Photosynthesis
Raven, Ch. 7
What percentage of the world’s photosynthesis goes to human use?
•	20%
•	35%
•	50%
•	60%

Global Carbon Cycle
A two part process:
•	Trapping light energy (light reactions)
•	Conversion of trapped energy into carbohydrates (dark reactions, or better, light independent reactions)
Some interesting history
•	Van Helmont (ca. 1600) shows that plants “don’t eat soil”
–	willow grows 74 kg using only 57 g. soil
•	Priestly (ca. 1700) shows depleted air is “restored” by vegetation
•	Ingenhousz (ca. 1780) shows restoration process is light dependent
•	van Niel and Hill (1930s) show that oxygen is derived from water, not CO2

Light wavelengths involved in photosynthesis is a limited range of wavelengths
•	Red and blue light
•	Why only this part of the spectrum?
–	Oddly this is part of the visible range

An overview of photosynthesis follows
•	Conversion of light energy into ATP and NADPH
•	Fixation of CO2 into what ultimately becomes sugar

Light Reactions
•	Two cooperating photosystems
–	Each with an antenna complex and reaction center
–	Photosystem II and Photosystem I

Light Reactions
•	Light energy is absorbed in PSII, e- taken from water and passed to PSI
–	Transfer of e- pumps protons into thylakoid lumen
•	Electrons are re-energized by PSI, which synthesizes NADPH
•	Protons moving through ATP synthase makes ATP

Noncyclic electron flow in PSI produces ATP, but no NADPH or O2

•	Significance may be to help balance ATP and NADPH production to support dark reactions

Dark Reactions
•	3 pathways:
–	C3
–	C4
–	CAM
•	All have the C3 pathway in common
–	C3 is typical of plants in intermediate habitats and plants living in shade

Overview

Schematic of C3 Pathway

To take out a C6 sugar, RuBP must be regenerated or system will stop
Photorespiration: a big problem with C3
Photorespiration
•	No energy yield
•	Reduced fixed carbon yield
•	Consumes RuBP
•	Greatest problem when temperatures are hot, causing kinetics to favor oxygenation

C4 Pathway prevents oxygenation at a cost
•	First fixation with PEP to acid
•	Transport of acid to bundle sheath
•	Decarboxylation and refixation using C3 in a low oxygen environment (bundle sheath)
•	Regeneration of PEP at an energy cost, limits C4 to bright environments
Summary of C4 Pathway

Benefits of C4
•	Higher rates of photosynthesis
•	Because PEP fixes CO2 more efficiently, can have lower levels of CO2 inside leaf
–	Stomata can be more closed, reducing water loss
–	Use less water per C fixed than C3
•	High N use efficiency
•	Tolerate high temperatures well (tropical and warm temperate climates, not deserts)
•	Examples: sugar cane, maize, prairie grasses 
CAM Pathway
•	Characteristic of cacti, sedum, bromeliads (pineapple family) of hot climates
•	Much like C4, but pathways occur at different times in leaf and stem cells.
–	Elaborate bundle sheath not involved
–	Store acid in vacuoles

Summary of CAM Pathway

CAM Advantages and Disadvantages
•	Confers great tolerance to drought
•	Very low rates of photosynthesis
•	Requires abundant light because costs are high

Analysis of Transpiration Data
Expected Outcome:
Stronger Evaporative Gradient
Rationale for Outcome:  
Stress Response?