In this lesson, we will explore the continuous chemical activities that occur within living cells. As discussed in Chapter 4, cells have a specific structure, and Chapter 6 highlighted the important role of enzymes in their functions. Living cells operate as “open systems,” meaning that substances are constantly entering and exiting. Inside the cells, materials are broken down, and new substances are formed, all driven by energy. In living organisms, energy exists in two forms: kinetic energy, which is actively used to perform work, and potential energy, which is stored for future use. This potential energy is stored in chemical bonds and is released as kinetic energy when those bonds are broken.

Bioenergetics and the Role of ATP

Bioenergetics is the study of energy relationships and transformations in living organisms. Organisms obtain energy by metabolizing the food they eat or produce. This food contains potential energy stored in its chemical bonds. When these bonds are broken, a significant amount of kinetic energy is released, some of which is stored as potential energy in ATP molecules, while the rest escapes as heat. The energy stored in ATP is then converted back into kinetic energy to perform various life activities.

Oxidation-Reduction Reactions

Life processes in organisms require a constant flow of energy, which involves acquiring, transforming, and using energy for activities like growth, movement, and reproduction. The direct source of this energy comes from oxidation-reduction reactions, or redox reactions, which involve the exchange of electrons between atoms. Losing electrons is called oxidation, while gaining electrons is called reduction.

Electrons can serve as an energy source depending on their arrangement in atoms. For instance, when electrons are associated with oxygen, they are stable and not good energy sources. However, if they are removed from oxygen and attached to other atoms like carbon or hydrogen, they become unstable and try to return to oxygen, releasing energy in the process. In living organisms, redox reactions often involve the loss and gain of hydrogen atoms. Since a hydrogen atom consists of one proton and one electron, losing a hydrogen atom means losing an electron (oxidation), while gaining a hydrogen atom means gaining an electron (reduction).

ATP: The Cell’s Energy Currency

The main energy currency of all cells is a nucleotide called adenosine triphosphate (ATP). ATP serves as the primary energy source for many cellular functions, including the synthesis of macromolecules (like DNA, RNA, and proteins), movement, transmission of nerve impulses, active transport, and processes like exocytosis and endocytosis.

ATP’s ability to store and release energy comes from its molecular structure, which consists of three parts: (a) adenine, a double-ringed nitrogenous base; (b) ribose, a five-carbon sugar; and (c) three phosphate groups arranged in a chain. The bonds connecting the phosphate groups, particularly the outermost bond, are high-energy bonds. When this bond breaks and an inorganic phosphate (Pi) is released, about 7.3 kcal of energy is released.

In typical energy reactions, only the outer bond breaks, converting ATP into adenosine diphosphate (ADP) and releasing one phosphate. Sometimes, ADP can further break down into adenosine monophosphate (AMP), releasing energy in the process:

Cells continuously recycle ADP by combining it with Pi to form ATP, a process that requires 7.3 kcal of energy per mole, which is obtained from the oxidation of food. In summary, ATP is generated through energy-releasing processes and broken down during energy-consuming processes, acting as a crucial energy transfer molecule in metabolic reactions.

Photosynthesis

Photosynthesis is the process through which glucose is synthesized from carbon dioxide and water using sunlight and chlorophyll, with oxygen produced as a by-product. This process is anabolic, meaning it builds complex molecules, and it is a crucial part of bioenergetics in living systems. Since ATP is a key energy currency in all organisms, it likely emerged early in the history of life. Photosynthesis is the most important biochemical pathway, as nearly all life depends on it. It involves a series of coordinated biochemical reactions that take place in plants, certain protists (like algae), and some bacteria. A simple general equation for photosynthesis can be represented as follows:

Limiting Factors in Photosynthesis

A limiting factor is any environmental condition whose absence or deficiency can reduce the rate of a metabolic reaction. For photosynthesis, several factors can act as limiting factors, including light intensity, temperature, carbon dioxide concentration, and water availability.

Effect of Light Intensity and Temperature

The rate of photosynthesis is directly influenced by light intensity; it decreases when light intensity is low and increases as light intensity rises. However, at very high light intensities, the rate of photosynthesis levels off. In the dark reactions of photosynthesis, three-carbon compounds are reduced to form carbohydrates, using hydrogen sourced from water.

The rate of photosynthesis also decreases with lower temperatures, although it can increase with rising temperatures up to a certain point. However, if light intensity is low, higher temperatures have little effect on the rate of photosynthesis.

Effect of Carbon Dioxide Concentration

As the concentration of carbon dioxide increases, the rate of photosynthesis also increases, but only until it is limited by other factors. If the carbon dioxide concentration becomes too high, the stomata (tiny openings on leaves) may close, which can reduce the rate of photosynthesis.

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