In competitive inhibition, an inhibitor molecule resembling the substrate binds to the active site and physically blocks the substrate from attaching.

It is important to understand that a single signaling molecule, initiating the same signal transduction pathway, can have different effects in different cell types. Also, there are only three types of membrane receptors (G protein-coupled receptors, ligand-gated ion receptors, and protein kinase receptors), but there are more than 700 known signaling molecules for G protein-coupled receptors alone. Each signaling molecule binds to a specific receptor, initiating a signal transduction pathway that is specific to the signaling molecule and cell type. The PKA produced by the epinephrine signal transduction pathway, then, is going to be different than that of the PKA produced by the adenosine pathway. Since epinephrine is associated with promoting the fight-or-flight response, PKA triggers the release of glucose from glycogen. The release of glucose provides a source of energy for fight-or-flight activities. Muscle fatigue, drowsiness, and reducing blood flow to the heart (as a result of vasoconstriction) do not promote the fight-or-fight response.

Caffeine is an adenosine signaling antagonist, blocking the effect of adenosine. When caffeine binds to the adenosine receptor, it blocks the G protein-coupled receptor from initiating the adenosine signaling pathway. As a result, PKA is not activated and drowsiness does not occur.

For the animal kingdom, the simplest invertebrates are the sponges, which have no body symmetry. As animals become more advanced, invertebrates move from having no symmetry to having radial symmetry. The most advanced invertebrates have bilateral symmetry. Nematoda are round worms and are more advanced than the cnidarian like the jellyfish; therefore nematodes should have bilateral symmetry, not radial symmetry. Likewise, the chordate or vertebrate are all characterized as having bilateral symmetry. The only incorrect grouping is [E].

When the adrenal cortex secretes glucocorticoids, many actions are initiated. Cortisol, for example, is released during times of stress. Gluconeogenesis (the process of creating glucose from non-carbohydrates) and lipolysis (the breakdown of fats) are both activated to generate more energy for the both during times of stress. Immunologic response, inflammatory response, glucose uptake by muscle and fat tissues, and protein construction are all restricted as these are energetically expensive and the body needs to place its energy reserves into other areas. Sodium reabsorption by the kidneys is regulated by secretion of mineralocorticoids of the adrenal cortex, such as aldosterone.

The microtubules are part of the cytoskeleton and are comprised of a specialized protein called tubulin. Microtubules are integral in cell division and cell movement. Cilia are a type of cell movement and are made up of microtubules.

The golgi body assists in the processing of proteins by modifying and sorting the protein. In addition, the golgi body is responsible for tagging and dispersing proteins that are destined to leave the cell. These proteins are packaged in transport vesicles that are then taken to the cell membrane, where the vesicle fuses with the membrane and ejects the protein outside of the cell. The ribosome and rough endoplasmic reticulum both help with protein synthesis but are not responsible for vesicle production. The mitochondria is the site of ATP production during cellular respiration and does not directly form vesicles. The smooth endoplasmic reticulum is essential in lipid production and detoxifying the cell.

After releasing trypsinogen from the pancreas, enteropeptidase cleaves the inactive protein at specific sites. As a consequence of this action trypsin is generated. This enzyme has ability to generate trypsinogen on its own and to release an active enzyme.

Histones are not responsible for methylating DNA(A) or helping topoisomerase enzymes bind. Histones interact with DNA through ELECTROSTATIC interactions, and an octet of histones plus a length of DNA forms a NUCLEOSOME and causes the DNA to form its tertiary structure and become supercoiled.

Cohesion is the attraction between like substances. Because of the hydrogen bonding between them, water molecules stick to other water molecules, holding them together to form a vertical column in plant xylem vessels. The water molecules in the column also demonstrate adhesion, the attraction between unlike substances, when they form weak hydrogen bonds with the cellulose and other molecules in the walls of the plant vessels. This mechanism, however, helps the water column oppose the downward pull of gravity.

Melatonin is a naturally produced hormone that helps to regulate the sleep-wake cycle. This hormone is produced by the pineal gland, a small cone-shaped gland located in the central region of the brain.

Neurons are at rest with a negative membrane potential relative to the outside of the cell. During an action potential, positively charged sodium ions rush into the cell making the membrane potential less negative (depolarization). Soon after, potassium (another positively charged ion) exits the cell and restores polarization. The presynaptic terminal reacts to the action potential by opening calcium channels. This calcium is required for the cellular machinery responsible for neurotransmitter release into the synapse.

Conjugation is the sexual reproduction of bacteria.

The term obligate means that this is a required condition that has to be met to ensure the viability of the organism. An obligate aerobe has to have oxygen present to be able to survive, like humans. Likewise, an organism that cannot live in the presence of oxygen would be an obligate anaerobe. Facultative anaerobes can survive in the presence of oxygen, but prefer aerobic environments. Permissive anaerobe is a term that does not exist in biology. Aerotolerant organisms do not require oxygen, but they are not poisoned by it either.

New genes are introduced into the bacterium through viral transduction.

A phospholipid with an unsaturated fatty acid tail (one with one or more double covalent bonds) creates a kink in the tail that spreads adjacent phospholipids apart, making the membrane more fluid. In contrast, phospholipids with saturated fatty acids pack more closely together and make the membrane more viscous. Answer choice A is incorrect because an increase in cytosol solute concentration will help the cytosol tolerate chilling, but it will not keep the plasma membrane fluid. Answer choice C is incorrect because glucose and other sugar molecules do not occur in the plasma membrane except as attached to phospholipids (glycolipids) or to proteins (glycoproteins). Furthermore, replacing phospholipids entirely with glucose molecules would destroy the integrity of the plasma membrane. Answer choice D is incorrect because the two phospholipid layers of the plasma membrane are asymmetric, each side specific to the direction it faces. Exchanging the phospholipids from one side to the other would damage that integrity.

The correct answer is B. The gap between the axon terminal and dendrite of the next neuron is called the synaptic cleft. The lipid-based coating that covers the neurons is the myelin sheath, and this sheath covers segments along the axon called internodes and the gaps between these internodes are called the Nodes of Ranvier.

This is a great example of a comprehension DAT biology question. All of the answer choices are plausible, but only one stands out like a sore thumb. A nonsense mutation leads to a premature stop codon in the mRNA, halting translation and the production of the protein, which would lead to a lack of activity. However, this is the least likely situation, as mutations are already very rare. It is much more likely that one of the other scenarios occurred. Leaving the introns (the non-coding regions of the DNA) in the mRNA would destroy the intended enzyme by not allowing it to properly fold, leading to inactivation. Introns are normally removed as mRNA is processed through the nucleus, directly extracting “pre-processed” mRNA would likely leave the introns in place. This is why scientists often use cDNA, or complementary DNA, which is the intended gene sans introns and can be directly translated. Most enzymes also undergo post-translational modifications to become active, such as phosphorylation or get cleaved at a specific environment to control their activity. This is especially useful in the production of pepsin in the stomach; producing an inactive version of pepsin (pepsinogen) allows us to control the activity of the enzyme so it does not end up eating the cell that produced it! Molecular chaperones are often present in cellular translation to help an enzyme fold properly.

In cellular respiration, glucose is broken down and energy is extracted and stored in NADH, FADH₂, and ATP. During the oxidative phosphorylation stage of respiration, electrons from NADH (and FADH₂) enter the electron transport chain (ETC) in the mitochondrion inner membrane. As this occurs, protons (H⁺) move across the membrane from the matrix into the intermembrane space, generating a proton gradient. At the end of the ETC, the electrons combine with a proton and oxygen to form H₂O. In contrast, the electrons in photosynthesis originate in H₂O from inside the thylakoids of chloroplasts and enter an ETC chain in the thylakoid membrane. As the electrons pass along the ETC, protons move into the thylakoids from the stroma, generating a proton gradient. At the end of the ETC, the electrons combine with NADP⁺ and a proton (H⁺) to form NADPH. Ultimately, the NADPH is used to generate glucose. For both respiration and photosynthesis, electrons pass through an ETC and generate a proton gradient. But for the origins and endpoints of the electrons involved, the two processes are the reverse of one another.

The enzymes in the lysosome are enzymatically active because the pH inside the lysosome is 5. The pH is maintained by pumping H⁺ ions (protons) into the lysosome by active transport proton pumps into the lysosome membrane.

Even though an amino acid doesn't have direct contact with the substrate, it still plays a role in the overall shape of the enzyme.

Both of the brothers are phenotypically identical in regards to their hair texture. Genotypically, the two are different, as one is homozygous dominant and the other is heterozygous for hair texture. Despite the genotype differences the brothers are phenotypically similar with their hair texture because the dominant allele is the one being expressed. In both the case of Hh and HH, the presence of the dominant allele results in straight hair. This is due to complete dominance. In incomplete dominance, the heterozygote phenotypically is a hybrid between the two parents (i.e. a red and white flower make a pink flower). Whereas with codominance the heterozygote is phenotypically like both parents, i.e. red and white flower makes white plant with red spots. Law of independent assortment and law of segregation refer to Mendelian genetics, but are not the explanations for the phenotypic similarity between the two genotypes.