Photosynthesis Chapter 10

Energy source Autotrophs: Producers Make own organic molecules Heterotrophs: Consumers

Photosynthesis

Organisms capture energy from sunlight Build food molecules Rich in chemical energy 6CO H 2 O ⇨ C 6 H 12 O 6 + 6H 2 O + 6O 2

Photosynthesis Captures only 1% of sun’s energy Provides energy for life

Photosynthesis

Leaf structure Stoma (Stomata) opening on leaf Exchange of gases. Chloroplasts Mesophyll layer of leaf

Chloroplasts Thylakoids: Internal membranes of chloroplasts Grana: Stacks of thylakoids Chlorophyll: Green pigment Captures light for photosynthesis Membranes of thylakoids

Chloroplasts Stroma: Semi-liquid substance Surrounds thylakoids Contain enzymes Make organic molecules from carbon dioxide

Chloroplasts

Fig. 10-3b 1 µm Thylakoid space Chloroplast Granum Intermembrane space Inner membrane Outer membrane Stroma Thylakoid

Chloroplasts Photosystem: Cluster of photosynthetic pigments Membrane of thylakoids Each pigment captures energy Photosystem then gathers energy Energy makes ATP, NADPH & organic molecules

NADP + Nicotinamide Adenine Dinucleotide Phosphate Coenzyme Electron carrier Reduced during light-dependent reactions Used later to reduce carbon Carbon dioxide forms organic molecules Photosynthesis is a redox reaction

Photophosphorylation Addition of phosphate group to ADP Light energy

Photosynthesis Occurs in 3 stages 1. Capturing energy from sun 2. Energy makes ATP Reducing power in NADPH 3. ATP & NADPH Power synthesis of organic molecules

Photosynthesis Light dependent reactions First 2 steps of photosynthesis Presence of light Light-independent reactions Formation of organic molecules Calvin cycle Can occur +/- light

Experimental history Jan Baptista van Helmont Plants made their own food Joseph Priestly Plants “restored” the air

Experimental history Jan Ingenhousz Sun’s energy split CO 2 Carbon & Oxygen Oxygen was released into air Carbon combined with water Make carbohydrates

Experimental history Fredrick Forest Blackman 1. Initial “light” reactions are independent of temperature 2. Second set of “dark” reactions are independent of light Dependent on CO 2 concentrations & temperature Enzymes involved in light-independent reactions

Experimental history C.B. van Neil Looked at light in photosynthesis Studied photosynthesis in Bacteria

C.B. van Neil CO 2 + 2H 2 S  (CH 2 O) + H 2 O + 2S CO 2 + 2H 2 A  (CH 2 O) + H 2 O + A 2 CO 2 + 2H 2 O  (CH 2 O) + H 2 O + O 2

C.B. van Neil O 2 produce from plant photosynthesis comes from splitting water Not carbon dioxide Carbon Fixation: Uses electrons & H + from splitting water Reduces carbon dioxide into organic molecules (simple sugars). Light-independent reaction

CO 2 + 2H 2 O  (CH 2 O) + H 2 O + O 2

Photosynthesis 1. Chloroplasts 2. Light-dependent reactions Sun’s energy makes NADPH & ATP 3. Light-independent reactions ATP & NADPH CO 2 into organic molecules

Light Fig H2OH2O Chloroplast Light Reactions NADP + P ADP i + ATP NADPH O2O2 Calvin Cycle CO 2 [CH 2 O] (sugar)

Sunlight UV light from sun Source of energy when life began UV light can cause mutations in DNA Lead to skin cancer

Light Photon: Packets of energy UV light photons have greater energy than visible light UV light has shorter wavelengths

Light Visible light Purple shorter wavelengths More energetic photons Red longer wavelengths Less energetic photons

Spectrum

Absorption Spectrums Photon of energy strikes a molecule Absorbed by the molecule or lost as heat Depends on energy in photon (wavelength) Depends on atom’s available energy levels Specific for each molecule

Pigments Molecules Absorb energy in visible range Chlorophylls & Carotenoids Chlorophyll a & b Absorb photons in the blue-violet & red light

Pigments Chlorophyll a main pigment of photosynthesis Converts light energy to chemical energy Chlorophyll b & carotenoids are accessory pigments Capture light energy at different wavelengths

Pigments

Chlorophyll b Carotenoids Chlorophyll a

Chlorophyll structure Located in thylakoid membranes A porphyrin ring with a Mg in center Hydrocarbon tail Photons are absorbed by the ring Absorbs photons very effectively Excites electrons in the ring

Chlorophyll structure

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Carotenoids Two carbon rings attached by a carbon chain Not as efficient as the Chlorophylls Beta carotene (helps eyes) Found in carrots and yellow veggies

Photosystems Captures the light Located on surface of the photosynthetic membrane Chlorophyll a molecules Accessory pigments (chlorophyll b & carotenoids) Associated proteins

Photosystems Consists of 2 components 1. Antenna (light gathering) complex 2. Reaction center

Photosystem 1. Antenna complex Gathers photons from sun Web of Chlorophyll a molecules Held by proteins in membrane Accessory pigments carotenoids Energy is passed along the pigments to reaction center

Photosystems 2. Reaction centers 2 special chlorophyll a molecules Accept the energy Chlorophyll a than passes the energized electron to an acceptor Acceptor is reduced (quinone)

Photosystem

Fig THYLAKOID SPACE (INTERIOR OF THYLAKOID) STROMA e–e– Pigment molecules Photon Transfer of energy Special pair of chlorophyll a molecules Thylakoid membrane Photosystem Primary electron acceptor Reaction-center complex Light-harvesting complexes

2 photosystems Photosystem I (older) Absorbs energy at 700 nm wavelength Generates NADPH Photosystem II (newer) Absorbs energy at 680 nm wavelength Splits water (releases oxygen) Generates ATP 2 systems work together to absorb more energy

Photosynthesis (Process) Light dependent reactions Linear electron flow Energy transfer Thylakoid membranes

Light dependent reactions Photosystem II (680 nm) Light is captured by pigments Excites an electron (unstable) Energy is transferred to reaction center (special chlorophyll) Passes excited electron to an acceptor molecule

Light dependent reactions PS II is oxidized Water splits (enzyme) Water donates an electron to chlorophyll Reduces PS II Oxygen (O 2 ) is released with 2 protons (H + )

Light dependent reactions Electron is transported to PS I (700 nm) Electron is passed along proteins in the membrane (ETC) Protons are transported across the membrane Protons flow back across the membrane & through ATP synthase Generate ATP

Light dependent reactions At the same time PS I received light energy Excites an electron Primary acceptor accepts the electron PS I is excited Electron from PS II is passed to PS I Reduces the PS I

Light dependent reactions PS I excited electron is passed to a second ETC Ferredoxin protein NADP + reductase catalyzes the transfer of the electron to NADP + Makes NADPH

Pigment molecules Light P680 e–e– Primary acceptor 2 1 e–e– e–e– 2 H + O2O2 + 3 H2OH2O 1/21/2 4 Pq Pc Cytochrome complex Electron transport chain 5 ATP Photosystem I (PS I) Light Primary acceptor e–e– P700 6 Fd Electron transport chain NADP + reductase NADP + + H + NADPH 8 7 e–e– e–e– 6 Fig Photosystem II (PS II)

Fig. 10-UN1 CO 2 NADP + reductase Photosystem II H2OH2O O2O2 ATP Pc Cytochrome complex Primary acceptor Primary acceptor Photosystem I NADP + + H + Fd NADPH Electron transport chain Electron transport chain O2O2 H2OH2O Pq

Enhancement effect

Fig Light Fd Cytochrome complex ADP + i H+H+ ATP P synthase To Calvin Cycle STROMA (low H + concentration) Thylakoid membrane THYLAKOID SPACE (high H + concentration) STROMA (low H + concentration) Photosystem II Photosystem I 4 H + Pq Pc Light NADP + reductase NADP + + H + NADPH +2 H + H2OH2O O2O2 e–e– e–e– 1/21/

Fig Key Mitochondrion Chloroplast CHLOROPLAST STRUCTURE MITOCHONDRION STRUCTURE Intermembrane space Inner membrane Electron transport chain H+H+ Diffusion Matrix Higher [H + ] Lower [H + ] Stroma ATP synthase ADP + P i H+H+ ATP Thylakoid space Thylakoid membrane

Photosystems Noncyclic photophosphorylation 2 systems work in series Produce NADPH & ATP Replaces electrons from splitting water System II (splits water)works first then I (NADPH)

Photosystems When more ATP is needed Plant changes direction Electron used to make NADPH in PS I is directed to make ATP

Calvin Cycle Named for Melvin Calvin Cyclic because it regenerates it’s starting material C 3 photosynthesis First organic compound has 3 carbons

Calvin cycle Combines CO 2 to make sugar Using energy from ATP Using reducing power from NADPH Occurs in stroma of chloroplast

Calvin Cycle Consists of three parts 1. Fixation of carbon dioxide 2. Reduction-forms G3P (glyceraldehyde 3-phosphate) 3. Regeneration of RuBP (ribulose 1, 5 bisphosphate)

Calvin Cycle 3 cycles 3 CO 2 molecules 1 molecule of G3P 6 NADPH 9 ATP

Fixation of carbon CO 2 combines with Ribulose 1, 5 bisphosphate (RuBP) Temporary 6 carbon intermediate Splits-forms 2- three carbon molecules 3-phosphoglycerate (PGA) Large enzyme that catalyses reaction (Rubisco) Ribulose bisphosphate carboxylase/oxygenase

Reduction Phosphate is added to 3- phosphoglycerate 1,3 Bisphosphoglycerate NADPH reduces the molecule Glyceraldehyde 3-phosphate (G3P)

Regeneration 5 molecules of G3P are rearranged to make 3 RuBP Uses 3 more ATP

Fig Ribulose bisphosphate (RuBP) 3-Phosphoglycerate Short-lived intermediate Phase 1: Carbon fixation (Entering one at a time) Rubisco Input CO 2 P P P P P ATP 6 6 ADP P P 6 1,3-Bisphosphoglycerate 6 P P NADP + NADPH i Phase 2: Reduction Glyceraldehyde-3-phosphate (G3P) 1 P Output G3P (a sugar) Glucose and other organic compounds Calvin Cycle 3 3 ADP ATP 5 P Phase 3: Regeneration of the CO 2 acceptor (RuBP) G3P

Fig. 10-UN2 Regeneration of CO 2 acceptor 1 G3P (3C) Reduction Carbon fixation 3 CO 2 Calvin Cycle 6  3C 5  3C 3  5C

Calvin Cycle 3 CO 2 enter cycle & combine with RuBP Generates 3 molecules more of RuBP & one G3P (glyceraldehyde 3- phosphate) G3P can be made into glucose & other sugars

Calvin Cycle Enzyme mediated 5 of these enzymes need light to be more efficient Net reaction 3CO ATP + 6NADPH ⇨ G3P + 8P i + 9ADP + 6NADP+

G3P Converted to fructose 6-phosphate (reverse of glycolysis) Made into sucrose Happens in cytoplasm Intense photosynthesis G3P levels rise so much some is converted to starch

Fig Light Reactions: Photosystem II Electron transport chain Photosystem I Electron transport chain CO 2 NADP + ADP P i + RuBP 3-Phosphoglycerate Calvin Cycle G3P ATP NADPH Starch (storage) Sucrose (export) Chloroplast Light H2OH2O O2O2

Photorespiration Hot Stoma in leaf close to avoid loosing water Carbon dioxide cannot come in Oxygen builds up inside Carbon dioxide is released G3P is not produced

Photorespiration Happens when Rubisco oxidizes RuBP (starting molecules of Calvin cycle) Oxygen is incorporated into RuBP Undergoes reactions that release CO 2 CO 2 & O 2 compete for same sight on the enzyme Under conditions greater than the optimal 25 0 C this process occurs more readily

C 4 Photosynthesis Process to avoid loosing carbon dioxide Plant fixes carbon dioxide into a 4 carbon molecule (oxaloacetate) PEP carboxylase (enzyme) Oxaloacetate is converted to malate Then taken to stroma for Calvin cycle Sugarcane and corn

CAM Process to prevent loss of CO 2 Plants in dry hot regions (cacti) Reverse what most plants do Open stoma at night Allows CO 2 to come in & water to leave Close them during the day.

CAM Carbon fix CO 2 at night into 4 carbon chains (organic acids) Use the Calvin cycle during the day.

Fig CO 2 Sugarcane Mesophyll cell CO 2 C4C4 Bundle- sheath cell Organic acids release CO 2 to Calvin cycle CO 2 incorporated into four-carbon organic acids (carbon fixation) Pineapple Night Day CAM Sugar Calvin Cycle Calvin Cycle Organic acid (a) Spatial separation of steps (b) Temporal separation of steps CO 2 1 2