Topic 2.9 Photosynthesis

6CO2 + 12H2O + energy from light → C6H12O6 + 6O2 + 6H2O 2.9 (U1) Photosynthesis is the production of carbon compounds in cells using light energy. Photosynthesis involves the conversion of light energy to chemical energy which is stored in glucose or other organic compounds; occurs in plants, algae and certain prokaryotes. 6CO2 + 12H2O + energy from light → C6H12O6 + 6O2 + 6H2O

Energy Conversion During photosynthesis, light energy from the sun is converted into chemical energy (organic molecules).

2.9 (U2) Visible light has a range of wavelengths with violet the shortest wavelength and red the longest. Light from the sun is normally referred to as white light but it is composed of a wide range of wavelengths including red, green, and blue that range from 400 to 700 nanometers. Light is visible to us but the wavelengths are invisible. These are the same wavelengths used by plants in photosynthesis.

2.9 (U3) Chlorophyll absorbs red and blue light most effectively and reflects green light more than other colors. Chlorophyll is a chemical substance (pigment) which can absorb light and is the main photosynthetic pigment. There are several forms of chlorophyll but they all appear green (green light is reflected and pass into the retina of our eyes).

Chlorophyll and light absorption Pigments are substances which absorb light and some pigments can absorb more colors or wavelengths than others. The colors they absorb depend on their structure. A pigment that absorbs all colors appears as black and a pigment that absorbs all colors except blue appears blue due to the fact that this part of the sunlight is not being absorbed. In other words, the remaining colors that are not absorbed are reflected and the color is perceived by the brain of the observer.

2.9 (S1) Drawing an absorption spectrum for chlorophyll and an action spectrum for photosynthesis. Sketch the absorption spectrum for chlorophyll and the action spectrum for photosynthesis in your sketchbook. Use the instructions on page 133 in your Oxford text to complete your drawings.

Absorption Spectrum of Chlorophyll Due to the structure of chlorophyll, it absorbs red and blue light better than green. Green light is reflected which makes chlorophyll, and as a result plants and chloroplasts, appear green.

2.9 (U4) Oxygen is produced in photosynthesis from the photolysis of water. Photolysis means to split using light (disintegration). This process is driven by energy absorbed through chlorophyll. This results in the formation of hydrogen and oxygen. The electrons are passed to a special chlorophyll molecule and the oxygen atom combines with another oxygen atom to form O2 which is released as a waste product through the stoma. H20  4e- + 4H+ + O2

2.9 (A1) Changes to the Earth’s atmosphere, oceans and rock deposition due to photosynthesis. First photosynthetic organisms were prokaryotes that existed 3,500 million years ago. Plants and algae evolved millions of years later and the process of photosynthesis has been increasing the oxygen concentration in the atmosphere ever since. Oxygen concentration rose 2% by volume between 2,400 mya and 2,200 mya and this is known as the Great Oxidation event.

2.9 (A1) Changes to the Earth’s atmosphere, oceans and rock deposition due to photosynthesis. A drop in the greenhouse effect due to a drop in methane and CO2 (greenhouse gases) caused by the Oxidation event lead to the first glaciation of the Earth. The increased oxygen also affected the oceans. The dissolved iron in the water was oxidized and precipitated onto the seabed forming a distinct rock formation known as the banded iron formation. These layers of iron oxide alternate with other minerals forming iron ores. There was a significant rise in the oxygen concentration about 750-635 mya to 20% that corresponds to the timing of the evolution of multicelluar organisms.

2.9 (U5) Energy is needed to produce carbohydrates and other carbon compounds from carbon dioxide. In order for plants to produce carbohydrates, carbon fixation (converting carbon as a gas into a solid) must occur and this requires energy in the form of ATP and hydrogen (from photolysis) are needed. The reaction is endothermic. The energy needed for photolysis comes from light energy (the sun) therefore photosynthesis can only occur in the light. The light energy gets converted into chemical energy (carbohydrates).

2.9 (U6) Temperature, light intensity and carbon dioxide concentration are possible limiting factors on the rate of photosynthesis. A limiting factor can limit the rate of a reaction if it is below the optimal level for the reaction. Photosynthesis can be limited by three limiting factors: temperature, light and carbon dioxide concentration.

Effect of Temperature There is an optimum temperature for photosynthesis. The rate of photosynthesis increases rapidly with an increase in temperature. Once the optimum temperature is exceeded the rate of photosynthesis falls rapidly.

Effect of Light Intensity When the intensity of light ranges from low to medium, the rate of photosynthesis is directly proportional to the intensity of light. Once the intensity of light is high there is a plateau in the rate of photosynthesis.

Effect of Carbon Dioxide Concentration There is no photosynthesis at very low CO2 concentrations. Between low and high concentrations of CO2 the rate of photosynthesis is positively correlated with the rate of CO2. If the rate of CO2 reaches very high concentrations there is a plateau in the rate of photosynthesis.

Topic 2.9 Skills 2.9 (S2) Design of experiments to investigate the effects of limiting factors on photosynthesis. 2.9 (S3) Separation of photosynthetic pigments by chromatograph (practical 4).