Bio #2

-*Identify at least two events in the cell cycle that must be completed successfully for daughter cells to share an identical complement of chromosomes.

1. All of the chromosomes must be replicated during S phase 2. The spindle apparatus must connect with the kinetochores of each sister chromatid in prometaphase 3. The sister chromatids of each replicated chromosome must be portioned in anaphase 4. The daughter chromosomes must be divided between two daughter cells by cytokinesis

*What are three attributes of mitochondria and chloroplasts that suggest they were one free-living bacteria?

1. Have their own small, circular (or linear in some species) chromosomes 2. They produce their own ribosomes 3. They divide in a manner that is independent of cellular division

*Predict how the following conditions would affect the production of O2, ATP, and NADPH and state whether noncyclic or cyclic electron flow would occur in each: (1) Only blur photons hit a chloroplast; (2) blue and red photons hit a chloroplast, but no NADP+ is available; (3) blue and red photons hit a chloroplast, but a proton channel has been introduced into the thylakoid membrane, so it is fully permeable to protons.

1. O2, ATP, and NADPH would be formed by noncyclic electron flow 2. No O2 or NADPH would be formed, but ATP may be made by cyclic electron flow 3. Initially, O2 and NADPH would be formed by noncyclic electron flow, but no ATP would be made. Without ATP, the Calvin cycle would halt and, once all the NADP+ is reduced to NADPH, noncyclic electron flow would switch to cyclic electron flow

-*A particular cell type spends 4 hours in G1 phase, 2 hours in S phase, 2 hours in G2 phase, and 30 minutes in M phase. If a pulse-chase experiment were performed with radioactive thymidine on an asynchronous culture of such cells, what percentage of mitotic cells would be radiolabeled 9 hours after the pulse?

A- 0% Adding the length of time spent in each phase allows you to determine that the cell cycle is 8.5 hours long. After 9 hours, all radiolabeled cells would have passed through a full cycle and be in either S phase or G2- none would be mitotic (in M phase)

*-If a reaction is exergonic, then which of these statements is true? a. the products have lower Gibbs free energy than the reactants b. energy must be added for the reaction to proceed c. the products have lower entropy (are more ordered) than the reactants d. the reaction always occurs quickly

A- the products have lower Gibbs free energy than the reactants

*-What do the light-capturing reactions of photosynthesis produce?

ATP and NADPH

*What does the chemiosmotic hypothesis claim?

ATP is generated using a proton-motive force that is produced by the electron transport chain

*Compare and contrast the structure of a generalized plant cell, animal cell, and prokaryotic cell. Which features are common to all cells? Which are specific to just prokaryotes, or just plants, or just animals?

All cells are bound by a plasma membrane, are filled with cytoplasm, carry their genetic information (DNA) in chromosomes, and contain ribosomes (the sites of protein synthesis). Some prokaryotes have organelles not found in plants or animals, such as a magnetite-containing structure. Plant cells have chloroplasts, vacuoles, and a cell wall. Animal cells contain lysosomes and lack a cell wall.

*Draw a chemical equation to represent the redox reaction that occurs when methane (CH4) burns in the presence of oxygen (O2). Identify the reactant that is being reduced and the reactant that is being oxidized. Of the four molecules that should be in your equation, point out the one that has the highest energy bonds.

Answer on paper

-Which of the following accurately describes a difference between prokaryotic and eukaryotic cells? a. Prokaryotic cells have fimbriae that allow the cell to swim whereas eukaryotic cells have flagella b. Eukaryotic cells are generally larger than prokaryotic cells c. Eukaryotic cells have organelles d. Prokaryotic cells have nuclei and eukaryotic cells have nucleoids

B- Eukaryotic cells are generally larger than prokaryotic cells

Suppose a drug were added to mitochondria that allowed protons to freely pass through the inner membrane. Which of the following mitochondrial activities would most likely be inhibited? a. The citric acid cycle b. Oxidative phosphorylation c. Substrate-level phosphorylation d. The electron transport chain

B- Oxidative phosphorylation

*-An investigator exposes chloroplasts to 700-nm photons and observes low O2 production, but high ATP production. Which of the following best explains this observation? a. The electrons from water are directly transferred to NADP+, which is used to generate ATP b. Photosystem II is not splitting water, and the ATP is being produced by cycling electrons via photosystem I c. The O2 is being converted to water as a terminal electron acceptor in the production of ATP d. Electron transport has stopped and ATP is being produced by the Calvin cycle

B- Photosystem II is not splitting water, and the ATP is being produced by cycling electrons via photosystem I

-*Which statement about the daughter cells following mitosis and cytokinesis is correct? a. They are genetically different from each other and from the parent cell b. They are genetically identical to each other and to the parent cell c. They are genetically identical to each other but different form the parent cell d. Only one of the two daughter cells is genetically identical to the parent cell

B- They are genetically identical to each other and to the parent cell

*-Which of the following results provided evidence of a discrete nuclear localization signal somewhere on the nucleoplasmin protein? a. the nucleoplasmin protein was small and easily slipped through the nuclear pore complex b. after cleavage of the nucleoplasmin protein, only the tail segments appeared in the nucleus c. removing the tail from the nucleoplasmin protein allowed the core segment to enter the nucleus d. the SRP bound only to the tail of the nucleoplasmin protein, bot the core segment

B- after cleavage of the nucleoplasmin protein, only the tail segments appeared in the nucleus

*-You have discovered an enzyme that appears to function only when a particular sugar accumulates. Which of the following scenarios would you predict to be responsible for activating this enzyme? a. The sugar cleaves the enzyme to form the active conformation b. the sugar is an allosteric regulatory molecule for the enzyme c. the sugar is a competitive inhibitor for the enzyme d. the sugar phosphorylates the enzyme to form the active conformation

B- the sugar is an allosteric regulatory molecule for the enzyme

*If you were to expose cells that are undergoing aerobic cellular respiration to a radioactive oxygen isotope in the form of O2, which of the following molecules would you expect to be radiolabeled? a. pyruvate b. water c. NADH d. CO2

B- water

The presence of many sac-like cristae results in a large amount of membrane inside mitochondria. Suppose that some mitochondria had few cristae. How would their output of ATP compare with that of mitochondria with many cristae? Justify your answer.

Because mitochondria with few cristae would have fewer electron transport chains and ATP synthase molecules, they would produce much less ATP than mitochondria with numerous cristae

Some biologists claim that photorespiration is an evolutionary "holdover," because rebisco evolved over a billion years ago when O2 levels were extremely low and CO2 concentrations relatively high. Do you agree with this hypothesis? Why or why not?

Because rubisco evolved in a high CO2, low O2 environment, which would minimize the impact of photorespiration, the hypothesis is credible. But once O2 levels increased, any change in rubisco that minimized photorespiration would give individuals a huge advantage over organisms with "old" forms of rubisco. There has been plenty of time for such changes to occur, making the "holdover" hypothesis less credible

Compare and contrast competitive inhibition and allosteric regulation.

Both are mechanisms that regulate enzymes; the difference is whether the regulatory molecule binds at the active site (competitive inhibition) or away form the active site (allosteric regulation)

*In step 3 of the citric acid cycle, the enzyme isocitrate dehydrogenase is regulated by NADH. Compare and contrast the regulation of this enzyme with the regulation of phosphofructokinase in glycolysis.

Both phosphofructokinase and isocitrate dehydrogenase are regulated by feedback inhibition, where the product of the reaction or series of reactions inhibits the enzyme activity. The differ in that phosphofructokinase is regulated by allosteric inhibition while isocitrate dehydrogenase is controlled by competitive inhibition

*Compare and contrast substrate-level phosphorylation and oxidative phosphorylation.

Both processes produce ATP from ADO and Pi, but substrate-level phosphorylation occurs when enzymes remove a "high-energy" phosphate from a substrate and directly transfer it to ADP, while oxidative phosphorylation occurs when electrons move through an ETC and produce a proton-motive force that drives ATP synthase

10- *-In antenna complexes, how is energy transferred among the pigment molecules?

By resonance

-Which of the following is not true of secreted proteins? a. They are synthesized using ribosomes b. They enter the ER lumen during translation c. They contain a signal that directs them into the lysosome d. They are transported between organelles in membrane-bound vesicles

C- They contain a signal that directs them into the lysosome

Cancer is primarily a disease of older people. Further, a group of individuals may share a genetic predisposition to developing certain types of cancer, yet vary a great deal in time of onset- or not get the disease at all. What conclusion could be drawn based on these observations? How does this relate to the requirements for a cell to become cancerous?

Cancer requires many defects. Older cells have had more time to accumulate defects. Individuals with a genetic predisposition to cancer start out with some caner-related defects, but this does not mean that the additional defects required for cancer to occur will develop

Using what you have learned about changed in Gibbs free energy, would you predict the ΔG value of catabolic reactions to be positive or negative? What about anabolic reactions? Justify your answers using the terms "enthalpy" and "entropy."

Catabolic reactions will often have a negative ΔG based on a decrease in enthalpy and increase in entropy. Anabolic reactions are the opposite-a positive ΔG that is based on an increase in enthalpy and decrease in entropy.

*Which process in plants generates the most ATP for driving cellular activities, photosynthesis or cellular respiration? Explain your answer

Cellular respiration generates the most ATP used to drive cellular activities. If you said photosynthesis, then you might be thinking that the ATP produced in the light reactions is used in cytosol. But the ATP produced by photosynthesis is used only in the stroma of chloroplasts for activities such as the production of G3P in the Calvin cycle.

Why are cyclins called cyclins? Explain their relationship to MPF ativity.

Cyclin concentrations change during the cell cycle. At high concentration, cyclins bind to a specific cyclin-dependent kinase (or Cdk), forming a dimer. This dimer becomes active MPF by changing its shape through the phosphorylation (activation site) and dephosphorylation (inhibitory site) of Cdk

-*Progression through the cell cycle is regulated by oscillations in the concentration of which type of molecule?

Cyclins

-If you were to expose glucose to oxygen on your lab bench, why would you not expect to see it burn as shown in Figure 8.6?

D- Activation energy is required for the sugar and oxygen to reach their transition state (energy, such as the thermal energy in fire, must be provided to overcome the activation energy barrier before the reaction can proceed)

What evidence suggests that during anaphase, kinetochore microtubules shorten at the kinetochore?

Daughter chromosomes were observed to move toward the poles faster than the marked regions of fluorescently labeled kinetochore microtubules

-Eukaryotic have which structure (s) that prokaryotic do NOT? a. DNA b. Ribosomes c. Lipid bilayer membrane d. Lysosomes e. Nuclear membrane

E- Nuclear membrane

What is most likely responsible for the reduction in death rates over the past several years in cancers of the breast and prostate? How is this related to the development of cancer?

Early detection of cancers leads to a greater likelihood of survival. The widespread implementation of breast and prostate exams allows for the identification and removal of benign tumors before they become malignant

-What does the first law of thermodynamics state?

Energy is conserved: it cannot be created or destroyed

-Evaluate each of the following defects. Which could lead to uncontrolled growth in cancer? True/False: The overexpression of MPF activity True/False: A nonfunctional Rb protein True/False: The overexpression of G1 cyclin True/False: A nonfunctional E3F protein

False True True False If the Rb protein is nonfunctional or G1 cyclin is overexpressed, E2F expression would immediately cause cells to enter S phase

-Which of the following cell structures would you expect to be most important in the growth of bacteria on the surface of your teeth?

Fimbriae (they are involved in bacterial attachment to surfaces and other cells, which would be important in the ability to grow on teeth)

*Early estimates suggested that the oxidation of glucose via aerobic respiration would produce 38 ATP. Based on what you know of the theoretical yields of ATP from each step, show how this total was determined. Why do biologists now think this amount of ATP/glucose is not achieved in cells?

For each glucose molecule, two ATP are produced in the citric acid cycle via substrate-level phosphorylation. A total of 10 NADH and 2 FADH2 molecules are produced from glycolysis, pyruvate oxidation, and the citric acid cycle. If each NADH were to yield 3 ATP, and each FADH2 were to yield 2 ATP, then a total of 34 ATP would be produced via oxidative phosphorylation. Adding these totals would result in 38 ATP. A cell will not produce this much ATP, because the proton-motive force is used in other transport steps and because of other issues that may reduce the overall efficiency

*In Figure 8.9, the energetic coupling of ATP hydrolysis and an endergonic reaction are show. If the hydrolysis of ATP releases 7.3 kcal of free energy, use the graph in this figure to estimate what you would expect the ΔG values to be for the uncoupled reaction and the two steps in the coupled reaction.

For the coupled reaction, step 1 has a ΔG of about-3kcal/mol and step 2 has a ΔG of about -3kcal/mol. The uncoupled reaction has a ΔG of about +1.3kcal/mol

In multicellular organisms, nondividing cells stay in G0 phase. For the cell, why is it better to be held in G1 rather than S, G2, or M phase?

G1 cells have not replicated their DNA in preparation for division

Compare and contrast how C4 plants and CAM plants separate the acquisition of CO2 from the production of sugar in the Calvin cycle.

In both C4 and CAM plants, atmospheric CO2 is brought in through stomata and first captured by fixing it to a 3-carbon molecule by PEP carboxylase. The C4 pathway and CAM differ in the timing of this first fixation step- it occurs during the day in C4 plants and during the night in CAM plants. They also differ in the location of the Calvin cycle with respect to this first fixation step. In C4 plants, the two processes occur in different cells, while in CAM plants they occur in the same cell, but at different times (Calvin cycle during the day)

*Where does the citric acid cycle occur in eukaryotes

In the matrix of the mitochondria

*-Why is chlorophyll green?

It absorbs wavelengths in only the blue and red parts of the visible spectrum

*Use what you know of the relationship between the light-capturing reactions and Calvin cycle to calculate the number of photons used to produce a new G3P and regenerate the RuBP. (Assume 1 ATP is produced for each pair of electrons used to form NADPH).

It would require 36 photons. To produce G3P from 3 CO2 molecules and regenerate RuBP, a total of 6 NADPH and 9 ATP are required. Each NADPH is made by exciting two electrons from water in photosystem II and then again in photosystem I. This means that 24 photons are required for 6 NADPH, which would also produce 6 ATP. To make 9 ATP, three more pairs of electrons are required for a total 18 electrons that must be excited by 36 photons.

-Enzymes are specific. Only molecules with the correct shape can fit into the enzyme. Just like only one key can open a lock, only one type of enzyme can speed up a specific reaction. This is called:

Lock and Key

Explain how microinjection experiments supported they hypothesis that specific molecule in the cytoplasm are involved in the transition from interphase to M phase. What was the control for this experiment?

Microinjection experiments suggested that something in the cytoplasm of M-phase cells activated the transition from interphase to M-phase. The control for this experiment was to inject cytoplasm from a G2-arrested oocyte into another G2-arrested oocyte

Why is the chlorophyll in chloroplasts less likely to produce fluorescence compared to extracted chlorophyll molecules?

Most of the energy captured by chlorophyll in chloroplasts is converted into chemical energy by reducing electron acceptors in ETCs. When pigments are extracted, the antenna complexes, reaction centers, and ETCs have been disassembled, so the energy is given off as fluorescence and/or heat

*After glucose is fully oxidized by glycolysis, pyruvate processing, and the citric acid cycle, where is most of its energy stored?

Most of the energy is stored in the form of NADH

Consider plants that occupy the top, middle, or ground layer of a forest, and algae that live near the surface of the ocean or in deeper water. Would you expect the same photosynthetic pigments to be found in species that live in these different habitats? Why or why not?How would you test your hypothesis?

No- they are unlikely to have the same complement of photosynthetic pigments. Different wavelengths of light are available in various layers of a forest and water depths. It is logical to predict that plants and algae have pigments that absorb the available wavelengths efficiently. One way to test this hypothesis would be to isolate pigments from species in different locations and test the absorbance spectra of each

*Explain the relationship between electron transport and oxidative phosphorylation. What does ATP synthase look like, and how does it work?

Oxidative phosphorylation is possible via a proton gradient that is established by redox reactions in the ETC. ATP synthase consists of a membrane-associated F0 unit and a F1 unit joined by a rotor shaft. When protons flow through the F0 unit, it spins the rotor shaft within the fixed F1 unit. This spinning shaft causes structural changes in the F1 that drives the synthesis of ATP from ADP and Pi

What conditions favor photorespiration? What are its consequences for the plant?

Photorespiration occurs when levels of CO2 are low and O2 are high. Less sugar is produced because (1) CO2 doesn't participate in the initial reaction catalyzed by rubisco and (2) when rubisco catalyzes the reaction with O2 instead, one of the products is eventually broken down to CO2 in a process that uses ATP

Why do plants need both chloroplasts and mitochondria? How do their roles differ in the cell?

Photosynthesis in chloroplasts produces sugar, which is used as a source of carbon for building organic molecules and energy for cellular respiration. Mitochondria harvest the energy stored in sugar to produce ATP, which is used to drive many cellular activities

Describe how vesicles move in a directed manner between organelles of the endomembrane system. Explain why this movement requires ATP.

Polarized cytoskeletal filaments (microtubules or microfilaments) are present between the organelles. End-directed motor proteins use ATP to move these transport vesicles between them

Why are most protein kinases considered regulatory proteins?

Protein kinases phosphorylate proteins. Phosphorylation changes a protein's shape, altering its function (activating or inactivating it). As a result, protein kinases regulate the function of proteins

*Explain the lock-and-key model of enzyme activity. What is incorrect about this model?

Reactions proceed when the reactant molecules (the key) fits snugly into the active site of an enzyme (the lock). Fischer's original model assumed that enzymes were rigid; in fact, enzymes are flexible and often change their shape after binding to substrates.

Describe the relationship between carbohydrates metabolism, the catabolism of proteins and fats, and anabolic pathways.

Stored carbohydrates can be broken down into glucose that enters the glycolytic pathway. If carbohydrates are absent, products from fat and protein catabolism can be used to fuel cellular respiration or fermentation. If ATP is plentiful, anabolic reactions use intermediates of the glycolytic pathway and the citric acid cycle to synthesize carbohydrates, fats, and proteins

*Most of the proteins that enter the nucleus possess a nuclear localization signal (NLS), even if they are small enough to pass through the nuclear pore complex unhindered. Why would a small protein have an NLS, when it naturally diffuses across the nuclear pore complex without one?

The NLS will be used to actively import the protein into the nucleus, leaving very little of the protein in the cytoplasm. Diffusion alone would not drive all the protein into the nucleus

*Compare and contrast the effects of removing growth factors from asynchronous cultures of human cells that are normal and those that are cancerous.

The absence of growth factors in normal cells would cause the cells to arrest in G1 phase, and eventually all the cells would be unlikely to depend on these growth factors, so those cells would not arrest and would continue through the cell cycle

-In addition to their protective function, carotenoids absorb certain wavelengths of light and pass the energy to other pigments via resonance. Based on this function, where would you expect carotenoids to be located in the chloroplast?

The antenna complex

*-How does pH affect enzyme-catalyzed reactions?

The concentration of protons affects an enzyme's folded structure and reactivity

*-At what point in photosynthesis is the electromagnetic energy of light first converted into chemical energy?

The conversion of light energy to chemical energy first occurs when electrons are transferred from excited pigments to an electron carrier in a photosystem

At what point in the light-capturing reactions is the electromagnetic energy of light converted into chemical energy? Where does this occur?

The conversion of light energy to chemical energy occurs when electrons are transferred from excited pigments to an electron carrier in the photosystems

Under normal conditions, what happens to the cell cycle if the chromosomes fail to separate properly at anaphase?

The cycle would arrest in M phase, and cytokinesis would not occur

Recently, researchers were able to measure movement that occurred in a single amino acid in an enzyme as reactions were taking place in its active site. The amino acid that moved was located in the active site, and the rate of movement correlated closely with the rate at which the reaction was taking place. Discuss the significance of these findings, using the information in Figures 8.10 and 8.13.

The data suggest that the enzyme and substrate form a transition state that requires a change in the shape of the active site, and that each movements corresponds to one reaction

In noncyclic electron flow, photosystems I and II function as an integrated unit. What connects the two photosystems?

The electron transport chain that accepts electrons from PS II. Plastocyanin is the molecule that transfers electrons from this chain to the PS I reaction center

Explain how electrons from water can be used to produce both ATP and NADH.

The electrons taken from water in PS II are excited twice by either photons or resonance energy. When excited in PS II, the electrons are transferred to PQ and used to build a proton-motive force that makes ATP. After reaching PS I, they are excite a second time and will either be used to reduce NADP+ to make NADPH (noncyclic) or be transported back to PQ to produce more ATP (cyclic)

*When a fruit fly embryo first begins to develop, a large cell is generated that contains over 8000 genetically identical nuclei. What is most likely responsible for this result?

The embryo passes through multiple rounds of the cell cycle, but cytokinesis does not occur during the M phases

*-Cells that line your intestines are known to possess a large number of membrane proteins that transport small molecules and ions across the plasma membrane. Which of the following cell structures would you expect to be required for this functions of the cells?

The endoplasmic reticulum (this is responsible for synthesizing the membrane proteins required for the transport of solutes across the plasma membrane)

What often happens to an enzyme after it binds to its substrate? Is this a permanent change?

The enzyme changes shape, but the change is not permanent. The enzyme shape will return to its original conformation after releasing the products

*Explain how feedback inhibition regulates metabolic pathways.

The final product of a pathway inhibits the activity of an enzyme early in the same pathway, thus reducing the activity of all subsequent steps in the pathway

*Describe the three phases of the Calvin cycle and how the products of the light-capturing reactions participate in this process.

The fixation phase is when CO2 is fixed to RuBP by rubisco to form 3-phosphoglycerate (3PGA). The reduction phase uses ATP to phosphorylate the carbons and NADPH to reduce them with high-energy electrons to form G3P. The regeneration phase uses more ATP to convert some of the G3P to RuBP to continue the cycle.

Describe the three phases of the Calvin cycle and how the products of light-capturing reactions participate in this process.

The fixation phase is when CO2 is fixed to RuBP by rubisco to form 3-phosphoglycerate. The reduction phase uses ATP to phosphorylate the carbons and NADPH to reduce them with high-energy electrons to form G3P. The regeneration phase uses more ATP to convert some of the G3P to RuBP to continue the cycle

*-What is a transition state?

The intermediate complex formed as covalent bonds in the reactants are being broken and re-formed during a reaction

*Upon analyzing a sample of cells form a patient, you find the lysosomes to be filled with undigested material. This observation makes you think that the lysosomes are not functioning properly. What are three different defects that could be responsible for malfunctioning lysosomes?

The patient may have a defect in: 1. a hydrolytic enzyme that breaks down polysaccharides 2. the process of adding mannose-6-phosphate signal to lysosomal hydrolases 3. the mannose-6-phosphate receptor in the Golgi apparatus Any of these three defects would result in lysosomes that are missing enzymes necessary to break down the polysaccharides

*-How does the hydrolysis of ATP result in the movement of a motor protein along a cytoskeletal filament?

The phosphate links to the motor protein and causes it to change shape, which results in the protein moving along the filament

Explain why substrate phosphorylation using ATP is an exergonic reaction. How does the phosphorylation of reactants result in driving reactions that would normally be endergonic?

The phosphorylation reaction is exergonic because the electrons in ADP and the phosphate added to the substrate experience less electrical repulsion, and thus have less potential energy, than they did in ATP. A phosphorylated reactant (i.e., an activated intermediate) gains enough potential energy to shift the free energy change for the reaction from endergonic to exergonic.

The enzymes found in peroxisomes are synthesized by cytosolic ribosomes. Suggest a hypothesis for how these proteins find their way to the peroxisomes.

The proteins must receive a molecular zip code that binds to a receptor on the surface of peroxisomes. They could diffuse randomly to peroxisomes or be transported in a directed way by motor proteins.

*George Palade's research group used the pulse-chase assay to dissect the secretory pathway in pancreatic cells. If they had instead performed this assay on the muscle cells, which have high energy demands and primarily consist of actin and myosin filaments, where would you expect the labeled proteins to go during the chase?

The radiolabeled proteins would likely be found in the cytosol and mitochondria. Cytosolic proteins would include proteins like the actin and myosin involved in muscle contraction. Since there is a high energy demand, you would predict that many proteins are required to produce ATP in mitochondria.

-*What major events occur during anaphase of mitosis?

The sister chromatids on replicated chromosomes separate, and the spindle poles are pushed farther apart

Propose an experiment that would determine if the NLS in nucleoplasmin is limited to this protein only or if it could direct other structures into the nucleus.

The tails cleaved from nucleoplasmin could be attached to the gold particles that were excluded from crossing the pore complex owing to their size. If these modified particles entered the nucleus, then the tail is not limited to the nucleoplasmin transport alone.

*-Molecular zip codes direct molecules to particular destinations in the cell. How are these signals read?

They bind to receptor proteins

Although all three cytoskeletal fibers constantly replace their subunits, only actin filaments and microtubules demonstrate differences in the rate of growth between the two ends. What is responsible for this difference, and why is this not observed in intermediate filaments?

This occurs in microfilaments and microtubules because they have ends that differ structurally and functionally- they have different filament growth rates. Intermediate filaments have identical ends, so there is no difference in the rate of assembly between the two ends.

*What is the primary function of the reactions that follow glycolysis in a fermentation pathway?

To regenerate NAD+ from NADH, so glycolysis can continue

-*After S phase, what comprises a single chromosome?

Two sister chromatids

Why do the absorption spectrum for chlorophyll and the action spectrum for photosynthesis coincide?

Wavelengths of light that are absorbed by chlorophyll trigger the light-capturing reactions

*Cyanide blocks complex IV of the electron transport chain. Suggest a hypothesis for what happens to the ETC when complex IV stops working. Your hypothesis should explain why cyanide poisoning in humans is fatal

When complex IV is blocked, electrons can no longer be transferred to oxygen, the final acceptor, and cellular respiration stops. Fermentation could keep glycolysis going, but it is unable to fuel a cell's energy needs over the long term. The low production of ATP would result in death, with those cells that lack the capacity for fermentation dying first

When does feedback inhibition occur?

When the product of a pathway feeds back to interact with an enzyme early in the same pathway to inhibit its function

Researchers can analyze the atomic structure of enzymes during catalysis. In one recent study, investigators found that the transition state included the formation of a free radical and that a coenzyme bound to the active site donated an electron to help stabilize the free radical. How would the reaction rate and the stability o the transitin state change if the coenzyme were not available?

Without the coenzyme, the free-radical-containing transition state would not be stabilized and the reaction rate would drop dramatically