Question

Light reaction and photochemical phase includes (a) Light absorption (b) Water splitting and release of oxygen (c) ATP and NADPH formation (d) All of these

Short answer

The correct answer is (d) All of these. This is because, in the light reaction and photochemical phase of photosynthesis, light is absorbed (with chlorophyll as the main pigment responsible for this), water molecules are split in a process known as photolysis (from which oxygen is released as a byproduct), and the energy-rich compounds ATP (Adenosine Triphosphate) and NADPH (Nicotinamide Adenine Dinucleotide Phosphate) are generated. These compounds are utilised in the dark phase of photosynthesis for the production of glucose.

Step-by-step solution

Understanding Light Absorption

Light absorption is the initial step of photosynthesis where chlorophyll and other pigments absorb light energy and convert it into chemical energy. During this process, electrons in the chlorophyll get excited and move to higher energy levels.

Explanation of Water Splitting and Oxygen Release

In the presence of light energy, water molecules are split into hydrogen ions, electrons, and oxygen in a process known as photolysis. The oxygen is then released into the atmosphere as a byproduct of photosynthesis.

ATP and NADPH Formation

The excited electrons from chlorophyll are used to create ATP and NADPH. ATP is generated first, followed by NADPH. These energy-rich compounds are used in the subsequent dark reaction phase of photosynthesis to produce glucose.

Making the Final Choice

Based on the discussion above, all these processes (light absorption, water splitting and oxygen release, ATP and NADPH formation) occur in the light reaction and photochemical phase of photosynthesis. Therefore, the correct option for this question is (d) All of these.

Key concepts

Light Reaction

The light reaction is the initial phase of photosynthesis where sunlight is absorbed by chlorophyll and other pigments in plant cells. This process takes place in the thylakoid membranes of the chloroplasts. During this stage, absorbed light energy is converted into chemical energy in the form of ATP and NADPH, which are crucial for the energy needs of plants.

The light reaction consists of multiple steps, including light absorption, water splitting, and the formation of energy-rich molecules like ATP. It is essential for plants as it provides the energy they later use in the dark reactions to make glucose.

Light Absorption

Light absorption is the first step in the photosynthesis process. During this phase, chlorophyll molecules in the plant leaves capture sunlight. Chlorophyll is the green pigment responsible for the absorption of light. It plays a crucial role in photosynthesis.

When light strikes the chlorophyll, it excites the electrons, causing them to move to a higher energy state. This excitation of electrons is necessary as it initiates the cascade of reactions that lead to the formation of ATP and NADPH. Without this step, the plant would not be able to convert the sun's energy into a form that can be used by its cells.

Water Splitting

Water splitting, or photolysis, occurs when light energy absorbed by chlorophyll causes the water molecules within the chloroplasts to split. This reaction is crucial as it produces the oxygen we breathe.

• Water divides into hydrogen ions, electrons, and oxygen.
• The oxygen is released into the atmosphere as a byproduct.
• The electrons are transferred to replace those lost by chlorophyll during light absorption.
• Hydrogen ions contribute to the formation of NADPH.

This process not only provides essential life-giving oxygen but also supplies the electrons and protons needed for forming ATP and NADPH, which drive the remainder of the photosynthetic reactions.

ATP Formation

ATP formation is a critical step within the light reaction where the energy from excited electrons is harnessed. After the electrons are excited by light absorption and replaced through water splitting, they travel through an electron transport chain. This journey generates proton gradients within the thylakoid membrane, a process known as chemiosmosis.

• The flow of protons back across the membrane drives the enzyme ATP synthase.
• ATP synthase catalyzes the synthesis of ATP from ADP and inorganic phosphate.
• ATP is then ready to power the dark reaction of photosynthesis.

This conversion of light energy to ATP is vital for the plant metabolism, as ATP serves as the primary energy currency within cells, enabling them to carry out numerous functions.