Essential Terms 16-30

Agonistic Behavior

A behavioral pattern in animal societies, agonistic behavior occurs when members of the same population have to compete for food or other resources. One example would be two male lions fighting over the carcass of a zebra.

AIDS

Acquired immunodeficiency syndrome is a disease that wreaks havoc on the immune system. Caused by the human immunodeficiency virus (HIV), it depletes the body's T-cells severely limiting the body's ability to fight infection. AIDS itself is never the direct cause of death; rather, it is the devastating effect of infections that the body can no longer fight off that ends up killing the afflicted.

Allantois

One of four extraembryonic membranes developed in birds, reptiles, and mammals, the allantois is the sac-like membrane that helps embryo exchange gases, and also stores liquid waste in the form of uric acid.

Alleles

Alleles are copies of a gene that show some difference from the original. Alternate forms, or alleles, of genes account for different traits. For example, there are alleles for blue eyes and alleles for brown eyes, but they are both alternate forms of the gene or eye color.

Allosteric regulators

substances that can either inhibit or activate enzymes. An allosteric inhibitor will bind to an allosteric site (a region of the enzyme other than the active site to which a substance can bind) and keep the enzyme in its inactive form while an allosteric activator will bind to an enzyme and induce its active form.

Alpha cell

a type of endocrine cell in the pancreas which secretes glucagon, which elevates glucose levels in the blood. These cells, as well as beta cells, are clustered in the islets of Langerhans.

Alternation of Generation

describes the way in which plants spend part of their lives haploids- possessing one set of chromosomes and part of their lives as diploids- possessing two sets of chromosomes. As haploids, plants are considered gametophytes, and as diploids, they are known as sporophytes. The gametophyte produces haploid gametes which join to form a diploid plant. The sporophyte in turn produces haploid spores via meiosis.

Altruistic Behavior

unselfish behavior that benefits other organisms in the group at the individuals expense because it advances the genes of the group. For example, when ground squirrels give warning calls to alert other squirrels puts itself at risk of being found by the predator.

Alveolus

part of the respiratory system; it is a tiny sac at the end of each bronchiole in the lungs. These sacs increase the ling's surface area to about 100 sq meters. Every time an individual inhales, oxygen is sent to the alveoli and is from there diffused into the capillaries. Carbon dioxide diffuses into the alveoli from capillaries and is expelled from the body when an individual exhales.

Amino Acids

type of organc compound that are critical to the synthesis of protein. They enzymatically attach to a tRNA molecule and combine with a ribosome. Each amino acid then links to its neighboring amino acid, forming a short, linear polymer chain called a peptide. This peptide then joins with other amino acids, lengthening the chain and forming polypeptides. The polypeptide keeps growing until it is released into the cell as a protein.

Ammonia (NH3)

by-product that occurs when cells break down proteins. Ammonia is toxic, and must be converted to a less poisonous substance in order to be excreted from the body. Birds and reptiles convert ammonia to uric acid, while most mammals convert it to urea.

Amnion

one of four extraembryonic membranes developed in birds, reptiles and mammals. The amnion protects the developing embryo by forming a fluid filled sac that envelops the embryo.

Amphibia

a class in the phylum Chordata, amphibia are cold-blooded animals that begin life breathing through gills before developing lungs. They are able to exchange gas through their moist skin and have a three-chambered heart. Some common amphibians are frogs, toads and newts.

Amphipathic Fatty Acids

type of lipid with both hydrophilic and hydrophobic regions. The tails of these lipids are considered hydrophobic regions. The tails of these lipids are considered hydrophobic, in that they do not mix with water. The phospholipid head is considered hydrophilic, because it does mix well with water. The negative charge of the phosphate head is drawn to the positively charged end of a water molecule.

Analogous Structures

features found in two different animals that have the same function but are structurally different and evolve independently of each other. One example is the eye. A human's eye and an insects eye have the same function, but they evolved completely independent of each other and have dissimilar structure.