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Photosynthesis Chapter 10
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Photosynthesis
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Photosynthesis Process where organisms capture energy from sunlight
Build food molecules Rich in chemical energy 6CO H2O C6H12O6 + 6H2O + 6O2
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Photosynthesis Captures only 1% of the energy of the sun
Uses it to provide energy for life
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Photosynthesis Autotrophs: Producers Make own organic molecules
Heterotrophs: Consumers
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Photosynthesis Plant leaves - Chloroplasts
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Chloroplasts Thylakoids: Internal membranes of chloroplasts
Membranes of thylakoids contain chlorophyll Green pigment that captures the light for photosynthesis Grana: Stacks of thylakoids
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Chloroplasts Stroma: Semi-liquid substance Surrounds the thylakoids
Contain enzymes Make organic molecules from carbon dioxide
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Chloroplasts
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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
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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
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Stoma (Stomata) Opening on the leaf Allows exchange of gases.
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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
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Photophosphorylation
Addition of phosphate group to ADP Light energy
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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
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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
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Experimental history Jan Baptista van Helmont
Plants made their own food Joseph Priestly Plants “restored” the air
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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
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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
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Experimental history C.B. van Neil
Looked at the role of light in photosynthesis Studied photosynthesis in Bacteria
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C.B. van Neil CO2 + 2H2S (CH2O) + H2O + 2S
CO2 + 2H2A (CH2O) + H2O + A2 CO H2O (CH2O) + H2O + O2
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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
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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
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i CO2 Light NADP+ ADP Calvin Cycle Light Reactions ATP NADPH
Fig H2O CO2 Light NADP+ ADP + P i Calvin Cycle Light Reactions ATP NADPH Chloroplast [CH2O] (sugar) O2
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Sunlight UV light from sun Important source of energy when life began
UV light can cause mutations in DNA Lead to skin cancer
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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.
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Light Visible light Purple has shorter wavelengths
More energetic photons Red has longer wavelengths Less energetic photons
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Spectrum
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Spectrum
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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
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Absorption spectrum Specific for each molecule
Range & efficiency of photons it is capable of absorbing
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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
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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
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Pigments
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Pigments Chlorophyll b Chlorophyll a Carotenoids
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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
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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
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Photosystems Captures the light
Located on surface of the photosynthetic membrane Chlorophyll a molecules Accessory pigments (chlorophyll b & carotenoids) Associated proteins
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Photosystems Consists of 2 components
1. Antenna (light gathering) complex 2. Reaction center
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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
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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)
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Photosystem
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(INTERIOR OF THYLAKOID)
Fig 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)
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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
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Photosynthesis (Process)
Light dependent reactions Linear electron flow Energy transfer Thylakoid membranes
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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
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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+)
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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
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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
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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
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Electron transport chain
Fig 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)
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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
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Enhancement effect
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Enhancement effect
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(low H+ concentration) Cytochrome complex Photosystem II Photosystem I
Fig 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 The light reactions and chemiosmosis: the organization of the thylakoid membrane Thylakoid membrane ATP synthase STROMA (low H+ concentration) ADP + ATP P i H+
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H+ Diffusion Electron transport chain ADP + P
Fig Mitochondrion Chloroplast MITOCHONDRION STRUCTURE CHLOROPLAST STRUCTURE H+ Diffusion Intermembrane space Thylakoid space Electron transport chain Inner membrane Thylakoid membrane Figure Comparison of chemiosmosis in mitochondria and chloroplasts ATP synthase Matrix Stroma Key ADP + P i ATP Higher [H+] H+ Lower [H+]
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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)
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Photosystems When more ATP is needed Plant changes direction
The electron used to make NADPH in PS I is directed to make ATP
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Calvin Cycle Named for Melvin Calvin
Cyclic because it regenerates it’s starting material C3 photosynthesis First organic compound has 3 carbons
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Calvin cycle Combines CO2 to make sugar Using energy from ATP
Using reducing power from NADPH Occurs in the stroma of the chloroplast
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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)
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Calvin Cycle 3 cycles 3 CO2 molecules 1 molecule of G3P 6 NADPH 9 ATP
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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
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Reduction Phosphate is added to 3-phosphoglycerate
1,3 Bisphosphoglycerate NADPH reduces the molecule Glyceraldehyde 3-phosphate (G3P)
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Regeneration 5 molecules of G3P are rearranged to make 3 RuBP
Uses 3 more ATP
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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)
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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)
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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
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Calvin Cycle Enzyme mediated
5 of these enzymes need light to be more efficient Net reaction 3CO ATP + 6NADPH G3P + 8Pi + 9ADP + 6NADP+
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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
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Electron transport chain
Fig H2O CO2 Light NADP+ ADP + P i Light Reactions: Photosystem II Electron transport chain Photosystem I RuBP 3-Phosphoglycerate Calvin Cycle ATP G3P Figure A review of photosynthesis Starch (storage) NADPH Chloroplast O2 Sucrose (export)
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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
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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
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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
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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.
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CAM Carbon fix CO2 at night into 4 carbon chains (organic acids)
Use the Calvin cycle during the day.
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(a) Spatial separation of steps (b) Temporal separation of steps
Fig Sugarcane Pineapple C4 CAM CO2 CO2 Mesophyll cell 1 CO2 incorporated into four-carbon organic acids (carbon fixation) Night Organic acid Organic acid Figure 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
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