Photosynthesis Flashcards
What is photosynthesis?
The conversion of light energy to chemical energy (carbohydrates) in plants.
6CO2 + 6H2O – Light –> C6H12O6 + 6O2
Where does photosynthesis occur?
In the chloroplasts
Describe the Light reaction stage of photosynthesis
occurs in the thylakoid membranes
light energy is captured by pigments and energy is transferred to a “reaction centre”
This flow generates NADPH
light energy also generates a proton gradient across the thylakoid membrane –generates ATP
ATP and NADPH needed to drive the synthesis of carbohydrates in stage 2, the carbon reaction.
Describe the Carbon reaction stage of photosynthesis.
occurs in the stroma of the chloroplasts
CO2 is converted to carbohydrates using the ATP and NADPH generated in the light reactions
What are the main pigments in plants and what wavelengths do they absorb?
Chlorophyll A (red - violet and orange light or 642 nm)
Chlorophyll B (blue and yellow or 372nm)
What range of light energy can be absorbed by plants?
approx 400-700nm
What does the antennae complex do?
Collects light and funnels the energy to a reaction centre
What is the combination of antennae complex and a reaction centre called?
Photosystem
Plants have 2 photosystems (PS1 and PS2), what is the main difference?
PS1 absorbs far-red light (P700)
PS2 absorbs red light (P680)
What else occurs at Photosystem 2?
Water is oxidised to oxygen
What are the 4 electron carriers from PS2 to PS1?
Pheophytin
Plastoquinone
Cytochrome
Plastocyanin
What excites P700 to a higher energy state?
Direct absorption of far red light
Energy transfer from antennae complex
Electron transfer from PS2.
What is photophosphorylation?
ATP is generated by an enzyme known as ATP synthase.
What-is photoprotection in plants?
The protection of cells to harsh sunlight, this can include layers over them that act as a sunscreen, or the formation of detoxifying enzymes.
What are the 3 stages in the carbon reaction of photosynthesis?
- Carboxylation - forms 3-PGA from RuBP (catalysed by rubisco)
- Reduction - forms G3P from previous product
- Regeneration - forms RuBP from previous product
What is photorespiration?
Occurs when there is more O2 available (from photosynthesis) to use over CO2, as Rubisco cannot identify between both.
This is a problem in hot, dry conditions as that forces the stomates to close to reduce water loss, which in turn, decreases the amount of oxygen available over that time.
Is wasteful, results in energy loss
How do plants combat photorespiration? What are the three categories of plants according to this defense?
C3 plants are that vast majority of plants, these plants photorespire without any combat as they experience cooler temperatures. They do however, have a metabolic pathway to partially recover the carbon lost.
C4 plants can combat to photorespire less, as they can push the CO2 into the rubisco environment and have a separate site of O2 production.
CAM plants are those that generally experience very hot and dry conditions, they do not respire at all. They have a similar system to C4, although they are better at concentrating CO2 in the rubisco environment by storing in the vacuole when not using. The plant’s stomata also remain closed during the day and open at night to reduce water loss.
What is the function of spongy mesophyll cells found below the palisade?
Their large pore spaces surrounding creates increased light scattering properties, which increases probability of absorption.
How do chloroplast move in low vs high light conditions and why?
In low light they move to cell surfaces parallel to the leaf plane to increase absorption.
In high light conditions they move to surfaces perpendicular to leaf plane, this decreases absorption by 15% and protects cells.
What are the ways in which manipulating photosynthesis could help increase yields?
- Manipulating rubisco could increase catalytic rate or reduce wasteful oxygenase activites
- Manipulating a more efficient pathway to recover the lost energy in oxygenation
- Engineering CO2 concentrating mechanisms (such as those found in CAM) into C3 plants?
- Broadening the wavelengths of usable light (such as some photosynthetic bacteria that can absorb up tp 1100nm
