CHAPTER 8

Photosynthesis

-synthesize carbohydrates.

  • 6CO2 + 12H2O + light energy → C6H12O6 + 6O2 + 6H2O

endergonic reaction

Trophic levels

Autotroph : Makes organic molecules from inorganic.
sources such as C02 and H2O

Photoautotroph: Use light as a source of energy. Green plants, algae, cyanobacteria

Heterotroph : Bacteria, protists, fungi, and animals

Chloroplast

-CO2 enters and O2 exits leaf
through pores called stomata

photosynthesis occurs internally
in leaves, in the mesophyll

-in plants and algae that carry out
photosynthesis. - Green pigment is chlorophyll

Outer and inner membrane -thylakoid membrane (Granum)

2stages :

Light reactions: Use light energy,e in thylakoid membranes,Produce ATP, NADPH and O2

Calvin cycle: in the stroma, Uses ATP and NADPH to incorporate CO2 into carbohydrates, Energy storage,Requires massive input of energy ,Product is glyceraldehyde-3-phosphate (G3P)

Absorption vs. Action spectru

Reactions That Harness:
Light Energy: electromagnetic radiation, waves,photons

Absorption spectrum: wavelengths that are absorbed by different pigments

Action spectrum: rate of photosynthesis by whole plant at specific wavelengths

  • Captured light energy to produce energy intermediate molecules, Thylakoid membranes of chloroplast contain :

Photosystem II: amazing redox machine nitial step,Excited electrons travel from PSII to PSI, Oxidizes water, generating O2 and H+, Releases energy

Two main components: Light-harvesting complex, which absorbs photons,Energy transferred- Reaction center, P680 → P680* , Water is oxidized , producing O2 gas

Three chemical products :

NADPH : Produced in the stroma

ATP:Produced in stroma

Oxygen, O2 :Produced in thylakoid lumen

ATP synthesis in chloroplasts : Achieved by photophosphorylation

H+ gradient generated three ways:

↑H+ by ETC pumping H+ into lumen

↓H + in stroma from formation of NADPH

↑H+ in thylakoid lumen by splitting of water

Noncyclic and cyclic electron flow :

Noncyclic: Electrons begin at PSII,y transfer to
NADPH in a linear process, Produces both ATP and NADPH

Cyclic : Electron cycling releases energy, Produces only ATP,return to PSI

Photosystem I : Primary role to make NADPH

Z scheme: Zigzag shape, Photosynthesis involves increases and decreases in the energy of an electron, a non-excited pigment molecule, starts with the lowest energy ,Photosystem I boosts the electron

Enhancement Effect : Simultaneous flashes,more than doubles the rate of photosynthesis -provided evidence for the existence of two photosystems

Phase 1 – Carbon fixation:CO2 into RuBP using rubisco -Phase 2 – Reduction and carbohydrate production :ATP is used to convert 3PG into (1,3-BPG), NADPH electrons reduce it to (G3P),Only 2 G3P molecules used for carbohydrates Phase 3 Regeneration of RuBP :10 G3P are converted into 6 RuBP using 6 ATP

Environmental conditions: Light intensity, Temperature,Water availability

Photorespiration Decreases the Efficiency
of Photosynthesis: Rubisco functions as a carboxylase, Rubisco can also be an oxygenase, in hot and dry environments, CO2 low and O2 high

CAM plants :

C3 or C4 : depends on the environment

warm dry climates, C4 plants conserve
water and prevent photorespiration.

cooler climates, C3 plants use less
energy to fix CO2,90% of plants are C3

C4 plants separate CO2 and Calvin cycle
using time

close their stomata during the day and open them at night,CO2 enters and is converted to malate ,Stomata close during the day to conserve water

Malate leaves central vacuole and is broken down into CO2

C4 Plants: Evolved a mechanism to minimize
photorespiration, making oxaloacetate in the first step of carbon fixation, Hatch-Slack pathway ,Leaves Mesophyll cells and Bundle-sheath cells