Photosynthesis

The Light Reactions Obtaining Energy – Autotrophs- organisms that use energy from sunlight or from chemical bonds in inorganic substances to make organic compounds like sugar – Photosynthesis- process used to convert light energy from the sun into chemical energy in the form of organic compounds, primarily carbohydrates – Heterotrophs- organisms that must get energy from food instead of directly from sunlight or inorganic substances – /CDA7_1/CDA7_1b/CDA7_1b.htm /CDA7_1/CDA7_1b/CDA7_1b.htm

Overview of photosynthesis – Can be divided into 2 stages Light reactions: light energy is converted to chemical energy, which is temporarily stored in ATP and the energy carrier molecule NADPH Calvin Cycle: organic compounds formed using CO2 and chemical energy stored in ATP and NADPH – Photosynthesis equation – /498596/CDA7_1/CDA7_1d/CDA7_1d.htm /498596/CDA7_1/CDA7_1d/CDA7_1d.htm

Capturing light energy – Light reactions occur in the thylakoids of chloroplasts Inside the chloroplast is a system of membranes called thylakoids that are arranged as flattened sacs Grana- stacks of thylakoids Stroma- fluid that surrounds grana

Capturing light energy – Pigments- compounds that absorb light – Chloroplast pigments Chlorophyll a- directly involved in light reactions Chlorophyll b- assists chlorophyll a in capturing light energy (accessory pigment) Carotenoids- yellow, orange, brown accessory pigments

Converting light energy into chemical energy – Photosystem- cluster of pigment molecules and the proteins that the pigment molecules are embedded in – Steps of light reactions 1. light energy forces electrons to enter higher energy level in photosystem II 2. These e- move along thylakoid membrane to primary e- acceptor

3. e- move through electron transport chain, losing energy; protons (H+) pumped into thylakoid 4. light energy forces electrons to enter higher energy level in photosystem I; e- from phtotsystem II replace photosystem I electrons: Photosystem II e- replaced by splitting water

5. e- from photosystem I move through a primary electron acceptor and then through another electron transport chain; e- combine with NADP+ to form NADPH 596/CDA7_1/CDA7_1f/CDA7_1f.htm 596/CDA7_1/CDA7_1f/CDA7_1f.htm

Making ATP – Chemiosmosis- process used to make ATP in the light reaction – The gradient of protons (H+) inside the thylakoid represent potential energy – These protons diffuse through ATP synthase, which uses this energy to add a phosphate to ADP to make ATP