2.9 photosynthesis

define photosynthesis

the use of light energy to produce organic molecules from inorganic raw material (CO2 and water)

name the main photosynthetic pigment in plants and the wavelengths it reflects and absorbs

chlorophyll
absorbs: red/blue
reflects: green

what is the light energy absorbed by chlorophyll used for?

to split water, generate ATP

what two materials derived from photolysis of water are used to fix CO2

ATP and hydrogen

where does the plant obtain the C02 and Hydrogen used to build up glucose molecules from?

the carbons is from the C02 absorbed from the atmosphere, the hydrogen that is split during photolysis

state the waste product of photosynthesis

oxygen

state the use of the other product of photosynthesis

substrate for cell respiration

name the two reactions in photosynthesis

light dependent reactions that convert light energy into chemical energy in the form of ATP
light independent reactions use the chemical energy from the ATP to sntheysis carbon compounds (glucose)

outline the differences in absorption of red, blue and green light by chlorophyll

chlorophyll is composed by a numbe rof pigments that absorb different colours of light -> mostly red and blue absorbed -> green mostly reflected

what is the energy transfer that takes place in photosynthesis?

light to chemical

explain the relationship between the action spectrum and the absorption spectrum of photosynthesis pigments

chlorophyll absorbs some wavelengths of light better than the others -> where there is the greatest percent absorption, the rate/efficiency of photosynthesis is best -> red and blue absorbed best, so rate of photosynthesis is highest at these wavelengths -> green mostly reflected so rate of photosynthesis is the lowest at these wavelengths

explain temperature as a limiting factor on photosynthesis

photosynthesis is controlled by enzymes, which is sensitive to temperature fluctuations as temperature increases, reaction rate will increase, as reactants have greater kinetic energy and more collisions result above a certain temperature, the rate of photosynthesis will decrease as essential enzymes begin to denature

explain light intensity as a limiting factor of photosynthesis

light is absorbed by chlorophyll, which convert the radiant energy into ATP -> as light intensity increases, reaction rate increases, as more chlorophyll are being photo-activated -> at a certain light intensity photosynthetic rate will plateau, as all available chlorophyll are saturated with light -> different wavelengths of light will have different effects on the rate of photosynthesis (e.g green light is reflected)

explain carbon dioxide as a limiting factor of photosynthesis

* carbon dioxide is involved in the fixation of carbon atoms to form organic molecules
* as carbon dioxide concentration increases, reaction rate will increase as more organic molecules are being produced
* at a certain concentration of CO2 photosynthetic rate will plateau, as the enzymes responsible for carbon fixation are saturated

Measuring CO2 Uptake (photosynthesis)

* Carbon dioxide uptake can be measured by placing leaf tissue in an enclosed space with water
* Water free of dissolved carbon dioxide can initially be produced by boiling and cooling water
* Carbon dioxide interacts with the water molecules, producing bicarbonate and hydrogen ions, which changes the pH (↑ acidity)
* Increased uptake of CO2 by the plant will lower the concentration in solution and increase the alkalinity (measure with probe)
* Alternatively, carbon dioxide levels may be monitored via a data logger

Measuring O2 Production

* Oxygen production can be measured by submerging a plant in an enclosed water-filled space attached to a sealed gas syringe
* Any oxygen gas produced will bubble out of solution and can be measured by a change in meniscus level on the syringe
* Alternatively, oxygen production could be measured by the time taken for submerged leaf discs to surface
* Oxygen levels can also be measured with a data logger if the appropriate probe is available

Measuring Biomass (Indirect Photosythesis Measurement)

* Glucose production can be indirectly measured by a change in the plant’s biomass (weight)
* This requires the plant tissue to be completely dehydrated prior to weighing to ensure the change in biomass represents organic matter and not water content
* An alternative method for measuring glucose production is to determine the change in starch levels (glucose is stored as starch)
* Starch can be identified via iodine staining (turns starch solution purple) and quantitated using a colorimeter