2015-10-28

Lecture 16 1

Cell Biology Lecture 223.16: Chloroplasts

Chloroplasts

2015-10-28

Lecture 16 2

Chloroplasts

• Convert solar energy into ATP

Chloroplasts • Photosynthesis can be broken up into two phases or stages:

-Light reactions - Solar energy converted to ATP -Dark reactions - ATP used to fix CO2 into usable forms

Light Reactions (Stage 1) • Take place in thylakoids • Solar energy is absorbed to increase energy of electrons • Involves electron transfer which produces ATP and NADPH (and O2 as a byproduct)

used in Dark Reactions (Stage 2)

2015-10-28

Lecture 16 3

Light Reactions - Photosynthetic Pigments

1. Chlorophyll

2. Carotenoids

Light Reactions - Photosynthetic Pigments

P680 P700

Fig 14-31

Mg2+

2015-10-28

Lecture 16 4

Light Reactions - Photosynthetic Pigments

Light Reactions - Photosynthetic Pigments

2015-10-28

Lecture 16 5

Light Reactions - Photosynthetic Pigments

Each photosystem has light capturing antennae complexes (with many chlorophyll molecules) and a reaction centre that converts light E into chemical E.

In the rx centre a dimers known as a special pair hold electrons at a lower E

2015-10-28

Lecture 16 6

Light Reactions - Photosynthetic Pigments

charge separation

Photosynthesis is a biosynthetic process to build organic molecules from CO2 (the production of O2 is biproduct) The plant needs E to do this in the form of ATP and the activated carrier NADPH The plant does this using 2 photosystems (photosystem II and photosystem I) and the electron transport chain

2015-10-28

Lecture 16 7

Light Reactions and Flow of Electrons PSII (responsible for ATP II) -catalyzes the extraction of e- from water (O2 is a biproduct) -feeds e- to a proton pump leading to ATP synthesis e- transport chain (including plastoquinone) PSI (responsible for NADPH production) -final electron acceptor for the e- transport chain -three together power ATP and NADPH production e- transferred from PSII to Q to cytochrome complex (H+ pump) and eventually to PSI to Fd and then to NADP+ (which is reduced to NADPH)

H+ gradient established here fuels ATP production

*

Light boosts E of e-

Light boosts E of e-

Light Reactions - Photosynthetic Pigments

2015-10-28

Lecture 16 8

Light reactions: -Solar energy converted to ATP -Dark reactions: -ATP used to fix CO2 into usable forms

Dark Reactions • Carbon fixation reactions • 3ATP + 2NADPH are consumed • For each CO2 converted to carbohydrate • Begins in the stroma (fluid of the chloroplast) and ends in the cytosol

Light and Dark Reaction

2015-10-28

Lecture 16 9

Dark Reactions

(a 5 carbon sugar)

Dark Reactions

Final product