3.8a + 3.8b Photosynthesis

08/01/2013 § 1 Comment

Our new unit, photosynthesis, is a direct link to the previous unit, respiration. As mentioned by one of the videos we watched during the respiration unit, photosynthesis is like the opposite of respiration. This is a very short unit, with five lectures, four blogs, [14 essay questions] and a test next week. Also, happy new year.

Photosynthesis is the process used by plants to make organic substances (food) by using light energy (from the sun) and inorganic substances like carbon dioxide and water. The first organisms to do this were the prokaryotes and that dates back 3,500 million years ago – so we (humans) obviously didn’t even exist yet. Eventually algae and plants also started carrying out photosynthesis and to this day, it reminds one of the oldest miracles of life. After a while, photosynthesis started to create an increasing amount of oxygen concentration in the atmosphere, eventually leading to the creation of the ozone layer. When this layer grew thick enough to protect [future] humans from the harmful UV rays of the sun, that was when oxygen-breathing eukaryotes started to evolve.

A huge part of photosynthesis is the absorption of sunlight. The sunlight we see – called visible sunlight – is a type of electromagnetic radiation. It’s a mixture of all of the different colours (of the rainbow) that we see every day, including blue, green, and red. Chemical substances called pigments are what absorb light. The main pigment used to absorb light in photosynthesis is chlorophyll. Chlorophyll absorbs larger amounts of red and blue light than green light, and instead, the green light is reflected back. This is why plants are green – we see the colors being reflected, so we see that they are green. Pigments that absorb all colours will look black to us, because they reflect no light, no color.

The energy absorbed by chlorophyll is used for

  • producing ATP
  • the photolysis of water, which is splitting water molecules and forms oxygen and hydrogen

Photosynthesis is affected by three main factors, those being: light intensity, carbon dioxide concentration, and temperature.

The effect of light intensity

Basically, the higher the intensity of the light, the more carbon dioxide is produced. However, at high light intensities, the rate of photosynthesis will slow down as light intensity increases and will eventually plateau and can no longer increase.

The effect of carbon dioxide concentration

There is “enough” carbon dioxide in the atmosphere to have an effect on photosynthesis in certain cases. At low concentrations and as the concentration rises, so does the rate of photosynthesis. At a high enough concentration of carbon dioxide, the rate of photosynthesis no longer increases and instead plateaus.


The effect of temperature

Similarly to how temperature affects enzymes, there is an optimum temperature in which organisms can do the highest rate of photosynthesis. At low temperatures, the rate is very small and as temperature rises, rate of photosynthesis also rises rapidly. At a certain point, photosynthesis reaches its peak and eventually decreases even more rapidly than it increased. This makes sense because the enzymes that do photosynthesis have been denatured after a certain point.



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
  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



Page 105, measuring the effect of temperature by data logging

1. Why was pH monitored during this experiment?

pH was monitored because it tracks the presence of protons, or the acidity of the solution. The presence of carbon dioxide lowers the pH so with that, we can track the amount of carbon dioxide produced with the fall (or rise) of pH.

2. Identify the independent and the dependent variable.

The independent variable is the temperature (°C) and the dependent variable is the pH (±0.05).

3. Deduce which variable should go on the x axis and which variable on the y axis if a graph of the results is plotted.

Temperature should be on the x-axis and pH should be on the y-axis. This is actually a bar graph of five bars showing the total amount of change in pH.

4. Discuss what conclusions can be drawn about the optimum temperature for photosynthesis in the green alga Chlorella.

The optimum temperature for photosynthesis in the green alga Chlorella is probably 25°C. This is because among the differences (4 hours – 0 hours), the difference of pH at 25°C is the largest, at a value of 28.

5. If this experiment is repeated using a different alga, the results may not be the same. Suggest reasons for differences in the results.

If a different alga was used for this experiment, the results may be similar but would most likely not be the same. This is because a different alga might have different enzymes and a different structure that will help it do more or less photosynthesis than the original alga.

6. Evaluate the methods used in the experiment and suggest improvements.

The increments between temperatures doesn’t seem to be large enough to show that much of a difference between the change of pH. There should be a larger range in the temperature so that we could see more of a difference in the pH. The experiment could also go on for longer than 4 hours to get more sufficient results.



2. The graph shows the results of a data logging experiment. Chlorella, a type of alga that is often used in photosynthesis experiments, was cultured in water, in a large glass vessel. Light intensity, temperature and the pH of the water were monitored over a three-day period. The changes in pH were due to changes in carbon dioxide concentration.. An increase in CO2 concentration causes a decrease in pH.

a) State the relationship shown in the graph between

i. light intensity and CO2 concentration

when there is a high level of intensity, there is a low concentration of CO2, and when there is a low level of intensity, there is a high level of CO2 (the two are opposite but the relationship between pH and light intensity is the same)

ii. temperature and CO2 concentration

When there is a higher temperature, there is a high pH, which means that there is low concentration of CO2. And vice versa, when there are lower concentrations of CO2, there is a high concentration of CO2.

b) Deduce, from the data in the graph, whether the effect of light intensity or temperature on carbon dioxide concentration is greater.

The effect of light intensity on carbon dioxide concentration is greater.

c) The graph shows both rises and falls in CO2 concentration. Explain the causes of

i. rises in CO2 concentration

In the first half of the day, there is a lot of light and a high temperature. At the higher temperature and with that amount of light, the alga is doing photosynthesis and building up the amount of CO2 in the glass vessel.

ii. falls in CO2 concentration

By the end of the day, when there is less light and cooler temperatures, the CO2 has already built up and accumulated in the glass vessel. This is when the alga doesn’t do photosynthesis and is idle.

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