Lecture 6 Photosynthesis Flashcards
Photosynthesis # of CO2 vs Cellular Respiration
Photosynthesis = 3CO2
Cellular Respiration = 1CO2
___ is always the mobile e- carrier to transfer e- carrier between the CC & P700
Plastocyanin
Sunlight + ATP + CO2 + H2O –(in chloroplasts)–> Glucose + O2
Photosynthesis
Glucose + O2 –(Cytoplast & Mitochondria)–> ATP + CO2 + H2O + heat
Cellular Respiration
Where does photosynthesis take place in ?
Chloroplasts
Where does cellular respiration take place in?
Cytoplasm + Mitochondria
Light E from sun by fixation of atmospheric carbo is converted into ____ energy
Chemical Energy
Energy transduction reaction (light reaction) produces ___ & ___
ATP & NADPH
Occurs in thylakoid membranes
e- carrier of photosynthesis
NADPH
___ & ___ fuel Calvin Cycle
ATP & NADPH
Produces sugars
Transpiration
Plant’s loss of water, mainly through the stomata of leaves
Pigments
Substances that absorb light
Used as E for organisms
Black = complete absorption
White = Complete reflection
Chlorophyll
Pigment that absorbs light in violet, blue & red spectrum
BUT reflects green
What are the 3 main chlorophyll pigments
- Essential
- Accessory
- Carotenoids: visible in autumn when chlorophyll breaks down (red/ fall colours)
All photosynthetic reactions occur in the __
Chloroplast
Chloroplast contains ___
Chlorophyll
Thylakoid membranes contain ___
Pigments
2 major reactions in photosynthesis
- Light Reaction
(E transduction reaction; Photolysis) - Light independent reaction
(Carbon Fixation Reaction)
Light E is absorbed by pigments via ___ ___ in the ____ ____
Antenna complexes / Thylakoid membranes
The light shines down and ___ an e- causing it to bounce around antenna complex
Excites
Excited e- are funneled by the antenna complexes to 2 transmembrane proteins known as: Photosystems __ & __
2 & 1 (in this order)
Why is PS2 before PS1?
PS2 was discovered first hence the order.
PS1 Light absorption peak
P700
PS2 Light Absorption Peak
P680
Photolysis
Separation of water molecules by the action of light E
Photo-sun
Lysis-cut
In photolysis, after pulling e- in pigment, excited e- is replaced by stealing e- from water molecules inside ___ ____
Thylakoid membranes
Photolysis creates concentration gradient of __ & __ within thylakoid
H+ & O2
Non-cyclic (Linear) e- Flow: PS2 & PS1
Starts at PS2, Ends at PS1
Excited e- from PS2 gets transferred to NADPH from PS1
Linear e- flow driven from 2 photochemical reactions
Light Reaction
Light + H2O = O2 + ATP + NADPH
Free energy charge = 51 kcal/mole
Total E from non-cyclic e- flow = 6 ATP & 6 NADPH
Calvin Cycle cost
E cost = 9 ATP & 6 NADPH
Why is the Calvin cycle E Cost a problem?
Because of unequal amounts of ATP/ NADPH means we will run out of ATP first
Cyclic (non-linear) e- flow
PS1 can work independently of PS2
Uses same machinery but reconfigure direction towards CC
Passes thru P700 in order to create just ATP without PS2
Light Independent Reaction
Uses ATP & NADPH generated from PS1 & PS2 + CO2
2nd major photosynthetic reaction AKA Carbon-Fixation Reaction
2nd in series of photosynthetic reactions
Stomata
Special openings in stems and leaves of terrestrial plants
- allows CO2 to enter plant cells
- knows when to open and close these openings
3 Different Carbon Fixation Pathways
- C3 - Calvin Cycle
- C4 - Hatch Slack
- CAM - Crassulacean Acid Metabolism
Carbon fixation pathway involving temperate plants with no special mods
C3 Plant
Carbon fixation pathway involving hot weather plants with separate fixation & Calvin cycle
C4 Plant
Carbon fixation pathway involving desert plants with temporal separation including night gas exchange and day photosynthesis
CAM Plant
3 Steps of C3 - Calvin Cycle
- Carbon Fixation
- PGA Reduction to PGAL
- RuBP Regeneration
Step 1 C3: Carbon Fixation
CO2 bind with RuBP, gets split and broken by enzyme Rubisco to create PGA
RuBP: ribulose 1,5-biphosphate (5-C molecule)
Step 2 C3: PGA Reduction (5 total)
- Uses most of ATP & NADPH produced by light reaction of photosynthesis
- Turning PGA to PGAL
- ATP & NADPH reduce 3PGA to G3P
- Taking ATP, moves phosphate around & turns into ADP
- From there we pull out, remove to create PGAL
- We oxidize to create PGAL
Step 3 C3: RuBP Regeneration
-Most PGAL generated in step 2 is used to regenerate RuBP
- 5/6 molecules creates a cyle
- Regeneration requires 3 ATP
- Remaining PGAL moved to cytosol, converted to sucrose
- PGAL stored in chloroplast, converted to starch
- Starch is stored in chloroplast in daylight, but exported to the rest of the plant as sucrose at night
Problems with C3 Calvin Cycle
Hot & dry condition plants cut stomata to prevent evaporative loss, CO2 stops moving altogether
Salvage Pathway C3
In absence of CO2, RuBP binds to O2 and initiates photorespiration
Photorespiration
Costly process that consumes O2 & starch, doesn’t produce ATP or NADPH and instead produces useless metabolite: Phosphoglycolate
What is formed during photorespiration and is metabolically useless?
Phosphoglycolate
Solution to photorespiration is known as which pathway?
C4 or Hatch-Slack Pathway
Hatch-Slack Pathway C4
Uses spatial separation, C4 plants employ both the C3 and C4 pathways of Carbon Fixation
Problem with Photorespiration
Very costly in E to restart Calvin cycle
2 types of chloroplast in C4 Plants
- Normal chloroplasts in mesophyll cells
- Grana-less chloroplasts in bundle sheath cells
Spatial Separation
Where carbon fixation and Calvin cycle occur in different types of cells
Main Advantages of C4 Pathway (3 total)
- PEP greater affinity for CO2 than RuBP
- PEP allows plant to function in conditions where CO2 is limited
- Can bind to ANY CO2 present, will not bind O2
In the C4 Pathway, CO2 binds to ___ instead of RuBP which creates Oxaloacetate
PEP
PEP stands for
Phosphoenolpyruvate
In C4 Pathway, oxaloacetate changes (reduced) to ___
Malate
In C4 Pathways, Malate is converted to __ & __
CO2 & Pyruvate
In C4 Pathways, CO2 enters ___ ___ and Pyruvate regenerates ___
Calvin Cycle & PEP
What restarts the entire C4 Pathway cycle?
PEP Regeneration
CAM stands for
Crassulasean Acid Metabolism
CAM Pathway
Uses temporal separation of carbon fixation
What plants primarily use CAM pathways?
Primarily in succulents (pineapple & cacti) in tropical locations
Stomata in CAM plants
In extreme heat the stomata in CAM plants are closed during the day but open at night
In CAM Pathway, CO2 is converted to ___
Malate
In CAM pathways, Malate is stored in ___ during ___
Vacuoles & Dark
When can Carbon fixation of CO2 occur in CAM Pathways?
Carbon fixation of CO2 occurs at night when CO2 can move into the plant through open stomata
In CAM pathway, Calvin cycle occurs during the day through conversion of ___ back into __ & __
Malate, CO2, Pyruvate
Calvin cycle during day conversion turns malate back into __ & __
CO2 & Pyruvate
Night (stomata open)
CAM Plants open their stomata during the night to minimize rate of transpiration.
- Start from chloroplast broken down into PEP
- Reacts with CO2, creates oxaloacetate
- Oxaloacetate is reduced to malate + stored as malic acid in vacuole
Day (stomata closed)
CAM plants keep stomata closed during daytime to reduce loss of water.
They can do this because they fix CO2 into organic acids during the night.
- Malic acid taken from vacuole, decarboxylated, produces CO2 to pyruvate
- CO2 goes to Calvin Cycle
- Pyruvate degraded further in Citric Acid Cycle (respiration) - creates more usable CO2