Photosynthesis and the Calvin Cycle Flashcards

Photosynthesis Overview

• Two major processes:

  • Light reactions

  • Calvin cycle

Light Reactions

• Convert sun's energy into chemical energy.

• Process:

  • Split water (H_2O).

  • Use electrons and hydrogen ions (H^+) to produce ATP and NADPH.

  • Release oxygen (O_2) as a byproduct.

• Equation:

  • H2O to ATP + NADPH + O2

• Oxygen released during photosynthesis originates from the splitting of water.

• Energy products (ATP and NADPH) are used in the Calvin cycle to fix carbon dioxide into carbohydrates.

Calvin Cycle

• Uses energy products from light reactions to fix carbon dioxide (CO_2) into carbohydrates.

• Output:

  • Glyceraldehyde-3-phosphate (G3P).

  • Two G3P molecules combine to form one glucose molecule.

    • 2 {G3P} = 1 {Glucose}

  • G3P serves as a building block for glucose and other organic molecules.

Photosystems in Light Reactions

• Two photosystems involved:

  • Photosystem II (PSII)

  • Photosystem I (PSI)

• Named in order of discovery, not order of action.

Photosystem II (PSII)

• Splits water molecules.

• Releases electrons, hydrogen ions (H^+), and oxygen (O_2).

• Electrons are used in the electron transport chain to produce ATP.

• H^+ ions contribute to a hydrogen ion gradient used for ATP synthesis.

Electron Transport Chain (ETC) in PSII

• Photons excite pigment complexes in PSII.

• Excited electrons are transferred to an electron acceptor in the reaction center.

• Reaction center chlorophyll a in PSII is P680.

  • P_{680}

• Excited electron from P680 is passed down the electron transport chain.

• Energy from electrons is used to pump H^+ ions across the thylakoid membrane, creating an H^+ ion gradient.

• H^+ ions then flow back through ATP synthase, driving the phosphorylation of ADP into ATP.

  • ADP + P_i to ATP

Photosystem I (PSI)

• Electrons from the electron transport chain of PSII are transferred to PSI.

• Reaction center chlorophyll a in PSI is P700.

  • P_{700}

• Excited electrons are passed down a short electron transport chain via ferredoxin to NADP+ reductase.

• NADP+ reductase uses electrons to reduce NADP+ into NADPH.

  • NADP^+ + 2e^- + H^+ to NADPH

Location of Photosystems

• Photosystems are located in the thylakoid membrane.

• Photosystems are pigment complexes that collect solar energy, acting like antennas.

Cyclic vs. Noncyclic Pathways

• Both cyclic and noncyclic pathways produce ATP.

• Noncyclic pathway also produces NADPH.

• If the rate of ATP production is not equal to the rate of NADPH production, the system shifts to cyclic pathway to produce ATP only.

• Cyclic pathway involves only photosystem I and does not split water like photosystem II does.

Noncyclic Pathway

• Occurs in the thylakoid membrane.

• Involves both photosystems II and I.

  • PSII captures light energy and ejects an electron from its reaction center chlorophyll a.

  • Electrons are sent to the primary electron transport chain to PSI.

  • Electrons lost by PSII are replaced by the splitting of water, forming oxygen and hydrogen ions.

• Hydrogen ions accumulate in the thylakoid membrane, creating an H^+ ion gradient.

• ATP synthase uses this gradient to produce ATP.

• In PSI, light energy is captured, and an electron is ejected and transferred to NADP+, forming NADPH.

Thylakoid Membrane

• Location of the noncyclic pathway.

• Stroma side: Outside the chloroplast.

• Thylakoid space: Inside the thylakoid membrane.

• Water is split at PSII.

• Electron transport chain pumps H^+ ions into the thylakoid space.

• PSI produces NADPH.

• ATP synthase phosphorylates ADP into ATP using the H^+ ion gradient.

Components of Photosystems

• Photosystem II (PSII)

  • Pigment complex and electron acceptors.

  • Receives electrons from water splitting.

  • Releases oxygen as a gas (O_2).

• Electron Transport Chain

  • Cytochrome complexes and plastoquinone.

  • Carry electrons between PSII and PSI.

  • Pump H^+ ions from the stroma into the thylakoid space.

• Photosystem I (PSI)

  • Pigment complexes and electron acceptors.

  • Located adjacent to NADP+ reductase.

  • Reduces NADP+ to NADPH.

• ATP Synthase

  • Channel for H^+ ion flow.

  • Flow of H^+ ions drives phosphorylation of ADP to ATP.

Cyclic Pathway

• Occurs when ATP production is not equal to NADPH production.

• Involves only photosystem I (P700).

• Electrons travel in a cyclic manner, reverting to photosystem I.

• ATP molecules are produced.

• Water is not required.

• NADPH is not synthesized.

• Oxygen is not evolved as a byproduct.

• Predominant in bacteria.

Phosphorylation Processes

• Photophosphorylation (in photosynthesis)

  • Utilizes light energy to convert ADP to ATP.

• Oxidative Phosphorylation (in respiration)

  • Electrons come from the food we eat, breaking down glucose.

  • Both involve an electron transport chain and chemiosmosis.

• Substrate-level Phosphorylation (in respiration)

  • Uses an enzyme to add ADP + Pi to yield ATP.

  • Less efficient than photophosphorylation and oxidative phosphorylation.

Photophosphorylation vs. Oxidative Phosphorylation

• Photophosphorylation: ATP production in photosynthesis using light energy.

• Oxidative Phosphorylation: ATP production in respiration using energy from glucose.

• Both involve an electron transport chain and chemiosmosis.

Noncyclic Photophosphorylation Recap

• Both photosystems I and II are used.

• P680 is the reaction center of chlorophyll a in PSII.

• Electrons travel in a noncyclic manner (Z scheme).

• Electrons from PSI are accepted by NADP+, forming NADPH.

• Both NADPH and ATP molecules are produced.

• Photolysis of water occurs.

• Oxygen is evolved as a product.

• Predominant in green plants.

ATP Production: A Deeper Dive

• The thylakoid space acts as a reservoir for H^+ ions.

• Each time water is oxidized, two H^+ ions remain in the thylakoid space.

• Transfer of electrons in the electron transport chain yields energy used to pump H^+ ions across the thylakoid membrane.

• H^+ ions move from the stroma into the thylakoid space.

• H^+ ions flow back across the thylakoid membrane via ATP synthase.

• The flow across ATP synthase energizes the phosphorylation of ADP into ATP.

• This method of producing ATP is called chemiosmosis because ATP production is tied to the establishment of a hydrogen ion gradient.

• Water splitting provides electrons for the electron transport chain to pump H^+ ions into the membrane.

• Electrons continue down the chain until they reach PSI.

• Electrons are then transferred to NADP+ reductase, which converts NADP+ to NADPH.

Calvin Cycle Overview

• Uses ATP and NADPH produced in the light reactions to fix carbon dioxide.

• Occurs in the stroma.

• Has three main stages:

  1. Carbon dioxide fixation

  2. Carbon dioxide reduction

  3. Regeneration of RuBP

• Is a cycle (starting molecule is regenerated).

Stages of the Calvin Cycle

1. Carbon Fixation

   • Starting molecule: Ribulose-1,5-bisphosphate (RuBP).

   • RuBP accepts carbon dioxide (CO_2).

   • Enzyme: RuBisCO (RuBP carboxylase).

     • For every 3 CO_2 molecules that enter, RuBisCO helps create an intermediate 6-carbon molecule

     • The 6-carbon molecule quickly splits into 6 molecules of 3-phosphoglycerate (3-PG).

2. Carbon Reduction

   • These 3-PGs then use ATP to change it to 6 molecules of 1,3-bisphosphoglycerate (1,3-BPG).

   • Then NADPH is used to convert those to our output molecule, which is glyceraldehyde 3-phosphate (G3P).

3. Regeneration of RuBP

   • Only one G3P leaves the cycle.

   • The other five G3P molecules are used to regenerate RuBP.

   • More ATP is invested to regenerate RuBP.

C3 Photosynthesis

• First output is a three-carbon molecule.

• Alternate Photosynthetic Strategies

  • C4 Plants

  • CAM Plants

    • Open stomata at night to take in carbon dioxide.

    • Store carbon dioxide as crassulacean acid.

    • During the day, release carbon dioxide to drive the Calvin cycle.

Overview of C3 Photosynthesis

• Carbon dioxide is attached to RuBP by RuBP carboxylase (RuBisCO).

• RuBisCO results in a six-carbon molecule that splits into two three-carbon molecules called 3-PG (3-phosphoglycerate).

• Do not confuse 3-PG with G3P.

Calvin Cycle Reactions

• RuBisCO catalyzes the reaction of carbon dioxide and RuBP to produce 3-phosphoglycerates.

• A phosphate group is transferred from ATP to 3-phosphoglycerate, producing 1,3-bisphosphoglycerate and ADP.

• The reduced NADPH transfers a proton reducing a molecule of 1,3-bisphosphoglycerate producing glyceraldehyde 3-phosphate and NADP+.

• RUBP is regenerated through a series of enzyme catalyzed reactions and then dephosphorylated using ATP to produce RuBP.

• For every complete cycle, one G3P is released.

• Three carbon dioxide molecules result one G3P molecule.

• In the reduction stage:

  • 3-PG is reduced to BPG.

  • BPG is reduced to G3P.

  • The G3P is reduced and chemically able to store more energy and complex molecules, such as glucose.

Key Concepts to Remember

• Stage 1: Carbon fixation

• Starting molecule: RuBP

• Enzyme: RuBisCO

• One G3P is released for every three carbon dioxide molecules fixed.

• The other five G3P molecules are used to regenerate RuBP.

• Know the three stages!

• Know the starting molecule!

• Know the output of G3P!