8.2c Photosynthesis; Limiting factors

15/01/2013 § 1 Comment

This next, and final blog, will focus on the limiting factors on the rate of photosynthesis, among other things. Say hello to the last biology blog post of the semester! (Only… three more semesters to go.)

Action spectra and absorption spectra

Well, basically an action spectra is a visual representation (a.k.a. a graph) that shows the rate of photosynthesis at each wavelength of light. In contrast, an absorption spectra shows the percentage of light absorbed at each wavelength by a pigment or a group of pigments. The difference is that photosynthesis only occurs where chlorophyll (or other photosynthetic pigments) can absorb light.

Quantum of light, or photons is the unit of light energy. The energy carried by photons is what excites electrons, which we now know is called photoactivation, and raises the electron to a higher level of energy. Once it is at this state, only then can it be absorbed (and pass through the transport chain). Different chlorophyll absorb in the red and blue parts of the spectrum but have slightly different properties. Other pigments, called accessory pigments absorb the rest of the wavelengths and transfer that energy to chlorophyll.

The concept of limiting factors

The three factors that affect the rate of photosynthesis are light intensity, carbon dioxide concentration, and temperature. We know this. We know how they affect the rate of photosynthesis. We know that they all have optimal levels of photosynthesis. But we didn’t know that if one of these factors were at a point lower than its optimal point of photosynthesis, it would then hinder the rate of photosynthesis and limit the organism. The factor that does this and is at the minimum point is therefore the limiting factor. Photosynthesis is determined by the rate of the current reaction taking the most time. That is the rate-limiting step, and all of the three limiting factors affect different rate-limiting steps.

  • light intensity: usually not the limiting factor because intensity is determined by the time of day (day/night), low intensities is when it could be the limiting factor, high intensities is when other factors are limiting
  • CO2 concentration: high concentrations are when other factors are limiting – because CO2 concentration in our atmosphere is low, CO2 is usually the limiting factor
  • temperature: low temperatures won’t let the enzymes work as fast as they can, intermediate temperatures are when other factors are limiting, high temperatures are pushing it – some enzymes denature

Essay Questions

  1. Outline the effect of temperature, light intensity and carbon dioxide concentration on the rate of photosynthesis. 6 marks
  2. Explain the effect of light intensity and temperature on the rate of photosynthesis. 8 marks
  3. Explain how the rate of photosynthesis can be measured. 7 marks
  4. Explain the role of water in photosynthesis. 4 marks
  5. Outline the light-dependent reactions of photosynthesis. 6 marks
  6. Explain photophosphorylation in terms of chemiosmosis. 8 marks
  7. Explain the reactions involving the use of light energy that occur in the thylakoids of the chloroplast. 8 marks
  8. Outline the light-independent reactions of photosynthesis. 8 marks
  9. Explain why the light-independent reactions of photosynthesis can only continue for a short time in darkness. 6 marks (I need to ask about this one, we learned it but I need to go over it again.)
  10. Explain how the light-independent reactions of photosynthesis rely on light-dependent reactions. 8 marks
  11. Outline the formation of carbohydrate molecules in photosynthesis starting from the absorption of light energy. 6 marks
  12. Compare the structure of a chloroplast and a mitochondrion in relation to function. 8 marks
  13. Draw a labelled diagram of the structure of a chloroplast as seen with an electron microscope. 4 marks
  14. Photosynthesis and transpiration occur in leaves. Explain how temperature affects these processes. 8 marks



(disguised as Rediscovering biology Question)

1. Compare the absorption of the two samples of pigments shown in the graph, including both the similarities and the differences.

Both pigments seem to be following a similar pattern in that both absorb a lot of blue light and a lot of red light – at least, more than the accessory pigments. Both pigments produce more blue light than red light. The red curve leaf also produces more green light than the purple curve leaf – around 20% more.

2. Deduce, with reasons, which curve shows the absorption of the pigments from the Fagus leaf and which shows the absorption of the pigments from the Acer leaf.

Since the Japanese maple leaf contained a red pigment, it is represented by the purple curve, which as a higher absorption percentage than the red curve at 700nm, which is the longest wavelength.

3 Suggest reasons for plants using pigments that absorb light in the range 400 – 700nm and not higher or lower wavelengths.

The light in the range of 400-700nm is the light we can see – visible light. Any light with a higher nm is too weak for the pigments to catch and any light with lower than 400 nm (the stronger wavelengths) pass right through the pigments because they are too strong.

4. Some algae growing on rocky beaches have a brown colour. Predict, with reasons, the curve that would be obtained if light absorption of their pigments were investigated.

This curve would start low and curve downwards towards the 600-700nm because that is the section of yellow, orange, and red colours. Since the rocks are brown (and I don’t know my colours very well but I’m pretty sure yellow, orange and red can make brown), it means that that is the colour reflected and isn’t absorbed. The higher percentage of light absorbed would stay below 600nm.


Chapter 9 Questions

 Now showing at « re: still faster than you » !


  • a) State the energy conversion that occurs during photosynthesis. The conversion of light energy to chemical energy.
  • b) Explain the property of light that is not normally visible, but can be seen in a rainbow. When light hits water droplets and reflect on the water, all of the pigments are reflected, which creates a rainbow. These are composed of different wavelengths of light, so white light is composed of all colours.
  • c) State which pigment is produced in the largest quantities by chloroplasts. Chlorophyll is produced in largest quantities and reflects green light.


  • a) State how plants produce and use hydrogen in photosynthesis. Plants do photolysis and split water molecules, creating O2 (oxygen) and hydrogen as byproducts. The oxygen becomes a waste product of photosynthesis and the protons in the hydrogen molecules contributes to creating a chemiosmotic gradient between the stroma and the thylakoid interior. More protons are pumped into the thylakoid interior from the stroma as charged electrons pass through the chain of electron carriers. The chemiosmotic gradient this creates helps ATP synthase photophosphorylate ADP into ATP.
  • b) Outline the reasons for the green coloration of the leaves of plants. Most, if not all, plants have chlorophyll in their chloroplasts, the pigment that absorbs many colours except for green. Instead, green is reflected, which is why our eyes tell us that leaves are green – it is what we see.
  • c) Explain three methods for measuring the rate of photosynthesis. One: pH can measure photosynthesis because it tracks the amount of protons (hydrogen) in the pigments. pH affects carbon dioxide so the change of pH can track the amount of carbon dioxide used. Two: biomass measures the plant’s physical growth, which is what photosynthesis is for! Three: Oxygen is a waste product of photosynthesis, the amount of oxygen measured can determine how much photosynthesis is being done.

~ Insert HL ribbon here ~

3. Figure 34 shows the effects of varying light intensity on the carbon dioxide absorption by leaves, at different, fixed carbon dioxide concentrations and temperatures.

a) Deduce the limiting factor for photosynthesis at:

  • i) W – light intensity
  • ii) X – CO2 concentration
  • iii) Y – temperature
  • iv) Z – light intensity

b) Explain why curves I and II are the same between 1 and 7 units of light intensity. Curves I and II are the same between 1 – 7 units of light intensity because the carbon dioxide concentration is the same for both samples, it is the limiting factor, and the only slight difference is the change in temperature.

c) Explain the negative values for carbon dioxide absorption when the leaves were in low light intensities. At a certain environment, or at certain points in all the three factors, photosynthesis would be very inefficient (obviously, because CO2 concentration, temperature, and light intensity are all not at their optimal level) especially because of the low light intensity. Also, instead of absorbing CO2 as part of photosynthesis, it would simply accumulate inside the chloroplasts.


  • a) Draw the structure of a leaf cell that can carry out photosynthesis at a rapid rate. I drew this during class therefore all the information in the below image is CORRECT. 
Retrieved via el iPhone de mi papá

Retrieved via el iPhone de mi papá

  • b) Outline the features of chloroplasts that allow them to be recognized in electron micrographs. Chloroplasts have an inner and outer membrane surrounding its stroma. It has ribosomes where proteins are made (haha!) and circular DNA (ahhh) that instructs the chloroplast what to do. There are also fat droplets and starch grain (which are large and irregularly shaped). The most important part though, are the structures of thylakoid and stacks of granum (or grana) where much of photosynthesis occurs. These looks like stacks of coins connected by a long rod.
  • c) Distinguish between the light-dependent and light-independent reactions of photosynthesis. Light dependent reactions include the reactions that happen in photosystems I and II and all depend on the energy absorbed by light. This light excites electrons, bringing them to a new level, and allows them to pass through the chain of electron carriers. Some products of light-dependent reactions include ATP and NADPH. Light independent reactions are quite different in that they don’t directly require the energy from the light absorbed by the chloroplast. Light independent reactions refer mostly to the Calvin cycle and the series of chemical reactions that produce sugar, using the ATP and NADPH produced from the light dependent reactions. (In that sense, light independent reactions are more like “indirectly light dependent reactions”.)

5. Water with mineral nutrients dissolved in it was sterilized and then placed in a 2dm3 fermenter. The temperature was kept at 25°C. The fermenter was kept in natural sunlight, but a lamp was also used to increase the light intensity. The lamp was controlled by an electronic timer, which switched it off at night. A light meter was placed against the side of the fermenter, near the base, to measure the intensity of light passing through the liquid in the fermenter. The maximum reading it could give was 1200 luxe. At the start of the experiment, a small quantity of Chlorella, a type of alga, was added to the fluid in the fermenter. Figure 35 shows the light intensity measured over the 45 days of the experiment.

a) The light intensity followed a similar pattern, every day from Day 12 onwards.

  • i) Outline the daily changes in light intensity over a typical day after Day 12. After the twelfth day, the light intensity starts to decrease in a downwards curve. In the first few days, intensity decreases by about 150 to 200 luxe, but later the amount decrease narrows down to about 100 and then less than 100. The curve starts to plateau after that point.
  • ii) Explain these daily changes in light intensity. After day 12, the bars are pointy, with a high peak, and a slightly lower peak. This is from when the light automatically turns off via the electronic timer. 

b) Each day there is a maximum light intensity. Outline the trends in maximum light intensity.

  • i) from Day 1 to Day 12 – The intensity remains high all throughout the twelve days.
  • ii) from Day 13 to Day 38 – Light intensity decreases exponentially up until day 38. (Decreases start as very large changes, and then the changes shrink).
  • iii) from Day 39 to Day 45 – Light intensity starts to plateau and remain at a constant level.

c) Explain why the light intensity when the light was switched on was lower at the end of the experiment than at the start. When the algae was added, more light would be absorbed. Initially, the light passes through the liquid in the fermenter but once the algae was there to absorb the water, it wouldn’t seem like the light was that intense to the light meter placed on the side of the fermenter as the light is absorbed by chlorophyll. 

d) Suggest reasons for the trend in maximum daily light intensity between Day 39 and Day 45. At this point in the experiment, all of the substances are used up and the nutrients that the alga had previously used at the start of the experiment are gone – what is left is probably the product of photosynthesis. In this sense, the light intensity will no longer change or decrease further because the alga and its rate of photosynthesis will remain unchanged.

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§ One Response to 8.2c Photosynthesis; Limiting factors

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