All MCAT Biology Resources
Example Questions
Example Question #141 : Organic Chemistry, Biochemistry, And Metabolism
Which organelle would have the most negative effect if its membrane were damaged?
Golgi body
Chloroplasts
Ribosomes
Mitochondria
Lysosomes
Lysosomes
The lysosomes contain an acidic environment and digestive enzymes. Damage to the membrane would allow hydrogen ions and these enzymes to escape into the cytoplasm of the cell, where they would do damage to the other cellular components.
Damage to a mitochondrion or chloroplast would affect energy production in the cell, but would not actively cause damage. Ribosomes don't have membranes.
Example Question #41 : Molecular Properties
Among the most important pH buffer systems in humans is the bicarbonate buffer, which keeps the blood at a remarkably precise 7.42 pH. The bicarbonate buffer system uses a series of important compounds and enzymes to make the system function. Figure 1 depicts the key reactions that take place.
The activity of this buffer system is mainly controlled by the renal and respiratory systems. The renal system excretes bicarbonate in the urine, while the respiratory system “blows off” carbon dioxide as needed. By balancing these two systems as needed, blood pH is maintained in such a narrow range.
What is the predicted molecular orbital hybridization state of the carbon in carbonic acid?
sp2
Pi
Sigma
sp3
sp
sp2
Carbons bound via one double bond are sp2 hybridized, as long as the remaining two bonds are each sigma bonds. Take the number of sigma bonds and subtract one for your exponent in the spx expression.
Example Question #1532 : Mcat Biological Sciences
When formic acid is completely reduced, methanol is formed.
What is the hybridization of the carbon in formic acid, compared to the carbon in methanol?
The carboxylic acid has sp2 hybridization and the alcohol has sp3 hybridization.
The carboxylic acid has sp3 hybridization and the alcohol has sp2 hybridization.
The carboxylic acid has sp3 hybridization and the alcohol has sp3 hybridization.
The carboxylic acid has sp2 hybridization and the alcohol has sp2 hybridization.
The carboxylic acid has sp2 hybridization and the alcohol has sp3 hybridization.
In order to find the hybridization of an atom, simply count the number of sigma bonds and lone pair electrons around the atom. The carbon in formic acid is double bonded to an oxygen, and has two single bonds. This means that it has sp2 hybridization. Upon being reduced to methanol, the carbon now has four single bonds surrounding it. As a result, the carbon now has sp3 hybridization.
Remember that a triple bond corresponds to sp hydrization, a double bond to sp2, and single bonds to sp3 for a carbon atom.
Example Question #145 : Organic Chemistry, Biochemistry, And Metabolism
One component of the immune system is the neutrophil, a professional phagocyte that consumes invading cells. The neutrophil is ferried to the site of infection via the blood as pre-neutrophils, or monocytes, ready to differentiate as needed to defend their host.
In order to leave the blood and migrate to the tissues, where infection is active, the monocyte undergoes a process called diapedesis. Diapedesis is a process of extravasation, where the monocyte leaves the circulation by moving in between endothelial cells, enters the tissue, and matures into a neutrophil.
Diapedesis is mediated by a class of proteins called selectins, present on the monocyte membrane and the endothelium. These selectins interact, attract the monocyte to the endothelium, and allow the monocytes to roll along the endothelium until they are able to complete diapedesis by leaving the vasculature and entering the tissues.
The image below shows monocytes moving in the blood vessel, "rolling" along the vessel wall, and eventually leaving the vessel to migrate to the site of infection.
Neutrophils make use of radical species to digest phagocytosed material. Which of the following is true of radical reactions?
Radical reactions always terminate by binding to the wall of the reaction container
Radical carbons are tetrahedral
Radical carbons have excess negative charge
Tertiary radical carbons are the least stable
Radical carbons are sp2 hybridized
Radical carbons are sp2 hybridized
Radical reactions have an sp2, trigonal planer carbon radical intermediate. Carbon radicals are most stable as tertiary species.
Example Question #1 : Covalent Bonding
2-butyne contains all of the following types of bonds except __________.
2-butyne has the following chemical structure.
The end carbons have hybridization (form single bonds only), while the middle two carbons have hybridization (involved in a triple bond). There are no hybridized carbons in this molecule.
Example Question #1533 : Mcat Biological Sciences
What carbon hybridizations can be found in one molecule of acetic acid?
only
only
and
and
and
Acetic acid has the formula . This compound is common enough that the IUPAC name will rarely be given, and you will need to recognize it by common name alone.
Acetic acid has two carbons in the molecule. The carbonyl carbon is the carbon that is double bonded to oxygen. This double bond gives the carbonyl carbon hybridization. The other carbon has four atoms attached to it, which gives it hybridization. Remember that the hybridization of an atom is dependent on the total number of bonded atoms, plus the number of lone electron pairs.
Example Question #1534 : Mcat Biological Sciences
Which of the following statements about the character of a bond is true?
The more "s" character a bond has, the more stable it is
The more "s" character a bond has, the longer the bond becomes
The more "s" character a bond has, the weaker it becomes
The more "s" character a bond has, the higher the energy in the bond
The more "s" character a bond has, the more stable it is
The character of a hybrid orbital is defined as the extent to which it resembles the unhybridized orbitals that created it. For example, an orbital has half "s" character and half "p" character and results from a triple bond. Remember that the more "s" character a bond has, the stronger, shorter, and more stable the bond will be. and hybridizations result in longer, weaker bonds (double bonds and single bond, respectively) due to increased contributions of "p" character.
Example Question #12 : Covalent Bonds And Hybrid Orbitals
Which of the letters above point at an atom that is hybridized?
A and C
C
B, C, and D
B and C
A and C
The answer is arrows A and C. The carbon that is pointed to by arrow C is hybridized. We see that its bond angles are at 120º (the full substituent points into the page) with a p-orbital that is involved in the pi bond of the carbonyl. Phenyl rings are also made up of carbons that are all hybridized. The nitrogen pointed to by arrow B has two electrons that are not shown, but cause the atom to be hybridized. The methyl groups denoted by arrow D are also hybridized.
We can quickly tell the hybridization of atoms by observing their double bonds and unbonded electrons. As a rule of thumb, any carbon, nitrogen, or oxygen involved in a double bond will be hybridized. Any of these atoms with no double bonds will be hybridized. Finally, nitrogens or carbons involved in triple bonds are hybridized.
Example Question #150 : Organic Chemistry, Biochemistry, And Metabolism
The concept of genomic imprinting is important in human genetics. In genomic imprinting, a certain region of DNA is only expressed by one of the two chromosomes that make up a typical homologous pair. In healthy individuals, genomic imprinting results in the silencing of genes in a certain section of the maternal chromosome 15. The DNA in this part of the chromosome is "turned off" by the addition of methyl groups to the DNA molecule. Healthy people will thus only have expression of this section of chromosome 15 from paternally-derived DNA.
The two classic human diseases that illustrate defects in genomic imprinting are Prader-Willi and Angelman Syndromes. In Prader-Willi Syndrome, the section of paternal chromosome 15 that is usually expressed is disrupted, such as by a chromosomal deletion. In Angelman Syndrome, maternal genes in this section are deleted, while paternal genes are silenced. Prader-Willi Syndrome is thus closely linked to paternal inheritance, while Angelman Syndrome is linked to maternal inheritance.
Figure 1 shows the chromosome 15 homologous pair for a child with Prader-Willi Syndrome. The parental chromosomes are also shown. The genes on the mother’s chromosomes are silenced normally, as represented by the black boxes. At once, there is also a chromosomal deletion on one of the paternal chromosomes. The result is that the child does not have any genes expressed that are normally found on that region of this chromosome.
The passage indicates that genomic imprinting relies on silencing genes by adding methyl groups to DNA sequences. Which of the following is true of methyl groups?
The central carbon is hybridized
The methyl group forms hydrogen bonds with DNA
The methyl group forms ionic bonds with DNA
The central carbon is hybridized
The central carbon is hybridized
The central carbon is hybridized
Methyl groups are substituents, and thus have a central carbon with four single bonds, one of which is a covalent attachment to the DNA substrate: . A central carbon with four bonds will be hybridized.
Note that methylation is a covalent interaction, making it much more permanent than dipole interactions between DNA and histones. This allows methylation to have long-term effects, and is one of the principles of epigenetic regulation and inheritance.
Example Question #2 : Bonds And Forces
For the compound shown below, the hybridization for carbon A is __________ and the hybridization for carbon B is __________.
Carbon A is hybridized because this carbon is only bonded to two other atoms. Carbon B is bonded to four atoms, and would therefore be hybridized.
Keep in mind that a carbon involved in a triple bond will always be hybridized, a carbon involved in a double bond will be hybridized, and a carbon involved only in single bonds will be hybridized.
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