Photosynthesis: The Process of Light-Dependent Reactions, Lecture notes of Construction

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The Light-Dependent Reactions

of Photosynthesis

MBB

Biochem H III Semester

Key Points

• Photosynthesis evolved as a way to store the energy in solar radiation as high-energy electrons in carbohydrate molecules.
• Plants, algae, and cyanobacteria, known as photoautotrophs, are the only organisms capable of performing photosynthesis.
• Heterotrophs, unable to produce their own food, rely on the carbohydrates produced by photosynthetic organisms for their energy needs.

The Importance of Photosynthesis

• The processes of all organisms—from bacteria to humans—require energy.
• To get this energy, many organisms access stored energy by eating food.
• Carnivores eat other animals and herbivores eat plants.
• But where does the stored energy in food originate?

• All of this energy can be traced back to the process of photosynthesis and light energy from the sun.
• Photosynthesis is essential to all life on earth. It is the only biological process that captures energy from outer space (sunlight) and converts it into chemical energy in the form of G3P.

The process of photosynthesis

• During photosynthesis, molecules in leaves capture sunlight and energize electrons, which are then stored in the covalent bonds of carbohydrate molecules.
• That energy within those covalent bonds will be released when they are broken during cell respiration.

• How long lasting and stable are those covalent bonds? The energy extracted today by the burning of coal and petroleum products represents sunlight energy captured and stored by photosynthesis almost 200 million years ago.

• Other organisms, such as animals, fungi, and most other bacteria, are termed heterotrophs (“other feeders”) because they must rely on the sugars produced by photosynthetic organisms for their energy needs.
• A third very interesting group of bacteria synthesize sugars, not by using sunlight’s energy, but by extracting energy from inorganic chemical compounds; hence, they are referred to as chemoautotrophs.

Overview of photosynthesis

• Photosynthesis is a multi-step process that requires sunlight, carbon dioxide, and water as substrates.
• It produces oxygen and glyceraldehyde-3- phosphate (G3P or GA3P), simple carbohydrate molecules that are high in energy and can subsequently be converted into glucose, sucrose, or other sugar molecules.

• These sugar molecules contain covalent bonds that store energy.
• Organisms break down these molecules to release energy for use in cellular work.

Photosynthesis and leaf

• In plants, photosynthesis generally takes place in leaves, which consist of several layers of cells.
• The process of photosynthesis occurs in a middle layer called the mesophyll.

• Each stoma is flanked by guard cells that regulate the opening and closing of the stomata by swelling or shrinking in response to osmotic changes.