Photosynthesis is the process within green plants and certain other organisms by which carbohydrates are synthesized from carbon dioxide (CO2) and water (H2O) using light as an energy source in the presence of chlorophyll and where oxygen (O2) is released as a byproduct. The net result of the process is light energy is converted into chemical energy in the form of carbohydrates.

Why is Chlorophyll Green? Chlorophyll is a pigment that absorbs certain wavelengths of light. Its color represents the colors of light that it reflects, not absorbs. In fact green light provides the least energy for the process of photosynthesis, making red and blue wavelengths the source of most of the energy plants require for photosynthesis.

The graph below shows the various wavelengths of light that are absorbed by the different pigments. The spikes at red and blue are because of the chlorophyll molecules. So Chlorophyll is green because it absorbs more light from the Red and blue sectors than from the green sector.

The lower spike at green is because of a group of pigments called carotenoids. These pigments many of which are forms of Vitamin A are responsible for the overall health of the plant in repelling disease accumulating toxins for later excretion. Notice most of the activity is in the higher energy ultra violet sector rather than the lower energy yellow and green sector. Also note that photosynthesis stops dramatically after the 700nm. This is known as the “Red Drop.”

<<Photosynthesis Photo 8.2>>

Note the one magnesium molecule is the difference between the blood of animals which replaces the magnesium with an iron molecule and the chlorophyll in plants.

Reaction Centers

The various coloured pigments absorb different light wave lengths. These various pigments are collectively known as antenna pigment molecules. Their position in the leaf is to collect the different wave lengths that are associated with different energy levels. They collect the light energy and channel the collected energy to the reaction center molecule.

A unit of several hundred antenna pigment molecules plus a reaction center is known as a photosynthetic unit.

There is 1 reactor molecule to about 250 chlorophyll molecules that actually turn units the collected light energy into units of chemical energy.

Think of it like building a house with all the workers bringing the bricks from all over the block to the work face so that the Head Brickie can start work. They refuse to work at night as they need sunlight to see what they are doing. Soon after the workers go home the Head Brickie runs out of bricks and he too must stop work. Turning to energy 1 solar panel will not run the lights at the MCG but if we had a thousand solar panels and all the energy was collected and sent to the MCG lights we would have enough to run the lights this is what is happening inside the leaf.

The large number of antenna pigment molecules in each photosynthetic unit enables its reaction center to be constantly supplied with high energy electrons. The reaction center is the only molecule in the photosynthetic unit that can pass this energy on to a series of enzymes for photophosphorylation or the production of high energy ATP using the energy from the sunlight.

The simple Chemical Equation for Photosynthesis is

6CO2 + 6H2O C6H12O6 + 6O2

While the equation above looks like a simple one step reaction, there are actually quite a few steps between the reactants and products. The complex reaction sequence of photosynthesis can be broken down into two reaction systems the light reaction and dark reaction. One relies directly on sunlight the light reaction while the dark reaction relies on the sunlight indirectly.

The Light Reactions: H2O O2 + ATP + NADPH

* Inside the thylakoid (Chapter 1) membranes of the granum (Chapter 1), water is split to produce negative oxygen electrons (e−), and positive hydrogen ions (H+).

Note the light reactions occur in the chloroplast while the dark reactions occur in the Mitochondria.

* This system depends on sunlight for its catalytic energy to activate the chemical equation.

* Light is absorbed by “chlorophyll a” which “excites” the electrons in the chlorophyll molecule.

* Electrons are passed through a series of carriers to produce adenosine triphosphate, ATP (high energy) and Nicotinamide adenine dinucleotide phosphate, NADPH.

The Dark Reactions: ATP + NADPH + CO2  C6H12O6

* In the stroma, ATP from the light reaction is used to split carbon dioxide, providing carbon to make sugars.

* Ultimately; glucose, C6H12O6 a stable, transportable, soluble and storable form of chemical energy, is produced.

* The dark reactions only continue as long as the light reactions can supply energy in the form of high energy ATP and NADPH, thus the dark reactions cease once sunlight is unavailable.

Factors Determining the Rate of Photosynthesis

1. Light Intensity

* Light-limited – At low light intensities photosynthesis is deprived of ATP energy. The system uses most of the energy the pigments capture and is therefore maximally efficient, but because there is less energy at lower light intensities, the rate of glucose production is also low. Under these conditions the rate may only slightly exceed the respiration rate, so the net photosynthetic production by the cells becomes less efficient.

* Light saturation – As the light intensity is raised, the rate of photosynthetic production increases. A plateau is reached at about one quarter the intensity of full sunlight 2500 foot candles in central Australia for most plants. Light saturation does not limit the capacity of chlorophyll to absorb light. It represents the maximum rate at which the dark reactions can utilize the ATP in the final reactions to split the Carbon Dioxide and produce glucose. A further increase in the energy supply becomes excess energy and it is converted to heat and wasted. (See plants mosaic Chapter 3)

2. Temperature

* The light reactions of photosynthesis are not temperature dependent.

* The dark reactions of photosynthesis are temperature dependent. It is an enzymatic processes with an optimum temperature.

The rate of photosynthesis in most plants increases only up to about 24C for shade plants and 36C in sun plants growing in arid zones. The rate levels out and then actually declines as the temperature approaches or exceeds human body temperature.

3. Carbon Dioxide

* The light reactions of photosynthesis are not dependent on CO2

* The dark reactions of photosynthesis are dependent on CO2.


1. Two basic raw materials are needed for photosynthesis. They are carbon dioxide and water
2. Two basic products of needed for photosynthesis. They are sunlight and chlorophyll
3. Water molecules are split during the light reaction of photosynthesis. 
4. Carbon dioxide molecules are split during the dark reaction of photosynthesis. 
5. The ultra violet and infrared range of light have the greatest affects on photosynthesis while the Green light range provides the least energy for photosynthesis. 
6. Pigments called caratenoids are responsible for the oranges and yellows when leaves change color in the fall. 
7. Reaction center molecules actually turn units of light energy into units of chemical energy which are collected by antennae pigments.
8. The production of ATP using the energy of sunlight is done during the light reaction
9. Light saturation occurs at about 40%; 1200 to 2500 foot candles, of the intensity of full sunlight. 
10. The light reaction of photosynthesis is not temperature dependent. 
11. The rate of photosynthesis in most plants increases only up to about 25oC. 
12. The net chemical equation for photosynthesis is 6CO2 + 6H2O = C6H12O6 + 6O2
13. The blue 400nm-500nm and red 650nm to 700nm wavelengths of light are the source of most of the energy for photosynthesis. 
14. A unit of several hundred antenna pigment molecules plus a reaction center is called a photosynthetic unit
15. The dark reactions of photosynthesis are dependent on CO2 levels in the atmosphere. 

16. The mesophyll cells contain the chloroplast and Mitochondria.

17. The light reactions occur in the chloroplasts using sunlight.

18. The dark reactions occur in the mitochondria using ADP.

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