8.3 Photosynthesis Flashcards
Where does photosynthesis take place?
In the chloroplast
Photosynthesis can be roughly divided into ___ processes
Two
What are the two processes that photosynthesis can be roughly divided into?
-Light-dependent reactions
-Light-independent reactions.
What happens in photosynthesis in the light dependent reactions?
The light-dependent reactions convert light energy into a flow of excited electrons.
What are the steps of the process of the light-dependent reactions converting light energy into a flow of excited electrons?
1) Photoactivation
2) Photolysis
3) Electron transport chain
4) Chemiosmosis
5) ATP synthesis
6) NADP reduction.
Where do the light-dependent reactions take place?
In the thylakoids (more specifically in the intermembrane space of the thylakoids), special membrane structures found within the chloroplast.
Just like in cellular respiration, some of the subsequent steps take place in another compartment of the ___. (reword)
Chloroplast
What happens to the energy generated in the light-dependent reactions of photosynthesis?
-It needs to be stored.
-Again, as is the case with cellular respiration, the energy is transferred to electron carriers, but in the chloroplast, it is in the reduced form of NADP (nicotinamide adenine dinucleotide phosphate).
-The light-dependent reactions also generate ATP.
What does each NADP molecule accept?
-It accepts two hydrogen atoms (i.e. two H + ions and two electrons) to form NADPH + H + (second plus is small).
-For simplicity, reduced NADP will be referred to as NADPH throughout the text.
What are the light-independent reactions called?
The Calvin cycle
Where does the Calvin cycle take place?
In the stroma (the cytoplasm of the chloroplast).
What are the steps of the Calvin cycle?
1) Carbon fixation
2) Carboxylation of ribulose bisphosphate (RuBP)
3) Triose phosphate production.
What happens to the NADPH and ATP produced in the light-dependent reactions?
They are used in the light-independent reactions.
When do light-independent reactions occur and stop?
-Although these reactions are called light-independent, in reality, they can only continue for a short while in the absence of light.
-Once the stock of NADPH and ATP runs out, light-independent reactions stop.
Diagram of the chloroplast: light-dependent and light-independent reactions (Calvin cycle)
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What is the difference between the electron carrier in photosynthesis and cellular respiration?
-Electron carrier in Photosynthesis: reduced form of NADP.
-Electron carrier in cellular respiration: reduced form of NAD.
What is photosynthesis?
The process during which an organism (usually a plant) uses light energy to carry out chemical reactions to produce sugars or other organic molecules.
What does visible light consist of?
Photons with a particular wavelength.
What do different wavelengths of the photon mean?
-A shorter wavelength (of the photon) means a higher energy content.
-So, a ‘blue’ photon has a higher energy level than a ‘red’ photon, because blue light has a shorter wavelength than red light.
-Chlorophyll is a light-sensitive molecule that can absorb photons with certain wavelengths.
-The absorption spectrum of chlorophyll shows this.
What is a photosystem made up of?
-Hundreds of chlorophyll molecules (up to 300) and other accessory pigments (around 30–40) aggregate (collect) together with a protein to form a protein complex called a photosystem.
-These pigments transfer all the energy that they have absorbed from light photons to a central chlorophyll a molecule that forms the reaction center of the photosystem.
What are accessory pigments?
-Any pigment, other than chlorophyll a, that can absorb light.
-Examples include chlorophyll b and carotenoids.
Examples of accessory pigments
Chlorophyll b and carotenoids
How many types of photosystems are there embedded in the thylakoid membrane?
Two
What are the two types of photosystems embedded in the thylakoid membrane?
Photosystem I and Photosystem II.
What is an important difference between Photosystem I and Photosystem II?
Photosystem I is sensitive to light wavelengths of 700 nm, while Photosystem II is sensitive to light wavelengths of 680 nm.
Diagram of Photosystem II, the photosystem that is the first to be activated by light
Which photosystem is the first to be activated by light?
Photosystem II
How do the chlorophyll molecules in Photosystem II become activated and what happens after this?
-They become activated by the photons (photoactivation) of light and pass on their activation energy to the reaction center, which in turn passes two excited electrons to the primary electron acceptor.
-This then passes the two electrons to plastoquinone, a hydrophobic electron carrier, which stays inside the thylakoid membrane to pass on the electrons to the next electron carrier; continuing all the way to photosystem I.
Describe the electron transport chain between photosystems
-The chlorophyll molecules in Photosystem II become activated by the photons (photoactivation) of light and pass on their activation energy to the reaction centre, which in turn passes two excited electrons to the primary electron acceptor.
-This then passes the two electrons to plastoquinone, a hydrophobic electron carrier, which stays inside the thylakoid membrane to pass on the electrons to the next electron carrier, continuing all the way to photosystem I.
-Photosystem II repeats this one more time, so that, in the end, the reaction centre has lost four electrons that must be replaced before a new cycle can begin.
-The reaction centre, because of its oxidised state (it has lost four electrons), now becomes a powerful oxidising agent.
-It is the reason that water molecules can be split or lysed (photolysis) to give up their electrons to the reaction centre.
-Photolysis of water generates electrons for use in the light-dependent reaction, because it constantly replaces electrons lost by Photosystem II.
What does the photolysis of water generate?
Electrons for use in the light-dependent reaction
What is photolysis of water?
-The splitting of water molecules into oxygen, hydrogen ions, and electrons in the presence of light.
-Light is not used to split the water molecules directly, but instead causes the loss of electrons from the reaction center (of Photosystem II), which then acts as an oxidizing agent to trigger the reaction.
Equation for the photolysis reaction
What is a byproduct of the photolysis reaction?
Oxygen
In Photosystem II, the electrons lost from the reaction center are replaced by ___.
Electrons derived from water.
Photosystems are found in the ___
Thylakoid membrane
What happens to the excited electrons channeled to plastoquinone from photosystem II?
They are passed on to other carriers that ultimately end up in Photosystem I, which in turn reduces NADP to NADPH.
What happens to the electrons and energy contained in NADPH?
They are used in the Calvin cycle to produce triose phosphates, the basis for all carbohydrate synthesis in photosynthetic organisms.
Describe the transfer of excited electrons between carriers
-This occurs between carriers in thylakoid membranes.
-Sufficient energy is released during electron transfer to enable ATP to be made from ADP and phosphate.
What happens as excited electrons are transferred along electron carriers and what does this cause?
-They drop in energy level to return to their stable ground state.
-This causes a release of energy, which is then used to pump hydrogen ions from the stroma into the thylakoid intermembrane space.
Explain how an electrochemical gradient is established across the thylakoid membrane
-As the thylakoid membrane is impermeable to protons, the hydrogen ions quickly accumulate and establish an electrochemical gradient.
-The proton gradient, resulting from this higher concentration of protons, provides the potential energy for an ATP synthase complex to drive the production of ATP, just like in cellular respiration.
-As hydrogen ions diffuse through the ATP synthase complex, sufficient energy is released to phosphorylate ADP to ATP.
-Since this process is ultimately driven by energy derived from photons (particles of light), it is referred to as photophosphorylation .
What is chemiosmosis?
This involves the pumping of protons (H + ions) into the intermembrane space of thylakoids by using energy released by electron transport along the ETC, followed by diffusion of protons into the stroma down a concentration gradient through ATP synthase to produce ATP.
Describe the transfer of electrons in photosystem I
-Photosystem I involves similar steps to Photosystem II.
-The excited electrons received (from Photosystem II) are ultimately transferred to ferredoxin, which in turn reduces NADP to NADPH.
-Each NADP molecule accepts two electrons from Photosystem I and two hydrogen ions from the stroma to form NADPH.
What is the ultimate acceptor of the electrons released from Photosystem II in the photophosphorylation process?
NADPH
Diagram of all the steps involving Photosystems I and II, the electron carriers, the generation of the reduced form of NADP, and the synthesis of ATP (electron transport chain and photophosphorylation)
What does the chemiosmotic process in chloroplasts involve?
Establishment of a proton gradient across the thylakoid membrane.
In mitochondria, chemiosmosis moves protons from the intermembrane space into the matrix, whereas in chloroplasts, chemiosmosis moves protons from ___
The thylakoid space to the stroma.
Chemiosmosis is the hydrogen ion (proton) gradient formed by the electron carriers in the ETC, which isolate protons in the thylakoid space.
What is carbon fixation?
-One of the most important biochemical reactions we know.
-It is the start of all carbohydrate compounds found in photosynthetic organisms.
How is atmospheric CO 2 ‘fixed’?
-By adding it to ribulose bisphosphate (RuBP).
-The reaction is catalyzed by a large enzyme, ribulose-1,5-bisphosphate carboxylase, commonly referred to as rubisco.
-This is the first step of what is known as the Calvin cycle.
Who was Melvin Calvin?
-A biochemist who worked out most of the details of this cyclic pathway.
-He was awarded the Nobel Prize in Chemistry in 1961.
The Calvin cycle takes place in the stroma of the chloroplasts, where there is a high concentration of the enzyme ___
Rubisco
Why is there a high concentration of rubisco in the stroma of the chloroplasts?
-There are two reasons for this: rubisco is a ‘slow’ enzyme and the Calvin cycle is rather inefficient due to its high energy requirement.
-Additionally, rubisco can be competitively inhibited by oxygen, binding to it in preference to carbon dioxide.
-So, to speed up the process, many molecules of rubisco enzyme are needed.
How many phases does the Calvin cycle consist of?
Four
What are the four distinct phases that the Calvin cycle consists of?
1) Carbon fixation
2) Reduction of glycerate 3-phosphate (3-PGA, 3 C atoms) to triose phosphate (3 C atoms)
3) Release of one molecule of triose phosphate (to be used for carbohydrate synthesis)
4) Regeneration of ribulose bisphosphate (5 C atoms).
What is glycerate 3-phosphate also known as?
3-phosphoglycerate, so it is abbreviated to 3-PGA.
What is glyceraldehyde 3-phosphate?
The three carbon triose phosphate molecule produced during the Calvin cycle.
Why is the Calvin cycle a costly process?
-To produce one molecule of triose phosphate that can be used to produce glucose and other carbohydrates requires three turns of the Calvin cycle.
-For each complete turn, three molecules of ATP and two molecules of reduced NADPH are needed.
Diagram of the Calvin cycle
You will need to know the names of all the intermediates and how ribulose bisphosphate is regenerated.
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What happens in the Calvin cycle when three carbon dioxide molecules react with three molecules of RuBP?
-Six triose molecules are formed.
-Of these, five are used to regenerate RuBP.
-Therefore, sugar is not directly formed through the Calvin cycle.
-Instead, triose phosphates can react together to form six-carbon sugar phosphates from which carbohydrates such as sugars and starch can be made.
Explain how RuBP is regenerated from triose phosphates
-This involves the rearrangement of the carbon atoms in three-carbon-containing molecules to form five-carbon molecules.
-This is a multi-step reaction catalyzed by enzymes.
-The rearrangement is followed by phosphorylation with ATP to add another phosphate group to each five-carbon molecule.
-This regenerates the required amount of RuBP, the original organic starting material for the cycle.
Why is the fixation of carbon dioxide by RuBP a rate-limiting step in the Calvin cycle?
The rate of the reaction using rubisco is directly affected by the concentration of carbon dioxide.
Summary of carbon fixation and the Calvin cycle
-In the light-independent reactions, a carboxylase (rubisco) catalyzes the carboxylation of ribulose bisphosphate.
-Glycerate 3-phosphate is reduced to a triose phosphate, using reduced NADP and ATP.
-Triose phosphate is used to regenerate RuBP and produce carbohydrates.
-Ribulose bisphosphate is reformed using ATP
Where do the enzymatic reactions of the Calvin cycle take place?
Stroma of the chloroplast
What is the primary function of the Calvin cycle?
To synthesise simple sugars from carbon dioxide.
The NADPH required for the Calvin cycle comes from ___
Reactions initiated in photosystem I.
What were Melvin Calvin’s idea and argument?
-He came up with the idea of using 14 CO 2 as the starting material for photosynthesis.
-This radioactive isotope of carbon had been recently discovered and allowed for a detailed analysis of the photosynthetic process.
-Calvin argued that, if CO 2 was the starting material, then the radioactive products should appear as soon as an organism, such as the green alga Chlorella, which uses CO 2 to carry out photosynthesis, was given access to the radioactive carbon source.
Explain the apparatus that Calvin used in his experiment
-Calvin used a lollipop apparatus, which allowed him to control when radioactive carbon was added to the algal soup (the green liquid in the lollipop container).
-He was able to take a sample at given times by draining some of the algal soup and letting the sample drop into boiling ethanol, which stops all reactions but does not affect the photosynthetic products.
Diagram of Calvin’s lollipop apparatus
Describe Calvin’s method in his experiment
-Samples collected at various time intervals were then used to run two-dimensional chromatography to separate the constituents.
-The chromatograms were exposed to X-ray films to make radiograms that capture the position of radioactive products made during photosynthesis.
-Calvin analyzed the samples and was able to deduce the starting point of the photosynthetic pathway and the subsequent products.
-For example, he would measure these products after 5s and after 30s exposure to 14 CO 2.
-Using this approach, he was able to deduce all the intermediate compounds in what is now known as the Calvin cycle.
Explain the results of Calvin’s experiment
-A large amount of 3-PGA compared to other detected compounds after 5 s suggests that 3-PGA is the first product in the carboxylation of RuBP.
-The results after 30 s highlight the variety of intermediate products (e.g, PGA, PEPA) made in the light-independent reaction.
A flask containing photosynthetic green algae and a control flask containing water with no algae are both placed under a bank of lights, which are set to cycle between 12 hours of light and 12 hours of dark.
The dissolved oxygen concentrations in both flasks are monitored.
Predict what the relative dissolved oxygen concentrations will be in the flask with algae compared to the control flask.
The dissolved oxygen in the flask with algae will be higher in the light, but lower in the dark.
When exposed to light, the algae will produce oxygen but, during the night, algae will respire and consume oxygen. The oxygen concentration in the other flask will remain the same.
Name the apparatus used by Calvin to elucidate (work out) the carboxylation of RuBP.
Lollipop apparatus
Describe the structure of the chloroplast
-The chloroplast is a double-membraned structure with thylakoids arranged in stacks.
-A single stack is called a granum (plural: grana).
-The thylakoids enclose a small space, the intermembrane space, in which the proton gradients needed for ATP synthase can be generated.
-The thylakoid spaces are small in volume, allowing the proton concentration to remain as high as possible.
What is the advantage of the thylakoid membranes?
-They provide a large surface area in which Photosystems I and II, ETC and ATP synthase are embedded.
-However, to optimise photosynthesis, the grana need to be exposed to the maximum amount of light.
Where are the thylakoids found?
They are embedded in the stroma.
What does the stroma contain?
-Many enzymes, substrates, NADPH and ATP molecules to keep the Calvin cycle turning.
-Sometimes starch grains can also be seen in the stroma.
Describe the DNA in the chloroplast
The chloroplast has its own DNA (usually a circular molecule, similar to a bacterial genome) and fully functioning protein translation machinery (70S ribosomes).
Diagram and transmission electron micrograph (TEM) of part of a chloroplast
Diagram of a whole chloroplast
What are the structures in the chloroplast?
-Thylakoid and grana
-Small thylakoid intermembrane space
-Stroma
-70S ribosomes
-Naked DNA
How are the thylakoids and grana adapted to their function?
Thylakoids and grana offer a large surface area where photosystems, ETC and ATP synthase are embedded and light-dependent reactions occur.
How is the small thylakoid intermembrane space adapted to its function?
The small volume of the thylakoid intermembrane space allows the fast generation of the H + gradient needed for chemiosmosis.
How is the stroma adapted to its function?
This contains rubisco, as well as other enzymes, substrates, NADPH and ATP molecules to keep the Calvin cycle turning.
How are the 70S ribosomes adapted to their function?
These synthesise some of the proteins and enzymes needed within the chloroplast.
How is the naked DNA adapted to its function?
This codes for some of the chloroplast proteins.
Drawing a chloroplast for the exam
You should be able to annotate a diagram to indicate the adaptations of a chloroplast to its function.
The pH of the thylakoid space has been measured, as have the pH of the stroma and the cytoplasm of a particular plant cell.
Which, if any, relationship would you expect to find?
The pH within the thylakoid is lower than that of the stroma.
The thylakoid space is where the protons (hydrogen ions) are collected, creating a high concentration of protons and thus a very low pH. This does not occur anywhere else in the cell, so all the options except #1 are incorrect.
What is used to reduce NADP in the light-dependent reactions of photosynthesis?
Electrons from Photosystem 1
What does photolysis generate?
Each water molecule releases two electrons, an oxygen atom, and two protons.
Which wavelength of electromagnetic radiation is absorbed efficiently by chlorophyll for
photosynthesis?
400-700nm
