An Intro to Photosynthesis

Chloroplasts, found in plant cells and not animal cells, conduct photosynthesis. There main function is absorbing sunlight and converting it into sugar molecules and ATP/NADPH for energy. Here’s a simple diagram of what a chloroplast looks like.


The chloroplasts are double membrane bound organelles and have a system of three membranes: the outer membrane, the inner membrane and the thylakoid system. The outer and the inner membrane of the chloroplast enclose a semi-gel-like fluid known as the stroma. This stroma makes up much of the volume of the chloroplast, the thylakoids system floats in the stroma. Here is some basic definitions of each part of the chloroplast:

Outer membrane – a semi-porous membrane, permeable to small molecules and ions, which diffuses easily. The outer membrane is not permeable to larger proteins.

Inner membrane – forms a border to the stroma. It regulates passage of materials in and out of the chloroplast. The fatty acids, lipids and carotenoids are synthesized in the inner chloroplast membrane.

Stroma – an alkaline, aqueous fluid which is protein rich and is present within the inner membrane of the chloroplast. The space outside the thylakoid space is called the stroma.

Thylakoid system – suspended in the stroma. The thylakoid system is a collection of membranous sacks called thylakoids. The chlorophyll is found in the thylakoids and is the sight for the process of light reactions of photosynthesis to happen. The thylakoids are arranged in stacks known as grana. Each granum contains around 10-20 thylakoids.
Thylakoids are interconnected small sacks. The membranes of these thylakoids is the site for the light reactions of the photosynthesis to take place.
So what is the process of photosynthesis?  The basic formula for photosynthesis is the exact opposite of cellular respiration:
6CO2 + 6H2O + Light –> C6H12O6 + 6O2
First, carbon dioxide (CO2) enters the leaves through stomata, where it diffuses and is absorbed into the stroma of the chloroplast.
The first step in photosynthesis is the light-dependent reactions:  These reactions take place on the thylakoid membrane inside the chloroplast.  During this stage light energy is converted to ATP (chemical energy) and NADPH (reducing power).
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As shown in the picture, the PSII is the first step (photosystem II). A photon of light is absorbed by a chlorophyll molecule in the light harvesting complex of PSII. An electron in the chlorophyll molecules becomes excited as a result of a higher level of energy.  The excited electron becomes unstable and is released. Another electron is released following the capture of another photon of light by the light harvesting complex and the transfer of energy to the reaction center. The electrons are transported in a chain of protein complexes, aka electron transport chain. (also question: is it also called oxidative phosphorylation in photosynthesis?). Next, water is used as the electron donor in oxygenic photosynthesis and is split into electrons (e-), hydrogen ions (H+, protons) and oxygen (O2). The hydrogen ions and oxygen are released into the thylakoid lumen. Oxygen is later released into the atmosphere as a by-product of photosynthesis. Hydrogen ions from the stroma are also tranferred and released into the thylakoid lumen. All these protons in the thylakoid lumen results in a high concentration gradient. As a result of the proton gradient in the lumen, hydrogen ions are transferred to ATP synthase and provide the energy needed for combining ADP and a phosphate group, producing ATP (!).

Screen Shot 2017-02-11 at 10.31.17 AM.png
Now for photosystem I: Now that some electrons have arrived in photosystem I, they are transferred to some mobile carriers. These carriers transport them to the final electron acceptor, and at this point, the electrons and a hydrogen ion are combined with NADP+ to produce NADPH. The lost electrons from PSI are replaced by electrons from PSII via the electron transport chain. So in the light dependent reactions, NADPH and ATP are the products. Here’s a picture I found really helpful.

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The second portion of the photosynthesis cycle is the Calvin Cycle. These reactions occur in the stroma of the chloroplast.  Energy from ATP and electrons from NADPH are used to convert carbon dioxide into glucose and other products.The Calvin cycle occurs in three phases. During the first phase, carbon fixation, carbon dioxide enters the cycle. An enzyme called RuBP carboxylase fixes carbon into ribulose biphosphate, which is a five-carbon sugar. This reaction produces a six-carbon molecule that splits into two molecules of 3-phosphoglycerate.

During the second phase of the cycle, called reduction (the gain of electrons), NADPH2 and ATP are used to convert 3-phosphoglycerate to a carbohydrate precursor to glucose and other types of sugars. This three-carbon molecule is called glyceraldehyde 3-phosphate.

Finally, the cycle begins the regeneration phase. During this third phase, ATP is used to convert some of the glyceraldehyde 3-phosphate to RuBP. The remaining molecules of the gly. 3-phosphate are used to produce fructose diphosphate, which is used to make starch, sucrose and other sugars. Although the Calvin cycle is a light-independent reaction, it relies indirectly on light reactions because these reactions produce the NADPH2 and ATP needed to complete the cycle.

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So that’s basically it! I would like to go more over the Calvin Cycle. What exactly do we need to know for the AP test? What molecules should we know? It’s hard to know how in depth our understanding should be.


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