3.8.1 Photosynthesis converting light energy to chemical energy.
3.8.2 Sunlight and electromagnetic radiation.
3.8.3 Chlorophyll
3.8.4 Absorption of light by chlorophyll
3.8.5 Light energy and the synthesis of ATP, photolysis, oxygen and hydrogen production.
3.8.6 Synthesis of organic molecules.
3.8.7 Measuring the rate of photosynthesis.
3.8.8 Factors affecting the rate of reaction.
Location: chloroplast or prokaryotic equivalent.
Reaction: Traps light energy (photons) and converts it into chemical energy.
Organisms: Prokaryotic and Eukaryotic
Substrate: Inorganic CO2 and H2O
Products: Organic compounds (sugars) and O2
Environments: Aquatic environments with light, terrestrial environments with light. There are even extremophiles that can photosynthesis at some extreme latitudes and altitudes. At extreme high temperatures we see photosynthesis in geothermal active regions.
Light form the sun is composed of a range of wavelengths (colours).
The visible spectrum to the left illustrates the wavelengths and associated colour of light.
Combined together these wavelengths give the 'white' light we associate with full sunlight.
The shortest wavelengths are the 'blues' which have more energy.
The longer wavelengths are the 'reds' which have less energy.
Chlorophyll is the main photosynthetic pigment. This is where light energy is trapped and turned into chemical energy.
The head of the molecule is polar and composed of a ring structure. At the heart of this ring structure is the inorganic ion magnesium. This is the light trapping region of the chlorophyll molecule.
The tail of the molecule is non polar and embeds itself in membranes in the chloroplast.
There are other pigments, reds, yellows and browns but these are only usually seen in the experimental chromatography or if you have been lucky enough to witness the autumnal colours of deciduous trees in a temperate climate.
The details of this image are not important and need not be learnt for the SL course.
The 'peaks' show which wavelength of light are being absorbed.
Look at the x-axis for colours of light absorbed at the 'peaks'.
The main colour of light absorbed by chlorophyll is red and blue.
The main colour reflected (not absorbed) is green.
Hence why so many plants are seen as green, the light is reflected from the chlorophyll to your eye.
(a) Light is absorbed by chlorophyll molecules (green) on membranes inside the chloroplast.
This is the light trapping stage in which photons of light are absorbed by the chlorophyll and turned into chemical energy (electrons).
(b) The chemical energy (electrons) is trapped in making ATP.
Photolysis(c):
Water used in photosynthesis is split which provides:
hydrogen for the formation of organic molecules. (C6H12O6)
oxygen gas is given off.
H+ from the splitting of water are combined with carbon dioxide to form organic compounds like sugar.
Bonds are formed between the carbon, hydrogen and oxygen using the energy from ATP (which came form the sun).
C, H, O are enough to form lipids and carbohydrates.
With a Nitrogen source amino acids and therefore proteins can be made.
Plants have this remarkable ability to manufactory all their own organic molecules and by definition all the basic organic molecules required by all life forms.
Processes like photosynthesis and respiration can be measured by either:
Depletion of substrate.
Accumulation of products
Investigation: Photosynthesis: Carbon dioxide + water ----> Organic molecule + Oxygen
The rate of photosynthesis can therefore be measured by:
Depletion of substrate which includes measuring how much carbon dioxide has been used or how much water is used.
Accumulation of product which might include measuring how much oxygen is produced or organic molecules (biomass) produced.
In this simple experiment the accumulation of oxygen is measure of rate of reaction.
Independent variable: Light Intensity or wavelength of light.
Dependent variable O2 vol against time
Method the collection of gas over water.
Specimen: Pond weed Elodea
The above set up represents a typical school laboratory experiment. Perhaps on a preparatory course for IB Biology you carried out this experiment. It is normal to count the bubbles per minute but it is possible to be more rigorous than this in determining and quantifying your dependent values. Spend some time revising the diagram, make modifications to improve the collection of valid and reliable data.
The effect of temperature on the rate of photosynthesis:
Photosynthesis is a biological reaction and like all other such reactions there are steps that require the presence of enzymes.
Temperature as we have already met is a change in the average kinetic energy of the particle.
The graph the left should look familiar as this is the same one covered in the section on the effect of temperature on the rate of an enzyme catalysed reaction.
(a) Increasing rate of photosynthesis as the kinetic energy of reactants increases.
(b) Maximum rate of reaction of photosynthesis at the 'optimal' temperature.
(c) Decrease in rate of photosynthesis as the enzymes become unstable and denature.
The effect of carbon dioxide concentration on the rate of photosynthesis:
Carbon dioxide is one of the reactants of the reaction so this graph is very much like the effect of substrate on the rate of reaction.
(a) O2 is used up as the plant is not photosynthesising but only respiring.
(b) As the concentration of the carbon dioxide (substrate) increases the rate of reaction increases.
(c) The atmospheric levels of carbon dioxide and the associate rate photosynthesis.
(d) Maximum rate of photosynthesis (see section e).
(e) The is a range of values for different plants reaching their saturation level with carbon dioxide. One the saturation level has been reached there is no further increase in the rate of photosynthesis.
The effect of light intensity on the rate of reaction.
Light energy absorbed by chlorophyll is converted to ATP and H+ see section 3.8.5.
At very low light levels (a) the plant will be respiring only not photosynthesising.
As the light intensity increases then the rate of photosynthesis increases.
At high light intensities the rate becomes constant, even with further increases in light intensity there are no increases in the rate.
The plant is unable to harvest the light at these high intensities and indeed the chlorophyll system can be damaged by very intense light levels.