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MASINGA C.L11What exactly is photosynthesis and where does it take place ?

MASINGA C.L2Photosynthesis is the process that occurs in green plants and some algae (Kingdom Protista). The process converts light energy into chemical energy, storing it in the bonds of sugar and occurs in distinctly two phases. A quick recap on the outlook of a plant structure before we move on.MASINGA C.L3

For the process of photosynthesis to take place the following is required:

* Light energy (ATP)* Water (H20)* Carbon dioxide (C02)

MASINGA C.L4The processPhotosynthesis takes place initially on the plant leaves which is the major site for the process as compared to the other parts of the plants (refer to labelled plant diagram in slide 3 showing all plant parts). A typical leaf includes the upper and lower epidermis, the mesophyll, the vascular veins as well as the stomata. The upper and lower epidermal are present for protection purposes hence the process does not take place there as these cells do not have chloroplasts. The mesophyll cells have chloroplast and this is where the process of photosynthesis takes place. The stomata are pores located in the lower epidermis and facilitate the exchange of air. They briefly allow CO2 in and O2 out. The vascular veins however are part of the plants transportation system, moving water and nutrients around the plant as needed. MASINGA C.L5

The chloroplastThe chloroplast includes the outer and inner membranes, intermembrane space, stroma, and thylakoids stacked in grana. Located and grooming in the membranes of the thylakoids, it is the green plastids found in green parts of a plant exposed in sunlight. Nevertheless it is energy from absorbed red and blue light unseen by the naked eye that therefore enables it to be used for photosynthesis. However the green light visible to the naked eye can not be absorbed by the plant hence is not the one used for photosynthesis.MASINGA C.L6

The photosynthesis process as mentioned initially has two distint phases which are light phase and dark phase.

The light phaseThe light phase of photosynthesis is light dependant and therefore takes place in the presence of light reaction within the thylakoid membrane, and converting light energy to chemical energy. In the thylakoid membrane chlorophyll and several other pigments such as beta-carotene are organized in clusters and also participate in this light reaction. Each of these differently-colored pigments then absorb colors of light according to their color meanwhile passing its energy to the central chlorphyll molecule for photosynthesis to occur. MASINGA C.L7

The dark phaseThe dark phase of photosynthesis is dependant on the absence of light and therefore takes place in the dark. Although the reaction doesnt directly need light in order to occur, but it does need the products of the light reaction (ATP and another chemical called NADPH). Moreover, energy harvested from the light phase reaction is stored by forming a chemical called ATP (adenosine triphosphate) which is a compound used by cells for energy storage.This reaction takes place in the stroma within the chloroplast, and converts CO2 into sugar. All in all the dark reaction involves a cycle called the Calvin cycle which plays a vital role in this phase.


The Calvin cycleThe Calvin cycle is a cycle that occurs within the dark phase, where the cycle spends ATP as an energy source and consumes NADPH2 as reducing power for adding high energy electrons to make the sugar. There are three phases of the cycle. In phase 1 (Carbon Fixation), CO2 is incorporated into a five-carbon sugar named ribulose bisphosphate (RuBP). The enzyme which catalyzes this first step is RuBP carboxylase or rubisco. It is the most abundant protein in chloroplasts and probably the most abundant protein on Earth. The product of the reaction is a six-carbon intermediate which immediately splits in half to form two molecules of 3-phosphoglycerate. In phase 2 ( Reduction), ATP and NADPH2 from the light reactions are used to convert 3-phosphoglycerate to glyceraldehyde 3-phosphate, the three-carbon carbohydrate precursor to glucose and other sugars. In phase 3 (Regeneration), more ATP is used to convert some of the of the pool of glyceraldehyde 3-phosphate back to RuBP, the acceptor for CO2, thereby completing the cycle. For every three molecules of CO2 that enter the cycle, the net output is one molecule of glyceraldehyde 3-phosphate (G3P). For each G3P synthesized, the cycle spends nine molecules of ATP and six molecules of NADPH2. The light reactions sustain the Calvin cycle by regenerating the ATP and NADPH2.MASINGA C.L9Diagrams representing the Calvin cycleMASINGA C.L10

The overall process of photosynthesis can be represented and concluded by the chemical equation:


Hence can also be summarized by the diagram:


The concept of photosynthesis has been clearly introduced, chemically presented, illustrated by diagrams and the processes clearly explained. However, we have to know why this process has to occur and what its benefits are. The benefits of photosynthesis are as follows:

* It is the source of the O2 we breathe* It is a significant factor in the concerns about deforestation * Plants produce glucose and respires it to provide energy for living organisms* Provides glucose used as a building block for making cellulose, which is used for producing cell walls for the plants structure and growth. * Store glucose as starch to be consumed in the diet of living organisms.* Combines with other molecules to make either fatty acids or amino acids needed by living organisms.

MASINGA C.L13Interesting facts about the photosynthesis processDid you know

* In the daytime, when the light reaction is occurring and ATP is available (but the stomates must remain closed), they take the CO2 from these organic compounds and put it into the Calvin cycle. These plants are called CAM plants, which stands for crassulacean acid metabolism after the plant family, Crassulaceae (which includes the garden plant Sedum) where this process was first discovered.

* Although photosynthesis is performed differently by different species, the process always begin when energy from light is absorbed by proteins called reaction centres that contain green chlorophyll pigments.


References1. Slide 1 pictures http://crispme.com and www.betterwallpaper.com (Accessed 03/03/2014)

2. Slide 2, 12 and 14 emoticons www.veryicon.com (Accessed 03/03/2014)

3. Slide 3 picture http://heightstechnology.edublogs.org/2009/10/05parts-of-a-plant (Accessed 03/03/2014)

4. Slide 5 picture www.tekura.school-nz (Accessed 03/03/2014)

5. Slide 6 picture http://passscience.blogspot.com/2010/09structure-of-cell-part-3.html (Accessed 03/03/2014)

6. Slide 7 picture http://frogsinmyformula.blogspot.com (Accessed 03/03/2014)

7. Slide 8 picture www.lisamorguess.com (Accessed 03/03/2014)

8. Slide 10 picture www.biologycorner.com and http://moodle.rockyview.ac.za (Accessed 03/03/2014)

9. Slide 11 picture www.homestead-farm.net/KidsLinks/photosynthesis.html (Accessed 03/03/2014)