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Photosynthesis : Location (Chloroplast),Structures and Resonance Transfer
Can you imagine a life without plants around us?
Never, because they carry out the great process of photosynthesis. It is the process which holds the life of organisms on the planet Earth.
Do you know the location of photosynthesis? Does photosynthesis take place only in green leaves?
Of course the answer is no. Photosynthesis also occurs on the other parts of the plant where chloroplast is present like a stem. So chloroplast is the site of photosynthesis.
Just imagine the size of chloroplast, it acts as a factory and carries out the great process of photosynthesis.
They are really awesome. Right?
So let's discuss the location of photosynthesis ie; chloroplast and their structure.
Table of Contents:
Location of Photosynthesis
Photosynthesis is the biological process in which an organism is capable of synthesising its own food using carbon dioxide and water in the presence of sunlight and chlorophyll. The location of photosynthesis is the chloroplast.
Chloroplast contains pigments like chlorophyll which can absorb the sunlight. Along with the chlorophyll, many other pigments also help in photosynthesis. So now we know that the location of photosynthesis is the chloroplast. Do chloroplast is present in every cell? To find the answer for this question let's observe the cross section of a dicot leaf.
You can find mesophyll cells in between the epidermal layers on leaves. These are the cells which contain chloroplast. So we can say that the photosynthesis takes place in chloroplasts present inside the mesophyll cells.
Mesophyll Cells
Leaves contain specialised cells called mesophyll cells which contain a large number of chloroplasts. They are a part of ground tissue and play an important role in photosynthesis. Because these cells contain many chloroplasts. Those parenchyma cells with chloroplast are called Chlorenchyma cells. We can find 20 – 100 chloroplasts that are present per cell. Chloroplasts contain photosynthetic pigments which are aligned towards the wall of the cells. So that they can receive the optimum quantity of sunlight.
Mesophyll Cells of Dicot and Monocot Leaves
Mesophyll cells of dicot and monocot leaves are different.
Mesophyll Cells of Dicot Leaves
Two types of mesophyll cells are present in the dicot leaf. They are the palisade parenchyma and spongy parenchyma.
Palisade parenchyma
Palisade parenchyma is present on the adaxial surface of the leaf. They possess columnar cells. The cells are arranged vertically and parallel to each other. They possess chlorophyll and perform photosynthesis.
Spongy parenchyma
Spongy parenchyma are oval or round cells. They are loosely arranged between palisade parenchyma and lower epidermis. They possess large intercellular spaces and air cavities. It allows air circulation inside the leaf. They possess few chloroplasts.
Mesophyll Cells of Monocot Leaves
Mesophyll cells of monocot leaves are also present between upper and lower epidermis. But they are not differentiated into spongy and palisade parenchyma. They only have a single type of cells made of Chlorenchyma cells. They form the photosynthetic tissue of the leaf.
Chloroplast
Chloroplast is the site of photosynthesis. It is seen inside the chlorenchyma cells of mesophyll tissues. Both the light and dark reactions are taking place here.
Structure of Chloroplast
Chloroplast is a double membranous structure. Between the outer and inner membrane it encloses the periplastidial space. Inside the chloroplast we can observe the membranous thylakoids that are immersed in the stroma. The division of labour is clearly visible inside the chloroplast.
Stroma is the matrix of the chloroplast where the synthesis of sugar takes place. Thylakoids are the coin-like structures containing chlorophyll. Each thylakoid has quantasomes. Each quantasome contains about 230 - 250 chlorophyll molecules. Each thylakoid is stacked one over the other, like the stacks of coins to form grana. The number of grana per chloroplast is 10 to 100. Each granum possesses 2 - 100 thylakoids. Pigments are positioned differently in the grana as photosystems.
Thylakoids are responsible for trapping the light. The energy needed for the dark reactions, ATP and NADPH are synthesised in the thylakoid membranes. Chloroplast is a semi autonomous organelle, it possesses its own DNA and 70S ribosomes.
Photosystems
If we zoom into the thylakoid membrane we could find the green structures. They are called Photosystems. Photosystems are a complex of proteins, photopigments and organic molecules embedded in the thylakoid membrane. There are 2 photosystems that take part in photosynthesis. Photosystem I and Photosystem II.
The two major parts of a photosystem are antenna complex or light harvesting complex (LHC) and reaction centre. LHC contains photopigments, proteins and accessory pigments (chlorophyll b, carotenoids or xanthophylls). Accessory pigments can absorb lights of different wavelengths. Reaction centre consists of the chief pigment ie; chlorophyll a.
Resonance transfer
The accessory pigments will absorb the light energy directly from the Sun. It forms part of the antenna complex. This energy is now transferred to the chlorophyll a in the reaction centre. Hence the antenna systems function to deliver energy efficiently to the reaction centres with which they are associated. Later this energy should flow through the PSI and PSII to produce the energy (ATP and NADPH) for the synthesis of sugar.
This process of energy transfer can be explained with the help of a tunic fork.
Experiment Using Tunic Fork
If you have done the experiments with a tuning fork, then you might have observed the resonance energy transfer between the forks. What happens if one tuning fork is struck and properly placed near another?
The second tuning fork will receive some energy from the first and will begin to vibrate. This is due to the transfer of resonance energy. The efficiency of the transfer of energy between the two tuning forks depends on their distance from each other. It also depends on their relative orientation, as well as their pitches or vibrational frequencies.
The resonance energy transfer in antenna complexes also takes place in a similar way as in the tuning fork. When the light hits on the photopigments of the antenna complex, then it will start to transfer energy to the next photopigment molecule. This process continues until the energy finally reaches the chlorophyll a of the reaction centre.
Practice Problems of Photosynthesis
1. Study the diagram carefully and label A, B, and C respectively?
a. Lumen, granum, and stroma
b. Granum, stroma, and lumen
c. Stroma, granum, and lumen
d. Stroma, lumen, and granum
Solution: The given diagram is the structure of a chloroplast. It is a double membranous structure. The fluid matrix enclosed by the inner membrane of the chloroplast is normally called the stroma. It is considered as the site of the dark reaction of photosynthesis. Disc-shaped structures called thylakoids are stacked above one another to form the granum and are present in the stroma. Each granum is interconnected by the stroma lamella. The space enclosed by the thylakoid membrane is considered as lumen. Photophosphorylation takes place inside the lumen of thylakoids. Hence the correct option is c.
2. Choose the correct statements about chloroplast:
1. The only double membrane bound organelle in the plant cells.
2. Contains stroma, grana and stroma lamellae.
3. iii. Sugar is synthesised in the stroma.
4. Membrane system traps the light energy.
a. Only i and ii is correct
b. Only iii and iv are correct
c. All are correct
d. All are correct except i
Solution: Chloroplast is not the only double membrane-bound organelle in the plant cells. Mitochondria, nucleus, chloroplasts are examples of double membrane-bound organelles present in the plant cells. Hence the correct option is d.
3. The accessory pigments of photosystems like chlorophyll b, xanthophylls and carotenoids play a role in _______________.
a. protecting chlorophyll a from photo-oxidation
b. enabling the chloroplast in utilising a wider range of wavelengths of incoming light
c. absorbing some amount of light and then transfer the energy to chlorophyll a
d. all of the above
Solution: The process of generation of light-dependent active oxygen species is known as photo-oxidation and it can damage chlorophyll a too. The accessory pigments could protect chlorophyll a from photo-oxidation. They also absorb a wide range of wavelengths of light, thus enabling the chloroplast in utilising a wider range of wavelengths of incoming light. The accessory thylakoid pigments like chlorophyll b, xanthophylls, and carotenoids also absorb light in the wavelengths where chlorophyll a cannot absorb and they transfer the energy to chlorophyll a. Hence the correct option is d.
4. Where does the sugar synthesis take place in a chloroplast?
a. Grana
b. Stroma
c. Stroma lamellae
d. Inner membrane
Solution: Sugar is synthesised in the stroma of the chloroplast which is later converted to starch.
Grana, stroma, stroma lamellae and inner membranes are some of the other components of the chloroplast. Hence the correct option is b.
FAQs of Photosynthesis
Question 1. What are the differences between the dicot and monocot mesophyll cells?
Answer: Two types of mesophyll cells are present in the dicot leaf. They are palisade parenchyma and spongy parenchyma. Palisade parenchyma are columnar cells that are present on the adaxial surface. Spongy parenchyma are oval or round cells. They are loosely arranged between palisade parenchyma and lower epidermis. Mesophyll cells of monocot leaves are also present between upper and lower epidermis. But they are not differentiated into spongy and palisade parenchyma.
Question 2. Explain the structure of chloroplast?
Answer: Chloroplast is a double membranous structure. It is a semi autonomous organelle and possesses its own DNA. It possesses 70S ribosomes. Inside the chloroplast there are membranous thylakoids that are immersed in the stroma. The division of labour is clearly visible inside the chloroplast. Stroma is the matrix of the chloroplast where the synthesis of sugar takes place. Thylakoids are the coin-like structures containing chlorophyll. Each thylakoid has quantasomes. Each quantasome contains about 230 - 250 chlorophyll molecules. Each thylakoid is stacked one over the other to form grana. The number of grana per chloroplast is 10 to 100. Each granum possesses 2 - 100 thylakoids. Pigments are positioned differently in the grana as photosystems.
Question 3. What do you mean by resonance transfer in a photosystem?
Answer: The resonance energy transfer in antenna complexes of photosystems also takes place in a similar way as in the tuning fork. When the light hits on the photopigments of the antenna complex, then it will start to transfer energy to the next photopigment molecule. This process continues until the energy finally reaches the chlorophyll a of the reaction centre.
Question 4. What is a photosystem?
Answer: The green structures on the thylakoid membranes are called Photosystems. Photosystems are a complex of proteins, photopigments and organic molecules embedded in the thylakoid membrane. There are 2 photosystems that take part in photosynthesis. Photosystem I and Photosystem II. The two major parts of a photosystem are antenna complex or light harvesting complex (LHC) and reaction centre.
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