Photosynthesis Chapter 10

Photosynthesis

Photosynthesis Process where organisms capture energy from sunlight Build food molecules Rich in chemical energy 6CO2 + 12H2O C6H12O6 + 6H2O + 6O2

Photosynthesis Captures only 1% of the energy of the sun Uses it to provide energy for life

Photosynthesis Autotrophs: Producers Make own organic molecules Heterotrophs: Consumers

Photosynthesis Plant leaves - Chloroplasts

Chloroplasts Thylakoids: Internal membranes of chloroplasts Membranes of thylakoids contain chlorophyll Green pigment that captures the light for photosynthesis Grana: Stacks of thylakoids

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

Chloroplasts

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

Chloroplasts Photosystem: Cluster of photosynthetic pigments In membrane of thylakoids Each pigment in system captures energy Photosystem then gathers energy Energy makes ATP, NADPH and organic molecules

Stoma (Stomata) Opening on the leaf Allows exchange of gases.

Nicotinamide Adenine Dinucleotide Phosphate NADP+ Nicotinamide Adenine Dinucleotide Phosphate Coenzyme Electron carrier Reduced during the light-dependent reactions Used later to reduce carbon in carbon dioxide to form 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 the sun 2. Energy makes ATP Reducing power in NADPH 3. ATP and NADPH Power synthesis of organic molecules from carbon dioxide

Photosynthesis Light dependent reactions First 2 steps of photosynthesis Take place in 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 the CO2 into Carbon & Oxygen Oxygen was released into the air Carbon combined with water to 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 CO2 concentrations & temperature Enzymes must be involved in the light-independent reactions

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

C.B. van Neil CO2 + 2H2S  (CH2O) + H2O + 2S CO2 + 2H2A  (CH2O) + H2O + A2 CO2 + 2H2O  (CH2O) + H2O + O2

C.B. van Neil O2 produce from green plant photosynthesis comes from splitting the water Not carbon dioxide Carbon Fixation: Uses H+ from spitting of water to reduce carbon dioxide into organic molecules (simple sugars). Light-independent reaction

Photosynthesis 1. Occurs in the chloroplasts 2. Light-dependent reactions use light to reduce NADP+ and manufacture ATP 3. ATP and NADPH will be used later in the light-independent reactions Incorporate carbon dioxide into organic molecules

i CO2 Light NADP+ ADP Calvin Cycle Light Reactions ATP NADPH Fig. 10-5-4 H2O CO2 Light NADP+ ADP + P i Calvin Cycle Light Reactions ATP NADPH Chloroplast [CH2O] (sugar) O2

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

Light Photon: Packets of energy UV light has photons with greater energy than visible light UV light has shorter wavelengths X-Rays have shorter wavelengths then UV & more energy.

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

Spectrum

Spectrum

Absorption Spectrums Photon of energy strikes a molecule Lost as heat or absorbed by the molecule Depends on amount of energy in the photon (wavelength) Dependent on the atom’s available energy levels

Absorption spectrum Specific for each molecule Range & efficiency of photons it is capable of absorbing

Pigments Molecules that are good absorbers of energy in the 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

Pigments Chlorophyll b Chlorophyll a Carotenoids

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

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 the sun Web of Chlorophyll a molecules Tightly held by proteins in the 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

(INTERIOR OF THYLAKOID) Fig. 10-12 Photosystem STROMA Photon Primary electron acceptor Light-harvesting complexes Reaction-center complex e– Thylakoid membrane Pigment molecules Transfer of energy Special pair of chlorophyll a molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID)

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 the pigments Excites an electron (unstable) Energy is transferred to the reaction center (special chlorophyll) Passes the excited electron to an acceptor molecule

Light dependent reactions PS II is oxidized Water splits (enzyme) Water donates an electron to the chlorophyll Reduces PS II Oxygen (O2) 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

Electron transport chain Fig. 10-13-5 Electron transport chain Primary acceptor Primary acceptor 4 7 Electron transport chain Fd Pq e– 2 e– 8 e– H2O e– NADP+ + H+ Cytochrome complex 2 H+ NADP+ reductase + 1/2 O2 3 NADPH Pc e– e– P700 5 P680 Light 1 Light 6 6 ATP Pigment molecules Photosystem I (PS I) Photosystem II (PS II)

Electron transport chain Electron transport chain NADP+ + H+ Fig. 10-UN1 H2O CO2 Primary acceptor Electron transport chain Primary acceptor Electron transport chain Fd NADP+ + H+ H2O Pq NADP+ reductase O2 Cytochrome complex NADPH Pc Photosystem I ATP Photosystem II O2

Enhancement effect

Enhancement effect

(low H+ concentration) Cytochrome complex Photosystem II Photosystem I Fig. 10-17 STROMA (low H+ concentration) Cytochrome complex Photosystem II Photosystem I 4 H+ Light NADP+ reductase Light Fd 3 NADP+ + H+ Pq NADPH e– Pc e– 2 H2O 1 1/2 O2 THYLAKOID SPACE (high H+ concentration) +2 H+ 4 H+ To Calvin Cycle Figure 10.17 The light reactions and chemiosmosis: the organization of the thylakoid membrane Thylakoid membrane ATP synthase STROMA (low H+ concentration) ADP + ATP P i H+

H+ Diffusion Electron transport chain ADP + P Fig. 10-16 Mitochondrion Chloroplast MITOCHONDRION STRUCTURE CHLOROPLAST STRUCTURE H+ Diffusion Intermembrane space Thylakoid space Electron transport chain Inner membrane Thylakoid membrane Figure 10.16 Comparison of chemiosmosis in mitochondria and chloroplasts ATP synthase Matrix Stroma Key ADP + P i ATP Higher [H+] H+ Lower [H+]

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 The 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 C3 photosynthesis First organic compound has 3 carbons

Calvin cycle Combines CO2 to make sugar Using energy from ATP Using reducing power from NADPH Occurs in the stroma of the 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 CO2 molecules 1 molecule of G3P 6 NADPH 9 ATP

Fixation of carbon Carbon dioxide combines with ribulose 1, 5 bisphosphate (RuBP) Temporary 6 carbon intermediate Two three carbon molecules called 3-phosphoglycerate (PGA) Ribulose bisphosphate carboxylase/oxygenase (Rubisco) is the large enzyme that catalyses the reaction

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. 10-18-3 Input 3 (Entering one at a time) CO2 Phase 1: Carbon fixation Rubisco 3 P P Short-lived intermediate 3 P P 6 P Ribulose bisphosphate (RuBP) 3-Phosphoglycerate 6 ATP 6 ADP 3 ADP Calvin Cycle 6 3 P P ATP 1,3-Bisphosphoglycerate 6 NADPH Phase 3: Regeneration of the CO2 acceptor (RuBP) 6 NADP+ 6 P i 5 P G3P 6 P Glyceraldehyde-3-phosphate (G3P) Phase 2: Reduction 1 P Glucose and other organic compounds Output G3P (a sugar)

3  5C 6  3C Calvin Cycle Regeneration of CO2 acceptor 5  3C Fig. 10-UN2 3 CO2 Carbon fixation 3  5C 6  3C Calvin Cycle Regeneration of CO2 acceptor 5  3C Reduction 1 G3P (3C)

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

Calvin Cycle Enzyme mediated 5 of these enzymes need light to be more efficient Net reaction 3CO2 + 9 ATP + 6NADPH G3P + 8Pi + 9ADP + 6NADP+

G3P G3P (glyceraldehyde 3-phosphate) Converted to fructose 6-phosphate (reverse of glycolysis) It is made into sucrose This occurs in the cytoplasm Intense photosynthesis G3P levels rise so much some is converted to starch

Electron transport chain Fig. 10-21 H2O CO2 Light NADP+ ADP + P i Light Reactions: Photosystem II Electron transport chain Photosystem I RuBP 3-Phosphoglycerate Calvin Cycle ATP G3P Figure 10.21 A review of photosynthesis Starch (storage) NADPH Chloroplast O2 Sucrose (export)

Photorespiration When hot the stoma in a leaf close to avoid loosing water Carbon dioxide cannot come in. Oxygen builds up inside Carbon dioxide is released G3P is not produced

Photorespiration Occurs when Rubisco oxidizes RuBP (starting molecules of Calvin cycle) Oxygen is incorporated into RuBP Undergoes reactions that release CO2 Carbon dioxide & oxygen compete for the same sight on the enzyme Under conditions greater than the optimal 250C this process occurs more readily

C4 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 the stroma for the Calvin cycle Sugarcane and corn

CAM Another process to prevent loss of CO2 Plants in dry hot regions (cacti) Reverse what most plants do Open stoma at night to allow CO2to come in & water to leave Close them during the day.

CAM Carbon fix CO2 at night into 4 carbon chains (organic acids) Use the Calvin cycle during the day.

(a) Spatial separation of steps (b) Temporal separation of steps Fig. 10-20 Sugarcane Pineapple C4 CAM CO2 CO2 Mesophyll cell 1 CO2 incorporated into four-carbon organic acids (carbon fixation) Night Organic acid Organic acid Figure 10.20 C4 and CAM photosynthesis compared Bundle- sheath cell CO2 CO2 Day 2 Organic acids release CO2 to Calvin cycle Calvin Cycle Calvin Cycle Sugar Sugar (a) Spatial separation of steps (b) Temporal separation of steps