PHOTOSYNTHESIS - Process That Converts Light Energy Into the Chemical Energy That

PHOTOSYNTHESIS

PHOTOSYNTHESIS - Process that converts light energy into the chemical energy that

is stored in sugars.

Light + 6CO2 + 6H2O → C6H12O6 + 6O2

Pigments - Molecules specialized for absorbing certain wavelengths of light.

Ex. chlorophyll, carotene, xanthophylls

- Leaves have different pigments to absorb the many different wavelengths

of light.

- Pigments other than chlorophyll are always present, but only seen when the

unstable chlorophyll is broken down and not regenerated during the autumn.

- Chlorophyll absorbs red and blue wavelengths of light, reflects green light.

LIGHT-DEPENDENT REACTIONS –

-  Occur in the thylakoids of the chloroplast. Process converts light

energy into chemical energy.

- Chlorophyll and other pigments are located in units called photosystems,

which are inside the thylakoid membrane. Photosystems absorb light

and energize electrons (e-).

-  Absorbs sunlight and uses this energy to help split water (photolysis), energize electrons, and produce the molecules ATP and NADPH.

(these are energy-storing molecules)

-  Oxygen is released as a product when water is split. H+ ions and

e- are also released from water.

- High-energy electrons are accepted by NADP+ to form NADPH

- ATP synthase (an enzyme) converts ADP into ATP. This reaction

is powered by the movement of H+ ions down the concentration

gradient.

(underlined words are products of the light-dependent reactions)

PHOTOSYSTEM II –

- Water is split to form H+ ions, oxygen, and electrons (e-)

-  Light absorbed by chlorophyll energizes electrons which will be passed

along the electron transport chain (ETC).

ELECTRON TRANSPORT CHAIN (ETC) –

- High energy e- are transferred to a series of electron carriers.

Energy from the e- is used to pump H+ ions across the thylakoid

membrane against the concentration gradient (into the thylakoid).

PHOTOSYSTEM I –

- Light absorbed here re-energizes the electrons. The e- (and H+ ions )

are accepted by NADP+ to form the energy-storing molecule called

NADPH.

ATP FORMATION –

- H+ ions inside the thylakoid flow down the concentration gradient

through a membrane protein called ATP synthase. The energy

released by falling H+ ions is used to power the change of ADP

into ATP.

LIGHT-INDEPENDENT REACTIONS –

- Occurs in the stroma of the chloroplast

- Includes the Calvin cycle which uses the energy stored in ATP and

NADPH to produce glucose from CO2

- Carbon Fixation incorporates inorganic CO2 from the air into a usable

organic compound.

- RuBisCo is an enzyme that catalyzes the reaction between CO2 and

organic RuBP (5-carbon sugar) in the carbon fixation reaction.

- 6 CO2 are required to produce 1 glucose molecule

- The first stable substance formed during the Calvin cycle is a 3-carbon

compound called PGA.

- High-energy phosphates from ATP and electrons from NADPH are

combined with PGA to form another 3-carbon compound called PGAL

(also known as G3P or GAP)

- For every 3 CO2 molecules that enter the Calvin cycle, 1 PGAL leaves

- A complex series of reactions, requiring ATP, rearrange other PGAL

molecules into RuBP molecules to keep the cycle going.

(Ex. 5 PGAL → 3 RuBP) (1 ATP needed for each RuBP produced)

- 2 PGALs can be combined to form glucose