Photosynthesis: Capturing Energy Chapter 8

Light Composed of photons – packets of energy Visible light is a small part of the electromagnetic spectrum All energy travels as waves Wavelength is the distance from 1 wave peak to the next Shorter wavelengths have more energy than longer wavelengths

Visible light: nm Energy from visible light is used in photosynthesis Why? Longer wavelengths don’t have enough energy; higher wavelengths have too much TV and radio waves Micro- waves Infrared Visible UV X-rays Gamma rays Color spectrum of visible light Red Orange Yellow Green Blue Violet 760 nm 700 nm 600 nm 500 nm 400 nm 380 nm One wavelength Longer wavelength Electromagnetic spectrum Shorter wavelength

Chloroplasts Organelles enclosed by a double membrane Located mainly within mesophyll cells inside a leaf Each cell contains chloroplasts This portion of the leaf has many air spaces and a high water vapor content

Palisade mesophyll Vein Stoma Spongy mesophyll

Outer membraneThylakoids Intermembrane space Thylakoid membrane Stroma Inner membrane Granum (stack of thylakoids) Thylakoid lumen

Chlorophyll Main photosynthetic pigment: chlorophyll a (initiates the light-dependent reactions) Accessory pigments: chlorophyll b, carotenoids (these absorb different wavelengths of light and pass the energy on to chlorophyll a ) These pigments are found in the thylakoid membranes of chloroplasts Pigments reflect the color of light we see; absorb the other colors Is green light used during photosynthesis? Why or why not?

Overview of photosynthesis Hydrogens from water reduce carbon dioxide; oxygen from water is oxidized Photosynthesis is a redox reaction: 6 CO H 2 O  C 6 H 12 O O 2 reduction oxidation

Overview… Two phases: 1 st : Light-dependent (‘photo’) Occurs in the thylakoids of the chloroplasts H 2 O is split and molecular oxygen is released Electrons energized by light produce ATP and NADPH which are both needed for the endergonic next phase 2 nd : Carbon-fixation (‘synthesis’) Occurs in the stroma of the chloroplasts ATP and NADPH provide the energy needed for the formation of carbohydrates

Overview… Light reactionsCarbon-fixation reactions Light reactions Calvin cycle ATP ADP NADPH NADP + H20H C0 2 carbohydrates

Light-dependent Reactions Occur in the thylakoids Energy is absorbed from light and converted to chemical energy stored in ATP and NADPH Oxygen is released Photosystem I and II both involved – these have similar pigments but different roles

Photosystems I and II Each system includes: Chlorophyll a molecules and associated proteins Multiple antenna complexes Photosystem I = P700 - wavelength absorbed Photosystem II = P680 - wavelength absorbed

Primary electron acceptor Photon Photosystem Chloroplast Thylakoid Antenna complexes Reaction center e–e–

Light reactions: Step #1 Light energy forces e - to a higher energy level in 2 chlorophyll a molecules of PS II (e - is excited) e - leave chlorophyll a (oxidation) a replacement e - is donated by H 2 O: 2H 2 O  4 H e - + O 2 This is noncyclic electron transport

Light reactions: Step #2 e - goes to a primary electron acceptor in the thylakoid membrane (reduction)

Light reactions: Step #3 e - donated from the primary electron acceptor to a series of molecules in the thylakoid membrane – the electron transport chain e - lose energy as they move through the chain – this energy moves H + into the thylakoid lumen this H + gradient will be used to produce ATP from ADP and P i using ATP synthase

Light reactions: Step #4 Light is absorbed by PS I e - leave chlorophyll a and go to another primary electron acceptor these e - are replaced by e - from the electron transport chain This is cyclic electron transport

Light reactions: Step #5 e - from the primary electron acceptors in PS I go to another electron transport chain on the stroma side of the thylakoid membrane e - with H + and NADP +  NADPH ATP + NADPH made during the light reactions are both needed to power the carbon-fixation reactions

Primary electron acceptor Primary electron acceptor NADPH NADP + H2OH2O ATP O2O2 ADP 1 2 Photosystem II(P680) Production of ATP by chemiosmosis H+H+ Ferredoxin Plastiquinone Cytochrome complex Plastocyanin 1/2+ 2 H+H+ PiPi A0A0 A1A1 FeS x FeS B FeS A Photosystem I(P700) (from medium) Electron transport chain Electron transport chain

ATP Synthesis and electron transport… the main ideas Electrons (e - ) move down the electron transport chain and release energy as they go Protons (H + ) move from the stroma to the thylakoid lumen, creating a proton gradient The greater concentration of H + lowers the pH The thylakoid membrane is impermeable to H + except through ATP synthase The flow of H + through ATP synthase generates ATP

Carbon-fixation reactions Also known as the Calvin Cycle or the light-independent reactions Occur in the stroma CO 2 + chemical energy from ATP and NADPH are used to make organic compounds – carbon is ‘fixed’

Three phases of the Calvin Cycle 1.CO 2 uptake 2.Carbon reduction 3.RuBP regeneration

Carbon fixation: Step #1 CO 2 uptake: CO 2 + RuBP*  unstable 6-C molecule, which splits  2 3-C molecules + 3 PGA** *RuBP = ribulose biphosphate (5 C) **PGA = 3 phosphoglycerate

Carbon fixation: Step #2 Carbon reduction: 2 molecules of 3-PGA are converted to 2 molecules of G3P* in a 2-part process, using the energy from ATP and the H + from NADPH from the light reactions * G3P = glyceraldehdye 3-phosphate

Carbon fixation: Step #3 RuBP regeneration: One molecule of G3P leaves the Calvin cycle to be converted into carbohydrates such as glucose or starch The other G3P molecule uses the energy from ATP and is converted back to RuBP The RuBP is returned back to the Calvin cycle

CO 2 molecules are captured by RuBP, resulting in an unstable intermediate that is immediately broken apart into 2 PGA PGA is phosphorylated by ATP and reduced by NADPH. Removal of a phosphate results in formation of G3P. Through a series of reactions G3P is rearranged into new RuBP molecules or another sugar Glucose and other carbohydrate synthesis 2 molecules of glyceraldehyde-3- phosphate (G3P) 6 molecules of ribulose bisphosphate (RuBP) CALVIN CYCLE Carbon reduction phase RuBP regeneration phase CO 2 uptake phase 12 NADPH 12 ADP 12 molecules of phosphoglycerate (PGA) 12 ATP 6 molecules of CO 2 10 molecules of G3P 6 molecules of ribulose phosphate (RP) ATP 6 ADP P P P P P PP P 12 molecules of glyceraldehyde-3- phosphate (G3P) NADP +

Adjustments based on weather… C 3 plants – use the C 3 pathway – the initial carbon fixation product is a 3-C sugar These plants must close their stomata during hot, dry weather to reduce water loss This reduces carb production Adaptations for hot, dry environments: C 4 plants – 1 st fix CO 2 into a 4-C compound CAM plants – fix CO 2 at night (cactus)

Table 8-2 p. 167