Photosynthesis 2 (MJC)
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The diagram shows the changes in pH of two structures X and Y of a chloroplast in the presence and absence of light.
Identify structures X and Y and account for the graph.
-Structure X is the stroma
while structure Y is the thylakoid space
- -In the presence of light, photoactivation would have occurred to result in chemiosmosis to occur at a higher rate due to there being both cyclic and non-cyclic photophosphorylation.
- -Protons are thus pumped from the stroma into the thylakoid space, resulting in the pH of X to increase while the pH of Y decreases.
Identify structures A,B and C and state the function of A.
Structure A- Granum
and it provides a large surface area for the attachment
of various proteins such as electro transport chain to allow for the maximum absorption of light.
- Structure B - Starch grain
- Structure C - Stroma
State the similarities between cyclic and non-cyclic photophosphorylation.
- -Both involves the release of electrons from the primary pigments (special chlorophyll a)
- -Energy released from redox reactions to synthesize ATP through chemiosomosis in both cyclic and non-cyclic.
- -A H+ concentration gradient is created across the thylakoid membrane in both photophosphorylation during chemiosmosis.
- - ATP generated in both photophosphorylation is passed to the light-independent reaction.
Explain how water can be a limiting factor on the rate of phosynthesis.
- -Low amounts of water can be a limiting factor due to the closure of the stomata which limits carbon dioxide uptake for photosynthesis.
- -Decreased carbon dioxide uptake results in a decrease in the rate of Calvin cycle, hence lowering the rate of photosynthesis.
What is the reducing power of NADPH used for in the chloroplast?
-It is used for the reduction of glycerate-3-phosphate into glyceraldehyde-3-phosphate.
Explain how light energy is converted into the chemical energy of ATP.
- - Accessory pigments absorb light energy and transfer it form one pigment to another until it is transferred to the special chlorophyll a molecule in the reaction centre of a photosystem.
- -This results in an electron in the chlorophyll a to be excited to higher level and is hence emitted, which then captured by a primary electron acceptor and then tranferred downhill through an electron transport chain.
- -The energy released through the redox reactions in the ETC is used for the process of chemiosmosis where protons are pumped across the thylakoid membrane from the stroma into the thylakoid space. This sets up a proton concentration gradient and protons will diffuse through the ATP synthease which synthesises ATP.
Describe how light plays a role in photophosphorylation.
- - Light is a source of energy needed to excite electrons in the photosystems to a higher level so that ATP and NADPH can be synthesized for the Calvin cycle.
- - Light is also needed for the photolysis of water to replace electrons lost from the PSII during non-cyclic photophosphorylation.
What is the role of NADP in photosynthesis.
- -NADP is a coenzyme / electron carrier
- -that serves as an electron and proton acceptor in the electron transport system from Photosystem I during photophosphorylation (light dependent stage) as electrons released from the `excited' chlorophyll in PSI are passed down the electron transport chain to NADP.
- -Reduced NADP carries the electron and proton from the light dependent stage and pass them
- to the Calvin cycle
- -For the reduction of Glycerate-3-phosphate to form glyceraldehyde-3-phosphate / triose phosphate.
- -This leads to the regeneration of NADP which then returns to the light dependent stage to be reduced again
Explain the region X and Y on the graph.
In region X, light intensity is a limiting factor hence when light intensity increases, rate of photosynthesis increases linearly with it.
At region Y, light is no longer a limiting factor and other factors such as carbon dioxide concentration becomes the limiting factor.
Explain carbon reduction in the Calvin cycle.
- -Glycerate-3-phosphate in the presence of ATP and NADPH from the light-dependent reaction is reduced to glyceraldehyde-3-phosphate
- -NADPH provides the reducing power while ATP provides energy
- -NADP and ADP is then recycled back into the thylakoid for the light dependent reaction.
Define the term chemiosmosis and its function.
- -Chemiosmosis is the process where the energy that is released from the passage of electrons down the chain of electron carriers of progressively lower energy levels is
- used for active transport of hydrogen ions from the stroma into thylakoid space via proton pumps.
- -This creates proton/H+ concentration gradient across the membrane.
- -H+ ions then diffuse back into the stroma, via ATP synthase which harnesses the proton-motive force to
- generate ATP.
- -This ATP generated is then passed to the light-independent reaction (Calvin cycle).
Describe the structure of the stroma and how these structures are related to it functions.
- - Presence of circular DNA that codes for electron carriers and enzymes needed for light dependent reaction
- - Presence of 70S ribosomes to carry out protein synthesis independently of the cell
- - Gel-like matrix which is the site of light-independent reaction and is associated with the thylakoid membranes so that the products of the light-dependent reaction can be channeled to the Calvin cycle.
Define the term photosynthesis.
Photosynthesis is a process by which green plants synthesize carbohydrates such as hexose sugars from inorganic raw materials (such as water, carbon dioxide and mineral salts) in the presence of chlorophyll and sunlight as an energy source. Oxygen is released in the process as a by-product.
Define the term limiting factor and explain how light colour limits photosynthesis.
- -Limiting factor is the one factor of many affecting a process that is nearest to its lowest value and hence is
- rate limiting.
- -Red or Blue Violet light will increase the rate of photosynthesis.
- -Green light will decrease rate of photosynthesis.
- -This is because the absorption spectrum of chlorophyll is the highest at the Red and Blue wavelengths and lowest at the green wavelength of light affecting the light dependent reactions.
Describe the structure of ATP synthease found on the thylakoid membrane.
- -Consist of F1 ATPase Head portion facing the stroma
- and a F0 transmembrane hydrophilic H+ channel embedded in thylakoid.
- -F0 Transmembrane hydrophilic channel embedded within the thylakoid membrane for the facilitated diffusion of H+ down its concentration gradient from thylakoid space to stroma.
- -The head portion rotates as hydrogen ions passes through and it uses the electrical potential energy released from electrons released to synthesise ATP.
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