Unit 3 (Photosynthesis)

Photosynthesis

The conversion of light energy to chemical energy

Autotrophs

Organisms that produce their own food (organic molecules) from their surroundings

• Plants are photoautotrophs

Heterotrophs

Organisms unable to make their own food, so they live off of other organisms

Evolution of Photosynthesis

First evolved in prokaryotic organisms

• Cyanobacteria: early prokaryotes capable of photosynthesis

• Oxygenated the atmosphere of early Earth

• Prokaryotic photosynthetic pathways were the foundation of eukaryotic photosynthesis

Site of photosynthesis

• Leaves are the primary location of photosynthesis in most plants

• Chloroplasts: location of photosynthesis that is found in the mesophyll (the cells that make up the interior tissue of the leaf)

• Stomata: pores in leaves that allow CO2 in and O2 out.

Structures of a Chloroplast

• Surrounded by a double membrane

• Stroma: fluid around thylakoids

• Thylakoid: from stacks known as grana

• Chlorophyll: green pigment in thylakoid membranes

Photosynthesis formula

 6CO₂ + 6H₂O + Light Energy → C₆H₁₂O₆ + 6O₂

Redox reactions

The complete or partial transfer of one or more electrons from one reactant to another

• Oxidation: loss of e^- (less negative)

• Reduction: gain of e^- (more negative)

Stages of photosynthesis

1. Light reactions (splits H20) → H20 is oxidized to O2

2. Calvin cycle (turns light energy into glucose) → CO2 is reduced to glucose

Light

Electromagnetic energy

• Made up of particles of energy called photons

• Travel in waves and at the speed of light

• Can be reflected, transmitted, or absorbed when interacting with matter

Wavelengths

The distance between two consecutive peaks (crests)

• The entire range is known as the electromagnetic spectrum

• 380-750nm is visible light

• Shorter wavelength: higher energy

• Longer wavelength: lower energy

Pigments

Can absorb visible light

• The color we see is the reflected wavelengths

Chlorophyll A

• Primary pigment

• Involved in the light reactions

• Blue/green pigment

• Absorbs blue and red, but reflects green

Chlorophyll B

• Accessory pigments

• Yellow/Green pigment

• Absorbs blue, orange, and red, but reflects yellow and green

Carotenoids

• Yellow/Orange pigment

• Photoprotection: carotenoids absorb and dissipate excessive light energy that could damage chlorophyll or interact with oxygen

Light Reactions overview

• Occur in the thylakoid membrane in the photosystems

• Convert solar energy to chemical energy

• Inputs:

  H2O

  ADP

  NADP+

• Outputs:

  O2

  ATP

  NADPH

Importance of Light in Chlorophyll

• Chlorophyll absorbs a photon that boosts electrons from a ground state to an excited state

• Electrons are unstable and fall back to the ground state

• Energy is released as heat

• Emits photons as fluorescence (rapid emission of light)

Photosystems

• Reaction center: proteins associated with chlorophyll a and an electron acceptor

• Light capturing complexes: pigments associated with proteins (antenna for the reaction centers)

Photosystem II

Reaction center that absorbs light at 680 nm

• Light energy (photon) causes an electron to go from an excited state to a ground state and repeats until it reaches the P680 pair of chlorophyll a molecules.

• The electron is transferred to an electron acceptor, forming P680+

• H20 is split into 2 → 2 e-

• P680+ is reduced → 2H+

• Released into the thylakoid space

• 1 oxygen atom bonds to another oxygen atom

Linear electron flow: each excited electron will pass from PSII to PSI via the ETC

Generation of ATP

• The “fall” of electrons from PS II to PS I provides energy to form ATP

• The proton (H+) gradient is a form of potential energy

Photosystem I

Reaction center that absorbs light at 700 nm

• Light energy excites electrons in P700, leaving it positively charged (P700⁺).

• The excited electrons move down a second ETC

• NADP⁺ reductase uses these electrons to turn NADP⁺ into NADPH.

Calvin Cycle

• The Calvin cycle is cyclic electron flow

• Uses ATP and NADH to reduce CO2 to sugar (G3P)

• For the net synthesis of 1 G3P molecule, the cycle must take place 3 times

• Inputs:

  3 CO2

  9 ATP

  6 NADPH

• Outputs:

  1 G3P

  9 ADP

  6 NADP+

Phase 1: Carbon Fixation

• CO₂ enters the cycle one at a time and attaches to RuBP.

• Catalyzed by Rubisco 

• This forms 3-phosphoglycerate.

Phase 2: Reduction

• Each molecule of 3-phosphoglycerate is phosphorylated by 6 ATP

• Becomes 1, 3-bisphosphoglycerate

• 6 NADPH molecules donate electrons to 1,3-bisphosphoglycerate and are reduced to G3P

• 6 molecules of G3P are formed, but only one is counted as a net gain

• The other 5 G3P molecules are used to regenerate RuBp

Phase 3: Regeneration of RuBp

• 5 G3P molecules are used to regenerate 3 molecules of RuBp 

• Uses 3 ATP for regeneration

• The cycle is now ready to take CO2 again

Problem for C3 Plants

• Photorespiration: on very hot days, plants close their stomata to stop water loss

• Causes less CO2 to be present and more O2

• Rubisco binds to O2 and produces ATP

• Produces CO2 but not sugar, which is bad for the plant

C4 Adaptations

Spatial separation of steps

• Stomata partially close to conserve water

• Mesophyll cells fix CO2 into a 4-C molecule

• Transferred to bundle sheath cells

• Releases CO2 to be used in the Calvin cycle

Cam Adaptations

• Open stomata at night and close during the day

• CO2 is incorporated into organic acids and stored in vacuoles

• During the day, light reactions occur, and CO2 is released from the organic acids and incorporated into the Calvin cycle