Exam #6 Flashcards
All of the following statements about plants cell organelles are true EXCEPT one. Which one is the EXCEPTION?
A: Most of the cytoplasmic volume is occupied by a single vacuole
B: Plant cell walls are composed of cellulose and function in maintaining cell shape
C: Plant cells possesses a cell wall but not a cell membrane
D: Adjacent plant cells contain channels allowing for intercellular communication
E: Plant cells have mitochondria, but do not have a centriole
C: Plant cells possesses a cell wall but not a cell membrane
A: Most of the cytoplasmic volume is occupied by a single vacuole
This is a true statement and therefore the answer choice is incorrect. A large central vacuole often occupies more than 80% of a plant cell’s cytoplasmic volume.
B: Plant cell walls are composed of cellulose and function in maintaining cell shape
This is a true statement and therefore the answer choice is incorrect. Plant cell walls contain cellulose, a polymer of glucose. They also function to protect the cell and maintain its shape.
C: Plant cells possesses a cell wall but not a cell membrane
This is a false statement and therefore the answer choice is correct. Plant cells contain both a cell wall as well as a cell membrane.
D: Adjacent plant cells contain channels allowing for intercellular communication
This is a true statement and therefore the answer choice is incorrect. Plasmodesmata are channels in between plant cells that allow communication from one cell to the next.
E: Plant cells have mitochondria, but do not have a centriole
This is a true statement and therefore the answer choice is incorrect. Plant cells use mitochondria to convert the glucose they produce into ATP. Plant cells do not have centrioles (but do possess microtubule organizing centers that carry out a similar function).
It can take several days before the effects of testosterone replacement therapy are noticed. Which of the following is a reason for why the intracellular binding of the steroid hormone testosterone is slow acting?
A: Steroids must generate a second messenger to propagate the signal
B: Steroids cannot cross the cell’s plasma membrane because they’re nonpolar
C: Steroids upregulate genes which must be transcribed and translated
D: Steroids dissolve completely in the blood and maintain a low concentration
E: Steroids are reversible competitive antagonists and must overcome other hormonal signals
C: Steroids upregulate genes which must be transcribed and translated
A: Steroids must generate a second messenger to propagate the signal
Incorrect. Intracellular steroid hormones bind directly to the DNA and do not require second messengers.
B: Steroids cannot cross the cell’s plasma membrane because they’re nonpolar
Incorrect. Steroid hormones are nonpolar and pass through the membrane.
C: Steroids upregulate genes which must be transcribed and translated
Correct. Intracellular steroid hormones act as transcription factors. There will only be an effect seen once the mRNA has been translated to protein, which is a slow process. This is in contrast to molecules which use secondary messengers, which can quickly amplify a cascade, leading to greater cellular effect much more quickly.
D: Steroids dissolve completely in the blood and maintain a low concentration
Incorrect. Steroid hormones are nonpolar and will not dissolve in polar blood.
E: Steroids are reversible competitive antagonists and must overcome other hormonal signals
Incorrect. Steroids do not bind as reversible competitive antagonists on receptors – they bind intracellularly as agonists to trigger a reaction in the cell. While they may have to overcome other hormonal signals, this is applicable to all types of hormones, and would not specifically explain why steroid hormones are slow acting.
Which of the following best describes the function of desmosomes?
A: They form a seal to prevent the passage of material between cells
B: They are channels that allow the passage of small molecules
C: They are narrow tunnels specific to plant cell exchange
D: They aid in providing cell-cell adhesion and mechanical stability
E: They allow a cell to adhere to the extracellular matrix
D: They aid in providing cell-cell adhesion and mechanical stability
A: They form a seal to prevent the passage of material between cells
Incorrect. The tight junctions form a seal to prevent the passage of material between cells.
B: They are channels that allow the passage of small molecules
Incorrect. The gap junctions allow for passage of ions and small molecules while preventing the cytoplasm of adjacent cells from mixing.
C: They are narrow tunnels specific to plant cell exchange
Incorrect. The plasmodesmata are narrow tunnels between plant cells that allow for exchange of material through cytoplasm around a narrow tube of endoplasmic reticulum known as the desmotubule.
D: They aid in providing cell-cell adhesion and mechanical stability
Correct. Desmosomes are found in multicellular tissue and help adjacent cells attach to each other. They are spot adhesions between cells, and provide mechanical stability, particularly in tissues subject to great mechanical stress (e.g. the skin and cervix).
E: They allow a cell to adhere to the extracellular matrix
Incorrect. Desmosomes function in cell to cell adhesion – not cell to external structures such as the extracellular matrix. Focal adhesions and hemidesmosomes carry out this function.
Gap junctions are the connections between two adjacent cells, allowing molecules and ions to directly pass from cell to cell. Intercalated discs in heart muscle are the structures that connect the cytoplasm of heart muscle together for a synchronous contraction. Part of the intercalated disc contains a gap junction.
Integrins are transmembrane proteins that send signals to the cell about its extracellular environment. The signals tell the cell whether it should grow, divide, differentiate, or undergo programmed cell death (apoptosis).
Which protein are the microfilaments of the cytoskeleton composed of?
A: Tubulin
B: Troponin
C: Keratin
D: Actin
E: Myosin
D: Actin
The cytoskeleton of the cell serves as the cell’s structural and motile units. The cytoskeleton is frequently remodeling, extending, retreating, contracting and relaxing.
Flagellin is the name of a protein building block that form the flagella in prokaryotes.
To summarize flagella:
Eukaryotic flagella are composed of microtubule proteins. Prokaryotic flagella are composed of flagellin proteins.
Intermediate filaments link up with the cell membrane embedded cell adhesion proteins desmosomes and hemidesmosomes. Desmosomes attach cells in a cell-cell adhesion. Hemidesmosomes attach cells in cell-extracellular matrix adhesions.
Keratin is a protein that concentrates in skin, hair and nails.
All of the following statements about viral replication are true EXCEPT for one. Which one is the EXCEPTION?
A: In the lysogenic cycle, the virus remains dormant in the host cell until triggered
B: When infecting cells, the viral capsid enters the cell, but not its genetic material
C: Viral genomes and protein are synthesized using host cell machinery
D: In the lytic cell cycle the virus will destroy the host cell
E: The viral genome may have RNA or DNA as its starting point for replication
B: When infecting cells, the viral capsid enters the cell, but not its genetic material
A: In the lysogenic cycle, the virus remains dormant in the host cell until triggered
This is a true statement and therefore the answer choice is Incorrect. During the lysogenic cycle, viral DNA is incorporated into the DNA of the host cell and remains in a dormant state as a provirus/prophage until external stimuli triggers it to begin the lytic cycle.
B: When infecting cells, the viral capsid enters the cell, but not its genetic material
This is a false statement and therefore the answer choice is correct. When a virus infects a cell, the viral genetic material must enter the cell so that it may be replicated for the assembly of new viruses. The capsid may or may not enter depending on the type of virus.
C: Viral genomes and protein are synthesized using host cell machinery
This is a true statement and therefore the answer choice is Incorrect. The virus makes use of the host cell’s machinery to produce nucleic acids and viral proteins that are assembled to form new viruses.
D: In the lytic cell cycle the virus will destroy the host cell
This is a true statement and therefore the answer choice is Incorrect. The lytic cycle is destructive, while the lysogenic cycle is not. A virus in the lysogenic cycle can be triggered by an environmental change to enter the lytic cycle, where the virions will burst out of the cell.
E: The viral genome may have RNA or DNA as its starting point for replication
This is a true statement and therefore the answer choice is Incorrect. Viral genetic material can either be DNA or RNA. HIV is a common example of a RNA virus that reverse transcribes to cDNA (complementary DNA) when in the host cell.
Viral coats are made up of a series of protein subunits called capsomeres. Capsomeres come together to form a protective protein coat called the capsid. A virus has no cell wall, no plasma membrane, nor any organelles. It only has its nucleic acid and a protein coat covering it. The virus can pick up a phospholipid envelope from a host cell’s membrane, however this is not a true cellular membrane belonging to the virus.
Viruses have two life cycles that they can rotate between, the lysogenic cycle and the lytic cycle.
In the lysogenic cycle the virus binds to the host and inserts its viral DNA into the host cells’ DNA chromosome. The viral DNA will be replicated whenever chromosomal DNA is replicated. The virus is considered dormant and does not harm the host while in the lysogenic stage.
In the lytic cycle the virus attaches to a host, inserts its DNA into that host, and takes over the host cell’s machinery. This includes making many copies of viral DNA and translating viral proteins. The many virions then break out of the host cell, destroying the host cell in the process.
A virus is able to switch between the lysogenic and the lytic cycle. A virus can infect a cell, enter the lysogenic cycle, remain dormant and replicate with the host chromosome, and eventually upon proper environmental stimulus, enter the lytic cycle and destroy the cell, and spread to other cells. The lytic cycle is often opportunistic, and will engage in favorable conditions.
Viruses exist as both DNA viruses and RNA viruses. A common example of an RNA virus is HIV.
HIV is a retrovirus. A retrovirus is a virus that stores its genetic material as RNA, will infect a host cell with that RNA, and then use a reverse transcriptase enzyme to convert its RNA genome into cDNA. cDNA is the abbreviation of complementary DNA, and it gets its name because the DNA is made as a complement to the RNA code. The cDNA will integrate into the host DNA and enter the lysogenic cycle.
When HIV (human immunodeficiency virus) enters the lytic cycle, it starts to destroy the host cells; the host cells in HIV are T helper cells (CD4 cells). As CD4 cell count decreases, the patient has decreasing immunity, and the condition has progressed to AIDS (acquired immune deficiency syndrome).
A scientist wants to track a specific molecule formed during oxidative phosphorylation. He places a special radioactive tag on the carbons of pyruvate molecules that will only become active once pyruvate is decarboxylated. Between glycolysis and the citric acid cycle, the tag becomes active and the scientist is able to observe the molecule of interest. Which molecule is the scientist tracking?
A. ATP B. Ethanol C. FADH2 D. Acetyl CoA E. lactate
Correct. Do not be overwhelmed by the experimental format presented in the question. Instead, isolate the relevant pieces of information that are presented to you: the molecule is formed during oxidative phosphorylation, specifically in between glycolysis and the citric acid cycle. As the question reveals, this is the step of pyruvate decarboxylation, where pyruvate is decarboxylated and is converted into acetyl CoA. This process also produces NADH and CO2. Answering this question correctly requires no special knowledge about the experiment itself – only what pyruvate is converted into.
Which of the following is correct regarding the light-dependent and dark reactions of photosynthesis?
A: The light dependent reactions fix carbon into glucose molecules
B: The dark reactions can occur in an environment without light
C: The dark reactions are responsible for synthesizing glucose
C: The dark reactions are responsible for synthesizing glucose
Photosynthesis can be broken into two separate stages: the light dependent reactions, and the light independent reactions (Calvin cycle).
This process begins with photosystem II. Photosystem II is also known as P680 because it absorbs light at the 680 nm wavelength. P680 receives a photon of light, capturing this light through the chlorophyll A pigment in the photosynthetic reaction center. Photosystem II is a powerful oxidizer; it can oxidize a water molecule, releasing electrons from H2O– H2O serves as the electron donor in photosynthesis. The process of splitting a water molecule through the power of a photon is called photolysis. Splitting a water molecule liberates O2, which is why plants produce oxygen.
Electrons are passed from photosystem II down the electron transport chain. As the electrons are passed from one carrier to the next, hydrogen is pumped from the chloroplast stroma into the lumen of the thylakoid creating a hydrogen chemiosmotic concentration gradient. There are more protons within the thylakoid lumen than in the chloroplast stroma.
The electrons pass to photosystem I, also called P700 because it absorbs best at 700 nm. A second photon of light is now used to re-excite the electrons, again bringing them to a higher energy level.
The electrons are passed from photosystem I, to another electron carrier, and then to an NADP+ reductase, which reduces NADP+ to NADPH.
The NADPH will be the carrier of the electrons, transferring electrons from the light dependent reactions to the Calvin cycle.
The hydrogen concentration gradient which has built up between the thylakoid lumen and the chloroplast stroma through the passing of electrons down the ETC powers ATP production. Hydrogen will travel down its gradient. It travels from the thylakoid lumen, into the chloroplast stroma, powering the ATP synthase to create ATP.
ATP and NADPH generated during the light dependent reactions will be used in the light independent reactions of photosynthesis, to create glucose.
The light independent reactions (also known as the Calvin cycle and sometimes the ‘dark reactions’), takes place in the chloroplast stroma. CO2 will be fixed into carbon-carbon bonds, which will produce glucose; this process is known as carbon fixation.
In ‘typical’ C3 carbon fixation, the enzyme RuBisCO fixes CO2 (1 carbon molecule) to ribulose biphosphate (RuBP, a 5 carbon molecule) forming a six carbon molecule.
Through a series of reactions using the energy from ATP and NADPH generated in the light dependent reactions, the six carbon molecule is worked on. Part of the six carbon molecule will undergo reactions (using more ATP) to regenerate RuBP, which can go and accept another CO2. This is why the Calvin cycle, is a cycle—because RuBP is integrated and then regenerated.
In total, the Calvin cycle must accept 6 CO2 in order to have sufficient carbon to generate one glucose molecule.
Non-cyclic photophosphorylation is the process we just described. It involves both photosystem II (PSII) and photosystem I (PSI), creating NADPH and ATP.
Cyclic photophosphorylation involves only PS I, not PS II. Here, electrons will move backwards in the electron transport chain, doubling back to an earlier electron carrier. The electrons then progress down the electron transport chain, returning to PS I, are re-excited by PS I, and sent backwards to the earlier electron carrier again. This will continue to pump protons into the thylakoid lumen and therefore produce ATP, but the electrons will not pass to NADP+, not NADP+ is reduced, and therefore no NADPH is generated when cyclic photophosphorylation is occurring.
Photorespiration is the process where O2 rather than CO2 binds to RuBisCO. If O2 binds, carbon fixation to glucose does not occur.
Oxygen is roughly 20% of atmospheric gas, whereas carbon dioxide is less than 0.05%. In spite of this, RuBisCO has relatively low levels of photorespiration, because of its high selectivity for CO2.
Important terms for you to remember when discussing adaptations to limit photorespiration:
Stomata are the pores mainly found in the bottoms of leaves. Stomata are the site of atmospheric gas (including CO2 and O2) exchange.
Guard cells surround the stomata, and control whether the stomata pore is open or closed.
Transpiration is when water evaporates out of the stomata, potentially desiccating the plant.
C3 photosynthesis is called C3 because CO2 will become a three carbon (C3) compound. This is conventional photosynthesis, and is what I discussed above. We can contrast this with C4 photosynthesis as well as CAM photosynthesis. C4 and CAM photosynthesis are both techniques that are used to prevent photorespiration.
C4 photosynthesis:
This method is called C4 photosynthesis because the CO2 molecule integrates into and becomes a four carbon compound first, before it bonds to RuBisCO later on.
PEP carboxylase has even lower affinity for O2 compared to RuBisCO, so even in the presence of O2, it is very unlikely to bind to oxygen. This is an advantage.
The enzyme PEP carboxylase takes CO2 and converts it into oxaloacetate. Oxaloacetate quickly turns into malic acid. Both oxaloacetate and malic acid are four carbon compounds, hence the name C4.
The malic acid will be transferred from the mesophyll cells where PEP carboxylase reaction has occurred, to the bundle sheath cells.
The bundle sheath cells are located in a different area in the leaf anatomy (they surround the vascular bundles of plants), where O2 concentration is much lower.
Here the malic acid can be decarboxylated to release CO2. The CO2 can now undergo the conventional Calvin cycle with RuBisCO, in an environment where O2 is not as prevalent, and RuBisCO has low risk of photorespiration.
CAM photosynthesis:
The C4 photosynthesis process isolates CO2 spatially. Spatial isolation means that CO2 is transported to a different location (a different space) to prevent photorespiration. C4 photosynthesis transports the CO2 to the bundle sheath cells.
CAM plants have a different method of isolation. CAM plants use temporal isolation. Temporal isolation is an isolation based on timing, as a means of preventing photorespiration. There is no spatial separation: the processes occur in the same part of the leaf, but the plant does different processes at different times.
During the day, CAM plants close stomata to prevent excessive loss of water, via transpiration, evaporation out of the stomates (keep in mind this will also limit new gases like O2 from entering the plant). At night, CAM plants have their stomata open, allowing CO2 to enter into the leaf. The same enzyme in C4 photosynthesis is used in CAM photosynthesis: PEP carboxylase will fixate CO2 into a four carbon molecule of oxaloacetate which converts into malic acid. In contrast, to the C4 pathway, rather than shuttle the malic acid to a different part of the leaf, the malic acid will be stored in a vacuole, for later use.
During the day, the malic acid will be shuttled out of the vacuole, CO2 will be decarboxylated from the malic acid, and the typical Calvin cycle will occur in a low O2 environment (stomates are closed). During the day the sun shines brightly, and ATP as well as NADPH are being produced plentifully.
Which enzyme synthesizes the short RNA sequence that is required for DNA polymerase binding during DNA replication?
A: RNase B: Isomerase C: Primase D: Helicase E: Endonuclease
C: Primase
A: RNase
Incorrect. RNase “ribonuclease” degrades RNA.
B: Isomerase
Incorrect. Isomerase is an enzyme that creates isomers.
C: Primase
Correct. Primase lays down an RNA primer which DNA polymerase requires in order to begin replicating DNA.
D: Helicase
Incorrect. Helicase ‘unzips’ the DNA helix.
E: Endonuclease
Incorrect. Endonuclease cleaves bonds in between nucleotides. The difference between endonuclease and exonuclease is where along the nucleotide polymer they act.
Endonuclease works within the interior of the polymer (endo means within). It will cleave the nucleotide at a phosphodiester bond somewhere in the polymer that is not at the ends.
Exonuclease works at the ends or outside of the polymer (exo means outside, think of exterior). Exonuclease cleaves nucleotides off the ends of a polymer, one at a time.
For the lagging strand of DNA replication these RNA primers are in between Okazaki fragments. These primers will need to be removed, DNA polymerase will need to synthesize new DNA at these locations, and the different fragments of DNA will need to be ‘stitched’ or ligated together by ligase.
Which of the following conditions would stimulate a cell to divide?
A: A high density of surrounding cells
B: A low cell surface area to volume ratio
C: Lack of anchorage to another surface
D: A failed cell cycle checkpoint during metaphase
E: The absence of extracellular growth factors
B: A low cell surface area to volume ratio
A: A high density of surrounding cells
Incorrect. A cell can sense the density of surrounding cells. High cell density is an indicator to a cell to limit/inhibit further cell division.
B: A low cell surface area to volume ratio
Correct. The plasma membrane is the interface where the cell exchanges nutrients and gases with the extracellular environment. If the cell surface area is low compared to the volume of the cell, the cell will be unable to perform sufficient nutrient exchange to support cell processes. This will stimulate the cell to undergo division.
C: Lack of anchorage to another surface
Incorrect. A cell can sense if it is anchored to a surface. Lack of anchorage is an indicator to a cell to limit/inhibit further cell division.
D: A failed cell cycle checkpoint during metaphase
Incorrect. A failed checkpoint would inhibit cell division. There are checkpoints after G1, G2, and metaphase of mitosis to ensure everything is correct before the cell proceeds to the next stage of the cell cycle (cell division).
E: The absence of extracellular growth factors
Incorrect. Extracellular growth factors stimulate a cell to divide.
Normal cell division in eukaryotic cells is regulated by cyclin and cyclin dependent kinases (CDK).
Cyclin allosterically activates CDK. CDK is a kinase, an enzyme which phosphorylates its substrate. CDK phosphorylated substrates signal to the cell that it is ready to progress to the next cell cycle stage.
Cyclin is a protein that cycles in amount through stages of synthesis and degradation. Because it cycles through stages of higher and lower amounts, it was named CYCLin.
When cyclin levels are high, CDK activity will be high, and conversely when cyclin is low, CDK activity is low.
The signal for RNA polymerase to end transcription in a eukaryotic cell is the:
A: binding of repressor proteins at the promoter
B: stop codon
C: addition of a 5’ cap
D: absence of transcription factors
E: terminator sequence
E: terminator sequence
A: binding of repressor proteins at the promoter
Incorrect. Binding of a repressor protein to the promoter would prevent transcription from beginning, but does not function to end transcription that has already begun.
B: stop codon
Incorrect. The stop codon is the signal for the ribosome to disassociate from mRNA during translation – it is not relevant to transcription.
C: addition of a 5’ cap
Incorrect. A 5’ GTP cap is added to the 5’ end of mRNA in eukaryotes before the mRNA leaves the nucleus, providing stability and an attachment point for ribosomes. It is not the signal for RNA polymerase to end transcription, but an mRNA processing step that takes place after the transcript is created.
D: absence of transcription factors
Incorrect. Transcription factors bind to the DNA to help attract RNA polymerase and stimulate the start of gene transcription. Their absence is not a signal for stopping transcription.
E: terminator sequence
Correct. The terminator sequence is the sequence in the DNA that signals the end of transcription and causes RNA polymerase to dissociate from the DNA.
Transcription factors are regulatory proteins that bind to DNA and affect the recruitment of RNA polymerases.
Transcription factors can either increase rates of transcription (up-regulation) or decrease rates of transcription (down-regulation).
In prokaryotic cells, prokaryotic core RNA polymerase is able to bind to prokaryotic DNA, but it lacks the ability to target promoter sites that are upstream of the gene to be transcribed. Prokaryotic core RNA polymerase will combine with sigma factor to form RNA polymerase holoenzyme. The sigma factor provides RNA polymerase holoenzyme the ability to target the promoter region of bacterial DNA.
In eukaryotic cells, RNA polymerase cannot directly detect the promoter region, and requires the binding of transcription factors in order to initiate transcription.
In prokaryotic cells (and sometimes in eukaryotic cells), a group of related genes can be under the control of one promoter site. This is known as an operon. An operator region of DNA regulates the site, and binds to either activator or repressor proteins (transcription factors). If the operon is activated, RNA polymerase will bind to the promoter site and the genes are transcribed. If the operator and operon is repressed, RNA polymerase cannot bind to the promoter site and the genes will not be transcribed.
Eukaryotic RNA polymerases rely on binding of transcription factor proteins at the promoter site to signal to the RNA polymerases where to bind. RNA polymerase cannot recognize the promoter site without the presence of upregulatory transcription factors in eukaryotes.
In addition to transcription factors binding at the promoter site, eukaryotic DNA also contain enhancer sites and silencer sites. These sites can be upstream, downstream or within the gene and transcription factors bind to them. Activator proteins bind enhancers. These elements will increase protein transcription. Repressor proteins bind silencers. These elements will decrease protein transcription.
Because enhancers and silencers can be far upstream or downstream from a gene, the DNA is thought to loop around so that the enhancer/silencer can colocalize with RNA polymerase.
Enhancers and silencers work even if the sequence of nucleotides is excised and flipped or if they are excised and moved to a different location within the nucleotide sequence. This is what makes them different than promoters, which have a very specific location and orientation.
In eukaryotes, the terminator sequence for genes that will become protein involves a poly A signal.
The poly A signal in the mRNA stimulates the polyadenylation of the 3’ end of the transcript (50-300 adenine nucleotides added).
Polyadenylation is part of the post-transcriptional modification or eukaryotic pre-mRNA into a processed mRNA, that can enter the cytoplasm to undergo protein translation.
Other post-transcriptional modifications are the addition of a 5’ cap, and intron splicing.
Colchicine is a drug used for the treatment of gout. Its mechanism of action inhibits microtubule polymerization by binding to tubulin. Consuming this drug before or during pregnancy would most likely have an adverse effect on all of the following cellular activities EXCEPT for one. Which one is the EXCEPTION?
A. Embryonic cell cleavage B. Mitotic spindle formation C. Egg transportation into the fallopian tube D. Cleavage furrow formation E. Fertilization
D. Cleavage furrow formation
If colchicine disrupts the polymerization of microtubules, the key to answering this question correctly is identifying all cell processes that would require microtubules, then eliminate them as answer choices. Microtubules may be required at the structural level of a cell (as in sperm or fimbriae), or in intracellular processes (mitotic spindle formation and cell division). Below, only cleavage furrow formation – which uses microfilaments rather than microtubules – would not by affected by inhibition of microtubules.
A. Embryonic cell cleavage: Recall that cell cleavage is the rapid division of cells that takes place following successful fertilization of an egg by sperm to form the zygote. When the zygote undergoes rapid cell division, the cells do not become larger, resulting in a cluster of cells that is the same size as the initial zygote but with less cytoplasm per cell – known as the morula. Cleavage officially ends once the hollow-centered blastula begins to form. Since cell cleavage involves cell division, and cell division relies on the formation of the mitotic spindle (requiring microtubule polymerization – see answer choice B for additional details), the answer choice is incorrect.
B. Mitotic spindle formation: Microtubule polymerization is a critical part of mitotic spindle formation during cell division. During prophase, microtubules begin connecting to the kinetochores of chromosomes, which allows for the chromosomes to begin aligning (metaphase). Since disruption of microtubule polymerization would prevent mitotic spindle formation from occurring, the answer choice is incorrect.
C. Egg transportation into the fallopian tube: In order for a fertilized egg to be implanted to the uterus, the egg must first be transported from the ovary to the uterus through the oviduct. The ovaries do not directly connect to the oviducts. The oviducts have fimbriae, and an ovulated egg is swept into the oviduct by the cilia of the fimbriae. Microtubules form a key structural and functional component of cilia. If microtubule polymerization were inhibited, cilia function would be inadequate and the egg would fail to make it to the uterus for successful implantation. As this process cannot occur if microtubule polymerization is disrupted, the answer choice is incorrect.
D. Cleavage furrow formation: During cytokinesis (the separation of the cell membrane and cytoplasm in the final stages of cell division), the cleavage furrow is the indented structure that separates the membrane into two separate cells. In animal cells, this indentation is formed by a contractile ring of microfilament proteins (actin and myosin), rather than microtubules (tubulin). Inhibition of microtubules would not be expected to affect formation of the cleavage furrow, therefore the answer choice is correct.
E. Fertilization: In order for fertilization to successfully take place, the sperm must be able to reach the egg. Recall that the structure of the sperm includes the midpiece and tail, both of which include the flagellum – a necessary component of sperm movement. As microtubules are a critical component of eukaryotic flagella, inhibited microtubule polymerization would negatively affect the ability of sperm to create motion. In addition, successful fertilization includes the eventual joining of sperm and egg chromosomes to form a diploid zygote, which would also require microtubules. As multiple processes in fertilization require microtubule polymerization, the answer choice is incorrect.
Topic: Anatomy and Physiology
All of the hormones below are a direct component of the menstrual cycle EXCEPT for one. Which one is the EXCEPTION?
A: Prolactin B: Estrogen C: Leutinizing hormone D: Follicle stimulating hormone E: Progesterone
A: Prolactin
A: Prolactin
Correct. Prolactin is involved in milk production, not the menstrual cycle.
B: Estrogen
Incorrect. Estrogen prepares the uterus for implantation.
C: Leutinizing hormone
Incorrect. Leutinizing hormone causes ovulation, and development of the corpus luteum.
D: Follicle stimulating hormone
Incorrect. Follicle stimulating hormone causes ovarian follicles to develop.
E: Progesterone
Incorrect. Progesterone prepares the uterus for implantation.
Where does chemical digestion of food begin in the human body, and via which specific enzyme?
A: Mouth via salivary amylase B: Esophagus via esophageal lipase C: Stomach via pancreatic trypsin D: Mouth via salivary peptidase E: Small intestine via pancreatic carboxypeptidase
A: Mouth via salivary amylase
The stomach continues chemical digestion. When a food bolus enters the stomach, the stomach is distended (stretched). This stretching is a signal for G cells of the stomach to release gastrin. Gastrin is a hormone which stimulates parietal cells of the stomach lining to release gastric juice (very acidic solution with high hydrochloric acid). Gastrin also stimulates chief cells of the stomach lining to secrete pepsinogen. Pepsinogen is a zymogen. A zymogen is the inactive precursor of an enzyme.
It is important for enzymes which digest proteins (like pepsin, trypsin and chymotrypsin) to not be active while in the cell—otherwise they could start digesting the proteins in the cell that produces them! Therefore, they are produced and stored in the cell as zymogens, and only activated when they enter the gastrointestinal area where they are meant to function.
Pepsin is the active form of pepsinogen, and pepsin is a protease (it digest peptide bonds in proteins to break a polypeptide protein into smaller amino acid chunks). If pepsin was active while in the chief cells which produce it, pepsin might start digesting the chief cells themselves! For this reason, the chief cells store the zymogen (inactive form) pepsinogen, which is secreted into the stomach, and activated by the gastric juice (secreted from parietal cells).
The chief cells of the stomach also secretes gastric lipase, which digests fats and lipids into simpler components.
The acidic, semi-digested mix of food leaving the stomach is known as chyme.
The esophagus is a transport tube which connects the pharynx to the stomach. It takes partially-digested food bolus from the pharynx, and contracts via peristalsis (a wave-like contraction) to send food down to the stomach.
The small intestine is involved in both digestion and absorption of the digested nutrients.
The small intestine is broken up into the duodenum (first), jejunum (second) and ileum (last).
Mnemonic: I think of a music DJ named Eye to remember the order of duodenum, jejunum, ileum. “Ladies n’ gentleman, tonight we have DJaaaaaayyyy Eyeee spinning tracks”.
The small intestine is not able to handle the acidity of the chyme from the stomach. The stomach produces mucin (a thick mucous) via mucous cells, and mucin protects the lining of the stomach from the acidity. However, the small intestine cannot equally tolerate this acidity. When the acidic, digested bolus of food from the stomach enters the small intestine, it must be neutralized.
Secretin is a hormone released by the duodenum in response to highly acidic gastric juice entering into the small intestine. Secretin causes the pancreas to secrete bicarbonate (HCO3–) into the duodenum. Bicarbonate is alkaline (basic) and will neutralize the acidic gastric juice that has entered the small intestine by way of the stomach.
When chyme enters the duodenum by way of the stomach, the fats and proteins in the chyme stimulate stimulate cells in the lining of the duodenum to release cholecystokinin (CKK). CKK has many effects:
It slows down gastric emptying (inhibits the transfer of stomach contents to the small intestine). This gives the small intestine more time to digest and absorb what has entered.
It stimulates the pancreas to release its digestive enzymes into the duodenum.
It stimulates the gall bladder to release bile.
Trypsin and chymotrypsin are proteases (enzymes which digest polypeptides into simpler forms, promoting amino acid absorption into the blood stream).
Both trypsin and chymotrypsin are initially secreted in an inactive form (a zymogen). When food enters the duodenum, glands in the duodenum release enteropeptidase. Trypsinogen is activated by enteropeptidase to active trypsin. Active trypsin then cleaves chmyotrypsinogen to form active chymotrypsin.
Bile is produced by the liver, and sent to the gall bladder for storage. When fat enter into the duodenum, the duodenum produces CKK, which causes the gall bladder to release bile.
Bile is not an enzyme, it is an emulsifier. It breaks down big fat globules into smaller globules, so fat breaking enzymes like lipases have more surface area to digest the fat.
Pancreatic lipase is an enzyme that functions to digest the small bits of fat (that bile emulsified) into smaller pieces. Fat is triglyceride. Pancreatic lipase digests triglycerides into glycerol and 3 fatty acid chains (the body can absorb and use these smaller components for cellular respiration).
After all of this ongoing digestion, the small intestine is able to absorb nutrients through its walls, into the blood stream. The wall of the small intestine has folds in it, known as villi. On the villi there are tiny projections known as microvilli. The villi and microvilli function to increase the surface area of the small intestine, which increases the amount of absorption the small intestine can undertake.
By the time the digested food has reached the large intestine (colon), most of the nutrients have already been absorbed out. The large intestine functions to absorb some remaining water and minerals (for example, sodium chloride), before the digested mass is evacuated as feces (through the rectum and anus).
Another function of the large intestine is vitamin production and vitamin absorption. The large intestine contains many commensal bacteria and mutualistic bacteria, within it. This includes bacteria that produce vitamin B and vitamin K, those that metabolize bile acid, and some that ferment fiber.
A child born with a suspected immune illness is brought to the doctor for diagnosis. After several tests, it is determined that the child has severely low macrophage levels, but all other aspects of his immune system are normal. Which immune function would one expect to be most directly compromised in the child?
A: Production of antibodies that circulate in blood
B: Phagocytosis of foreign substances
C: Recognition and response to allergens
D: Rapid response to previously encountered antigens
E: Formation of the membrane attack complex
B: Phagocytosis of foreign substances
A: Production of antibodies that circulate in blood
Incorrect. Plasma cells produce antibodies, so this function would not be directly be compromised. Indirectly, macrophages do act as antigen presenting cells that could activate B-cells to produce antibodies, but this would not be the primary impact of lacking macrophages.
B: Phagocytosis of foreign substances
Correct. Macrophages phagocytose foreign bodies, infectious cells, cancerous cells, microbes, cell debris and other non-native entities within the body. Without macrophages, all of these functions would be severely compromised.
C: Recognition and response to allergens
Incorrect. Mast cells are responsible for recognition of allergens (via attached IgE molecules) and response (through the release of histamine and other chemicals).
D: Rapid response to previously encountered antigens
Incorrect. Memory B cells function to rapidly respond to an antigen that has previously been recognized via accelerated antibody synthesis. Memory T cells also play a role in the response to invasions by a previously encountered pathogen.
E: Formation of the membrane attack complex
Incorrect. The complement system forms the membrane attack complex.
Plasma cells produce antibodies. Plasma cells are derived from B cells. B cells come in two varieties: plasma B cells and memory B cells. Plasma B cells are the type of B cell that produces an antibody. Antibodies are also known as immunoglobulins. An antibody’s role in the immune system is either to act as a marker to tag an antigen for removal (for example, marking an antigen to signal it should be phagocytosed), they can activate the complement system, or antibodies can coat a pathogen and neutralize it.
An antibody is produced in response to a specific antigen. The antigen is the unique substance that an antibody will be complimentary to.
Macrophages are phagocytic cells. Phago- means ‘to eat’, and macrophages eat or engulf other things (like foreign substances, bacteria or cancer cells), and digest these components as a component of the acute immune response.
Innate immunity is the body’s first line defense, and is a nonspecific response. Innate immunity begins with physical and physiological barrier s that prevent infection from entering the body: skin, mucous membranes, cilia in the lungs, stomach acidity, lysozyme, etc. Lysozyme is an enzyme which nonspecifically breaks down bacterial cell walls—killing microbes. It is found in secretions like tears, saliva and mucous.
Innate immunity continues with the inflammatory response. There are five signs associated with inflammation: heat, redness, swelling, loss of function and pain.
Inflammation is associated with certain innate immune response cells like neutrophils, monocytes/macrophages, eosinophils, and basophils/mast cells (all discussed below).
The final main aspect of innate immunity is the complement system. The complement system is a series of blood proteins which ‘turn on’ each other in a cascading series of activations, through the release of cytokines (intercellular signaling molecules). The complement system can lyse bacterial membranes, trigger inflammation, activate the adaptive immune response, target antigens for removal, and help clear immune complexes (an antibody bound to an antigen). The Membrane Attack Complex is formed by the complement system. Five complement proteins join together to form the Membrane Attack Complex (MAC), which can puncture the cell membrane of infecting cells.
While the innate immune response is nonspecific, the adaptive immune response is specific. The innate immune response is a generalized approach whereas the adaptive immune response is a targeted approach. The adaptive immune response mechanism is explained in detail, below.
Acronym for remembering the relative number of leukocytes (white blood cells) circulating in the blood from highest number of cells to lowest number of cells:
Never Let Monkeys Eat Bananas
Never = Neutrophils
Let = Lymphocytes
Monkeys = Monocytes/Macrophages
Eat = Eosinophils
Bananas = Basophils (which form mast cells)
Neutrophils: Innate immunity. Neutrophils are the most numerous white blood cell, and are often the first white blood cells to respond to a site of injury/infection. Neutrophils phagocytose and release granules to capture/kill foreign bodies and microbes.
Lymphocytes: Three types of lymphocytes: natural killer cells, B cells and T cells.
Innate immunity lymphocytes: Natural killer cells.
Adaptive immunity lymphocytes: B and T cells.
Natural killer cells (NK) are a part of the innate immunity, and share many similarities to cytotoxic T cells (CD8) which are part of the adaptive immunity. As the adaptive immune system requires activation, but the innate immune system is always ‘on’, NK cells are often first to act, and CD8 cells will come in to help after once the adaptive immunity has been stimulated. NK cells therefore have a speed advantage as they don’t need to be activated before they can do their job. However, CD8 cells have a specificity advantage, and will be more targeted as they have been activated to the specific antigen.
CD8 and NK cells share techniques in how they destroy pathogenic cells.
Both NK and CD8 cells release:
Perforin, which perforates (poke holes in) pathogenic cell membranes, causing cell lysis (cell breakdown).
Granzymes, a protease which stimulates a target cell to undergo apoptosis (programmed cell death)—useful for killing cancerous cells.
The adaptive immunity lymphocytes are B cells and T cells. They are both produced in the bone marrow from blood stem cells. B cells mature in the bone marrow, T cells travel to the thymus to mature.
There are two types of T cell: cytotoxic T cells (CD8 cells) which were mentioned above, and helper T cells (CD4 cells).
CD4 cells cannot recognize antigens that are ‘freely floating’, they require activation by interfacing with a processed antigen whose epitope is bound to the MHC of an antigen presenting cell.
Antigen presenting cells (APC) are cells that can take an antigen from the pathogen to be destroyed, process that antigen, and attach the antigen to a major histocompatibility complex (MHC) on the APC’s membrane.
MHC are a complex of cell surface proteins that bind to antigens and present the epitope to adaptive immune cells requiring activation. Epitopes are the ‘important part’ of an antigen that is unique and recognizable by the immune cells. This processed epitope on an APC can then be ‘shown’ to cells that require immune activation in the adaptive immune response.
The APCs that activate cells of the adaptive immune response are mainly B cells, dendritic cells and macrophages.
Once activated the CD4 cell can activate other lymphocytes of the adaptive immune response: B cells (both plasma cells and B helper cells), cytotoxic T cells (CD8), and also recruits and activates additional helper T cells (CD4), which ramps up the immune response.
Cytotoxic T cells (CD8) kill tumor, and virus infected cells. Any nucleated cell that is pathogenic (whether it was infected by a virus or has become cancerous) will present an epitope on its surface via an MHC. Activated CD8 cells will recognize the epitope presented by the MHC complexes and will destroy this cell.
Memory T cells are memory cells that remember what antigen they were activated against. If there is another encounter with the same antigen, the memory T cells will help the adaptive immune response to ‘turn on’ more quickly (Memory B cells have a similar role).
B cells function in three main ways:
B cells can be activated to become plasma cells. Plasma cells will produce antibodies to the specific antigen of the pathogen. Antibodies are also known as immunoglobulin. An antibody’s role in the immune system is either to act as a marker to tag an antigen for removal (for example, marking an antigen to signal it should be phagocytosed), they can activate the complement system, or antibodies can coat a pathogen and neutralize it.
An antibody is produced in response to a specific antigen. The antigen is the unique substance that an antibody will be complementary to.
B cells can serve as antigen presenting cells (discussed above).
And, B cells can become memory B cells that are long term cells, so that if there is another encounter with the same antigen, the immune response can occur more quickly.
Monocytes/ Macrophages: part of the innate immunity. Macrophages are phagocytic cells. Phago- means ‘to eat’, and macrophages eat or engulf other things (like foreign substances, bacteria and even cancer cells), and digest these components. Macrophages are not able to target specific antigens—they just ‘eat things’ that should not be in the body in a nonspecific way. A major role of the macrophage is as an antigen presenting cell.
This immune cell is called a monocyte when circulating in the blood. When the monocyte enters the tissue, it differentiates, and is now referred to as a macrophage.
Eosinophils: Eosinophil cell cytoplasm is filled with granules and are granulocytes. Eosinophils contain a variety of proteins that are damaging to both pathogens as well as host tissues. Eosinophils are important in fighting parasites to the host.
Basophils/Mast cells: Like eosinophils, basophils/mast cells are granulocytes. The cell cytoplasm of these cells is filled with granules that can be released through degranulation. These cells are also packed with histamine as well as heparin, and are important mediators of inflammation/the allergic response. Histamine causes vasodilation, and heparin helps to prevent clots from forming—both of which promote more blood entry. Basophils leave bone marrow as mature cells and remain circulating in the blood, whereas mast cells leave the bone marrow and circulate the blood as immature cells, only maturing when they enter the tissue.
The posterior pituitary gland releases which hormone, produced by which organ?
A: Adrenocorticotropic hormone produced in the anterior pituitary
B: Oxytocin produced in the posterior pituitary
C: Melatonin produced in the pineal gland
D: Growth hormone produced in the posterior pituitary gland
E: Antidiuretic hormone produced in the hypothalamus
E: Antidiuretic hormone produced in the hypothalamus
A: Adrenocorticotropic hormone produced in the anterior pituitary
Incorrect. Adrenocorticotropic hormone is produced and released by the anterior pituitary gland.
B: Oxytocin produced in the posterior pituitary
Incorrect. Oxytocin is produced by the hypothalamus and released by the posterior pituitary gland.
C: Melatonin produced in the pineal gland
Incorrect. Melatonin is produced and released by the pineal gland of the brain.
D: Growth hormone produced in the posterior pituitary gland
Incorrect. Growth hormone is produced and released by the anterior pituitary gland.
E: Antidiuretic hormone produced in the hypothalamus
Correct. Antidiuretic hormone is produced by the hypothalamus and release by the posterior pituitary gland.
The posterior pituitary gland is also known as the neurohypophysis because it has a direct neuronal connection with the hypothalamus.
Axons extend from the hypothalamus directly down to the posterior pituitary. The hypothalamus is the producer of oxytocin and antidiuretic hormone. However, it is the posterior pituitary gland that stores and releases these hormones, when stimulated.
The anterior pituitary gland is also known as the adenohypophysis and connects to the hypothalamus via a hypophyseal portal system. A portal system is when a capillary bed is connected to another capillary bed through a portal vein. The hypothalamus sends releasing hormones to the anterior pituitary gland via the hypophyseal portal system to stimulate the release of anterior pituitary hormones.
Hypothalamic releasing hormones include:
Thyrotropin Releasing Hormone (TRH), which stimulates release of thyroid stimulating hormone (TSH, discussed below).
Corticotropin Releasing Hormone (CRH), which stimulates release of ACTH (discussed below).
Gonadotropin releasing hormone (GnRH) a tropic hormone that will cause release of luteinizing hormone (LH) and follicle stimulating hormone (FSH) from anterior pituitary gland.
The anterior pituitary gland releases tropic hormones. Tropic hormones are hormones which target and act on other endocrine glands, which will release their own hormones.
Releasing hormones (discussed above) from the hypothalamus are also considered tropic hormones, as they are hormones which target the anterior pituitary gland.
Hormones the anterior pituitary gland produces can be remembered with the acronym FLAT PiG.
Follicle Stimulating Hormone (FSH) Leutinizing Hormone (LH) Adrenocorticotropic Hormone (ACTH) Thyroid Stimulating Hormone (TSH) Prolactin Ignore Growth Hormone
Follicle Stimulating Hormone (FSH) function: In females, stimulates follicles in the ovary to develop; this stimulates production of female sex hormones (progesterone and estrogen). In males, FSH stimulates sperm to develop.
Luteinizing Hormone (LH): In females, a surge of luteinizing hormone causes ovulation of an egg and formation of the corpus luteum; it also stimulates production of the female sex hormones (estrogen and progesterone). In males, luteinizing hormone stimulates production of testosterone by acting on Leydig cells.
Mnemonics:
In females: Follicle stimulating hormone stimulates the follicle to develop—this makes intuitive sense based on the name follicle stimulating hormone. Luteinizing hormone causes ovulation of an egg and formation of the corpus luteum—this makes intuitive sense based on the name luteinizing hormone.
In males: We know follicle stimulating hormone is abbreviated FSH. The acronym FSH looks like the word FiSH and sperm swimming look like fish. So FSH helps development of sperm. Luteinizing hormone is abbreviated as LH. We can think about luteinizing hormone (LH) as making men look Large and Hairy (L and H, just like LH)—testosterone makes men Large and Hairy, and luteinizing hormone (LH) stimulates testosterone production.
Adrenocorticotropic Hormone (ACTH) stimulates the adrenal gland (a gland that sits above the kidney) to release corticosteroids (the steroid hormones the adrenal gland makes in the adrenal cortex). Adrenal CORTEX produces CORTICOsteroids. The corticosteroids are the glucocorticoids like cortisol, and the mineralocorticoids like aldosterone.
Aldosterone functions to increase salt and water reabsorption back into the body, as well as secretion of potassium into the filtrate in the distal tubules and collecting duct of the kidneys.
The adrenal cortex also plays a role in producing androgens (male sex hormones). In females, the level of androgen production by the adrenal cortex can in some instances be significant; in males, adrenal production of androgens is secondary compared to the levels produced by the testes.
Thyroid Stimulating Hormone stimulates the thyroid gland to release its thyroid hormones: triiodothyoronine (T3) and thyroxine (T4).
Thyroid hormones function to increase the metabolic rate of cells (the amount of energy it is producing from the nutrients it has).
T3 differs from T4 based on the number of iodines that are attached to the molecule. T3 has three iodine atoms attached to it (hence the name T3) and T4 has four iodine atoms attached to it (hence the name T4).
T3 is the active form of the hormone—it is around 4x as potent in stimulation as a hormone than T4.
However, most circulating thyroid hormone is T4 as T4 has a longer half life, therefore it lasts longer in the body. T4 is converted to T3 in body tissues. T4 is a prohormone for T3.
Prolactin’s most direct function in the female body is to stimulate lactation (milk secretion from mammary glands) in mammals, in response to infant suckling. Mnemonic: prolactin stimulates lactation.
Growth Hormone (GH) is also known as somatotropin (somato- means cells of the body, -tropin means hormone). Growth hormone stimulates the cells of the body to grow, reproduce and divide.
Humans (typically) have four parathyroid glands on the back of the thyroid gland. The parathyroid glands release parathyroid hormone (PTH), which stimulates the kidney to reabsorb more calcium. PTH also causes the bones to release calcium by indirectly stimulating osteoclasts. Osteoclasts are bone resorbing (breaking down) cells. If bone is being resorbed, calcium is being liberated, and blood calcium levels increase.
Calcitonin is a hormone which lowers calcium levels in the blood (calcitonin ‘tones down’ calcium). Calcitonin is secreted by the parafollicular cells of the thyroid gland. Calcitonin limits calcium reabsorption in the kidney’s nephrons and limits calcium absorption in the intestines. Calcitonin also causes bones to take up calcium by stimulating osteoblasts to build more bone (osteoblasts build bone). Building bone consumes calcium phosphate, lowering calcium in the blood. Calcitonin also inhibits osteoclasts. If less bone breakdown is occurring, less calcium is being released from bone.
Calcitonin has the opposite function to parathyroid hormone.
(Note: all processes in the body have great complexity. Above is a summary of the high-level function of PTH and calcitonin—they do have other, additional effects).
The pineal gland is a pea sized gland in the brain that releases melatonin. Melatonin is a hormone that regulates circadian rhythms, which sets the sleep/wake cycle, as well as day/night blood pressure, body temperature and hormone production changes. The pineal gland and the subsequent circadian rhythms it sets are fine tuned by light. The presence of light indicates day and the absence of light indicates night (that’s why you need to stop your DAT at the right time of the evening, so your body knows it is time to go to sleep!)
