Photosynthesis

Ecological Significance

 Photosynthesis- the process that converts light energy to chemical energy  Autotrophs- “self + feeders”  Photoautotrophs- “light + self + feeders”  Heterotrophs- “ other + feeders”  Consumers- eat other things  Decomposers- break down dead organic material

Chloroplast Structure  Leaf Structure  The leaf is the major site of photosynthesis  Stomata- small pores in the leaf where CO 2 enters an O 2 and H 2 O leave  Mesophyll  Tissue in center of the leaf  Where chloroplasts are found

Chloroplast Structure  Double Membrane  Stroma- fluid inside the inner membrane but outside the thylakoid  Thylakoid interconnected membranous sacks  Granna (granum)- stacks of thylakoids  Thylakoid Space- the space inside the thylakoid  Chlorophyll- green pigment found inside (embedded in) the thylakoid membrane

Chemical Context of Photosynthesis  General Equation 6CO 2 + 6H 2 O + light energy → C 6 H 12 O 6 + 6O 2  Redox Reaction  Reverse of cellular respiration  Electrons increase in potential energy as they move from water to sugar  Anabolic reaction- builds up large organic molecules from smaller ones  Requires energy that is supplied by the sun

Light and Pigments  Pigments- molecules that absorb light  Visible Light- 750nm-380nm (ROYGBIV)  Absorbance vs. Transmittance/Reflection  Absorbance- wavelengths that are absorbed (colors that you don’t see)  Transmittance- wavelengths that bounce off or go through a substance (colors that you do see)

Plant Pigments  Chlorophyll a- reflects blue-green wavelengths (all others are captured)  Chlorophyll b- reflects yellow-green wavelengths (all others are captured)  Carotenoids- reflects yellow and orange wavelengths  Major function of carotenoids= photoprotection- absorbs excessive light energy that could otherwise be harmful

The Big Picture of Photosynthesis

Light Reactions  “photo” = light  Also known as: Light Dependent Reactions  Location: thylakoid membrane (photosystems I & II and the electron transport chain)  Input: light (photons) + H 2 O + NADP + + ADP + P  Output: O 2 + NADPH + ATP  Reaction: light + H 2 O → O 2

Calvin Cycle  “synthesis”= make, put together  Also known as: Light Independent Reactions, Dark Reactions  Location: stroma  Input: NADPH + ATP + CO 2  Output: G3P (later converted to glucose) + NADP + + ADP + P  Reaction: CO 2 → C 6 H 12 O 6

The Light Reaction  Photosystems- a protein complex embedded in the thylakoid membrane that contains two parts:  Reaction Center- protein complex that holds a special chlorophyll a molecule and a primary electron acceptor  Light-harvesting complex- pigment molecules that capture light and transfer excited electrons to the special chlorophyll a molecues

Two Kinds of Photosystems  Photosystem II:  Special chlorophyll a= P680  Functions 1 st  Photosystem I:  Special chlorophyll a= P700  Functions 2 nd

P680 and P700  Very unstable when oxidized so electrons must be replenished immediately  P700 is replenished with electrons moving down the ETC from P680  P680 is replenished by the splitting of water molecules

Noncyclic Electron Flow

Cyclic Electron Flow  the calvin cycle uses more ATP than NADPH  a build up of NADPH triggers the switch from noncyclic to cyclic electron flow  Uses only PSI  Electrons cycle back from Ferredoxin (Fd) to the cytochrome complex to replenish P700  Does not produce NADPH or O 2 but does produce ATP

Chemiosmosis in the Chloroplast  Energy from sunlight, not food  Build up of H + occurs in the thylakoid space  Enzyme: ATP synthase  Overall Result of the Light Reactions: solar power generates ATP and NADPH providing chemical energy and reducing power to the Calvin cycle

Overall Result of the Light Reaction  Solar power generates ATP and NADPH providing chemical energy and reducing power to the Calvin cycle

The Clavin Cycle  Input: 3 CO 2 (one at a time) + 9 ATP + 6NADPH  Output: glucose

Three Major Steps of the Calvin Cycle  Carbon Fixation- carbon is incorporated into organic forms from carbon dioxide  CO 2 is converted into carbohydrates using the energy provided by ATP and NADPH from the light reactions

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