Test: Biology - 5 - MCAT MCQ

Chyme is the semi-fluid mass of partly digested food that is formed in the stomach during the process of digestion. When food enters the stomach, it is mixed with gastric juices, which contain enzymes and hydrochloric acid. The muscular contractions of the stomach wall, known as peristalsis, help to mix the food with gastric juices and break it down into smaller particles.

As the food particles mix and churn with the gastric juices, they gradually form a semi-fluid mass called chyme. The chyme is then slowly released from the stomach into the small intestine for further digestion and absorption of nutrients.

It is important to note that while some digestion occurs in the stomach, the majority of nutrient absorption takes place in the small intestine. The stomach primarily functions to store food, initiate protein digestion through the action of enzymes like pepsin, and mix the food with gastric juices to form chyme.

Therefore, the formation of chyme occurs in the stomach (option E) as a result of the mechanical and enzymatic breakdown of food during the process of digestion.

Muscles are composed of specialized contractile units called sarcomeres. Sarcomeres are the functional units of muscle fibers and are responsible for muscle contraction. They are made up of chains of protein filaments, including actin and myosin, which slide past each other to generate muscle contraction.

The chains of sarcomeres are organized within muscle fibers into long cylindrical structures called myofibrils. Myofibrils are thread-like structures that extend the entire length of a muscle fiber and are responsible for the striated appearance of skeletal and cardiac muscles. They are composed of repeating units of sarcomeres, which are aligned end to end.

The sarcolemma is the plasma membrane of a muscle cell, also known as a muscle fiber. It surrounds and encloses the sarcoplasm, which is the cytoplasm of the muscle cell. However, sarcolemma is not involved in the organization of sarcomeres within the muscle.

Sarcoplasm is the cytoplasm of muscle cells and contains various cellular components including myofibrils. While myofibrils are located within the sarcoplasm, they specifically refer to the chains of sarcomeres organized in a sequential manner.

Myoblasts are the embryonic cells that fuse together during development to form mature muscle fibers. They are involved in the initial formation of muscle tissue but do not directly organize the chains of sarcomeres.

Fascia is a connective tissue that surrounds and separates muscles and other organs. It provides support and structure but is not directly involved in the organization of sarcomeres.

Therefore, the correct answer is D. Myofibrils, as they are the structures that organize the chains of sarcomeres within muscle fibers.

Adipose tissue, also known as body fat or fatty tissue, is a specialized type of connective tissue primarily responsible for storing and releasing energy in the form of fat. It consists of adipocytes, which are specialized cells that contain large lipid droplets.

The main role of adipose tissue is to store excess energy in the form of triglycerides, which are composed of fatty acids and glycerol. When the body needs energy, adipose tissue releases stored fat by breaking down triglycerides into fatty acids and glycerol through a process called lipolysis. These fatty acids are then released into the bloodstream and used as a source of energy by various tissues and organs in the body.

Adipose tissue also serves other functions in the body, including insulation and protection. It helps to insulate the body by providing a layer of fat that acts as a thermal barrier, helping to regulate body temperature. Additionally, adipose tissue cushions and protects organs by providing a layer of padding around them, helping to absorb shock and minimize physical damage.

Implantation is a critical step in the process of human reproduction. It occurs after fertilization, when the developing embryo, in the form of a blastocyst, attaches itself to the lining of the uterus, known as the endometrium.

After fertilization, the zygote undergoes multiple cell divisions and forms a hollow ball of cells called a blastocyst. The blastocyst then travels through the fallopian tube and reaches the uterus. During implantation, the blastocyst attaches itself to the thickened and enriched endometrial lining of the uterus.

Implantation involves several complex processes. The blastocyst first makes contact with the endometrium and then burrows into it, eventually becoming embedded in the endometrial tissue. This establishes a connection between the developing embryo and the maternal blood supply, which is crucial for the exchange of nutrients, oxygen, and waste products.

Implantation typically occurs around 6 to 7 days after fertilization, and it marks the beginning of pregnancy. Following successful implantation, the endometrium undergoes further changes to support the growth and development of the embryo, leading to the formation of the placenta.

The other options listed are not accurate descriptions of implantation. B. The fusion of sperm nuclei with egg nuclei is known as fertilization, which occurs prior to implantation. C. Loss of the lining of the endometrium is associated with menstruation, not implantation. D. The gastrula stage refers to a later developmental stage, after implantation has already taken place.

Therefore, the correct answer is A. The blastocyst becomes embedded in the endometrium. Implantation is the crucial process in which the developing embryo attaches itself to the uterine lining to establish pregnancy.

The acceptor stem is a specific region found in tRNA molecules. tRNA molecules are involved in protein synthesis and play a crucial role in bringing amino acids to the ribosomes during translation.

The acceptor stem is located at one end of the tRNA molecule and contains a specific sequence of nucleotides. This sequence is important for recognizing and binding to a specific amino acid. The acceptor stem typically consists of a short double-stranded region followed by a single-stranded loop.

During protein synthesis, the acceptor stem of tRNA pairs with the corresponding codon on mRNA through base pairing. This pairing ensures that the correct amino acid is added to the growing polypeptide chain.

The other options listed are not associated with the acceptor stem. B. rRNA (ribosomal RNA) is a component of ribosomes and is involved in protein synthesis but does not have an acceptor stem. C. siRNA (small interfering RNA) is involved in gene regulation and silencing of gene expression. D. mRNA (messenger RNA) carries the genetic information from DNA to the ribosomes for protein synthesis but does not possess an acceptor stem. E. hnRNA (heterogeneous nuclear RNA) is an intermediate molecule involved in the processing of mRNA but does not have an acceptor stem.

Therefore, the correct answer is A. tRNA. The acceptor stem is a characteristic feature of tRNA molecules and is involved in the recognition and binding of specific amino acids during protein synthesis.

Gastrulation is the stage in development of an embryo, in which three distinct germ layers (ectoderm, endoderm and mesoderm) are formed. It takes place next to cleavage, in which blastula formation occurs from zygote. After gastrulation is neurulation which is the typical organogenesis.

Vitellogenin is a yolk protein that is produced in the liver of female amphibians. It is an essential component of egg development and provides the necessary nutrients for the growing embryo. Vitellogenin is synthesized and secreted by the liver cells (hepatocytes) in response to hormonal cues during the reproductive cycle of female amphibians.

The synthesis of vitellogenin is triggered by hormones such as estrogen and progesterone, which are released during the breeding season. These hormones stimulate the liver to produce and secrete vitellogenin into the bloodstream. From the bloodstream, vitellogenin is taken up by the developing oocytes (eggs) in the ovary, where it is processed and incorporated into the yolk.

So, the correct answer is C. Liver. The liver is the site of vitellogenin synthesis in amphibians, and it plays a crucial role in providing the necessary nutrients for egg development.

In plant reproductive structures, the stamen is the male reproductive organ. It consists of the filament and the anther. The anther produces and releases pollen, which contains the male gametes (sperm cells). The staminate flowers specifically refer to flowers that bear only stamens and do not have functional pistils (female reproductive organs).

Unisexual flowers are those that contain either male or female reproductive structures but not both. In the case of staminate flowers, they are exclusively male and only have stamens. These flowers do not possess pistils, which are the female reproductive organs responsible for producing and receiving the pollen for fertilization.

Therefore, the correct answer is A. Unisexual male flower bearing stamens. The term 'staminate' is used to describe flowers that are specifically male and possess stamens but lack functional pistils.

Parturition refers to the process of giving birth to a baby or delivering a fetus. It involves a series of physiological and mechanical events that occur in the mother's body to expel the fetus from the uterus and into the outside world. Parturition is a complex process involving uterine contractions, cervical dilation, and the expulsion of the fetus through the birth canal.

Implantation (option A) is the process by which the fertilized egg (zygote) attaches to the wall of the uterus, typically occurring about 6-7 days after fertilization.

Lactation (option B) is the production and secretion of milk from the mammary glands in the breasts after childbirth, enabling the nourishment of the newborn.

Fertilization (option D) is the process of fusion between a sperm and an egg to form a zygote, which occurs in the fallopian tubes.

Therefore, the correct answer is C. Parturition, which specifically refers to the process of delivering the fetus during childbirth.

Juxtacrine signaling is a form of cell signaling in which cells directly interact with each other through physical contact. In this type of signaling, molecules present on the surface of one cell bind to receptors on the surface of an adjacent cell, transmitting signals and triggering specific cellular responses.

Endocrine signaling (option A) involves the release of signaling molecules (hormones) into the bloodstream, which then travel to target cells or tissues located at a distance from the signaling cell.

Paracrine signaling (option B) occurs when signaling molecules are released by a cell and act on nearby cells in the immediate vicinity of the signaling cell.

Intracrine signaling (option D) involves the production of signaling molecules that act within the same cell, without being released into the extracellular space.

Therefore, the correct answer is C. Juxtacrine signaling, which describes the direct contact and interaction between cells for communication.

Leucoplasts are a type of plastid found in plant cells. They are responsible for the synthesis and storage of various substances such as starch, oils, and pigments. Leucoplasts are involved in functions specific to plant cells, such as photosynthesis and storage of nutrients.

On the other hand, animal cells do not contain leucoplasts. Animal cells lack plastids, which are specialized organelles found in plant cells. Animal cells do have other organelles such as the nucleus (option A), Golgi apparatus (option B), lysosomes (option C), and mitochondria (option E).

Therefore, the correct answer is D. Leucoplast, as it is a specific organelle found in plant cells but not in animal cells.

Atrial systole is the shortest phase of cardiac cycle. Atrial systole gets completed in 0.1 seconds. Duration of atrial diastole is 0.7 seconds. Duration of ventricular systole and diastole is 0.3 seconds and 0.5 seconds respectively.

Entropy is a measure of the disorder or randomness in a closed system. It is a thermodynamic quantity that describes the distribution of energy within a system or the number of possible arrangements of a system's particles.

In a closed system, the entropy tends to increase over time, leading to an increase in disorder or randomness. This concept is related to the second law of thermodynamics, which states that the entropy of an isolated system will always increase or remain constant but will never decrease.

Enthalpy (option B) is a measure of the total heat content of a system and is not directly related to disorder or randomness. Potential energy (option C) and kinetic energy (option D) are forms of energy associated with the position or motion of objects and are not specific measures of disorder.

Therefore, the correct answer is A. Entropy.

The state of a neuron that inhibits the generation of an action potential is called hyperpolarized. Hyperpolarization occurs when the membrane potential of the neuron becomes more negative than its resting potential.

During hyperpolarization, the neuron's cell membrane becomes more permeable to potassium ions (K⁺), which leads to an increased efflux of K⁺ ions from the cell. This efflux of positive ions makes the inside of the neuron more negative, moving it further away from the threshold required to initiate an action potential.

Depolarization (option A) refers to a change in the membrane potential towards a more positive value, typically caused by the influx of positive ions such as sodium (Na⁺). Repolarization (option B) is the process of restoring the membrane potential back to its resting state after depolarization. Resting potential (option D) is the stable membrane potential of a neuron when it is not actively transmitting signals.

Therefore, the correct answer is C. Hyperpolarized.

Cholera toxin is produced by the bacterium Vibrio cholerae and is responsible for the characteristic symptoms of cholera, a severe diarrheal disease. Cholera toxin primarily targets and affects G proteins.

G proteins are a family of proteins that play a crucial role in signal transduction pathways within cells. They act as molecular switches, relaying signals from cell surface receptors to intracellular signaling pathways. Cholera toxin specifically targets and modifies the alpha subunit of the G protein complex, preventing it from turning off adenylyl cyclase, an enzyme involved in the production of cyclic AMP (cAMP).

By continuously activating adenylyl cyclase and increasing cAMP levels within cells, cholera toxin disrupts the normal ion and fluid balance in the intestines, leading to increased secretion of chloride ions and water into the intestinal lumen, causing the characteristic watery diarrhea seen in cholera.

Therefore, the correct answer is A. G proteins.