (sugars) and are called, photoautotrophs. Plants take in carbon dioxide and water from the atmosphere, absorb light energy from the sun, and produce glucose (a simple sugar) and oxygen. 6 CO2 + 6 H2O + light energy C6H12O6 + 6 O2 carbon dioxide + water + light energy glucose + oxygen Photosynthesis occurs in the cells of the leaves of plants in organelles called chloroplasts. The process of photosynthesis is divided into two separate parts: I. The light-dependent reactions: occurs in the thylakoids of chloroplasts. The thylakoids contain the green light energy absorbing pigment, chlorophyll. II. The light-independent reactions: Also called the Calvin Cycle or the dark reactions. Occurs after the light-dependent reactions in the stroma of the chloroplast. The stroma is the fluid in the space between the thylakoids in the chloroplast. Chloroplast:
Overview of Photosynthesis: The Light-Dependent Reactions: Where: thylakoids of the chloroplasts Starting materials (reactants): sunlight, water Products: ATP, NADPH, O2. ATP and NADPH carry high energy electrons to the Calvin Cycle where they are used to build sugars. What happens?: Chlorophyll in the thylakoids absorbs light energy from the sun. The absorbed light energy excites the electrons in chlorophyll to a higher energy level. Water is split releasing more high energy electrons and O2 into the atmosphere. The high energy electrons are stored in the electroncarrying molecules ATP and NADPH. ATP and NADPH basically act as energy shuttles, transporting energy from the light-dependent reactions to the light-independent reactions.
The Details of the Light-Dependent Reactions: 1. Light energy (photons) is absorbed by the chlorophyll in photosystem II. The photosystems are just a group of molecules and chlorophyll that absorb light energy at different wavelength ranges. The energy from the sunlight is transferred to chlorophyll, meaning the electrons in chlorophyll become excited to a higher energy level. 2. Water is split and more high energy electrons are generated as well as hydrogen ions (H+) and oxygen (O2). O2 is released into the atmosphere. The hydrogen ions gather in the interior space of the thylakoid. There is a greater concentration of hydrogen ions in the inner thylakoid space than in the stroma, creating a concentration gradient. The high energy electrons generated from the splitting of water are shuttled down the electron transport chain to photosystem I. The electron transport chain is a series of proteins embedded in the thylakoid membrane that act like a conveyor belt for high energy electrons, transporting them from photosystem II to photosystem I. 3. More light energy is absorbed in the photosystem I, essentially re-energizing the electrons passed down the electron transport chain from photosystem II. 4. The hydrogen ions diffuse down their concentration gradient from high concentration (in the interior thylakoid space) to low concentration (in the stroma) through a membrane protein channel and enzyme called ATP synthase. This diffusion does not require any energy. ATP synthase actually captures the energy generated by hydrogen ions flowing down their concentration gradient and uses this energy to make ATP. ATP is a high energy molecule that
drives all cellular processes and will be used as an energy source later in the light-independent reactions. 5. Electrons passed down the electron transport chain and a hydrogen ion is added to NADP+ to make NADPH. NADPH is a high-energy electron-carrying molecule. It stores energy that will later be used in the light-independent reactions. The Light-Independent Reactions: Where: stroma of the chloroplasts Starting materials (reactants): CO2 from the atmosphere, ATP and NADPH transferred over from the light-dependent reactions Products: Sugars (glucose, sucrose, starch) What Happens?: The light independent reactions do not require light from the sun. In the stroma of the chloroplasts, six carbon sugars are built out of carbon derived from CO2 from the atmosphere. The electron-carrying molecules, ATP and NADPH, are the source of energy used to build up six carbon sugar molecules out of CO2. The Details of the Light-Independent Reactions: 1. 3 CO2 are added to a five carbon sugar called RuBP to form 6 three-carbon molecules called 3- phosphoglycerate (PGA). 2. ATP from the light-dependent reactions converts PGA into a higher-energy three-carbon
molecule called 3-phosphoglycerate (G3P). 3. NADPH from the light-dependent reactions adds a phosphate group (P) to G3P, thereby making it more energized. 4. The G3P with the phosphate group attached leaves the cycle. Once two of these G3Ps leave the cycle, they combine to form one six carbon sugar, such as glucose. 5. The remaining five G3P are converted to RuBP using energy from ATP from the light-dependent reactions. Remember, RuBP was the five carbon sugar that we combined with CO2 at the beginning of the cycle. So RuBP is regenerated in the Calvin Cycle.