BIOL 102 W4: Photosynthesis

Reactants of photosynthesis

CO2, H2O, light

Products of photosynthesis

O2, glucose

Light reaction function

Produces ATP from light energy for the Calvin cycle by reducing NADPH

What is the thylakoid

Small compartment in chloroplasts where light energy is captured

What is the thylakoid membrane

Outer layer that holds chlorophyll

Grana

Stacks of thylakoids

Lumen

Hollow fluid filled space inside the thylakoid

What is a photosystem

Site that captures light energy and excites electrons

Important molecules in the reaction centre

Chlorophyll a molecules, primary electron acceptor, electron donor

Autotrophs

Sustain themselves without consuming anything derived from another species

Producers of the biosphere

Autotrophs

Heterotrophs

Unable to produce their own food and must feed off other organisms

Consumers of the biosphere

Heterotrophs

Decomposers of the biosphere

Consumers of organic remains like feces and fallen leaves

Chloroplast

Organelle that facilitates photosynthesis

Endosymbiont theory

States that the original chloroplast was a prokaryote symbiotically living within a eukaryotic ancestor

Mesophyll

Interior leaf tissue

Stomata

Holes in the leaf where CO2 enters and O2 exits

Veins

Transport water to leaves and sugar to roots/non-photosynthetic parts

Chloroplast structure

Envelope of 2 membranes surrounding stroma

Stroma

Cellular fluid within chloroplasts

Thylakoids

Membrane-bound sacs separating stroma from the thylakoid space

Grana

Stacks of thylakoids

Total equation of photosynthesis

6CO₂ + 12H₂O + light energy = C₆H₁₂O₆ + 6O₂ + 6H₂O

Direct product of photosynthesis

3C sugar used to produce glucose

Net equation of photosynthesis

6CO₂ + 6H₂O + light energy = C₆H₁₂O₆ + 6O₂

O₂ production

Comes from splitting of water

Role of light energy

Excites electrons as they move from water to sugar

NADP+ role

Accepts electrons and protons from water driven by light absorption in chlorophyll

Location of light reactions

Thylakoid membrane

Where is P680

PSII

Phases of the light reactions

Photoexcitation at PSII, photolysis, ETC, photoexcitation at PSI, NADPH production, photophosphorylation

Photoexcitation at PSII

Chlorophyll absorbs photons, exciting an electron which is captured by primary electron acceptor oxidizing P680 to P680+

Photolysis

Enzyme splits water whose electrons replenish P680+, O2 is waste, protons accumulate in lumen

Electron transport chain

Excited electrons flow from PSII to plastiquinone, Cyt complex, plastocyanin, then PSI as protons are pumped into lumen creating proton-motive force

Photoexcitation at PSI

Photon excites P70 and electrons pass through a shorter ETC to ferredoxin

NADPH production

NADP+ reductase transfers 2e and H+ rom stroma reducing NADP+ to NADPH

Photophosphorylation (ATP synthesis)

Protons flow down their gradient through ATP synthase, phosphorylating ADP

Location of the Calvin Cycle

Stroma

Photorespiration

Rubisco is inefficient and fixes O2 instead of CO2 when O2 is more concentrated, wasting ATP and CO2 without producing sugar

Steps in the Calvin Cycle

Carbon fixation, reduction, regeneration of RuBP

Products of the Calvin Cycle

1 net G3P for every 3 turns and 3 CO2

Carbon fixation (CC)

Rubisco attaches each CO2 to a 5C sugar called RuBP creating an unstable 6C intermediate that immediately splits into 6 molecules of 3-PGA

Reduction (CC)

6 3-PGAs are phosphorylated into 6 1,3-BGPs then reduced to 6 G3Ps with 1 exiting and the other 5 repeating

Regeneration of RuBP

The 5 remaining G3P are used to regenerate the original 3 RuBPs

Chlorophyll a

Converts light energy into chemical energy during light-dependent reactions, absorbing mostly blue-violet and red-orange light

Chlorophyll b

Greenish yellow accessory pigment that expands a plant’s usable spectrum

Carotenoids

Red, orange, and yellow pigments that absorb blue-green light and dissipate excess light as heat to prevent damage

Phycobilins

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