Question

Plants produce oxygen when they photosynthesize. Where does the oxygen come from? a. splitting water molecules b. ATP synthesis c. the electron transport chain d. chlorophyll

Short answer

The oxygen produced by plants during photosynthesis comes from the splitting of water molecules (option a) during the light-dependent reactions. This process is called photolysis, and it results in the release of oxygen as a byproduct.

Step-by-step solution

Option a - Splitting water molecules

During the light-dependent reactions of photosynthesis, light energy is absorbed by chlorophyll and other pigments in the thylakoid membrane of chloroplasts. This energy is used to split water molecules into hydrogen ions, electrons, and oxygen. This process is called photolysis. The produced oxygen is then released as a byproduct of photosynthesis. So, the oxygen comes from the water molecules that are split during this process.

Option b - ATP synthesis

ATP synthesis is the process of producing ATP (adenosine triphosphate) molecules, which serve as the energy currency for cells. During photosynthesis, ATP is produced by the light-dependent reactions, and it is used as a source of energy for the light-independent reactions (Calvin cycle) to reduce CO2 to carbohydrates. While ATP synthesis is an essential step in photosynthesis, it does not produce oxygen.

Option c - The electron transport chain

The electron transport chain is a series of protein complexes located in the thylakoid membrane that helps transfer electrons during photosynthesis. This transfer of electrons drives the production of ATP and NADPH, which are used in the carbohydrate synthesis in the light-independent reactions of the Calvin cycle. However, the electron transport chain itself is not responsible for producing oxygen.

Option d - Chlorophyll

Chlorophyll is a pigment that is responsible for capturing light energy, which is essential for initiating photosynthesis. Chlorophyll absorbs light energy, which excites electrons, leading to the photolysis of water and the release of oxygen. While chlorophyll plays a crucial role in photosynthesis, it is not directly responsible for the production of oxygen. Based on the information above, the correct answer is: a. splitting water molecules

Key concepts

Light-dependent Reactions

In photosynthesis, the light-dependent reactions are the first stage where sunlight is absorbed to begin the process of energy conversion. These reactions occur in the thylakoid membranes of chloroplasts.

They depend on light because they capture energy from sunlight to produce energy-rich molecules like ATP and NADPH.

• ATP provides energy for various cellular processes.
• NADPH facilitates the reduction of carbon dioxide in the light-independent reactions.

The absorbed light energizes electrons, which trigger a chain of reactions that ultimately split water molecules, release oxygen, and drive ATP and NADPH formation.

Photolysis

Photolysis is the process of using light energy to split water molecules into hydrogen ions, electrons, and oxygen. In the light-dependent reactions of photosynthesis, photolysis is crucial for oxygen production.

During photolysis:

• Water molecules absorb energy from light absorbed by chlorophyll.
• The water molecule breaks down into electrons, hydrogen ions, and oxygen gas.
• The oxygen produced is released into the atmosphere.

This splitting of water molecules not only provides necessary electrons but also leads to the byproduct of oxygen we breathe.

Chlorophyll

Chlorophyll is the green pigment found in the chloroplasts of plants, algae, and some bacteria. It plays a pivotal role in capturing light energy required for photosynthesis.

Here's how it works:

• Chlorophyll absorbs mostly blue and red wavelengths of light while reflecting green, which is why plants appear green.
• It becomes 'excited' when it absorbs sunlight, passing this energy to electrons.
• These energized electrons are then used in subsequent reactions, including those in photolysis and the electron transport chain.

Chlorophyll is essential for initiating the light-dependent reactions that lead to oxygen production.

Electrons

Electrons play a critical part in the light-dependent reactions of photosynthesis. When chlorophyll absorbs light, electrons within it gain energy and move to higher energy states.

These high-energy electrons:

• Move through the electron transport chain, a series of proteins that create a flow of electricity.
• As they move along this chain, they're used to pump hydrogen ions and generate ATP.
• These electrons also help transform NADP+ into NADPH by reducing it, which is used in the Calvin cycle.

The electron's journey is vital as it powers the formation of energy molecules and ultimately supports the synthesis of oxygen and carbohydrates.

Oxygen Production

Oxygen production is a key outcome of photosynthesis, specifically occurring during the light-dependent reactions. The process of splitting water, or photolysis, results in the formation of oxygen.

Main points about oxygen production include:

• Oxygen is released as a byproduct when water molecules are split.
• This process not only releases oxygen but also provides essential electrons and protons for further reactions.
• The oxygen produced is essential for the survival of most living organisms as it forms part of the atmospheric oxygen we depend on.

Thus, through photosynthesis, plants not only nourish themselves but also provide oxygen crucial for life on Earth.