Photosynthesis Overview  Only 5% of the light that hits earth's surface is converted to organic compounds by photosynthesis. 6 CO 2 (g) + 6 H 2 O(I) + light energy  C 6 H 12 O 6 (aq) + 6 O 2 (g)

Light  Light travels in wave packets called photons  Light from the sun is a mixture of photons of different energies (called wavelengths, measured in nanometers).  Photons are captured in plants by plastids.

Chloroplasts  Photosynthesis factories for plants and algae  Store excess glucose as starch. Structure:  2 membranes (like mitochondria)  Interior space filled with stroma (protein-rich fluid)  In the stroma are thylakoids (membrane bound sacs)

Thylakoids  Grana: stack of thylakoids (~60 grana per chloroplast with thylakoids each)  Lamellae: unstacked thylakoids that link grana together.  Lumen: interior of the thylakoid, water- filled  The thylakoid membrane contains light gathering (photosynthetic) pigments

Pigments  Green plants contain chlorophyll a and b  both absorb blue and red light and reflect green light  So we assume blue and red light is used in photosynthesis  Chloropyhll a is used to transfer energy from light into chemical energy.  Chlorophyll b is an accessory pigment. It absorbs photons that a misses.

Accessory Pigments  In addition to chlorophyll b there are other accessory pigments. Their job is to absorb light that can damage chlorophyll and lose it as heat instead.  Carotenoids: appear yellow/orange (ie. Carrots).  Betacarotene protects our eyes  Xanthophylls: reflect yellow light, also found in the thylakoid

Photosystems  Photosynthetic pigments are embedded in the thylakoid membrane clustered in groups called photosystems.  A photosystem contains a chlorophyll a molecule called a reaction centre, surrounded by accessory pigments called the antenna complex.  The antenna complex harvests the light and passes it to the reaction centre.

P I (P700) and P II (P680)  Two types of photosystems exist, Photosystem I (PI) and PII.  They have identical reaction centres, but they are associated with different protein molecules.  As a result they absorb light at slightly different wavelengths.  The reaction center in PI is called P700; absorption peaks at 700nm  PII reaction centre is P680, absorption peaks at 680nm