P HOTOSYNTHESIS The Dark and Light Reactions

E NERGY IS ESSENTIAL TO ALL LIVING THINGS. All energy on earth originates in nuclear reactions in the sun.

P RODUCERS PLANTS AND OTHER ORGANISMS CAPABLE OF CARRYING OUT PHOTOSYNTHESIS 6C H 2 0+ energy  C 6 H

P HOTOSYNTHESIS : 6 CO H 2 O + light energy  C 6 H 12 O 6 + 6O 2

P HOTOSYNTHESIS T AKES P LACE IN THE C HLOROPLAST

L IGHT R EACTIONS : OCCUR ON THE THYLAKOID MEMBRANES convert light and water into chemical energy produce O 2 as a waste product

H OW L IGHT W ORKS Visible radiation drives the light reactions Light is a type of energy called electromagnetic energy. Different pigments embedded in thylakoid membranes

D IFFERENT PIGMENTS ABSORB LIGHT OF DIFFERENT WAVELENGTHS. The wavelengths absorbed by the light harvesting pigments are mainly in the blue and red range This is why most plants appear green The light harvesting pigments of chloroplasts are 1. chlorophyll a: absorbs in blue and red 2. chlorophyll b: absorbs in blue and orange 3. carotenoids: absorbs mainly blue-green range

Having all these different pigments broadens the range of visible light that is usable for photosynthesis. Only chlorophyll a passes electrons to the electron transport chain and thus is directly involved in the light reactions. The other pigments convey the light energy to chlorophyll a.

T HE LIGHT R EACTIONS Photosystems I and II

S TEP 1: Pigments in photsystems II absorb light Energy from the light is absorbed by electrons (increasing energy level) High-energy electrons are passed on to the electron transport chain H 2 O molecules provide new electrons to the chlorophyll to replace the ones that were lost Enzymes on the inner surface of the thylakoid membrane break up each H2O molecule into 2 high-energy electrons Oxygen is released into the air as oxygen gas 2 H+ ions are released inside the thylakoid membrane

S TEP 2: High-energy electrons move through the electron transport chain from photosystem II to photosystem I b.Energy is used from the electrons to transport the H+ from the stroma into the inner thylakoid

S TEP 3: Pigments in photosystem I use energy from light to reenergize the electrons b.NADP+ picks up high-energy electrons on the outer surface of the thylakoid membrane plus a H+ ion and becomes NADPH

S TEP 4: H+ ions released during water-splitting and electron transport the inside of the thylakoid membrane becomes positively charged and the outside negatively charged b.The difference in charges provides the energy to make ATP

S TEP 5: H+ ions CANNOT directly cross the membrane b.ATP synthase (protein) allows H+ ions to pass through it c.As H+ ions pass through this protein, the protein rotates and ATP synthase binds to ADP and a phosphate group – producing ATP

Water splits into H+ ions electrons and Oxygen Electrons H+ ions Oxygen WATER PHOTOLYSIS: the splitting of water

P HOTOSYNTHESIS : T HE D ARK R EACTIONS ATP + NADPH 2 + CO 2  C 6 H 12 O 6

D ARK R EACTIONS : OCCUR IN THE STROMA Carbon dioxide is split, providing carbon to make sugars. The ultimate product is glucose. While this system depends on the products from the light reactions, it does not directly require light energy.

C ARBON F IXATION 3 CO 2 molecules attach to 3, 5 carbon sugars (RuBP) A 6 carbon sugar is formed and O 2 is released This reaction is catalyzed by the enzyme Rubisco. This 6 carbon sugar rearranges and splits into 2, 3 carbon sugars (PGA)

R EDUCTION 6 ATP contribute phosphate groups 8 NADPH drop off electrons These are used to make the high energy compound G3P

R EGENERATION 1 G3P is used to form glucose The others are used to regenerate the original 5 carbon sugar 9 ATP and 6 NADPH are used per turn of the calvin cycle These are regenerated by Light Reactions 2 G3P = Glucose