Lecture 6: Photosynthesis Flashcards
Photosynthetic organisms
- photoautotrophs
primary producers that convert SOLAR energy into CHEMICAL energy through complex organic molecules - use some of organic molecules as own energy sources
What are the two stages of photosynthesis
1) Light dependent (z cycle)
- pigment molecules capture light energy which is used to synthesize ATP and NADPH
2) Light independent reactions (Calvin cycle)
- uses energy in NADPH (form light dependent) and ATP to convert CO2 from inorganic to organic form (glucose), or other carbon skeletons needed in macromolecules
*** VIA CARBON FIXATION
The process of photosynthesis is the opposite of…
etc
Light reactions
Photosystem II (PSII) absorbs light, exciting electrons from water molecules, splitting them into oxygen, protons, and electrons. The excited electrons are passed to the electron transport chain.
Electrons lose energy as they move down the chain, and this energy is used to pump protons across the membrane, creating a proton gradient for ATP synthesis.
Photosystem I (PSI) absorbs more light, re-exciting the electrons. These high-energy electrons are used to reduce NADP+ to NADPH.
Calvin cycle
Carbon fixation: CO₂ combines with RuBP (ribulose bisphosphate) to form 3-PGA.
Reduction: 3-PGA is converted into G3P (glyceraldehyde-3-phosphate) using ATP and NADPH.
Regeneration: Some G3P molecules are used to regenerate RuBP, allowing the cycle to continue.
Where does photosynthesis take place for pro/euk
euk=chloroplasts
pro=plasma membrane/cytosol, lack chloroplasts
Structure of chloroplast
- double membraned
stroma- fluid filled environment in the inner membrane
thylakoid membrane: complex of flattened internal membrane compartments
- stacks: grana
- connections between grand: lamellae
- compartment enclosed by thylakoids: thylakoid lumen
lumen means
space
purpose of thylakoids
- absorb light through chlorophyll and carotenoids (pigments)
- electron transfer
- ATP synthesis by ATP synthase
Purpose of stroma
space around thylakoids..
- site of Calvin cycle
The photosynthetic Apparatus
a) electrons in pigment molecules absorb light energy
b) chlorophyll and carotenoid pigments cooperate in light absorption
c) photosynthetic pigments are organized into photosystems (that catalyze the conversion of light energy to chemical energy)
Excitation
- electron at ground state will move to excited state which is unstable
absorbs energy If moving away from nucleus
releases energy when moving towards nucleus
Fate 1 of excitation
1) energy released as heat or as light (fluorescence), electron returns to ground state
fluroscence
- Light of a longer wavelength:
IN EXCITATION FATE 1:
-electron can just release that energy as light
- emmitted won’t have an equal amount of energy (longer wavelength) than what was absorbed, meaning the electron will return to ground state by emitting a less energetic photon
Fate 2 of excitation
energy transferred to a neighbouring pigment molecules by inductive resonance
Inductive resonance:
transfer excites second pigment and the first pigment returns to ground state
The transfer of solar energy from one pigment molecule to other
Fate 3 of excitation
Excited state electron itself is transferred to nearby electron accepting molecule, the primary acceptor
- complete transfer of excited electron and energy so it can energize other molecules such as NADP+ or pump protons to increase membrane potential
* chlorophyll a is the only pigment that can do this *
Pigments
chlorophylls- major photosynthetic pigment
carotenoids- accessory pigments that absorb light energy and pass it to chlorophyll
Chlorophyll a
- harvests light in blue/violet/red light (absorbs these wavelengths the strongest)
- only one to give excited electrons to primary acceptor (fate 3)
What do chlorophyll and carotenoids do together
- absorb photons during photosynthesis
Englemen’s Experiment
- most growth of aerobic algae (spirogyra) in
violet/blue wavelength and red wavelength because the algae is doing the most photosynthesis (O2 is a byproduct) - least growth in green/yellow wavelength, meaning the least amount of photosynthesis would occur which is why its reflected and plants appear green
As more plants enter dormancy, pigments
die off, hence the colour of leaves during fall and winter
photosystems
- photosynthetic organisms that capture solar energy (photons of light to oxidize a reaction centre chlorophyll with the electron being transferred to primary electron acceptor)
- each photosystem is composed of a large antenna complex of pigments that surrounds a central reaction centre (primary electron acceptor)
PSII with P680
PSI with P700
* numbers indicate the wavelength of light they absorb the most *
Linear electron transport
IN thylakoid membrane
1) Light energy is absorbed by chlorophyll, exciting electrons to a higher energy state.
2) Excited electrons are transferred to an electron transport chain (ETC), starting with Photosystem II (PSII).
3) Electrons lost by PSII are replaced by splitting water molecules, releasing oxygen as a byproduct.
4) As electrons move through the ETC (which includes plastoquinone, cytochrome b6f, and plastocyanin), protons (H⁺) are pumped into the thylakoid lumen, creating a proton gradient.
- use photon to re-excite electrons because they lost 1/2 their energy at this point (when meeting with P700 *
5) Electron sits on Ferredoxin (outer membrane of thylakoid) and donates electrons to NADP+ to NADPH which is catalyzed by NADP+ reductase (brought to krebs) ** using 2e- from etc and proton from aq environment **
6) ATP Synthase: Driven by H+ force via electrochemical gradient: three things contribute:
1) Splitting of water in lumen
2) PQ (hydrophobic core) picks up electrons and protons for neutrality, and give up H+ in lumen
3) NADPH redox forms NADP+ by losing H+ to strong side
Summarized Transport
1) P680 Redox
2) Plastoquinone Pool Redox
3) Electron transfer from cytochrome complex and shuttling by plastocyanin
4) Redox of P700
5) Electron transfer to NADP+ by ferradoxcin (NADP+ Reductase)
Chemiosmotic synthesis of ATP
- uses proton motive force established across thylakoid membrane to synthesize ATP
- H+ force (via osmosis)
- uses atp synthase
The linear use of light to synthesize ATP
- not spontaneous since you use light energy
- Positive enthalpy: since the e- energy in NADPH is greater than in H2O
To get 1 electron down the ETC from PS2 to NADP+ takes…
2 photons of light
- one photon absorbed by each photosystem
2H2O = 4H+ + 4e-+ O2
- For 4 electrons, need a total of 8 photons of light
Cyclic electron transport
- PS1 can operate independently of PS2 by using cyclic electron flow (to get extra ATP)
- electron transport from PS1 to ferredoxin is not followed by electrons going to NADP+ reductase complex
- reduced ferredoxin donates electrons back to the plastoquinone pool
- USE THIS TO MAKE MORE ATP WHICH IS IMPORTANT IF THE ORGANISM UNDERGOES CHANGES TO ENVIRONMENT
which comes first in the transport chain: PS1 OR PS2
PS2, named based on time they were discovered
Cyclic E- Transport summarized
NO NADPH
YES ATP
What happens to CO2 in Calvin cycle
reduced and converted into organic substances (because co2 is an inorganic form)
- NADPH = provides electrons and H+
- ATP = provides additional energy
What is carbon fixation
Capturing CO2 molecules with the key enzyme RUBISCO
- Rubisco acts as a carboxylase in Calvin cycle
- will do either carboxylation or oxygenation
Calvin cycle summary
1) carbon fixation: CO2 will be fixated onto RUBP via rubisco enzyme to form a 6 C intermediate that is too unstable and will break down to form 2 3-PGA
2) ATP and NADPH from the light dependent reactions will convert the 3-PGAs into G3P
3) ATP and NADPH will be used to regenerate RUBP to keep the cycle going
4) G3P can be used to form glucose and other carbohydrates for fuel
How many times is Calvin undergone
three times to produce a single three-carbon GA3P molecule (check textbook)
and six times to produce a six-carbon glucose molecule.
G3P
starting point for synthesis of
- sucrose
- starches
- cellulose
- amino acids
- fatty acids and lipids
- proteins
- nucleic acids
photorespiration
Rubisco binds to 02 instead of con leading to a wasteful by product forming and u d in photosynthetic efficiency bc of ROS
O2 is a
competitive inhibitor
Glycolate is
toxic
Limit photorespiration
- pump bicarbonate anion into cells (converting into co2) ALGAE
- spatially separate c4 cycle from Calvin cycle (do them in diff spaces) [C4]
- temporally separate c4 cycle from carbon cycle (not at same time) [C4]
Photosynthesis Dilemma
problem: photorespiration and water loss
- leaf covered in waxy cuticle: prevents water loss BUT prevents rapid diffusion of gases into leaf
- stomata: control high rates of gas exchange + minimizes h2o loss [pores] CO2 IN O2 OUT
DILEMMA: need to open stomata to let in CO2 but need to keep them closed to conserve water
C4 Photosynthesis
- increases concentration of CO2 relative to O2 near Rubisco to minimize photorespiration
- CO2 is combined with PEP to produce 4-C intermediate (oxyloacetate)
- PEP enzyme has no oxygenate activity
- When O2 conc is low, oxyloacetate is oxidized to release CO2 (converted to malat–bundle sheath cells–decarboxylated)
- CO2 enters Calvin by binding to rubisco
type of cycle: Calvin
C3 cycle because a 3C intermediate is produced first (3PGA)
Example of spatial separation of C4 and Carbon cycle
- CO2 is captured in mesophyll cells close to the surface of leaves where PEP carboxylase is NOT affected by high O2 conc
- Malate goes deeper in tissue (converted from OAA) and then decarboxylates in bundle sheath cellsoxidized to pyruvate
- This is a place where O2 is less abundant and CO2 is more concentrated, photorespiration is reduced and the Calvin cycle is enhanced
Temporal separation of C4 and carbon
CAM PHOTOSYNTHESIS
- night: plants open stomoata to capture co2 with c4 cycle
- day: plants close their stomata to conserve water, and malate is oxidized to release CO2 in chloroplasts
+use o2 to cellular respiration
+ stomata close as temp increases
+ conc of o2 is lower
Z scheme summarized
NOTE: chlorophyll a will absorb light energy that excites electrons that are brought to ETC starting with PS2
1) H2O will split at PS2 where they will release O2, H+, and electrons that will be used for the ETC
2) PS2 will excite the electrons by using light energy
3) Electrons move to PQ where they will be held
4) Electrons will be sent to cytochrome complex where they will be used in the etc to pump H+ into the thylakoid lumen
5) Electrons will be sent to another intermediate, PC
6) Electrons will be brought to PS1 where they will be re-energized with light energy
7) Electrons will be used to reduce NADP+ to NAPDH
bundle sheath cells
photosynthetic cells around leaf
mesophyll cells
tissue located between the two epidermal cell layers of the leaf
shorter wavelength blue light vs red light
red light has longer wavelengths and thus photons of less energy
3 possibilities when light interacts with matters
1) reflected off the object
2) transmitted through the object
3) absorbed by the object
- light is absorbed when the energy of a photon is transferred to an electron within a molecule which excites the electron
molecules that are efficient at absorbing photons of specific wavelengths
pigments
- they have a region of carbon atoms are covalently bonded to each other with alternating single and double bonds (conjugated system)
- results in delocalization of electrons=electrons are available to absorb energy
autotrophs
chemoautotrophy
drive conversion of co2 into organic form through light
chem…- bacteria version, use chemical compounds for energy like H2S and Fe2+
photosynthesis is not present in which domain
arches, but halo bacteria do harvest light energy to pump protons across membrane for ATP synthesis
- but light energy isn’t use to convert CO2 to organic form so they aren’t photosynthetic
the energy of a photon
must match the amount of energy needed to move a delocalized (not associated with an atom) electron from its ground to a specific excited state
if energies of photon and electron don’t match
photon of light isn’t absorbed or reflected and it instead transmitted through the molecule or reflected off molecule
what determines the colour of pigments
ability to absorbed specific wavelengths
how can we measure the rate of photosynthesis
rate of O2 produced or CO2 consumed
where are the enzymes of the Calvin cycle found
cytosol
absorption spectrum
plot of how much light is intercepted by the pigment (absorbed) as a function of wavelength
chlorophyll has 2 excited states
1) one that matches energy of a blue photon
2) matches energy of a red photon
action spectrum
plot of effectiveness of light of a particular wavelength in driving a process
what process gets a chlorophyll molecule into a state where it readily gives up an electron (oxidized)
absorption of light
P680, P700 are examples
NADPH production does what
enhances the gradient across the thylakoid membrane
the process of using light to generate ATP
photophospohrylation
P1 can function independently of P2
cyclic electron transport
-Fd donates electrons back to PQ pool so its continually reduces and oxidized to keep moving P+ across membrane
unlike linear electron transport, NADPH is not formed
during cyclic electron transport
How does the plant control which pathway electrons should be sent out
sensory proteins in chloroplast control ratio of ATP and NADPH
how many photons of light are required to generate 1 molecule of NADPH by linear electron transport
2 photons
the Calvin cycle is what type of reaction
endergonic, needs light to occur
- requiring abundance of energy supplied by ATP and electrons in NADPH
3 turns of Calvin
= 3 CO2 into 3 RuBp to produce 6 3-PG
- 6 ATP and NADPH are consumed
- 5 G3P are used to regenerate 3 RuBP, requiring 3 ATP
what role does the chloroplast genome play in the synthesis of RuBisCo
The chloroplast genome encodes the large subunit of RuBisCo, essential for assembling the enzyme complex, while the nuclear genome encodes its small subunits.
O2 binds to RuBisCo
- makes enzyme act as an oxygenate instead of carboxylase
- forms toxic glycolate which leads to reactions releases co2 (wasteful)
algae pump bicarbonate ion purpose
prevention of rubisco binding to O2 (photorespiration)
- because in aquatic environment there isn’t enough CO2 to saturate RuBisco, algae use bicarbonate to prevent photorespiration (converted in cytosol)
even though solubility of gas decrease as temperature increases, co2’s solubility decreases faster
which photosystem makes o2
2
CHECK GRADE 12 BIOLOGY PHOTOSYNTHESIS FLASHCARDS FOR C3,C4,AND CAM
ok
difference of g3p in cell resp and photosynthesis
- cell resp: product of catabolic pathways
- photo: is used by anabolic pathways
