Lecture 12: Photosynthesis Flashcards
What is the chemical reaction for photosynthesis?
6CO2 + 6H2O —- C6H12O6 + 6O2
Does respiration have a negative delta G or does photosynthesis?
-Respiration does, photosynthesis has a positive delta G that requires energy to be put in the reaction
Where does photosynthesis get its energy from?
Sunlight
Where does photosynthesis occur?
Chloroplasts
How many chloroplasts can be found in plant leaf?
40-50 chloroplasts per plant leaf
Why are plant leaves so thin?
To capture sunlight more easily
Where do the dark reactions occur?
The storm
The Calvin cycle does not require light directly but what does it require?
-ATP
- NADH
Both produced by light reactions
Where do the light reactions occur?
Thylakoid membrane
What are some similarities between aerobic respiration and photosynthesis?(3)
- Both have an ETC
- Both have ATP synthase
- Citric acid cycle and Calvin cycle are opposite of each other
What does the citric acid cycle do vs the Calvin cycle?
Citric Acid Cycle: Oxidizes sugar and produces CO2
Calvin Cycle: Uses CO2 to build sugars
What are some differences between aerobic respiration and photosynthesis?(2)
- Photosynthesis uses light energy to split water into oxygen, protons and electrons used to initiate the ETC
- Photosynthesis uses NADPH
What’s the difference between NAD and NADP?
NADP has a phosphate group attached to it
Why is NADPH used in photosynthesis?
-NADH is used for catabolic reactions
-NADPH is used for anabolic reactions(helps support plant growth as it can be used to drive reactions producing fatty acids, and other polymers)
Are NADH and NADPH both found in plants and animals?
Yes
Why is the NADPH concentration kept high?
Used for reductions to make sugars
Why is the NAD concentration kept high?
NAD is used to oxidize sugar
Are short wavelengths high energy or low energy?
HIGH energy
Are long wavelengths high energy or low energy?
LOW energy
What wavelengths does photosynthesis absorb? what are these wavelengths called?
-400nm -700nm
-This is the visible light spectrum
What does visible light do to electrons?
Lifts them from the ground state to the excited state
Why are microwaves not used?(low energy)
Only induce vibrational energy
Why are gamma rays not used? (high energy)
Cause damage by ionization
What happens when an excited electron is brought back down to the ground state?
-Excited electrons contain potential energy
-This energy is released when brought back down to the ground state
What do the pigments chlorophyll and B carotene have that allows their electrons to be excited?
Both chlorophyll and B carotene have alternating bonds that have delocalized electrons that can be easily excited by certain visible wavelengths
Why are leaves green ?
Chlorophyll found in plants absorb blue and red light and reflects green light
What is the action spectrum?
A spectrum that shows at what wavelengths plants best absorb light and produce oxygen (oxygen is a waste product of photosynthesis, so this how’s how much photosynthesis is being done)
What does the solar spectrum show?
-Describes the type of electromagnetic radiation that arrives on earth
-Shows most sunlight arrives in the visible spectrum
Why did humans evolve on earth and not another planet?
-On other planets the sun radiates gamma rays or infrared rays which could not sustain human life
Why do leaves change from green to red in the fall?
Before the leaves drop to the ground, trees try to recoup their investments and try to shuttle chlorophyll and other chemicals back into their trunk and that leaves carotenoids in the leaves which leaves the leaves as red/orange
What can happen to an excited electron(not in photosynthesis)?
Excited electrons decay and fall back to ground state resulting in the emission of light (wavelength is lower as some is lost as heat)
What is resonance energy transfer?
- Two molecules such as chlorophyll and carotenoid are close together
- One molecule like chlorophyll is excited by light
- Then it falls back down to the ground state the excited electron within chlorophyll also falls back to the ground state
- This releases a wavelength slightly longer than before(energy lost due to heat) .
- This wavelength then immediately excites the neighbouring carotenoid molecule(energy is transferred not the electron).
What is successive electron transfers?
- When light hits a chlorophyll molecule the electron within it is excited
- This causes the electron to be transferred to the neighbouring molecule
- This initiates a redox reaction and reduces the neighbouring molecule
- The chlorophyll now lacks an electron and must get one from a water molecule
Does resonance energy transfer occur in photosynthesis?
Yes, occurs in the light harvesting complex
Does successive electron transfer occur in photosynthesis?
Yes it does it initiates the electron transport chain
What is found within the light harvesting complex?
Multiple chlorophylls and carotenoids sitting next to each other
What does there need to be multiple chlorophylls and carotenoids in one place?
Individual chlorophylls are excited too rarely
-Its very rare that a photon of light hits a chlorophyll molecule directly
Why does each transfer of resonance energy create a longer wavelength ?
Because energy is lost as heat as the light wave is transferred
What is the light harvesting complex also called?
Antenna
What is the reaction centre?
Made up of chlorophyll it is the last one to receive an energy transfer in the antenna
Does the reaction centre get a long or short wavelength?
Long wavelength of low energy since it is the last energy transfer and heat is lost amongst energy transfers
What is the reaction centre connected to?
The ETC
What wavelength must the reaction centre accept?
680 nm
How many energy transfers usually occur before reaching the reaction centre?
Depends
Could be 1 if the wavelength absorbed is initially large
Could be 25 if the wavelength is initially very small
It is transferred until it reaches 680 nm
How is the electron transferred from the reaction centre to the ETC?
It is transferred by an electron carrier called pheophytin
What does chlorophyll do after losing its electron?
Gets one from water
What is plastiquinone?
A protein that transfers electrons
Are there proton pumps in the ETC?
Yes there are proton pumps that create a proton gradient to create ATP via ATP synthase
where is water split in the ETC ?
Water is split at PSII and contributes to the proton gradient
How many protons does each water molecule splitting produce?
4 Protons and 4 electrons
How does plastoquinone contribute to the protein gradient?
Plastoquinone takes up one proton from the outside when it gets reduced by the electron. It then gives up the proton on the inside contributing to the proton gradient
What happens at PSII?
-water gets split
-pheophytin gets reduced and transfers electrons to photosystem one
What wavelength of light does PSI accept ?
700 nm
Why are there two photosystems?
One photon of light is not enough energy to make both ATP and NADPH
Non cyclic electron transport Z scheme?
Both photosystems are used
How is NADPH created?
PSI transfers the excited electron through a bunch of energy carriers and finally to ferrodoxin, an enzyme then transfers the electron from ferrodoxin to NADP+ generating NADPH
What is the cyclic electron transport ?
-Only one photosystem is used
When does the cyclic electron transport occur?
When plants have made enough glucose and only want to make ATP
How does the cyclic electron transport occur?
-Electrons are shuttled from ferredoxin to plastoquinone instead of NADP+
-They then run down to PS1 and ATP is created
What does rubisco do ?
It is an enzyme that catalyzes the reaction to fix carbon(its basically is the reason every molecule and life on earth)
What is the Calvin cycle?
Dark Reactions
What are the three steps of the Calvin cycle?
- Fixation of carbon
- Reduction takes place to form G3P(NADPH is used up in this step and ATP)
- Regeneration of RuBP from G3P
How many times must the Calvin cycle run to remove on G3P molecule?
The cycle must run three times and thus one G3P molecule requires 3 carbon dioxide molecules
What is G3P used for?
To make sugar and amylose. After the Calvin cycle it is usually made into fructose and glucose and then other carbohydrates
What are stomata?
- Tiny holes on plant leaves that can open and close to let CO2, water and oxygen diffuse in/out
When do stomata close?
-When its really hot to avoid evaporation
What happens if stomata close for too long?
-Oxygen accumulates in the leaves and CO2 depletes can cause problems such as photorespiration
What is photorespiration?
-When rubisco enzyme reacts binds with oxygen which is very wasteful and serves no purpose(ATP is used up )
Why can rubisco interact with O2 ?
Because it evolved when there was barely any oxygen on earth
Difference between light and dark reaction?
Light:
- Produce ATP
-Produce NADPH
-USe both photosystems
-Thylakoid membrane
Dark:
-Stroma
-Uses NADPH
-Uses ATP
-No water split
-No O2 released
-Only makes sugar
-Only uses PSI
How is oxygen production measured directly?
By shining different wavelengths of lights on plants and measuring oxygen production
How is oxygen production measured indirectly?
Oxygen-seeking bacteria will move toward wavelengths where plants produce the most oxygen
What is the absorption spectrum?
Shows what wavelengths pure pigments absorb
-A peak corresponds to an excited electron in the chlorophyll
What is the strongest oxidizing agent in biology?
Chlorophyll packing an electron, it is strong enough to split water and take its electron
What releases oxygen during photosynthesis?
Splitting of water
How many things contribute to the proton gradient in photosynthesis?
- Splitting of water
- Reduction of ubiquinone
- NADP+ removes protons on the opposite side of the gradient, decreasing the concentration
How is ATP made in photosynthesis? What is it called?
-Photophosphorylation
-Through the ATP synthase
