Photosynthesis: Using Light to Make Food (Edited)

The process that converts solar energy into chemical energy - Photosynthesis: The process that converts solar energy into chemical energy Producers of the Biosphere: Plants and other autotrophs Plants are photoautotrophs: They use the energy of sunlight To make organic molecules from: Water and carbon dioxide

Figure 10.1

Biology and Society: Plant Power for Power Plants All food consumed by humans can be traced back to: Photosynthetic plants. An “energy plantation” Is a renewable energy source. Copyright © 2007 Pearson Education, Inc. publishing as Pearson Benjamin Cummings

Figure 7.1

Photosynthesis occurs in: In land: Plants: Predominant producer of food on land In aquatic environment: Algae Some unicellualr protists, e.g. egulina Some prokaryotes: Cynobacteria Sulfur bacteria

(b) Multicellular algae (c) Unicellular protist (a) Plants (b) Multicellular algae (c) Unicellular protist 10 m (d) Cyanobacteria (e) Purple sulfur bacteria Figure 10.2

Chloroplasts: Sites of Photosynthesis Occurs in chloroplasts. Chloroplasts Are found in the interior cells of leaves. Contain a thick fluid called stroma. Contain membranous sacs known as thylakoids.

Figure 7.3

The Overall Equation for Photosynthesis The reactants and products of the reaction

Photosynthesis as a Redox Process Photosynthesis is a redox process Water is oxidized Carbon dioxide is reduced

A Photosynthesis Road Map Photosynthesis is composed of two processes: The light reactions: Convert solar energy to chemical energy (ATP & NADPH). The Calvin cycle: Makes sugar from carbon dioxide.

Figure 7.4

Split water releasing oxygen The light reactions The light reactions Occur in the grana Split water releasing oxygen Convert light energy into chemical energy by: Producing ATP and Forming NADPH

Forms sugar from carbon dioxide, using: The Calvin cycle Occurs in the stroma Forms sugar from carbon dioxide, using: ATP for energy, and NADPH as reducing power

An overview of photosynthesis CO2 Light LIGHT REACTIONS CALVIN CYCLE Chloroplast [CH2O] (sugar) NADPH NADP  ADP + P O2 Figure 10.5 ATP

Sunlight is a type of energy called: The Nature of Sunlight Sunlight is a type of energy called: Radiation or electromagnetic energy. The full range of radiation is called: The electro-magnetic spectrum. Wavelength: Is the distance between the crests of waves Determines the type of electromagnetic energy

The visible light spectrum includes: The colors of light we can see The wavelengths that drive photosynthesis

Figure 7.5

Photosynthetic Pigments: The Light Receptors Substances that absorb visible light They reflect light, which include the colors we see Light Reflected Chloroplast Absorbed light Granum Transmitted

The spectrophotometer Is a machine that: Sends light through pigments Measures the fraction of light transmitted at each wavelength

Chloroplasts contain several pigments: Chloroplast Pigments Chloroplasts contain several pigments: Chlorophyll a: Is the main photosynthetic pigment Chlorophyll b: Is an accessory pigment Carotenoids: An accessory pigment (protective)

How Photosystems Harvest Light Energy Light also behaves as photons A photons is: A discrete quantity of energy. The shorter the wavelength the greater the photon energy

When chlorophyll molecules absorb photons: Electrons in the pigment gain energy Electrons are raised from ground state to excited (unstable) state Electrons lose excess energy (return to ground state) The energy is released (light/heat) and used.

Figure 7.9

Thylakoid-Membrane Photosystems A photosystem is an organized complex of: Chlorophyll molecules Other small organic molecules Proteins It is composed of : A reaction center complex surrounded by: Several light-harvesting complexes

(INTERIOR OF THYLAKOID) The Photosystem Primary election acceptor Photon Thylakoid Light-harvesting complexes Reaction center Photosystem STROMA Thylakoid membrane Transfer of energy Special chlorophyll a molecules Pigment THYLAKOID SPACE (INTERIOR OF THYLAKOID) Figure 10.12 e–

Thylakoid-Membrane Photosystems The reaction center: Include a pair of special chlorophyll a molecules Contains a molecule called “primary electron acceptor” The primary electron acceptor is capable of : Accepting electrons, & Becoming reduced

Thylakoid-Membrane Photosystems Light harvesting complex: Surrounds the reaction center Consists of various pigments molecules These pigments molecules: Include chlorophyll a, b, & carotenoid They are bound to proteins

(INTERIOR OF THYLAKOID) The Photosystem Primary election acceptor Photon Thylakoid Light-harvesting complexes Reaction center Photosystem STROMA Thylakoid membrane Transfer of energy Special chlorophyll a molecules Pigment THYLAKOID SPACE (INTERIOR OF THYLAKOID) Figure 10.12 e–

Thylakoid-Membrane Photosystems The thylakoid membrane is populated by two photosystems: Water-splitting photosystem NADPH-producing photosystem Both photosystems: Cooperate in the light reaction of photsynthesis Use light energy to generate : ATP and NADPH

(INTERIOR OF THYLAKOID) The Photosystem Primary election acceptor Photon Thylakoid Light-harvesting complexes Reaction center Photosystem STROMA Thylakoid membrane Transfer of energy Special chlorophyll a molecules Pigment THYLAKOID SPACE (INTERIOR OF THYLAKOID) Figure 10.12 e–

The light-harvesting complexes: Each complex consists of pigment molecules bound to particular proteins It transfers photon energy from one pigment molecule to another within itself Eventually funnels the photon energy to the reaction center

The reaction center complex: When its chlorophyll molecule absorbs energy: One of the molecule electrons gets pumped up And captured by the primary electron acceptor

Linear (Noncyclic) Electron Flow Noncyclic (linear) electron flow: Is the primary pathway of energy transformation in the light reactions Produces NADPH, ATP, and oxygen

Linear (Noncyclic) Electron Flow Figure 10.13 Photosystem II (PS II) Photosystem-I (PS I) ATP NADPH NADP+ ADP CALVIN CYCLE CO2 H2O O2 [CH2O] (sugar) LIGHT REACTIONS Light Primary acceptor Pq Cytochrome complex PC e P680 e– + 2 H+ Fd reductase Electron Transport chain Electron transport chain P700 + 2 H+ + H+ 7 4 2 8 3 5 1 6

Chemiosmosis in Chloroplasts and Mitochondria Generate ATP by the same basic mechanism, chemiosmosis Mitochondria transfer chemical energy from food molecules to ATP, whereas Chloroplasts transform light energy into chemical energy in ATP

The spatial organization of chemiosmosis Differs in chloroplasts and mitochondria Key Higher [H+] Lower [H+] Mitochondrion Chloroplast MITOCHONDRION STRUCTURE Intermembrance space Membrance Matrix Electron transport chain H+ Diffusion Thylakoid Stroma ATP P ADP+ Synthase CHLOROPLAST Figure 10.16

In both organelles Redox reactions of electron transport chains generate a H+ gradient across a membrane ATP synthase Uses this proton-motive force to make ATP

The light reactions and chemiosmosis: the organization of the thylakoid membrane REACTOR NADP+ ADP ATP NADPH CALVIN CYCLE [CH2O] (sugar) STROMA (Low H+ concentration) Photosystem II H2O CO2 Cytochrome complex O2 1 1⁄2 2 Photosystem I Light THYLAKOID SPACE (High H+ concentration) Thylakoid membrane synthase Pq Pc Fd reductase + H+ NADP+ + 2H+ To Calvin cycle P 3 H+ 2 H+ +2 H+ Figure 10.17

The Calvin cycle uses ATP and NADPH to convert CO2 to sugar Is similar to the citric acid cycle Occurs in the stroma

The Calvin cycle has three phases Carbon fixation Reduction Regeneration of the CO2 acceptor

The Calvin cycle Figure 10.18 Input Light 3 CO2 CALVIN CYCLE (G3P) Input (Entering one at a time) CO2 3 Rubisco Short-lived intermediate 3 P P Ribulose bisphosphate (RuBP) 3-Phosphoglycerate 6 P 6 1,3-Bisphoglycerate 6 NADPH 6 NADPH+ Glyceraldehyde-3-phosphate 6 ATP 3 ATP 3 ADP CALVIN CYCLE 5 1 G3P (a sugar) Output Light H2O LIGHT REACTION ATP NADPH NADP+ ADP [CH2O] (sugar) CALVIN CYCLE Figure 10.18 O2 6 ADP Glucose and other organic compounds Phase 1: Carbon fixation Phase 3: Regeneration of the CO2 acceptor (RuBP) Phase 2: Reduction

Alternative mechanisms of carbon fixation have evolved in hot, arid climates On hot, dry days, plants close their stomata Conserving water but limiting access to CO2 Resulting in less production during such season

Water-Saving Adaptations of C4 and CAM Plants C3 plants (Soybean, oat, wheat, rice): Use CO2 (Calvin cycle) directly from the air. Are very common and widely distributed. Low production during hot season Stomata closed to conserve water Less CO2 and less photosynthesis

C4 plants (Corn, sorghum, sugarcane) Close their stomata to save water during hot and dry weather. Can still carry out photosynthesis. Enzymatic binding of CO2 into 4-C compound 4-C compound donates CO2 to Calvin cycle in a nearby cell

C4 leaf anatomy and the C4 pathway CO2 Mesophyll cell Bundle- sheath cell Vein (vascular tissue) Photosynthetic cells of C4 plant leaf Stoma Mesophyll C4 leaf anatomy PEP carboxylase Oxaloacetate (4 C) PEP (3 C) Malate (4 C) ADP ATP Sheath Pyruate (3 C) CALVIN CYCLE Sugar Vascular tissue Figure 10.19

CAM plants (Pineapple, cacti, succulent plants) Open stomata only at night to conserve water Carry photosynthesis during the day CO2 is incorporated into 4-C compound 4C compound donates CO2 to Calvin cycle during the day

The CAM pathway is similar to the C4 pathway Spatial separation of steps. In C4 plants, carbon fixation and the Calvin cycle occur in different types of cells. (a) Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cells at different times. (b) Pineapple Sugarcane Bundle- sheath cell Mesophyll Cell Organic acid CALVIN CYCLE Sugar CO2 C4 CAM CO2 incorporated into four-carbon organic acids (carbon fixation) Night Day 1 2 Organic acids release CO2 to Calvin cycle Figure 10.20

The Importance of Photosynthesis: A Review A review of photosynthesis Light reactions: • Are carried out by molecules in the thylakoid membranes • Convert light energy to the chemical energy of ATP and NADPH • Split H2O and release O2 to the atmosphere Calvin cycle reactions: • Take place in the stroma • Use ATP and NADPH to convert CO2 to the sugar G3P • Return ADP, inorganic phosphate, and NADP+ to the light reactions O2 CO2 H2O Light Light reaction Calvin cycle NADP+ ADP ATP NADPH + P 1 RuBP 3-Phosphoglycerate Amino acids Fatty acids Starch (storage) Sucrose (export) G3P Photosystem II Electron transport chain Photosystem I Chloroplast Figure 10.21

The Environmental Impact of Photosynthesis

Photosynthesis has an enormous impact on the atmosphere. It swaps O2 for CO2.

Organic compounds produced by photosynthesis Provide the energy and Building material for ecosystems