IB Biology - Curriculum Notes 

2.9 Photosynthesis   - Photosynthesis uses the energy in sunlight to produce the chemical energy needed for life

∑ - Understandings:

∑ - Photosynthesis is the production of carbon compounds in cells using light energy.

Living organisms require complex carbon compounds to carry out life processes and build the structures in their cells
Photosynthesis involves the conversion of light energy into chemical energy (carbohydrates, lipids, protein and nucleic acids).
Chloroplasts absorb light energy from the sun and convert this energy into chemical energy (glucose) to be used by the organisms for energy.

∑ - Visible light has a range of wavelengths with violet the shortest wavelength and red the longest.

Light from the sun is composed of a range of wavelengths.
The visible spectrum is the portion of the electromagnetic spectrum that is visible to or can be detected by the human eye.
Electromagnetic radiation in this range of wavelengths (380 to 750 nm) is called visible light.
All these wavelengths together form white light, with violet/blue colours having shorter wavelengths (more energy) and red colours having longer wavelengths (less energy)


∑ - Chlorophyll absorbs red and blue light most effectively and reflects green light more than other colours.

Sunlight is a mixture of different wavelengths of visible light, which we see as colours.
The two main colours of light that are absorbed by chlorophyll are red and blue light.
The main colour that is reflected in the green light, which is why most leaves look green.

 ∑ - Oxygen is produced in photosynthesis from the photolysis of water.

Photolysis is one of the first and most important steps in the light-dependent reactions of photosynthesis
Two water (H2O) molecules are split by photons of light to produce 4 e- + 4H+ + O2

∑ - Energy is needed to produce carbohydrates and other carbon compounds from carbon dioxide.

Plants convert inorganic CO2 into organic carbohydrates through the process of photosynthesis
Carbon dioxide + Water --> CH2O (carbohydrates) + oxygen
Energy is required for this reaction to occur
Light energy from the sun is used and converted into chemical energy
The reactions are generally endothermic

 ∑ - Temperature, light intensity and carbon dioxide concentration are possible limiting factors on the rate of photosynthesis.

Light intensity, CO2 concentration, and temperature can all be limiting factors for the rate of photosynthesis
If any of these factors is below their optimal level, they can be limiting; however, only one of these factors can be limiting at one time
This is usually the factor that is the furthest away from its optimal level
This is the only factor that can increase the rate of photosynthesis
As this factor gets closer to its optimal level, the limiting factor can change to one of the other factors


At low temperatures, the rate of photosynthesis is very low.
Because photosynthesis requires enzymes, as the temperature increases the amount of kinetic energy in the reactants increases, thereby increasing the rate of photosynthesis.
This rate increases until an optimum temperature is reached. In plants, this optimum temperature is usually between 25º and 35º C.
After the optimum temperature is reached, the rate of photosynthesis drops dramatically, because the temperature can cause the enzymes to denature (lose their shape and active site)

The graph of photosynthesis rate vs. temperature is very similar to the enzyme/temperature graph.

Light Intensity

Light is used to produce ATP and split water by photolysis to form H+ ions and oxygen.
As light intensity increases the rate of photosynthesis also increases.
At low light intensities, an increase in light causes a drastic increase in the rate of photosynthesis.
As light intensity increases the rate of photosynthesis begins to level off and becomes constant.
As light intensity increases further there is no change in the rate of photosynthesis as enzymes are working at their maximum rate.


Carbon Dioxide Concentration

CO2 is an essential molecule in the formation of organic molecules.
At low CO2 concentrations, an increase in the amount of CO2 will increase the rate of photosynthesis. At very low levels, no photosynthesis will take place
As the CO2 concentration increases, the rate of photosynthesis begins to plateau.
At high levels of CO2 concentration, the rate of photosynthesis remains constant unless light intensity is increased to create more ATP or temperature is increased to provide more kinetic energy.


*** Do the data-based questions on page 134***

Applications and skills:

∑ - Changes to the Earth’s atmosphere, oceans and rock deposition due to photosynthesis.

Early bacterial life introduced oxygen to the atmosphere about 3.5 billion years ago.
As the first free oxygen was released through photosynthesis by cyanobacteria, it was initially soaked up by iron dissolved in the oceans and formed red coloured iron oxide, which settled to the ocean floor.
Over time, distinctive sedimentary rocks called banded iron formations were created by these iron oxide deposits. Once the iron in the oceans was used up, the iron oxide stopped being deposited and oxygen was able to start building up in the atmosphere about 2.4 billion years ago.
This was known as the “Great Oxidation Event”
The oxygen remained at about 2% until about 700 mya. Then there was then a significant rise in oxygen until it reached about 20%.
This lead to a huge increase in species as multicellular organisms evolved

B - Skill: Drawing an absorption spectrum for chlorophyll and an action spectrum for photosynthesis.

The electromagnetic spectrum consists of the entire range of electromagnetic radiation.
The part of the spectrum that is involved in photosynthesis is called the visible light spectrum.
An action spectrum is the rate of photosynthesis plotted against wavelength of light. It shows which wavelength of light is most effectively used during photosynthesis.
The highest rates of photosynthesis occur at red and blue wavelengths.
The absorption spectrum shows the % of light absorbed by the photosynthetic pigments in chloroplasts at each different wavelength.
The graphs are very similar because photosynthesis occurs when light is absorbed by the chlorophyll pigments; therefore the wavelengths that have the greatest rates of absorption will also have high rates of photosynthesis.
Green wavelength of light is reflected and therefore has a very low % absorption level on the absorption spectra (this is why most leaves are green). 

B  Skill: Design of experiments to investigate the effect of limiting factors on photosynthesis. -

B  Skill: Separation of photosynthetic pigments by chromatography. (Practical 4)

These would be two separate practicals. You could use the first investigation to carry out a full lab or a Mock IA in preparation for the internal assessment.

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