ATP-ADP Cycle Overview

ATP-ADP Cycle

• Adenosine triphosphate (ATP) is the cell's energy currency, powering various cellular functions (mechanical work, transport, chemical reactions).
• ATP consists of a ribose sugar, adenine, and three phosphate groups.
• The last two phosphate bonds in ATP store high-energy bonds.
• Hydrolysis of these bonds releases energy, breaking ATP into adenosine diphosphate (ADP) and inorganic phosphate (Pi).
• The cell recycles ATP by phosphorylating ADP, using energy from catabolic processes.
• ATP powers three main types of cellular work: chemical, transport, and mechanical work.

Hydrolysis of ATP

• ATP hydrolysis breaks the phosphate bonds, releasing energy.
• This energy is derived from the negative charge repulsion of phosphate groups, not from the bonds themselves.
• Hydrolysis of ATP leads to a change in protein shape and ability to bind other molecules.
• This shape change powers various cellular processes.

Regeneration of ATP

• Catabolic (exergonic) reactions provide energy required for ATP regeneration from ADP and inorganic phosphate.
• ATP regeneration is rapid; humans would need nearly their body weight in ATP daily without regeneration.

Photosynthesis

• Pigments capture solar energy for photosynthesis.
• Chlorophyll is the primary photosynthetic pigment in plants.
• Chlorophyll absorbs blue and red light and reflects green light, resulting in plants appearing green.
• Chlorophyll is found in thylakoid membranes inside chloroplasts.
• Multiple chlorophyll types (e.g., chlorophyll a, chlorophyll b) capture more wavelengths of sunlight.

Photoexcitation of Chlorophyll

• Chlorophyll molecules absorb photons of light.
• This excites an electron to a higher energy level.
• The excited electron returns to its original energy level, releasing energy as heat or light.
• This process is crucial for transferring energy in photosynthesis.

Photosystems

• Photosystems are protein complexes in thylakoid membranes that contain pigments and proteins.
• They capture light energy and convert it to chemical energy.
• Photosystem II and Photosystem I are the two main types of photosystems.