1

Controlling the Internal

Environment II: Salt and water

balance

Keywords (reading p. 936-949)• Ammonia toxicity

• Urea

• Uric acid

• Osmoconformer

• Osmoregulator

• Passive transport

• Facilitated diffusion

• Active transport

• Osmoregulation by an

aquatic invertebrate

• Osmoregulation in marine

fish

• Osmoregulation in

freshwater fish

• Water loss on land

• Permeable and

impermeable body surfaces

• Kangaroo rate water

balance

• anhydrobiosis

The internal environment

• In most animals, the majority of cells are

bathed by internal fluids rather than the

environment

• This is advantageous since there can be

control of substrates needed for metabolism

Consider the origin of life: started

out as enzymes in the primordial

sea

Rates of reactions were

determined by the concentrations

of substrates in the environment

The first proto-organism enclosed

it’s enzymes inside a membrane

and became a cell

2

Control of substrate concentration

Products do not diffuse away

• Good because reactions will

work better and you don’t

lose the products

• Good because you can keep

out molecules that you don’t

want

• Bad because there can be

osmotic problems

• Bad because hazardous by

products can stay in the cell

Hazardous products

• Most species of

most phyla live

in the ocean

• Some live in

freshwater

• Fewer live on

land

Therefore the internal chemical

environment is controlled

• A. Avoiding buildup of toxic chemicals

– Dealing with ammonia

• B. Osmoregulation - controlling internal

solutes

A. Avoiding buildup of toxic

chemicals

Hazardous products

• A major source of hazardous products is the

production of nitrogenous wastes

• Ammonia (NH3) is a small and very toxic

molecule that is normal product of protein

and amino acid breakdown

• If you are an aquatic organism, ammonia

can readily diffuse out of the body and this

is not a problem

3

Ammonia toxicity is a problem

for terrestrial animals

• Ammonia does not readily diffuse away

into the air.

• The strategy of terrestrial animals is to

detoxify it then get rid of (excrete) it.

Ammonia can be converted to urea

which is 100,000 times less toxic

• Mammals, most amphibians, sharks, some

body fishes

The drawback of using urea

• Takes energy to synthesize

• Still need to use water to “flush it out”

Some animals cannot afford to

use water to excrete urea

• These animals use excrete uric acid instead

Uric acid

• Since uric acid is not

very soluble in water, it

can be excreted as a

paste.

• Less water is lost

• Disadvantages:

– Even more costly to

synthesize.

– Loss of carbon

4

Who uses uric acid?

• Birds, insects, many reptiles, land snails

• Related to water use, but also reproduction

• Eggs - N wastes from embryo would

accumulate around it if ammonia or urea are

used. Uric acid precipitates out.

B. Osmoregulation - controlling

internal solutes

Osmolarity

• Osmolarity = # of solutes per volume

solution

• Often expressed in moles (6.02 x 1023

atoms/molecules) per liter.

• 1 mole of glucose = 1 mole of solute

• 1 mole of NaCl = 2 moles of solute

Osmotic problems

• Humans have internal solute concentration

(osmolarity) of 300 milliosmoles per liter

(mosm/L)

• The ocean is 1000 mosm/L

1000 mosm/L300 mosm/L

What would happen if your body

surface is water permeable and you fall

into the sea

http://www.yout

ube.com/watch?

v=Ym1rvwP-

po4&feature=rel

ated

http://www.yout

ube.com/watch?

v=gWkcFU-

hHUk&feature=

related

Jellyfish in the ocean

• Keep solutes at 1000 mosm/L no water loss

or gain.

• A relatively simple solution

1000 mosm/L

1000 mosm/L

jellyfish

5

Optimal cell conditions

• Na+ is detrimental to cell function

• K+ less detrimental than Na+

Life in freshwater - hydra living

in a pond

0 mosm/L

0 mosm/L

Green hydra

• Can the same strategy of matching the

environmental osmolarity be used?

Hydra living in a pond

• If external osmolarity is very low like 0

mosm/L, hydra cannot maintain an internal

osmolarity of 0 mosm/L

• Why is this?

• Consequently freshwater animals will most

likely have a higher osmolarity than the

environment.

What happens to freshwater

organisms?

• Water from the environment is continually

entering tissues.

• The diffusion gradient favors loss of solutes

• Therefore there is a need to regulate solutes

and water

Two ways to deal with osmotic

problems

• Keep your internal concentrations the same

as the environment (osmoconformer)

• Regulate your internal concentrations

(osmoregulator)

Solute regulation

• Transport solutes across the body surface

– Note: even in the jellyfish example, there is ion

regulation. Although the internal fluids have the

same osmolarity as seawater, they do not have

the same composition

6

Ways molecules get across membranes Passive transport:

Diffusion

• Works for lipid soluble

molecules and gases

• No good for most water soluble

molecules and ions

http://www.youtube.com/watch?v=Q

qsf_UJcfBc&feature=related

Passive transport:

Facilitated diffusion

• Generally used for ions, larger

molecules, non-lipid soluble

molecules.

• Must be a gradient favoring

diffusion

http://www.youtube.com/watch?v=s0p1

ztrbXPY&feature=related

Active transport

• Works for ions and

molecules like glucose or

amino acids

• Can transport against a

gradient.

• Costs energy, usually

ATP

http://www.youtube.com/watch?v=ST

zOiRqzzL4

In this diagram, how might

sodium get across the membrane?

• A) diffusion

• B) active transport

• C) facilitated

diffusion or active

transport

Na+Na+

Na+

Na+

In this diagram, how might

sodium get across the membrane?

• A) diffusion

• B) active transport

• C) facilitated

diffusion or active

transport

Na+Na+

Na+

Na+

Na+Na+

Na+

Na+

Na+Na+

Na+

Na+

7

In this diagram, how might

sodium get across the membrane?

• A) diffusion

• B) active transport

• C) facilitated

diffusion or active

transport

Na+Na+

Na+

Na+

Na+

Na+

- - - - - - - - - - - - -

+ + + + + + + + + +

In this diagram, how might steroids

get across the membrane?

• A) diffusion

• B) active transport

• C) facilitated

diffusion

• D) all of the above

steroidsteroid

steroid

steroidsteroid

In this diagram, how might steroids

get across the membrane?

• A) diffusion

• B) active transport

• C) facilitated

diffusion

• D) all of the above

steroidsteroid

steroid

steroidsteroid

steroidsteroid

steroid

steroidsteroid

steroidsteroid

steroid

steroidsteroid

Responses of soft-bodied invertebrates

to changes in salinity

• Marine invertebrates can often be exposed

to salinity changes (e.g., tidepool drying

out, estuaries)

• If salts enter the body, pump them out using

transporters

• If salts are leaving body, take them up from

the environment using transporters

• Or just let your internal concentrations

follow changes in the environment

Dumping/pumping amino acids

• One way to respond while keeping internal

ion concentrations the same is to pump

amino acids out.

• Often used by bivalves living in estuaries

– Clams, oysters, mussels

8

aaaa

aaaa

aa

aa

aa aa

1000 mosm/L

1000 mosm/L

Estuary - high tide

aaaa

aaaa

aa

aa

aa aa

500 mosm/L

1000 mosm/L

Estuary - low tide

aaaa

aaaa

aa

aa

aa aa

500 mosm/L

500 mosm/L

Estuary - low tideAdvantages of amino acid

osmoregulation

• Changing amino acid concentrations is less

disruptive on internal processes (enzyme

function).

• Costs: pumping amino acids (can involve

ATP), loss of amino acids (carbon and

nitrogen)

Osmoregulation in other aquatic

organisms

• Example: fishes maintain internal

concentration of solutes

• Body volume does not change

• Involves energetic cost of active transport

• In bony fishes this can be 5% of metabolic

rate

Marine fishes

9

Marine fishes• Problem: lower internal osmolarity than

seawater

• Water will leave body, sea salts will go in

• Solution: Fish drink large amounts of

seawater, then transport out ions (Na+, Cl-)

at their gill surface or in urine (Ca++, Mg++,

SO4--).

Freshwater fishes

Freshwater fishes

• The opposite situation: tendency to lose

solutes and gain water

• Solutions: take up salts in food and by

active transport across gills

• Eliminate water via copious dilute urine

production

Water balance on land

• Unlike aquatic animals, terrestrial animals

don’t lose or gain water by osmosis

• However, water loss or solute gain can be a

major problem

• Cells are maintained at around 300 mosm/L

• Humans die if they lose 12% of their body

water

Why not just prohibit water loss?

• Impermeable surfaces: waxy exoskeleton

(insects), shells of land snails, thick skin

(vertebrates).

• Not all surfaces can be impermeable

because gas exchange must also occur.

• Evaporation across respiratory surfaces is

only one of the two main causes of water

loss

– The other is urine production

Drinking

• Replenishes water that is lost

• Water can also be gained by moist foods

• What if there is no water to drink?

10

Desert kangaroo ratDesert kangaroo rat does not

drink

• Don’t lose much water

– Special nasal passages

– Urine doesn’t contain much water

• Recovers almost all of the water that results

from cellular respiration

• Note

comparison is

relative not

absolute

• Greater

proportion of

water intake of

K rat is from

metabolism

• Low

proportion of

K rat water

loss is in urine

Anhydrobiosis: Tardigrades

(water bears)

• Can lose 95% of their body water