Photosynthesis

- the transfer of energy from sunlight to organic molecules

- occurs in green plants, algae and some bacteria

- involves a complex series of chemical reactions

Chloroplasts

- the organelle in plants where photosynthesis takes place

- double outer membrane- stroma (solution)- thylakoids (interconnected stacks of grana)

Sunlight

- electromagnetic spectrum

Pigment - compound that absorbs light

Chloroplast pigments:- chlorophylls (mostly a and b)- carotenoids (yellow, orange, brown)

Accessory pigments (chlorophyll b, carotenoids)

Electron Transport

The energy from sunlight is transferred from one molecule to another through electrons

Photosystem - clusters of pigment molecules

Step 1: light is absorbed by chlorophyll a in photosystem II, which causes the electrons to be “excited”

Step 2: excited electrons move to primary electron acceptor

Step 3: excited electrons move through electron transport chain

Step 4: light is absorbed by chlorophyll in photosystem I, and excited electrons move through etc

Step 5: excited electrons combine with NADP+ to form NADPH (will be used in next step

Chemiosmosis

ATP is produced

NADPH and ATP is used in the next part of photosynthesis - the Calvin Cycle

The Calvin Cycle

Produces organic compounds using the energy from ATP and NADPH formed during the light reactions.

The Calvin cycle “fixes” carbon from CO2.

Step 1: CO2 diffuses into the stroma. An enzyme combines a CO2 molecule with a 5-carbon molecule (RuBP). The 6-carbon molecule splits into a pair of 3-carbon molecules (PGA)

Step 2: PGA is converted into PGAL using ATP and NADPH.

Step 3: Some PGAL molecules can be used to make organic compounds

Equation for Photosynthesis

6CO2 + 6H2O + light energy -->

C6H12O6 + 6O2

Three turns of the Calvin cycle require nine ATP and six NADPH.

Mr. Durand Photosynthesis Song

Photosynthesis Song

Alternative Pathways

C3 plants - plant species that fix carbon exclusively through the Calvin cycle.

C4 plants - plant species that fix CO2 into four-carbon compounds- plants that partially close their stomata during the hottest part of the day-examples - corn, sugar cane, crabgrass

CAM plants - plant species that fix CO2 at night when their stomata are open-grow fairly slowly-lose less water than C3 or C4 plants-examples - cactuses, pineapples

Rate of Photosynthesis

Affected by:- light intensity - rate increases then levels off to a plateau- CO2 levels - rate increases then levels off to a plateau- temperature - rate increases, peaks at a certain temp, then decreases