Carbon fixation is a critical process in the synthesis of biological molecules, particularly in autotrophic organisms. It involves the incorporation of inorganic carbon (typically from carbon dioxide in the atmosphere) into organic compounds, a fundamental step in the process of photosynthesis. This process occurs during the Calvin Cycle in plants and certain bacteria. The enzyme RuBisCO catalyzes the reaction where CO2 is attached to a five-carbon sugar, ribulose-1,5-bisphosphate (RuBP), forming a six-carbon compound that quickly splits into two molecules of 3-phosphoglycerate. These molecules are then converted into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar. G3P serves as the building block for various organic molecules, including glucose, which is a primary energy source for living organisms. Carbon fixation is significant because it is the initial step in converting inorganic carbon into organic forms, making carbon available for use in various biological molecules necessary for life. Without carbon fixation, the carbon cycle would be incomplete, and the synthesis of key organic molecules would be severely impeded.

Environmental pollutants can significantly affect the synthesis of molecules in living organisms, often leading to detrimental effects. Pollutants can disrupt the availability and balance of essential elements required for molecular synthesis. For example, heavy metals like lead or mercury can replace essential metals in enzymes, rendering them ineffective and disrupting metabolic pathways. Air pollutants such as sulfur dioxide and nitrogen oxides can alter the pH of soil and water, affecting nutrient uptake and hindering processes like nitrogen fixation and photosynthesis. Pesticides and other chemicals can accumulate in the food chain, affecting the health and metabolic functions of organisms at various trophic levels. Additionally, pollutants can cause oxidative stress, leading to damage of cellular components, including proteins, lipids, and nucleic acids, thus interfering with the synthesis and function of these molecules. The impact of environmental pollutants on molecule synthesis is a critical concern, as it can lead to a cascade of ecological imbalances and health issues in living organisms, including humans.

The nitrogen cycle plays a pivotal role in the synthesis of biological molecules. Nitrogen is a fundamental component of amino acids (the building blocks of proteins) and nucleic acids (DNA and RNA). However, atmospheric nitrogen (N2) is not directly usable by most living organisms. Through the nitrogen cycle, N2 is converted into biologically available forms, such as ammonia (NH3) or nitrate (NO3-), via nitrogen fixation by certain bacteria and archaea. These forms of nitrogen are then assimilated into organic compounds by plants. When animals consume plants, they obtain nitrogen in forms that can be incorporated into their own proteins and nucleic acids. Additionally, the decomposition of organic matter and waste products returns nitrogen to the soil, further facilitating its availability for molecule synthesis. The nitrogen cycle is, therefore, essential for the continuous availability of nitrogen in a form that can be used for the synthesis of key biological molecules, sustaining the life processes of all organisms.

Cellular respiration is a critical process that closely relates to the synthesis of molecules from environmental atoms. In cellular respiration, glucose—a molecule synthesized from environmental carbon, hydrogen, and oxygen atoms—is broken down to produce ATP, the primary energy currency in cells. This process involves a series of reactions occurring in the mitochondria, starting with glycolysis in the cytoplasm, followed by the Krebs cycle and the electron transport chain. During these stages, glucose is systematically deconstructed, and its stored energy is converted into a usable form (ATP). This energy is then utilized in various cellular activities, including the synthesis of new molecules. For instance, the energy from ATP is essential for driving the endergonic reactions in anabolic pathways, such as protein synthesis, nucleotide assembly, and lipid construction. Thus, cellular respiration not only provides the energy necessary for these synthesis processes but is also intrinsically linked to the cycle of molecule formation and breakdown in living organisms.

Autotrophs and heterotrophs differ significantly in their methods of synthesizing molecules from environmental atoms. Autotrophs, such as plants and certain bacteria, are capable of synthesizing organic molecules directly from inorganic substances. They primarily use photosynthesis, where they convert carbon dioxide and water into glucose and oxygen using sunlight. This process enables them to synthesize carbohydrates from simple atoms and molecules available in their environment. Heterotrophs, on the other hand, cannot produce organic compounds from inorganic sources. Instead, they must consume organic materials to obtain the necessary atoms for molecule synthesis. Animals, fungi, and most bacteria fall into this category. They rely on ingesting or absorbing organic compounds, breaking them down, and then reassembling the constituent atoms into molecules necessary for their own biological functions. This fundamental difference in molecule synthesis underpins the ecological relationship between autotrophs and heterotrophs, with autotrophs serving as the primary producers in ecosystems and heterotrophs as consumers or decomposers.