Chapter 10: Photosynthesis Photosynthesis transforms solar light energy into chemical bond energy stored as sugar

Autotrophs vs heterotrophs Autotrophic organisms synthesize their own organic food from inorganic raw materials * photoautotrophs * chemoautotrophs Heterotrophic -consumers * Animalsand decomposers

Structure of a chloroplast Chloroplasts are primarily in cells of mesophyll, green tissue in the leaf’s interior. Parts to know: Intermembrane space – between the inner and outer membranes Thylakoids – flattened membranous sacs inside the chloroplast thylakoid membranes- contain chlorophyl /light reactions/ chemiosmosis thylakoid space- inside the thylakoid. grana – stacks of thylakoids Stroma – fluid outside the thylakoids

Properties of Light  electromagnetic energy - composed of discrete particles called “photons” with energy dependent on wavelength

 Visible light is only a small portion of the electromagnetic spectrum -blue and red are the two wavelengths most effectively absorbed by chlorophyll -green is least effective (reflected)

Photosynthetic Pigments  Pigments are substances that absorb visible light  Every pigment has a characteristic absorption spectrum

 Chlorophyll a is the blue/green light- absorbing pigment that participates directly in the light reaction  Accessory Pigments include: - Chlorophyll b (a yellow-green pigment similar to chlorophyll a) - Carotenoids (yellow and orange)

- The absorbed photon boosts one of the pigment molecule’s electrons in its lowest energy state (ground state) to an orbital of higher potential energy (excited state)

Photosystems  Photosystems are the light-harvesting complexes of the thylakoid membrane  Photosystems contain chlorophyll a, chlorophyll b, and carotenoids

Photosystem components 1) Antenna Complex - several hundred chlorophyll a, chlorophyll b, and carotenoid molecules to absorb photons and pass energy from molecule to molecule - different pigments have different absorption spectra

2) Reaction-center Chlorophyll - only one of the many chlorophyll a molecules in each complex transfers an excited electron to initiate the light reactions. Location is the reaction center

3) Primary electron acceptor located near the reaction center, a primary electron acceptor molecule traps excited electrons released from the reaction center chlorophyll

Photosystems  The reaction center of photosystem I has a chlorophyll a molecule known as P700 (absorbs best at 700 nm)  The reaction center of photosystem II has a chlorophyll a molecule known as P680 (absorbs best at 680 nm)

Noncyclic Electron Flow  Noncyclic electron flow involves both photosystem I and photosystem II. * occurs in the thylakoid membrane to split water into H +, electrons, and oxygen produces ATP, NADPH, and O 2

1) Photons excite photosystem I and electrons are transferred from P700 to the primary electron acceptor 2) These electrons are passed on to ferredoxin (an iron containing protein) and are passed on to NADP+, producing NADPH (reducing power for Calvin cycle)

3)Photosystem II supplies electrons to fill the electron “holes” in photosystem I  Electrons ejected from P680 are trapped by photosystem II primary electron acceptor  These electrons are passed down an ETC embedded in the thylakoid membrane.

 As excited electrons move to P700, protons are picked up on one side (stroma) and deposited on the other side (thylakoid space)

 An ATP synthase enzyme in the thylakoid membrane uses the proton-motive force to make ATP. noncyclic photophosphorylation

How do we replace these missing electrons from photosystem II?  We get them from water!  A water-splitting enzyme extracts electrons from water and passes them to the P680 reaction center

II. Cyclic Electron Flow  Cyclic electron flow only involves Photosystem I. This process: - produces ATP - does not produce NADPH or O 2

1) Photons excite photosystem I and electrons are transferred from P700 to the primary electron acceptor 2) The electrons are not passed on to NADP+ Reductase (which would produce NADPH). Instead the electrons are passed down the ETC back to P700. [Fig ]  This process is called cyclic photophosphorylation

Why do plants need this cyclic pathway?  To create more ATP for the Calvin cycle.  NADPH concentration might influence whether electrons flow through cyclic or noncyclic pathways?

Comparison of Chemiosmosis in Chloro. & Mitoch. Similar:  electrons passed on to more electronegative carriers  ATP synthase complexes  electron carriers (quinones and cytochromes) similar

Different:  Chloroplast use light energy, mitochondria use chemical energy to produce ATP  Chloroplasts pump protons from stroma to thylakoid compartment… mitochondria from matrix to intermembrane space

Calvin Cycle  The Calvin cycle uses the ATP and NADPH produced from the light reaction to reduce CO 2 to sugar

Phase I: Carbon Fixation  Each CO 2 attaches to a five-carbon sugar (RuBP) – ribulose bisphosphate - This reaction is catalyzed by rubisco ribulose bisphosphate carboxylase - The product of this rxn is an unstable 6- carbon intermediate that immediately splits into two 3-carbon molecules (3- phosphoglycerate)

Phase II: Reduction ATP phosphorylates and NADPH reduces 3 phosphoglycerate to Glyceraldehyde 3 Phosphate (G3P)  G3P stores more potential energy than 3-phosphoglycerate  For every 3 CO 2 that enter the Calvin cycle, six G3P are produced, only one of which can be counted as a net gain.

 The cycle begins with three 5-carbon molecules - a total of 15 carbons  The six G3P molecules produced contain 18 carbons (a net gain of 3 carbons from CO 2 )  One G3P molecule exits the cycle… what happens to the other five?

Phase III: Regeneration of Starting Material (RuBP)  The other five G3P are recycled through a complex series of rxns to regenerate three molecules of the original 5-carbon sugar (RuBP)  These rxns require 3 more ATP (why we needed cyclic electron flow )

Summary of Calvin Cycle:  The net synthesis of one G3P molecule requires: ATP? 9 NADPH molecules? 6 CO 2 ? 3  G3P is used to construct glucose and other carbohydrates  The Calvin cycle uses 18 ATP and 12 NADPH and 6 CO2 to produce one glucose 

1)Carbon enters the Calvin cycle as CO 2 and leaves as sugar glyceraldehyde 3- phosphate (G3P) 2)ATP is the energy source, while NADPH is the reducing agent 3) Calvin cycle is similar to the Kreb cycle in that the starting material is regenerated

Photorespiration Rubisco can also accept Oxygen – results in the loss of 2-carbon fragments which peroxisomes and mitochondria convert to carbon dioxide Leaches carbon away from Calvin cycle!!!

Alternate photosynthesis C 4 Plants 1) fix incoming CO2 to an organic acid in mesophyll cells(via *PEP carboxylase*) 2) deliver this acid directly to bundle sheath cells, in which the Calvin cycle occurs Cost: extra cells (bundle sheath) and enzymes Gain: reduced photorespiration

C A M Plants (crassulacean acid metabolism) 1) fix incoming CO2 to an organic acid only during night (when stomata are open) 2) close stomata during day and feed CO2 to Calvin cycle as light reactions occur Cost: extra enzymes and CO2 availability Gain: reduced photorespiration AND transpiration