photosynthesis in detail light reactions and calvin cycle honors biology
TRANSCRIPT
Photosynthesis in Detail
Light Reactions and Calvin CycleHonors Biology
The Two Stages of Photosynthesis: A Preview
• Photosynthesis consists of two processes1) The Light reactions
•NEEDS LIGHT• Light Dependent Reactions
2) The Calvin cycle•A.k.a- Dark Reactions or Light Independent Reactions •DOES NOT NEED LIGHT
The Light ReactionsOccur in the grana (& thylakoids)
Convert solar energy to chemical energy
Chlorophyll absorbs solar energy Split water release oxygen gas (a by-product) produce ATP (using chemiosmosis) Forms NADPH from NADP+ (an e- acceptor)
Temporarily stores high energy e-’s“Electron shuttle bus”
The Calvin Cycle
Occurs in the stromaForms SUGAR from carbon dioxideCarbon fixation occurs (CO2
fixed carbon) using ATP for energy and NADPH
for reducing power (adding e-s to fixed carbon) Fixed carbon carbohydrate (glucose)
An overview of photosynthesis
H2O CO2
Light
LIGHT REACTIONS
CALVINCYCLE
Chloroplast
GLUCOSE(sugar)
NADPH
NADP
ADP+ P
O2
ATP
G3P
Starch
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• Occurs in the chloroplast• Divided into 2 sets of reactions:
- Light Dependent Reaction- Calvin Cycle (Dark Reaction)
Photosynthesis
Photosynthesis
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• Requires Light!!• Occurs in the thylakoid/grana (in chloroplast)• Electron Transport Chain (ETC) makes ATP and
NADPH sends it to the Calvin Cycle WASTE PRODUCT: O2 Gas
Light Reaction
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Part 1 – Light Reactions You need to be able to draw this picture!
1. Photosystem II2. ETC3. Photosystem I
H2O Splits
What are the main steps in the light reactions?
• Uses energy from sunlight• Splits water into H+ and O2
• Converts ADP into ATP and NADP+ into NADPH– Use sunlight and water– Produces Oxygen, NADPH, and ATP
WRITELight Rxn Sequence of Events…
1) Photosystem II2) ETC3) Photosystem I
How is light absorbed in the thylakoid?
• Photosystems absorb light– Clusters of chlorophyll and proteins trap energy
from the sun– Energy is transferred to electrons makes
“excited” electrons
What are Electron Carriers?• Compound (NADP+) that can accept a pair of
high-energy electrons and transfer them to another molecule
• NADP+ grabs/carries 2 electrons and a H+ becomes NADPH
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Photosystem II• Photosystem II Steps →1) Chlorophyll and other light-
absorbing pigments absorb energy from the sun
• Energy transferred to e- high energy e- leave the chlorophyll and go to the ETC (electron transport chain)
2) H2O molecules are broken down –- Electrons come from H2O–- O-, H+ and e- are separated –For every TWO H2Os we get 4e-,
4H+, and 1 O2
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Photosystem II (cont’d)• 3) The excited e- jump from protein to protein and
enter the ETC (electron transport chain) on the thylakoid membrane• As e- jump through the ETC, they lose energy• This energy pumps/pulls H+ from the outside (stroma) to
the inside (lumen - aka - thylakoid space)• H+ ions start to build up inside the thylakoid
• Next, e- move to Photosystem I
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Photosystem I• Photosystem I → 4) More energy is absorbed from sunlight– Energy is added to e- from Photosystem II– Excited e- leave the molecules
5) E- are added to NADP+ which then becomes NADPH
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Photosystem I (cont’d)• Now, remember…those H+ ions are building up inside the
lumen6) H+ ions diffuse through a protein channel embedded in the
thylakoid membrane– The protein channel is part of an enzyme called ATP Synthase
7) As the H+ ions flow through the protein channel, the enzyme (ATP Synthase) makes ATP by adding a P group to ADP
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Light Reaction Summary…
• Summary: – Chemiosmosis The process that uses the proton gradient and ATP
synthase to make ATP– e- travels from PS II PS I added to NADP+ makes NADPH– Both ATP (via ATP Synthase) and NADPH are produced
During the light reaction thesethen enter theCalvin Cycle
NADP+ is the final e- acceptor in photosynthesis
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Calvin Cycle!
Second part of PS Occurs in the stroma of the chloroplast Can occur with or without light
Three turns shown above
Calvin Cycle
Summary of Steps:• ATP and NADPH used for
energy• CO2 is taken into the plant• High-energy sugars are
made • Also known as the sug ar
factory
Carbon dioxide & Calvin Cycle• Enters cycle from the atmosphere• Carbon from the carbon dioxide molecule is
used to make glucose (C6H12O6)• Plants then use the glucose for:
• Food!• Plant energy! – Stored in bonds of the
glucose molecule
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Calvin Cycle!Main job:
• Incorporates CO2 and rearranges the C’s to form sugar using the energy (ATP/NADPH) from the Light Reaction
• The CC “fixes” CO2
- Carbon enters as CO2 and leaves as sugar
• Goal is to produce a sugar (G3P)
• Each turn “fixes” one molecule of carbon, so one G3P takes 3 turns of the Calvin Cycle
INPUT: Each turn needs:
- 3 ATP- 2 NADPH- 1 CO2
Three turns shown above
Carbon “fixing” – process of turning CO2 into a solid compound
The Calvin cycle has three phases
1. Carbon fixation2. Reduction3. Regeneration of the CO2
acceptor (RuBP)
The Calvin Cycle Steps
CARBON FIXATION1. CO2 enters cycle and attached to a 5-
carbon sugar called ribulose biphosphate (RuBP) forming 6-C molecule (unstable)
Enzyme RUBISCO catalyzes reaction
2. Unstable 6-C molecule immediately breaks down to 3 3-C molecules called 3-phosphoglycerate (3-PGA)
REDUCTION3. Each 3-phosphoglycerate (3-PGA) gets an
additional phosphate from ATP (from LIGHT RXN) becomes 1,3 phosphoglycerate
4. NADPH reduces 1,3 phosphoglycerate to Glyceraldehyde-3-phosphate (G3P)– G3P = a sugar that stores
potential energy – Every 3 CO2 yields 6 G3P’s BUT only 1
can be counted in net gain for carbohydrate (GLUCOSE) production
REGENERATION OF CO2 ACCEPTOR (RuBP)
5. The C- skeletons of 5 G3P molecules are rearranged into 3 RuBP molecules
–ATP is used !!!!
The Calvin cycle
Phase 1: Carbon fixation
Phase 2:Reduction
Phase 3:Regeneration ofthe CO2 acceptor(RuBP)
Also known as PGAL
NOTE:
MORE ATP is needed
than NADPH!!
Calvin Cycle Overview
For 1 G3P molecule made 9 ATP molecules are used 6 NADPH molecules are used
G3P (aka PGAL)= starting material to make other organic molecules (glucose, starch, etc.)
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Calvin Cycle Steps:1) Enzyme Rubisco
attaches CO2 to RuBP creates 6 x 3-PGA
2) 6 x PGA are reduced to = 6 x G3P (sugar) via ATP/NADPH
3) 1 x G3P exits as glucose
4) 5 x G3P remain in the cycle. ATP rearranges the atoms 3 x RuBP
Note: RuBP and Rubisco are NOT the same thing!
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6CO2 + 6H2O + sunlight C6H12O6 + 6O2
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Rate of Photosynthesis
• The rate of PS is affected by 3 factors: – Light intensity– Amount of CO2
– Temperature
• Why wouldn’t roots need chloroplasts?
Root cells don't have chloroplasts, because chloroplasts catch sunlight! Since roots are underground, they are not exposed to the sun!
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Alternate Pathways• If it is too hot out, the plant will close the stomata so that
it doesn’t lose too much water and become dehydrated– However this eliminates the gas exchange!!
• SO the levels of CO2 drop and the levels of O2 increase– This results in…. PHOTORESPIRATION
• Photorespiration adds oxygen to the Calvin Cycle instead of carbon dioxide
- This makes NO sugar or ATP- This wastes all of the plants resources!
• Two types of plants: 1) CAM 2) C4 plants
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Alternate Pathways
• Therefore, certain plants, (cacti, pineapple, etc.) only open their stomata at NIGHT! – This prevents them from drying out and still lets
them get CO2!
What is wrong with this picture?
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For the test, you will need to know these pictures and labels:
• Flower Reproductive • Leaf Structure• Chloroplast • Light Reactions• Calvin Cycle
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Exit Slip-April 9, 2014