The rate at which a plant can photosynthesise depends on factors in the environment that surrounds it. On a warm, sunny afternoon, photosynthesis will be more rapid than on a cool, shady morning.
More oxygen will be produced and more carbon dioxide used. But photosynthesis cannot increase beyond certain limits. The eff ect of light, temperature and carbon dioxide in the air can be measured
experimentally, varying one factor while keeping the others the same, and graphs such as those in Figure 3.25 (overleaf) can be drawn.
An increase in light intensity, when all other variables are unchanging, will produce an increase in the rate of photosynthesis that is directly proportional to the increase in light intensity. However, at a certain light intensity, enzymes will be working at their maximum rate, limited by temperature and the availability of carbon dioxide. At very high light intensities, light absorption (and therefore the rate of photosynthesis) reaches its maximum and cannot increase further. At this point, the graph reaches a plateau (Figure 3.25a).
Increasing temperature also increases the rate of photosynthesis as the frequency and energy of molecular collision increases (Figure 3.25b).
Photosynthesis has an optimum temperature above which the rate will decrease sharply as enzymes are denatured, or the plant wilts and is unable to take in carbon dioxide.
An increase in the concentration of carbon dioxide causes the rate of photosynthesis to increase, as carbon dioxide is a vital raw material for the process. At very high concentrations, the rate will plateau as other factors such as light and temperature limit the rate of reaction (Figure 3.25c).
Figure 3.24 The rate of oxygen production can be used as a direct measure of the rate of photosynthesis.
The eff ects of temperature, light and carbon dioxide concentration are well known to horticulturalists who grow crops in glasshouses.
Commercial producers of cucumbers and tomatoes keep their glasshouses warm and well lit. They may also introduce carbon dioxide to boost photosynthesis to its maximum rate, thereby increasing crop production and profi ts.
Photosynthesis is described in more detail in Chapter 8.
17 What is photolysis?
18 Which colours of the spectrum are used in photosynthesis?
19 Describe two ways in which the rate of photosynthesis can be measured.
Figure 3.25 These graphs show the effects on photosynthesis of varying light intensity, carbon dioxide concentration and temperature.
Light intensity
experiment 3 25°C; 0.4% CO2
experiment 2 25°C; 0.04% CO2 experiment 1 15°C; 0.04% CO2 (0.04% CO2 is about atmospheric concentration)
Rate of photosynthesis
Light intensity
Rate of photosynthesis
0 5 10 15
Temperature (°C)
c The rate of photosynthesis at
different temperatures, carbon dioxide concentrations and light intensities a The rate of photosynthesis at different light
intensities and constant temperature
b The rate of photosynthesis at different temperatures and constant light intensities
Rate of photosynthesis
20 25
low light intensity high light intensity
Light intensity
experiment 3 25°C; 0.4% CO2
experiment 2 25°C; 0.04% CO2 experiment 1 15°C; 0.04% CO2 (0.04% CO2 is about atmospheric concentration)
Rate of photosynthesis
Light intensity
Rate of photosynthesis
0 5 10 15
Temperature (°C)
c The rate of photosynthesis at
different temperatures, carbon dioxide concentrations and light intensities a The rate of photosynthesis at different light
intensities and constant temperature
b The rate of photosynthesis at different temperatures and constant light intensities
Rate of photosynthesis
20 25
low light intensity high light intensity
3 THE CHEMISTRY OF LIFE 65
End-of-chapter questions
1 Which of the following statements is correct?
A The most frequently occurring chemical elements in living organisms are carbon, hydrogen, oxygen and calcium.
B A water molecule can form hydrogen bonds with other water molecules due to its polarity.
C Sweating cools an animal because body heat is required to break the covalent bonds in the water in the sweat, allowing it to evaporate.
D Increasing substrate concentration causes enzymes to denature. (1)
2 Which of the following statements is correct?
A The components of a nucleotide are a sugar molecule attached to two phosphate groups and a base.
B In a molecule of DNA, the bases thymine and uracil are held together by hydrogen bonds.
C During the process of transcription, tRNA molecules bond to mRNA using complementary base pairing.
D During DNA replication, new nucleotides are added using the enzyme
DNA polymerase. (1)
3 Outline how monosaccharides are converted into polysaccharides. (2)
4 State why each step in a biochemical pathway often requires a separate enzyme. (2)
5 The unicellular green alga Phaeodactylum tricornutum is photosynthetic. Cell biologists genetically modifi ed this organism by adding a glucose transporter gene. The modifi ed and unmodifi ed algae were grown in a nutrient medium under a series of diff erent conditions and the growth rate of the cells was measured.
light and no glucose in the medium 12
10 8 6 4 2 0
0 2 4 6 8 0
Time / days
2 4 6 8
unmodified algae
Cell density/107 cells cm–3 modified algae
Key
source: L A Zaslavskaia, et al., adapted (2001) Science, 292, pages 2073–2075
= light and glucose
in the medium
= dark and glucose
in the medium
=
a State the role of glucose in the metabolism of cells. (1) b Deduce where you would expect to fi nd the glucose transporter protein in the modifi ed algae cells. (2) c Compare the eff ect of light on the modifi ed and the unmodifi ed cells. (2)
Commercially, unmodifi ed algae are grown in shallow sunlit ponds or illuminated containers. The cells only grow in the top few centimetres. However, the modifi ed algae can grow at any depth.
d Explain why the modifi ed algae can grow at any depth whereas the unmodifi ed algae can only grow
at the surface. (3)
(total 8 marks)
© IB Organization 2009
6 The rate of carbon dioxide uptake by the green succulent shrub Aeonium goochiae can indicate the amount of photosynthesis taking place in the plant. This rate was measured at 15 °C and 30 °C over a 24-hour period.
The units of carbon dioxide absorption are mg CO2 h−1.
The results are shown below. The centre of the graph corresponds to −2 mg CO2 h−1 and the outer ring is +2.5 mg CO2 h−1.
0600 (morning) 0300(early morning)
0900 (late morning)
0500
0700 0400
0800 0200
1000 0100
1100 2300 2.5
2 1.5 1 0.5 0 –0.5 –1 –1.5 –2
1300 2200
1400 (late evening)2100
(early afternoon)1500 2000
1600 1900
1700
2400 (midnight)
(midday)1200 30°C
15°C
1800 (evening) Key
source: adapted from www.biologie.uni-hamburg.de/d-online/e24/9.htm
a Identify a time that carbon dioxide uptake was the same at both temperatures. (1)
b State the maximum rate of carbon dioxide uptake at 15 °C. (1)
c Compare the rate of carbon dioxide uptake at each temperature in daylight and darkness. (3)
d Suggest why the carbon dioxide uptake may at times be negative. (1)
(total 6 marks)
© IB Organization 2009
4 GENETICS 1 67