carbon dioxide water glucose water oxygen Photosynthesis 6CO2 + 12H2O C6H12O6 + 6H2O + 6O2 carbon dioxide water glucose water oxygen Reactants: 6CO2 + 12H2O Products: C6H12O6 + 6H2O + 6O2 Photosynthesis vs. Cellular Respiration: 6CO2 + 12H2O C6H12O6 + 6H2O + 6O2 C6H12O6 + 6O2 6CO2 + 12H2O

Discovery of Photosynthesis The work of many scientists led to the discovery of how photosynthesis works: Jan Baptista van Helmont (1580-1644) Joseph Priestly (1733-1804) Jan Ingen-Housz (1730-1799) F. F. Blackman (1866-1947)

Early discoveries Joseph Priestly: Candle with bell jar and mouse experiment – He concluded that air is necessary for the growth of a plant. He discovered the fact that plants restore oxygen in the air. Jan Ingenhousz: Experiment with aquatic plant in light and dark – He concluded that sunlight is essential for plant processes that purify the air. Julius Von Sachs: Green parts of plant make glucose and store as starch. T.W. Engelmann: Spilt light using prism into 7 colours (VIBGYOR) - Green algae Cladophora placed in a suspension of aerobic bacteria - Bacteria were used to detect the sites of O2 evolutions. Cornelius van Niel: He did experiment with purple and green bacteria and demonstrated photosynthesis is a light dependent process with hydrogen from H2O reduces CO2 to carbohydrates. He concluded that oxygen comes from H2O, and not from CO2.  Finally, the correct equation for photosynthesis was discovered.                          6CO2 + 12H2O C6H12O6 + 6H2O + 6O2

Discovery of Photosynthesis C. B. van Niel, 1930‘s proposed a general formula: CO2+H2A + light energy CH2O + H2O + 2A H2A is the electron donor van Niel identified water as the source of the O2 released from photosynthesis Robin Hill confirmed van Niel’s proposal that energy from the light reactions fuels carbon fixation

Photosynthesis Photosynthesis is divided into two reactions: 1. Light-dependent reactions capture energy from sunlight reduce NADP+ to NADPH produce ATP occur in chloroplasts 2. Carbon fixation reactions use ATP and NADPH to synthesize organic molecules from CO2 Occurs in stroma

AN OVERVIEW OF PHOTOSYNTHESIS The light reactions convert solar energy to chemical energy Produce ATP & NADPH Light Chloroplast NADP ADP + P The Calvin cycle makes sugar from carbon dioxide ATP generated by the light reactions provides the energy for sugar synthesis The NADPH produced by the light reactions provides the electrons for the reduction of carbon dioxide to glucose Calvin cycle Light reactions

Photosynthesis Overview Photosynthesis takes place in chloroplasts Thylakoid membrane – internal membrane arranged in flattened sacs contain chlorophyll and other pigments Grana – stacks of thylakoid membranes Stroma – semiliquid substance surrounding thylakoid membranes

Fig. 8.1-1

Energy from Light Reactions Fuels Carbon Fixation Photon: a particle of light acts as a discrete bundle of energy energy content of a photon is inversely proportional to the wavelength of the light Photoelectric effect: removal of an electron from a molecule by light occurs when photons transfer energy to electrons Light energy is absorbed by pigments: molecules that absorb visible light

Energy from Light Reactions Fuels Carbon Fixation

Pigments Pigments: molecules that absorb visible light chlorophyll a chlorophyll b carotenoids Each pigment has a characteristic absorption spectrum: the range and efficiency of photons it is capable of absorbing.

Different pigments absorb light differently

Pigments Chlorophyll a – primary pigment in plants and cyanobacteria absorbs violet-blue and red light Chlorophyll b – secondary pigment absorbing light wavelengths that chlorophyll a does not absorb

Pigments Structure of pigments: Porphyrin ring: complex ring structure with alternating double and single bonds magnesium ion at the center of the ring photons excite electrons in the ring electrons are shuttled away from the ring

Pigments Accessory pigments: secondary pigments absorbing light wavelengths other than those absorbed by chlorophyll a increase the range of light wavelengths that can be used in photosynthesis include: chlorophyll b, carotenoids, phycobiloproteins carotenoids also act as antioxidants

Photosynthesis: Photosystems Photosystem: Enzyme complexes for photosynthesis enzymes use light to oxidize H2O and reduce CO2 to carbohydrates Located in the thylakoid membrane of plants, algae and cyanobacteria or the cytoplasmic membrane of photosynthetic bacteria

Photosystem Organization A photosystem consists of: 1. an antenna complex of hundreds of accessory pigment molecules 2. a reaction center of one or more chlorophyll a molecules Energy of electrons is transferred through the antenna complex to the reaction center

Photosystem Organization At the reaction center, the energy from the antenna complex is transferred to chlorophyll a This energy causes an electron from chlorophyll to become excited chlorophyll absorbs a photon, looses an electron The excited electron is transferred from chlorophyll a to an electron acceptor, quinone Water donates an electron to chlorophyll a to replace the excited electron

Light-Dependent Reactions In chloroplasts, two linked photosystems are used in noncyclic photophosphorylation 1. Photosystem I reaction center pigment (P700) with a peak absorption at 700nm 2. Photosystem II reaction center pigment (P680) has a peak absorption at 680nm This system produces energy, O2, and NADPH for biosynthesis of carbohydrates

Light-Dependent Reactions Photosystem II acts first: accessory pigments shuttle energy to the P680 reaction center excited electrons from P680 are transferred to the cytochrome/b6-f complex - connects photosystems II and I electron lost from P680 is replaced by an electron released from hydrolysis: the splitting of water

Light-Dependent Reactions The b6-f complex is a series of electron carriers an electron transport chain electron carrier molecules are embedded in the thylakoid membrane protons are pumped into the thylakoid space to form a proton gradient

Light-Dependent Reactions Photosystem I: receives energy from an antenna complex energy is shuttled to P700 reaction center excited electron is transferred to a membrane-bound electron carrier electrons are used to reduce NADP+ to NADPH electrons lost from P700 are replaced from the b6-f complex

Z Diagram of Photosystems I and II Fig. 8.13 Z Diagram of Photosystems I and II

Fig. 8.13-1

Fig. 8.13-2

Fig. 8.13-3

Light-Dependent Reactions ATP is produced via chemiosmosis: ATP synthase is embedded in the thylakoid membrane protons have accumulated in the thylakoid space protons move into the stroma only through ATP synthase ATP is produced from ADP + Pi

Carbon Fixation Reactions Calvin cycle biochemical pathway that allows for carbon fixation incorporates CO2 into organic molecules occurs in the stroma uses ATP and NADPH as energy sources

Carbon Fixation Reactions To build carbohydrates, cells need: 1. Energy provided by ATP from light-dependent reactions 2. Reduction Potential: hydrogen atoms provided by NADPH from photosystem I

Carbon Fixation Reactions The Calvin cycle has 3 phases: 1. Carbon Fixation RuBP + CO2 2 molecules PGA 3 phosphoglycerate 2. Reduction PGA is reduced to G3P glyceraldehyde-3-phosphate 3. Regeneration of RuBP G3P is used to regenerate RuBP

Carbon Fixation Reactions Carbon fixation – the incorporation of CO2 into organic molecules occurs in the first step of the Calvin cycle: ribulose-bis-phosphate + CO2 2(PGA) 5 carbon molecule 1 carbon 3 carbons Reaction is catalyzed by rubisco

Carbon Fixation Reactions Glucose is not a direct product of the Calvin cycle 2 molecules of G3P leave the cycle each G3P contains 3 carbons 2 G3P are used to produce 1 glucose in reactions in the cytoplasm

Carbon Fixation Reactions During the Calvin cycle, energy is needed. The energy is supplied from: 18 ATP molecules 12 NADPH molecules

Photorespiration Rubisco has 2 enzymatic activities: 1. Carboxylation – the addition of CO2 to RuBP favored under normal conditions 2. Photorespiration – the oxidation of RuBP by the addition of O2 favored in hot conditions CO2 and O2 compete for the active site on RuBP.

Photorespiration When Rubisco reacts with O2 instead of CO2 Occurs under the following conditions: Intense Light (high O2 concentrations) High heat Photorespiration is estimated to reduce photosynthetic efficiency by 25%

Why high heat? When it is hot, plants close their stomata to conserve water They continue to do photosynthesis  use up CO2 and produce O2  creates high O2 concentrations inside the plant  photorespiration occurs

C2 oxidative carbon cycle: Input 4C Output 3C 75% C recovery rate

C4 Photosynthesis Certain plants have developed ways to limit the amount of photorespiration C4 Pathway* CAM Pathway* * Both convert CO2 into a 4 carbon intermediate  C4 Photosynthesis

Fig. 8.19-1

Fig. 8.19-2

Photorespiration Some plants can avoid photorespiration by using an enzyme other than rubisco. PEP carboxylase adds CO2 to phosphoenolpyruvate (PEP) a 4 carbon compound is produced CO2 is later released from this 4-carbon compound and used by rubisco in the Calvin cycle

Photorespiration C4 plants use PEP carboxylase to capture CO2 CO2 is added to PEP in mesophyll cell the resulting 4-carbon compound is moved into a bundle sheath cell where the CO2 is released and used in the Calvin cycle

Photorespiration CAM plants CO2 is captured at night when stomata are open PEP carboxylase adds CO2 to PEP to produce a 4 carbon compound this compound releases CO2 during the day CO2 is then used by rubisco in the Calvin cycle

C4 Pathway CO2 is fixed into a 4-carbon intermediate Has an extra enzyme– PEP Carboxylase that initially traps CO2 instead of Rubisco– makes a 4 carbon intermediate

C4 Pathway The 4 carbon intermediate is “smuggled” into the bundle sheath cell The bundle sheath cell is not very permeable to CO2 CO2 is released from the 4C malate  goes through the Calvin Cycle C3 Pathway

How does the C4 Pathway limit photorespiration? Bundle sheath cells are far from the surface– less O2 access PEP Carboxylase doesn’t have an affinity for O2  allows plant to collect a lot of CO2 and concentrate it in the bundle sheath cells (where Rubisco is)

CAM Pathway Fix CO2 at night and store as a 4 carbon molecule Keep stomates closed during day to prevent water loss Same general process as C4 Pathway

How does the CAM Pathway limit photorespiration? Collects CO2 at night so that it can be more concentrated during the day Plant can still do the calvin cycle during the day without losing water

Factors affecting photosynthesis Light Water Temperature Wind speed CO2 concentration Blackman proposed the law of limiting factors in 1905. According to this law, when a process depends on a number of factors, its rate is limited by the pace of the slowest factor.