Race to the Slopes #13

Dichlorophenolindophenol (DCPIP) is a chemical dye. When DCPIP is chemically reduced, it changes color from blue to clear. DCPIP can be used as an electron acceptor in experiments that measure the rate of electron transport through the electron transport chain. A student performed an experiment to study the effects of a chemical, DCMU, on photosynthesis. The student prepared four tubes with a liquid buffer and chloroplasts that had been extracted from spinach leaves. The student then added DCPIP to three of the tubes and added DCMU to one of them. Additionally, tube 3 was wrapped in tin foil. The contents of each tube are shown in the table. The student then incubated each tube for 60 minutes and measured the absorbance (A600) of each solution at five-minute intervals. The absorbance readings of each solution are shown in Figure 1.

Which of the following best explains how DCMU affected the reaction?

• DCMU acts as a second buffer in the reaction.

• DCMU acts as an additional carrier of electrons from photosystem II to DCPIP.

• DCMU acts as an additional source of electrons to the light reaction of photosynthesis.

• DCMU acts as an inhibitor to the movement of electrons within the light reaction of photosynthesis.

Answer explanation

The lack of change in absorbance over the length of the tubes containing DCMU indicates that it prevents the flow of electrons. These results are similar to those observed in tube 3, when the lack of light prevent electrons from flowing.

Dichlorophenolindophenol (DCPIP) is a chemical dye. When DCPIP is chemically reduced, it changes color from blue to clear. DCPIP can be used as an electron acceptor in experiments that measure the rate of electron transport through the electron transport chain. A student performed an experiment to study the effects of a chemical, DCMU, on photosynthesis. The student prepared four tubes with a liquid buffer and chloroplasts that had been extracted from spinach leaves. The student then added DCPIP to three of the tubes and added DCMU to one of them. Additionally, tube 3 was wrapped in tin foil. The contents of each tube are shown in the table. The student then incubated each tube for 60 minutes and measured the absorbance (A600) of each solution at five-minute intervals. The absorbance readings of each solution are shown in Figure 1.

Which of the following claims is best supported by the experimental results?

• Light is required for the electron transport chain to transfer electrons.

• Water, not carbon dioxide, is the source of electrons used in the light-dependent reaction of photosynthesis.

• Carbon dioxide is the source of carbon used by green plants to build carbohydrates.

• DCPIP provides a significant source of electrons to the electron transport chain of the light reaction in the absence of light.

Answer explanation

Tube 2 showed a reduction in absorbance while tube 3 did not, which suggests that light was required for the reaction

Dichlorophenolindophenol (DCPIP) is a chemical dye. When DCPIP is chemically reduced, it changes color from blue to clear. DCPIP can be used as an electron acceptor in experiments that measure the rate of electron transport through the electron transport chain. A student performed an experiment to study the effects of a chemical, DCMU, on photosynthesis. The student prepared four tubes with a liquid buffer and chloroplasts that had been extracted from spinach leaves. The student then added DCPIP to three of the tubes and added DCMU to one of them. Additionally, tube 3 was wrapped in tin foil. The contents of each tube are shown in the table. The student then incubated each tube for 60 minutes and measured the absorbance (A600) of each solution at five-minute intervals. The absorbance readings of each solution are shown in Figure 1.

Which of the following best justifies the use of tube 2 as a control treatment?

• It was a negative control for the accuracy of the spectrophotometer, ensuring that an accurate reading for treatment cuvette 2 would be made.

• It was a negative control for the chemical stability of the chlorophyll suspension, ensuring that changes in absorbance could only be attributed to changes in chlorophyll content

• It was a positive control for the change in DCPIP color associated with changes in light intensity

• It was a positive control for measuring the effect of DCMU on the reaction

Answer explanation

Tubes 4 and 2 differed only by the presence of DCMU so any changes in the outcome of the experiment could be attributed only to DCMU

Dichlorophenolindophenol (DCPIP) is a chemical dye. When DCPIP is chemically reduced, it changes color from blue to clear. DCPIP can be used as an electron acceptor in experiments that measure the rate of electron transport through the electron transport chain. A student performed an experiment to study the effects of a chemical, DCMU, on photosynthesis. The student prepared four tubes with a liquid buffer and chloroplasts that had been extracted from spinach leaves. The student then added DCPIP to three of the tubes and added DCMU to one of them. Additionally, tube 3 was wrapped in tin foil. The contents of each tube are shown in the table. The student then incubated each tube for 60 minutes and measured the absorbance (A600) of each solution at five-minute intervals. The absorbance readings of each solution are shown in Figure 1.

In which of the following tubes did the greatest reduction of DCPIP occur after 60 minutes?

• Tube 1

• Tube 2

• Tube 3

• Tube 4

Answer explanation

Over the 60-minute experiment, the absorbance of tube 2 decreased, indicating that DCPIP was reduced.

Which of the following best describes the hydrolysis of carbohydrates?

• The removal of a water molecule breaks a covalent bond between sugar monomers.

• The removal of a water molecule forms a covalent bond between sugar monomers.

• The addition of a water molecule breaks a covalent bond between sugar monomers.

• The addition of a water molecule forms a covalent bond between sugar monomers.

Answer explanation

The hydrolysis of carbohydrates involves the addition of a water molecule, which breaks the covalent bond between monomers.

Which of the following best explains how molecules such as O2 and CO2 can move across the membrane of a cell?

• The majority of the cell membrane contains protein channels that allow this type of molecule into the cell.

• The majority of the cell membrane is nonpolar, which allows small, nonpolar molecules to freely cross.

• The phospholipids of the membrane are tightly packed, so only small molecules and ions can fit between phospholipids.

• ATP is hydrolyzed to provide energy to help O2 and CO2 move against their concentration gradient and across the membrane.

Answer explanation

Small, nonpolar molecules can pass through the cell membrane by simple diffusion because the membrane is composed of phospholipids with nonpolar tails.

Which of the following best describes a characteristic of the process shown in Figure 1 that is unique to prokaryotes?

• The mRNA is synthesized in a 5' to 3' direction

• A single strand of the DNA is being used as a template for the transcription of the mRNA.

• The translation of the mRNA is occurring while the mRNA is still being transcribed.

• The enzyme that is transcribing the mRNA is RNA polymerase.

Answer explanation

t. In eukaryotes, RNA transcripts need to be modified before they can be translated, but prokaryotes can translate the beginning of an mRNA molecule while the end is still being transcribed.

Which of the following describes a mutation that would lead to an increase in the frequency of nondisjunction?

• A mutation affecting checkpoint 1 proteins that forces cells to enter G0

• A mutation affecting checkpoint 2 proteins that allows cells to divide with DNA damage

• A mutation affecting checkpoint 3 proteins that prevents attachment of spindle fibers

• A mutation affecting checkpoint 2 proteins that prevents duplication of the chromosomes

Answer explanation

If the cells move into anaphase without the spindle fibers properly attached to the chromosomes, the chances of nondisjunction occurring will increase.

A mutation in the gene coding for a single-polypeptide enzyme results in the substitution of the amino acid serine, which has a polar R group, by the amino acid phenylalanine, which has a nonpolar R group. When researchers test the catalysis of the normal enzyme and the mutated enzyme, they find that the mutated enzyme has much lower activity than the normal enzyme does.

Which of the following most likely explains how the amino acid substitution has resulted in decreased catalytic activity by the mutated enzyme?

• The substitution decreased the mass of the enzyme so that the mutated enzyme binds more weakly to the substrate than the normal enzyme does.

• The substitution altered the secondary and tertiary structure of the enzyme so that the mutated enzyme folds into a different shape than the normal enzyme does.

• The substitution caused many copies of the mutated enzyme to cluster together and compete for substrate to bind

• The substitution caused the directionality of the enzyme to change such that the amino terminus of the normal enzyme has become the carboxy terminus of the mutated enzyme

Answer explanation

The replacement of a polar amino acid with a nonpolar amino acid will affect the folding of the enzyme because there will be different interactions between the R groups