LET’S TALK LIFE SCIENCES (GRADE 10)

PHOTOSYNTHESIS

MASINGA C.L 1

What exactly is

photosynthesis and

where does it take

place ?

MASINGA C.L 2

Photosynthesis 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.L 3

For the process of photosynthesis

to take place the following is

required:

* Light energy (ATP)

* Water (H20)

* Carbon dioxide (C02)

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The 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 plant’s transportation system, moving water and nutrients

around the plant as needed.

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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.

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The photosynthesis process as mentioned initially has two

distint phases which are light phase and dark phase.

The light phase

The 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.

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The dark phase

The dark phase of photosynthesis is dependant on the

absence of light and therefore takes place in the dark.

Although the reaction doesn’t 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.

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The Calvin cycle

The 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.L

9

Diagrams representing the Calvin cycle

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The overall process of photosynthesis can be represented and concluded by

the chemical equation:

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Hence can also be summarized by the diagram:

WHAT ARE THE

BENEFITS OF

PHOTOSYNTHESIS?

MASINGA C.L 12

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 plant’s 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.

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Interesting facts about the photosynthesis

process

Did 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.

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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)

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