Photosynthesis Carbon dioxide + water glucose + oxygen sunlight absorbed by chlorophyll Plants use sunlight to turn water and carbon dioxide into glucose.

Plant leaves have many types of cells!

Typical Dicot Leaf Cross-Section Cuticle Epidermis Palisade Parenchyma Vascular bundles Guard Cells Spongy Parenchyma Stoma

Plant Cells Chloroplasts

Plant Cells Chloroplasts

Visible light is only a small part of the electromagnetic spectrum (all forms of light).

Wavelength of Light (nm) 400 500 600 700 Short wave Long wave (more energy) (less energy)

LIGHT behaves as if it were composed of "units" or "packets" of energy that travel in waves. These packets are photons. The wavelength of light determines its color.

Different pigments absorb light differently

Chlorophyll: A Light Absorbing Pigment The Solar Panel Chemical!

A. Cyclic Electron Flow SUN e- Photons Photosystem I ATP produced Primary Electron Acceptor e- ATP produced by ETC Photosystem I Accessory Pigments SUN Photons

B. Noncyclic Electron Flow P700 Photosystem I P680 Photosystem II Primary Electron Acceptor ETC Enzyme Reaction H2O 1/2O2 + 2H+ ATP NADPH Photon 2e- SUN

PHOTOSYNTHESIS O2 as a byproduct of photosynthesis Photolysis: replaces lost electrons by splitting water

At each step along the transport chain, the electrons lose energy. Sun Light energy transfers to chlorophyll. At each step along the transport chain, the electrons lose energy. Chlorophyll passes energy down through the electron transport chain. Energized electrons provide energy that to ADP splits H2O bonds P forming ATP H+ NADP+ oxygen released NADPH for the use in light-independent reactions

Calvin cycle Incorporation of CO2 - carboxylation rxn Ribulose bisphosphate carboxylase/oxygenase (Rubisco) very abundant protein (40% leaf soluble protein)

Light-independent reactions. Notice where ATP and NADPH are used up. 1 Carbon fixation combines CO2 with RuBP. 6 CO2 2 G3P synthesis uses energy. 6 12 6 RuBP C3 cycle (Calvin-Benson cycle) PGA 3 RuBP synthesis uses energy and 10 G3P. 12 ATP 12 ADP 6 ADP 12 NADPH 12 6 ATP 12 G3P NADP+ 4 G3P available for synthesis of carbon compounds such as glucose. glucose (or other molecules)

Calvin Cycle (C3 fixation) 6CO2 6C-C-C-C-C-C 6C-C-C 6C-C-C-C-C 12PGA RuBP 12G3P (unstable) 6NADPH 6ATP C-C-C-C-C-C Glucose (6C) (36C) (30C) C3 glucose

protects against light damage Photorespiration depends on light “wastes” CO2 protects against light damage favored by high O2, low CO2 and warm temperatures 9/12/07

C2 oxidative carbon cycle: Input 4C Output 3C 75% C recovery rate 2 x 5C 1 x 3C 2 x 2C Chloroplast 1 x 3C C2 oxidative carbon cycle: Input 4C Output 3C 75% C recovery rate Peroxisome 2 x 2C 2 x 2C 1 x 3C Mitochondrion

Leaf Anatomy In C3 Plants In C3 plants (those that do C3 photosynthesis), all processes occur in the mesophyll cells. Mesophyll cells Bundle sheath cells Image taken without permission from http://bcs.whfreeman.com/thelifewire|

C3 and C4 Leaf structure

C4 Pathway Affinity of PEP Carboxylase for CO2 is much higher than its affinity for O2. Image taken without permission from http://bcs.whfreeman.com/thelifewire|

C4 plants use the C4 pathway CO2 is captured with a highly specific enzyme. CO2 PEP C4 Pathway 4-carbon molecule AMP ATP within mesophyll chloropast pyruvate CO2 O2 PGA rubisco CO2 stoma C3 Cycle bundle- sheath cells RuBP G3P Almost no photorespiration occurs in hot, dry conditions. glucose In a C4 plant, both mesophyll and bundle-sheath cells contain chloroplasts. within bundle-sheath chloropast Lots of glucose is synthesized. C4 plants essentially store carbon for hot times of the day. Guess what pathway many weeds use?

9/12/07

C4 Photosynthesis C4 plants need more light quanta than C3 plants to fix CO2

CAM vs C4 Syndrome

C4 Plants C3 CO2 C-C-C PEP C-C-C-C Malate ATP C-C-C Pyruvic Acid Mesophyll Cell CO2 C-C-C PEP C-C-C-C Malate ATP Bundle Sheath Cell C-C-C Pyruvic Acid C-C-C-C CO2 C3 Malate Transported glucose Vascular Tissue

CAM Plants Night (Stomates Open) Day (Stomates Closed) Vacuole C-C-C-C Malate CO2 C3 C-C-C Pyruvic acid ATP PEP glucose

PHOTOSYNTHESIS What affects photosynthesis? Light intensity: as light increases, rate of photosynthesis increases CO2 uptake (mmol m-2 s-1) Fluence rate (mmol m-2 s-1)

Light response curves Light saturation point Light saturation for an individual leaf is ~ 1/3 - 1/2 photon flux of full sunlight BUT At the whole plant level P/S is rarely saturated even in full sunlight Slope = max. quantum yield for CO2 assimilation

CO2 response curve of photosynthesis: Net Ps Compensation point CO2 diffusion Biochem limits: light-harvesting, Rubisco (N), RuBP (P) 5.6

PHOTOSYNTHESIS What affects photosynthesis? Temperature: Temperature Low = Rate of photosynthesis low Temperature Increases = Rate of photosynthesis increases If temperature too hot, rate drops

250 350 700 Atmospheric CO2 (ppm) C4 C3 CO2 uptake rate 9/12/07

C3 versus C4 plants C3 C4 Photorespiration Yes Not detectable CO2 compensation point (mL CO2 l-1) 20 – 100 0 – 5 Temperature optimum (oC) 20 – 25 30 – 45 Quantum yield as a function of temp. Declining Steady Transpiration ratio 500 – 1000 200 – 350 Light saturation (mmole photons m-2 s-1) 400 – 500 Does not saturate C3 plants are favoured in environments where water is plentiful, temperature and light levels are moderate (temperate climates) C4 plants are favoured in environments where water is limiting and light and temperatures are high (tropical / subtropical habitats)