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What is a limiting factor?
- A limiting factor for a metabolic process is the factor that is present at the lowest or least favourable level that limits the process.
- At any given moment, the rate of a metabolic process is limited by the factor that is present at its least favourable value.
Higher temp causes what change to rubisco?
More oxygen compete for the active site of rubisco, slowing down photosynthesis because CO2 is less readily fixed.
How does lots of light intensity affect levels of GP, RuBP and TP?
- More excitation of electrons, so more photophosphorylation in light-dependent stage.
- More ATP and reduced NADP produced for use in independent stage
- More GP reduced and phosphorylated to TP
- More TP phosphorylated to RuBP
- [So, More RuBP (ATP used to convert from TP) and more TP, but less GP because more GP is being converted to TP by the ATP and reduced NADP]
How does little light intensity affect levels of GP, RuBP and TP?
- No light = light-dependent stage will cease and so will stop light-independent stage too.
- GP cannot be changed to TP (need products of light-dependent stage)
- Levels of TP fall
- Levels of GP accumulates
- Levels of RuBP fall
- Less CO2 fixed, subsequently, less new GP formed.
How does lots of CO2 affect levels of GP, RuBP and TP?
- More CO2 fixation
- More GP (maybe more amino acids or fatty acids)
- More TP (maybe more hexose sugars)
- More regeneration of RuBP
- HOWEVER, open stomata means more water loss (from transpiration) and plant may wilt, if water loss is greater than uptake. Leads to stress response which closes the stomata, reducing CO2 and therefore rate of photosynthesis.
Effect of low CO2 on levels of GP, RuBP and TP? How much is low?
- Below 0.01%
- RuBP (CO2 acceptor) will accumulate.
- Less GP
- Less TP
Effect of high temperature on levels of RuBP, GP and TP?
- Will not have that much of an effect upon rate of light-dependent reaction as it is not dependent upon enzymes (except from photolysis of water).
- Will alter rate of independent stage, because it is a series of biochemical steps, each catalysed by specific enzyme.
- Increased temperature will, at first, increase rate.
- However, above 25oC, oxygenase activity of rubisco increase more than carboxylase activity increases. More photorespiration than photosynthesis.
- So, ATP and reduced NADP are wasted.
- Reduced overall rate of photosynthesis.
- V.high temp also denature enzymes
- Increased temp may lead to more water loss by transpiration, so may lead to closure of stomata and hence reduced photosynthesis.
Effect of low temp on levels of RuBP, GP and TP?
- All reactions in Calvin cycle catalysed by enzymes.
- All enzymes will work slower - reactions slower.
- Level of GP, RuBP and TP fall.
How does light affect rate of photosynthesis? Light causes...
- Stomata to open - CO2 can diffuse in
- Trapped by chlorophyll - excites electrons
- Photolyses water molecules to produce protons.
List 3 ways we can measure the rate of photosynthesis experimentally.
- Volume of O2 produced
- Rate of uptake of CO2
- Rate of increase in dry mass of plants
What are the limitations of measuring volume of oxygen produced per m inute, to measure rate of photosynthesis?
- Some of oxygen produced by photosynthesis will be used by plant for its respiration.
- There may be some dissolved nitrogen in the gas collected.
What 2 experiments can you do to measure rate of photosynthesis?
- Use the apparatus photosynthometer (or Audus microburette) to measure bubble size, for oxygen production.
- Measure change in density of leaf discs, which will reveal amount of O2 produced to make them less dense.
Which plant would you use in the experiment with the photosynthometer?
Describe how to set up the experiment using the photosynthometer?
- Fill apparatus with water. Make sure no air bubbles are present and that it is air-tight.
- Put pondweed in a test tube and put test tube in water bath (20 degrees).
- Add sodium hydrogencarbonate solution, which provides CO2.
- Measure distance of light source from test tube. (try to use low heat bulb)
- Leave apparatus for 5-10mins to acclimatise.
How does this experiment work?
- Gas given off by plant over time collects in flared end of capillary tube.
- Syringe used to move air bubble into part of the capillary tube against the scale.
- Use scale to measure length of airbubble. (if radius known, can also measure volume by length x pi'r'squared).
- Experiment should be repeated at same light intensity and averages used.
- Do this for each distance.
How can light intensity be calculated from the distance of the light source from the plant?
How would you keep the other variables constant?
- Use water bath to keep the temperature constant.
- Do it in a dark room, so that the only light source is from the equipment you have used in experiment.
- Include the same amount of sodium hydrogencarbonate solution in the water so that each plant gets the same amount of CO2.
- The length of plant cut should be equal - and same size and shape. (eg. amount of leaves or part of plant).
Limitations with this experiment?
- Gas collected not pure oxygen (CO2 and nitrogen may be in it too).
- Oxygen collected may not be representation of oxygen produced by photosynthesis. O2 may be used for respiration.
- Oxygen may escape collection. eg. slip through between tube and capillary tube, or it may get caught on leaves.
- Ambient light
- Fluctuations in temp. (Could be improved by using thermostatically-controlled water bath)
- Hard to measure air bubble? (Could be improved by using gas syringe).
How would you use the photosynthometer experiment for testing the 3 factors?
- Light intensity: Vary distance of light source and do 1/d2
- Temp: change temp of water bath. [not wholly accurate as warmer water reduce solubility of oxygen gas]
- CO2: Vary number of drops of sodium hydrogencarbonate solution added.
Describe how the experiment with the lead disc density can be done.
- 1. Cut discs of leaf from cress cotyledons.
- 2. Put 5/6 leaf discs in syringe and half-fill with diulte sodium hydrogencarbonate solution,
- 3. Hold syringe upright, place finger over end, and pull plunger. Air spaces in spongy mesophyll replaced by sodium hydrogencarbonate solution. Density increases, and sink to bottom.
- 4. Transfer contents into small beaker. Illuminate from above.
- 5. Measure how long it takes for one leaf disc to float to the top of solution. Reciprocal of time taken (1/t) is measure of the rate.
- 6. Repeat, and then at different light intensities.