Download WATER BALANCE IN PLANTS AND HUMANS Ch 44 and 39 I

WATER BALANCE IN PLANTS AND
HUMANS
Ch 44 and 39
I. Purpose of maintaining water balance
 To keep an organism in osmotic
balance
o Land animals and plants are
hypotonic to their environment
and would tend to lose water
o They have mechanisms to
prevent excess water loss
II. Water balance in Mammals
 Involves the kidney
A. Structure of the mammalian kidney
 Composed of millions of
nephrons
 Each nephron filters blood and
reabsorbs water and ions
 Each nephron consists of a long
tube
 The long tube and the millions of
nephrons increases surface area
for water reabsorption
 Each nephron is surrounded by
capillaries to increase surface
area for water reabsorption
B. parts of a nephron
 Glomerulus: a mass of capillaries
surrounded by Bowmans capsule
that filters fluid from blood. Fluid
contains urea, water, ions, glucose,
amino acids
 Loop of Henle: reabsorbs molecules
back into blood
o Water: by osmosis
o ions, glucose and amino acids:
by active transport
 Collecting tubule: collects waste to be
transported to ureter and bladder
C. How kidneys filter
 Renal artery brings blood to kidney
 Blood travels to the Glomerulus
where it is filtered, removing ions,
urea, glucose and amino acids
 Water follows because blood has
become hypotonic
 In the loop of Henle, ions like Na+
are reabsorbed back into the blood
by active transport
 Water follows because filtrate has
become hypotonic
 In the collecting tubule, the filtrate
leaves the kidney through the ureter
to the bladder
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D. Endocrine Control of Water balance
1. Dehydration: high blood osmolality
 Means not enuf water in blood
Osmoreceptors in hypothalamus
pick this up and:
Produce
Sensation of
Thirst
act on
pituitary
to secrete ADH
We drink
acts on kidney
Tubules to
Reabsorb more
Back into
Blood
Blood osmolality lowers and we get
negative feedback to hypothalamus
URINE IS MORE CONCENTRATED!
2. Excess water from overhydration: low
blood osmolality
Osmoreceptors in hypothalamus
detect this and cause pituitary to
secrete less ADH which means less
water reabsorption into blood
URINE IS DILUTE!
E. Other mechanisms used by land
Animals
 Exoskeleton in insects
 Dead skin in mammals
 Scales
 Animals that live in deserts often
are nocturnal
III. Water Balance in land plants
 Plants must balance the rate of
photosynthesis with the rate of
transpiration
 Both processes require open stomata
 Problem: if water is limited, excessive
transpiration will cause water column
to break and plant will die
 Problem: if stomata close to prevent
excessive transpiration, photosynthesis
will stop and plant will die
 Mechanisms to prevent excess
transpiration in plants that live in dry
areas (see notes ch 36):
 Thicker waxy cuticle on plants
 control of stomata opening in
plants
 sunken stomata
 stomata located on lower surface
of leaves