ATP-ADP Cycle Overview

Study Notes

Adenosine triphosphate (ATP) is the energy currency of cells, powering essential cellular processes like mechanical work, transport, and chemical reactions.
ATP comprises adenine, ribose sugar, and three phosphate groups.
High-energy bonds link the last two phosphate groups, releasing energy upon hydrolysis (breaking with water).
The recycling process converts ATP to ADP (adenosine diphosphate) and back, constantly regenerating usable energy.

ATP's structure is composed of ribose sugar, adenine, and a chain of three phosphate groups.
ATP is crucial for energy coupling, linking exergonic reactions to drive endergonic reactions.
ATP powers three main types of cellular work: chemical, transport, and mechanical.
- Chemical work: synthesis of polymers from monomers.
- Transport work: pumping of substances across membranes against the direction of spontaneous movement.
- Mechanical work: processes like cilia beating and muscle contraction.
ATP is also used in RNA synthesis.

Hydrolysis breaks the terminal phosphate bond in ATP, releasing inorganic phosphate (Pi) and forming ADP.
The released energy comes from the instability created by the negative charges of the phosphate groups being crowded together.
This energy release is key to driving endergonic reactions.

ATP hydrolysis directly performs work, often by shifting the shape of a protein, thereby affecting its ability to bind to another molecule.
The energy released from ATP hydrolysis can create heat as well as mechanical force used to drive other processes.
Shivering in cold environments uses ATP to generate heat in order to maintain cell functionality.

ATP is continually regenerated by the addition of phosphate to ADP.
Exergonic (catabolic) processes provide the energy needed for this regeneration, converting ADP back to ATP.
This constant cycling of ATP and ADP is essential for cell function, providing a continuous supply of usable energy.
Humans would use nearly their body weight in ATP each day if not continuously regenerated.

Pigments capture solar energy for photosynthesis.
Chlorophyll is the most important pigment, absorbing blue and red light and reflecting green light, hence its green appearance.
Accessory pigments (like chlorophyll b and carotenoids) absorb other wavelengths of light and transfer their energy to chlorophyll.
The process of light absorption by pigments is essential for photosynthesis.

Photosystems are complexes of pigments and proteins in the chloroplast responsible for capturing light energy and transferring electrons.
Photosystems I and II work together, both containing chlorophyll, to perform vital reactions.
Photosystem II uses P680 to absorb 680nm wavelengths.
Photosystem I uses P700 to absorb 700nm wavelengths.