Photosynthesis

Photosynthesis is a process that first occurred nearly 3 billion years ago. And the very first photosynthetic organisms were not plants, but organisms called stromatolites, which were similar to today’s cyanobacteria.

Prior to the emergence of these organisms, the earth’s atmosphere was devoid of oxygen, making life almost impossible. But life did exist in the form of anaerobic microorganisms, and these were extremophiles.

However, these anaerobic microorganisms began to die out as these ancient photosynthetic organisms began producing oxygen, which was particularly toxic to these microbes. And since then, oxygen started to gradually accumulate in the earth’s atmosphere, rising up to the levels we are familiar with today.

Life on earth would have been completely different, if not for these ancient photosynthetic organisms.

Table of Contents

Photosynthesis Definition

Photosynthesis is defined as a biological process used by all green plants and a few other organisms to synthesize nutrients by using carbon dioxide and water. The by-product of this process is oxygen.

The word “photosynthesis” is derived from the Greek words phōs (pronounced: “fos”) and σύνθεσις (pronounced: “synthesis“)

Phōs means “light” and σύνθεσις means, “combining together.” This means “combining together with the help of light.”

The definition of photosynthesis also applies to organisms besides green plants. These include several prokaryotes such as cyanobacteria, purple bacteria and green sulfur bacteria. These organisms exhibit photosynthesis just like green plants.

Photosynthesis Diagram

Photosynthesis diagram highlighting the requirements (sunlight, water and CO2) and by-products (oxygen)

This biological process is also used by algae to convert solar energy into chemical energy. Oxygen is liberated as a by-product, and light is considered as the major factor to complete the process of photosynthesis.

This process occurs usually when plants use light energy to convert carbon dioxide and water into glucose and oxygen gas. Leaves are made up of small cells which have tiny structures known as chloroplasts.

Each chloroplast contains a green coloured pigment called chlorophyll. Light energy is absorbed by chlorophyll molecules whereas carbon dioxide and oxygen enter through the tiny pores of stomata located in the epidermis of leaves.

Oxygen is considered to be one of the most important by-products of this process. Nearly all living organisms on earth depend upon this essential element. Another by-product of this process is sugars such as glucose and fructose.

These sugars are then sent to the roots, stems, leaves, fruits, flowers, and seeds. In other words, these sugars are used by the plants as an energy source, which helps them to grow. These sugar molecules then combine with each other to form more complex carbohydrates like cellulose and starch. The cellulose is considered as the structural material that is used in plant cell walls.

Photosynthesis Equation

Photosynthesis reaction involves two reactants, carbon dioxide and water. These two reactants yield two products, namely, oxygen and glucose.

Photosynthesis Equation: 6CO2    +   6H2O  —>  C6H12O6  + 6O2

Unlike plants, some photosynthetic bacteria do not produce oxygen as the by-product of photosynthesis. Such bacteria are called anoxygenic photosynthetic bacteria. The bacteria that do produce oxygen as a by-product of photosynthesis are called oxygenic photosynthetic bacteria.

Photosynthesis Reaction

A visual representation of the photosynthesis reaction

 

Process of Photosynthesis

At the cellular level,  the process of photosynthesis takes place in cell organelles called chloroplasts. These organelles contain a green coloured pigment called chlorophyll, which is responsible for the characteristic green colouration of the leaves.

Structure of Chloroplast

Structure of Chloroplast. Note the presence of the thylakoid

 

Where does photosynthesis take place?

As already stated, photosynthesis occurs in the leaves and the specialized cell organelles responsible for this is the chloroplast. Structurally, a leaf comprises a petiole, epidermis and a lamina. The lamina is used for absorption of sunlight and carbon dioxide during the process.

During the process of photosynthesis, carbon dioxide enters through the stomata, water is absorbed by the root hairs from the soil and is carried to the leaves through the xylem vessels. Chlorophyll absorbs the light energy from the sun to split water molecules into hydrogen and oxygen.

The hydrogen from water molecules and carbon dioxide absorbed from the air are used in the production of glucose. Furthermore, oxygen is liberated out into the atmosphere through the leaves as a waste product.

Glucose is a source of food for plants which provide energy for the growth and development, while the rest is stored in the roots, leaves, and fruits for their later use.

Pigments are other fundamental cellular components of photosynthesis. They are the molecules which impart colour and they absorb light at some specific wavelength and reflect back the unabsorbed light. All green plants mainly contain chlorophyll a, chlorophyll b, and carotenoids which are present in the thylakoids of chloroplasts and help them in capturing the light energy. Chlorophyll-a is the main pigment.

 

Structure of Chlorophyll

Structure of chlorophyll

Structure of Chlorophyll consists of 4 nitrogen atoms that surround a magnesium atom. A hydrocarbon tail is also present

Stages of Photosynthesis

Chlorophyll is a green pigment found in the chloroplasts of the plant cell and in the mesosomes of cyanobacteria. This green colour pigment plays a vital role in the process of photosynthesis by permitting plants to absorb energy from sunlight. Chlorophyll is a mixture of chlorophyll-a and chlorophyll-b.

The structure of the chlorophyll is similar to the structure of haemoglobin. It is a double membrane organelle, with the central atom magnesium, bonded by large organic molecules composed of carbon, hydrogen, oxygen and nitrogen. Photosynthesis comprises two phases

Stages of Photosynthesis

Stages of Photosynthesis in Plants depicting the two phases – Light reaction and Dark reaction

 

Light-dependent reaction (or) Light reaction

Photosynthesis begins with the light reaction which is carried out only during the day in the presence of sunlight. In plants, the light-dependent reaction takes place in the thylakoid membranes of chloroplasts. The Grana, membrane-bound sacs like structures present inside the thylakoids functions by gathering light and is called photosystems.

These photosystems have large complexes of pigment and proteins molecules present within the plant cells which plays the primary role during the process of light reactions.  There are two types of photosystems: photosystem I and photosystem II.

Under the light-dependent reactions, the light energy is converted to ATP and NADPH which are used in the second phase of photosynthesis. During the light reactions, ATP and NADPH are generated by two electron transport chains, water is used and oxygen is produced.

The chemical equation in the light reaction can be reduced to:

2H2O + 2NADP+ + 3ADP + 3Pi → O2 + 2NADPH + 3ATP

Light-independent reaction (or) Dark reaction

 

 

Calvin cycle

Calvin Cycle (Dark Reaction)

 

Dark reaction is also called carbon-fixing reaction. It is a light-independent process in which sugar molecules are formed from the water and carbon dioxide molecules. The dark reaction occurs in the stroma of the chloroplast where they utilize the NADPH and ATP products of the light reaction.

Plants capture the carbon dioxide from the atmosphere through stomata and proceed to the Calvin cycle. In the Calvin cycle, the ATP and NADPH formed during light reaction drive the reaction and convert 6 molecules of carbon dioxide into one sugar molecule or glucose.

The chemical equation for the dark reaction can be reduced to:

3CO2 + 6 NADPH + 5H2O + 9ATP → G3P + 2H+ + 6 NADP+ + 9 ADP + 8 Pi

* G3P – glyceraldehyde-3-phosphate

Importance of Photosynthesis

  • It is essential for the existence of life on planet earth both as a source of food, shelter, and energy.

  • This process is responsible for supplying all of the earth’s organic compounds and the energy required to sustain life on the planet.

  • It is also responsible for maintaining and balancing the atmosphere’s oxygen and carbon dioxide levels and protects our planet from global warming

Factors affecting Photosynthesis

There are several factors that are required at an optimum level for a constant rate of photosynthesis.

  • Light Intensity: Increased light intensity results in the high rate of photosynthesis and low light intensity would be considered as a low rate of photosynthesis.

  • The concentration of CO2: Higher concentration of carbon dioxide helps in increasing the rate of photosynthesis. Usually,0.03 to 0.04 per cent concentration of carbon dioxide is adequately for photosynthesis.

  • Temperature: For efficient execution of the process, it is important to have an optimum temperature that ranges between 25° to 35° C.

  • Water: As water is an important factor in this process, lack of water can lead to a problem of intake of carbon dioxide. The scarcity of water leads to the refusal of stomatal opening to retain the amount of water they have stored inside.

  • Pollution: Industrial pollutants and other particles may settle on the leaves surface. This can it blocks the pores of stomata which makes it difficult to take in carbon dioxide.

Just like how we Human beings and animals consume food to gain energy to stay alive and active, plant do synthesis their own food through the Biological process of Photosynthesis.

Important Questions on Photosynthesis:

1. What is Photosynthesis?

Photosynthesis is a biological process utilized by all green plants to synthesise their own nutrients. The process requires solar energy and other components such as water and carbon dioxide. The by-product of this process is oxygen

2. What is the significance of Photosynthesis?

During this process, oxygen gas is liberated out into the environment and is utilised by humans, animals, and other living species during the process of respiration.

3. List out the factors influencing Photosynthesis?

There are several factors that affect the rate of photosynthesis. Light intensity, water, soil pH, carbon dioxide concentration, temperature and other climatic conditions are the main factors affecting the rate of photosynthesis.

4. What are the different stages of Photosynthesis? 

Photosynthesis takes place in two stages:

  • Light-dependent reactions also called the light reaction and occur during the day time

  • Light-independent reaction also called the dark reaction.

5. What is the Calvin cycle?

The Calvin cycle is also called the light-independent reactions. The complete process of the Calvin cycle takes place in the stroma of the chloroplasts.

6. Write down the Photosynthesis Equation.

6CO2    +   6H2O  —>  C6H12O6  + 6O2

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During what stage of photosynthesis is O2 produced?