Plant Physiology

1. some of the reasons can be named: 

- The conditions of all the reaction are in normal cell conditions (temperature, pressure...)

- The catalysts are limited there is no such a catalyst can be use to transform CO2 and H2O into glucose 1 one reaction or shorter

-  Also not enough energy for the reaction to occur. 

2. Maybe. I want to correct the equation a little bit: 2 H₂O + 2 NADP⁺ + 10 H⁺ + 3 ADP + 3 P_i + light → 2 NADPH + 12 H⁺ + 3 ATP + O₂

I have a question in the reaction, why we apply H+ and the product also have H+?

Maybe, the reaction will be

2 H₂O + 2 NADP⁺ + 3 ADP + 3 P_i + light → 2 NADPH + 2 H⁺ + 3 ATP + O₂

Because the electrons will transfer at internal compound right?

 Hi everyone, I want to add up some ideas about the first question from mrs.Van Anh, maybe it is correct or not, I still want to share :)))

- Firstly, if plant synthesis glucose from its monomers( which is maybe fast and less complicated) this means that its monomers will have to bond with each other by dehydrolization ( release H+ and OH-) to form H20. and this increase of H20 inside the cell somehow can prevent the diffusion of CO2 into the cell, while 02 still be there=> this can lead to PHOTORESPIRATION, a harmful process since it using up the products of photorespiration and decrease its efficiency

- Secondly, the Calvin Cycle is rather a cycle than a process, which means that it self regenerates its initial materials: RuBP => It's a very stable . As the components of the cycle are all recycled, the Rubisco can just keep picking up carbon dioxide and shooting out sugars, turning an inorganic gas into an energy molecule useful for life.=> in totally wiser and energysaving than 1 step synthesize

For the question 1, I think there are some reasons:

- It  is dependent on the following environmental factors: light intensity, temperature, and the availability of carbon dioxide, water, and certain minerals. 

- Photosynthesis is not a single process but consists of a number of photochemical and enzymatic reactions. 

Eg. The chloroplasts are crowded with multiple layers of membranes, the lamellae, composed of proteins and lipids. The protein matter includes some of the enzymes and coenzymes used in the photosynthetic process; the lipid portion contains two types of chlorophyll, along with other pigments that assist in absorbing light energy.

=> Therefore, it needs to have some reactions to prepare for the process of glucose synthesis is more easier.

• I agree with Nguyễn Thùy Linh on that the "chosen" pathway must be the one that the cell conditions can sustain, such as if energy is not dispersed the waste heat during reaction can damage the cell.
• I would like to add another point, based on the theory of evolution by natural selection. The plant, as well as the chloroplast and its mechanism, is not designed but gradually "selected". The more efficient pathways, on every aspect, among all pathways around, will be more sucessful in reproducing, and outnumber the lesser ones. Among the victors, there appear mutations that compete again. The cycle continues up to that a most efficient mechanism for glucose production is formed. Because the process emerged from random roundabouts like that, it is complex and efficient.

1. In my opinion, in order to synthesize glucose, we need some resources (for example ATP, NADPH). In photosynthesis, it has 2 reactions: light reactions and Calvin Cycle. After occurring light reaction, it produces ATP and NADPH which are material for Calvin Cycle. After Calvin cycle, it produces glucose. I think that the reason why to synthesize glucose, we need a lot of reactions.

1. Why in plant (an in living organism), the synthesis of glucose (and other compound) is really complex? 

This is my opinion about the below question:

The calvin cycle is one of the most sustainable production lines. 

There are 6 process happening at the same time so they create 12 G3P meaning that 36 carbons exist, the precise number needed to manufacture sugar, and rebuild those RuBPs. Of the 12 G3P, two are formed carbon glucose chain. Then the others carbons to recreate those six RuBPs, that requires 30 carbons the exact number contained by the remaining 10 G3PS.

The calvin cycle generates the precise numbers of element and process required to keep this production turning endlessly.

If nature uses single carbon, it wouldn't be nearly as efficient or successful at using energy to manufacture the materials. Cycle creates many loops that repeatedly reuse and rebuilt ingredients. 

My opinion is quite similar to Hien Minh's second idea, just more detailed. 

I think in the first question, each stage in the Calvin cycle has a specific role. For example, after the reduction stage, the 6 molecules GA3P divide into 2 part, 5 molecules participate in the regeneration of RuBP and 1 molecule is used to synthesise glucose; In the regeneration stage, because molecule GA3P only has 3 C so it needs a complex process to return to 6 C.

The complex process don't focus only on the final product but it creates different compounds with very small changes. The internal compounds are not only used in one synthesis but can be reused in another. Otherwise, subtances are combined and collapsed all the time to produce various compounds . Living organisms can not have enough energy and time to synthesize each of them . Therefore, although there are more reactions but time and energy will be saved since steps  are very simple and effortless required.

In photosynthesis there is a light reaction (light dependent reaction) and a dark reaction (calvin cycle or light dependent reaction). The light-dependent reaction produces ATP + NADPH + O2. The product of the Calvin cycle will be sugar. So I think that's why the plants are really complex

So,the key enzyme in the Calvin Cycle is Rubisco(PUBP carboxylase) that catalyzes the transformation of the 5-carbon sugar ribulose-5-phosphate and the single-carbon carbon dioxide to two 3-carbon 3-phosphoglycerates. Rubisco accounts for 40% of the protein content of the chloroplast and is likely the most abundant protein on Earth. The Calvin Cycle is an important part of the Carbon Cycle. Carbon dioxide is fixated from the air or water in which the organism lives in through the process of photosynthesis. In this process, plants harness the energy of the Sun to convert carbon dioxide and water into the byproduct oxygen and energy-rich carbohydrates. These energy-rich carbohydrates are used for a variety of purposes by the plant, for the production of sugars, lipids, amino acids, and other compounds needed for plant metabolism and growth. These carbohydrates are truly essential for the growth and development of a plant. They are truly essential, and are then obtained by other heterotrophs, and the cycle continues.

• I also think that the intermediate compounds of the cycles are for the synthesis of not only glucose but also of many other products, such as other sugars or cellulose or the backbone of DNA and RNA.

Q2. I think the above reaction is generalized but if full it would be: 

2 H₂O + 2 NADP⁺ + 10 H⁺ + 3 ADP + 3 P_i + light → 2 NADPH + 12 H⁺ + 3 ATP + O₂

Personally, I think there are 3 reasons why in plant, the synthesis of glucose (and other compound) is complex:
- Firstly, the synthesis of these compound needs a large amount of energy. If compounds are created quickly through one reaction, there is not enough energy.
- Secondly, to make sure the processes are suitable for the tolerance of living organism, it is necessary to divide them into several reactions.
- Thirdly, every activity in living organism is limited because of its enzymes and proteins. Enzymes in plant have particular function for small reactions, so if the synthesis process shortens, more energy is require and enzymes are unable to catalyse.
- Moreover, human can create these compound by processes which include fewer reactions. However, they still use plant instead of that way. It's because the waste from these processes is harmful and noxious, and they waste a lot of money and energy, while plant just need light, water, and CO2 - an inorganic gas causing pollution. So plant is really helpful.

Q1. The synthesis of glucose is really complex. 

Firstly, we as seen in the calvin cycle, to make one G3P molecule that can exit the cycle and go towards making glucose takes 3 stages.

Stage 1. Carbon fixation 

- A CO2 combines with 5 RuBP to have two molecules of a three-carbon compound, 3-phosphoglyceric acid (3-PGA). 

Stage 2. Reduction

- Converting one molecular 3-PGA into a THREE-CARBON sugar called glyceraldehyde 3-phosphate (G3P) that MAKE TO GLUCOSE in next stage. 

The last stage is recyled 5 in 6 molecular G3P to regenerate the RuBP. However, regeneration requires ATP and involves a complex network of reactions. 

So the conclusion is that a cycle produces a molecule G3P contains three fixed carbon atoms, BUT,  ONE GLUCOSE molecule has 6 CARBON. Therefore, It takes two times turning cycle to create two molecules G3P for produce one molecule of glucose

I think your ideas are all good and reasonable so I don't repeat. I just want to add ideas for question 1 that I came up with (however they seem not so related to yours). In order to synthesize 'natural product', we can not stick 2 carbon atoms together like playing Lego, we need functional groups (we can only add/reduce 1 or 2 carbon atoms each period because in carbohydrate there usually the first or last carbon can react to form new C-C bond, others have only -OH), different reactants at each period and those reactants are not able to survive all together in a cell (they will react immediately).

Organisms do not always have such reactants so they create their own way by using enzymes and complex proteins (through a very long trial-and-error process) but it turns out a good way because they can control the configuration of every single carbon (carbohydrate always has many chiral centers, when you change one carbon, you get a new sugar) to get what they want (things we have to make a big effort). However, I think human can't do that just because we don't have such a long time like nature.

1. The synthesis of glucose (and other compound) in plant is really complex because:

 Firstly, a portion of the light energy absorbed by chlorophylls and carotenoids is eventually stored as chemical energy via the formation of chemical bonds (e.g: between glucose's compositions). This conversion of energy from one form to another makes the synthesis of organic compounds complex.

Secondly, those reactions are properly organized to fit the morphology and physiology of the plants after several years of evolution.

Thirdly, energy is stored step-by-step in each reaction but not storing at the same time which causes a burden on the photosynthetic apparatus and additional steps provide essential starting materials for the others so they are strictly related together.