AP Biology Notes Outline Chapter 10: Photosynthesis Photosystems are light-harvesting complexes in the thylakoid membranes of chloroplasts. o Each photosystem consists of a reaction center containing chlorophyll a and a region of many atenna pigment molecules that funnel energy into chlorophyll a. Two types of photosystems cooperate during photosynthesis: o Photosystem I o Photosystem II Named in the order they were discovered – however, PS II occurs first, followed by PS I. o PS I absorbs light best in the 700nm range (so called P700). o PS II absorbs light best in the 680nm range (so called P680). There are 2 stages in Photosynthesis: o Light dependent reactions o Light independent reactions (Calvin Cycle) BOTH REQUIRE LIGHT (SOMEWHAT): Even the dark reactions in most plants occurs during daylight – because that is the only time the light reactions can operate AND the dark reactions depend on the light reactions!!! OVERVIEW OF LIGHT DEPENDENT REACTIONS: require presence of light occur in thylakoids of chloroplasts
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AP Biology Notes Outline Chapter 10: Photosynthesis
Photosystems are light-harvesting complexes in the thylakoid membranes of chloroplasts.o Each photosystem consists of a reaction
center containing chlorophyll a and a region of many atenna pigment molecules that funnel energy into chlorophyll a.
Two types of photosystems cooperate during photosynthesis:
o Photosystem Io Photosystem II
Named in the order they were discovered – however, PS II occurs first, followed by PS I.
o PS I absorbs light best in the 700nm range (so called P700).
o PS II absorbs light best in the 680nm range (so called P680).
There are 2 stages in Photosynthesis:o Light dependent reactionso Light independent reactions (Calvin Cycle)
BOTH REQUIRE LIGHT (SOMEWHAT): Even the dark reactions in most plants occurs during daylight – because that is the
only time the light reactions can operate AND the dark reactions depend on the light reactions!!!
OVERVIEW OF LIGHT DEPENDENT REACTIONS: require presence of light occur in thylakoids of chloroplasts use energy from light to produce ATP and
NADPH (a temporary, mobile energy source that helps store even more energy)
oxygen gas is produced as a by-product
OVERVIEW OF LIGHT INDEPENDENT REACTIONS (CALVIN CYCLE) do not require light directly – so also known
as the Dark Reactions or the Calvin Cycle take place in the stroma of chloroplasts ATP and NADPH produced during light dependent reactions are used to make glucose
AP Biology Notes Outline Chapter 10: Photosynthesis
Light is absorbed by PS I and PS II in the thylakoid membranes and electrons flow through electron transport chains.o There are 2 possible routes for electron flow:
AP Biology Notes Outline Chapter 10: Photosynthesis
How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 2)
How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 3)
Photolysis – water gets split apart, providing electrons to replace those lost from chlorophyll a in PS II.
Photolysis splits water into two electrons, two protons, and one oxygen atom.
Two oxygen atoms combine to produce one O2 molecule, which is released into the air as waste.
Electron Transport Chain – Electrons from PS II pass along an electron transport chain consisting of plastoquinone (PQ), and a complex of cytochromes and ultimately end up in PS I.
This flow of electrons is exergonic and provides energy to produce ATP by chemiosmosis.
Because this ATP synthesis is powered by light, it is called PHOTOPHOSPHORYLATION.
AP Biology Notes Outline Chapter 10: Photosynthesis
Electron Transport in Photosynthesis – To Accompany Layer 3
Protons that were released from water during photolysis are pumped by the thylakoid membrane from the stroma into the lumen (thylakoid space).
ATP is formed as these protons diffuse down the gradient from the thylakoid space, through ATP-synthase channels, and into the stroma.
The ATP produced here provides the energy to power the Calvin cycle.
NADP becomes reduced when it picks up the two protons that were released from water in PS II. Newly formed NADPH carries hydrogen to the Calvin cycle.
How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 4 & 5)
Photosystem I – energy is absorbed by PS I. Electrons from the head of chlorophyll a become energized and are captured by a primary electron receptor.
Electrons that escape from chlorophyll a are replaced with electrons from PS II (instead of water).
This electron chain contains ferrodoxin and ends with the production of NADPH, not ATP.
Remember: the light reactions use solar power to generate ATP and NADPH, which provide chemical energy and reducing power, to the sugar-making reactions of the Calvin Cycle.
AP Biology Notes Outline Chapter 10: Photosynthesis
Under certain conditions…photoexcited electrons take a short-circuit path called cyclic electron flow.
o Involves Photosystem I only…no production of NADPH and no release of oxygen…but ATP is produced.
During Cyclic Electron Flow, photo-excited electrons travel from PS II electron transport chain to PS I, to a primary electron acceptor, and then back to the cytochrome complex in PS II.
o THE SOLE PURPOSE OF CYCLIC PHOTOPHOSPHORYLATION IS TO PRODUCE ATP!!!
Why Cyclic?o Because noncyclic electron flow produces ATP and NADPH in roughly equal quantities…but Calvin cycle consumes
more ATP than NADPH. Cyclic flow makes up the difference for more ATP needed.
Cyclic vs. Noncyclic Electron Flow Noncyclic – pg. 186
o uses Photosystem II, and ETC (with the electron carrier plastoquinone, Pq) , Photosystem I, and another ETC using an iron-containing protein called ferredoxin.
o produces ATP and NADPH Cyclic – pg. 187
o uses only Photosystem I and the second ETC – no production of NADPH and no release of Oxygeno DOES produce ATP to be used to make up the difference needed due to Calvin cycle demands.
Comparison of Chemiosmosis in Cell Respiration & Photosynthesis
AP Biology Notes Outline Chapter 10: Photosynthesis
PHASE 3 – REGENERATION
In a series of reactions, the carbon skeletons of 5 molecules of G3P are rearranged by the last steps of the Calvin cycle into three molecules of RuBP….the RuBP is now prepared again to receive CO2…and the cycle continues.
Light Reactions:-carried out by molecules in thylakoid membranes
-convert light E to chemical E of ATP and NADPH
-split H2O and release O2 to the atmosphere
Calvin Cycle Reactions:-take place in stroma
-use ATP and NADPH to convert CO2 to the sugar G3P
-return ADP, inorganic phosphate, and NADP+ to the light reactions
AP Biology Notes Outline Chapter 10: Photosynthesis
Normal Pathway - C3 Plants Initial fixation of carbon occurs using rubisco – enzyme in Calvin cycle that adds CO2 to RuBP (ribulose biphosphate) Produces first organic product called 3-phosphoglycerate (thus called C3 plant)
o Ex. rice, wheat, soybeans THESE PLANTS PRODUCE LESS FOOD WHEN THEIR STOMATA CLOSE ON HOT, DRY DAYS.
o The declining level of CO2 in the leaf starves the Calvin cycle.o Making matters worse, rubisco can accept O2 in place of CO2 – and as O2 concentrations overtake CO2 concentrations, rubisco adds O2
to the Calvin cycle instead of CO2. The product formed splits, leaves the chloroplast, and is broken down by mitochondria and peroxisomes – KNOWN AS
PHOTORESPIRATION! PHOTORESPIRATION GENERATES NO ATP AND NO SUGAR!!!!
All plants do not use RuBP directly to fix their carbon!o The environmental factors that foster photorespiration are hot, dry, bright days – which cause stomata to close.o In certain plant species, alternate modes of carbon fixation that minimize photorespiration have evolved:
C4 PLANTS CAM PLANTS
C4 Plants Modification for DRY ENVIRONMENTS (combats
photorespiration)o Spatial separation of processes (mesophyll
cells v/s bundle-sheath cells) In C4 plants, a series of steps precedes the Calvin
cycle that pump CO2 deep into the leaf in bundle-sheath cells.o Requires unique leaf anatomy – see page 192
CO2 enters the mesophyll cell of the leaf and combines with a 3-carbon molecule PEP, to form the 4-carbon molecule oxaloacetate. The enzyme that catalyzes this reaction does not combine with oxygen and therefore can fix CO2 more efficiently!
C4 photosynthesis minimizes photorespiration and enhances sugar production
o Ex. Sugarcane, corn, members of the grass family
CAM Plants Adaptation for dry environments (combats photorespiration). CAM
stands for crassulacean acid metabolism. Temporal separation – night v/s day:
o Succulents, cacti, pineapples Open stomata during night and close them during day Take up CO2 during night and make organic acids (this is the mode of
carbon fixation…)o Mesophyll cells of CAM plants store organic acids in their
vacuoles until day, when light reactions can supply ATP and NADPH for Calvin Cycle
During day, the CO2 is released from the organic acids
AP Biology Notes Outline Chapter 10: Photosynthesis
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