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Catherine Guarini

on 1 February 2014

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Transcript of Photosynthesis

Chloroplasts of plants use
to capture light energy and convert it to chemical energy.
Autotrophs produce their organic molecules from the environment and CO2
: photosynthesis happens in plants, algae, other protists and some prokaryotes

: from oxidizing inorganic substances it harvests energy (Sulfur, Ammonia).
Introduction to Autotrophs
consume biosphere
feed on plants and animals
feed on dead organisms and on organic litter
completely dependent on photosynthesis
If a plant has green in it, it has chloroplast.
Leaves are the major site of photosynthesis
Colour of a leaf comes from

As shown in he picture CO2 enters the
plant while O2 exits. This happens through microscopic pores called
in the leaf.
Veins in the plant deliver H20 from the roots and carry sugar from mesophyll cells to other parts of the plant.
Chloroplast has two membranes. The membranes are around the stroma.
There are sacks in the stroma called
Thylakoid Lumen
is the aqueous space in the thylakoid.
When thylakoids are stacked they are called
Through light the chlorophyll produces organic compounds, O2, CO2 and H2O
Pathway of Photosynthesis
The O2 in given off by plants comes from H2O not CO2
Hydrogen extracted from wter is incorporated into sugar. The oxygen is then released into the atmosphere.
Water is split and the electrons are transferred with H+ from water to CO2, making sugar.
Light boosts energy of electrons as they move from water to sugar
Converting Light Energy Into Chemical Energy
Calvin Cycle
incorporates CO2 into an organic molecule and uses energy from the light reaction to reduce the new carbon piece to sugar.
In the light reaction light energy is absorbed by chlorophyll in the thylakoids.
This drives the transfer of electrons and hydrogen from water to
which forms
NADPH which is an electron acceptor provides energized electrons which reducing energy to the Calvin Cycle.
The light reaction also produces ATP by
to be later used in the Calvin Cycle.
It begins with the incorporation of CO2 into an organic molecule via
carbon fixation
New piece of carbon is the backbone is reduced with electrons produced by NADPH.
ATP is also powers parts of the Calvin Cycle.
The Calvin Cycle occurs in the
Converting Light Reaction Energy to
Light travels in rhythmic waves called
The thylakoids have several pigments that differ in absorption.
Chlorophyll a
: is the dominant pigment and absorbs best in red and blue wavelengths. And absorb the green pigment the least
Only chlorophyl a participates directly in the light reaction.
Accessory photosynthetic pigments absorb light and transfer the energy to chlorophyll a
Chlorophyll a
Chlorophyll b
Has a different absorption spectrum than chlorophyll a
Funnels the energy from wavelengths to chlorophyll a
Funnels energy from other wavelengths to chlorophyll a.
Participates in photoprotection against excess light
Chlorophyll b is transferred to B-carotene then to chlorophyll a where it enters either
Photosystem 2 or 1
, continuing to the reaction center.
In both chlorophyll a and b an electron from magnesium in
porphyrin ring
that is excited
Act as a light-gathering "antenna complex" which contain chlorophyll a, b and carotenoid molecules.
Photosystem 2
Photosystem 1
Has a reaction center with an absorbtion peak of 680nm.
Has a reaction center with an absorption peak of 700nm
The difference between their reaction centers and their absorption spectra is that it lies not in the chlorophyll, but in the proteins associated with each reaction center.
Photosystem 2 comes before photosystem 1 because it was discovered second.
Can also be called PII or P680.
Can also be called PI and P700.
Comes after Photosystem 2 but was discovered first.
Two routes for electron flow:
non cyclic
Both photosystems work together, using light energy, in order to produce ATP and NADPH.
Here it is visible that Both the photon and the H2O are passed through both photosystems and another photon being added at PI to create NADPH. Through chemiosmosis ATP is also formed.
Cyclic and Non Cyclic
Non Cyclic electron flow is the predominant route produces both ATP and NADPH.
When photosystem II absorbs light, an excited electron is captured by the primary electron acceptor, leaving the reaction center oxidized.
An enzyme extracts electrons from water and supplies them to the oxygen atom which combines with another to form CO2.
Photo excited electrons pass along an electron transport chain before ending up at an oxidized Photosystem I reaction center.
As those electrons pass along the transport chain, their energy is harnessed to produce ATP.
Noncylic photophosphorelation is similar to the process on oxidative phosphorealtion.
At the bottom of the electron transport chain, the electrons fill a hole in an oxidized PI center.
The hole is created when protons excite electrons on the Photosystem I complex.
Excited electrons are captured by a second electron acceptor which carries them to the second electron transport chain
Electrons are passed from the transport chain to NADP+.
NADPH will carry the power that is slowly reducing from high energy electrons to the Calvin Cycle.
Alternative Pathways?
Electrons from Photosystem one can take an alternate pathway called
Cyclic Electron Flow
Electrons venture from reaction center to oxidzed P700 chlorophyll
As along the electron transport chain they generate ATP by
cyclic photophosphorylation
Protons are pumped across the membrane as electrons are passed along a series of electronegative carriers. Which builds the proton-motive force in the form of an H+ gradient across the membrane.
ATP synthase molecule generates ATP as H+ defuses back to the membrane
The protein gradient or pH gradient moves across the thylakoid membrane
When illuminated the pH in the thylakoid drops to about 5 while the pH in the stroma increases to 8. The difference is H+concentration.
Noncyclic electron flow pushes electrons from low potential energy, to NADPH, where they have high potential energy. ATP is also produced and oxygen is a byproduct.
Here we can see that the flow goes through ATP Synthase to get to low concentration when it was in high concentration where the energy will be higher.
The Calvin Cycle
Regenerates the stored material after molecules enter and leave the cycle.
CO2 enters the cycle and leaves sugar
It spends the energy of the reducing power of electrons carried by NADPH to make sugar.
The sugar formed by the Calvin Cycle is not glucose, but it is
glyceraldehyde-3-phosphate (G3P)
1 carbon is fired each round of the calvin cycle
The Calvin Cycle Pt.2
In order to form one glucose it would require 6 cycles and the fixation of 3 CO2's
There are three phases to the Calvin Cycle:
1. Carbon fixation phase- CO2 attaches to 5 carbon sugar (RuBP)
2. Reduction- each 3-phosphoglycerate receives a group of ATP and forms 1,3-biophosphoglycerate. NADPH reduces 1,3-biophosphoglycerate to glyceraldehyde-3-phosphate. reduced to a carbonyl group.
3. Regeneration of CO2 acceptor- 5 G3P molecules that are recycled to form 3 RuBP. Must spend 3 ATP molecules to complete the cycle.
Alternative Mechanisms
Overall 9 ATP are used and 6 NADPH are used to form 1 G3P.
Problems in terrestrial plants is that they face dehydration issues.
Stomata are both the major route for gas exchange and for the evaperative loss of water. On hot, dry days plants close the stomata to keep water from evaporating and this causes problems for photosynthesis due to the fact the stomata needs to be open.
In C3 plants fixation of CO2 occurs in Rubisco and results in a three carbon compound, 3-phosphoglycerate. (e.g. Wheat, rice and soy beans)
When the stomata is closed CO2 levels drop as it is being consumed in the Calvin Cycle. While this is occurring O2 levels rise,as the light reaction converts light to chemical energy.
C4 Plants
Lets take a journey to the tropics where plants such as sugar cane grows.
An example of a C4 plant is sugar cane...
C4 Plants
C4 plants first fix CO2 first in a 4 carbon compound.
7 000 other plants including sugarcane can take this path.
Mesophyll in C4 plants incorporate CO2 into organic molecules.
Phosphoenolpyruvate carboxylase adds CO2 to phosphoeolpyruvate (PEP) to form oxaloacetate.
CAM Plants...
CAM Plants
Crassulacean acid metabolism plants for long.
These plats open their stomata at night instead of during the day. At night these plants fix CO2 into a verity of acid in mesophyll cells. During the day the light reaction supply ATP + NADPH to the Calvin cycle which releases CO2.
Plants such as pineapples and cacti are characterized as CAM plants
Wrap Up of Photosynthesis
Video on how it all works in sequence.
Thanks For Watching
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