Breakdown & Excretion of N-Compounds

amino acids TCA cycleNH3

deamination

pyrimidines alanine + NH3 + CO2

(cytosine, thymidine, uracil)

purines uric acid allantoin/allantoic acid(adenine, guanine)

urea NH3 + CO2

NH3 (NH4+) - ammonotelic

• diffuses rapidly & used for ion exchange• toxic• mainly in aquatic spp.

Urea - ureotelic - from purine degradation orsynthesized via Krebs ornithine cycle.• less toxic & used as organic osmolyte• highly soluble - requires water for excretion

Uric Acid - uricotelic• water conservative - sparingly soluble• crystallizes - excreted as paste

Excretory Products

Ammonia-Excreting Animals (Ammonotelic)

• Toxic – only small quantities stored in body

• Requires considerable water to excrete

• Gills typical site for excretion in aquatic animals

• Isopods, some land snails, crabs---can excrete NH3 through volatization into air

Acid-Base Balance

Intracellular (and therefore extracellular) pHmust be regulated within narrow limits.

In humans, arterial blood pH at core temp.= 7.4

Convulsions and death result from pH > 7.7alkalosis - inc. in blood pH

Coma and death result from pH < 6.8acidosis - dec. in blood pH

Deleterious effects of changes in pHresults from titration of charges on

proteins:

_+

_

_

_

_ __

+

+

+

H+ H+

H+

H+ H+

H+

Alters the 2°, 3° and 4° structures of protein

Urea-Excreting Animals (Ureotelic)

• More soluble, less toxic than NH3

• Better deal…get rid of 2 nitrogen molecules

O

H2N—C—NH2

• Formed by ornithine-urea cycle

Uric Acid-Secreting Animals (Uricotelic)

• Major nitrogenous end-product in most terrestrial arthropods, reptiles, birds

• Advantage – 4 nitrogen molecules, but requires energy to synthesize

• Advantage – does not contribute plasma osmotic pressure

Sample Questions

The model presented in lecture to explain active transport based on the Na+/K+ pump:a) included conformation changes in membrane proteins that serve as transportersb) suggested that the two ions are exchanged in a 1:1 ratioc) did not require the participation of ATPd) is based on the premise that once Na+ is moved across a membrane against its gradient, K+ follows passively

What is the equilibrium potential for K+ if its concentration is 152 mM in compartment B (consider thisto be “inside”) and 143 mM in compartment A (outside):

a) 1.485 mv c) -1.53 mvb) 1.53 mv d) -2.65 mv

Cell volume regulation in animals:a) is facilitated by the presence of a cell wall that provides a barrier against the influx of water and solutes b) is solely dependent on the active transport of substances across the cell membranec) depends heavily on hormones that alter the membrane’s permeability to waterd) involves regulating the intracellular osmotic and ionic concentration in response to changes in the extracellular fluid

More on next slide

A marine teleost fish will:a) lose salts to its environmentb) gain water from its environmentc) remove excess salt via chloride cells in the gillsd) produce urine that is more concentrated than its blood plasma

If you increase the colloidal osmotic pressure in the glomerular capillaries:a) the net filtration pressure will increaseb) the glomerular filtration rate will decreasec) plasma proteins will move from the glomerular capillaries into Bowman’s capsuled) the osmotic gradient between the cortex and medullary portions of the kidney will be eliminated

This animal has a 3-chambered heart, scales, cortical nephrons only, and a U/P ratio that cannot exceed1.0. The majority of its nitrogenous wastes is eliminated as:

a) ammonia c) uric acidb) guanine d) urea