Difference Between Photosystem 1 And Photosystem 2: Explanation, Types, Steps And FAQs

What are Photosystems?

Photosystems are the pigment-protein complexes located in the thylakoid membranes of chloroplasts. They play a critical role in capturing the energy from light to drive electron transport, which results in the formation of ATP and NADPH. These are products essential for the Calvin cycle and glucose synthesis in plants. There are two types of photosystems: Photosystem 1 and Photosystem 2, which differ in their function and composition.

Photosystems are a very basic concept both in plant physiology and bioenergetics. The significance of photosystems is not only associated with their role in the mechanism of photosynthesis but has far-reaching applications in bioengineering or biological engineering projects like artificial photosynthesis in research, photovoltaic systems development for renewable energy, etc.

Structure of Photosystems

The structure of photosystems is complex and highly specialised. They include the capture and conversion of light energy to the extreme. Each photosystem is the optimal structure for carrying out the light-dependent reactions as well as absorbing the light it needs during its particular range of wavelengths.

A photosystem consists of following components:

Photosystem I (PSI) – Structure and Function

The key features of photosystem I are:

• Reaction center pigment: P700 (absorbs at 700 nm, far-red region).

• Core complex: 6 to 25 chlorophyll molecules and several proteins called the core antenna complex.

• Primary electron acceptor: An iron-sulfur protein (ferredoxin).

• Location: Found in the stroma lamellae.

• Reaction: NADP⁺ + H⁺ + 2e⁻ → NADPH

Photosystem II (PSII) – Structure and Function

The key features of photosystem II are:

• Reaction center pigment: P680 (absorbs at 680 nm, red region).

• Core complex: There is significant variation in the chlorophyll and carotenoid pigments.

• Primary Electron Acceptor: Plastoquinone.

• Special feature: Contains Oxygen Evolving Complex (OEC) which is responsible for water-splitting and evolution of oxygen.

• Location: Found in the thylakoid membrane, across the stack of grana.

• Reaction: 2H₂O → 4H⁺ + 4e⁻ + O₂

Table – Difference Between Photosystem I and Photosystem II

The difference between photosystem I and photosystem II is included in the table below:

Mechanism of Photosystem Function (Z-Scheme Overview)

The electron transport chain of photosystems is a cascade of redox steps that process light energy, eventually converting it into the chemical energy in the coenzymes ATP and NADPH.

• PS II absorbs light energy of wavelength 680 nm and transfers electrons to plastoquinone (PQ).

• Water molecules split to replace lost electrons and release oxygen.

• Electrons donated to plastocyanin (PC) and cytochrome b6f complex.

• PS I absorb light by P700, electrons get excited and transport to the primary electron acceptor, ferredoxin.

• Catalysation of the formation of NADPH by the enzyme ferredoxin-NADP⁺ reductase.

• ATP synthesised via ATP synthase due to proton gradient.

Photosystem Variations in Different Plant Types

Photosystems may have slight variations from one type of plant to the other, making some more efficient or compatible with certain environments than others.

Photosystem 1 vs Photosystem 2 NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Types of Photosystems (PSI & PSII)

• Mechanism of Photosystem

• PS I vs PS II

Practice Questions for NEET

Q1. In photosynthesis, oxygen is produced by

1. Photosystem I from carbon dioxide.

2. Photosystem II from carbon dioxide.

3. Photosystem I from water.

4. Photosystem II from water.

Correct answer: 4) Photosystem II from water.

Explanation:

The splitting of water is associated with PS II. The water splits into H+, [O], and electrons. This produces oxygen, one of the net products of photosynthesis. The water splitting complex is associated with PS II, which itself is physically located on the inner side of the membrane of the thylakoid. The electrons required to replace removed electrons from photosystem I are provided by photosystem II.

2H2O ------ 4H+ + O2 + 4e-

Hence, the correct answer is option 4) photosystem II from water.

Q2. PS- I comprised of

a) p - 680, chlorophyll a - 695, p -700, chlorophyll - b and carotenoids

b) Chlorophyll a - 670, chlorophyll a - 690, chlorophyll b and phycobilin

1. Both a and b are correct

2. Both a and b are incorrect

3. a is correct

4. b is correct

Correct answer: 3) a is correct

Explanation:

Light wavelengths have the pigments affected by them divided into two; PS-I and PS-II. PS- I comprise p - 680, chlorophyll a - 695, p -700, chlorophyll - b and carotenoids PS - II consists of chlorophyll a - 670, chlorophyll a - 690, chlorophyll b and phycobilin

Hence, the correct answer is option 3) a is correct.

Q3. ____________is a light-harvesting complex made up of proteins and numerous chlorophyll a, chlorophyll b, and carotenoid molecules embedded in the thylakoid membrane of plants.

1. Antenna Complex

2. Reaction Centers

3. Energy-fixing reaction

4. Chemiosmosis

Correct answer: 1) Antenna Complex

Explanation:

Antenna Complex is a light-harvesting complex made up of proteins and numerous chlorophyll a, chlorophyll b, and carotenoid molecules embedded in the thylakoid membrane of plants.

The photosystem's reaction center is where energy from absorbed light is directed to excite the molecules of chlorophyll.

Moving ions across a semipermeable membrane to produce ATP is known as chemiosmosis. Chemiosmosis is the process by which hydrogen ions move from the thylakoid space into the stroma via ATP synthase during photosynthesis.

Hence, the correct answer is option 1) Antenna complex is a light-harvesting complex made up of proteins and numerous chlorophyll a, chlorophyll b, and carotenoid molecules embedded in the thylakoid membrane of plants.

Also Read:

• MCQs on Photosystems

• An Overview of Photosystem

• C4 and CAM Pathway