Photosynthesis Ch. 7

Photosynthetic Organisms Photosynthesis coverts solar energy into the chemical energy of a carbohydrate. Photosynthetic organisms are called autotrophs because they produce their own food. With a few exceptions, it is possible to trace any food chain back to plants and algae.

Photosynthesis takes place in the green portions of plants. The raw materials for photosynthesis are water and carbon dioxide. The roots absorb water, while carbon dioxide enters a leaf through stromata. Then CO2 and H2O diffuse into chloroplasts, which carries 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 its semifluid interior called the stroma. A different membrane system within the stroma forms flattened sacs called thylakoids. The thylakoid membrane contains chlorophyll and other pigments capable of absorbing solar energy.

The Photosynthesis process The overall process of photosynthesis can be represented by the following equation:

In the reaction, CH2O represents carbohydrates. Photosynthesis is a redox reaction Lose of electrons is oxidation and gain of electrons is reduction. It takes a lot of energy to reduce CO2. Solar energy 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 the electron transport chain. As the electrons move, energy is released and captured to 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 reduce carbon dioxide to a carbohydrate in the stroma of chloroplasts. These reactions can occur during both day and night. During the Calvin Cycle, CO2 is taken up and then reduced to a carbohydrate that can later be converted to glucose. Chemical Energy (ATP, NADPH)  Chemical Energy (carbohydrate)

Pigments and Photosystems Pigment molecules absorb wavelengths of light. Most pigments absorb only some wavelengths and reflect or transmit the others. The pigments in chloroplasts are capable of absorbing portions of visible light called their absorption spectrum.

Pigments and Photosystems Photosynthetic organisms differ in the type of chlorophyll they contain. Chlorophyll a and chlorophyll b play a prominent role in photosynthesis. 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 when chlorophyll breaks down. A photosystem consists of a pigment complex and electron acceptor molecules within the thylakoid membrane. It’s the “antenna” for gathering solar energy.

Electron Flow in Light Reactions The light reactions utilize 2 photosystems, called photosystem I (PS I) and photosystem II (PSII). During light reactions, electrons usually follow a noncyclic 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 as water splits, releasing oxygen. The electron transport chain (ETC), consisting of plastoquione and cytochrome complexes, carries electrons from PS II to PS I via redox reactions. PS I, which also consists of a pigment complex and electron acceptor molecules, is adjacent to NADP reductase. The ATP synthase complex which has a channel and a protruding 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 calvin cycle. During this step, CO2 is attached to RuBP (ribulose-1,5-bisphosphate) to form a 6 carbon molecule. The resulting 6 carbon molecule is then split into two 3 carbon molecules. RuBP carboxylase is the enzyme that speeds this reaction and makes up 20-50% of the protein content in chloroplasts.

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 used to reform 3 molecules of RuBP.