Photosynthesis: Location of Pigments, Chloroplast, Photosystems, Pigments, Practice Problems and FAQs 

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.

ALSO READ: What is plant? 

Let’s understand more about these pigments and photosystems in plants. 

Table of Contents

• Location of Pigment
• Chloroplast
• Photosystems
• Types of Photosystems
• Practice Problems of Photosynthesis an Overview
• FAQs Photosynthesis an Overview

Location of Pigment

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 

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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Photosystems

They are a complex of proteins, photopigments, and organic molecules embedded in the thylakoid membrane of chloroplasts. It helps in capturing the sunlight. 

Components of Photosystems

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. 

Reaction Centre 

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. 

Antenna Complex

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. 

Types of Photosystems 

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:

• Photosystem I
• Photosystem II

Difference Between Photosystem I and II

Photosynthetic Pigments

They have the ability to absorb light energy at specific wavelengths. They are of two types as follows: 

• Chlorophylls 
• Carotenoids

Chlorophylls 

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.

Structure of Chlorophyll Molecules

 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. 

Carotenoids

They are yellowish to orange pigments associated with the chlorophylls. They are of two types as follows:

• Carotenes 
• Xanthophylls

Carotene 

They are orange coloured pigments. They are considered as the precursor of vitamin A. 

Xanthophylls 

They are yellow coloured pigments which act as accessory pigments. 

Practice Problems of Photosynthesis an Overview 

Question 1. Photosynthesis occurs in

a. Chloroplast
b. Golgi body
c. Endoplasmic reticulum
d. Nucleus

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

a. Pheophytin
b. Ferredoxin
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. 

FAQs of Photosynthesis an Overview 

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:

• Absorption of light energy by the photosynthetic pigment chlorophyll. 
• Conversion of light energy into chemical energy form, splitting of the water molecules into hydrogen and oxygen. 
• Reduction of carbon dioxide leads to the formation of carbohydrates like glucose.
•  

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:

• Water
• Carbon dioxide
• Light
• Chlorophyll

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.

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