Photosynthesis Overview

Slide 1: Introduction to Photosynthesis

• Definition: Photosynthesis is the process that uses solar energy to convert carbon dioxide (CO2) and water (H2O) into sugar and oxygen (O2).

  • General Formula: 6CO2 + 6H2O -> C6H12O6 + 6O2

Slide 2: Energy Transformations

• Light Reactions: Convert solar energy into short-term chemical energy.(proton Gradient)

  • Products: ATP and NADPH.

• Calvin Cycle: Converts short-term chemical energy (ATP, NADH) into long-term chemical energy (sugar).

  • Calvin cycle = Biochemical reactions

Slide 3: Chlorophyll

• Function: A pigment that absorbs light, specifically red and blue wavelengths, and transmits and reflects green light.

  • A pigment is a chemical that absorbs light

Slide 4: Light Reactions

• Reactions that directly require light

• Location: Occurs in the thylakoids, in the grana chloroplasts.

• Thylakoid:Green photosynthetic membrane, contains chlorophyll and other pigments

• Purpose: Convert solar energy into short term chemical energy (ATP and NADPH).

  • Solar energy

  • Electron transport/Redox reactions

  • Proton Gradient

  • ATP?NADH

    • Reactants(IN): H2O, ADP + P, NADP+

    • Products(OUT): O2, ATP, NADPH

Slide 5: Calvin Cycle (Biochemical Reactions)

• Definition: Light-independent reactions; they do not directly us light.

  • May occur in light or dark, but usually during the day

• Location: Stroma(Liquid) of chloroplasts.

• Purpose: Fix CO2 into sugar.

  • Uses ATP + NADPH to make sugar from CO2 + H2O

    • Carbon Fixation: Convert inorganic C to organic C compounds

      • Convert short term chemical energy (ATP, NADPH) to long term chemical energy (Sugar) (Store it as starch and ship it to other cells)

        • Reactants(IN): CO2 + C5 = RuBP, ATP, NADPH

        • Products(OUT): C6 = Glucose, ADP + P, NADP+

Slide 6: Photosynthetic Pigments

• Types: Chlorophyll a, Chlorophyll b, Carotenoids.

• Chlorophyll a: essential pigment, part of the reaction center of PSI and PSII

• Accessory Pigments: absorbs wavelengths of light chlorophyll a can’t, so can use broader range of light for photosynthesis

  • Chlorophyll b, carotenoids, xanthophylls

    • Role: Accessory pigments absorb light wavelengths that chlorophyll a cannot, broadening the range of light used for photosynthesis.

• Leaves are green due to chlorophyll and accessory pigments, absorbs mainly red and blue wavelengths

  • Other wavelengths (green) are reflected or transmitted

• Action spectrum: Rate of Photosynthesis vs. Wavelength

  • Photosynthesis uses most wavelengths of light (except green) because of the absorbance of the accessory pigments and chlorophyll a

Slide 7: Light Harvesting Complex

• Structure: Composed of antenna complexes that absorb light and transfer energy to the reaction center of photosystems.

• Antenna Complex = Light harvesting complex

  • Chlorophyll a molecules are in the reaction center of the photosystem

  • Accessory pigments are in the antenna complex or light harvesting complex of the photosystem

  • Light is absorbed by accessory pigments in the antenna complex and transferred to the reaction center of the photosystem

    • 2 Types of Reaction Center:

      • PSI : P700

      • PSII: P680

        • PSI = Photosystem I

        • PSII = photosystem II

Slide 8: Electron Transport Chain

• Process: Involves the transfer of electrons from PSII to PSI, generating a proton gradient used for ATP synthesis and split H2O → O2.

• Electron transport from PSI to ferredoxin to the sysntesis of NADPH

• Light reactions: Electrons transport from PSII to PSI

  • Antenna complex absorbs a photon of light and transfers it to the reaction center of photosystem II

• Absorption of light by PSII; the excited electron of chlorophyll a in p680 is easily donated to another molecule

• Electron Transport from PSII to PSI

  • After PSII absorbs a photon of light the excited electron is passed down chain until it reaches PSI

    • PSII → plastoquinon (PQ or QB ) → cytochrome complex → plastocyanin (PC) → PSI

      • During Electron Transport H+s are pumped across membrane

  • Electrons from H2O go to PSII H+s from H2O are part of H+ gradient

• Purpose: Generate a proton Gradient!

  • H+s moved from stroma to thylakoid lumen during elecron transport (cytochrome complex)

• Water Splitting: Produces O2 and contributes to the proton gradient.

  • 2H2O → O2 + 4H+ + 4e-

    • PSII has lost an electron and can’t absorb another photon of light until the electron is replaced

    • Electrons from water go to fill “hole” in PSII

    • 4H+ contribute to H+ gradient

    • O2 is made as a by product

  • Location: Oxygen evolving complex (OEC), part of PSII

• Purpose: releases H+s into thylakoid lumen

  • Thylakoid lumen: pH 7 → pH 4

  • Stroma: pH 7 → pH 8

• H+ gradient will be used by ATP synthase to make ATP

• Light Reactions: Electron Transport from PSI

  • Antenna Complex absorbs a photon of light and transfers it to the reaction center of photosystem I

• Absorption of Light by PSI

  • The excited electron of chlorophyll a in P700 is easily donated to another molecule

• Electron Transport from PSI:

  • After PSi absorbs a photon of light the excited electron is passed down chain until it raches ferredoxin

  • PSI → ferredoxin (Final e- acceptor)

  • Ferredoxin reacts with an enzyme to make NADPH (Redox reaction)

  • Enzyme: Ferredoxin NADP Reductase (FNR)

• PSII: H2O split to make O2 and H+ Electron transport from PSII to PSI synthesis of NADPH and ATP

Slide 9: ATP Synthesis

• Electron Transport generates a proton gradient

  • Energy in proton gradient used to make ATP by enzyme ATP Synthase

• Chemiosomosis: Oxidative Phosphorylation

Slide 10: Calvin Cycle Phases

• Phase 1: Carbon fixation (CO2 + RuBP(C5) -> 2 PGA(C3)).

  • Enzyme: Rubisco = Ribulose bisphosphate carboxylase oxygenase

    • Carboxylase = add CO2. Oxygenase = Add O2

  • Location: Stroma of chloroplast

• Carboxylase Activity: Carbon Fixation

  • CO2 + Ribulose bisphosphate (C5) → 2 Phosphoglyceric acid (C3)(Goes to calvin cycle to make sugar)

  • Oxygenase Activity: Photorespiration (BAD!!!)

    • O2 + Ribulose bisphosphate (C5) → Phosphoglyceric acid (C3)(Goes to calvin cycle to make sugar) + Phosphoglycolate (C2) (Cell spends ATP to salvage)

• Reactants: CO2 , RuBP (C5)

• Products: 2 PGA (C3)

• Enzyme: Rubisco = protein = enzyme

• Phase 2: Sugars are Rearranged and Reduced

  • PGA (C3) → → → G3P (C3)

  • Uses the ATP and NADPH made in the light reacions

• Phase 3: Regeneration of Ribulose Bisphosphate (RuBP).

Slide 11: Types of Photosynthesis

• C3 Photosynthesis: Most common, efficient under moderate conditions but prone to photorespiration.

  • Most plants

• 1st stable compound made: C3 sugar

• 1st Reaction: CO2 + RuBP C5 → 2 PGA C3

  • 2PGA = Phosphoglycotic acid

• Enzyme: Rybisco

• Example: Soybean, Most plants

• Advantage: Most efficient type of photosynthesis

• Disadvantage: Photorespiration (BAD! WASTEFUL)

• C3 Leaf

  • Mesophyll = photosynthetic tissu palisade and spongy mesophyll

• C4 Photosynthesis: Adapted to hot, dry environments, minimizes photorespiration.

  • Corn, Sugar cane

• 1st stable compound : C4 OAA (Ocaloacetate)

• 1st Reaction: CO2 + PEP (C3) → OAA (C4)

• Enzyme : PEP carboxylase (Adds CO2)

  • PEP: Phosphoenol Pyruvate

  • OAA: Oxaloacetate

• Advatage: No Photorespiration

• Disadvantage: Uses more energy to make sugar

  • Need to mkae special structures = the bundle sheath

  • In Mesophyll:

    • CO2 + PEP C3 → OAA C4

    • Enzyme: PEP carboxylase no photorespiration because PEP carboxylase can’t react with O2

    • OAA C4 → Malate C4,Malate shipped to Bundle sheath cells

  • In Bundle sheath

    • Malate C4 → CO2 + Pyruvate C3

    • Calvin Cylce: CO2 + RuBP C5 → 2 PGA C3

      • Enzyme: Rubisco

    • No photorespiration because high CO2 (20-100x) in bundle sheath cells! No photorespiration = no waste!

  • Bundle Sheath Cells: Contain Rubisco, make sugar in the calvin cycle (high CO2 so no photorespiration)

• CAM Photosynthesis: Opens stomates at night to store CO2 as malate, preventing dehydration and photorespiration.

  • Carries out Calvin Cycle during Day

    • Desert plants, cacti, sedum, “Living stones” (slow growing desert plants)

• Not all plants are C4 plants due to optimal conditions C3 plants can compete C4 plants (high CO2, wet environment, moderate temperature)

  • C4 plants need an extra 2 ATP to fix C into a C6 sugar

  • C4 plants must expend energy to produce extra enzymes and build bundle sheath cells

• CAM Photosynthesis: Night

  • Stomates open, CO2 eneters leaf

  • CO2 + PEP C3 → OAA C4

  • Enzyme: PEP carboxylase

  • OAA C4 → Malate C4

  • Malate shipped out of chloroplast stored in central vacuole

• CAM photosynthesis: Day

  • Stomates close to prevent dehydration

  • Malate shipped from vacuole back to chloroplast

  • Malate C4 → CO2 + pyruvate C3

  • CO2 used in calvin cycle to make sugar

• CAM Photosynthesis: Results

  • CAM plants carry out photosynthesis during Day with stomates closed

  • No Photorespiration: High CO2 because CO2 was stored as malate overnight

  • No dehydration: Plant keep stomates closed during day to prevent excess water loss

• C4 vs CAM Photosynthesis

  • Same reactions, different locations or time

  • Initial CO2 fixation: CO2 + PEP (C3) = OAA (C4)

  • Enzyme: PEP Carboxylase

  • CO2 is stored as a C4 sugar. Later C4 broken down to release CO2, to be used in calvin cycle to make glucose

• Photorespiration:

  • why does it occur? Rubisco reacts with O2 instead of with CO2 in C3 plants

    • Rubisco can use oxygen as a substrate when CO2 levels are low and oxygen levels are high

  • When is it a problem? When hot + Dry

    • When relatively high O2/CO2 ratio

    • When stomates close: high temperature low water (CO2 goes down)

    • When stomates close no new gases enter leaf, use up CO2 make more O2 ( O2 goes up)

  • Advantages: NONE

  • Disadvantages: Breaks down sugar and wastes energy, fixed CO2

  • Oxygenase Activity of Rubisco: O2 + RuBP (C5) → PGA (C3) + Phosphoglycolate (C2) ( Salavage reaction spend energy)

• Compensation Point:

  • Plants carry out 2 metabolic processes;

    • Photosynthesis: Uses CO2 + H2O to produce O2 + sugar

    • Respiration: Uses O2 + Sugar to produce CO2 + H2O

  • Compensation Point: When amount of O2 made by photosynthesis exactly matchs amount of O2 used by respiration

• C3 vs C4 Photosynthesis

  • C3 plants out compete C4 plants:

    • Cool/ moderate temeprature, high humidity, low to average light intensity

    • C4 photosynthesis costs more energy to make sugar and other structures

  • C4 plants out compete C3 plants:

    • Hot dry conditions, high light intensity

    • No photorespiration in C4 plants

Slide 12: Environmental Factors

• Temperature: High rate as temperature increase, up to a point

  • Cool temperature → lower rate

• Light Intensity: Lower rate at low light intensity (shade) and Higher rate at high light intensity, up to a point

• Humidity/Water Availability: Rate increases with high humidity/water

Page 13: Summary of Photosynthesis

• Connection: Photosynthesis and respiration are interconnected processes that utilize and produce gases (O2 and CO2) in plants.

• Importance: Understanding photosynthesis is crucial for comprehending plant biology, ecology, and the global carbon cycle.