Unit 2 Chapter 10: Photosynthesis Flashcards
What is photosynthesis
- the endergonic suite of reactions that reduces carbon dioxide to glucose(or other sugars), O2 and H2O with the help of H2O and light energy
- consists of the light reactions which obviously depend on light and the calvin cycle which depends on the products from the light reactions
What do the light reactions produce
oxygen from H2O, ATP
What does the calvin cycle produce
sugar from CO2
How are the light reactions and calvin cycle linked
by electrons that are released when H2O splits to form O2
What is the electron carrier in photosynthesis
NADPH
Where does photosynthesis occur
chloroplasts
Describe the structure of chloroplasts
filled with vesicle-like thylakoid, which are stacked into grana, inside a thylakoid is the lumen and surrounding the thylakoids is the lumen
How many membranes does a chloroplast have
2 like the mitochondria
Describe chlorophylls
2 types: a and b
- absorb blue and red light, reflect and transmit green light
- give plants their green colour
Describe carotenoids
- absorb blue and green light
- appear yellow, orange or red
- accessory pigments: absorb light and give energy to chlorophyll
- Beta carotenes and xanthophylls
- found in chloroplasts
- extend the range of wavelengths that can drive photosynthesis
- protect chlorophylls by accepting or stabilizing free electrons to prevent free radicals that degrade molecules
- photosynthesis stops without these
What is the action spectrum
wavelengths that drive the light reactions
What are the main photosynthetic pigments
chlorophylls
What are favonoids
- “natural sunscreen”
- accessory pigment that protects plants from radiation by absorbing UV light
- found in vacuoles
- necessary for photosynthesis
What is the structure of chlorophyll a and b
- both have a long isoprene tail, and a head that has a long ring with a magnesium in it where light is absorbed
- a has a CH3, b has a CHO in the same spot
- tail keeps the molecule in the thylakoid membrane
What happens when a photon of light is absorbed by chlorophyll
photons energy is transferred to an electron in the chlorophyll’s head, the electron gets excited and jumps to a higher energy state (only if the difference between its energy states is equal to the energy of the photon)
What happens if an electron simply falls back to its ground state after being excited
energy is released as heat and light; called florescence
What is the antenna complex
proteins and chlorophyll molecules embedded in the thylakoid membrane
What is resonance
when a red or blue photon strikes a pigment molecule in the antenna complex, energy is absorbed and electron is excited
this energy is passed to a nearby chlorophyll where another electron is then excited
-once energy is transferred og electron falls back to ground state
What is the reaction centre
complex of several proteins, pigments and other cofactors
What occurs at the reaction centre
excited electrons are transferred to a specialized chlorophyll molecule that acts as an electron acceptor
-when this special chlorphyll is reduced electromag. energy is transformed into chem energy and cannot be re-emitted as fluorescence
What are the 3 “fates” for excited electrons
- fall back to ground state and cause fluorescence (isolated pigments)
- induce resonance (antenna complex)
- transfer to an electron acceptor in a redox. reaction (reaction centre)
Which photosystem is first and how does it work
- photosystem 2 (680nm)
- light is absorbed in antenna complex which transmits energy to reaction centre
- pheophytin accepts excited electron from chlorophyll
- electrons from pheophytin are passed to an ETC in the thylakoid membrane (onto PQ, onto the cytochrome complex)
- proton gradient created which drives the synthesis of ATP
Where is the proton gradient created in the light reactions
inside the thylakoid lumen
What is the production of ATP from ADP and a phosphate group called in plants
photophosphorylation
Where do the electrons that enter photosystem 2 come from and what do they do
H2O (splitting of water), replace the lost electron that was lost due to excitement
How does photosystem 1 work
- 700nm
- pigments in antenna complex absorb photons and pass energy to reaction centre
- electrons excited
- high energy electrons pass through ETC and then to ferredoxin and then to NADP+ reductase
- NADP+ reductase moves 2 electrons and a proton to NADP+ which reduces it to form NADPH
How do photosystems 2 and I interact (NON-CYCLIC)
- Z-scheme
1. photons excite electrons in chlorophyll of PS2’s antenna complex
2. energy transferred to reaction centre and P680 passes electrons to pheophytin, electrons are replaced on P680 by splitting of H2O
3. reduced pheophytin transfers electrons to ETC where a bunch of redox rxns decrease their pot. energy
4. using energy from the redox reactions PQ drives photons into the thylakoid lumen creating a gradient which drives ATP prod.
5. after completing PS2’s ETC, electrons are passed to PC which picks up an electron from cytochrome complex and gives it to PS1 (replacing P700’s lost electron)
6. then more photons hit P700 and its excited electron binds to ferredoxin which are then passed to NADP+ reductase and NADPH is produced
What is cyclic photophosphorylation
- produces only ATP
- PS1 moves electrons back to ETC in PS2 to make ATP via photophosphorylation
- occurs when ATP levels are low and NADPH levels are high
Where is the site of ATP production
the stroma
What is the difference between NAD and NADP
NADP: used for anabolic pathways
NAD: used for catabolic pathways
What is carbon fixation
addition of CO2 to an org.compound
- redox rxn where C in CO2 is reduced
- CO2 reacts with ribulose 1,5-biphosphate (RuBP) to produce 3-phosphoglycerate (6 C molecule breaks in half to make to 3 C phosphoglycerates)
What are the 3 steps of the calvin cycler
- carbon fixation
- reduction
- regeneration
What is the reduction phase
3-phosphoglycerate is phosphorylated by ATP (6 ATP reduced to ADP) and then reduced by NADPH (6 NADPH oxidized) to make 6 G3P, (1 leaves to go make sugar, 5 others are sent to regeneration)
What is the regeneration phase
3 ATPs reduced to ADP, 5 G3Ps made back into RuBP
Where does the calvin cycle take place
stroma
How many times does the calvin cycle turn
1 turn fixes 1 molec of CO2, 3 turns make 1 G3P
What is the most abundant protein in the world
rubsico
Which reaction starts the transformation of CO2 gas into sugars
reaction between CO2 and RuBP catalyzed by rubsico
Describe rubsico
- cube shaped with 8 active sites
- slow
- catalyzes reaction between CO2 and RuBP
- inefficient because it catalyzes addition of O2 to RuBP as well as the addition of CO2 to RuBP
- since O2 and CO2 have to compete, CO2 fixation is slowed
What is photorespiration
- occurs in chloroplasts
- occurs when rubisco acts on O2 and not CO2
- consumes energy (ATP and O2) and releases fixed CO2 “undoes photosynthesis”
- declines photosynthesis
- makes one 3-phosphoglycerate for calvin cycle and one 2-phosphoglycolate (the wasteful one)
How does CO2 enter plants
- stoma contains pores guarded by guard cells
- if CO2 is low, proton pumps in guard cells establish a charge gradient so K+ ions move into guard cells; water flowing across osmotic gradient causes guard cells to swell and create a pore
What is C3 photosynthesis
regular photosynthesis, RuBP +CO2 react to form two 3-phosphoglycerates (3C sugars)
-rubsico from bundle sheath cells
What is C4 photosynthesis
- PEP carboxylase fixes CO2 in mesophyll cells makes a 4C org.acid
- these acids travel to bundle sheath cells via plasmodesmata
- the acids release a CO2 which rubisco uses to make 3-phosphogylcerate and drive the calvin cycle
- these reactions need ATP but also increase conc. of CO2, ensuring less O2 binds to rubsico
- used in hot dry conditions when stoma need to stay closed, and photosynthesis slows so O2 levels rise form cellular resp and photorespiration increases
What are some examples of c4 plants
corn, sugar cane, crabgrass
What is CAM photosynthesis
- increases CO2 and involves 4 carbon acids
- occurs at diff time than photosynthesis
- at night CAM plants open their stoma and take in tons of CO2 which is fixed to the org acids and stored in vacuoles
- during the day the molecules are process and CO2 is released for calvin cycle
- ie, cacti
How is photosynthesis regulated
- light increases it
- high amounts of sugar inhibit
- rubisco activated when light is available and inhibited when theres low CO2
- low phosphate levels stimulate calvin cycle
What happens to sugar made from photosynthesis
- 1 G3P from calvin cycle makes glucose and fructose into sucrose (cytosol)
- alternative makes glucose into starch (chloroplast)
-slow photosynthesis makes sucrose, fast makes starch
