Photosynthesis: Capturing and Converting Energy In the process of photosynthesis, plants convert the energy of sunlight into the energy in chemical bonds of sugars and starches (carbohydrates). carbon dioxide + water glucose + oxygen 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2 sunlight

Photosynthesis Photosynthesis involves two sets of reactions: the light-dependent & light-independent reactions The light-dependent reactions 1. L ight absorption : P hotosynthesis takes place in the chloroplast. The chloroplast contain saclike photosynthetic membranes that have chlorophyll and other pigment molecules. This membrane is called the thylakoid.

Photosynthesis: Capturing and Converting Energy

Chlorophyll and the pigment molecules are used in what’s called a photosystem. Green plants contain two photosystems (II & I) Photosynthesis starts when light is absorbed by one of the pigment molecules in photosystem II. This energy is passed among pigment molecules until it reaches the reaction center. High energy electrons are released from here.

Photosynthesis: Capturing and Converting Energy

2. Electron transport After release from the reaction center, the high-energy electrons are moved by a series of electron carriers in the photosynthetic membrane. This process is called electron transport.

Photosynthesis: Capturing and Converting Energy At the end of this process, the high-energy electrons are passed to the electron carrier NADP +, converting it to NADPH. This compound will be used later in the light-independent reactions.

Photosynthesis: Capturing and Converting Energy 3. Oxygen production The high-energy electrons removed from the chlorophyll and used in electron transport (mentioned above), are replaced by electrons from a water molecule. To get the electrons the water molecule is split thereby leaving a molecule of oxygen gas which is released into the air.

Photosynthesis: Capturing and Converting Energy 4. ATP Formation The splitting of water molecules to replace the electrons results in the build up of hydrogen ions on the inside of the membrane. This results in the photosynthetic membrane being positively charged on the inside and negatively charged on the outside.

Photosynthesis: Capturing and Converting Energy This difference in charge is an energy source that is used to make ATP from ADP. ATP (adenosine triphosphate) is the compound that stores energy in cells

Photosynthesis: Capturing and Converting Energy

Adenosine Triphosphate (ATP)

ATP

Photosynthesis: Capturing and Converting Energy Summary: The light reaction uses sunlight and water to produce oxygen and two energy-storing compounds, NADPH and ATP.

Investigating Photosynthesis Early researchers wondered where the mass of a plant comes from as it grows. In the 17th century Jan van Helmont, based on his experiment, concluded that most of the mass a plant gains comes from water. Van Helmont's experiment accounts for the hydrate or water portion of the carbohydrate produced by photosynthesis.

Investigating Photosynthesis But, where does the carbon portion of the carbohydrate come from?