molecules

(noun)

A molecule is an electrically neutral group of two or more atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their lack of electrical charge. However, in quantum physics, organic chemistry, and biochemistry, the term molecule is often used less strictly, also being applied to polyatomic ions.

Related Terms

Examples of molecules in the following topics:

  • Riboswitches

    • Riboswitches are naturally occurring RNA molecules that can regulate gene expression.
    • Riboswitches are specific components of an mRNA molecule that regulates gene expression.
    • The riboswitch is a part of an mRNA molecule that can bind and target small target molecules.
    • An mRNA molecule may contain a riboswitch that directly regulates its own expression.
    • Hence, the existence of RNA molecules provide evidence to the RNA world hypothesis that RNA molecules were the original molecules, and that proteins developed later in evolution.

  • Chemoautotrophs and Chemoheterotrophs

    • Chemotrophs are a class of organisms that obtain their energy through the oxidation of inorganic molecules, such as iron and magnesium.
    • Chemoautotrophs are able to synthesize their own organic molecules from the fixation of carbon dioxide.
    • The energy required for this process comes from the oxidation of inorganic molecules such as iron, sulfur or magnesium.
    • Chemoheterotrophs, unlike chemoautotrophs, are unable to synthesize their own organic molecules.
    • They do, however, still obtain energy from the oxidation of inorganic molecules like the chemoautotrophs.

  • Adaptive Immunity and the Immunoglobulin Superfamily

    • The defining characteristics of adaptive immunity are specificity for distinct molecules and an ability to "remember" and respond more vigorously to repeated exposures to the same microbe.
    • These membrane molecules function as B cell receptors for antigens.
    • Molecules are categorized as members of this superfamily based on structural features shared with immunoglobulins, which are also known as antibodies.
    • Members of the IgSF include cell surface antigen receptors, co-receptors, and co-stimulatory molecules of the immune system, molecules involved in antigen presentation to lymphocytes, cell adhesion molecules, certain cytokine receptors, and intracellular muscle proteins.
    • Each of the plasma cells manufactures millions of identical antibody molecules and pours them into the bloodstream.

  • MHC Polymorphism and Antigen Binding

    • MHC molecules display a molecular fraction called an epitope and mediate interactions of leukocytes with other leukocytes or body cells.
    • Major histocompatibility complex (MHC) is a cell-surface molecule encoded by a large gene family in all vertebrates.
    • To clarify the usage, some of the biomedical literature uses HLA to refer specifically to the HLA protein molecules and reserves MHC for the region of the genome that encodes for this molecule, but this is not a consistent convention.
    • This imposes a very specific link between the MHC molecule and the peptide, and it implies that each MHC variant will be able to bind specifically only those peptides that are able to properly enter in the cleft of the MHC molecule, which is variable for each allele.
    • The variations in the MHC molecules (responsible for the polymorphism) are the result of the inheritance of different MHC molecules, and they are not induced by recombination, as it is the case for the antigen receptors.

  • Types of Catabolism

    • Catabolism is the set of metabolic processes that break down large molecules.
    • These include breaking down and oxidizing food molecules.
    • All these different forms of metabolism depend on redox reactions that involve the transfer of electrons from reduced donor molecules such as organic molecules, water, ammonia, hydrogen sulfide or ferrous ions to acceptor molecules such as oxygen, nitrate or sulfate.
    • In animals these reactions involve complex organic molecules being broken down to simpler molecules, such as carbon dioxide and water.
    • Next, these smaller molecules are taken up by cells and converted to yet smaller molecules, usually the acetyl coenzyme A (acetyl-CoA), which releases some energy.

  • Unresolved Questions About the Origins of Life

    • The question of how simple organic molecules formed a protocell is largely unanswered.
    • There is substantial understanding of how inorganic molecules can give rise to somewhat simple building blocks of life in the process known as abiogenesis.
    • However, the question of how simple organic molecules form a protocell is largely unanswered.
    • Chiral molecules can be synthesized, but in the absence of a chiral source or a chiral catalyst, they are formed in a 50/50 mixture of both enantiomer.
    • A scheme depicting how organic molecules might be synthesized in space.

  • Type III and Type IV Secretion

    • Type III and IV secretion systems are utilized by pathogenic bacteria to transfer molecules from the bacterial cell to the host cell.
    • The secretion of specific molecules allows for adaptation to occur, thereby promoting survival.
    • Effector molecules secreted include proteins, enzymes or toxins.
    • The type IV secretion systems can either secrete or receive molecules.
    • The type III secretion system is characterized by the ability to inject secretory molecules into the host eukaryotic cell.

  • RNA Regulation and Antisense RNA

    • Antisense RNAs are single-stranded RNA molecules that can bind and inhibit specific mRNA translation to protein.
    • There are specific types of RNA molecules that can be utilized to control gene regulation, including messenger RNAs (mRNAs), small RNAs such as microRNAs and lastly, antisense RNAs.
    • Antisense RNAs are single-stranded RNA molecules that exhibit a complementary relationship to specific mRNAs.
    • Antisense RNAs are utilized for gene regulation and specifically target mRNA molecules that are used for protein synthesis.

  • Elements of Life

    • In such a reducing atmosphere, electrical activity can catalyze the creation of certain basic small molecules (monomers) of life, like amino acids.
    • A fundamental question is about the nature of the first self-replicating molecule.
    • The polymerization of nucleotides into random RNA molecules might have resulted in self-replicating ribozymes (RNA world hypothesis).
    • Using this apparatus, and using conditions thought to approximate the conditions on pre-biotic earth, they were able to catalyze the molecules of life like amino acids.

  • T Cell Receptors

    • T lymphocytes have a dual specificity: they recognize polymorphic residues of self major histocompatibility complex (MHC) molecules, which accounts for their MHC restriction; they also recognize residues of peptide antigens displayed by these MHC molecules, which is responsible for their specificity.
    • MHC molecules and peptides form complexes on the surface of antigen presenting cells (APCs).
    • T cells also express other membrane receptors that do not recognize antigens but participate in responses to antigens; these are collectively called 'accessory molecules'.
    • The physiologic role of some accessory molecules is to deliver signals to the T cells that function in concert with signals from the TCR complex to fully activate the cell.