11.3 - The Light-independent Reaction Flashcards
What are the products of the light-dependent reaction of photosynthesis. What are these used for
1) ATP
2) reduced NADP
—used—> to reduce glycerine 3-phosphate in the 2nd stage of photosynthesis
Where does the light independent stage take place
stroma of the chloroplasts
What does the light-independent stage rely on
- it requires the products of the lightdependent stage
- however, it will therefore rapidly ceases when light is absent.
Who worked out the details of the light independent reaction
Melvin Calvin + co-workers
—> hence: ‘Calvin cycle’
Describe the stages of the Calvin cycle
1) CO2 from atmosphere diffuses into the leaf through stomata and dissolves in water around the walls of the mesophyll cells. Then diffuses through the CSM —> cytoplasm —> chloroplast membranes —> stroma of the chloroplast.
2) In the stroma: the CO2 reacts with the 5-carbon compound ribulose bisphosphate (RuBP), catalysed by an enzyme called ribulose bisphosphate carboxylase, otherwise known as rubisco.
3) This reaction produces two molecules of the 3-carbon glycerate 3-phosphate (GP).
4) Reduced NADP from the light-dependent reaction is used to reduce glycerate 3-phosphate to triose phosphate (TP) using energy supplied by ATP.
5) The NADP is re-formed and goes back to the light-dependent reaction to be reduced again by accepting more protons.
6) Some triose phosphate molecules are converted to organic substances that the plant requires such as starch, cellulose, lipids, glucose, amino acids, and nucleotides..
7) Most triose phosphate molecules are used to regenerate ribulose bisphosphate using ATP from the light-dependent reaction
How is the chloroplast adapted to carrying out the light-independent reaction of photosynthesis
- The fluid of the stroma contains all the enzymes needed to carry out the light-independent reaction. Stromal fluid is membranebound in the chloroplast which means a chemical environment which has a high concentration of enzymes and substrates can be maintained within it — as distinct from the environment of the cytoplasm.
- The stroma fluid surrounds the grana and so the products of the light-dependent reaction in the grana can readily diffuse into the stroma.
- It contains both DNA and ribosomes so it can quickly and easily manufacture some of the proteins involved in the light-independent reaction.
Describe the role of ribulose bisphosphate (RuBP) in the Calvin cycle.
It accepts / combines with a molecule of CO2 (to produce 2 molecules of glycerate-3-phosphate).
State how the reduced NADP from the light-dependent reaction is used in the lightindependent reaction.
It is used to reduce (donate hydrogen) glycerate-3phosphate to triose phosphate.
Apart from reduced NADP, which other product of the lightdependent reaction is used in the light-independent reaction
ATP
State precisely where in a plant cell the enzymes involved in the Calvin cycle are found.
Stroma of the chloroplasts.
Light is not required for the Calvin cycle to take place. Explain therefore why the Calvin cycle cannot take place for long in the absence of light.
- The Calvin cycle requires ATP and reduced NADP in order to operate.
- Both are the products of the light-dependent reaction, which needs light.
- No light means no ATP or reduced NADP are produced and so the Calvin cycle cannot continue once any ATP or reduced NADP already produced have been used up.
Describe how a limiting factor works
At any given moment, the rate of a physiological process is limited by the factor that is at its least favourable value.
What is the compensation point.
When there will be no net exchange of gases into or out of the plant
Carbon dioxide is present in the atmosphere at a concentration of around….
- 0.04%
- so is often the factorthat limits the rate of photosynthesis under normal conditions.
The optimum concentration of carbon dioxide for a consistently high rate of photosynthesis is…
- 0.1%
- growers of some greenhouse crops, such as tomatoes, enrich the air in the greenhouses with more carbon dioxide to provide higher yields.
- However, CO2 concentration affects enzyme activity, in particular the enzyme that catalyses the combination of ribulose bisphosphate with carbon dioxide in the light-independent reaction.
Provided that other factors are not limiting, the rate of photosynthesis increases in direct proportion to the temperature. Between the temperatures of 0 °C and 25°C the rate of photosynthesis increases by ____ for each __°C rise in temperature.
- 2x
- 10°C
State one measurement that could be taken to determine the rate of photosynthesis in this experiment.
Volume of oxygen produced / CO, absorbed.
Name the factor that is limiting the rate of photosynthesis over the region marked A on the graph. Explain your answer.
Light intensity — because an increase in light intensity produces an increase in photosynthesis over this region of the graph.
In the spring a commercial grower of tomatoes keeps his greenhouses at 25 °C and at a carbon dioxide concentration of 0.04%. The light intensity is 4 kilolux at this time of year. Using the graph, predict whether the tomato plants would grow more if the carbon dioxide level was raised to 0.1% or if the temperature was increased to 35 °C. Explain your answer.
Raising the CO, level to 0.1% — because this increases the rate of photosynthesis more than increasing the temperature to 35 °C.
Explain why there is no advantage in the grower heating his greenhouses on a dull day.
Because light is limiting photosynthesis and so an increase in temperature will not increase the rate of photosynthesis.
Using your knowledge of the light-independent reaction, explain why, at 25 °C, raising the level of carbon dioxide from 0.04% to 0.1% increases the amount of glucose produced.
More CO, is available to combine with RuBP to form more GP, then more triose phosphate and ultimately more glucose
The rate of photosynthesis in an aquatic plant such as Canadian pondweed (Elodea) can be found by…
measuring the volume of oxygen produced by using an apparatus called a photosynthometer
What is a photosynthometer set up with care to avoid
- air bubbles
- makes sure it is air tight
What conditions are necessary
- The water bath is used to maintain a constant temperature throughout the experiment and can be adjusted as necessary.
- Potassium hydrogencarbonate solution is used around the plant to provide a source of carbon dioxide.
- A source of light, whose intensity can be adjusted, is arranged close to the apparatus, which is kept in an otherwise dark room.
- The apparatus is kept in the dark for two hours before the experiment begins.
- The light source is switched on and the plant left for 30 minutes to allow the air spaces in the leaves to fill with oxygen.
- Oxygen released by the plant during photosynthesis collects in the funnel end of the capillary tube above the plant.
After 30 minutes this oxygen is drawn up the capillary tube by gently withdrawing the syringe until its volume can be measured on the scale, which is calibrated in mm - The gas is drawn up into the syringe, which is then depressed again before the process is repeated at the same light intensity four or five times, and the mean volume of oxygen produced per hour
is calculated. - The apparatus is left in the dark for 2 hours before the procedure is repeated with the light source set at a different light intensity.
Explain why the apparatus needs to be airtight
Because any air escaping from or entering the apparatus will respectively decrease or increase the volume of gas measured, which will give an unreliable result.
Explain why the temperature of the water bath needs to be kept constant.
So that any changes in the rate of photosynthesis can be said to be the result of changes in light intensity and not changes in temperature.
Suggest an advantage of providing an additional source of carbon dioxide.
To ensure there is sufficient CO, and so it does not limit the rate of photosynthesis
Suggest a reason for carrying out the experiment in a room that is dark except for the light source.
To prevent other light falling on the plant as this may fluctuate and will affect the light intensity and hence the rate of photosynthesis, leading to an unreliable result.
Suggest why the plant is kept in the dark before the experiment begins.
To prevent photosynthesis and to allow any oxygen produced before the experiment begins, to disperse.
Suggest why measuring the volume of gas produced by the plant in this experiment may not be an accurate measure of photosynthesis.
Because the volume of oxygen produced will be less than that produced by photosynthesis as some of the oxygen will be used up in cellular respiration / dissolved oxygen (and other gases) may be released from, or absorbed by the water.
