Biology - MCAT Biological and Biochemical Foundations of Living Systems

Hyperventilation is a classic example of a process that can cause alkalosis, or basicity of the bloodstream. Hyperventilation can cause a net loss of CO2. Low levels of CO2 can cause respiratory alkalosis via reduction of carbonate in the blood. Lactate—also known as lactic acid—is a product of anaerobic respiration, and decreases blood pH. Increased levels of urea and creatinine indicate renal acidosis, a form of metabolic acidosis, which occurs when the kidney does not remove enough acid from the body.

The normal pH of blood is around 7.3, which means blood is normally slightly basic. A truly neutral pH is 7.0. Whenever the pH of blood is under 7.3 (not 7.0) it is considered acidosis, and so our answer is either 7.2 or 6.8.

The Bohr Effect describes hemoglobin's affinty for oxygen as a function of blood pH and carbon dioxide content. An increase in CO2 concentration will lower the blood pH, causing the hemoglobin affinity for oxygen to reduce. High temperature also causes oxygen to be released from hemoglobin, but is not related to the Bohr Effect.

Think about when you're exercising. Your blood has a reduced O2 concentration and an elevated CO2 concentration. These factors allow hemoglobin to release more oxygen in the muscles to faciliate ATP production and maintain energy levels.

Calcium plays a huge role in the regulation of muscle contraction. Without the presence of calcium, the myosin binding sites on the actin filaments are blocked by tropomyosin and muscle contraction cannot occur. Stimulation from the nervous system causes a chain reaction that releases large stores of calcium from the sarcoplasmic reticulum to regulate muscle contraction.

Sodium ions play an essential role in initiating the chain reaction that eventually leads to calcium release, but is not stored in the sarcoplasmic reticulum. Potassium plays a role in regulating membrane potential, but also is not stored in the sarcoplasmic reticulum. Protons are essential to mitochondrial function and play a crucial role in myocyte metabolism, but are not linked to the sarcoplasmic reticulum or contractile function.

Muscle contraction is regulated by blocking the myosin binding sites of actin and selectively exposing them when contraction is supposed to occur. Tropomyosin binds actin fibers and recruits troponin to perform this blocking function. Troponin will bind calcium ions, change conformation, and move tropomyosin out of the way of the myosin binding sites to allow contraction to occur. The most likely mutation described in the question is one that causes troponin to have a decreased affinity for calcium, thus never allowing the myosin binding sites to be exposed under normal physiological concentrations.

In a typical muscle cell, tropomyosin is bound to an active site on actin. This prevents muscle contraction because the myosin head cannot bind to actin active site. Muscle contraction is initiated when the sarcoplasmic reticulum releases calcium ions into the cytoplasm of the muscle cell. Calcium ions bind to and activate troponin. Activated troponin molecules subsequently remove tropomyosin from the active site on actin. This allows muscle contraction to occur because the myosin head can now bind to the active site on actin and initiate a power stroke to shorten the sarcomere.

An individual with depleted calcium ions in his sarcoplasmic reticulum will not activate troponin and, therefore, will have reduced muscle tone and strength.

The question states that muscle cramps occur because myosin heads remain attached to the active site on actin; therefore, you are looking for a molecule that is responsible for the detachment of the myosin head from actin. Recall that binding of ATP to the myosin head releases the myosin head from the actin binding site. This allows tropomyosin to re-attach to actin and causes the muscle to relax. The ATP that is bound to the myosin head dissociates into ADP and inorganic phosphate, allowing the myosin head to enter its high-energy state and prepare for another contractile stroke. Once tropomyosin is released again from the actin filament, the ADP and inorganic phosphate on the myosin head are released, the myosin head attaches to actin, and the cycle continues.

Calcium is essential for muscle contraction because it allows for the removal of tropomyosin from the actin binding sites. Depletion of calcium, however, would cause the actin sites to be blocked, preventing contraction from occurring (as opposed to the sustained contraction of a muscle cramp). Depleted sodium may result in fewer action potentials at the neuromuscular junction. This would also inhibit muscle contraction, rather than sustain it. Muscular microtears can occur during exercise, but are unrelated to muscle cramps.

Organelles are only found in eukaryotic cells which set them apart from prokaryotic cells.

Cytoplasm is found in all types of cells. Flagella is found in all cells that are mobile. Spores can be found in bacteria but also fungi. Ribosomes are found in prokaryotic cells albeit smaller in size compared to those in eukaryotic cells.

This question is asking about one of the differences between eukaryotes and prokaryotes. While both eukaryotes and prokaryotes use DNA to encode their genetic information, making "DNA" incorrect, remember that prokaryotes lack membrane-bound organelles. \This makes both "ribosomes" and "phospholipid cell membrane" incorrect, because neither of them are membrane-bound organelles. We are left with "nucleus," which is the correct answer.

One of the key differences between prokaryotes and eukaryotes is that prokaryotes have no membrane bound organelles, therefore, prokaryotes would not have mitochondria. Prokaryotes have a plasma membrane for protection, ribosomes for protein production, RNA in the form of mRNA, tRNA, and rRNA, and DNA polymerase to replicate their DNA.

The characteristic that defines a eukaryote is the presence of membrane bound organelles, such as the nucleus, mitochondria, Golgi body, and chloroplasts. Both eukaryotes and prokaryotes have ribosomes, DNA and RNA, and and electron trasnsport chain. Bacteria and plants have cell walls but not all eukaryotes do, so it is not a defining characteristic.

Prokaryotes do not have membrane-bound organelles, such as the nucleus or mitochondria. In fact, endosymbiotic theory suggests that mitochondria evolved from symbiotic prokaryotic relationships. Prokaryotes are often associated with all of the other characteristics listed.

Membrane bound organelles are specific only to eukaryotes. Circular DNA is only present in prokaryotes. Rough endoplasmic reticulum is only present in eukaryotes, as all prokaryotic ribosomes are cytoplasmic. Although prokaryotes have circular DNA, both eukaryotes and prokaryotes have double-stranded DNA.

Since the obligate anaerobe grows well, we know that both bacteria are grown in an anaerobic environment.

A facultative anaerobe can grow in an aerobic or anaerobic environment. It simply grows better when oxygen is present; therefore, we would see slower growth from a facultative anaerobe than an obligate anaerobe when there is no oxygen present. If the bacteria were aerobic, it would not grow at all in the anaerobic environment.

Prokaryotes like Neisseria do not have mitochondria, while eukaryotic human cells do. In fact, prokaryotes probably gave rise to the first mitochondria via the theory of endosymbiosis.

Bright pink cells under Gram stain are characteristic of Gram negative status. This is associated with an outer lipid membrane and reduced peptidoglycan. The reduction in peptidoglycan is what leads to a failure to retain the Gram stain, which would have colored the cells purple.

Round bacterial cells are considered cocci. In this case, with two cells, it would be classified specifically as a diploccocus.

Prokaryotes like Neisseria do not have mebrane bound organelles, including nuclei. Their DNA is stored in the cytosol without the benefit of extensive sequestration.