The Chemical Basis of Photosynthesis

1 · Anchor

Key terms before we begin

Van Niel’s Contribution

Photosynthesis as a light-dependent reaction using hydrogen donors.

Water as the Hydrogen Donor

Water is the source of oxygen in green plants.

The Balanced Chemical Equation

The overall process of photosynthesis in green plants.

2 · Explain

Understanding the concept

For a long time, scientists knew plants made food using sunlight, carbon dioxide, and water. However, they didn't fully understand how the oxygen we breathe was released.

A scientist named Cornelius van Niel studied purple and green bacteria. He noticed that these bacteria perform photosynthesis differently than green plants. Instead of using water ( $H_2O$ ) as a source of hydrogen, they use hydrogen sulfide ( $H_2S$ ). When these bacteria make food, they release sulfur or sulfate instead of oxygen.

Van Niel realized that photosynthesis is a process where a "hydrogen donor" (a substance that gives away hydrogen) is used to reduce carbon dioxide into carbohydrates. In green plants, that donor is water. Because water is split during this process, oxygen is released as a byproduct. This proved that the oxygen released by plants comes from water, not from carbon dioxide.

We can summarize the entire process of photosynthesis in green plants with this balanced chemical equation:

6CO_2 + 12H_2O \xrightarrow{\text{Light}} C_6H_{12}O_6 + 6H_2O + 6O_2

In this equation, $C_6H_{12}O_6$ is glucose (the sugar plants make for food). Notice that we use 12 molecules of water because the process is a complex, multi-step reaction.

Cornelius van Niel, through his studies on purple and green sulfur bacteria, demonstrated that photosynthesis is essentially a light-dependent reaction. He proposed that hydrogen from a suitable oxidizable compound reduces carbon dioxide to carbohydrates.

His general equation for this process is:

2H_2A + CO_2 \xrightarrow{\text{Light}} 2A + [CH_2O] + H_2O

In this equation, $H_2A$ represents the oxidizable hydrogen donor.

In green plants, $H_2O$ acts as the hydrogen donor and is oxidized to release $O_2$ . Van Niel inferred that the oxygen evolved by green plants originates from water, not from carbon dioxide. This hypothesis was later confirmed using radioisotopic techniques.

The balanced empirical equation for photosynthesis in oxygen-evolving organisms is:

6CO_2 + 12H_2O \xrightarrow{\text{Light}} C_6H_{12}O_6 + 6H_2O + 6O_2

Here, $C_6H_{12}O_6$ represents glucose. It is important to note that this equation represents the overall process, which is a multi-step biochemical pathway rather than a single reaction.

3 · Apply

Practice questions

3A. Direct MCQ

According to Cornelius van Niel’s experiments, what is the source of the oxygen released during photosynthesis in green plants?

A

Carbon dioxide ( $CO_2$ )

B

Glucose ( $C_6H_{12}O_6$ )

C

Water ( $H_2O$ )

D

Sulfur compounds

3B. Assertion–Reason

Assertion

In purple and green sulfur bacteria, oxygen is not evolved during photosynthesis.

Reason

These bacteria use $H_2S$ as a hydrogen donor instead of $H_2O$ .

A

Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

B

Both Assertion and Reason are true but Reason is NOT the correct explanation of Assertion.

C

Assertion is true but Reason is false.

D

Assertion is false but Reason is true.

3C. Trap / Close-Option Question

Which of the following correctly represents the balanced chemical equation for photosynthesis in green plants?

A

$6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

B

$6CO_2 + 12H_2O \rightarrow C_6H_{12}O_6 + 6H_2O + 6O_2$

C

$CO_2 + H_2O \rightarrow CH_2O + O_2$

D

$6CO_2 + 6H_2O + 6O_2 \rightarrow C_6H_{12}O_6 + 6H_2O$

· Reinforce

Lock it in

Van Niel’s Inference: Photosynthesis is a light-dependent reaction where a hydrogen donor reduces $CO_2$ .
Oxygen Source: The $O_2$ released by green plants comes from the splitting of $H_2O$ .
Bacterial Photosynthesis: Uses $H_2S$ as a donor; releases sulfur/sulfate instead of $O_2$ .
Key Equation: $6CO_2 + 12H_2O \xrightarrow{\text{Light}} C_6H_{12}O_6 + 6H_2O + 6O_2$

* **Note:** Photosynthesis is a multi-step process, not a single-step reaction. ---

05 · Images Required

Images Required

1. Comparison of Hydrogen Donors in Photosynthesis

Type: comparison

Purpose: To visually contrast how green plants use water ( $H_2O$ ) to release oxygen, while sulfur bacteria use hydrogen sulfide ( $H_2S$ ) to release sulfur, reinforcing Van Niel's core discovery.