Photosynthesis: The Core of Life on Earth
What is Photosynthesis?
Photosynthesis is the chemical process by which green plants, certain algae, and some bacteria capture light energy from the sun and convert it into chemical energy. Plants contain a green pigment called chlorophyll, located in the chloroplasts of their cells. This pigment absorbs sunlight’s energy and uses it to produce carbohydrates like glucose from water and carbon dioxide. As a byproduct, photosynthesis releases oxygen into the atmosphere, crucial for life and respiration.
This photochemical process is fundamental to life as we know it. Photosynthesis provides energy for all living beings, including humans (indirectly through the food chain) and herbivores (directly).
The Chemical Reaction of Photosynthesis
The equation represents six molecules of water plus six molecules of carbon dioxide, in the presence of sunlight and chlorophyll, producing one glucose molecule and six molecules of oxygen. The oxygen is then released into the atmosphere. The glucose produced is stored as starch, cellulose, and other essential carbohydrates that contribute to plant structure. Photosynthesis also produces other organic substances like proteins and lipids necessary for plant cell life, growth, and reproduction.
Where Does Photosynthesis Occur?
Photosynthesis takes place in thylakoids, flattened sacs or vesicles within the chloroplast, immersed in a solution called stroma. Photosynthetic pigments like chlorophyll, carotenoids, and xanthine reside in the thylakoid membrane. Light-harvesting reactions occur within the thylakoids. These thylakoids form stacks called grana within the chloroplasts.
The Two Phases of Photosynthesis
Photosynthesis occurs in two phases: the photochemical phase (light-dependent) and the biochemical phase (carbon dioxide fixation).
Photochemical Phase
This phase occurs in the chloroplast’s thylakoids. Here, chlorophyll captures sunlight’s energy, which is then stored in two molecules: ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate). These molecules facilitate the conversion of water and carbon dioxide into reduced organic compounds like glucose, releasing oxygen. Light initiates the breakdown of water, releasing electrons.
Biochemical Phase
This phase occurs in the chloroplast’s stroma. ATP and NADPH are used to assimilate atmospheric CO2 to produce substances, primarily glucose. ATP stores significant energy and belongs to the nucleotide group, consisting of a nitrogenous base (adenine), a five-carbon monosaccharide (ribose), and a phosphate group with high-energy bonds. Through catabolism (breaking down complex molecules into simpler ones), energy stored in phosphate bonds is released.
Mechanism of Photosynthesis
- Leaves capture sunlight using chlorophyll, the green pigment in the thylakoids of chloroplasts.
- Carbon dioxide enters through the stomata of the leaves.
- Roots absorb water and minerals (raw sap), transporting them to the leaves through the stem.
- Hydrogen and oxygen from water combine with carbon dioxide to create glucose and oxygen. Oxygen is released into the atmosphere.
- Plants use glucose as food and store some for later use.