W6 Self-Assessment Questions

In the context of nutrition, what does "essential" mean?

"Essential," when used to describe dietary amino acids, minerals, or vitamins, does not mean that other nutrients are less important or useful. Instead, "essential" in this context means that these nutrients must be obtained in the diet; the organism cannot synthesize them on its own. In other words, it is "essential" that these nutrients are consumed.

What is the role of ADH in the regulation of urine concentration?

ADH increases urine concentration by increasing the permeability of the collecting duct to water. In the presence of ADH, aquaporins are inserted into the membrane of the cells lining the collecting duct; they increase the reabsorption of water and concentrate the urine. In the absence of ADH, the collecting duct is impermeable to water, so water remains in the filtrate and the urine is dilute.

Why do mammals convert ammonia to urea rather than simply excreting it, as fishes do?

Ammonia excreted by fishes is rapidly diluted to nontoxic levels in the surrounding water. Mammals convert ammonia to urea, which is much less toxic than ammonia; therefore urea can be concentrated and stored before being eliminated.

How do animals gain and lose water and electrolytes?

Animals gain water and electrolytes through drinking and the foods they eat. Freshwater animals also gain water through their gills. Additionally, water is generated by cellular respiration. Water and electrolytes are lost through urine, feces, evaporation from the lungs, and sweating. Most marine animals lose water through their gills as well. Specialized glands in some animals can also aid in removal of excess electrolytes.

What are the similarities and differences between the foregut and hindgut fermentation?

Both foregut and hingut fermenters rely n bacteria in the gut to help break down plant materials into nutrients that can be absorbed. Foregut fermenters maximize nutrient absorption by processing the plant material extensively within several chambers of the stomach (where the bacteria reside) beside it reaches the small intestine. By contrast, hindgut fermenters pass the ingested material through the small intestine before it is fermented by the bacteria residing in the colon and cecum, reducting the ability of the host animals to extract nutrients from the plant material they ingest.

Some animals, such as rabbits, whose digestive systems include hindgut fermentation, re-ingest the products of digestion. What function do you think this serves?

By eating the products of the cecum, these animals can pass the partially digested food through the small intestine a second time, where additional nutrients are absorbed.

Muscle, fat, and glycogen are all reservoirs of energy. In which order are they used during a prolonged fast?

Glycogen stores are used first, then fat, and finally muscle protein.

How do the kidneys help to regulate blood volume and blood pressure?

If blood pressure drops, cells within the juxtaglomerular apparatus release renin into the blood. The release of renin, in turn, leads to activation of angiotensin II, which causes blood vessels to constrict, increasing blood pressure. In addition, angiotensin II causes release of aldosterone, which triggers increased reabsorption of water and electrolytes in the distal convoluted tubule and collecting duct, leading to an increase blood volume and blood pressure.

Name two animals that are osmoconformers and two animals that are osmoregulators. What is the difference between the two types of animals?

Osmoconformers are animals that regulate their internal osmotic pressure by maintaining their internal solute concentration at a level similar to that of their environment. Osmoconformers include animals such as sharks and marine invertebrates such as sea stars. Some marine vertebrates are also osmoconformers, including hagfish, lampreys, rays, and coelacanths. All freshwater animals, including fishes and amphibians, and all terrestrial animals, including humans, are osmoregulators: they actively regulate their internal osmotic pressure, expending considerable energy to maintain an internal solute concentration that is different from that of their environment. The largest group of marine vertebrates, the teleosts, are also osmoregulatorss.

A solution of water and dissolved ions is separated by a selectively permeable membrane that allows the passage of water but not small ions. The concentration of sodium is higher on one side of the membrane than on the other side. However, there is no net movement of water across the membrane. How is this possible?

Osmosis depends on the difference in total solute concentration on the two sides of a membrane. There is no net movement of water molecules across a selectively permeable membrane with a high concentration of a particular solute on one side of a membrane, if this concentration is matched by a similar concentration of some other solute on the other side of the membrane. For example, the concentration of sodium ions may be high on one side of a selectively permeable membrane and low on the other side, whereas the distribution of potassium ions may be just on reverse. In this case, the total solute concentration is the same on the two sides of the membrane, and the result is no net water movement.

The rocks and sediments in the gizzards of birds break apart food mechanically. Which structures in vertebrates have a similar function?

Rocks and sediments in gizzards break apart food mechanically, as do teeth in vertebrates.

The filtrate is dilute at the start and the end of the loop of Henle. What, then, is the function of the loop of Henle?

The loop creates a concentration gradient from the cortex to the medulla, which is important for subsequent water reabsorption from the collecting ducts. The loops also leave urea as the main solute.

What are the main sites of digestion of protein, carbohydrates, and fats, and which enzymes are involved in breaking down each of these?

The main site of protein digestion in the stomach, where the enzyme pepsin works to break down proteins into smaller peptides. Proteins are further digested by trypsin in the small intestine. Digestion of carbohydrates begins in the mouth when food mixes with salivary amylase, which breaks down sugars and starches. Carbohydrates are further digested in the small intestine by pancreatic amylase. Fats are digested in the mouth, stomach, and small intestine by lingual, gastric, and pancreatic lipases, respectively.

What are the three steps in which animals secrete wastes?

The three steps of waste excretion are filtration, reabsorption, and secretion. Filtration is the passing of solutes, wastes, and water from the blood into an extracellular space, creating a filtrate. Reabsorption returns water and necessary solutes from the filtrate to the blood. Secretion is the active transport of excess solutes and toxins from the blood into the filtrate.

What are the three forms of nitrogenous waste? How is each an adaptation for the environment in which an animal lives?

Three forms of nitrogenous waste are ammonia, urea, and uric acid. Ammonia is produced by most aquatic animals, including fishes. Although ammonia is highly toxic, fishes can excrete waste in this form because it can be easily dilutes in their aquatic environment; they do no have to expend additional energy converting this waste into a less toxic form. Terrestrial animals convert ammonia to less toxic forms of nitrogenous waste. Mammals, many amphibians, sharks, and some bony fishes excrete nitrogen in the form of urea, which can be stored in a concentrated form and excreted with water in the urine. Urea is less toxic than ammonia, but requires energy to produce it and water to eliminate it. In some animals that live in hot, dry climates, ammonia is converted into uric acid, which is much less toxic and does not dissolve in water, so waste can be excreted with limited water loss. Birds, insects, reptiles, and land snails excrete waste in the form of uric acid.

What happens when a cell is placed in a hypotonic solution (one with a solute concentration lower than that of the cell)? What happens when a cell is placed in a hypertonic solution (one with a solute concentration higher than that of a cell)?

When a cell is placed in a hypotonic solution, water moves into the cell by osmosis and the cell swells or bursts. When a cell is placed in a hypertonic solution, water leaves the cell by osmosis and the cell shrinks.