Photosynthesis Ch. 7 Photosynthetic Organisms
Photosynthesis coverts solar energy into thechemical energy of a carbohydrate. Photosynthetic organisms are called autotrophsbecause they produce their own food. With a few exceptions, it is possible to trace anyfood chain back to plants and algae. Photosynthesis takes place in the green portionsof plants.
The raw materials for photosynthesis are waterand carbon dioxide.The roots absorb water,while carbon dioxide enters a leaf throughstromata. Then CO2 and H2O diffuse into chloroplasts, whichcarries out photosynthesis. Carbon Dioxide and Water then diffuse into chloroplasts,which are the organelles that carry out photosynthesis. A double membrane surrounds a chloroplast, and itssemifluid interior called the stroma. A different membrane system within the stroma formsflattened sacs called thylakoids. The thylakoid membrane contains chlorophyll and otherpigments capable of absorbing solar energy. The Photosynthesis process
The overall process of photosynthesis can berepresented by the following equation: In the reaction, CH2O represents carbohydrates.
Photosynthesis is a redox reaction Lose of electrons is oxidation and gain of electrons isreduction. It takes a lot of energy to reduce CO2.Solarenergy is not used directly to provide the energy,but instead is used to generate the ATP needed. Solar Energy Chemical Energy (ATP, NADPH)
Two Sets of Reactions Light Reactions These reaction only occur when solar energy is present. The solar energy energizes electrons to move down theelectron transport chain. As the electrons move, energy is released and capturedto produce ATP molecules. Solar Energy Chemical Energy (ATP, NADPH) Chemical Energy (ATP, NADPH) Chemical Energy (carbohydrate)
Two Sets of Reactions Calvin Cycle Reactions Named after Melvin Calvin. He discovered the enzymatic reactions that reducecarbon dioxide to a carbohydrate in the stroma ofchloroplasts. These reactions can occur during both day and night. During the Calvin Cycle, CO2 is taken up and thenreduced to a carbohydrate that can later be convertedto glucose. Chemical Energy (ATP, NADPH) Chemical Energy(carbohydrate) Pigments and Photosystems
Pigment molecules absorb wavelengths of light.Most pigmentsabsorb only some wavelengths and reflect or transmit theothers. The pigments in chloroplasts are capable of absorbing portionsof visible light called their absorption spectrum. Pigments and Photosystems
Photosynthetic organisms differ in the type of chlorophyll theycontain. Chlorophyll a and chlorophyll bplay a prominent role inphotosynthesis. Chlorophyll a and b absorb violet, blue, and red light better than others. Green light is reflected by chlorophyll giving plants their green color. Carotenoids play an accessory role. Carotenoids, which are shades of yellow and orange, absorb the violet- blue-green range.These pigments become noticeable in fall whenchlorophyll breaks down. A photosystem consists of a pigment complex and electron acceptormolecules within the thylakoid membrane.Its the antenna forgathering solar energy. Electron Flow in Light Reactions
The light reactions utilize 2 photosystems, calledphotosystem I (PS I) and photosystem II (PSII). During light reactions, electrons usually follow anoncyclic pathway that begins with PS II. Electron Flow in Light Reactions
PS II, which consists of pigment complexes and electron- acceptor molecules, receives electrons from water aswater splits, releasing oxygen. The electron transport chain (ETC), consisting ofplastoquione and cytochrome complexes, carries electronsfrom PS II to PS I via redox reactions. PS I, which also consists of a pigment complex andelectron acceptor molecules, is adjacent to NADPreductase. The ATP synthase complex which has a channel and aprotruding ATP synthase, is an enzyme that joins ADP + P. Plants are Carbon Dioxide Fixers
Step 1: Fixation of Carbon Dioxide Carbon dioxide fixation is the first step in the calvincycle.During this step, CO2 is attached to RuBP(ribulose-1,5-bisphosphate) to form a 6 carbonmolecule.The resulting 6 carbon molecule is then splitinto two 3 carbon molecules. RuBP carboxylase is the enzyme that speeds thisreaction and makes up 20-50% of the protein content inchloroplasts. Plants as Carbon Dioxide Fixers
Step 2: Reduction of Carbon Dioxide 3PG (3-phosphoglycerate is reduced to G3P (glyceraldehyde-3- phosphate) in 2 steps: Plants of Carbon Dioxide Fixers
Step 3: Regeneration of RuBP For every 3 turns of the Calvin cycle, 5 molecules of G3P are usedto reform 3 molecules of RuBP.