Photosynthesis and Energy Conversion

Section 4: Photosynthesis and Energy Conversion

In a plant factory, artificial light sources continuously provide energy for plant growth. In nature, however, all life depends on the sun for growth. The input, capture, and conversion of solar energy are fundamental to the operation of the biosphere. Photosynthesis is the only biological pathway capable of capturing and converting light energy. Therefore, some refer to photosynthesis as the "most important chemical reaction on Earth."

Whether in a plant factory or in nature, plants rely on specific substances and structures to capture light energy.

Pigments and Structures for Capturing Light Energy

For higher plants, leaves are the main organs for photosynthesis. Most of these plant leaves are green, indicating the presence of green pigments. Occasionally, albino seedlings without green pigments can be seen in cornfields (Figure 5-10). These albino seedlings die once the nutrients stored in their seeds are depleted. This suggests that pigments in leaves may be related to the capture of light energy.

Pigments for Capturing Light Energy

What pigments are present in green leaves?

We explore this through the following experiment.

There are four pigments in green leaves, classified into two categories:

Chlorophyll (about 3/4 of the total)

• Chlorophyll a (blue-green)
• Chlorophyll b (yellow-green)

Carotenoids (about 1/4 of the total)

• Carotene (orange-yellow)
• Xanthophyll (yellow)

How do these four pigments differ in their light absorption?

Scientists conducted experiments placing a prism between pigment solutions and sunlight. Sunlight is composed of composite light with different wavelengths, which disperse into different colored bands (Figure 5-11) when passing through a prism, known as a spectrum. By exposing pigment solutions to different colored lights, scientists obtained the absorption spectra of these pigments.

The experimental results show that chlorophyll a and chlorophyll b mainly absorb blue-violet and red light, while carotene and xanthophyll mainly absorb blue-violet light (Figure 5-12). These pigments absorb light at different wavelengths but all contribute to photosynthesis.

Where are these pigments located within cells?

In the early 19th century, scientists isolated chlorophyll from plant cells but did not initially understand their distribution within plant cells. It was later discovered that chlorophyll is not evenly distributed throughout the plant cell but concentrated in a smaller structure known as chloroplasts.

Structure of Chloroplasts Suitable for Photosynthesis

Under an optical microscope, chloroplasts from rice, apples, and other angiosperms appear generally flat, elliptical, or spherical in shape. However, finer structures inside chloroplasts are only observable under an electron microscope.

Observing chloroplast structure under an electron microscope (Figure 5-13), chloroplasts are enveloped by double membranes and contain numerous thylakoids. Each thylakoid is stacked into disc-like vesicles called grana, where the four pigments responsible for absorbing light energy are located on the thylakoid membranes. The space between grana is filled with stroma.

What other functions do chloroplasts serve besides absorbing light energy?

Engelmann's experiments directly proved that chloroplasts absorb light energy for photosynthesis, releasing oxygen. Combined with other experimental evidence, scientists concluded that chloroplasts are the sites of photosynthesis.

On the extensive membrane surfaces inside chloroplasts, many pigment molecules that absorb light energy are distributed, as well as enzymes necessary for photosynthesis in thylakoid membranes and chloroplast stroma. This forms the structural basis for chloroplasts to capture light energy and carry out photosynthesis.

Principles and Applications of Photosynthesis

We know that photosynthesis is the process by which green plants, through chloroplasts, utilize light energy to convert carbon dioxide and water into organic compounds that store energy and release oxygen. This process can be summarized by the following chemical equation, where (CHO) represents sugars:

CO2 + H2O + light energy → (CH2O) + O2

Everywhere in nature, there is carbon dioxide, water, and sunlight. However, only cells capable of photosynthesis can synthesize organic compounds from them. Chloroplasts are responsible for this miraculous process.

Principles of Photosynthesis

How do chloroplasts convert light energy into chemical energy? And how do they store chemical energy in organic compounds like sugars? Does the oxygen released in photosynthesis come from water or carbon dioxide in raw materials? Let's analyze some experiments conducted by scientists.

The experiments mentioned earlier demonstrate that the oxygen in the oxygen released during photosynthesis comes from water, and the production of oxygen and the synthesis of sugars are not the same chemical reaction but proceed in stages. In fact, the process of photosynthesis is highly complex, involving a series of chemical reactions. Depending on whether light energy is needed, these reactions can be broadly categorized into two stages: light reactions and dark reactions (now also known as carbon reactions).

Light Reaction Stage

The first stage of photosynthesis, requiring light, is called the light reaction stage. The light reaction stage occurs on the thylakoid membranes inside chloroplasts.

The light energy absorbed by photosynthetic pigments in chloroplasts serves two main purposes:

1. Splitting water into oxygen and H, where oxygen is released directly as oxygen molecules, and H combines with oxidized coenzyme II (NADP+) to form reduced coenzyme II (NADPH). NADPH, as an active reducing agent, participates in chemical reactions in the dark reaction stage and also stores some energy for use in the dark reaction stage.
2. Under the catalytic action of relevant enzymes, providing energy to facilitate the reaction of ADP with Pi to form ATP. Thus, light energy is converted into chemical energy stored in ATP. These ATP molecules participate in the chemical reactions for synthesizing organic compounds in the second stage.