In science, “fixation” describes a process of making something stable or usable. Its specific meaning varies widely across scientific fields, shifting depending on the context. This concept’s application ranges from preserving microscopic tissues to altering the genetic makeup of entire populations.
Preserving the Microscopic World: The Science of Tissue Fixation
Tissue fixation is the chemical process of preserving biological tissues from decay to maintain them in a life-like state. This preservation is foundational for histology and pathology because it halts biochemical reactions like autolysis or putrefaction. A successful fixation hardens the tissue, which allows it to be thinly sliced for microscopic examination, and prepares it to accept stains that make cellular components visible.
The most common method is chemical fixation. Cross-linking fixatives, such as formaldehyde, work by creating chemical bridges between proteins that stabilize the fine structure of the cells and the extracellular matrix. Formaldehyde is widely employed because it penetrates tissues efficiently.
Precipitating fixatives, including alcohols, work by removing water from the tissues, which denatures and precipitates proteins. This method is often faster but can cause more significant structural changes. The choice of fixative and the duration of the process depend on the tissue type and the intended analysis, to ensure the target molecules are not destroyed or masked.
Two main techniques are used to apply these chemicals: immersion and perfusion. Immersion involves submerging the tissue sample in the fixative solution. For whole organs, perfusion is used, where the fixative is introduced through the circulatory system for rapid and uniform distribution.
Life’s Foundation: Understanding Carbon Fixation
Carbon fixation is the process where inorganic carbon, primarily atmospheric carbon dioxide (CO2), is converted into organic compounds by living organisms. This conversion forms the foundation of nearly every food chain. Through this mechanism, organisms build essential molecules like sugars, which store energy. The process also has a significant role in the global carbon cycle by removing CO2 from the atmosphere.
The most recognized pathway is photosynthesis, which occurs in plants, algae, and some bacteria. During photosynthesis, organisms use energy from sunlight to power chemical reactions inside organelles called chloroplasts. Here, CO2 is incorporated into an organic molecule in a series of steps called the Calvin cycle.
The Calvin cycle has three main stages: fixation, reduction, and regeneration. In the first stage, a CO2 molecule is attached to a five-carbon acceptor molecule, ribulose-1,5-bisphosphate (RuBP). This reaction is catalyzed by the enzyme RuBisCO, and the resulting compound immediately splits into two three-carbon molecules.
Following fixation, these three-carbon molecules are reduced. Using energy from ATP and NADPH, the molecules are converted into a three-carbon sugar called glyceraldehyde-3-phosphate (G3P). The remaining G3P molecules stay in the cycle and are used to regenerate the initial RuBP acceptor molecule, a process that also consumes ATP. The G3P that exits the cycle is a versatile building block that the cell can use to synthesize glucose and other essential organic molecules.
Nourishing Ecosystems: The Process of Nitrogen Fixation
Nitrogen fixation is the process that converts atmospheric nitrogen (N2), a gas unusable by most living things, into reactive compounds like ammonia (NH3). This conversion is fundamental for life because nitrogen is a required component of amino acids and nucleic acids. Although the atmosphere is 78% nitrogen, its strong triple bond makes it difficult for organisms to break apart.
Biological nitrogen fixation is primarily carried out by specialized microorganisms. Some microbes, like cyanobacteria, are free-living in soil and water. Others have formed symbiotic relationships with plants, such as the Rhizobium bacteria that live in nodules on the roots of legumes. Inside these nodules, the bacteria are protected from oxygen, which inhibits the nitrogen-fixing enzyme nitrogenase, and are supplied with carbohydrates by the plant.
Beyond biological processes, nitrogen can also be fixed through high-energy natural events like lightning. The most significant non-biological source today is industrial fixation through the Haber-Bosch process. This method combines atmospheric nitrogen and hydrogen gas under high temperature and pressure to produce ammonia for synthetic fertilizers. The development of this process has had a profound effect on global agriculture, enabling food production to support a much larger human population.
Evolution’s Turning Point: Genetic Fixation in Populations
In population genetics, fixation is when a single allele becomes the only version of that gene in a population, reaching a 100% frequency. At this point, all individuals are homozygous for that allele. This marks an endpoint in the gene’s evolutionary trajectory for that population, as all other variants have been lost.
Two primary mechanisms drive an allele toward fixation: natural selection and genetic drift. Natural selection can lead to fixation when an allele provides a significant survival or reproductive advantage. Over generations, individuals carrying this beneficial allele are more likely to reproduce, passing the allele on until it outcompetes other variants.
Genetic drift describes random fluctuations in allele frequencies that occur from one generation to the next due to chance. This effect is particularly strong in small populations, where an allele with no particular advantage can become fixed simply because its carriers happened to produce more offspring. Similarly, a beneficial allele can be lost from a small population before it has a chance to become established.
The consequence of genetic fixation is a reduction in genetic variation. If the fixed allele is adaptive, the population becomes better suited to its environment. If fixation results from genetic drift, it may not be adaptive and could even be mildly harmful. Over long timescales, the fixation of different alleles in separate populations is a part of how new species diverge.