MCAT Biology w/ Hormones

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Review biology terms and hormones

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 cardiac muscle 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 reproduce semen: 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.

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.


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 macromolecules and 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.


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


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, and muscle 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 where substrates (reactants) bind and a chemical reaction is facilitated.

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.


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)


anterior pituitary gland


fat cell

Adrenal medulla

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 and prolon the effects of sympathetic stimulation in the body.

Adrenergic tone

A constant input to the arteries that keeps them somewhat constricted to maintain a basal level of blood pressure.

Adrenocoricotropic hormone (ACTH)

A trop hormone produced by the anterior pituitary gand that targets the adrenal cortex, stimulating it to relase corisol and aldosterone.

Afferent arteriole

The small artery that carries blood toward the capillaries of the glomerulus.

Afferent neuron

A neuron that arries information (action potentials) to the central nervous system; a sensory neuron.


A blood protein produced by the liver. Albumin helps to mantain blood osmotic pressure (oncotic pressure)


The principal mineralocorticoid secreted by teh adrenal cortex. This steroid hormone targets the kidney tubules and increases renal reabsorption of sodium [and excretion of potassium]. (this causes ADH to be secreted & increased water comes out, increasing blood pressure indirectly).

Alimentary canal

Also known as the gastrointestinal (GI) tract of the digestive tract, the alimentary canal is the long muscular "tube" that includes the mouth esophagus, somatch, small intesitne, and large intestine.

Allosteric regulation

The modifaction of enzyme activity through interactino of molecules with specific sites on the enzyme other than the active site (called allosteric sites)


(singular alveolus.) Tiny sacs, with walls only a single cell layer thick found at the end of the respiratory bronchiole tree. Alveoli are the site of gas exchange in the respiratory system.


A version of a gene. For example, the gene may be for eye color, and the allels include those for brown eyes, those for blu e eyes, those green eyes, etc. At most, dploid organsims can posses only two alleles for a given gene, one on each of the two homologous chromosomes.

Amino Acid

The monomer of a protein; amino acids hae an amio group on one end fo the molecule and a carboxylic acid group on the other, and of the of 2 different side chains.

Amino acid acceptor site

The 3' end of a tRNA molecule that binds an amino acid. The nucleotide sequence at this end is CCA

Aminoacyl tRNA

A tRNA with an amino acid attached. This is made by an animoacyl-tRNA synthetase specific to the amino acid being attache.d


A sac filled with fluid (aminotic fluid) that surroudns and protects a developing embryo.


The characteristics of amolecule that has both polar (hydrophilic) and non-polar hydrophobic) regions, e.g. phospholipids, bile, etc.


An enzyme that digests starch into disaccharides. Amylase is secreted by salivary glands and by the pancreas.


The process of bulidng complex structures out of simpler precursors, e.g. synthesizing protiens from amino acids.

Analogous structures

Physical structures in two different organism that have funcitonal similarity due to their evoluntion in a common environment, but have different underlying structure. Analogous structures arise from convergent evolution.

Anal sphincter

The valve that controls the release of feces from the recturm. It has an internal part made of smooth muscle (thus involuntary) and an external part made of skeletal muscle (thus voluntary).


The third phase of mitosis. During anaphase, replicated chromosmes are split apart at their centromeres (the sister chromatids are separated from each other) and moved to opposite sides of the cell.

Anaphase I

The third phase of meiosis I. During anaphase I the rplicated homologous chromosomes are separated (the tetrad is split) and pulled to opposite sides of the cell.

Anaphase II

The third phase of meiosis II. During anaphase II the sister chromatids are finally spearated at their centromeres and puled to opposite sides of teh cell. Note that anaphase II is identical to mitotic anaphase, excep the number of chromosmes was reduced by half during meiosis I.


Mal sex hormones. Testosteron is the primary androgen.


A normal blood protein produced by the liver, angiotensin is converted to angiotensim I by renin (secreted by kidney when blood pressur falls). Angiotensin I si further onverted to angiotensim II by ACE (angiotensin converting enzyme). Angiotensin II is a powerful systemic vasocontrictor ans stimulator of aldosterone relase, both of which result in an increase in blood pressure.


Something that acts to oppose the action of something else. For example, muscles that move a join in oppoiste direction are said to be antagonists.

Anterioir pituitary gland

Also known as the adenohypophysis, the anterior pituitary is made of gland tissue and makes and secretes six different homrones: FSH, LH, ACTH, prolactin, TSH, and growth hormone. The anterior pituitary is controlled b yreleasing and inhibiting factors from the hypothalamus.

Antibody (Ab)

Also called immunoblobins, the antibodies are protiens secreted by B-cells upon activation that bind in a highly specific manner to foreign proteins (such as those found of the surface of pathogens or transplanted tissues). The foreign proteins are called antigens. Antibodies generally do not directly destroy antigens, rather they mark them for destruction through other methods, and can inativate antigens by clumping them together or by convering necessary active sites.


A sequence of three nucleotides (found int he anticodon loop of tRNA) that is complementary to a specific codon in mRNA. The codon to which the anticodon is complementary specifies the amino acid that is carried by that tRNA.

Antidiuretic hormone (ADH)

Also called vasopressin, this hormone is produced in the hypothalamus and secreted by teh posterior pituitary gland. It tartes teh kidney tubules, increasing their permeability to water, adn thus increasing water retention by the body. Also raises blood pressure by inducing moderate vasoconstriction.

Antigen (Ag)

A molecule (usually a protein) capable of initiating an immune repsonse (antibody production).

Antigen presenting cell

Cells that possess MHC II (B cells and macrophages) and are able to display bits of ingested antigen on their surface in order to activate T cells. See also "MHC"

Antiparallel orientation

The normal configuration of double-stranded DNA in which the 5' end of oen strand is paired with the 3' end of the other


A carrier protein that transports two molecules acrss the plasma membrane in opposite directions.


The largest artery in teh body; the aorta carries oxygenated blood away from the left ventricle of the heart.


A mass of lymphatic tissue at the befenning of the large intestine that helps trap ingested pathogens.

Aqueous humor

A thin, watery fluid found in teh anterior segment of the eye (between the lens and the cornea). THe aqueous humor is constantly produced and drained, adn helps to bring nutrients to the lesn and corena, as well as to remove metabolic wastes


A function in the reproductive system, controlled by the parasympathetic nervous system, that includes erection (via dilation of erectile arteries) and lubrication.


A blood vessel that carries blood away from the heart chambers. Arteries have muscular walls to regulate blood flow and are typically high-pressure vessles.

A site

Amino-acyl tRNA site; the site on a ribosome where a new amino acid is added to a growing peptide.

ATP synthase

A protein complex foudn in the inner membrane of the mitochondira. It is essentially a channel that llows H+ ions to flow from teh intermembrane space to the matrix (down teh gradeint produced by the enyzmes complexes of the electron transport chain); as the H= ions flow through the channel, ATP is synthesized from ADP and Pi

Atrioventricular bundle (AV) bundle

Also known as the Bundle of His, this is the first portion of the cardiac conduction system, after the AV node.

Atrioventricular (AV) node

The second major node of the cardiac conduction system (after the SA node). The cardiac impulse is delayed slightly at teh AV node, allowing the ventricles to contract just after the atria contract.

Atrioventricular valves

The valves in the heart that separte the atria from teh ventricles. The tricuspid valve separates teh right atrium from the right ventricel, and the bicuspid (mitral) valves separates the left atrium from the left ventricle. These valves close at the beginning of systole, preventing the backflow of bloo dfrom ventricles to atria, and producing the first heart sound (lub).


One of the two small chambers in the heart that receive blood and pass it on to the ventricles. The right atrium receives deoxygenated blood from teh body through the superior and inferiro vena cavae, adn the left atrium receives oxygenated blood from teh lungs through the pulmonary veins.


The first step in viral infection. Attachemen of a virus to its host is very specific and is also known as adsorption.

Auditory tube

The tube that connects the middle ear acity with the pharynx; also known as the Eustachian tube. Its fucntion is to equalize midle ear pressure with atmospheric pressure so that pressure on boths sides of the tympanic membrane is the same.

Autoimmune reaction

An immune reaction directed against normal (necessary ) cells.Fo example, diabets melitus (typeI) is an autoimmun reaction directed against teh beta cells of the pancrease (destorying them and preventing insulin secretion) and aginst insulin itself.

Autonomic nervous system (ANS)

The division of the periperal nervsous system that innervates and cotnrols the visceral organs (everything but the skeletal muscles). It is also knowns as the involuntary nervous system and an be subdivided into the sympathetic and parasympathetic divisions.


A chromosome that does not determine gender (is not a sex chromosome). Humans have two sex chromsomes and 22 autosomes.


An organism that makes its own, typically using CO2 as a carbon source.


A bacterium that cannon survive on minimal medium (glucose alone) because it lacks the ability to syntheisze a molecule it needs to live (typically an amino acid). Auxotrphs must ave the needed substance (the auxiliary trophic substance) added to their medium in order to survive. The are typically denoted by teh susbstance they require followed by a "-" sign in superscript. For example, a bacterium that cannot syntehisze leucine would be a leucine auxotroph, and would be indicated as leu- (w/ a superscripth, though)


Lacking a blood supply; cartialge is an example of this


A long projection off the cell body of a neruon down which an action potential can be propagated.


A bacterium having a rod-like shaped (plural = bacilli).


A virus that infects a bacterium.


A sensory receptor that responds to hcanges in pressure; for example, there are baroreceptors in the carotid arteries and the aortic ach that monitor blood pressure.

Basement membrane

A layer of collagen fibers that separates epithelial tissue from connective tisse (example of epithelial cells in digestive tract) - they are actual connective tissue.

Basilar membrane

The flexible membrane in teh chochlea that supports the organ of Corti (structure which contains the hearing receptors). The fibers of the basilar membrane are short and stiff near the oval windown and long and fleaxible near the apex of the cochlea. This difference in structure allows the basilar membrane to help trasnduce pitch.

B cell

A type of lymphocyte that can recognize (bind to) an antigen adn secrete an antibody specific for that antigen. When activated by binding an antigen, B cells mature into plasma cells (that secreted antibody) and memory cells (that patrol the body for future encounters with that antigen). - must be activated by Helper T cell also, though.


HCO3-. THis ion results from the dissociation of carbonic acid, together wiht carbonic acid forms the the major blood buffer system. Bicarbonate is also secreted by teh pancreas to neutralize stomach acid in the intestines.


A green fluid made from cholesterol and secreted by teh liver. It is stored and concentrated in the gallbladder. Bile isn an amphipathic molecule that is secreted itno the small intestine when fats are present, adn serves to emulsify the fats for better digestion by lipases.

Binary fission

An asexual method of bacterial reproduction that serves only to increase the size of the population; ther is no introduciton of gnetic diversity. THe bacterium simply grows in size until it has doubled its cellular components, then it replicates its genone and splits into two.

Bipolar neuron

A neuron with a single axon and a single dendrite, often projecting from opposite sides of the cell body. Bipolar neurons are typically associated with sensory organs; an example is the bipolar neuron in the retina of the eye. - note that one axon may innervate many different muscles, or other things.


A fluid-filled sphere formed about 5 days after fertilization of an ovum that is made up of an outer ring of cells and inner cell mass. THis is the structure that implants in the endometrium of the uterus.

Bohr effect

The tendency of certain factors to stablize the hemoglobin in the tense conformation, thus reducing its affinity for oxygen and enhancing the relase of oxygen to the tissues. The factors include increased PCO2, increase temperature, increased bisphosphoglycerate (BPG), and decreased pH. Note that the Bohr effect shifts the oxy-hemolobin saturation curve to the right.

Bone marrow

A non-bony material that fills the hollow spaces inside bones. Red bone marrow is found in regiosn of spongy bone and is the site of blood cell (red and white) production. Yellow bone marrow is found in the diaphysis (shaft) of long bones, is mostly flat, and is inactive.

Bowman's capsule

The region of the nephron that surrounds the glomerulus. The capsule ollects the plasma that is filtered from teh capillaries in the glomerulus.


Very small air tubes int eh respiratory system (diameter 0.5 - 1.0 mm). The walls of the bronchioles are made of smooth muscle (thus involunatry) to help regulate air flow.

Brush border enzymes

Enzymes secreted by the mucosal cells lining the intestine. The brush border enzymes are disaccharides adn dipeptidases taht digest the smallest peptides and carbohydrates into their respective monomers.

Bulbourethral galnds

Small paired gland found inferior to the prostate in males and at the posterior end of the penile urethra. They secrete an alkaline mucus on sexual arousal that helps toneutralize any traces of acidic urine the urethra that might be harmful to sperm.


A hormone produced by the C-cells of the thyroid gland that decreases serum calcium levels. It targets teh bones (stimulates osteoblasts), the kidneys (reduces calcium reabsorption), and the small intestine (decreases calcium absorption).


A hormone produced from vitamin D that acts in essentially the same manner as parathyroid hormone.


A cyoplasmic Ca2+-binding protein. Calmodulin is particularly important in smooth muscle cells, where binding of Ca2+ allows calmodulin to activate myosin light-chian kinase, the first step in smooth muscle cell contraction.


Very small tube or channel, such as is found between lacunae (connecting them together) in compact bone.


An incrase in the fragility of the membranes of sperm cells when exposed to the female reproductive tract. Capacitation is required sot aht the acrosomal enzymes can be relased to faciliate fertilization.


The smalles of all blodo vessles, typically having a diamtere just large neough for blood cells to pass through in single file. Capillaries have extremelyu thin walls to faciliate the exchange of material between the blood and the tissues.


The outer protein coat of a virus (the whole coat)


Molecules made from monosaccharides that serve as the primary source of cellular energy,. Carbohydrates can also act as cell surface markers (good thing to remember).

Carbonic anhydrase

An enzyme present in erythrocytes (as well as in other places) that catalyzes the conversion of CO2 and H2O into carbonic acid (H2CO3).

Cardiac conduction system

The specialized cells of the heart that spontaneously initiate action potentials and transmit them to the cardiac muscle cells. The cells of the conduction system are essentially cardiac muscle cells, but lack the contractile fibers of the muscle cells, tus they are able to transmit impulses (action ptnetials) more quickly and efficiently that cardiac muscle tissue. The cardiac conduction system includes the SA node, the internodal tract, he Av node, the AV bundle, the right and left bundle branches, and the Purkinje fibers.

Cardiac muscle

The muscle tissue of the heart Cardiac muscle is striated, uninucleate, and under involuntary control (controlled by teh autonomic nervous system). Note also that cardiac muscle is self-stimulatory, and autonomic control serves only to modify the intrinsic rate of contraction.

Cardiac output

The volume of blood pumped out of the heart in one minute (vol/min); the product of the stroke volume (vol/beat) and the heart rate (beat/min). Cardiac output is directly proportional to blood pressure**.

Carrier protein

An integral membrane protein that undergoes a conformational change to move a molecule from one side of the membrane to another. See also 'uniporter', 'antiporter', and 'symporter'.


A strong connective tissue with varying degrees of flexibility. (1) Elastic cartilage is the most flexible, forming structures that reuqire support but also need to bend, such as the epiglottis and outer ear. (2) Hyaline cartilage is more rigid than elastic cartilage, and forms the cartilages of the ribs, the respiratory tract, and all joints. (3) Fibrocartilage is the least flexible of them all, and forms very strong connections, such as the public symphysis and the intervertebral disks.


The process of breaking down large molecules into smaller precursors, e.g. digesion of starch into glucose.


The primary enzyme in peroxisomes; catalse catalyzes the hydrolysis of hydrogen peroxide (H2O2) into water and oxygen.


Something that increases the rate of a chemical reaction by reducing the activation energy for that reaction. The free energy of reaction remains unchanged.


Complementary DNA. DNA produced synthetically by reverse trascribing mRNA. Because of eukaryotic mRNA splicing, cDNA contains no inrons.


The first part of the large intestine.

Cell surface receptor

An integral membrane proteint hat binds extracellular signaling molecules, suchas hormones and peptides.

Central canal

The hollow center of an osteon, also known as a Haversian canal. The central canal contains blood vessels, lymphatic vessels, lymphatic vessels, and nerves. Bone is laid down around the central canal in concentric rings called lamellae.

Central chemoreceptors

Receptors in the central nervous system that monitor the pH of cerebrospinal luid to help regulate ventilation rate.

Central Nervous System

The subdivision of the nervous system consisting of the brain and spinal cord.


A structure composed of a ring of nine microtube triplets, found in pairs in the MTOC (microtubule organizing center) of a cell. The centrioles duplicate during the cell division, and serve as the organizing center for the mitotic spindle.


A structure near the middle of eukaryotic chromosomes to which the fibers of the mitotic spindle attach during cell division.


The region of teh brain that coordinates and smooth skeletal muscle activity.

Cerebral cortex

A thin (4 mm) layer of gray matter on the surface of the cerebral hemispheres. The cerebral cortex is the conscious mind, and is functionally divided into four pairs of lobes: the frontal lobes, the parietal lobes, the temporal lobes, and the occipital lobes.

Cerebrospinal fluid

A clear fluid the circulates around through the brain and spinal cord that helps to physially support teh brain and act as a shock absorber, and taht also exchanges nutrients and wastes with teh brain and spinal cord.

Ceruminous gland

A gland that secretes a waxy product, found in the external ear canal.


The opening to the uterus The ervix is typically plugged with a sticky acidic mucus during non-fertile times (to form a barrier against the entry of pathogens), however during ovulation the mucus becomes more watery and alkaline to facilitate sperm entry.

Channel protein

An integral protein that selectively allows molecules across the plasma membrane. See also entries under 'ion channel', 'voltage-gated channel', and 'ligand-gated channel'.

Chemical synapse

A type of synapse at which a chemical (a neurotransmitter) is released from teh axon of a neuron into the ysnaptic cleft where it binds to receptors on the next structure in sequence, either another neuron or an organ.


A sensory receptor that responds to specific chemicals. Some examples are gustatory (taste) receptors, olfactory (smell) receptors, and central chemoreceptors (responds to pH changes in teh cerebrospinal fluid).


Movement that is directed by chemical gradients, such as nutrients or toxins. (seen in some bacteria)


An organism that relies on a chemical source of energy (such as ATP) instead of light (which phototrophs).

Chief cells

Pepsinogen-secreting cells foudn at teh bottom of the gastric glands


A poysaccharide found in the cell walls of fungi and in the exoskeletons of insects.

Cholecystokinin (CCK)

A hormone secreted by the samll intestine (duodenum) in response to the presence of fats. It promotes release of bile from the gallbladder and pancreatic juice from the pancreas,and reduces stomach motility.


A large, ring shaped lipid found in cell membranes. Cholesterol is the precursor for steroid hormones, and is used to manufacture bile salts.


A mature, cartilage cell.


The portion of the placenta derived from the zygote.


The darkly pigmented middle layer of the eyeball, found between teh sclera (outer layer) and the retina (inner layer).


A single piece of double-stranded DNA; part of the genome of an organism. Prokaryotes have circular chromosomes and eukaryotes have linear chromosomes.


A type of lipoprotein; the form in which absorbed fats from the intestines are transported to the circulatory system.


Partially digested, semiliquid food mixed with digestive enzymes and acids in the stomach.


One of the main pancreatic proteases; it is activated (from chymotrypsinogen) by trypsin.


A hair-like structure on teh cell surface composed of microtubules ina '9+2' arrangement (nine pairs of microtubles surrounding 2 single microtubules in the center). Teh microtubules are conneted with a contractile protien called dynein. Cilia beat in a repetitive sweeping motion, which helps to move substances along the surface of the cell. They are particularly important in the respiratory system, where they sweep mucus out of the trachea and up to the mouth and nose.

Ciliary muscles

Muscles that help focus light on teh retin by controlling the curvature of the lens of the eye.

Circular smooth muscles

The inner layer of smooth muscle in the wall of the digestive tract. When the circular muscle contracts, the tube diameter is reduced. Certain areas of the circular muscle are thickened to act as valves (sphincters).


A fibrous protein found on the intracellular side of the plasma membrane (also associated with the Golgi complex) that helps invaginate the membrane. Typically cel surface receptors are associated with clathrin-coated pits at the plasma membrane binding of the ligan to the receptor trigger invagination (example: cholesterol uptake via lipoprotein endocytosis).


The rapid mitotic division of a zygot that being within 24-36 hours after fertilization


A bacteria having a round shape (plural = cocci)


The curled structure in the inner ear that contains the membranes and hair cells that transduce sound waves into action potentials.


A situation in which a heterozygote displays the phenotype associated with each of the alleles, e.g., human blood type AB.


A group of three nucleotides taht is specific for a particular amino acid, or that specifies 'stop translating'


An **organic molecuel taht associates non-covalently with an enzyme, and that is required for the proper functioning of the enzyme.


An **inorganic molecule that associates non-covalently with an enzyme and that is required for the proper functioning of the enzyme


A protein fiber with a unique triple-helix that gives it great strength. Tissues with a lot of collagen fibers are typically very strong, e.g. bone, tendons, ligaments, etc.

Collecting duct

The portion of the nephron where water reabsorption is regulated via antidiuretic hormone (ADH). Several nephrons empty into each collecting duct, and this is the final region through which urine must passon its way to the ureter.

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