Before we go into all the complex details of the photosynthesis in higher plants NCERT chapter, let's start from the basics. Many of you may already know what photosynthesis is all about. However, you need to clear your concept in a better way to go ahead with the chapter. Let's discuss what exactly photosynthesis is? 

 

What is photosynthesis? 

It is the method in which the plants prepare their food. They do so by converting light energy into chemical energy, which they later utilise. It is crucial for life on earth as it helps in the release of oxygen. 

 

Now that you've understood what photosynthesis is all about, let's move further with all the other concepts surrounding this term. If you can be thorough with them, you can easily find photosynthesis in higher plants NCERT solutions. Let's have a look at what we know about this concept so far. 

We know that photosynthesis requires sunlight. It is an anabolic process that is regulated by an enzyme. It is a physio-chemical process wherein green plants use sunlight or light energy to synthesise organic compounds. 

 

The chemical energy produced from photosynthesis is stored as carbohydrates like starch, sucrose, etc. The reactants involved are carbon dioxide and water, and the products are oxygen and carbohydrates.

It releases oxygen which is the basis of life on earth. Due to the presence of chlorophyll and sunlight, the process of photosynthesis is possible. It takes place in the green part of the leaves.

Some of the early experiments that were associated with photosynthesis are: 

 

• The first experiment was conducted by J.B. Priestly in 1770. He took two bell jars for his experiment. In one jar, he put a mouse and a candle. In the other jar, he kept a green plant with the mouse and candle. He observed that the rat in the first jar died due to suffocation, and the candle was extinguished too. However, in the second jar, the rat survived. The flame of the candle kept burning too. 

 

In conclusion, the air in the second jar was purified by the green plant. It produces oxygen that let the rat and candle survive. It was then that he discovered oxygen in 1774.

• In 1854, Julius Von Sachs discovered that green plants produce glucose that is later stored as starch. It is the first product of the photosynthesis reaction.

 

• T.W. Engelman discovered the effect of different colours and it's wavelength on photosynthesis. He used a prism for his experiment. It was split into its component colours. An alga named Cladophora was illuminated with the same prism light. In the split spectrum, he observed the growth of bacteria in the red and blue spectrum of light. 

 

• Cornelius Van Neil discovered the actual reaction that takes place with photosynthesis. He researched the purple and green sulphur bacteria. He discovered that hydrogen reduces carbon dioxide to form sugar.

Photosynthesis takes place mainly in the chloroplasts of the eukaryotic photoautotrophs. Now, let's understand the location and mechanism of the same. Chloroplasts are green plastids that are present in mesophyll cells under the leaves. Please note that we are considering a green plant here.

Within the chloroplast, many membranous organelles are present. They include stroma, grana, lamellae, and fluid stroma. This whole system is responsible for generating energy in the form of ATP and NADPH so that the plant can utilise it later. 

 

In the stroma, enzymes work towards extracting sugar while incorporating carbon dioxide. The reaction where light energy is reduced to energy is known as a light reaction. Light is necessary for this process. The process wherein carbon dioxide is reduced to sugar does not need light. It is called a dark reaction.

 

The major pigments present in leaves are:

Chlorophyll is said to be the chief pigment involved in photosynthesis, as maximum absorption takes place in the red and blue regions. 

Other pigments are involved in absorbing light and transferring it to chlorophyll a. They are called accessory pigments.

Light Reaction involves numerous steps. They include: 

• • Light absorption 
  • Water splitting 
  • Oxygen release 
  • Formation of ATP and NADPH 

The pigments are organised into two photosystems that are light-harvesting complexes (LHC). It consists of photosystem I and photosystem II. 

 

The LHC consists of many pigments except chlorophyll a in each of the photosystems. The pigments present in both the photosystems absorb different wavelengths of light. The reaction centre is the single molecule of chlorophyll a. The highest peak in PS I is 700 nm and is called P700. Similarly, the peak in PSII is called P680.

In PSII, the reaction centre absorbs light and makes the electron excited. Later, they get picked by an electron acceptor and then it passes to the electron transport system that consists of cytochromes. 

Through the transport chain, the electrons are passed to the pigment in PSI. The same process takes place here, and the electrons get excited. They are passed to another molecule that possesses a greater redox potential.

The electron transfer takes place in a downhill direction. Energy release also takes place. ADP is converted to ATP and NADP+ is reduced to NADPH. It forms a z shape and is known as Z-scheme.

Chemiosmotic Hypothesis of ATP Formation 

It was proposed in 1961 by Mitchell. The ATP is synthesised by the presence of a proton gradient on the membrane of the thylakoid or mitochondria. It takes place by: 

1. The splitting of water molecules to give out hydrogen ions or protons 

2. As there is the transfer of electrons across the PS, protons get transported across the membranes

3. Now comes the NADP reductase enzyme. It is located on the stromal side of the membrane

4. Protons help in reducing NADP+ to NADPH and Hydrogen ions 

5. The protons are removed from the stroma to create a proton gradient and pH in the lumen 

6. Later, the proton gradient is broken because of proton movement across the membrane channel of F0 of the ATPase enzyme

7. The movement of electrons produces ATP and NADPH 

8. It is used to fix carbon dioxide to form sugar

The C4 Pathway is also called Slack Pathway as Hatch and Slack discovered it in 1965-67. It is functional in plants like sugarcane, maize, etc. 

 

In this pathway, the first stable compound that is formed is known as Oxaloacetic Acid or AOO. It is a 4-carbon compound. It is also called the C4 Pathway. It is said that all C4 Pathway plants have Kranz Anatomy. 

 

The carbon dioxide acceptor is Phosphophenol Pyruvate which is a 3-carbon molecule. It is present in the mesophyll cells of the leaves.

 

The OAA that is formed in the mesophyll cells forms malic acid or aspartic acid. They are 3-carbon compounds. It is returned to the mesophyll cells for PEP formation.

 

There is the release of carbon dioxide at the bundle sheath. It enters the Calvin Cycle, where it forms sugar in the presence of RuBisCO.

It is the oxygenation of RuBP in the presence of sunlight or light energy. It is completely light-dependent. Through this process, the release of carbon dioxide takes place. 

 

When the oxygen concentration is increased from 2% to 21%, the photorespiration decreases the rate of photosynthesis. 

 

In the presence of light and high Oxygen concentration, RuBisCO binds with Oxygen: 

 

 RuBisCO + O2 ---> PGA + phosphoglycolate 

 

This pathway includes Chloroplast, Mitochondria, and Peroxisomes. Photorespiration does not take place in C4 plants.

The factors affecting photosynthesis are:

• Light: As the intensity of light increases, the rate of photosynthesis increases too.

• Temperature: It does not influence directly. However, high temperatures can cause denaturation of enzymes which can affect the dark reaction.

• CO2 Concentration: As it increases, the rate of photosynthesis increases too.

• Water: If the amount of water increases, the rate of photosynthesis does not increase proportionally as a saturation point is reached.

1.Observe the leaves of a plant that are under the shade and compare them with leaves of the same plant which are under sunlight. Which of them are dark green and why? 

The leaves under sunlight are dark green as the light reaction is taking place in them. The leaves in the shade are undergoing a dark reaction.

2. What is photosynthesis in higher plants? 

The physio-chemical reaction in green plants wherein light energy from the sun is converted to chemical energy, which can be utilised later by the plant, is called photosynthesis.

3. Why is photosynthesis important in class 11? 

It is an important topic because it deals with the main mechanism of photosynthesis and how plants release oxygen into the environment.