Photosynthesis handwritten notes


We all know that the plants produce food by themselves and we must have also learnt about it in lower classes before. If you thought of the word photosynthesis, you guessed it absolutely right. In this article we are going to be talking about photosynthesis in brief. 

Handwritten notes are attached at the end of this article, don't forget to check them out.

What is photosynthesis ? 

Photosynthesis is the process where glucose is synthesized from inorganic materials like CO2 and H2O using sunlight and chlorophyll. It is a biochemical, intracellular and anabolic process. The plants which can produce their own food by the help of photosynthesis are known as autotrophs and the ones which depends on these plants are called as heterotrophs

During this process, oxygen is formed as a by-product with carbohydrates where this oxygen is released in the atmosphere. Many people thought that the oxygen released comes from CO2, but when it was experimented it was known that the oxygen released comes from water. 

When the water molecule splits into half it is known as photolysis of water and from this split water molecule, O2 is released. Equation of photosynthesis is as follows:


The process of photosynthesis is an oxidation-reduction process where CO2 is reduced and H2O is oxidised to form the reserve food material i.e carbohydrates. The energy used during this process is in the form of ATP (Adenosine triphosphate). It converts the radiant energy from the sun into chemical energy. 

Site of photosynthesis:

Photosynthesis takes place in chloroplast. Chloroplast is present within the mesophyll cells which are present only on the green parts of the plants like leaves, stem, etc.. The chloroplast is a double membrane organelle. 


The chloroplast consist of jelly like fluid which is called the stroma which carries our dark reaction in it. There are many small disc or coin like structure called thylakoids. These thylakoids are placed on one another forming stacks which is called the grana

The membrane of the thylakoid contains chlorophyll which forms photosynthesis units called quantasomes. This membrane carries out light reaction

The chloroplast is located on the outer margin, parallel to the cell wall of the mesophyll cells. They have a broad surface. This position helps in easy diffusion of CO2 from the atmosphere to the chloroplast.
 

Chloroplast:

Chloroplast are green colour plastids which can be of any shape depending on the plant. It can be star-shaped, ribbon-shaped, cupshaped, etc.. Chloroplast is only found in plants and protista. It is made up of lipoproteins. It is a semi-autonomous cell organelle. The chloroplast contains proteins, lipids, chlorophyll, pigments, vitamins and certain metal ion traces


It consist of two membranes - Outer and inner membrane and both these membranes as a whole is known as peristromium. Each membrane is 4 to 6nm thick and the space between both the membranes is of 25 to 75 Å

In between the two membrane there is some space called the periplastidal space or outer chamber. The portion on the inside is called the inner chamber which consist of stroma which is a colourless and colloidal fluid. 

There is presence of enzymes in the stroma which takes active part in the process of photosynthesis. It contains DNA, RNA, 70S type of ribosome and osmophilic granules of vitamins K. Stroma is the site for dark reaction. The DNA is called as plastidome and is circular, closed and is a naked ring. Since chloroplast has nuclei acid it is known as an organelle which can self replicate and self regulate.

Inside the chloroplast, thyalkoids are stacked on one another which makes a grana. The thylakoids are membrane bound compartments in the chloroplast. The grana is where light reaction and ATP synthesis takes place. The grana is interconnected to each other by membranous lamellae called stroma lamellae or Fret channels

The thylakoids have a grainy appearance as there are small units of quantasomes present on it. These quantasomes are called the photosynthetic units because it contains chlorophyll-a, chlorophyll-b, carotenes and xanthophylls which forms a pigment system and also these pigments are important for photosynthesis.  

Mechanism of photosynthesis:

The mechanism of photosynthesis has two stages - Light dependent reaction (Light reaction) and light independent reaction (Dark reaction). 

Light dependent reaction:

It is the first phase of photosynthesis which takes place in the grana of the chloroplast. It is also called as light reaction as it only takes place when there is sufficient light in order to trap it. It was discovered by Arnon and he said that light reaction takes place in two steps - Cyclic photophosphorylation and non-cyclic photophosphorylation

Photophosphorylation is synthesis of ATP from ADP and inorganic phosphate (iP) by the help of solar energy (the energy which comes from the sun). 

Non-cyclic photophosphorylation:

It involves photolysis of water and the flow of electrons is only in one direction. The electron flow is continuous. It flows from water to PS-II, from PS-II to PS-I via cytochrome. Cytochrome are redox-active protein which are involved in transportation of electrons. 


The electron which was expelled from the reaction centre will never come back to the same reaction centre. Non-cyclic photophosphorylation is more efficient than cyclic photophosphorylation as the end product is both ATP and NADPH2. 

Cyclic photophosphorylation:

Cyclic photophosphorylation involves only one photosystem i.e. PS-I and the flow of electron is in a cyclic manner. Meaning, the electron expelled from the reaction action centre will come back to the same reaction centre unlike non-cyclic photophosphorylation. Photolysis of water is not involved. 


Photolysis of water:

Splitting of water molecule under the presence of light is called photolysis of water. Magnesium, Calcium and chloride ions play an important role in the process of photolysis of water. ATP and NADPH2 are formed in this process and it also evolves oxygen

This is only involved in non-cyclic photophosphorylation. It takes place under favourable conditions like aerobic conditions, high CO2 concentration and enough light intensity. It is seen in all plants which evolve O2 but is absent in bacteria. 

Light independent reaction:

This is the second phase of photosynthesis and is also known as dark reaction. It takes place in the stroma of the chloroplast and uses the assimilatory power which was created in the first phase of photosynthesis, hence it can be said that sunlight is used indirectly. 

Suggested article: Light independent reaction

The main aim of this process is to form carbohydrates from CO2 by reduction by the help of the energy which is provided by assimilatory power. It is also known as CO2 reduction, CO2 assimilation, CO2 fixation or Carbon reaction

In different plants CO2 is fixed in different ways. There are three main types - Calvin cycle, Hatch-Slack cycle and CAM cycle

Calvin cycle:

Calvin cycle is found in most of the plants and the plants which carry out this cycle are known as C3 plants. This cycle was proposed by Dr. Melvin Calvin. He used a radioactive isotope of carbon (C14) to determine the sequence of dark reaction. 


The first stable product formed in this cycle is a 3 carbon compound which is phosphoglyceric acid. In this process CO2 is fixed to form carbohydrates hence its known as CO2 fixation process. There are four major steps of this process, which are as follows:
  1. Carboxylation of RuBP 
  2. Reduction of PGA 
  3. Sugar formation and glycolytic reversal 
  4. Regeneration of RuBP. 
Hatch-Slack cycle:

Hatch-Slack cycle is to avoid the process of photorespiration and the loss of photosynthetic pigments. This cycle is also called C4 cycle and the plants which carry out this cycle are known as C4 plants. This cycle is more efficient than C3 cycle. 



CAM cycle:

CAM stands for Crassulacean Acid Metabolism. In CAM plants the metabolism is different at day and at night. Following are the steps which takes place at night in CAM plants:
  1. The stomata is open at night which takes in CO2
  2. This CO2 is picked by PEP which then forms 4-carbon oxaloacetic acid which is then reduced to 4-carbon malic acid
  3. The formed malic acid accumulates in the centre of the vacuole at night and stays there until the day break. 
Following are the steps which takes place in day in CAM plants:
  1. The stomata closes in the day. 
  2. The malic acid which was formed at night is now used as a source of CO2 in day. 
  3. After a while the malic acid leaves the vacole and is broken down to form pyruvate, releasing CO2 which is taken up by Calvin cycle for the synthesis of sugar. 
  4. Pyruvate may also form sugar by the process of reverse glycolytic reaction


Significance of photosynthesis:

Photosnythesis plays a very important role in order to keep all the living organisms alive. It produces food not only for itself but also it is available to all the other heterotrophic organisms which rely on this process. All living organisms, directly or indirectly, depend on the process of photosynthesis. 

It releases O2 which is needed to survive for all aerobic organisms, maintains the balance of CO2 and O2 in the atmosphere and also helps to purify the air by taking in CO2 and giving out O2. It provides, hiding and nesting place for animals. There would be no ecosystem without photosynthesis. 

Economically important products like rubber, resins, timber, cotton, etc. are formed by the process of photosynthesis. Apart from this, it also provide coal, petroleum and natural gas which are again economically important. 

The molecules of O2 which are released during this process, form a layer of ozone, 15 to 35 km above the earth's surface which absorbs harmful ultraviolet rays and stops it from entering the earth's surface. These rays can cause many diseases like skin cancer, etc.. 

Handwritten notes:

Here are my handwritten notes for the chapter Photosynthesis:

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