All Flashcards
What is the general balanced equation for photosynthesis?
6CO2 + 6H2O —> C6H12O6 + 6O2
- Is photosynthesis endergonic or exergonic? Explain
Photosynthesis is an endergonic reaction, as it requires input of energy to synthesize carbohydrates. This is because the energy is absorbed and creates more energy. Exergonic reactions release energy instead of consuming it.
- .What is the main function of CO2 in photosynthesis?
Enters the calvin cycle and is added to ribulose 1,5- bisphosphate and CO2 is converted into C-containing organic compounds (ex. glucose)
- What is the main function of H2O in photosynthesis?
Provides electrons for P680 to send them through the antenna complex to the ETC and H+ ions enter the lumen to create a proton gradient.
- Where does the O2 molecule come from during photosynthesis?
It comes from P680 splitting two water molecules.
- Define transpiration
The process by which plants give off water vapor (evaporates) through the stomata in their leaves.
- Describe stomata. How, when, and why do they open and close?
Stomata: regulates gas exchange between the plant and environment and control of water loss. Open:
- when K+ ions diffuse into guard cells, water moves in by osmosis, guard cells swell, and the stoma opens
- when open there is some water loss
- to enable controlled gas exchange Close:
- when K+ ions diffuse out of guard cells, water moves out by osmosis, guard cells become flaccid, and the stoma closes
- when the plant is at risk of losing too much water, the stomata remain closed
- Light Reactions:
a. Describe photoexcitation, photosystem I (P700) and photosystem II (P680)
Photoexcitation: when photons are absorbed by photosystem II in the antenna complex and gets transferred to the P680 molecule; one electron for every photon absorbed moves from the ground state to an excited state
Photosystem 1 (P700): when the electrons from the cytochrome complex are transferred to P700 via plastocyanin. Another photon is required for an electron to be released from P700 and pulled to the primary electron acceptor in Photosystem I.
Photosystem 2 (P680):
very electronegative and can split water molecules
P680* is when the electrons from the photons of light are being absorbed by P680.
Then P680 becomes oxidized, losing electrons that go up the antenna complex, becoming P680 +. Finally, it becomes neutral after the electrons from water go into it to regain electrons.
for every 1 H2O molecule, 2e- go to P680, 2H+ remain in the thylakoid lumen, and 1/2 O2 is a waste product.
THATS HOW WE BREATHE!!
Final electron acceptor in light dependant reaction: NADPH
b. Describe non-cyclic electron flow
Non-cyclic electron flow is a process in photosynthesis where light energy is absorbed by two photosystems (PS II and PS I) in a sequence. First, light hits Photosystem II (PS II), exciting electrons. These high-energy electrons are passed through an electron transport chain, creating a flow of protons (H+) across the membrane, which generates a proton gradient. As a result, ATP is made. Meanwhile, the electron that left PS II is replaced by splitting water molecules (photolysis), producing oxygen and more electrons. Next, the electrons reach Photosystem I (PS I), where they get re-energized by light and are used to reduce NADP+ to NADPH. The key reason it’s called “non-cyclic” is because the electrons don’t return to PS II or recycle through the system; they are permanently transferred to NADP+, forming NADPH, which is used in the Calvin cycle.
c. Describe chemiosmosis:
Chemiosmosis in photosynthesis is when protons (H+) flow back into the stroma through ATP synthase, which uses this flow to convert ADP and phosphate into ATP. This happens after light energy drives electrons through an electron transport chain, creating a proton gradient across the thylakoid membrane. The ATP produced is used in the Calvin cycle to make sugars.
d) What is the purpose of NADPH & ATP?
NADPH: NADPH is the final electron acceptor in photosynthesis, carrying electrons to the Calvin cycle to help convert carbon dioxide into glucose.
ATP: ATP provides energy and phosphate groups, which are used to drive chemical reactions and build molecules in the Calvin cycle. Together, NADPH and ATP fuel the process of making sugars from carbon dioxide
e. Describe cyclic electron flow and why it happens
In this process, after light excites electrons in Photosystem I, instead of being passed to NADP+ to make NADPH, the electrons are sent back through the electron transport chain to PS I. when the electrons are transferred to ferredoxin, instead of being oxidized and transferring electrons to NADP+, ferredoxin transfers the electrons to PQ
pumps H+ into the lumen which is used to drive ATP synthesis.
This looping of electrons allows the plant to produce more ATP without producing NADPH, which helps balance the energy needs of the cell, especially when more ATP is required for the Calvin cycle.
3 ways to create a proton motive force:
- protons taken into the lumen by redox reactions of PQ
- protons are added due to water splitting inside the lumen
- protons are removed from the stroma to make NADPH, which decreases the concentration of protons in the stroma compared to inside the lumen (cytochrome complex)
- Light independent reactions/Calvin Cycle: a. Describe carbon fixation
- 3 CO2 reacts with RuBP (5-carbon) to form two 3- carbon molecules of 3-phosphoglycerate. (18 Carbons)
- Then, through chemical reactions creates 1-3 bisphosphoglycerate - The Splits into 2 3-phosphoglycerate molecules
b. Describe reduction & oxidation reactions – for the ‘key players’
P680: reduces to P680 with electrons and water-splitting complex to be oxidized again -oxidized when it transfers high-energy electrons
P700: reduced via electrons from plastocyanin. a photon provides more energy. oxidized and electron gets pulled to PEA in photosystem
NADP+: reduced; electrons are transferred to NADP+ to form NADPH in the light dependent reactions.
NADPH: oxidized in calvin cycle to form NADP+
malate in C4: oxidized to pyruvate; when it diffuses from the mesophyll cell into the bundle-sheath cells, and enters chloroplasts.
malate in C3: oxidized; when malic acid diffuses from the vacuoles into the cytosol. Oxaloacetate: reduced to form malate
c. Why is Rubisco considered one of the most important enzymes on Earth?
it helps in the making of organic molecules that are consumed by many of the worlds organisms - catalyzes the reactions of CO2 + RuBP to 3-phosphoglycerate
d. Describe how RuBP is regenerated
- The remaining 5 G3P are combined and rearrange to form 3 molecules of RuBP - 5 mol. G3P x 3 carbons / G3P = 15 carbons
- rearranged to 3 molecules of RuBP — 3 RuBP is 15 carbons
- How many G3P molecules are required to make 1 glucose molecule?
2 G3P molecules
- Why is G3P a key intermediate in photosynthesis?
It helps in the production of glucose and the regeneration of RuBP.
- How many CO2 molecules are required to make 1 glucose molecule? How many turns of the Calvin cycle is needed for this?
6 CO2 for 1 glucose and 2 cycles and 6 turns.
- Define photorespiration
an unfavorable process which decreases the production of sugars by photosynthesis. Instead of Rubisco reacting with co2, it will react with oxygen which will waste energy, as the whole point is to fix co2 not oxygen by itself.
- Describe how CAM plants are unique
- they are water storing plants
- stomata is open at night and closed during the day (temporal separation)
Weather conditions for c3, c4, and cam plants:
C3: cool, moist environments
C4: inhabit hot, dry environments;
CAM: inhabit hot, dry desert environments during the day and cool environments at night.
- Explain how the Calvin Cycle is affected for C3 plants during
sunny, how, and dry days AND during low light conditions.
Sunny Days: the efficiency of carbon fixation can be reduced due to photorespiration
- increased rate of photorespiration
- the Calvin cycle is more active as it captures CO2 to produce sugars Dry Days: - stomata closes to conserve water and CO2 intake reduces. Low light conditions: reduced production of ATP and NADH
- decrease in photosynthesis due to limited light access
- insufficient energy to power the enzyme-driven processes, such as the fixation of CO2 by RuBisCO
- Explain how both C4 plants and CAM plants have adapted to survive sunny, hot, and dry days.
C4 captures CO2 in mesophyll cells and then transporting it to bundle sheath cells. This increases the CO2 concentration around the enzyme RuBisCO, reducing photorespiration. As a result, C4 plants use water and CO2 more effectively, helping them survive in hot, dry conditions. CAM plants store CO2 at night and use it for photosynthesis during the day, keeping their stomata closed in the heat to save water.
Examples of C3. C4, ,AND CAM PLANTS:
C3: wheat, rice
C4: corn, sugarcane
CAM: cacti, pineapple
- Would a green plant grow better under a green light or a red light? Explain.
A green plant would grow better under red light compared to green light. This is because plants primarily absorb light in the blue (~450-495 nm) and red (~640-680 nm) regions of the spectrum for photosynth light is mostly reflected by the plant, which is why plants appear green to us. The chlorophylls in plants absorb light most efficiently in the red and blue wavelengths. Therefore, red light provides more energy
photosynthesis, and the plant would grow better under it.
- How do the atmospheric concentrations of CO2 and O2 influence the rate of photorespiration? Explain why this is important.
Photorespiration is a process that occurs when RuBisCO (the enzyme responsible for fixing carbon in the Calvin cycle) mistakenly fixes oxygen (O₂) instead of carbon dioxide (CO₂). The effect of CO₂ and
photorespiration is as follows:
• High CO₂ concentrations reduce photorespiration because RuBisCO is more likely to bind with CO₂, leading to proper carbon fixation in the Calvin cycle.
• High O₂ concentrations increase photorespiration because RuBisCO may bind to O₂ instead of CO₂, leading to the production of a wasteful byproduct and consuming energy without producing sugar.
Why this is important:
• Photorespiration wastes energy and carbon, reducing the efficiency of photosynthesis.
• In environments with low CO₂ or high O₂ (such as hot, dry conditions), plants experience increased photorespiration, which lowers overall productivity. This is one of the reasons why some plants (lik
CAM plants) have evolved mechanisms to minimize photorespiration and increase photosynthetic efficiency.
- Where would you expect to find more C4 plants: in southern
Ontario or northern Ontario? Explain your reasoning.
You would expect to find more C4 plants in southern Ontario. Here’s why:
• C4 plants are adapted to hot and dry environments where photorespiration is a significant problem (because of high temperatures and low CO2 concentrations). In these conditions, the C4 pathway is
efficient than the C3 pathway because it concentrates CO2 in specialized cells (bundle sheath cells), reducing the likelihood of RuBisCO fixing oxygen instead of carbon dioxide.
• Southern Ontario has a warmer climate compared to northern Ontario, with longer growing seasons and higher temperatures, making it a more suitable environment for C4 plants.
• Northern Ontario has cooler temperatures and shorter growing seasons, which makes it less ideal for C4 plants, as their photosynthetic advantage is most beneficial in warmer climates. Therefore, C3
more common in northern Ontario.
- How are the light-dependent reactions of photosynthesis and the Calvin cycle dependent on each other?
The light-dependent reactions and the Calvin cycle are intimately linked and depend on each other:
• Light-dependent reactions (occurring in the thylakoid membranes) use light energy to produce ATP and NADPH, which are essential for the Calvin cycle.
• The Calvin cycle (occurring in the stroma) uses ATP and NADPH produced by the light-dependent reactions to fix carbon and produce glucose.
In other words, the light-dependent reactions provide the energy carriers (ATP) and reducing power (NADPH)necessary for the Calvin cycle to convert CO2 into sugars. Without the ATP and NADPH from reactions, the Calvin cycle cannot function effectively.
- Diuron is a herbicide that is no longer registered for use in Canada. It is a photosystem II inhibitor. What does this mean for a plant?
Diuron is a photosystem II (PSII) inhibitor, which means it blocks the electron transport chain in the light-dependent reactions of photosynthesis. Here’s what happens to a plant treated with Diuron:
• Photosystem II is involved in the first step of the light-dependent reactions, where it absorbs light energy and uses it to split water molecules, releasing oxygen and electrons.
• By inhibiting PSII, Diuron prevents the transfer of electrons, disrupting the production of ATP and NADPH, which are essential for the Calvin cycle.
• As a result, the plant cannot efficiently carry out photosynthesis, leading to stunted growth, reduced energy production, and eventually death if exposure continues.
- A botanist takes a sample of chloroplasts from a plant. The plant has been thriving under normal conditions in the greenhouse. Upon examining the chloroplasts und
microscope, the botanist notices that they contain a very high concentration of ATP but very little NADPH. What are some possible explanations for the high concentrati
ATP?
There are a few possible explanations for the high concentration of ATP but very little NADPH in the chloroplasts:
1 Cyclic Photophosphorylation: The plant might be primarily relying on cyclic photophosphorylation, where only ATP is produced and NADPH is not generated. In cyclic photophosphorylation, elect
cycled back to Photosystem I, creating ATP but not NADPH.
2 Inhibition of NADP+ Reductase: The enzyme responsible for reducing NADP+ to NADPH (NADP+ reductase) could be inhibited or malfunctioning, preventing the production of NADPH even thoug
still produced by photophosphorylation.
3 An Imbalance in the Light Reactions: There could be an imbalance in the light reactions where there is excessive production of ATP (perhaps due to increased light intensity or specific environmenta
without corresponding NADPH production.
- Scientists routinely create knockout organisms to determine the function of a gene. Suppose that a plant has the gene responsible for NADP+ reductase enzyme
“knocked out.” What effect would this have on the cells of the plant, in terms of their ability to perform each of the following processes:
a) Non-cyclic ATP synthesis
b) Cyclic ATP synthesis
c) Calvin cycle
• a) Effect: Non-cyclic ATP synthesis (the normal light-dependent reaction) would still occur, but the plant would not produce NADPH, as the NADP+ reductase is required to reduce NADP+ to
NADPH. The ATP would be produced, but the Calvin cycle could not function efficiently without NADPH.
• b) Effect: Cyclic ATP synthesis would still occur, as it does not rely on NADPH. In cyclic photophosphorylation, electrons are cycled through Photosystem I to produce ATP, and no NADPH is
generated, so this process would still be functional.
• c) Effect: The Calvin cycle would be significantly impaired or unable to function because it relies on NADPH to reduce 3-phosphoglycerate to glyceraldehyde-3-phosphate. Without NADPH, the
Calvin cycle cannot proceed, and the plant cannot synthesize sugars effectively.
- Two fields contain a variety of different weeds: some undergo C3 photosynthesis, others undergo C4 photosynthesis, and the rest undergo CAM photosynthesis.
One field is sprayed with a herbicide that inhibits rubisco. The other field is sprayed with a herbicide that inhibits PEP carboxylase. After one week, what would you
expect to see happening in the two fields?
• Herbicide that inhibits rubisco: This would primarily affect C3 plants, as rubisco is essential for fixing carbon in the C3 pathway. C3 plants would have significantly reduced photosynthetic
activity, leading to poor growth or death. C4 and CAM plants, which use alternative enzymes for carbon fixation (PEP carboxylase), would be less affected.
• Herbicide that inhibits PEP carboxylase: This would primarily affect C4 and CAM plants, as they use PEP carboxylase to initially fix CO2 into a 4-carbon compound. C3 plants, which do not
rely on PEP carboxylase, would not be affected as much. C4 and CAM plants would exhibit reduced photosynthesis and growth.
