CARNES AP BIO



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← 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

← 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

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← 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:

▪ Noncylic photophosphorylation

▪ Cyclic photophosphorylation

← Photophosphorylation is a method of generating ATP by using light to add P to ADP

o Occurs in Light Reactions

← When a photon strikes a pigment molecule, the energy is passed from molecule to molecule until it reaches the reaction center.

o Here, an excited electron from the reaction center chlorophyll is captured by a specialized molecule call the primary electron acceptor.

← During noncyclic photophosphorylation, electrons enter two electron transport chains, and ATP and NADPH are formed.

← The process begins in PS II and proceeds to PS I.

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How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 1)

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)

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)

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← 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 Oxygen

o DOES produce ATP to be used to make up the difference needed due to Calvin cycle demands.

Comparison of Chemiosmosis in Cell Respiration & Photosynthesis

Remember:

In mitochondria…oxidative phosphorylation.

In chloroplast…photophosphorylation.

The thylakoid membrane of the chloroplast pumps protons from the stroma into the thylakoid space, which functions as the H+ reservoir.

The thylakoid membrane makes ATP as the hydrogen ions diffuse from the thylakoid space back to the stroma through ATP synthase.

Thus…ATP forms in the stroma, where it is used to help drive sugar synthesis during the Calvin cycle.

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← Calvin cycle can be divided into 3 phases:

o Phase 1: Carbon Fixation

o Phase 2: Reduction

o Phase 3: Regeneration of CO2 acceptor (RuBP)

PHASE 1 – CARBON FIXATION

CO2 is incorporated through stomata and attached to RuBP (catalyzed by enzyme rubisco).

← Product of reaction is 6-carbon intermediate so unstable that it splits in half to form two molecules of 3-phosphoglycerate.

PHASE 2 – REDUCTION

← Each molecule of 3-phosphoglycerate receives additional phosphate group from ATP to become 1,3 biphosphoglycerate.

← Pair of electrons donated from NADPH reduces 1,3 to G3P (a sugar)….notice for every 3 molecules of CO2 there are six molecules of G3P

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.

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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

USEFUL ANIMATION:



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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.

Energy is absorbed by PS II.

Electrons from the double bonds in the head of chlorophyll a become energized and move to a higher energy level.

They are captured by a primary electron acceptor.

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.

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.

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

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