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Transcript of Photosynthesis
By: Tiffany Tran
The Process of Photosynthesis
There are two parts to this process
1, LIGHT REACTIONS
2. DARK REACTIONS/ the CALVIN CYCLE
History: Discovering Photosynthesis
The Calvin Cycle-
the Second Step
Please turn on your speakers or put in your earphones to hear this song! :)
Photosynthesis: phōs meaning "light", and synthesis, "putting together". Thus photosynthesis is a process used mostly by plants to convert solar energy into chemical energy which is stored in carbohydrate molecules. During the process, oxygen gas is released as a waste product.
Let's start with the
anatomy of the plant:
Vascular System which are made up of the shoot system and the root system are responsible for transporting minerals, vitamins, water throughout the plant.
As for the leaves, they are the sites for photosynthesis
Leaves are the centers of capturing sunlight
Light is a form of electromagnetic radiation that travels as wave packets called photons.
Photons possess a wavelength that is inversely proportional to their energy. Photons long wavelengths= low energy and short wavelengths= high energy.
General formula for photosynthesis:
CO2 + H20
CH20 + O2
the leaves' structure and arrangement on stems and branches maximizes the surface area exposed to sunlight and limits the distance that gases (ie.CO2) need to travel to reach the choroplasts
The inside of the leaf
The site of photosynthesis happens within the chloroplasts,
Chloroplast are a magnesium containing, type of plastids or organelles found within the mesophyll cells.
Photosynthesis actually takes place
within the thylakoids. Thylakoids are membrane-bound sacs which are stacked into granum. Granum is attached to attached to other thylakoids through bridges called lamallae. The stroma fills most of the thylakoid which is a protein-rich semiliquid material. (The sugar produced by photosynthesis goes into the stroma.) The thylakoid membrane contains the light-gathering pigment molecules and electron transport chain. They enclose an interior thylakoid lumen which will contain the H+ ions for the electrochemical gradient for ATP synthesis.
To undergo this process, the plant cell must contain chlorophyll. Plants use CO2 gas, water (liquid) and light energy to make sugars (stored chemical energy) and O2 gas.
openings on the surface of a leaf that allow for the exchange of gases between air spaces in the leaf interior and the atmosphere
water- resistant, waxy layer outside leaf that protects the leaf from excessive absorption of light and evaporation of light
layer that allows light to pass through to the mesophyll cells
system of tubes and cells that transport water and minerals and carbohydrates
are photosynthetic cells arranged into tightly packed sheaths around the veins of a leaf
photosynthetic tissue of a leaf, located between the upper and lower epidermis.that most chlorophyll resides
light-absorbing green-colored pigment that begins the process of photosynthesis
There are two main types of chlorophyll: chlorophyll a and chorophyll b.
Chlorophyll a: carries a methyl group which Chlorophyll a also transfers resonance energy in the antenna complex, ending in the reaction center where specific chlorophylls P680 and P700 are located
Chlorophyll b, like chlorophyll a aids in absorbing energy, It carries an aldehyde and is merely an accessory pigment, absorbing photons that chlorophyll a absorbs poorly or not at all.
Clusters of photosynthetic pigments called photosystems embedded in thylakoids, absorb photons of particular wavelengths. Chlorophyll a and b play a huge role in absorbing energy. It absorbs most energy from wavelengths of violet-blue and orange-red light. It also reflects green/yellow light, and as such contributes to the observed green color of most plants.
Step 1- Light Reactions
This process begins when photons strike a photosynthetic membrane; divided into steps
photoexcition: absorption of light
electron transport: transfer of energy through series of membrane bound carriers which creates an electrochemical gradient
chemiosmosis: the making of ATP
Photosystems II and I are clusters of protein and accessory pigment molecules which are the sites of absorbing light.
-Consists of antenna complex (web of chlorophyll molecules that transfer energy to the reaction centre) and reaction centre (transmembrane protein complex containing chlorophyll a whose electrons absorb light energy.
Products: ATP, NADPH, O2 gas
ATP- adenosine triphosphate
NADPH- nicotineamide adenonsine dinucleotide phosphate
ADP + P --> ATP
NADP+ H+ --> NADPH
This non-cyclic electron flow process starts when a photon in the correct energy range (best at wavelength of 680 nm) hits photosystem II (aka PSII or P680-because it best absorbs at 680 nm). When a photon strikes PSII, it excites an electron of chlorophyll 680. The excited e- is captured by the primary electron acceptor pheophytin and through redox reactions is transferred to plastoquinone.
At the same time a Z protein associated with PSII splits water into oxygen, hydrogen ions electrons. Two of these electrons are used to replace the missing electrons in chlorophyll P680. Oxygen leaves the chloroplast as a byproduct and the protons remain in the thylakoid space, adding to the H+ gradient.
The electrons are transferred from PSII to bc-f complex via the electron acceptor plastoquinone which physically moves between the two e- destination
the cytochrome b6-f complex is an enzyme which transfer electrons between the two reaction complexes from Photosystem II to Photosystem I, whereby introducing protons into the thylakoid space.
Plastocyanin is a copper-containing protein involved in electron-transfer
Photosystem 1 is struck by photons, releasing two electrons . The two electrons from PSII replaces the two electrons lost in photosystem I.
The chlorophyll a molecule in the reaction centre of photosystem I is called P700 because its absorption spectrum peaks at the wavelength of 700 nm.
The two electrons are from PS1 pass through another electron trtransport chain containing an iron-containing protein called ferredoxin. FD carry the 2 e- to the NADP reductase
The enzyme NADP reductase uses the two electrons and H+ ions from the stroma to reduce NADP+ to NADPH.
The NADPH produced is ready to move on to the next stage of Photosynthesis, the Calvin cycle.
The last step of the light reactions is the production of ATP. Through the process, an electrochemical gradient has been created through the accumulation of H+ ions inside the thylakoid space. As protons move through the ATPase complex from the thylakoid lumen into the stroma, ATP is created. This process is called photophosphorylation (b/c light is required for the establishment of the proton gradient).
The ratio of protons to ATP formed has been found to be 4 H+ per ATP. The ATP are ready to move into the Calvin Cycle.
NADPH and ATP
for more info watch:
For further clarification:
in the thylakoid
-takes place in the stroma thylakoid
- a cycle of reactions that convert carbon dioxide into carbohydrate molecules that does not require light, (thus is also called "dark reactions"
Occurs in 3 phases:
Phase 1- Carbon fixation
- CO2 addes to an already exising 5-carbon molecule, ribulose 1,5 bisphosphate, RuBP, to form a higher unstable 6-carbon intermediate. Almost instantly, this intermediate splits into two 3-carbon molecules called 3- phosphoglycerate (PGA).
-if this process occurs three times, 3CO2 react with 3 molecules of RuBP to produce 6 molecules of 3-phoshoglycerate.
-These reactions catalyze by the enzyme rubisco (which is a large enzyme that works very slowly. Therefore there are many molecules of rubisco to catalyze the reactions more efficiently)
- this reaction is exergonic, owing to the high level of chemical potential energy in RuBP in relation to PGA.
Phase 2- Reduction Reactions
-each six molecules of PGA is phosphorylated by an ATP for form six molecules of 1,3-bisphosphoglycerate.
-then, a pair of electrons from each of six NADPH molecules reduce six molecules of 1,3-bisphosphoglycerate to six molecules of glyceraldehyde 3-phosphate, a sugar.
One molecule of glyceraldehyde 3-phosphate exits the cycle as a final product.
Phase 3- RuBP Regeneration
-out of the six molecules of glyceraldehyde 3-phosphate (G3P), one exits the cycle and the 5 remaining molecules are rearranged to regenerate three molecules of RuBP.
-Three molecules of ATP are used
Thus this process is a cycle.
Where does the molecule of glyceraldehyde 3-phosphate?
As the cycle continues, the G3P molecules that leace are used to synthesize larger sugars such as glucose and other carbohydrates.
Plants store glucose in form of starch mainly in the seeds. Some plants such as the stem tubers and root tubers store glucose in their specialised organs. In a potato, glucose molecules are bound together in a long chain.
The overall equation for the Calvin Cycle:
3 RuBP + 3CO2 +9ATP +6NADPH +5H20
9ADP + 8P + 6NADP + G3P +3 RUBP
** NOTE **
For net synthesis of one G3P molecule: CALVIN CYCLE uses- nine molecules of ATP and six molecules of NADPH. THE LIGHT REACTIONS: for every ATP, there needs to be four H+. For every 4 H+ ions, there is 4 light reactions,
2. Ms. Musalem's notes
Nelson, Thomson. Nelson Biology 12. Canada: Thomson Nelson, 2003. Print.
3. Youtube, Khan academy
Other methods of photosynthesis:
1. Cyclic electron flow- uses only PSI, e- is passed to Fd then to b6-f complex and back to chlorophyll P700. This generates a photon gradient for ATP synthesis bu does not release e- to generate NADPH.
2. CAM plants (lack of water), open their stomata at night. The CO2 is stored as the four-carbon acid malate, and then used during photosynthesis during the day.
3. C4 plants (the cycle we have been learning about is C3), have the same cycle except they go through another cycle before the Calvin cycle. In this cycle, PEP carboxylase grabs the CO2 molecule (more efficient than rubisco). This cycle readily gives the Calvin cycle the CO2.
In conclusion, the relationship between plants and human beings form the basis of life! This complicated process of photosynthesis that plants undergo using light energy and our waste CO2 gas, makes our necessities: O2 and (some plants) food.
I hope this presentation does justice to this amazing system :)
ribulose 1,5- bisphosphate
Musalem, Yenny. Photosynthesis Notes. Toronto : Print.