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CHAPTER 8: Photosynthesis - Coggle Diagram
CHAPTER 8: Photosynthesis
Photosynthesis
Light energy is captured and used to synthesize molecules carbohydrates such as Glucose.
Equation is C02 + H20 + Light energy -> C6H12O6 + O2 + H20
Where the CO2 is reduced, while, the H20 is oxidized.
Photosynthesis powers the biosphere
Biosphere
regions on the surface of the Earth and atmosphere where living organisms exist
Largely driven by the photosynthetic power of green plants
Energy cycle
Cells use organic molecules for energy and plants replenish those molecules using photosynthesis
Trophic Levels
Heterotroph
Must eat food (organic molecules from their environment) to sustain life
Autotroph
Makes organic molecules from inorganic soruces
Photoautotroph
Use light as a source of energy
Green plants, algae, cyanobacteria
Chloroplast
Organelle in plants and algae that carries out photosynthesis
Green pigment found in plants is called chlorophyll
Majority of photosynthesis occurs internally in leaves, in the mesophyll
Carbon dioxide enters and oxygen exits the leaves through pores, also called as the stomata.
Chloroplast Anatomy
Outer and inner membrane separated by intermembrane space
A third membrane, the thylakoid membrane contains pigment molecules
Granum - a stack of thylakoids
Fluid-filled region between thylakoid membrane and inner membrane is the stroma
Two Stages of Photosynthesis
Light reactions
Use light energy
Takes place in thylakoid membranes
Produce ATP, NADPH, and O2
Calvin Cycle
Occurs in stroma
Uses ATP and NAPDH to incorporate CO2 into carbohydrates.
Photosystems I and II
Captured light energy can be transferred to other molecules to produce energy intermediate molecules for cellular work
Thylakoid membranes of chloroplast contain two distinct complexes of molecules
Photosystem I (PSI): Was the first one to be discovered
Photosystem II (PSII): First step in the process of Photosynthesis
Light excites pigment molecules in both PSII and PSI
Photosystem II
The initial step in photosynthesis
Excited electrons travel from PSII to PSI
Oxidizes water, generating O2 and H+
Releases energy in electron transport chain (ETC)
Energy used to make H+ electrochemical gradient
Photosystem I
Primary role to make NADPH
Addition of H+ to NADP+ contributes to H+ gradient by depleting H+ from the stroma.
Three chemical products in Photosynthesis
Oxygen (O2)
Produced in thylakoid lumen by oxidation of H20 by PSII
Two electrons transferred to P680+ molecules
NADPH
Produced in the stroma from high-energy electrons that start in PSII and are boosted in PSI
NADP+ + 2 electrons + H+ -> NADPH
ATP
Produced in the stroma by ATP synthase using the H+ electrochemical gradient
Synthesizing Carbohydrates via Calvin Cycle
Calvin Cycle (aka Calving-Benson Cycle)
CO2 incorporated into carbohydrates
Requires massive input of energy
For every 6 CO2 incorporated, 18 ATP and 12 NADPH must be used.
Product is glyceraldehyde-3-phosphate
Glucose is later amde from G3P in separate process.
Phases of Calvin Cycle
Phase 1 - Carbon Fixation
CO2 incorporated into RuBP using rubisco
Reaction product is a six-carbon intermediate that splits into two 3-phosphoglycerate molecules (3PG)
Phase 2 - Reduction and Carbohydrate Production
ATP is used to convert 3PG into 1,3-bisphosphoglycerate (1,3-BPG)
NADPH electrons reduce it to glyceraldehyde-3-phosphate (G3P)
10 G3P molecules must be used for the regeneration of RuBP while only 2 are used for carbohydrates
Phase 3 - Regeneration of RuBP
10 G3P are converted into 6 RuBP using 6 ATP
Variations in Photosynthesis
C4 Plants
Evolved a mechanism to minimize respiration
make oxaloacetate in the first step of carbon fixation
Leaves have two-cell layer organization
Mesophyll Cells
CO2 enters via stomata and 4 carbon compound formed (PEP carboxylase does not promote photorespiration)
Bundle-sheath Cells
4 carbon molecule transferred that releases steady supply of CO2, minimizing photorespiration
C3 Plants
make 3PG
90% of plants are C3 plants
C3 plants use less energy to fix CO2 in cooler climates
CAM plants
Where CAM plants open their stomata at night and the CO2 enters and is converted into malate
Crassulacean Acid Metabolism
Stomata closes during the day to conserve water and the oxaloacetate is converted to malate
Malate is broken down into CO2 to drive the calvin cycle during the day.