Mcat Repaso Biologia

5' cap

a methylated guanine nucleotide added to the 5' end of eukaryotic mRNA. The cap is necessary to initiate translation of mRNA

A band

The band of the sarcomere that extends the full length of the thick filament. The A band includes regions of thick and thin filament overlap, as well as a region of thick filament only. A bands alternate with I bands to give skeletal and cardiacmuscle a striated apperance. The A band does not shorten during muscle contraction.

Absolute refractory period

A period of time following an action potential during which no additional action potential can be evoked regardless of the level of stimulation. (usually because Na+ channel closed whle K+ efflux)

Accessory glands

The three glands in the male reproductive system that reproducesemen: the seminal vesicles, the prostate, and the

Accessory organs

(1) In the GI tract, organs that play a role in digestion but not directly part of the alimentary canal. These include the liver, the gallbladder, the pancreas, adn the salivary glands.

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Acetylcholine (Ach)

The neurotransmitter used throughout the parasympathetic nervous system as well as the neuromuscular junction.Acetylcholinesterase (AChE)

The enzyme that breaks down acetylcholine in the synaptic cleft.

Acetyl-CoA

The first substrate in teh Krebs cycle, produced primarily from the oxidation of pyruvate by the pyruvate dehydrogenase complex, however acetyl-CoA is also produced during fatty acid oxidation and protein catabolism.

Acid hydrolases

Enzymes that degrade various macromoleculesand that require an acidic pH to function properly. Acid hydrolases are found within the lysosomes of cells.

Acinar cells

Cells that make up exocrine galnds, adn that secrete their products into ducts. For example, in the pancreas, acinar cells secrete digestive enzyme; in the salivary glands, acinar cells secrete saliva.

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Acrosome

A region at the head of a sperm cell that containsdigestive enzyems which, when released during the acrosome reaction, can facilitate penetration of the corona radiata of the egg, and subsequently, fertilization

Actin

A contractile protein. In skeletal and cardiac muscle, actin polymerizes (along with other proteins) to form the thin filaments. Actin is involved in many contractile activities, such as cyotkinesis, pseudopod formation, andmuscle contraction.

Action potential

A localized change in a neruon's or musce cell's membrane potential that can propogate itself away from its point of origin. Action potentials are an all-or-none process mediated by the opening of voltage-gated Na+ and K+ channels when the membrane is brought to the threshold potential; opening of the Na+ channels causes a characteristic depolarization,while opening of the K+ channels repolarizes the membrane.

Activation energy (Ea)

The amount of energy required to produce the transition state of a chemical reaction. If the activation energy for a reaction is very high, the reaction occurs very slowly. Enzymes (and other catalysts) increase reaction rates by reducing activation energy.

Active site

The 3D site of an enzyme wheresubstrates (reactants) bind and a chemical reaction is facilitated.

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Active transport

The movement of molecules through the plasma membrane against their concentration gradients. Active transport requires input of cellular energy, often in the form of ATP. An example is the Na+/K+ ATPase in the plasma membrane of all cells.

Adenine

One of the four aromatic bases found in DNA and RNA;also a component of ATP, NADH, and FADH2. Adenine is apurine; it pairs with thymine (in DNA) and with uracil (in RNA)

Adenohypophis

anterior pituitary gland

Adipocyte

fat cell
The inner region of the adrenal gland. The adrenal medulla is part of the sympathetic nervous systme, and releases epinephrine (adrenaline) and norepinephrine into the blood when stimuated. These hormones augment...