Need Help?

Get in touch with us

bannerAd

Ecosystem and Energy Flow

Aug 27, 2022
link

Key Concepts

• Photosynthesis – process, stages

• Light reaction – process, stages, products

• Dark reaction – process, stages, products

• C4 Plants

• C4 cycle – process, stages, significance

parallel

Physiological Metabolism – C4 Cycle (Hatch and Slack Pathway) 

Introduction:

Photosynthesis (photo – light; synthesis – to join) is the process through which green plants make use of energy from sunlight to make their food. In the presence of sunlight, chloroplasts of green plants synthesize glucose using water and carbon dioxide. During photosynthesis, water gets oxidized, and carbon dioxide gets reduced to form carbohydrates. Photosynthesis forms the basis of all types of food chains and food webs. Photosynthesis produces starch and oxygen required for the survival of an organism. Therefore, photosynthesis supports all forms of life on Earth. 

Photosynthesis 
Process of photosynthesis in plants 
Process of Photosynthesis

Photosynthesis in Plants: 

Photosynthesis in plants 

Photosynthesis is an important process that takes place in green plants. In the presence of sunlight, plants combine carbon dioxide and water to produce carbohydrates. Oxygen is the most important by-product of photosynthesis. In the presence of light energy, water is split to release oxygen. This process is known as the photolysis of water. Water molecules get split up during photosynthesis to release oxygen, and carbon dioxide forms carbohydrates. Oxygen produced during photosynthesis comes from a water molecule and not from carbon dioxide. In plants, photosynthesis takes place in the green part. Mostly green leaves carry out the photosynthesis process. Sometimes other green parts of plants, such as green stems and floral buds, are also involved in photosynthesis. Chloroplasts are specialized organelles in which the photosynthesis process takes place. 

Stages of Photosynthesis: 

Photosynthesis involves two important processes, 

  1. Light-dependent reaction (Light reaction) 
  1. Light-independent reaction (Dark reaction/Calvin cycle) 

Though the entire process of photosynthesis takes place in chloroplast, light and dark reactions occur at different sites. The light reaction occurs in grana, and the dark reaction occurs in stroma regions of the chloroplast.  

 Process of photosynthesis in chloroplasts

Light-dependent Reaction: 

The light-dependent reaction is also known as the primary photochemical reaction, Hill’s, or Arnon’s cycle. The light reaction takes place at a faster rate than the dark reaction. During the light-dependent reaction, light energy gets converted into chemical energy. Light-dependent reactions occur in the thylakoid (thylakoids are stacked up to form grana) region of the chloroplast. Thylakoids contain chlorophyll molecules. Chlorophyll pigments are green in color. These pigments absorb different wavelengths of light and convert them into chemical energy through photosynthesis. 

parallel

The light-dependent reaction involves four important stages. 

  1. Absorption of light energy 
  1. Splitting of water molecules 
  1. Release of oxygen 
  1. Formation of energy-carrying molecules – ATP and NADPH 

Products of Light Reaction: 

During light reactions, solar energy gets converted into chemical energy and is stored in the form of energy-carrying molecules – ATP and NADPH. During light reactions, water is split up to release oxygen. High energy carrier molecules, ATP, and NADPH are required for the dark reaction, the next stage of photosynthesis.  

Light and dark reactions inside chloroplast 

Dark Reaction/Calvin Cycle: 

The second stage of photosynthesis is the light-independent reaction. It is also known as the dark reaction, Calvin cycle, or C3 cycle. The Calvin cycle refers to converting carbon dioxide to carbohydrates or the fixation of atmospheric carbon dioxide by plants through photosynthesis. Plants that undergo this cycle of reactions are known as C3 plants (For example, rice, wheat, barley, peanuts, spinach, etc.). 

The overall purpose of the Calvin cycle is to convert atmospheric carbon dioxide into carbohydrates (sugars). The Calvin cycle takes place in the stroma of chloroplasts without light. The Calvin cycle uses ATP and NADPH produced by the light reaction to convert carbon dioxide into carbohydrates. 

 Phases of the Calvin Cycle: 

Calvin cycle involves three important phases. 

  1. Carbon fixation 
  1. Reduction 
  1. Regeneration of ribulose 1, 5-bisphosphate (RuBP) 

During the first phase of the Calvin cycle, carbon dioxide reacts with ribulose 1, 5-bisphosphate (RuBP). This reaction is catalyzed by the enzyme ribulose biphosphate carboxylase/oxygenase or RUBISCO(RuBp). Two molecules of three-carbon molecule known as 3 – phosphoglycerate (3– PGA) are produced during this phase. In the reduction phase of the Calvin cycle, 3-PGA molecules (from the carbon fixation phase) are converted into glyceraldehyde-3-phosphate (G3P). ATP and NADPH are used for this reaction. As NADPH gets reduced (donates electrons), this reaction is termed as reduction phase. During the third phase, ribulose 1, 5-bisphosphate (RuBP) gets regenerated, and the cycle restarts. 

Products of Calvin Cycle: 

  • After each turn of the Calvin cycle, one molecule of carbon gets fixed. 
  • After three turns of the Calvin cycle, one molecule of G3P is exported into the cytoplasm. 
  • G3P molecules that leave the Calvin cycle are used for the production of glucose/fructose/sucrose/starch.  
  • Two G3P molecules combine together to form one glucose molecule. Therefore six turns of the Calvin cycle are required to produce one molecule of glucose. 
Three stages of the Calvin cycle 

C4 Cycle (Hatch and Slack Pathway): 

In 1966, Marshall Davidson Hatch and Charles Roger Slack discovered the C4 cycle. C4 cycle is also known as the beta carboxylation pathway or co-operative photosynthesis. C4 cycle is observed in plants present in the dry tropical region. In order to grow in a hot and dry environment, plants have adapted themselves by following the C4 pathway. The first stable product of the C4 cycle is oxaloacetate. It is a four-carbon compound. Hence the pathway is named C4 cycle. 

Significance of C4 Cycle: 

C4 cycle is a kind of adaptation by plants to survive in a hot and dry environment. C4 plants have more advantages than C3 plants. C4 plants do not exhibit the process of photorespiration. (Photorespiration refers to the oxidation of RuBp in the presence of oxygen). Photorespiration begins when RUBISCO takes up oxygen instead of carbon dioxide. In order to reduce the loss of water, C3 plants close their stomata (pores present in leaves). Photorespiration takes place during this time. C3 plants lose an enormous amount of energy through photorespiration. In order to avoid energy loss through photorespiration, certain plants adapt themselves to C4 and CAM pathways. The crop yield also increases through this adaptation. 

C4 Plants: 

Plants that exhibit this C4 cycle are known as C4 plants. Example: Sugarcane, maize, sorghum. C4 plants have some special features. Their leaves have a special type of structure known as Kranz anatomy (Kranz – wreath). Mesophyll cells are arranged as ring-like clusters around the bundle-sheath cells in these leaves. The leaves are also observed to have additional characteristics such as more number of chloroplasts, thick walls without intercellular space, and walls that are not permeable to gaseous exchange. C4 plants have the ability to withstand high temperatures and high light intensity. C4 plants manage to grow even with very low water content. C4 plants do not exhibit the process of photorespiration. (Photorespiration refers to the oxidation of RuBp in the presence of oxygen). C4 plants have the ability to carry out the photosynthesis process even under very low carbon dioxide concentration. 

Leaves of C4 plants are found to have dimorphic chloroplasts, i.e., two different types of chloroplasts.  

  1. Granal Chloroplasts 
  1. Agranal Chloroplasts 

Granal Chloroplasts (Mesophyll chloroplasts) 

Chloroplasts present in the mesophyll cells are smaller in size. Grana is well developed in these chloroplasts. These chloroplasts do not store starch. 

Agranal Chloroplasts (Bundle Sheath chloroplasts) 

Chloroplasts present in the bundle sheath cells are larger in size. These chloroplasts do not possess grana. Several starch grains are stored in these chloroplasts. 

Kranz Anatomy 
Fig. No.9: Leaf Anatomy of C3 and C4 plants 
Anatomy of C3 and C4 leaves 

Process of C4 Cycle: 

In C4 plants, light and dark reactions occur in different regions. The light reaction takes place in the mesophyll cells, while dark reaction takes place in the bundle-sheath cells. Calvin cycle/C3 cycle is commonly observed in all photosynthetic plants. Whereas the C4 cycle is observed only in C4 plants. C4 pathway is also observed to occur before the C3 pathway.  

C4 cycle involves four important steps. 

  1. Carboxylation 
  1. Break down 
  1. Splitting  
  1. Phosphorylation 
The C4 Cycle 

Carboxylation: 

Carboxylation or carbon dioxide fixation occurs in the chloroplast stroma of mesophyll cells. After the atmospheric carbon dioxide enters the leaf, it combines with phosphoenolpyruvate (PEP), a three-carbon compound. This reaction is catalyzed by the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase) in the presence of water. This reaction results in the formation of oxaloacetic acid (OAA), a four-carbon compound. This is the first stable product of the cycle. 

Break down: 

Oxaloacetate formed in the carboxylation step is broken down into aspartate and malate. Aspartate and malate are then diffused from mesophyll cells to bundle sheath cells. This reaction is catalyzed by enzymes, transaminase, and malate dehydrogenase. 

Splitting: 

Malate and aspartate are split into carbon dioxide and pyruvate (three-carbon molecules) inside the bundle sheath cells. The carbon dioxide formed enters the Calvin cycle that takes place in the bundle sheath cells.  

Phosphorylation: 

Pyruvate formed during the decarboxylation reaction is transported back to the mesophyll cells.  In mesophyll cells, pyruvate reacts with ATP and regenerates phosphoenol pyruvate (PEP). Thus, the cycle is repeated again. 

Difference between C3 and C4 plants:  

 Difference between C3 and C4 plants
 Difference between C3 and C4 plants

Summary

• Photosynthesis is the process through which chloroplasts of green plants synthesize glucose by making use of water and carbon dioxide in the presence of sunlight.

• During photosynthesis, water gets oxidized, and carbon dioxide gets reduced to form carbohydrates.

• Photosynthesis involves two important processes – light reaction and dark reaction.

• The light reaction occurs in grana, and the dark reaction occurs in stroma regions of the chloroplast.

• The light-dependent reaction involves four important stages – absorption of light energy, splitting of water molecules, releasing oxygen, and forming energy-carrying molecules – ATP and NADPH.

• Light reaction results in the conversion of light energy into chemical energy through the production of ATP and NADPH.

• The Calvin cycle takes place in the stroma of chloroplasts without light.

• The overall purpose of the Calvin cycle is to convert atmospheric carbon dioxide into carbohydrates (sugars). The Calvin cycle uses ATP and NADPH produced by light reactions for this purpose.

• Calvin cycle involves three important phases – Carbon fixation, reduction, and regeneration of Ribulosel, 5-bisphosphate. Six turns of the Calvin cycle are required to produce one glucose molecule.

• C4 cycle is observed in plants present in the dry tropical region.

• C4 cycle is a kind of adaptation by plants to survive in a hot and dry environment.

• C4 plants have more advantages than C3 plants. C4 plants do not exhibit the process of photorespiration.

• Leaves of C4 plants are found to have dimorphic chloroplasts – granal and agranal.

• Leaves of C4 plants are found to have Kranz anatomy.

• In C4 plants, the C4 pathway takes place in the mesophyll cells, while the C3 pathway takes place in the bundle-sheath cells. Whereas in C3 plants Calvin cycle occurs in the mesophyll cells.

• C4 cycle involves four important steps – carboxylation, break down, splitting and phosphorylation.

 • Carbon dioxide produced during the C4 cycle enters into the Calvin cycle.

Comments:

Related topics

Character Displacement : Abstract and History

Introduction:  CHARACTER DISPLACEMENT   Abstract  Introduction  Character displacement favors the evolution of novel resource use or reproductive traits, drives divergence between sympatric and allopatric conspecific populations, and both initiate and finalize the process of speciation. Despite the significance of character displacement, research has been largely focused on whether it occurs or not. However, it is needed […]

Read More >>

Process of Natural Selection and Evolution

Key Concepts • Natural selection • Variation • Adaptation • Process of natural selection Introduction Natural selection is one of the important mechanisms of evolutionary change and is the process responsible for the evolution of adaptive features in various species. It is a force that causes groups of organisms to change over time and it […]

Read More >>

Release of Energy – Detailed Explanation

Introduction Release of Energy   Food web organisms transmit energy from producers to consumers. Organisms require energy to complete complicated activities. The great majority of energy in food webs comes from the Sun and is turned (processed) into chemical energy via the photosynthesis process in plants. When molecules are broken down during respiration in plants, a […]

Read More >>

Formation of Food Molecule – Types, Importance

Key Concepts Food Molecules Carbohydrates Fats/Lipids Proteins Process of photosynthesis Importance of photosynthesis Step involved in photosynthesis Introduction Food Molecules   Food is made up of many biological molecules that provide us with energy and include chemicals that we require to develop and repair ourselves and assist our cells to work in our bodies. Carbohydrates and […]

Read More >>

Other topics