Explanation

Gap junctions are specialized structures that allow direct communication between adjacent cells.

They are formed by connexin proteins that create channels enabling the exchange of ions, metabolites, and other small molecules between cells. 

Explanation

Sharks, along with other cartilaginous fish-like rays, retain urea in their bodies as part of their osmoregulatory strategy.

This helps them maintain osmotic balance in marine environments by making their internal body fluids isotonic with the surrounding seawater. 

Explanation

Angiotensin III is part of the renin-angiotensin system, which regulates blood pressure and fluid balance.

The juxtaglomerular cells, located in the kidneys, release renin in response to a decrease in sodium chloride concentration in the distal tubules.

Renin then catalyzes the conversion of angiotensinogen to angiotensin I,

Which is further converted to angiotensin II, and subsequently to angiotensin III.

This cascade helps in regulating blood pressure and sodium balance.

Explanation

Capacitation is the physiological process that sperm must undergo to gain the ability to penetrate and fertilize an egg.

This process occurs after the sperm enters the female reproductive tract

They involve a series of biochemical changes that enhance the sperm's motility, and its ability to bind to the egg.

It is distinct from spermiogenesis (the final stage of sperm development), the acrosome reaction (which occurs later during fertilization), and fertilization itself

Explanation

Luteinizing Hormone (LH) is a glycoprotein hormone.

It is composed of two subunits, alpha and beta, with attached carbohydrate groups.

LH plays a crucial role in regulating the reproductive system, particularly in triggering ovulation in females and stimulating testosterone production in males.

It is not a steroid, peptide, or lipid, but specifically a glycoprotein.

Explanation

Horny structures, like the horns of cattle, are made of keratin and do not shed off.

Unlike antlers, which are shed annually, horns are permanent structures that continue to grow throughout the animal's life

The other options—scales, feathers, and fur—are either shed or replaced regularly.

Explanation

Sea cucumbers belong to the class Holothuroidea within the phylum Echinodermata.

Unlike some other echinoderms like starfish (class Asteroidea), sea cucumbers do not have arms that are sharply set off from a central disc

Explanation

The class Solenogastres (also known as Solengasters) lacks a shell and possesses sclerites, which are small, hard, plate-like structures embedded in the skin.

These organisms are part of the phylum Mollusca, specifically in the class Solenogastres.

Explanation

The primary type of lipid present in the plasma membrane is phospholipids.

They form the fundamental structure of the membrane, creating a bilayer that is essential for membrane function and fluidity.

Explanation

The fluid mosaic model describes the structure of the plasma membrane of both prokaryotic and eukaryotic cells.

This model illustrates the membrane as a dynamic and flexible structure with various proteins floating in or on the fluid lipid bilayer.