Excretion

AP Biology

Unit 6

Osmolarity

• Osmolarity = moles of osmotically active

particles per liter of solvent

• 1 M Glucose = 1 Osmolar solution

• 1 M NaCl = 2 Osmolar solution

– 2 osmotically active particles because NaCl

dissociates to become Na+ and Cl- in water

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Question…

• What would happen if your body did not

maintain proper osmolarity?

• You would either have an excess of water

(bloated) or too little water (dehydrated)

• Cells wouldn’t have the correct balance of

solutes and H2O � won’t function properly

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Osmoconformers

• Animals whose internal osmolarity changes

in relation to their external environment

– Equilibrate with the environment

– There are limits to this– too high or too low

will cause death

• Marine invertebrates

• Ex. Brine Shrimp (Artemia)

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Osmoregulators

• Animals who can maintain their internal

osmolarity at a particular level regardless of

the external environment

• Ex. Fish, humans, lots of other animals ☺

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Question…

• How does the environment one lives in

affect how osmoregulation takes place?

– Depending on the environment one lives in,

osmoregulation can be very different

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Saltwater Fish

• Challenge: Prevent too much water from

leaving the body (to go into the outside

environment)

– Higher osmolarity outside compared to inside

of body

• Solution…

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Freshwater Fish

• Challenge: Prevent too much water from

coming into the body from the outside

– Higher osmolarity inside body compared to

outside

• Solution…

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Birds: Salt Glands

• Many birds who live by the

sea may take in sea water

along with the food they eat

• They get rid of the extra salt

in their bodies by excreting

it through nasal salt glands

� sneeze or shake off the

salt droplets

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Nitrogen Waste

• Nitrogenous wastes are a type

of metabolic waste that must be

removed from the body.

• Carbohydrates broken down

into CO2 and H2O

• Fats broken down into CO2 and

H2O

• Proteins and Nucleic Acids

broken down into NH2 groups

(urea, ammonia, uric acid)

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Ammonia

• Ammonia is the most common nitrogen waste

• Toxic at certain concentrations

• To prevent toxicity to the animal ammonia must be

– continuously excreted (keep internal levels low) OR

– Converted to a nontoxic molecule (urea or uric acid) before excretion

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Excretion in Aquatic Animals

• For most aquatic animals, excreting

ammonia is not an issue - why?

– Ammonia is highly soluble in H2O, diffuses

away rapidly (won’t stay concentrated around

them)

– Aquatic animals continuously lose ammonia

from their bodies through diffusion across their

gill membranes

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Ammonotelic

• Animals that excrete

nitrogen waste mostly as

ammonia are

ammonotelic

• Ex. Aquatic

invertebrates, bony fish

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Question…

• Why don’t terrestrial animals and some

aquatic animals just excrete dilute ammonia

in liquid?

– Since ammonia is toxic even at fairly low

levels, it would have to use a lot of water to

dilute it

– Too much water loss = dehydration

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Ureotelic

• Animals that excrete

nitrogen waste mostly

as urea are ureotelic

• Ex. Mammals (us!),

amphibians, sharks,

rays, some bony fish

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Uricotelic

• Animals that excrete nitrogen waste mostly as uric acid are uricotelic

• Helps conserve H2O because it isn’t very soluble in water � semi solid

• Ex. Birds, reptiles, insects, some amphibians

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Excretory Process

• The main steps in producing urine (fluid

waste) are:

• Filtration

• Selective Reabsorption

• Secretion

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Filtration

• nonselective process in which water and small solutes are filtered across a membrane into the excretory system

– Small solutes include salts, nitrogen wastes, sugars, amino acids

– Filtrate is the liquid produced from this step

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Selective Reabsorption

• Useful/”good” molecules are

reabsorbed back into the body

from the excretory system

– Sugars, amino acids, some salts,

– By active transport

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Secretion

• More waste (toxins, extra salts, etc) are transported into the filtrate

– By active transport

• Selective reabsorption and secretion also causes water to move in /out of filtrate

• Urine = whatever is left of the filtrate after it has completed all 3 steps

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Protonephridia

• Excretory system found in

flatworms (platyhelminthes)

• Consists of a series of tubules

that dead end in the body,

open up to nephridiopores on

the side of body

• Dead ends contain flame cells

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Protonephridia

1. The cilia in the flame cells cause water and solutes to enter from the interstitial fluid

2. The beating of the cilia causes the filtrate to flow down the tubule towards the nephridiopore

3. As the filtrate flows, it is modified (water, solutes reabsorbed)

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Metanephridia

• Excretory system found in

earthworms (annelids)

• Each segment of the worm

has 2 metanephridia in it

• Due to pressure from blood

(closed circulatory

system), water and solutes

are pushed from the blood

into the coelomic fluid

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Metanephridia

1. Coelomic fluid enters the metanephridia at an opening called the nephrostome

2. As the fluid passes through the metanephridia, it is altered (water, solutes, reabsorbed)

3. Urine is excreted through the nephridiopore

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Malpighian Tubules

• Excretory system in insects

• 100 - 200 tubules attached to the midgut and hindgut of digestive system

• Open circulatory system doesn’t allow insects to produce filtrate through filtration

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Malpighian Tubules

1. Uric acid, and ions (Na+ and

K+) are actively transported

into the Malpighian tubules

� H2O follows by osmosis

2. Na+ and K+ are actively

transported back into

coelom from hindgut

3. Uric acid precipitates out of

solution � H2O returns to

coelom (by osmosis), uric

acid is excreted

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