Photosynthesis

Evolution of Photosynthesis

• Earliest life-form survived by metabolizing high energy inorganic molecules
  • Harvesting only a small fraction of the potential energy which is available
• Three billion years ago some organisms developed photosynthetic capabilities
  • Transformation of electromagnetic energy into chemical energy

Evolution of Oxygenic Photosynthesis

• Some organism developed the capability of utilizing water as an electron donor
  • A by-product of this process was the release of molecular oxygen
• Oxygen accumulation in the atmosphere (1.5-2 billion years ago) allows for the evolution of high efficiency aerobic respiration.
• Main mechanism for the efficient extraction of energy from food

Energy Acquisition Strategies

• Autotrophs organisms (plants and bacteria) that are capable of producing organic nutrients from inorganic substances.
  • Exclusively dependent upon photosynthesis
• Heterotrophs organisms (animals) that must obtain organic nutrients from the environment
  • Directly or indirectly dependent upon photosynthesis

Photosynthetic Reactions

• The basic equation for the conversion of electromagnetic energy into glucose is: 6CO2+12H2O+ light---> C6H12O6+6H2O +O2
  • It was thought for a long time that light energy was being used to split CO2 to release O2 and produce carbohydrates
• This was disproved by vanNeil (1930) with his experiments with photosynthetic sulfur bacteria that used H2S as the raw material of photosynthesis which release elemental sulfur as a by-product rather than O2
  

Photosynthetic Storage of Energy

• Photosynthetic reactions conserve energy in two ways
  • Phosphorylation: ADP + Pi---> ATP
    • Light dependent Reactions
  • Carbohydrate formation: CO2--> sugar
    • Light independent reaction

Electromagnetic Energy

• The electromagnetic spectrum is a continuum of energy
  • Visible light constitutes a very narrow range of of this energy (between 400 and 700 nm of wavelength)
    • The shorter the wavelength the more energy
    • The energy is inversely proportionate to wavelength

Photosynthesis and Light

• Not all wavelengths are effective for photosynthesis
  • Chlorophyll is the molecule that traps light energy for use in photosynthesis
    • There are several kinds of chlorophyll, each with distinct characteristics
• Light striking an object can either pass through it (transmission), be reflected or be absorbed
  • We can see both transmitted and reflected light

Site of Photosynthesis

• In plants the photosynthetic processes occur in the chloroplast.
  • Chloroplasts are an enveloped organelle consisting of an outer and inner membrane
  • The envelope surrounds a protein rich matrix that is referred to as the Stroma
    • Site of carbon dioxide fixation (Light Independent Reactions)
  • Series of membranes suspended in the stroma are the Thylakoids
    • Site of energy capture and conversion (Light Dependent Reactions)

Chlorophyll

• Chlorophyll is the green pigment which we see when we observe a plant leaf
  • Chlorophyll can not be absorbing green wavelengths or we would not see that color
  • Chlorophyll must be absorbing some wavelengths of light or we would perceive the leaf as white.
    • Reflection or transmission of all wavelengths in visible light

Photosynthetic Action Spectrum

• Extracted chlorophyll exposed to light of specific wavelengths in a spectrophotometer can precisely indicate which wavelengths are being absorbed.
• Accessory pigments can absorb additional wavelengths and contribute to energy capture during photosynthesis.
  • Carotenoids, xanthophylls, etc.

Organization of Chlorophyll Pigments

• Chlorophyll and accessory pigments are arranged in association with proteins and form Light Harvesting Complexes (LHCs)
• LHCs (specifically LHC II) has 3 identical subunits each containing:
  • 7 chlorophyll a molecules
  • 5 chlorophyll b molecules
  • 2 carotenoid molecules

Photosynthetic units

• Photosynthetic units consist of between 5 and 10 LHCs
  • Serve to funnel light energy into a specialized set of chlorophylls
• One group of specialized chlorophyll a molecules are distinct from the others and serve as Reaction center molecules.
  • Reaction centers are the site where energy conversion begins

Effect of light on Chlorophyll

• Photonic energy that strikes a chlorophyll molecule is transferred to an electron
  • The electron is excited to a higher unstable energy level
• Only photons of specific energy levels that match the difference in energy to a discrete electron energy levels will accellerate an electron to that level
  

Fate of an Excited Electron

• Excited electrons rapidly loss there elevated unstable state by returning to the lowest available ground state
  • Energy loss can occur as the re-emission of energy in the form of light (Fluorescence)
  • The energy can be transferred to another chlorophyll molecule through a process referred to as Resonance Transfer.
    • Resonance can continue until it reaches a reaction center

Harvesting the Energy of Excited Electrons

• There are two general mechanism for the harvest of excited electron energy
  • Cyclic Photosynthesis which yield enough energy to drive the process of cyclic photophosphorylation
• Non-cyclic Photosynthesis which in addition to photophosphorylation yields a reduced electron acceptor (NADPH) which is used in anabolic processes
  

Cyclic Photophosphorylation

• In cyclic photophosphorylation the photosynthetic reaction center absorbs light maximally at 700 nm
• The excited electrons are transferred from P700 to a FeS protein and then through a series of membrane bound redox enzymes (ferridoxin->cytochrome b->cytochrome f->plastocyanin->P700)
• The energy released drives the development of an electrochemical gradient across the membrane

Cyclic Photophosphorylation

• Cyclic photophosphorylation was the first elaborate photosynthetic system to evolve
  • Cyclic photophorylation is still dominant in bacterial systems today
• Not very efficient of the 25 kcal/mol of energy gained by the excitation of the electron only 3.4 kcal/mol of energy are available for use
• Present days organisms primarily use cyclic photophosphorylation to augment the noncyclic pathway

Non-cyclic Electron Flow

• The non-cyclic photosynthetic process is comprised of two interacting photosystems
  • Photosystem I (PS I) with a reaction center that absorbs maximally at 700nm (P700)
  • Photosystem II (PS II) with a reaction center that absorbs at 680 nm (P680)

Non-cyclic Electron Transport

• Non-cyclic photosynthesis starts similar to the cyclic process with a photon event exciting an electron within a chlorophyll molecule of the photosythetic unit to a higher energy level
• The excited state is passed through the antennae complex to the reaction center (P680)
  • The reaction center electrons donate an electron to a primary electron acceptor (pheophytin)
• Pheophytin passes to the electron to a electron transport chain
• The energy of the electron is slowly dissipated as it moves toward lower energy states via the redox reaction of the electron transport chain
• The electron is then contributed to PS I, where a second photon event again elevates the energy status of an electron to another acceptor
• The excited electron is then passed through a series of redox proteins and finally reduces NADP---> NADPH

Source of Electrons and Energy Coupling

• Since electron are removed from PSII and transported through PSI to NADPH, this leaves PS II electron deficient
  • The electron deficiency is satisfied by electron pulled by an enzymatic process from water
    • This reaction results in 2 free H+ ions and molecular O2
• During the electron passage between PS II and PS I the release of energy is used to build a chemiosmotic gradient across the thylakoid membrane used in ATP formation

Light Independent Reactions of Photosynthesis

• The light independent reaction of photosynthesis occur in the chloroplast stroma
  • These reactions can occur in both the presence or absence of light but require the input of ATP and NADPH
• The light independent reaction are responsible for the fixation of CO2 into carbohydrates

Calvin Cycle (Reductive Pentose Cycle)

• Calvin cycle is composed of a series of enzymes that utilize the energy conserved in the light dependent reaction to reduce atmospheric CO2 to glucose
  • These are a series of endergonic reactions
• The cycle starts with a 5C ribululose bisphosphate (RuBP) molecule being complexed with the CO2 by an enzyme Ribulose 1,5 bis-phosphate carboxylase (Rubisco)
  • The result of this reaction is an unstable 6C compound that is rapidly split into 2 phosphoglyceric acid molecules: PGA (3C)
  • PGA is then phosphorylated and reduced to form phosphoglyceraldehyde (PGAL)
    • PGAL is stable end product of photosynthesis
      • 2 PGAL contribute to the synthesis of 1 glucose
  • This type of photosynthesis is referred as C3
  • To produce 1 PGAL molecule the Calvin cycle move turn 3 times
    • 5 of every 6 PGAL is utilized to regenerate the starting ribulose bis-phosphate (RuBP)

Photorespiration

• Rubisco is also capable of adding molecular O2 to the RuBP, instead of CO2 as it does in the Calvin cycle
  • This addition results in the wasteful oxidation of the high energy intermediate of carbohydrate sysnthesis
    • No energy is derive from this process
  • The balance between photosynthesis and photorespiration is dependent upon the concentrations of CO₂ and O ₂ within the leaf

Abiotic Factors that Affect the CO2/O2 Ratio

• Temperature of the leaf
• High temeratures cause leaves to attempt to conserve water by closing their stomata
• Stomatal closure will allow photosynthesis to draw down the concentration of CO₂ while increasing the O₂ content
  • This relationship shifts the balance towards wasteful photorespiration
• Water deficiency work in a similar manner

Strategies for Avoiding Photorespiation

• C₄ photosynthesis: development of Krantz leaf anatomy (mesophyll and bundle sheath cells)
  • CO₂ is initial fixed in the mesophyll cells as a 4C organic acid by the enzyme PEP carboxylase
• This organic acid is then pumped into the bundle sheath cell where it is decarboxylated, releasing CO₂ , increasing the concentration
  • Calvin cycle occurs in the bundle sheath cells
    

Crassulacean Metabolism

• Alternate method of avoiding photorespiration
  • Found in many succulent plants
• CO₂ is initial fixed into a organic acid (malic acid) during the night while temperatures allow for the stomata to be opened with minimal water loss
• Malic acid is decarboxlated during the daytime when the light reaction can supply ATP and NADPH

• This page is maintained by James C. Pushnik; jpushnik@ecst.csuchico.edu