Photosynthesis: Autotrophs and Heterotrophs

Photosynthesis

• Synthesis occurs by light energy

Autotrophs

• Can use water and CO₂ solely to produce their own carbohydrates.
• The carbohydrates made can be used directly or made into other molecules for processes.
• Plants, algae, and cyanobacteria are examples of autotrophs.

Heterotrophs

• Cannot produce their own carbohydrates.
• Consumption of autotrophs or other heterotrophs is needed to gain molecules to power activities.
• Animals, fungi, and most bacteria are examples of heterotrophs.

Photosynthesis Equation

• Sunlight + 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Respiration Equation

• C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + free energy

Origins of O₂ in Photosynthesis

• O₂ comes from H₂O and not CO₂
• Original Equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
• Revised Equation: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O

Pathways of Photosynthesis

• Light reactions (aka light-dependent reactions) which convert light energy into chemical energy, producing ATP and NADPH.
• Carbon-fixation reactions (aka light-independent reactions) which use ATP and NADPH with CO₂ to produce carbohydrates; anabolic/building of complex carbon-containing compounds.

Chloroplast

• Chemical energy gets used to convert CO₂ and H₂O to carbohydrate inside the stroma.
• The stroma is the area lying outside of the thylakoid membranes.
• Thylakoid membranes are the sites where light energy gets harvested by chlorophyll (green pigment).
• The green pigment then gets converted into chemical energy.
• Granum is a stack of thylakoids.

Energy in Light

• The amount of energy found in electromagnetic radiation has an inverse relationship with its wavelength.
• Longer the wavelength, lower is its energy.
• Shorter the wavelength, the greater is the energy.

Photons

• Particles of light are energy packets.
• Molecules can experience three outcomes when meeting photons:
  • The photon may bounce off (scattered or reflected).
  • The photon may pass through (transmitted).
  • The photon may be absorbed (adding energy to the molecule).

Electron Excitation

• An increase in energy boosts electrons further from the nucleus.
• The electrons become unstable, and the molecule becomes more chemically reactive.

Photosynthetic Pigments

• A molecule's ability to absorb specific wavelengths will be characteristic of that molecule in question.
• Pigments can absorb wavelengths in the visible spectrum.
• Certain wavelengths get absorbed and others scattered when white light falls on a pigment.

Absorption Spectrum

• Chlorophyll absorbs red and blue light.
• Green is the remaining light.

Action Spectrum

• Rate of biological activity an organism carries out against the wavelengths of light

Chlorophyll

• Light absorbed by the complex ring structure of a chlorophyll molecule.
• Hydrocarbon tails secure chlorophyll molecules to hydrophobic proteins inside the photosystem.

Chlorophyll Forms

• Chlorophyll a and b

Photosystems

• Contain light-harvesting complexes, also known as antenna systems.
• Arrangement of pigment molecules occurs in a specific orientation adjacent to one another.
• Embedded in a complex of proteins that span the thylakoid membrane via hydrophobic tails.
• Chlorophyll a, b, and accessory pigments.
• Accessory pigments help absorb light energy at more wavelengths.
• Energy from an absorbed photon elevates an electron to an excited state.
• The electron releases its energy which gets absorbed by an adjacent pigment molecule.
• Energy gets passed from pigment to pigment until chlorophyll molecules at the reaction center of the photosystem gains said energy.
• The reaction center will convert light energy into chemical energy.
• The reaction center passes up its excited electron to a chemical acceptor.
• Reaction center chlorophylls lose their excited electrons in a redox reaction and becomes positively charged.
• Reaction center chlorophylls become oxidized, and the acceptor molecule becomes reduced.
• Oxidation is losing an electron.
• Reduction is gaining an electron.

Thylakoid Reactions

• Coordinated photosystems, each featuring a reaction center, collaborate to produce ATP and NADPH.
• Light capture
• Energy transfer
• Electron transport
• ATP synthesis

Light Capture

• Pigments harvesting

Energy Transfer

• Photosystem II
• Excited chlorophylls in the reaction center yields energetic electrons to reduce two chemical acceptor molecules.
• Chlorophylls now lack the electron, causing instability because of the tendency to obtain a replacement from another molecule.
  • The replenishing electrons come from water.
• Photosystem I

Electron Transport

• H₂O (2e⁻) → Photosystem II → Plastoquinone → Cytochrome → Plastocyanin → Photosystem I → Ferrodoxin → NADP+ reductase (2e⁻) → NADP+.
• NADPH is the final product.
• Initial electron donor is water.
• End electron acceptor is NADP+.

ATP Synthesis

• Also known as photophosphorylation, ATP synthesis from light energy.
• Maintenance of H+ concentration gradient across thylakoid membrane (electrochemical gradient).
• Electron carriers actively transport H+ from the stroma to the thylakoid lumen (inside the thylakoid membrane).
• Water oxidation creates more H+ in the thylakoid lumen.
• NADP+ reduction removes H+ in the stroma.

ATP Synthase

• Refers to the movement of protons moving out from the thylakoid lumen and into the stroma
• ATP creation

Photosystems - Products

• Photosystem I and II

Cyclic Electron Transport

• The linear (noncyclic) pathway for electron transport might not provide sufficient ATP for carbon fixation
• Electron cycling in cyclic electron transport delivers more ATP.
• Photosystem I (excited e⁻)→ Ferrodoxin → Cytochrome → Plastocyanin → Photosystem I.
  • Light energy fuels excited electrons.
• Creates 1x H+ transported into thylakoid lumen per cycle.
• Causes additional ATPs to be synthesized.

Calvin Cycle

• Rubisco is main enzyme- ribulose bisphosphate carboxylase/oxygenase
• Rubisco is the most common protein in the world
• 50% of leaf proteins are rubisco
• Three processes
  • Fixation of CO2
  • Reduction of 3PG
  • Regeneration of RuBP

Fate of G3P

• In six rounds of the Calvin Cycle, two molecules of Glyceraldehyde-3-phosphate are produced.
• Some of the G3P leaves out from the chloroplast for the cytoplasm.
  • G3P is transformed into hexoses (glucose and fructose).
• Part of the G3P gets applied to synthesize glucose inside the chloroplast.
  • It is stored as starch and saved for nighttime use.

Photorespiration

• Rubisco has a reaction rate of approximately 3 conversions of CO₂ per second.
  • This is very slow.
• Rubisco can also bind to CO₂ and O₂
  • Rubisco is a carboxylase when CO₂ is added to RuBP.
  • Rubisco is an oxygenase when O₂ is added to RuBP.
• Oxygenase Activity:
  • RuBP + O₂ → phosphoglycolate -- 3PG → Calvin cycle.

Photorespiration Attributes

• Phosphoglycolate (C2 molecule) inhibits the Calvin cycle.
• Several conversion reactions for phosphoglycolate exist.
• Overall Reaction: 2 Phosphoglycolate (C4) + O₂ → 3PG (3 carbons) + CO₂
  • Carbon experiences a 25% loss.

Rubisco Affinities

• Display a higher affinity for CO₂ than O₂.
• The affinity in Earth's atmosphere favors photorespiration 21% O₂, 0.04% CO₂.
• At atmospheric concentrations, CO₂ fixation is favored.
• At lower CO₂ concentration, O₂ is favored.

Factors Changing CO₂ Levels

• During the day, stomata shut down to help conserve H₂0.
• Gas exchange halts:
  • CO₂ consumption occurs in the Calvin Cycle.
  • O₂ is produced by Photosystem II.

Photorespiration Properties

• Occurs in the light.
• Consumes O2.
• Releases CO2.