Trapping Energy from Sunlight: Chloroplast and Chlorophyll

In this article, we’ll learn about Photosynthesis, Trapping energy from sunlight with the role of chloroplast and chlorophyll. Let’s begin with an introduction.

Introduction

An ecosystem is a community of living organisms in a particular geographic area. It is composed of biotic (living) and abiotic components (non-living).  Plants are the producers of an ecosystem. Green plants make use of chlorophyll pigment to trap solar energy and undergo the process of photosynthesis.

With the use of solar energy and inorganic compounds, plants manufacture their own food. Hence, plants are known as autotrophs (auto – self ; trophos- feeder). Other living organisms cannot produce their own food. These organisms are known as heterotrophs. Heterotrophs, directly or indirectly, depend on plants for their food.

In simple terms, photosynthesis (photo – light; synthesis – to join) is the process through which green plants make use of energy from sunlight to make their own food. Green plants make use of light energy, inorganic materials, namely carbon dioxide and water to produce sugar and oxygen.

This process forms the basis of all types of food chains and food webs. Photosynthesis produces starch and oxygen required for the survival of an organism. Therefore, photosynthesis supports all forms of life on Earth. In simple terms, plants are the source of food for almost all living organisms on Earth.

Photosynthesis Process

Organisms that make their own food through photosynthesis are known as photoautotrophs. Plants, algae and cyanobacteria undergo photosynthesis. Protists such as euglena also undergo photosynthesis.

During the process of photosynthesis, light energy gets converted into chemical energy and is stored in the form of glucose/sugar. During photosynthesis, inorganic compounds such as sunlight, carbon dioxide and water are utilized to produce oxygen, water and glucose.

Photosynthesis in Plants

Photosynthesis is an important process that takes place in green plants. In the presence of sunlight, plants combine carbon dioxide and water to produce carbohydrates. Oxygen is the most important by product of photosynthesis. In the presence of light energy, water is split to release oxygen.

This process is known as photolysis of water. During the process of photosynthesis, water molecules get split up to release oxygen, and carbon dioxide forms carbohydrates. Oxygen produced during photosynthesis comes from water molecules and not from carbon dioxide.

In plants, photosynthesis takes place in the green part. Mostly green leaves carry out the photosynthesis process. Sometimes other green parts of plants, such as green stems and floral buds are also involved in photosynthesis.

Green leaves have specialized cells known as mesophyll cells. Mesophyll cells contain chloroplasts (Chloros = green, plastos = moulded). Chloroplasts are specialized organelles in which the photosynthesis process takes place.

Photosynthetic Pigments

Chloroplasts have membrane bound compartments known as thylakoids. Thylakoids contain pigments to absorb light energy. These pigments absorb different wavelengths of light and convert them into chemical energy through photosynthesis. In higher plants, these photosynthetic pigments could be classified into two classes,

1. Chlorophyll
2. Carotenoids

Chlorophyll

The term chlorophyll is coined from the Greek words, khloros – green and phyllon- leaves. Chlorophyll pigments are green colored and are involved in photosynthesis. They act as photoreceptors and carry out photochemical reactions of photosynthesis. Chlorophyll pigments absorb maximum light in the blue, violet, and red regions of the visible spectrum.

They reflect green light and therefore leaves appear in green color. Chlorophyll pigments could be classified into different types, namely chlorophyll a, b, c, d, e and f. Chlorophyll-a is bluish-green in color.

They are found in higher plants, cyanobacteria, and algae. Chlorophyll-b is yellow-green in color. They are found in green algae and higher plants. Brown algae, dinoflagellates, and diatoms are found to have chlorophyll-c.
Only red algae are found to have chlorophyll-d. Chlorophyll-e is a rare pigment which is identified in some golden algae. Chlorophyll-f has been identified in recent times. It is found to absorb infrared light.

Carotenoids

Carotenoids are composed of carotene and xanthophyll. Carotenoid pigments absorb light in the regions of the spectrum that are not absorbed by chlorophyll pigments. Carotenoids transfer the absorbed light energy to chlorophyll pigments which in turn is used for photosynthesis.

Chloroplast Structure

Chloroplasts contain two important regions, namely grana and stroma. Disc-shaped structures present in chloroplasts are known as thylakoids. Thylakoids contain chlorophyll molecules. These thylakoids are stacked up to form grana.

The homogeneous matrix in which grana are embedded are known as stroma. Photosynthetic enzymes, starch grains, ribosomes and DNA are present in stroma.

Photosystems

Among all photosynthetic pigments, chlorophyll-a is the most essential pigment that traps solar energy. All autotrophic plants except photosynthetic bacteria are found to have chlorophyll-a pigment. Chlorophyll-a is also known as reaction center.

Other types of chlorophyll pigments and carotenoids are collectively referred to as accessory pigments or harvesting centre. The primary role of accessory pigments is to absorb light energy and transfer it to chlorophyll – a pigment for photosynthesis.

Chlorophyll-a (reaction center) and accessory pigments are packed together into functional units known as photosystems. Photosystems are classified into two different types namely, PSI and PSII. Each photosystem is composed of around 250 – 400 chlorophyll-a molecules.

In PS I maximum absorption of light energy occurs at a wavelength of 700 nm. This reaction centre with chlorophyll-a is referred to as P700 (P-pigment). In PS II maximum absorption of light energy occurs at a wavelength of 680 nm. The reaction centre of PS II with chlorophyll a is referred to as P680.

The rate of photosynthesis varies with the wavelength of light energy absorbed by chlorophyll pigments. Maximum photosynthesis occurs in the red and blue regions of the spectra. Whereas photosynthesis is very little with yellow and green light as leaves reflect these rays.

Stages of Photosynthesis

Photosynthesis involves two important processes,

1. Light dependent reaction (Light Reaction)
2. Light-independent reaction (Dark Reaction / Calvin Cycle)

Though the entire process of photosynthesis takes place in chloroplast, light and dark reactions occur at different sites. Light reaction takes place in grana and dark reaction takes place in stroma regions of chloroplast.