Category: Biology Dictionary A

Definition Analogous structures are similar structures that evolved independently in two living organisms to serve the same purpose. The term “analogous structures” comes from the root word “analogy,” which is a device in the English language where two different things on a basis of their similarities. Analogous structures are examples of convergent evolution, where two…

Definition Anaerobic respiration is the type of respiration through which cells can break down sugars to generate energy in the absence of oxygen. This is in contrast to the highly efficient process of aerobic respiration, which relies on oxygen to produce energy. Molecular oxygen is the most efficient electron acceptor for respiration, due to its…

Anaerobic Organism Definition Anaerobic organisms are those that live an anoxic environment – one which lacks oxygen. While most living things require oxygen to survive – they’re aerobic – oxygen can actually be toxic to anaerobic organisms. The great majority of organisms produce energy molecules called ATP (adenosine triphosphate) through a process of aerobic cellular…

Anabolism Definition Anabolism collectively refers to all the processes of chemical reactions that build larger molecules out of smaller molecules or atoms; these processes are also known as anabolic processes or anabolic pathways. The opposite of anabolism is catabolism, the set of processes that breaks down larger molecules into smaller ones. Anabolism and catabolism are…

Amphipathic Definition An amphipathic molecule is a molecule that has both polar and non-polar parts. Phospholipids, for example, have non-polar fatty acid “tails” and polar phosphate “heads.” “Polarity” is an important property of molecules that determines how they will interact with other molecules. Polarity is created when some atomic nuclei in a molecule attract electrons…

Amoeba Definition An amoeba is a highly motile eukaryotic, unicellular organism. Typically belonging to the kingdom protozoa, it moves in an “amoeboid” fashion. As such, microbiologists often use the term “amoeboid”, to refer to a specific type of movement and amoebae interchangeably. Interestingly, amoebae are not a distinct taxonomic group and are, instead, characterized based…

Amniotic Fluid Definition Amniotic fluid is the clear liquid that surrounds a developing fetus in the mother’s womb. It is formed from the mother’s plasma (or the fluid part of blood cells) as it diffuses past the fetal membranes and succumbs to the forces of osmosis and hydrostatic pressure. Amniotic Fluid Overview Visually speaking, the…

Amniotes Definition Amniotes are vertebrate organisms which have a fetal tissue known as the amnion. The amnion is a membrane derived from fetal tissue which surrounds and protects the fetus. The amnion can be found within the egg, as in lizards and birds, or the amnion can simply enclose the fetus within the uterus. Amniotes include…

Amnion Definition The amnion is an extraembryonic membrane that surrounds a developing amniote embryo. It acts as a protective sac along with three other extraembryonic membranes: the chorion, the yolk sac, and the allantois. The membranes are then enclosed further by a shell (in birds, reptiles, and some mammals) or in a uterus (in most…

Ammonification Definition Ammonification is part of the five-step nitrogen cycle, which is crucial for providing living organisms with the essential nitrogen that they need. Ammonification itself takes place thanks to the existence of decomposers, which break down animal and plant cells into simpler substances, making nutrients available in the ecosystem. The process of ammonification converts organic…