Photosynthesis In Higher Plants

Dark Reaction and Photorespiration

Haven’t you always read plants need sunlight to make their food by the process of photosynthesis? But, do you know that photosynthesis does not need sunlight in all its steps? What does photosynthesis have to do with photorespiration? Let us know!

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Photorespiration

Photosynthesis is divided broadly into two main steps

  • Light Reactions: These reactions occur in the presence of sunlight and they are responsible for the generation of energy molecules such as ATP and NADPH.
  • Dark Reactions: These reactions do not need sunlight and occur in its absence. Dark reactions use the energy molecules that were generated during the light reactions. It is in these reactions that food is actually synthesized.

While photosynthesis occurs, there is another process that is simultaneously occurring which is called as photorespiration. The reason these two processes occur at the same time is that photorespiration uses oxygen while giving out carbon dioxide and the dark reaction of photosynthesis uses carbon dioxide while giving out oxygen. Thus, these processes are often studied together to understand the efficiency of the plant system.

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Dark Reaction Of Photosynthesis

(Image Source: roboplant.wordpress.com)

The dark reaction of photosynthesis is not dependent on sunlight but it does not mean it does not occur in its presence. The dark reaction occurs both in the presence and absence of sunlight. This reaction occurs in the stroma of the chloroplasts which are cell organelles known as the kitchen of the cells.

The dark reaction was discovered by three scientists namely Calvin, Benson and Bassham and so this cycle is also called as the Calvin- Benson- Bassham cycle. This reaction is also called by many other names such as C3 cycle or reductive pentose phosphate cycle. The plants that have Calvin cycle are known as C3 plants.

As mentioned earlier, the dark reaction is the actual phase where food is synthesized by the plants and it uses the energy generated by the light reactions.

Phases of Dark Reaction

  • Carbon Fixation: The CO2 taken in by the plant combines a 5 carbon compound known as ribulose 1,5- biphosphate to form two molecules of 3-phosphoglycerate(3PGA).  This step is catalyzed by the enzyme ribulose biphosphate carboxylase or RUBISCO(RuBp)
  • Carbon Reduction: In this phase, 3PGA uses 6 ATP molecules to form an isomer of 3PGA called as 1,3- phosphoglycerate. In the next step, 6NADPH to form glyceride-3 – phosphate.
  • RuBp Regeneration: This step’s responsibility is to regenerate the RuBp which is what the reaction began with. At the same time, it also synthesizes glucose which is stored as food. Glyceride -3- phosphate uses 3 ATP molecules to convert into ribulose-1,5- biphosphate which enters a new cycle of dark reaction by combining with the incoming CO2.

Photorespiration

(Image Source: educalingo.com)

Photorespiration is a special type of respiration that is found in C3 plants that is different from the normal respiration that occurs in all plants in their mitochondria. It occurs in the presence of sunlight whereas mitochondrial respiration is independent of it.

Photorespiration occurs in three organelles of the plant cells namely: Chloroplast, peroxisomes, and mitochondria. It is an oxidative process where glycolate is oxidized to release CO2. Photorespiration is said to be influenced by the CO2 concentration in the plant, the temperature of the atmosphere and the oxygen levels.

It is important to mention here that the enzyme RUBISCO has an affinity for both CO2 and O2 and its name changes depending on who it is binding to. When it binds to oxygen, it is called ribulose biphosphate oxygenase. Though the affinity of RUBISCO to oxygen is said to be more than that for CO2, it depends upon the concentration of O2 and CO2 and temperature conditions which ultimately decide what RUBISCO binds to.

In the first step of photorespiration, the oxygen that is available to the plant is taken in and it combines with RUBISCO to form one molecule of 3-phosphoglycerate and one molecule of 2- phosphoglycerate. The 3-phosphoglycerate enters the dark reaction of photosynthesis or the Calvin cycle where it is converted to RuBP.

The 2-phosphoglycerate is converted to phosphoglycolate in the peroxisome. This phosphoglycolate is dephosphorylated in the chloroplast in the presence of the enzyme phosphatase and forms glycolate. This glycolate now travels back to the peroxisome where it is oxidized to glyoxylate.

This glyoxylate which is then further synthesized into amino acids glycine, serine, ammonia and carbon dioxide is given out. The amino acids get further synthesized into other by-products.

Question For You

Q: Which of the following factors does NOT increase the affinity of RUBISCO to O2?

(a)    Increased CO2 concentration in the plant       

(b) Increased temperature

(c)     Increased O2 levels                                             

(d) Decreased Water levels in the plant

Sol. (d) Decreased Water levels in the plant. Increase in the CO2 concentration in the cells, increased oxygen levels and an increased temperature (especially during the afternoon times for the tropical plants) causes increased affinity of RUBISCO to oxygen and leading to increased photorespiration. Water levels in the plant have no effect on RUBISCO.

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Ranjeeta

How to remember the dark reaction in biology

junaid Khan
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junaid Khan

that so easy to remember it has a special formula through which you can remember them easily

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