Photosynthesis
Lecture 7
Objectives I
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Explain how the process of photosynthesis is involved in net primary production and an increase in plant biomass.
Draw a sketch of a chloroplast, identify the structural components, and indicate where the light reactions and the Calvin cycle take place.
List the inputs and outputs of the light reactions and the Calvin Cycle.
Objectives II
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Describe how the light reactions and Calvin Cycle are linked
Describe the role of enzymes in the process of photosynthesis.
Explain how the following factors affect photosynthesis: stomatal opening, temperature, light intensity, water availability, and CO2 availability.
Explain what the plant does with the carbohydrate that is produced by photosynthesis.
Photosynthesis
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Captures solar energy
Store as chemical energy in sugars
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Food
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Bioenergy
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Global Carbon Cycle
Photosynthesis :
light
CO2  H 2 O
Carbohydrates  O 2
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Respiration :
O 2 Carbohydrates 
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Reactions are COUPLED
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Carbon is CYCLED
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Reactions are inefficient
CO2 H 2 OHeat ATP
Net Primary Productivity
NPP=GPP−R a
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GPP = Gross primary productivity
= Total amount of photosynthesis
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Ra = Autotrophic (Plant) Respiration
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NPP = Total amount of new plant biomass
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On average NPP ~ ½ GPP
What is sunlight?
Why are plants green?
Light­Dependent Reactions
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Convert light energy into chemical energy
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Occurs on thylakoid membranes
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2H 2 O2NADP 3 ADP3P i light  O 2 2 NADPH 2 ATP
Water
Energy storing compounds
Oxygen
Stored energy
NADP
Summary of Light Reactions
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Light energy enters chloroplast
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Flow of electrons
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Splitting water provides electrons
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Release of oxygen from water
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Accumulation of H+ in thylakoid spaces
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Reduction of NADP to NADPH using H+
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Production of ATP
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ATP synthase enzyme
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Powered by flow of H+ from thylakoid spaces
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ATP + P → ATP
Calvin Cycle
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Cyclic series of reactions
3CO29ATP6NADPH  PGAL9 ADP9 P i 6 NAPD
Phosphoglyceraldehyde
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2 PGAL ­> 1 Glucose
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Enzyme mediated
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Coupled to light reactions
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RuBisCo
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Most abundant protein on Earth
Large fraction of leaf nitrogen
Rate limiting step of Calvin Cycle
Calvin Cycle Summary
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Addition (fixing) of gaseous CO2 to RUBP, a 5 carbon compound in the stroma of chloroplasts.
Energy input from ATP
(ATP from the light reactions ­­­> ADP + Phosphate back to the light reactions)
Addition of H+ and energy from NADPH
(NADPH from the light reactions ­­­> NADP back to the light reactions
Production of carbohydrates for storage, transport, and biosynthesis
Recycling of 5 carbon compound (RUBP) to fix more CO2
Leaf Anatomy
Stomata
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Regulate CO2 uptake and H2O loss
Can't photosynthesize with stomata closed
Trade­off of drought risk
Thermal regulation
C4 & CAM plants store CO2 for later use Limits to Photosynthesis
Limits to Photosynthesis
Limits to Photosynthesis
Limits to Photosynthesis
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Water
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Low soil moisture → Close stomata
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Low humidity → Close stomata
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Actual limitation is CO2
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Photosynthetic reactions themselves are never water limited
Minerals
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Nitrogen → RUBISCO, protein
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Phosphorus → ATP, nucleic acids
Lecture activity
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Get into groups of one or two From your understanding of biomes and the basic equation of photosynthesis, write down the factors you predict to be limiting the rate of photosynthesis and Net Primary Productivity (NPP) in:
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Boreal forest (navy blue)
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Deserts (tan)
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Tropical Forest
(bright green)
PRINT & SIGN YOUR NAME
on the top of your paper
Limits to global NPP