Why are the leaves of a plant mostly green in colour?
What do plants eat? Do they eat food like us?
Do these questions come to your mind also?
Plant leaves are mostly green in colour because they cannot absorb green colour rather they reflect it. They also have a green coloured pigment known as chlorophyll in their cells.
Plants have a self built kitchen which is able to synthesise food for them, in which using some core ingredients like CO2, water and sunlight energy, it synthesises sugars and O2. This O2 is utilised in respiration by living beings.
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Let’s understand more about these pigments and photosystems in plants.
Table of Contents
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. 20 – 100 chloroplasts are present per cell normally. Chloroplasts contain photosynthetic pigments which are aligned towards the wall of the cells such that they receive the optimum quantity of sunlight.
Chloroplast is covered by an envelope, which is made up of double membrane. A periplastidal space is present between the two membranes. Stroma is the matrix which is enclosed by the membrane. Inside the stroma a membrane system called lamellae or thylakoids are present. It contains DNA, RNA, ribosomes, enzymes, proteins, fat globules and starch grains. Chloroplast DNA is circular and naked. It contains 70S ribosomes. Thylakoids are flattened membranous sacs. The membranes of thylakoids are called fret membranes. Photosynthetic pigments are embedded in the thylakoid membranes. Thylakoids at places are aggregated to form stacks of coin-like structures called grana. The thylakoids present outside the grana are called intergranal thylakoids or stroma thylakoids. 40 - 60 grana normally present in a chloroplast. The space enclosed by the grana is called loculus.
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They are a complex of proteins, photopigments, and organic molecules embedded in the thylakoid membrane of chloroplasts. It helps in capturing the sunlight.
They are made up of pigment molecules and proteins. Each photosystem has a photo centre or reaction centre which is surrounded by more than 200 pigment molecules.
The photo centre has a special chlorophyll a molecule. The reaction centre has an absorption peak which differs in the photosystems. The PS I has an absorption peak at 700 nm and the PS II has an absorption peak at 680 nm.
The light harvesting pigment molecules are of two types namely antenna molecules and core molecules.
Core Molecules - They pick up energy from antenna molecules and pass it to the photocentre.
Antenna Molecules - They absorb light of various wavelengths but shorter than that of photocentre. On absorption of light energy, they get excited and push electrons to their outer orbital. The excited molecules pass their energy to core molecules by resonance energy transfer.
There are 2 photosystems that take part in photosynthesis. They are named based on their discoveries instead of the sequence of functions in photosynthesis as follows:
Chlorophyll a, present in the reaction centre, has an absorption maxima at 700 nm.
Chlorophyll a, present in the reaction centre, has an absorption maxima at 680 nm.
Known as P700 based on the absorption wavelength
Known as P680 based on the absorption wavelength
Lies in the stacked regions of the thylakoid membrane.
Lies in the unstacked regions along with the ATP synthase and protrudes into stroma lamellae
Molecular oxygen is not released
Molecular oxygen is released by photolysis of water
Participates in both cyclic and non-cyclic photophosphorylation
Participates only in non-cyclic photophosphorylation
They have the ability to absorb light energy at specific wavelengths. They are of two types as follows:
They are of five types a, b, c, d and e. Chlorophyll a is found in all photosynthetic organisms except bacteria. It is called a universal or primary photosynthetic pigment. The other chlorophyll molecules are considered accessory pigments as they transfer light energy to chlorophyll a.
A chlorophyll molecule has a porphyrin head and a phytol tail. Porphyrin head is made up of four pyrrole rings which are linked by methine groups. The centre possesses an Mg2+ion.
They are yellowish to orange pigments associated with the chlorophylls. They are of two types as follows:
They are orange coloured pigments. They are considered as the precursor of vitamin A.
They are yellow coloured pigments which act as accessory pigments.
Question 1. Photosynthesis occurs in
b. Golgi body
c. Endoplasmic reticulum
Solution: Photosynthesis occurs in the plant cell organelles called chloroplast. They are double membrane structures. Hence option a is correct.
Question 2. The electrons from the excited chlorophyll molecules of Photosystem II are first accepted by
c. Cytochrome f
d. Cytochrome b
Solution: Pheophytin is the first electron carrier that accepts electrons from the excited chlorophyll molecules of Photosystem II or PS II. Hence option a is correct.
Question 3. Maximum photosynthesis occurs in
a. Blue light
b. Red light
c. White light
d. Green light
Solution: Maximum photosynthesis occurs in the red light region of the visible spectrum. Hence option b is correct.
Question 4. The optimum temperature for photosynthesis is
a. 5 - 35℃
b. 10 - 15℃
c. 35 - 40℃
d. 20 - 25℃
The optimum temperature for photosynthesis is 20 - 25℃. Hence the correct answer is option d.
Question 1. What are the 3 major events of photosynthesis?
Solution : The three major events that occur during the process of photosynthesis are as follows:
Question 2. What do you understand about the term ‘photosynthesis’?
Solution: The term ‘photo’ means ‘sunlight’ and ‘synthesis’ means ‘to produce’.
Thus by photosynthesis, plants synthesise sugar and energy utilising water, air and sunlight in the presence of chlorophyll.
Question 3. Which four raw materials are used in photosynthesis?
Solution: The raw materials required for photosynthesis are as follows:
Question 4. Write the equation for photosynthesis?
Solution: Photosynthesis is the process in which plants take in CO2, water and convert them into organic compounds (glucose) in the presence of sunlight and chlorophyll.
|The Living World||Biological Classification||Plant Kingdom|
|Animal Kingdom||Morphology of Flowering Plants||Anatomy of Flowering Plants|
|Structural Organization in Animals||Cells: The Unit of Life||Biomolecules|
|Cell Cycle and Division||Transport in Plants||Mineral Nutrition|
|Photosynthesis in Higher Plants||Respiration in Plants||Plant Growth and Development|
|Digestion and Absorption||Breathing and Exchange of Gases||Body Fluids and Circulation|
|Excretory Products and their Elimination||Locomotion and Movement||Neural Control and Coordination|
|Chemical Coordination and Integration|