Which of the following describes the general function of cytokines in the immune system?

Correct Answer: A

Rationale: The correct answer is A: 'They communicate between cells to instigate an immune response.' Cytokines are signaling molecules that act as messengers between cells in the immune system, playing a vital role in coordinating and regulating immune responses. They communicate with various immune cells to initiate appropriate responses against pathogens. Choice B is incorrect because cytokines do not inhibit blood clotting but rather regulate immune responses. Choice C is incorrect as cytokines do not bind to specific pathogens to increase their mass; instead, they regulate the immune response. Choice D is also incorrect because cytokines do not transport pathogens trapped in mucus to the stomach for destruction; they primarily function as signaling molecules within the immune system.

Correct Answer: A

Rationale: The correct answer is the smooth endoplasmic reticulum. This organelle lacks ribosomes on its surface, unlike the rough endoplasmic reticulum, which is involved in protein synthesis. The enzymes present in the smooth endoplasmic reticulum are responsible for synthesizing lipids such as phospholipids and steroids. Choice B, ribosomes, are responsible for protein synthesis and not lipid synthesis. Choice C, rough endoplasmic reticulum, is involved in protein synthesis due to the presence of ribosomes. Choice D, Golgi apparatus, is responsible for modifying, sorting, and packaging proteins for secretion, not lipid synthesis.

Correct Answer: A

Rationale: Progesterone plays a key role in the regulation and preparation of the endometrial lining of the uterus for potential pregnancy. It also inhibits contractions of the uterus as the ovum is released and stimulates milk production in the breasts. However, the expression of secondary sexual characteristics, such as enlarged breasts, is primarily influenced by estrogen, not progesterone. Therefore, choice A is the correct answer as it is not a function directly attributed to progesterone in the female reproductive system.

Correct Answer: C

Rationale: The hypodermis, also known as the subcutaneous tissue, is the deepest layer of the skin. This layer contains adipocytes (fat cells) that act as an energy reserve by storing excess energy in the form of fat. When energy is needed, these stored fats can be released into circulation to be used by the body's cells. The epidermis is the outermost layer of the skin, primarily responsible for providing a protective barrier. The dermis lies between the epidermis and hypodermis and contains blood vessels, nerves, and structures like hair follicles and sweat glands. The stratum basale is the deepest layer of the epidermis, responsible for cell renewal and regeneration.

Correct Answer: C

Rationale: Neurotransmitters send chemical messages across the gap between one neuron and another through a structure called the synapse. The synapse is a specialized junction where the axon of one neuron meets the dendrite or cell body of another neuron. Neurotransmitters are released from the axon terminal of the presynaptic neuron and travel across the synaptic cleft to bind to receptors on the postsynaptic neuron, transmitting the signal between the two neurons. Choice A, Schwann cell, is incorrect as Schwann cells are responsible for producing myelin sheath around axons in the peripheral nervous system, not for transmitting neurotransmitters between neurons. Choice B, ganglion, is incorrect as ganglia are clusters of nerve cell bodies outside the central nervous system and do not directly participate in the transmission of chemical messages between neurons. Choice D, axon, is incorrect as the axon is a long, slender projection of a neuron that conducts electrical impulses away from the cell body and towards the axon terminals, where neurotransmitters are released into the synapse, but it is not the structure across which neurotransmitters travel to communicate between neurons.