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Kit Tabañera

on 14 January 2014

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Light-independent reaction ( Calvin Cycle)
Equation for Photosynthesis
Introduction to Photosynthesis
The process of photosynthesis produces ATP from ADP and Pi by using the energy from light to excite electrons that are passed along an electron transport chain.

Coupled with the transfer of electrons is the pumping of hydrogen ions and the splitting of water molecules.

Complexes found in a
Photosynthesis Electron Transport Chain:
Photosystem I - passes electrons through ET chain of acceptors that generate NADPH
Cytochrome b6f - also known as plastoquinol-plastocyanin reductase; transfers electrons between the two reaction complexes from Photosystem II to Photosystem I, whereby introducing protons into the thylakoid space.
Photosystem II - has chlorophyll a with no or few chlorophyll b and can accept a photon from the antenna directly and an electron from PS II.
Ferredoxin NADP Reductase (FNR) - transfers an electron from each of two ferredoxin molecules to a single molecule of the two electron carrier NADPH.
ATP Synthase - complex that uses the potential energy of flowing hydrogen ions to make ATP
Three Mobile Carriers that transports the electrons:
Plastiquinone Qb - transports the protons to the lumen of thylakoid discs, while the electrons continue through the electron transport chain into the cytochrome b6f protein complex.
Plastocyanin - functions as an electron transfer agent between cytochrome f of the cytochrome b6f complex from photosystem II and P700+ from photosystem I
Ferredoxin - accepts electrons produced from sunlight-excited chlorophyll and transfers them to the enzyme ferredoxin:NADP+ oxidoreductase

Light-dependent reaction
Light-independent reaction or Calvin Cycle

Light-dependent: converts light energy into chemical energy; produces ATP molecules to be used to fuel light-independent reaction, NADPH and Oxygen ( O2).

Light-independent: uses ATP produced to make simple sugars.

2 Phases of Photosynthesis
1.) A photon of light hits a chlorophyll molecule surrounding the Photosystem II complex; this creates resonance energy that transferred to neighboring chlorophyll molecules.

2.) When this energy reaches the reaction center embedded in Photosytem II, an electron is released. The reaction center Chlorophyll contains electrons that can be transferred when excited, one photon is needed to excite each of the electrons in this chlorophyll.

3.) Once excited 2 electrons are transferred to Plastiquinone Qb, the first mobile carrier, in addition to the 2 electrons Qb also picks up 2 protons from the stroma.

4.) The 2 electrons lost from Photosystem II are replaced by the splitting of water molecules; water splitting also releases hydrogen ions into the lumen. This contributes to a hydrogen ion gradient similar to the one created by mitochondrial ET.

5.) After 2 water molecules have been split, 1 molecule of molecular oxygen is created.

6.) Plastiquinone Qb then transfers the 2 electrons to the Cytochrome b6f complex. The protons it picked up are released into the lumen, this transfers are coupled with the pumping of two more hydrogen ions into the lumen's space by Cytochrome b6f.

7.) The eletrons are next transferred to Plastocyanin, the second mobile carrier.

8.) Next the electrons are transferred from Plastocyanin to the Photosystem I complex. It is here where photons again energize each electron and propel their transfer to Ferredoxin.

9.) Ferredoxin then transfers the electrons to FNR.

10.) After 2 electrons are transferred to FNR, NADPH is made by adding the 2 electrons and a hydrogen ion to NADP.

11.) The gradient created by the electron transport chain is utilized by ATP synthase to create ATP from ADP and Pi. This is similar to way ATP is synthesized in the mitochondria. ATP, NADPH and Molecular Oxygen are the final vital products of photosynthesis.

Occurs in the Thylakoid membranes
During the light reaction, there are two possible routes for electron flow:
A. Cyclic Electron Flow

B. Noncyclic Electron Flow

Pigments absorb light energy & excite e- of Chlorophyll a to produce ATP



Photosystem I








by ETC

Cyclic Electron Flow

Occurs in the thylakoid membrane
Uses Photosystem II and Photosystem I
P680 reaction center (PSII) - chlorophyll a
P700 reaction center (PS I) - chlorophyll a
Uses Electron Transport Chain (ETC)
Generates O2, ATP and NADPH

Noncyclic Electron Flow

Oxygen comes from the splitting of H2O, not CO2

H2O 1/2 O2 + 2H+

Requires light
Occurs in chloroplast (in thylakoids)
Chlorophyll (thylakoid) traps energy from light
Light excites electron (e-)
Kicks e- out of chlorophyll to an electron transport chain
Electron transport chain: series of proteins in thylakoid membrane
Energy lost along electron transport chain
Lost energy used to recharge ATP from the combination of ADP and Pi

NADPH produced from e- transport chain
Stores energy until transfer to stroma
Plays important role in light-independent reaction

Total byproducts: ATP, NADP, O2

Light-dependent reaction (Light Reaction)
Occurs in the thylakoid membrane.
Uses Photosystem I only
P700 reaction center- chlorophyll a
Uses Electron Transport Chain (ETC)
Generates ATP only


Cyclic Electron Flow

1.) The cyclic electron flow also called the cyclic phosphorylation, involves an electron transfer chain that starts from a pigment complex called photosystem I in the chlorophyll.

2.) It passes from the primary acceptor to the ferredoxin (iron-sulfur proteins).

3.) From there it is transferred to a complex of two cytochromes and then to plastoquinone (quinone molecule which is a part of the transport chain of electrons in light reactions).

4.) After which it returns to the chlorophyll.

This electron transport chain produces a proton-motive force (PMF), which leads to the pumping of H+ ions across the membrane and results in a concentration gradient that is used to power ATP synthase during chemiosmosis (movement of ions). The cyclic pathway is seen in bacterial photosynthesis.
Carbon Fixation (light independent reaction)
C3 plants (80% of plants on earth)
Occurs in the stroma
Uses ATP and NADPH from light reaction as energy
Uses CO2
To produce glucose: it takes 6 turns and uses 18 ATP and 12 NADPH.

Remember: C3 = Calvin Cycle

Powers ATP synthesis
Takes place across the thylakoid membrane
Uses ETC and ATP synthase (enzyme)
H+ move down their concentration gradient through channels of ATP synthase forming ATP from ADP and Pi

Note: Chemiosmosis is a very significant process in photosynthesis. This is the process that will synthesize ATP in the light stage. This is the diffusion of ions through special channels in the membranes.


The second stage of photosynthesis, which takes place in the stroma of the chloroplast, can occur without the presence of sunlight. In this stage, known as the Calvin Cycle, carbon molecules from CO2 are fixed into glucose (C6H12O2). The reactions of the Calvin Cycle is as follows:

1. A five-carbon sugar molecule called ribulose bisphosphate, or RuBP, is the acceptor that binds CO2 dissolved in the stroma. This process, called CO2 fixation, is catalyzed by the enzyme RuBP carboxylase, forming an unstable six-carbon molecule. This molecule quickly breaks down to give two molecules of the three-carbon 3-phosphoglycerate (3PG), also called phosphoglyceric acid (PGA).

2. The two 3PG molecules are converted into glyceraldehyde 3-phosphate (G3P, a.k.a. phosphoglyceraldehyde, PGAL) molecules, a three-carbon sugar phosphate, by adding a high-energy phosphate group from ATP, then breaking the phosphate bond and adding hydrogen from NADPH + H+.

3. Three turns of the cycle, using three molecules of CO2, produces six molecules of G3P. However, only one of the six molecules exits the cycle as an output, while the remaining five enter a complex process that regenerates more RuBP to continue the cycle. Two molecules of G3P, produced by a total of six turns of the cycle, combine to form one molecule of glucose.
Types of Photosynthesis
Called C3 because the CO2 is first incorporated into a 3-carbon compound.
Stomata are open during the day.
RUBISCO, the enzyme involved in photosynthesis, is also the enzyme involved in the uptake of CO2.
Photosynthesis takes place throughout the leaf.
Adaptive Value: more efficient than C4 and CAM plants under cool and moist conditions and under normal light because requires less machinery (fewer enzymes and no specialized anatomy)..
Most plants are C3
C3 Photosynthesis : C3 plants.
C4 Photosynthesis : C4 plants.
Called C4 because the CO2 is first incorporated into a 4-carbon compound.
Stomata are open during the day.
Uses PEP Carboxylase for the enzyme involved in the uptake of CO2. This enzyme allows CO2 to be taken into the plant very quickly, and then it "delivers" the CO2 directly to RUBISCO for photsynthesis.
Photosynthesis takes place in inner cells (requires special anatomy called Kranz Anatomy)

Adaptive Value:
Photosynthesizes faster than C3 plants under high light intensity and high temperatures because the CO2 is delivered directly to RUBISCO, not allowing it to grab oxygen and undergo photorespiration.
Has better Water Use Efficiency because PEP Carboxylase brings in CO2 faster and so does not need to keep stomata open as much (less water lost by transpiration) for the same amount of CO2 gain for photosynthesis.

C4 plants include several thousand species in at least 19 plant families. Example: fourwing saltbush pictured here, corn, and many of our summer annual plants.
CAM Photosynthesis : CAM plants Crassulacean Acid Metabolism Plants
Called CAM after the plant family in which it was first found (Crassulaceae) and because the CO2 is stored in the form of an acid before use in photosynthesis.
Stomata open at night (when evaporation rates are usually lower) and are usually closed during the day. The CO2 is converted to an acid and stored during the night. During the day, the acid is broken down and the CO2 is released to RUBISCO for photosynthesis

Adaptive Value:
Better Water Use Efficiency than C3 plants under arid conditions due to opening stomata at night when transpiration rates are lower (no sunlight, lower temperatures, lower wind speeds, etc.).
May CAM-idle. When conditions are extremely arid, CAM plants can just leave their stomata closed night and day. Oxygen given off in photosynthesis is used for respiration and CO2 given off in respiration is used for photosynthesis. This is a little like a perpetual energy machine, but there are costs associated with running the machinery for respiration and photosynthesis so the plant cannot CAM-idle forever. But CAM-idling does allow the plant to survive dry spells, and it allows the plant to recover very quickly when water is available again (unlike plants that drop their leaves and twigs and go dormant during dry spells).

CAM plants include many succulents such as cactuses and agaves
and also some orchids and bromeliads.
Note: The process of chemiosmosis in the mitochondria and in the chloroplast are the same the difference is that:

1.) In the mitochondria, protons are pumped from the matrix to the intermembrane space while in the chloroplasts, protons are pumped from the stroma to the thykaloid space.

2.) Creation of ATP in the mitochondria is the point of cellular respiration while in the chloroplasts, the ATP is just the first step that is needed in the use of the Dark reactions.
The following steps outline the process of Chemiosmosis in chloroplasts:

1. H+ ions (protons) accumulate inside thylakoids

2. a pH and electrical gradient across the thylakoid membrane is creates...

3. ATP synthase generate ATP

4. The Calvin Cycle produces G3P using NADPH and CO2 and ATP

Autotrophic Process: Plants and plant-like organisms make their energy (glucose) from sunlight.
6CO2 + 6H2O + sunlight C6H12O6 + 6O2
Plants use sunlight to turn water and carbon dioxide into glucose. Glucose is a kind of sugar.
Plants use glucose as food for energy and as a building block for growing.
Stored as carbohydrate in their bodies.
Autotrophs make glucose and heterotrophs are consumers of it.

Photosynthesis-starts to ecological food webs!

Makes organic molecules (glucose) out of inorganic materials (carbon dioxide and water).
It begins all food chains/webs. Thus all life is supported by this process.
It also makes oxygen gas!!

Why is Photosynthesis important?

Plant leaves have many types of cells!

-site where light-dependent reactions occur
- site where light-independent reactions or the Calvin Cycle occur
Six molecules of carbon dioxide plus six molecules of water, produce one molecule of sugar and six molecules of oxygen.
What does the Equation mean:
Photosynthesis occurs in two stages. In the first stage, light-dependent reactions capture energy of light and use it to make the energy-storage molecules called ATP and NADPH. During the second stage, the reactions then use these products to capture and reduce carbon dioxide.
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