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Flashcards in Biology2 Deck (292):

Oval window

The membrane that separates the middle ear from the inner ear.


Ovarian cycle

The 28 days of the menstrual cycle as they apply to events in the ovary. The ovarian cycle has three subphases: the follicular phase, ovulation, and the luteal phase.



The female primary sex organ. The ovary produces female gametes (ova) and secretes estrogen and progesterone.



The release of a secondary oocyte (along with some granulosa cells) from the ovary at the approximate midpoint of the menstrual cycle (typically around day 14). Ovulation is triggered by a surge in LH.



A four-carbon molecule that binds with the two-carbon acetyl unit of acetyl-CoA to form citric acid in the first step of the Krebs cycle.



To attach oxygen, to remove hydrogen, or to remove electrons from a molecule.


Oxidative phosphorylation

The oxidation of high-energy electron carriers (NADH and FADH2) coupled to the phosphorylation of ADP, producing ATP. In eukaryotes, oxidative phosphorylation occurs in the mitochondira.



A hormone released by the posterior pituitary that stimulates uterine contractions during childbirth and milk ejection during breastfeeding.


Pacemaker potential

A self-initiating action potential that occurs in the conduction system of the heart and triggers action potentials (and thus contraction) in the cardiac muscle cells Tee pacemaker potential is triggered by the regular, spontaneous depolarization of the cells of the conductions system, due to slow inwar leak of positive ions (Na+ and Ca2+). Because the SA node has the fastest leak, it typically reaches the threshold for the pacemaker potential before any other region of the conduction system, and thus sets the pace of the heart.



An organs in the abdominal cavity with two roles. The first is an exocrine role: to produce digestive enzymes and bicarbonate, which are delivered to the small intestine via the pancreatic duct. The second is an endocrine role: to secrete insulin and glucagon into the bloodstream to help regulate blood glucose levels.


Pancreatic duct

The main duct of the pancreas. The pancreatic duct carries the exocrine secretions of the pancreas (enzymes and bicarbonate) to the small intestine (dueodenum).



An organism that requires the aid of a host organism to survive, and that harms the host in the process.


Parasympathetic nervous system

The division of the autonomic nervous system known as the 'resting and digesting' system. It causes a general decrease in body activities such as heart rate, respiratory rate, and blood pressure, and an increase in blood flow to the GI tract and digestive function. Because the preganglionic neurons all originate from either the brain or the sacrum, it is also known as the craniosacral system.


Parathyroid hormone (PTH)

A hormone produced and secreted by the parathyroid glands that increases serum calcium levels. It targets the bones (stimulates osteoclasts), the kidneys (increases calcium reabsorption), and the small intestine (increases calcium absorption).


Parietal cells

Cells found in gastric glands that secrete hydrochloric acid (for hydrolysis of ingested food) and gastric intrinsic factor (for absorption of vitamin B-12).


Partial pressure

The contribution of an individual gas to the total ppressure of a mixture of gases. Partial pressures are used to describe the amounts of the various gases carried in the bloodstream.


Passive transport

Movement across the membrane of a cell that does not require energy input from the cell. Passive transport relies on concentration gradients to provie the driving force for movement, and includes both simple and facilitated diffusion.



The percentage of individuals with a particular genotype that actually displays the phenotype associated with the genotype.



The second step in viral infection, the injection of the viral genome into the host cell.



A protein-digesting enzyme secreted by the chief cells of the gastric glands. Pepsin is secreted in its inactive form (pepsinogen) and is activated by gastric acid. It is unusual in that its pH optimum is around 1-2; most of these enzymes in the body function best at neutral pHs


Peptide bond

The bond formed between the carboxyl group of one amino acid and the amino group of another.


Peptide hormone

A hormone made of amino acids (in some cases just a single, modified amino acid). Peptide hormones are generally hydrophilic and cannot cross the plasma membranes of cells, thus receptor for peptide hormones must be found on the cell surface. An exception is thyroxine, which is hydrophobic enough to enter the cells easily. Binding of a peptide hormone to its receptor usually triggers a second messenger system within the cell.



A complex polymer of sugars and amino acids; the substance from which bacterial ell walls are made.



The flow of blood through a tissue; ischeia is when there is no blood flow, anoxia when there is no O2 available (ischemia is more dangerous b/c of waste build-up)


Peripheral chemoreceptors

Receptors in the carotid arteries and the aorta that monitor blood pH to help regulate ventilation rate.


Peripheral membrane protein

A protein that is associated with the plasma membrane of a cell, but that is not embedded in the lipid bilayer. Peripheral proteins typically associate with embedded proteins through hydrogen bonding or electrostatic interactions.


Periperal nervous system

All parts of the nervous system except for the brain and spinal cord.


Peripheral resistance

The resistance to blood flow in the systemic circulation. Peripheral resistance increases if arteries constrict (diameter decreases), and an increase in peripheral resistance leads t o an increase in blood pressure.


Periplasmic space

The space between the inner and outer cell membranes in Gram-negative bactera. The peptidoglycan cell wall is found in the periplasmic space, and this space sometimes contains enzymes to degrade antibiotics.



A wave of contraction that sweeps along a muscular tube, pushing substances along the tube (e.g., food through the digestive tract, urine through the ureters, etc.)



Small organelles that contain the hydrogen peroxide produced as a byproduct of lipid metabolism. Peroxisomes convert hydrogen peroxide to water and oxygen by way of the enzyme catalase.



The non-specific uptake of solid material by a cell accomplished by englufing the particle with plasma membrane and drawing it into the cell.



A passageway leading from behind the nasal cavity to the trachea. The pharynx is divided into three regions, named for their location. The nasopharynx is behind the nasal cavity, the oropharynx is behind the oral cavity, and the laryngopharynx is behind the larynx. The nasopharynx is a passageway for air only, but the oropharynx and laryngopharynx are passageways for both air and food; consequently they are lined with a much thicker layer of cells to resis damage due to abrasion.



The physical characterisitcs resulting from the genotype. Phenotypes are usually described as dominant or recessive.



The enzyme that catalyzes the phosphorylation of fructose-6-phosphate to form fructose-1-6-bisphosphate in the third step of glycolysis. This is the main regulatory step of glycolysis. PFK is feedback-inhibited by ATP.



The primary membrane lipid. Phospholipids consist of a glycerol molecule esterified to two fatty acid chains and a phosphate molecule. Additional, highly hyrohpilic groups are attached to the phosphate, making this molecule extremely amphipathic.



A receptor that responds to light



An organism that utilizes light as its primary energy source.



A long projection on a bacterial surface involved in an attachment, e.g., the sex pilus attaches F+ and F- bacteria during conjugation.



The non-specific uptake of liquid particles into a cell by invagination of the plasma membrane and subsequent 'pinching off' a small bit of the extracellular fluid.



An organ that develops during pregnacy, derived in part from the mother and in part from the zygote. The placenta is the site of exchange of nutrients and gases between the mother's blood and the fetus' blood. The placenta is formed during the first three months of pregnancy.


Placental villi

Zygot-derived projections that extend into the endometrium of the uterus during pregnancy. Fetal capillaries grow into the placental villi, which are surrounded by a pool of maternal blood. THis facilitates nutrient and gas exchange between the mother and the fetus, without actually allowing the blood to mix.



A clear area in a lawn of bacteria. Plaques represent an area where bacteria are lysing (dying) and usually caused by a lytic virus.



The liquid portion of blood; plasma contains water, ions, buffers, sugars, proteins, etc. Anything that dissolves in blood dissolves in the plasma portion.


Plasma cell

An activated B cell that is secreting antibody.



A small, extrachromosomal (outside the genome), circular DNA molecule found in prokaryotes.



Extremely small pseudo-cells in the blood, important for clotting. They are not true cells, but are broken-off bits of a larger cell (a megakaryocyte).


Pleiotropic gene

A gene that has effects on several different characteristics.



The membranes that line the surface of the lungs (visceral pleura) and the inside wall of the chest cavity (parietal pleura).


Pleural pressure

The pressure in the (theoretical) space between the lung surface and the inner wall of the chest cavity.


Point mutation

A type of mutation in DNa where a single base is substituted for another.


Polar body

A small cell with extremely little cytoplasm that results from the unequal cytoplasmic divsion of the primary (produces the first polar body) and the secondary (produces the second polary body) oocytes during meiosis (oogenesis). The polar bodies degenerate.


Poly-A tail

A string of several hundred adenine nucletodies added to the 3' end of the eukaryotic mRNA.


Poycistronic mRNA

mRna that codes for several different proteins by utliizing different reading frames, nested genets, etc. Polycistronic mRNa is a characteristic of prokaryotes.



A molecule formed by joining many monosaccharides together. POlysaccharides are typically energy-storage molecules (glycogen in animals, starch in plants) or structural molecules (cellulose in plants, chitin in exoskeletons).



The fertilization of an oocyte by more than one sperm. This occurs in some animals, but in humans, blocks to polyspermy exist (the fast block and the slow block) so that only a single sperm can penetrate the oocyte.



A subset of a species consisting of members that mate and reproduce with one another.



A pathway through a plasma membrane that restrics passage based only on the size of the molecules. Pore are made from porin proteins.


Portal systems

A system of blood vessels where the blood passes from arteries to capillaries to veins, then through a second set of capillaries, and then through a final set of veins. THere are two portal systems in the body, the hepatic portal system and the hypothalamic portal system.


Posterior pituitary gland

Also known as the neurohyophysis, the posterior pituitary is made of nervous tisssue and stores and secretes two hormones made by the hypothlamus; oxtytocin and ADH. The posterior pituitary is controlled by action potentials from the hypothalamus.


Postganglionic neuron

In the autonomic division of the PNS, a neuron that has its *cell body located in the autonomic ganglion* (where a preganglionic neuron synapses with it) and whose axon synapses with the target axon.


Potassium leak channel

An ion channel specific for potassium found in the plasma membrane of all cells in the body. Leak channels are constitutively open and allow their specifi ion to move across the membrane according to its gadient. Potassium leak channels allow potassium to leave the cell.


Power stroke

The step in the sliding filament theory during which yosin undergoes a conformaitonal change to its low energy state, in the process dragging the thin filaments (and the attached Z lines) toward the center fo the sarcomere. NOte that power stroke requires ATP only indirectly: to se the myosin molecule in its high-energy conformation during a different step of the sliding filament thoery.


Preganglionic neuron

In the autonomic divison of the PNS, a neuron that has its *cell body located in the CNS*, and whose axon extends into the PNS to synapse with a second neuron at an autonoic ganglion. (The second neuron's axon synapses with the target axon)


Primary active transport

Active transport that relies directly on the hydrolysis of ATP.


Primary bronchi

The first branches of the trachea. There are two primary bronchi, one for each lung.


Primary immune response

The first encounter with an antigen, resuling in activated B cells (antibody secretion) and T cells (cellular lysis and lymphocyte proliferation). The primary immune response takes approximately ten days, which long enough for symptoms of the infection to appear (because initial activation takes long time).


Primary oocytes

Diploid cells resulting from the activation of anoogoium; primary oocytes are ready to enter meiosis I. remember: cyte means ready to undergo meiosi


Primary spermatocytes

Diploid cells resultinf rom the activation of a spermatogoium; primary spermatocytes are ready to enter meiosis I. remember: cyte means ready to undergo meiosis.



An RNA polymerase that creates a primer (made of RNA) initiate DNa replication. DNA pol binds to the primer and elongates it.


Productive cycle

A life cycle of animal viruses in which the mature viral particles bud from the host cell, acquiring an envelope (a coating of lipid bilayer) in the process.



A steroid hormone produced by the corpus luteum in the ovary during the second half of the menstrual cycle Progesterone maintains and enhances the uterine lining for the possible implantation of a fertilized ovum. It is the primary hormone secreted during pregnancy.



An organism that lacks a nucleus or any other memrane-bound organelles. All prokaytes belong to the Kingdom Monera (not protista!)



A hormone secreted by the anterior pituitary that tarets the mammary glands stimulating them to produce breastmilk.


Proliferative phase

The second phase of the uterine (endometrial) cycle, during which the endometrium (shed off during menstration is rebuilt). This phase of the cycle is under the control of estrogen, secreted from the follicle developing in the ovary during this time period. The proliferative phase typically lasts from day 6 to day 14 of the menstrual cycle.



The sequence of nucleotides on a chromosome that activates RNA polymerase so that transcription can take place. The promoter is found upstream of the start site, the location where transcription actually takes place.



The first phase of mitosis. During prophase the replicated chromosomes condense, the spindle is formed, and the nuclear envelope breaks apart into vessicles.


Prophase I

The first phase of meiosis I. During prophase I the replicated chromosomes condense, homologous chromsomes pair up, crossing over occurs between homologous chromosomes, the spindle is formed, and the nuclear envelope breaks apart into vesicles. Prophase I is the longest phase of meiosis.


Prophase II

The first phase of meiosis II. Prophase II is identical to mitotic prophase, except that the number of chromosomes was reduced by half during meiosis I.



A receptor that responds to changes in body position, such as stretch on a tendon, or contraction of a muscle. These receptor allow us to be consciously aware of the position of our body parts.



A small gland encircling the male urethra just inferior to the bladder (only reproductive structure not paired). Its secretion contain nutrients and enzymes and account for approximately 35% of the ejaculate volume.


Prosthetic group

A non-protein, but organic, molecule (such as vitamin) that is covalently bound to an enzyme as part of the active site.



Molecules made by connecting amino acids via peptide bonds. Proteins are synthesized (translated) by ribosomes, and function as enzymes, carriers, structrual fibers, cell surface receptors, channels, porters, hormones, etc.


Proximal convoluted tubuel

The first portion of the nephron tubuel after the glomerulus. THe PCT is the site of most reabsorption; all filtered nutrients are reabsorbed here as well as most of the filtered water.


P site

Peptidyl-tRNA site; the stie on a ribosome where the growing peptide (attached to a tRNA) is found during translation.



Salivary amylase


Pulmonary artery

The blood vessel that carries deoxygenated from the right ventricle of the heart to the lungs.


Pulmonary circulation

The flow of blood from the heart, through the lungs, and back to the heart.


Pulmonary edema

The collection of fluid in the alveoli, particularly dangerous because it impedes gas exchange. Common causes of pulmonary edema are increased pulmonary blood pressure or infection of the respiratory system.


Pulmonary vein

One of several vessels that carry oxygenated blood from the lungs to the left atrium of the heart.



A hole in the center of the iris of the eye that allows light to enter the eyeball. The diameter of pupil is controlled by the iris in response to the brightness of the light.


Purine bases

Aromatic bases found in DNA and RNA that are derived from purine. They have a double rightn structure and include adenine and guanine.


Purkinje fibers

The smallest (and final) fibers in the cardiac conduction system. The Purkinje fibers transmit the cardiac impulse to the ventricular muscle.


Pyloric sphincter

The valve that regulates the passage of chyme from the stomach into the small intestine.


Pyrimidine bases

Aromatic bases found in DNa and RNA that have a single-ring structure. They include cytosine, thymine, and uracil.


Pyruvate dehydrogenase complex

A group of three enzymes that decarboxylates pyruvate, creating an acetyl group and carbon dioxide. The acetyl group is then attached to coenzyme A to produce acetyl-CoA, a substrate in the Krebs cycle. In the process, NAD+ is reduced to NADH. The pyruvate dehydrogenase complex is the second stage of cellular respiration.


Pyruvic acid

The product of glycolysis; 2 pyruvic acid (pyruvate) molecules are produced from a single glucose molecule. In the absence of oxygen, pyruvic acid undergoes fermentation and is reduced to either lactic acid or ethanol; in the presence of oxygen, pyruvi acid is oxidized to produce acetyl-CoA, which can enter the Krebs cycle.


Receptor-mediated endocytosis

A highly specific cellular uptake mechanism. The molecule to be taken up must bind to cell surface receptor found in a clathrin-coated pit.



The allele in a heterozygou genotype that is not expressed; the phenotype resulting from possession of two recessive alleles (homozygous recessive).


Recombination frequency

The RF value, the percentage of recombinant offspring resulting from a given genetic cross. The recombination frequency is proportional to the physical distance between genes on a chromosome. If a recombination frequency is low, the genes under consideration may be linked.



The final portion of the large intestine.



To remove oxygen, to add hydrogen, or to add electrons to a molecule.


Reflex arc

A relatively direct connection between a sensory neuron and a motor neuron that allows an extremely rapid response to a stimulus, often without conscious brain involvement.


Relative refractory period

The period of time following an action potential when it is possible, but difficult, for the neuron to fire a second action potential due to the fact that membrane is further from theshold potential (hyperpolarized).


Release factor

A cytoplasmic protein that binds to a stop codon where it appears in the A-site of the ribosome. Release factors modify the peptidyl transferase activity of the ribosome, such that a water molecule is added to the end of the completed protein. This releases the finished protein from the final tRNA, and allows the ribosome subunits and mRNA to disassociate.


Renal absorption

The movement of a substance from the filtrate (in the renal tuble) bak into the bloodstream. Reabsorption reduces the amount of a substance in the urine.


Renal tubule

The portion of the nephron after the glomerulus and apsule; the region of the nephron where the filtrate is modified along its path to becoming urine.



An enzyme secreted by the juxtaglomerular cells when blood pressure decreases. Renin onverts angiotensinogen to angiotensin I.



The duplication of DNA


Relication fork(s)

The site(s) where the parental DNA double helix unwinds during replication.


Replication bubbles

Multiple sites of replication found on large, linear eukaryotic linear eukaryotie chromosomes.



The return of membrane potential to normal resting values after a depolarization of hyperpolarization.


Repressible enzyme

An enzyme whose transcription can be stopped by an abundance of its product (as opposed to inducible enzymes). Usually part of anabolism of product.



A regulatory protein that binds DNA at a specific nucleotide sequence (sometimes known as the operator) to prevent transcription of downstream genes.


Residual volume

The volume of air remaining in the lungs after a maximal forced exhalation, typically about 1200 mL.



A function the reproductive system (conrolled by the sympathetic nervous system) that returns the body to its normal resting state after sexual arousal and orgasm.


Respiratory acidosis

A drop in blood pH due to hypoventilation (too little breathing) and a resulting accumulation of Co2.


Respiratory alkalosis

Arise in blood pH due to hyperventilation (excessive breathing) and a resulting decrease in CO2.


Resting membrane potential

An electrical potential established across the plasma membrane of all cells by the Na+/K+ ATPase and the K+ leak channels. IN most cells, the resting membrane potential is approximately -70 mV with respect to the outside of the cell.


Restriction endonuclease

A bacterial enzyme that recognizes a specific DNA nucleotide sequence and that cuts the double helix at a specific site within the sequence.



The innermost layer of the eyeball. The retina is made up of a layer of photoreceptors, a layer of bipolar cells, and a layer of ganglion cells.



A chemical derived from vitamin A found in the pigment proteins of the rod photoreceptors of the retina. Retinal changes conformation when it absorbs light, triggering a series of reactions that ultimately result in an action potential being sent to the brain.



A virus with an RNA genome (e.g. HIV) that undergoes a lysogenic life cycle in a host with a double stranded DNA genome. In order to integrate its genome with the host cell genome, the virus must first reverse trasncribe its RNA genome to DNA.


Reverse transcriptase

An enzyme that polymerizes a strand of DNA by reading an RNA template (an RNA dependent DNa polymerase); used by retrovirus in order to integrate their genome with the host cell genome.



A structure made of two protein subunits and rRNA; this is the site of protein synthessis (translation) in a cell. Prokaryotic ribosomes (also known as 70S ribosomes) are smaller than eukaryotic ribosome (80S ribosomes). The S value refers to the sedimentation rate during centrifugation.


RNA dependent RNA polymerase

A viral enzyme that makes a strand of RNA by reading a strand of RNa . All prokaryotic and eukaryotic RNa polymerases are DNa dependent; they make a strand of RNa by reading a strand of DNA.


RNA polymerase

An enzyme that transcribes RNa. Prokaryotes have a single RNA pol, while eukaryotes have three; in eukaryotes, RNA pol I transcribes rRNA, RNA pol II transcribes mRNA, and RNA pol III transcribes tRNA.



Photoreceptors in the retina of the eye that respond to dim light and provide us with black and white vision.


Rough endoplasmic reticulum

A large system of folded membranes within a eukaryotic cell that has ribosomes bound to it, giving a rough appearnce. These ribosomes synthesize proteins that will ultimately be secreted from the cell, incorporated into the plasma membrane, or transported to the Golgi apparatus or lysosome.



Ribosomal RNA; the type of RNA that associates with ribosomal proteins to make a functional ribosome. It is thought that the rRNA has the peptidyl transferase activity.


Rule of addition

A statistical rule stating that the probability of either of two indpendent (and mutually exclusive) events ocuring is the sum of their individual probabilities minus the probability of them both occuring together.


Rule of multiplication

A statistical rule stating that the probability of two independent events occuring together is the product of their individual probabilities.


Saltatory conduction

A rapid from of action potential conduction along the axon of a neuron in which the action potential appears to jump from nodde of Ranvier to node of Ranvier.



An organism (such as a fungus) that feeds of dead plants and animals.



The plasma membrane of a muscle cell.



The unit of muscle contraction. Sarcomeres are bounded by Z lins, to which thin filaments attach. Thick filaments are found in the center of the sarcomere, overlapped by thin filaments over one another during contraction reduces the distance between Z lines, shortening the sarcomere.


Sarcoplasmic reticulum

The smooth ER of a muscle cell, enlarged and specialized to act as a Ca2+ reservoir. The SR winds around each myofibril in the muscle cell.


Schwann cell

One of the two peripheral nervous system supporting (glial) cells. Schwann cells from he myelin sheath on axons of peripheral neurons.



The white portion of teh tough outer layer of the eyeball


Sebaceous gland

Oil-forming glands found all over the body, especially on the face and neck. The product (sebum) is released to the skin surface through hair follicles.


Seondary active transport

Active transport that releies on an established concentration gradient, typically set up by a primary active transporter. Secondary active transport relies on ATP indirectly.


Secondary immune response

A subsequent immune response to previously encountered antigen that results in antibody production and T cell activation. The secondary immune response is mediated by memory cells (produced during the primary immune respone) and is much faster and stronger than the rpimary response, typicaly taking only a dya or less. THis is not long enough for the infection to become established, and symptoms do not appear, thus the person is said to be "immune" to that particular antigen.


Secondary oocyte

A haploid cell resulting from the first meiotic division of oogenesi (not that the cytoplasmic division in this case is unequal, producing one large cell with almost all of they cytoplasm - the secondary oocyte- and one smaller cell with virtually no cytoplasm - the first polar body.). The secondary oocyte (along with some follicular cells) is released from the ovary during ovulation.


Secondary spermatocytes

Haploid cells resulting from the first meiotic division of spermatogenesis. Secondary spermatocytes are ready to enter meiosis II.


Secondary sex characteristics

The set of adult characteristics that develop during puberty under the control of the sex steroids. In males the secondary sex characteristics include enlargement and maturation of the genitalia, growth of facial, body, and pubic hair, increased muscle mass, and lowering of the voice. In females, the characteristics include the onset of menstruation and the menstrual cycle, enlargement of the breasts, widening of the pelvis, and growth of pubic hair.


Second Law of Thermodynamics

The entropy (disorder) of the universe (or system) tends to increase.


Second messenger

An intracellular chemical signal (such as cAMP ) that relays instructions from the cell surface to enzymes in the cytosol.



A hormone secreted by the small intestine (duodenum) in response to low pH (e.g., from stomach acid). It promotes the release of bicarbonate from the pancreas to act as a buffer.



(1) The secretion of useful substances from a cell, either into the blood (endocrine secretin) or into a cavity or onto the body surface (exocrine secretion). (2) in the nephron, the movement of substances from the blood to the filtrate along the tubule. Secretion increases the rate at which substances can be removed from the body.


Secretory phase

The third phase of the uterin (endometrial) cycle, during which the rebuilt endometrium is enhanced with glycogen and lipid stores. The secretory phase is primarily under the controll of progestone and estrogen (secreted from the copus luteum during this time period), adn typically lasts from day 15 to day 28 of the menstrual cycle.



An alkaline, fructose-rich fluid produced by three different glands in the male reproductive tract and released during ejaculation. Semen is very nourishing for sperm.


Semicircular canals

Three loop-like structures in the inner ear that contain sensory receptors to monitor balance.


Semiconservative replication

DNA replication in which each of the parental strands is read to make a complementary daughter strand, ethus each new DNa molecule is composed of half the parental molecule paired with a newly synthesized strand.


Semilunar valves

The valves in the heart that separate the ventricles from the arteries. The pulmonary semilunar valve separates the right ventricle from the pulmonary artery, and the aortic semilunar valve separates left ventricle from the aorta. These valves close at the end of systole, preventing the backflow of blood from arteries to ventricles, and producing the second heart sound.


Seminal vesicles

Paired glands found on the posterior external wall of the bladder in males. Their secretions contain an alkaline mucus and fructose, among other things, and make up approximately 60% of the ejaculate volume.


Seminiferous tubules

Small convoluted tubules in the testes where spermatogenesis takes place.


Sertolli cells

Cells that form the walls of the seminiferous tubules and help in spermatogenesis Sertoli cells are also called susenacular cells.



Plasma with the *clotting factors removed*. Serum is often used in diagnostic tests because it does not clot.


Sex-linked rait

A triat determined by a gen on either the X or Y chromosomes (the sex chromosomes).


Shine-Dalgarno sequence

The prokaryotic ribosome-binding site on mRNA, found 10 nucleotides 5' to the start codon.


Signal recognition particle (SRP)

A cytoplasmic protein that recognizes the signal sequences of proteins destined to be translated at the rough ER. It binds first to the ribosome translating the protein with the signal sequence then to an SRP receptor on the rough ER>


Signal sequence

A short sequence of amino aids, usually found at the N-terminus of a protein being translated, that directs the ribosome and its associated mRNa to the membranes of the rough ER where trasnlation will be completed. Signal sequences are found on membrane-boudn proteins, secreted proteins, and proteins destined for other organelles.


Signal transduction

The intracellular process triggered by the binding of a ligand to its receptor on the cell surface. Typically this activates seond messenger pathways.


Silent mutation

A point mutation in which a codon that specifies an amino acid is mutated into a new codon that specifies the same amion acid.


Simple diffsuion

The movement of a hydrophobic molecule across the plasma membrane of cell, down its concentration gradient. Since the molecule can esialy interact with the lipid bilayer, no additional help (such as a channel or pore) is required.


Single strand binding proteins

Proteins that bind to and stabilize the signle strands of DNA exposed when helicase unwinds the double helix in preparation for replication.


Sinoatrial (SA) node

A region of specialized cardiac muscle cells in the right atrium of the heart that initiate the impules of heart contraction; for this reason the SA node is knownas the 'pacemaker' of the heart.


Sister chromatid

Identical copies of a chromosome, produced during DNA replication and held together at the centromere Sister chromatids are separated during anaphase of mitosis.


Skeletal muscle

Muscle tissue that is attached to the bones. SKeletal muscle is striated multinucleate, and under voluntary control.


Siding filament theory

The mechanism of contraction in skeletal and cardiac muscl cells. It is a series of four repeated steps: (1) myosin binds actin, (2) myosin pull actin toward the center of the sarcomere (3) myosin releases actin, and (4) myosin resets to its high-energy conformation.


Slow block to polyspermy

Also known as the cortical reaction, the slow block invovles an increase in intracellular [Ca2+] in the egg, which causes the release of cortical granules near the egg plasma membrane. This results in the hardening of the zona pellucida and its separation from the surface of the egg, preventing the entry of more than one sperm into the egg.


Small intestine

The regino of the digestive tract where virtually al digestion and absorption occur. It is subdivided into three regions: the duodenum, the jejunum, and the ileum.


Smooth Endoplasmic Reticulum

A network of membranes inside eukarytoic cells invovled in lipid synthesis (steroid in gonads), detoxification (in liver cells), and/or Ca2+ storage (muscle cells).


Smooth muscle

Muscle tissue found in the walls of hollow organs, e.g., blood vessels, the digestive tract, the uterus, etc. Smooth muscle is non-striated, uninucleate, and under involuntary control (controlled by the autonomic nervous system).



The cell body of a neuron.


Somatic nervous system

The division of the peripheral nervous system that innervates and controls the skeletal muscles; also known as the voluntary nervous system.


Spatial summation

Integration by a postsynaptic neuron of inputs (EPSPs and IPSPs) from multiple sources.



A haploid but immature cell resulting from the second meiotic division f spermatogenesis. Spermatids undergo significant physical changes to become mature sperm (spermatozoa).



Sperm production; occurs in human males on a daily basis from puberty until death. Spermatogenesis results in the production of four mature gametes (sperm) from a single precursor cell (spermatogonium). For maximum sperm viability, spermatogenesis requires cooler temperatures and adequate testosterone.



A diploid cell that can undergo mitosis to form more spermatogonium, and can also be triggered to undergo meiosis to form sperm.


S phase

The phase of the cell cycle during which the genome is replicated.


Sphincter of Oddi

The valvecontrolling release of bile and pancreatic juice into the bloodstream.



A blood pressure cuff



A bacterium having a spiral shape (plural = spirochetes)



An abdominal organ that is considered part of the immune system. THe spleen has four functions: (1) it filters antigen from the blood (2) it is the site of B cell maturation, (3) it stors blood, and (4) it destroys old red blood cells.



One type of eukaryotic mRNA processing in which introns are removed from the primary transcript and exons are ligated together. SPlicing of transcripts can be different in different tissues.


Spongy bone

A looser, more porous type of bone tissue found at the inner core of the epiphyses in long bones and all other bone types. Spongy gone is filed with red bone marrow, important in blood cell formation.


Start site

The location on a chromosome where transcription begins.


Steroid hormone

A hormone derived from cholesterol. Steroids are generally hydrophobic and can easily cross the plasma membrane of cells, thus receptors for steroids are found intracellularly. Once this steroid binds to its receptor, the receptor-steroid complex acts to regulate transcription in the nucleus.



The portion of the digestive tract that stores and grinds food. Limited digestion occurs in the somach, and it has the lowest pH in the body (1-2).


Stop codon

A group of nucleotides that does not specify a particular amino acid, but instead serves to notify the ribosome that the protein being translated is complete. The stop codons are UAA, UGA, and UAG. They are also known as nonsense codons.


Stroke volume

The volume of blood pumped out the heart in a single contraction.



The layer of connective tissue directly under the mucosa of an open body cavity.



The reactants in an enzyme-catalyzed reaction. Substrate binds at the active site of an enzyme.


Sudoriferous gland

A sweat gland located in the dermis of the skin. Sweat consists of water and ions (including Na+ and urea) and is secreted with temperatures rise.



(1) The integration of input (EPSPs and IPSPs) from many presynaptic neruons by a single postsynaptic neuron, either temporaly or spatially. Summation of al input can either stimulate the postsynaptic neuron and possibly lead to an action potential, or it can inhibit the neuron, reducing the likelihood of an action potential. (2) The integration of single muscle twitches into a sustained contraction (tetany).



A method of DNA protection utilized by prokaryotes in which their large circular chromosome is coiled upon itself.



An amphipathic molecule secreted by cells in the alveoli (type 2 alveolar cells) tha reducs surface tension on the inside of the alveolar walls. This prevents the alveoli from collapsing upon exhale and sticking together, thus reducing the effort required for inspiration.


Sympathic nervous system

The division of the autonomic nervous system known as the "fright or flight" system. It causes a genera increase in body activities such as heart rate, respiratory rate, and blood pressure, and an increase in blood flow to skeleltal muscle. It causes a general decrease in digestive activity. Because al of its preganglionic neurons originate from the thoraci or lumbar regions of the spinal cord, it is also known as the thoracolumbar system.



A carrier protein that transports two molecules across the plasma membrane in the same direction. For example, the Na+-glucose cotransporter in intestinal cells is a symporter.



A neuron-to-neuron, neuron-to-organ, or muscle to cell-to-muscle cell junction.



Pairing of homologus chromosomes in a diploid cell, as occurs during prophase I of meiosis.


Synaptic cleft

A microscopic space between the axon of one neuron and the cell body or dendrites of a secon neruon, or between the axon of a neuron and an organ.



A large multinucleate cell, typically formed by the fusion of many smaller cells during development (e.g. a skeletal muscle cell), or formed by nuclear division in the absence of cellular division.



Something that works together with another thing to augment the the second thing's activity. For example, a uscle that assists another muslce is said to be a syngergist. An enzyme that helps another enzyme is a synergist.


Synovial fluid

A lubricating, nourishing fluid found in joint capsules.


Systemic circulation

The flow of blood from the heart, through the body (not including the lungs), and back to the heart.



The period of time during which the ventricles of the heart are contracted.


Systolic pressure

The pressure measured in the arteries during contraction of the ventricles (during systole).


T cell

A type of lymphocyte. The major subtypes of T cells are the helper T cells (CD4) and the killer T cells (CD8, or cytotoxic T cells). Helper T cells secrete chemicals that help killer Ts and B cells proliferate. Killer T cells destroy abnormal self-cells (e.g., cancer cells) or infected cells.



The cerebral hemispheres.



A specialized region at the ends of eukaryotic chromosmes that contains several repeats of a particular DNA sequence. These ends are maintained (in some cells) with the help of a special DNA poymerase called telomerase. In cells that lack telomerase, the telomeres slowly degrade with each round of DNA replication (as the RNA primer, is not replaced and the 5' of the new DNA would not exist); this is though to contribute to the eventual death of the cell.



The fourth (and final) phase of mitosis. During telophase the nuclear envelope reforms, chromosomes decondense, and the mitotic spindle is disassembled.


Telophase I

The fourth of meiosis I. Telophase I is identical to mitotic telophase, except that the number of chromosoms is now reduced by half. After this phase the cell is considered to be haploid. Note however, that the chromosomes are still replicated, and the sister chromatids must still be separated during meiosis II.


Telophase II

The fourth and final phase of meiosis II. Telophase II is identical to mitotic telophase, except that the number of chromosomes was reduced by half during meiosis. I.


Temporal summation

Summation by a postsynaptic cell of input (EPSPs or IPSPs) from a single source over time.



Strong bands of connective tissue that connect skeletal muscle to bone.



A genetic cross between an organism displaying a recessive phenotype (homozygous recessive) and an organism displaying a dominant phenotype (for whic the genotype is unknown), done to determine the unknown genotype.



The primary male sex organ. The testes are suspended outside the body cavity in the scrotum and have two functions (1) produce sperm, and (2) secrete testosterone.



The primary androgen (male sex steroid). Testosterone is a steroid hormone produced and secreted by the interstitial cells of the testes. It triggers the development of secondary male sex characteristics during puberty (including spermatogenesis) and maintains those characteristics during adulthood.



A smooth sustained muscle contraction, such as occurs in skeletal muscle when stimulation frequency is high enough (this is the normal type of contraction exhibited by skeletal muscle).



A pair of replicated homologous chromosomes. Tetrads form during prophase I of meiosis so that homologous chromosomes can exchange DNA in a process known as 'crossing over.'



The central structure of the diencephalon of the brain. the thalamus acts as a relay station and major integrating area for sensory impulses.


Thecal cells

A layer of cells surroudning the granulosa cells of the follicles in an ovary. Thecal cells help produce the estrogen secreted from the follicle during the first phase of the ovarian cycle.



A receptor that responds to changes in temperature.


Theta replication

DNA replication in prokaryotes, so named because as replication proceeds around the single, circular chromosome, it takes on the appearnce of the Greek letter theta.


Thick filament

In skeletal and cardiac muscle tissue, a filament composed of bundles of myosin molecules. The myosin head groups attach to the thick filaments and pull the toward the center of the sarcomere during muscle contraction.


Thin filament

In skeleta and cardiac muscle tissue, a filament composed of actin, tropomyosin, and troponin. Thin filaments are attached to teh Z lines of the sarcomers and slide over thick filaments during muscle contraction.



A blood clot that forms in an unbrokened blood vessel. Thrombi are dangerous they can break free and begin travelin in the bloodstream (become an embolus). Emboli ultimately become stuck in a small vessel and prevent adequate blood delivery to tissues beyond the sticking point, leading to tissue death. A brain embolism cna lead to stroke, a heart embolism to a heart attack, and a pulmonary embolism to respiratory failure.



One of the four aromatic bases found in DNA. Thymine is a pyrimidine; it pairs with adenine.



An immune organ located near the heart. THe thymus is the site of T cell maturation and is larger in children and adolescents.


Thyroid stimulating hormone (TSH)

A tropic hormone produced by the anterior pituitary gland that targets the thyroid gland, stimulating it to produce and release thyroid hormone.



Also called thryoid hormone, thyroxine is produced and secreted by follicle cells in the thyroid gland. it targets all cells in the body and increases overall body metabolism.


Tidal volume

The volume of air inhaled and exhaled in a normla, resting breath, typically about 500 mL.


Tight junction

Also called occluding junctions, tight junctions form a seal between cells that prevents the movement of substances across the cell layer, except by diffusion through the cell membranes themselves. Tight junctions are found between the epithelial cells lining the intestines and between the cells forming the capillaries in the brain (the blood-brain barrier).


Tolerant anaerobe

An organism that can survive in the presence of oxygen (oxygen is not toxic), but that does not use oxygen during metabolism (anaerobic metabolism only).



Paired masses of lymphatic tissue near the back of the throat that help trap inhaled or swallowed pathogens.



An enzyme that cuts one or both strands of DNa to relieve the excess tension caused by the unwinding of the helix by helicase during replication.


Total lung capacity

The maximal volume of air that the lungs can contain. Total lung capacity is the sum of the vital capacity and the residual volume, and is typically about 6000 mL (6L).



Having the ability to become anything; a zygote is totipotent.



The main air tube leading into the respiratory system. The trachea is made of alternating rings of cartilage and connective tissue.



The enzymatic process of reading a strand of DNA to produce a complemenetary strand of RNA



The transfre by a lysogenic virus of a portion of a host cell genome to a new host.


Transition mutation

A point mutation in which a pyrimidine is susbstituted for a pyrimidine, or a purine is substituted for a purine.



The process of reading a strand of mRNA to synthesize protein. Protein translation takes place on a ribosome.


Transmembrane domain

The portion of an integral membrane protein that passes through the lipid bilayer.


Transversion mutation

A point mutation in which a pyrimidine is substitued for a purine, or vice versa.



Transfer RNA; the type of RNA that carries an amino acid from the cytoplasm to the ribosome for incorporation into a growing protein.


tRNA loading

The attachment of an amino acid to a tRNA (not that this a specific interaction). tRNa loading requires two high-energy phosphate bonds.



The outer ring of cells of a blastocyst. The trophoblast takes part in the formation of the placenta.


Tropic hormone

A hormone tha tcontrols the release of another hormone.



A helical protein that winds around actin helices in skeletal and cardiac muscle cells to form the thin filament of the sarcomere. In the absence of Ca2+, tropomyosin covers the myosin-binding sites on actin and prevents muscle contraction. When calcium is present, a conformation change in tropomyosin occurs so that the myosin-binding sites are exposed and muscle contraction can occur.



A globular protein that ssociated with tropomyosin as part of the thin filament of the sarcomere. Troponin binds Ca2+, which causes the conformaiton change in tropomyosin required to expose the myosin-binding sites on actin and initiate muscle contraction.



The main protease secreted by the pancreas; trypsin is activated (from trypsinogen) by enterokinase, and subsequently activates other pancreatic enzymes.


T tubules

Also called transverse tubules, these are deep invaginations of the plasma membrane found in skeletal and cardiac muscle cells. These invaginations allow depolarization of the membrane to quickly penetrate to the interior of the cell.


Tympanic membrane

The membrane that separate the outer ear from the middle ear. The tympanic membrane is also known as the eardrum.


Umbilical cord

The cord that connects the embryo of a developing mammal to the placenta in the uterus of the mother. The umbilical cord contains fetal arteries (carry blood toward the placenta) and veins (carry blood away from the placenta). The umbilical vessels derive from the allantois, a structure that develops from the embryonic gut.



A carrier protein that transports a single molecule across the plasma membrane.


Universal acceptor

A person with blood type AB+. Because this person's red blood cells possess all of the typical blood surface proteins, they will not display an immune reaction if transfused with any of the other blood types.


Universal donor

A person with blood type O-. Because this person's red blood cells possess none of the typical blood suface proteins, they cannot initiate an immune reaction in a recipient.



Toward the 5' end of an Rna transcript (the 5' end of the DNA coding strand). The promoter and start sites are upstream.



One of the four aromatic bases found in RNA. Uracil is pyrimidine; it pairs with adnenine.



A waste product of protein dbreakdown, produced by the liver and relased into the bloodstream to be eliminated by the kidney.



The tubes that carry urine from the kindeys to the bladder.



The tube that carries urine from the bladder to the to outside of the body. In males it also carries semen and sperm during ejaculation.


Urinary sphincter

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


Uterine tubes

Also called falopian tubes, these tubes extend laterally from their side of the uterus and serve as a passageway for the ocyte to travel from the ovary to the uterus. This is also the normal site of fertilization. Severing of the uterine tubes (tubal ligation) results in sterility of the femlae.



The muscular femal organ, in which a baby develops during pregnancy.



The deliberate exposure of a person to an antigen in order to provoke the primary immune response and memory cell production. Typically the antigens are those normally associated with pathogens, thus if the live pathogen is encountered in the future, the seconday immune response can be initiated, preventing infection and symptoms.


Vagal tone

The constant inhibition provided to the heart by the vagus nerve. Vagal tone reduces the intrinsic firing rate of teh SA node from 120 beats/minute to around 80 beats/minute.



The birth canal; the stretchy, muscular passageway through which a baby exits the uterus during childbirth.


Vagus nerves

Cranial nerve pair X. The vagus nerves are very large mixed nerves (They carry both sensory input and motor input) that innervate virtually every visceral organ. They are especially important in transmitting parasympathetic input to the heart and digestive smooth muscle.


Vasa recta

The capillaries that surround the tubules of the nephron. The vasa recta reclaims reabsorbed substances, such as water and sodium ions.


Vas deferens

A thick muscular tube that connects the epididymis of the testes to the urethra. Muscular contractions of the vas deferns during ejaculation ehp propel the sperm outward. Severing of the vas deferens (vasectomy) results in sterility of the male.



A blood vessel that carries blood toward the heart chambers. Veins do not have muscular walls, have valves to ensure that blood flows in one direction only, and are typically low-pressure vessels.


Vena cava

One of two large vessels (superior and inferior) that return deoxygenated blood to the right atrium of the heart.


Venous returns

The amount of blood returned to heart by the vena cavae.



One of two large chambers in the heart. The ventricles receive blood from the atria and pump it out of the lungs of the heart. The right ventricle has thing walls and pumps deoxygenated blood to the lungs through the pulmonary artery. The left ventricle has thick walls and pumps deoxygenated blood the body through the aorta.


Vestibular glands

Paired glands near the posterior side of the vaginal that secrete an alkaline mucus upon sexual arousal. The mucus helps to reduce the acidity of the vagina (which could be harmful to sperm) and lubricates the vagina to facilitate penetration.



(Singular:villus). Folds of the intestinal mucosa that project into the lumen of the intestine; vili serve to increase the surface area of the intestine for absorption.



A nonliving, intracellular parasite. Viruses are typically just pieces of nucleic aid surrounded by a protein coat.


Vital capacity

The maximum amount of air that can be forcibly exhaled from the lungs after filling them to their maximum level, typically about 4500 mL



One of several different nutrietns that must be consumed in the diet, and generally not synthesized in the body. Vitamins can be hdyrophobic (fat-solube) or hydrophilic (water-soluble).


Vitreous humor

A thick, gelatinous fluid found in the posterior segment of the eye (between the lens and the retina). The vireous humor is only produced during fetal development and helps maintain intraocular pressure (the pressure inside the eyeball).


Voltage-gated ion channel

An ion channel that is oepend or closed based on the electrical potential across the plasma membrane. Once opened, the channel allows ions to cross the membrane according to their concentration gradients. Examples are the Na+ and K+ voltage-gated channels involved in the action potential of neurons.


White matter

Myelinated axons


Wolffian ducts

Early embryonic ducts that can develop into male internal genitalia under the proper stimulation (testosterone).


Yolk sac

An embryonic structure particularly important in egg-laying animals because it contains the yolk, the only source of nutrients for the embryo developing inside the egg. IN humans, the yolk sac is very small (since mammals get their nutrients via the placenta) and is the site of synthesis of the first red blood cells.


Z lines

The ends of a saromere.


Zona pellucida

A thick, transpartent coating rich in glycoproteins that surrounds an oocyte.



A diploid cell formed by the fusion of two gametes during sexual reproduction.



An inactive precursor of an enzyme, activated by various methods (acid hydrolysis, cleavage by another enzyme, etc.)