Lecture 33: Photosynthesis Flashcards
Tuesday 3rd DEcember 2024
What does all free energy needed by biological systems arise from?
Solar energy that is trapped by photosynthesis
Globally, how much free energy does photosynthesis store per year?
Globally, photosynthesis stores about 10^18 kJ of free energy every year. That is about 200 billion tons of carbon fixed into carbohydrates and other organic compounds
What does the light reaction of photosynthesis produce?
reducing power (NADPH) and ATP
What is the waste product of photosynthesis?
Oxygen
Is photosynthesis endergonic or exergonic?
Endergonic, absorbs energy and requires a lot of free energy
The ΔG0 for photosynthesis is + 480 kJ/mol. What does this tell us about photosyntheisis?
That it will not run if energy is not put in.
What process introduced oxygen into the atmosphere?
Photosynthesis
Technically, is photosynthesis feasible under standard conditions?
No, this is why lots of solar energy has to be inputted into the reaction
Where does photosynthesis occur?
In the chloroplasts
How many plasma membranes do chloroplasts have?
2, they are double membraned
What are grana?
stacks of thylakoids
Why do the thylakoid membranes have a large surface area?
To increase the surface area for more proteins
What 2 reactions can photosynthesis be divided into?
The light dependant reaction and the light independent reaction (calvin cycle)
Give an overview of the light dependant reaction
- Capture of photons/light
- Conversion of light energy into ATP and NADPH
- Split water to release oxygen as a by-product.
Give an overview of the light independant reaction
- Use ATP and NADPH to fix Carbon dioxide
and produce sugars
Is it true that the shorter the wavelength, the higher the energy?
Yes
Give the equation for the speed of light
Speed of light
C = 𝝺 * 𝝼
Wavelength * frequency
What is the equation for calculating the energy of a photon from its wavelenght?
E= h⋅c/λ
What is h
h = Planck’s constant (6.6 x10-34 J·s)
What is c
c = speed of light (3x108 m/s)
What is 𝛌
𝛌 = wavelength (e.g. 700 nm )
What ion does chlorophyll have in the middle?
Magnesium / Mg2+
Is chlorophyll hyrdophobic or hydrophilic?
Chlorophyll is hydrophobic
What are the 2 types of chlorophyll?
Chlorophyll a and chlorophyll b
Does chlorophyll absorb green light?
No, chlorophyll reflects green light. This why leaves appear green.
What are the main pigments that chlorophyll absorbs?
red and blue light
What makes chlorophyll hydrophobic?
Its hydrophobic side chains/phytol chain is very hydrophobic
Describe the hill reaction
- In the hill reaction, chloroplasts were isolated and it was found that when you shine light on to the chloroplasts, they will evolve oxygen, so long as an electron acceptor is available. They used a compound called fericyanide as their electron acceptor.
- Showed that the primary event in photosynthesis is the light-driven energy requiring transfer of an electron from one substance to another. `
Light —> Thylakoid membrane —> reaction centre —>
‘Photons causes electron to go from
ground state to excited state
The excited state can be transferred
until trapped in the reaction centre.’ Is this true ?
Yes
What happens when a photon reaches the reaction centre?
- A photon excites an electron in the reaction center.
- The electron is transferred to a primary electron acceptor.
- The reaction center is replenished with a new electron from water.
- The excited electron’s energy is used to produce ATP and NADPH in subsequent steps.
What are the 2 different types of reaction centres (each work with photons of different wavelenghts)?
Photosystem II (PSII) and Photosystem I (PSI)
What is the ‘red drop’?
The “red drop” is a phenomenon observed in photosynthesis that refers to a drop in the rate of photosynthesis when light is shifted to longer wavelengths. Higher rate at 680 nm with PSII and when both red and blue light is available. Just red = slower rate.
What wavelength of light does PSII absorb?
680nm
What wavelength of light does PSI absorb?
700nm
Is it true that ‘Photosynthetic rate in the presence of both 670 nm and 700 nm light is greater than the sum of the individual rates at 600 and at 700 nm’?
Yes. Hence, the 2 light reactions must react.
Describe the steps of the light dependant stage of photosynthesis
-Photons are captured
- Conversion of light energy to ATP and NADPH
- Electrons reach terminal receptor NADP+ and this results in the creation of NADPH.
- Proton gradient created by electron transport used to make ATP
- Oxygen is a waste product
Name one accessory pigment that also absorbs light during photosynthesis
Beta-carotene
What initiates the electron transport chain?
Excited electrons
when the electron in PSII gets excited and leaves the photosystem, where does the replacement electron come from?
Water
when the electron in PSI gets excited and leaves the photosystem, where does the replacement electron come from?
plastocyanin (PC)
When the photosystems absorb light, what happens to their redox potentials?
Their Redox potentials decrease, increasing the likelihood of them giving up their electrons.
What does PSII pass electrons on to ?
Plastaquinone (PQ)
What are the problems with PSII?
Photons remove 1 e- at a time
Plastoquinone (the acceptor) takes 2 e-, one at a time (via semiquinone) PQ +2e- ➞ PQH2
Water needs to donate 4e- to make one molecule of oxygen
solved by: e- taken off Mn cluster one at a time until 4 are received from water
(energy from the electrons is used to move 4H+ into the thylakoid lumen)
Describe the light dependant reaction
- PSII extracts electrons from water.
- Electrons move via plastoquinone (PQ) to cytochrome b6f. The cytochromre can only carry one electron at a time. PQH2 unloads e- via Q cycle.
- Electrons are evnetually transferred to plastocyanine, also a 1e- carrier (Cu protein).
- 2 H+ are transferred to the thylakoid lumen, creating a gradient.
- PSI
- Electrons re-excited by photons. Excited electron goes on to ferredoxin and is replaced by an electron from plastocyanine.
- Final electron acceptor: NADP+, reduced to NADPH. Further decreases proton concentration in stroma, as one proton has now been taken up by NADP.
(NADP+ is the final electron acceptor. Needs two electrons to reduce to NADPH. 2Fd transfer 1e- each to the enzyme).
- Proton gradient drives ATP synthase, synthesizing ATP.
Where does the energy for photoysnthesis come from?
Light
Where does the energy for respiration come from?
NADH
What is complex III in respiration similar to in photosynthesis?
Complex III is similar to Cyt bf
In the mitochondria, where is the low concentration of protons?
In the matrix
In the chloroplast, where is the high concentration of protons?
In the lumen
similarities in electron transport in mitochondria and chloroplasts
- Both use a source of energy, whether that be light or chemical.
- Both systems utilize the gradient of reducing potentials (from negative to positive) to transfer electrons
- Both rely on the generation of a proton gradient for ATP synthesis.
-
Describe cyclic Photophosphorylation
If ATP demand exceeds NADPH demand, Feredoxin will no longer give electrons to enzymes that reduce NADP and electrons will be put back into the Cytochrome bf complex in PSI.
What does cyclic photophosphorylatio produce?
It produces just ATP, instead of ATP AND NADPH.
Describe Calvin’s experiment
- Calvin and his colleagues sought to determine the exact biochemical pathway by which
CO is incorporated into organic molecules. - Used algae in a disc
- Shone light on to the algae
- This allowed photosynthesis to occur
- He would then feed the algae with radioactive carbon dioxide to see where the carbon dioxide would end up.
- Found that if he harvested his algae 5 seconds after giving them the carbon dioxide, he got only one product, which was 3-phosphoglycerate.
- 3-phosphoglycerate is the first compound made from harvesting carbon dioxide.
- We eventually established the entire process, which is known as the calvin cycle.
What products of the light dependant stage are used in the light independent stage?
ATP and NADPH
Where does the light independent reaction occur?
In the chloroplast stroma
What is the first product of the Calvin cycle?
3-phosphoglycerate
In the calvin cycle, is carbon dioxide reduced to sugars?
Yes
What 2 forms does RuBP have?
Its ketone form and its enol form (alkene group)
Which form of RuBP is reactive in photosynthesis?
the enol form
What happens when carbon dioxide is fixed onto RuBP?
A very unstable 6 carbon compound is formed. But this immediately reacts with water. This cleaves (breaks) the bond between the 2 carbons and we end up with 2 molecules of 3-phosphoglycerate
(3PG). (the first official molecule)
What catalyses the fixation of Carbon dioxide to RuBp?
The enzyme RuBisCo
What is the world’s most abundant enzyme?
The world’s most abundant enzyme is RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase).
Steps of the Calvin cycle
- Carbon fixation. Carbon dioxide is fixed onto RuBP. 2 molecules of 3-phosphoglycerate
(3PG) are produced. - Reduction of 3PG to Glyceraldehyde-3-phosphate (G-3-P) using energy from ATP and Hydrogen from NADPH.
- The Glyceraldehyde-3-phosphate (G-3-P) can then be pulled out to produce sugars OR RuBP can be regenrated. This regenration requires ATP.
How many CO2’s need to go in before a sugar can be withdrawn?
3
What is the overall balanced equation of the calvin cycle?
6 CO2 + 18 ATP + 12 NADPH + 12 H2O →C6H12O6 + 18 ADP + 18 Pi + 12 NADP+ + 6 H+
How much ATP and NADPH need to go in before a sugar can be withdrawn?
6 of both
to fix 6 CO2 and make an hexose (e.g. glucose):
12 ATP and 12 NADPH spent from 3PG to G-3-P
6 ATP spent from G-3-P to RuBP
to fix 6 CO2 and make an hexose (e.g. glucose):
12 ATP and 12 NADPH spent from 3PG to G-3-P
6 ATP spent from G-3-P to RuBP
What is the full name of Rubisco?
Ribulose 1,5 bisphosphate carboxylase/oxygenase
Why is rubisco one of the most dissapointing enzymes?
- Has very slow catalytic activity (~3 reactions/second).
- Often fixes O2 instead of CO2
(photorespiration), wasting ATP and NADPH. This is because Rubisco evolved before there was any oxygen around.
-
What adaptions are there to minimise photorespiration? (instead of C3)
C4 photosynthesis and CAM Photosynthesis.
C4
- C4 photosynthesis is a specialized photosynthetic pathway that evolved to efficiently fix carbon dioxide in conditions of low carbon dioixide and high oxygem, usch as hot, arid environments.
- It minimises photorespiration
- In C4 photosynthesis, Carbon fixation and the Calvin cycle occur in different cell types: mesophyll cells and bundle sheath cells.
- In the mesophyll cells, CO2
is captured and fixed by the enzyme PEP carboxylase (high affinity for CO2), producing a 4-carbon compound, oxaloacetate. - Oxaloacetate is converted to another 4-carbon compound, malate, which is more stable and can be transported easily.
- Malate is transported from the mesophyll cells to the bundle sheath cells, which are located deeper in the leaf and are insulated from the high oxygen levels in the mesophyll.
- Inside the bundle sheath cells, malate is decarboxylated (releases CO2), regenerating a 3-carbon molecule, pyruvate, and releasing CO2 for use in the Calvin cycle.
-The released CO2 is now fixed by RuBisCO in the Calvin cycle to form sugars. This occurs in an environment where O2 levels are low, minimizing photorespiration.
- The pyruvate formed in the bundle sheath cells is transported back to the mesophyll cells, where it is converted back to PEP using ATP, completing the cycle.
Why does C4 photosynthesis require additional ATP compared to C3 photosynthesis?
C4 photosynthesis requires additional ATP compared to C3 photosynthesis because of the energy used to regenerate PEP.
On average, 2 extra ATP molecules are needed per
CO2 fixed.
is C4 photosynthesis common in many tropical plants?
Yes
‘Malate can even be generated during night and stored for use in the day time in some plants, such as cacti’. is this statement true?
Yes
Describe CAM photosynthesis
- Stomata open at night to fix CO2, stored as malate. (less sunlight at this time, so plant less likely to release oxygen as a result of photosynthesis)
- Calvin Cycle operates during the day using stored CO2.
How can the carbon cycle be coupled to gluconeogenesis?
- Calvin cycle will produce G-3-P, which will be transported acrossthe chloroplast membrane via an antiporter .
- It will then be converted to glucose, then sucrose for transport, as sucrose is a mobile sugar that can be easily transported around a plant.
What can excess glucose in a plant be converted into?
Starch
Where is starch found in plants?
In stroma in chloroplasts
In seeds and tubers like potatoes
What is starch made up of?
amylose and amylopectin
What is starch similar in structure to?
glycogen. But amylose is unbranched, and ha fewer branchpoints. This means that starhc degrades slower than glycogen
What is the overall efiicnecy of photsynthesis?
30%
WHat is the difference bvetwee ATP synthase in respiration and in photosynthesis?
In respiraiton, the ATP synthase is in its standard orientation, but in photosynthesis, the ATP synthase is inverted.
Photosynthesis proton gradeint?
Lumen → Stroma
Respiration proton gradeint?
Matrix → Intermembrane Space
