PHOTOSYNTHESIS• Converts light energy into chemical

energy through a complex series of biochemical reactions

• 6CO2 + 6H2O -> C6H12O6 + 6O2

• Photosynthesis occurs inside the chloroplasts

• White light from the sun is composed of different colors of different wavelengths – visible spectrum

CHLOROPLAST STRUCTURE

• Outer, double membrane• Inner Membrane system:

– THYLAKOIDS – flattened sacs– GRANA – stacks of thylakoids

• Light harvesting pigments embedded in thylakoid membrane

• Surrounding thylakoids, liquid: STROMA

• Photosynthesis depends upon green pigment CHLOROPHYLL (absorbs light in the blue-violet and orange-red and reflects light in green region

• Accessory pigments help harvest light energy

LIGHT DEPENDENT RXNS

• Visible light (traveling in “photons”, packets of energy) is changed into chemical energy

• H2O is split into O2 and H• PS I and II absorb light energy• This light energy is transferred to

reaction center, a Chlorophyll a that donates e- to electron carrier

LIGHT DEP. RXNS cont’d

• Lost e- from PSII is replaced by e- from H2O

• At end of electron flow, electrons combine with NADP+ to form NADPH

• As electrons flow along electron transport chain, protons build up inside thylakoids

H3C HC1"/,H C C H CHJ H CH3 H H H H H C CH1J,,/HJI I I I I I I I II! I/c, /C"", /c""' /C, /C, /C""' /c, /C"", /c~ /c, /c" /CH1H1C C C C C C C C C C C /C"I I I I I I I I I I H3C CHJH1C, /C H H H H H CHJ H CH3 HC "H1 CH3f,'-carotene

LIGHT DEP. RXNS cont’d

• These built up protons will diffuse down concentration gradient through ATP synthase

• TAH-DAH!! ATP is made!!!• PRODUCTS:

– O2, ATP and NADPH!!!!

CALVIN CYCLE• Pathway that produces organic

compounds, using energy stored in ATP and NADPH from the light reactions

• Occurs in the stroma

• CO2 is “fixed” into organic compounds

• RuBP (ribulose bisphosphate) is the 5 carbon sugar that CO2 is bound to by the enzyme rubisco

More CALVIN…

• The new 6 carbon molecule is immediately split into 2 3-carbon molecules (PGA)

• PGA converted to PGAL by addition of the phosphate from ATP and the hydrogen from NADPH

• ADP, NADP+ and phosphate are used again in the light reactions to form more ATP and NADPH

• To make one molecule of G3P, three turns of the cycle + 3 CO2 molecules

• CO2 is fixed to RuBP by rubisco – produces unstable intermediate -> 3-phosphoglycerate

• RuBP is regenerated – one per each trip through the cycle

• Uses 9 ATP and 6 NADPH

ALTERNATIVE PATHWAYS• Calvin cycle plants = C3 because of PGAL

that is formed (3 carbon)• Water loss through stomates is big

problem• When air is hot and dry, stomates close

to prevent water loss

• BAD THING – CO2 levels fall and O2 levels rise, resulting in carbon fixation inhibition

C4 PATHWAY

• During hottest part of day, C4 plants partially close stomates

• A special enzyme fixes CO2 into 4-carbon compounds that is stored in bundle sheath cells and can then enter the Calvin cycle

• Corn, sugar cane and crabgrass

CAM Photosynthesis• Adaptation to hot, dry climates• Open stomates at night and close them

during the day (minimizes water loss)• CO2 that enters at night is fixed into a

variety of organic compounds and stored in vacuoles; in morning, stomates close and CO2 is then released during the day and enters Calvin cycle

• These plants grow very slowly – cactuses, pineapples

Rates of Photosynthesis

• 4 limiting factors:– Light intensity, temperature, [CO2], [O2]

Active site of Rubisco can bind to O2 or CO2: Photorespiration – results in release of previously fixed CO2 that would otherwise remain in organic form

RATES OF PHOTOSYNTHESIS

• As light intensity increases, so does rate of photosynthesis

• Levels off at a max rate, when all electrons are excited

• Same thing for CO2 levels• Temperature increase, rate increases to

a point; then, enzymes denature and stomates close to prevent water loss, thus decreasing rate at high temperatures