photosynthesis

All living things need energy to live. They take this energy from food. But, have you ever seen plants eating food? Then, how do they get their energy? Plants get their energy through a process called photosynthesis which allows them to make their food. Read on to learn more about this interesting topic.

Learning Objectives

In this lesson, we will learn about the following

1. Definition of photosynthesis
2. Chemical reactions that occur during photosynthesis
3. Steps in the process of photosynthesis that enables phototrophs to make their food
4. Two main stages of photosynthesis – light reaction and dark reaction
5. Four different types of pigments involved in photosynthesis

Photosynthesis is a process by which phototrophs convert light energy into chemical energy, which is used to fuel cellular activities. It takes in the chloroplasts through pigments such as chlorophyll a, chlorophyll b, carotene, and xanthophyll.

All green plants and a few other autotrophic organisms rely on photosynthesis to use carbon dioxide, water, and sunlight to synthesize nutrients. Oxygen is the by-product of photosynthesis.

Just like green plants, some other organisms also carry out photosynthesis. These include prokaryotes like purple bacteria, cyanobacteria, and green sulfur bacteria.

Photosynthesis reaction can be shown as:

Carbon dioxide + Water ========= Glucose + Oxygen

                                              (Sunlight)

Importance of photosynthesis

Photosynthesis is essential for the existence of all life on Earth. It plays an important role in the food chain. It allows plants to create their food thus, making them primary producers.

Photosynthesis also releases oxygen into the atmosphere. Oxygen is needed by most organisms to survive.

Site of photosynthesis

Photosynthesis takes place in chloroplasts in plants and blue-green algae. All green parts of a plant, including the green stems, green leaves, and sepals contains chloroplasts.

Chloroplasts are present only in plant cells and are located within the mesophyll cells of leaves.

Photosynthesis Equation

In photosynthesis, carbon dioxide and water react to form two products, namely, oxygen and glucose. This is an endothermic reaction.

6CO₂ + 6H₂O = C₆H₁₂O₆ + 6O₂

Unlike plants, not all bacteria produce oxygen as the by-product of photosynthesis. Bacteria that do not produce oxygen as the by-product of photosynthesis are called anoxygenic photosynthetic bacteria. Bacteria that do produce oxygen as a by-product of photosynthesis are called oxygenic photosynthetic bacteria.

 

Photosynthetic pigments

There are four different types of pigments present in leaves:

1. Chlorophyll a
2. Chlorophyll b
3. Xanthophylls
4. Carotenoids

Steps of the process

At the cellular level, the photosynthesis process takes place in cell organelles called chloroplasts. These organelles contain a green-colored pigment called chlorophyll, which is responsible for the characteristic green coloration of the leaves.

Structurally, a leaf comprises of petiole, epidermis and a lamina. The lamina is used for the absorption of sunlight and carbon dioxide during photosynthesis.

• During the process of photosynthesis, carbon dioxide enters through the stomata, water is absorbed by the root hairs from the soil and is carried to the leaves through the xylem vessels. Chlorophyll absorbs the light energy from the sun to split water molecules into hydrogen and oxygen.
• The hydrogen from water molecules and carbon dioxide absorbed from the air is used in the production of glucose. Furthermore, oxygen is released into the atmosphere through the leaves as a waste product.
• Glucose is a source of food for plants that provide energy for growth and development while the rest is stored in the roots, leaves, and fruits for their later use.
• Pigments are other basic cellular components of photosynthesis. They are the molecules that impart color and they absorb light at some specific wavelength and reflect back the unabsorbed light. All green plants mainly contain chlorophyll a, chlorophyll b, and carotenoids which are present in the thylakoids of chloroplasts. It is primarily used to capture light energy. Chlorophyll-a is the main pigment.

Two stages of photosynthesis

The process of photosynthesis occurs in two stages:

• Light-dependent reaction or light reaction
• Light independent reaction or dark reaction

 

Light Reaction of Photosynthesis or Light-dependent Reaction

• Photosynthesis begins with the light reaction. It is carried out only during the day in the presence of sunlight. In plants, the light reaction takes place in the thylakoid membranes of chloroplasts.
• Inside the thylakoid membranes, there are sac-like structures called Grana which gather light.
• The light energy is converted to ATP and NADPH which are used in the second phase of photosynthesis. Water is used and oxygen is produced.

The chemical equation in the light reaction of photosynthesis can be reduced to:

2H₂O + 2NADP + 3ADP + 3Pi = O₂ + 2NADPH + 3ATP

Dark Reaction of Photosynthesis or Light-independent Reaction

• This is also called a carbon-fixing reaction.
• It is a light-independent process in which sugar molecules are formed from the water and carbon dioxide molecules.
• The dark reaction occurs in the stroma of the chloroplast where they utilize the NADPH and ATP products of the light reaction.
• Plants capture the carbon dioxide from the atmosphere through stomata and proceed to the Calvin cycle.
• In the Calvin cycle, the ATP and NADPH formed during light reaction drive the reaction and convert six molecules of carbon dioxide into one sugar molecule or glucose.

The chemical equation for the dark reaction is

3CO₂ + 6NADPH + 5H₂O + 9ATP = G3P + 2H + 6NADP + 9ADP +8Pi

(Where G3P is glyceraldehyde-3-phosphate)