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Chapter Introduction
Living Systems as Compartments
3.1 Exchanged Materials
3.2 Membrane as Barrier
How Cells Exchange Materials
3.3 Diffusion and Osmosis
3.4 Passive and Active Transport
Exchange in Multicellular Organisms
3.5 Gas Exchange in Water
3.6 Adaptation to Life on Land
3.7 Waste Removal
3.8 Human Urinary System
Chapter Highlights
Chapter Animations
Learning Outcomes
By the end of this chapter you will be able to:
A Discuss the structure and function of membranes
in living organisms.
B Describe how materials are exchanged across
membranes.
C Explain how various organisms are adapted to
maintain water balance while processing
nitrogenous wastes.
D Relate the structure of the human nephron to
its function.
Exchanging Materials with the Environment
 How do living organisms
exchange materials with
their surroundings?
 What molecular processes
are responsible for
exchange?
A scuba diver breathing under water with the
aid of an apparatus
Exchanging Materials with the Environment
• The surface of an organism
is a barrier against
destructive forces.
• Food, water, waste, and
communication signals must
be allowed to pass through
the barrier if the organism is
to survive.
A scuba diver breathing under water with the
aid of an apparatus
Living Systems as Compartments
3.1 Exchanged Materials
• Their cytoplasm, or interior, of cells is surrounded
by a wall made of carbohydrates and proteins and a
membrane made largely of phospholipids.
• Materials needed for
life must pass into
this compartment to
be useful.
A Bacillus megaterium bacterium (x30,500)
Living Systems as Compartments
3.1 Exchanged Materials (cont.)
• Organisms and their cells need water.
• Cells need the correct balance of ions, such as
sodium (Na+), magnesium (Mg+2), calcium (Ca+2),
hydrogen (H+), chloride (Cl–), and potassium (K+).
• Carbon dioxide is needed in autotrophs to build
food molecules.
• Nutrients must enter cells to supply energy and
building material for cell components.
• Some hormones are needed to transmit messages.
• Wastes, such as ammonium ion (NH4+), must exit.
Living Systems as Compartments
3.2 Membrane as Barrier
• Membranes are composed of two thin, fluid, layers
of phospholipids and proteins.
• Not all molecules are equally soluble in a membrane.
– The nonpolar phospholipid tails of the lipid bilayer
tend to repel charged particles such as ions but
allow fat-soluble molecules to pass.
– Usually, the polarity, size, and electric charge of
molecules determine whether they can pass
through a membrane.
A selectively permeable membrane
Living Systems as Compartments
3.2 Membrane as Barrier (cont.)
• Charged molecules such as the ions H+ or Ca+2
can pass through only with the help of special
proteins, called transport proteins, that are
embedded in the membrane.
• Proteins and other very large molecules cannot pass
through a membrane without special processes.
• By limiting entry, a membrane is selectively
permeable, which means that it regulates the
exchange of materials in a very specific way.
Living Systems as Compartments
3.2 Membrane as Barrier (cont.)
• The structure of membranes is complex and allows
them to perform many functions in the cell.
• Some proteins, called glycoproteins, are embedded
in membranes have sugars attached to them.
• Sugars also can be attached to the heads of
membrane lipids (glycolipids).
• Glycoproteins and glycolipids act as antennae that
receive chemical messages from other cells.
The fluid-mosaic model of a membrane’s structure
How Cells Exchange Materials
3.3 Diffusion and Osmosis
• Diffusion refers to the movement of molecules
from an area of higher concentration to an area of
lower concentration.
• Diffusion is a random process, and the entropy of
the system increases as it occurs.
How Cells Exchange Materials
3.3 Diffusion and Osmosis (cont.)
• A concentration gradient exists when there is a
difference in concentration of molecules across
a distance.
• Diffusion is a basic process underlying the
movement of molecules into and out of cells.
How Cells Exchange Materials
3.3 Diffusion and Osmosis (cont.)
Molecules move from an area of higher concentration to an area of
lower concentration until the concentration is the same throughout. In
(a), a crystal of potassium permanganate (KMnO4) was dropped into a
glass of water. The molecules diffuse through the water (b) until they
are evenly distributed throughout (c).
How Cells Exchange Materials
3.3 Diffusion and Osmosis (cont.)
• Concentration gradients across cell membranes
provide potential energy to drive many cellular
processes.
• The potential energy is based on the concentration
gradient of substances.
• If the substance in question is charged, an electric
potential also forms across the membrane.
How Cells Exchange Materials
3.3 Diffusion and Osmosis (cont.)
• Movement of water down its concentration gradient
is a special form of diffusion called osmosis.
• If the concentration of water outside the cell is higher
than inside, water moves in, and the cell swells.
• If the concentration of water is higher inside the cell
than outside, water is driven out and the cell shrinks.
• Outward pressure of a cell against its cell wall is
called turgor.
How Cells Exchange Materials
3.3 Diffusion and Osmosis (cont.)
The motion of molecules in a glass container is random, but the net result is
movement from an area of higher concentration to one of lower concentration.
Initially, a barrier separates the
two bulbs, with gas molecules
(and potential energy)
concentrated on the right side.
When the barrier is removed,
molecules begin to appear in
the left-hand bulb.
How Cells Exchange Materials
Initially the cells are in a
solution with the same
concentration of dissolved
material as is found inside the
cells. This is called an
isosmotic solution. The
animal cell can survive only
fairly small variations from
this concentration.
How Cells Exchange Materials
3.3 Diffusion and Osmosis (cont.)
• The rate of diffusion, including osmosis, depends
on the size of the concentration gradient and the
surface area relative to the enclosed volume.
How Cells Exchange Materials
3.4 Passive and Active Transport
• Organisms must establish and maintain
concentrations of materials inside their cells that may
differ from concentrations resulting from diffusion.
• Membranes are permeable to many substances only
with the help of transport proteins, which assist
movement passively or actively.
– Passive transport involves diffusion without any
input of energy.
– Active transport moves substances against their
concentration gradients and thus requires energy.
How Cells Exchange Materials
3.4 Passive and Active Transport (cont.)
• Simple diffusion of neutral molecules such as
oxygen or carbon dioxide into or out of a cell is a
form of passive transport.
• Facilitated diffusion is passive transport that occurs
with the help of transport proteins in the membrane.
• Facilitated diffusion makes transport more specific
and speeds up the rate, but it does not work against
the gradient.
How Cells Exchange Materials
3.4 Passive and Active Transport (cont.)
• Active transport requires energy to move substances,
in addition to the help of transport proteins.
• Sources of energy include the hydrolysis of ATP
and coupling the movement of one substance
against its gradient to the movement of another
down its gradient.
How Cells Exchange Materials
3.4 Passive and Active Transport (cont.)
• Maintaining specific gradients across cell
membranes is essential to keep internal
conditions in a range that permits life functions.
• Many necessary substances could not enter or
leave cells without active transport.
Passive and active transport
How Cells Exchange Materials
3.4 Passive and Active Transport (cont.)
• To move very large molecules such as proteins into
or out of a cell, the cell membrane folds around the
substance to be transported, making a pocket to
carry it in or out of the cell.
– Endocytosis is a useful way for unicellular
organisms or very simple multicellular organisms
to get food into their internal environment.
– Exocytosis helps cells remove waste materials
and specific molecules into the external
environment.
How Cells Exchange Materials
3.4 Passive and Active Transport (cont.)
Large molecules are transported into a cell by endocystosis (a), and out
of a cell by exocytosis (b). Both processes require energy.
Exchange in Multicellular Organisms
3.5 Gas Exchange in Water
• Cellular respiration is an important supply of energy
for metabolism and other cell activities in most
organisms.
• Oxygen is essential for cellular respiration, and
carbon dioxide is given off as a waste product.
• The correct balance of these two important
molecules must be regulated carefully.
Exchange in Multicellular Organisms
3.5 Gas Exchange in Water (cont.)
• Gas exchange happens by diffusion across a
membrane when the gases are dissolved in water.
• As with most exchange processes, efficiency
requires a large surface area relative to volume.
• In fish, breathing through gills is very efficient
because they have a large surface area made up of
many fine, threadlike filaments.
Exchange in Multicellular Organisms
3.5 Gas Exchange in Water (cont.)
Fish gills are thin filaments supported by bony
structures and richly supplied with blood vessels.
Each filament is made of disks that contain numerous
capillaries. Water flows past these disks in directions
opposite (countercurrent) to the flow of blood through
the capillaries. A covering over the gills, called the
operculum, protects the delicate filaments.
Exchange in Multicellular Organisms
3.6 Adaptation to Life on Land
• Obtaining oxygen on land poses several challenges:
– Organisms living on land are constantly battling
the tendency to dry out.
– Land organisms must dissolve gases in water
on the exchange membrane.
• Many species of land organisms have evolved
exchange surfaces in an interior space which
protects the surface from excess evaporation
caused and still allows a large area for exchange.
Exchange in Multicellular Organisms
3.6 Adaptation to Life on Land (cont.)
• Some land-dwelling organisms have no special
gas-exchange organs.
Planaria (flatworms) (a), and earthworms (b), have no special gasexchange organs. Gases are exchanged directly through their skin.
Exchange in Multicellular Organisms
3.6 Adaptation to Life on Land (cont.)
• Insects use a system of small, branched air ducts
to carry oxygen throughout the body.
In insects, gas exchange occurs through branching air tubes called
tracheae (singular: trachea). Air flows in and out of tracheae through
openings called spiracles. The spiracles can close to retain water and
keep foreign particles out.
Exchange in Multicellular Organisms
3.6 Adaptation to Life on Land (cont.)
• Lungs are the organs of gas exchange in many land
animals, including humans.
• Lungs minimize the effects of drying out by
eliminating the one-way flow of oxygen that is so
efficient in gills.
• Because the concentration difference is not great,
the gas-exchange efficiency of lungs is much less
than that of gills.
Exchange in Multicellular Organisms
3.6 Adaptation to Life on Land (cont.)
• The air you breathe passes through your nose,
where it is filtered by hairs lining the nasal cavities,
moistened, and warmed.
• It then travels through
branched passageways to
reach millions of
microscopic cavities in the
lungs called alveoli.
Scanning electron micrograph of alveoli, x415.
Capillaries in the alveolar walls provide a close
relationship between blood and air.
Exchange in Multicellular Organisms
3.6 Adaptation to Life on Land (cont.)
• Oxygen and carbon dioxide diffuse across the
alveolar walls and the walls of the capillaries.
• The numerous alveoli of the lungs provide an
enormous amount of surface area for gas exchange.
Exchange in Multicellular Organisms
3.6 Adaptation to Life on Land (cont.)
Exchange in Multicellular Organisms
3.6 Adaptation to Life on Land (cont.)
• Another water-conservation strategy of terrestrial
(land-dwelling) organisms involves barriers that limit
the permeability of the outside of the organism itself.
• Air-breathing vertebrates and arthropods, plants, and
fungi all have surface waxes and lipids that minimize
water loss by evaporation.
• In plants, cells along the surface of a leaf secrete a
waxy substance that forms a water-repellent
covering called the cuticle.
Exchange in Multicellular Organisms
3.6 Adaptation to Life on Land (cont.)
• In plants, gases normally move into and out of the
leaf tissue through openings known as stomates on
the leaf surface.
• Each stomate is surrounded by a specialized pair of
guard cells which bend apart when swollen with
water, opening the stomate.
• This opening allows carbon dioxide to diffuse in and
water vapor and oxygen to exit. The loss of water by
this pathway is called transpiration.
Exchange in Multicellular Organisms
3.6 Adaptation to Life on Land (cont.)
• As osmosis results in the loss of water from the
guard cells, they shrink and draw toward one
another, closing the stomate.
Guard cells act as gates around the stomates in the leaf surface. When
open (a), they allow water vapor to escape and carbon dioxide to enter the
leaf. When water loss in the plant is higher than its replacement, the guard
cells droop toward one another. This action closes the stomate (b).
Exchange in Multicellular Organisms
3.7 Waste Removal
• Organisms living in fresh water constantly must rid
themselves of excess water.
Contractile vacuoles in Paramecium rid the cell of excess water. The
vacuoles (a) expand as water fills them through radiating canals (b, c).
The vacuoles then contract and eject the water from the organism (d).
Exchange in Multicellular Organisms
3.7 Waste Removal (cont.)
• In addition to water, a variety of waste products must
be removed from cells and organisms, including
excess salts and carbon dioxide.
• The exchange of materials, including the removal of
wastes, is essential to maintaining homeostasis, the
balanced and controlled conditions in the internal
environment of an organism.
Exchange in Multicellular Organisms
3.7 Waste Removal (cont.)
• In relatively simple organisms
such as sponges and Hydra,
each cell simply excretes its
wastes directly through the
external surface.
• In more complex animals,
special organs have
evolved for excretion and
maintaining water balance
in larger organisms.
Exchange in Multicellular Organisms
3.7 Waste Removal (cont.)
• Metabolism produces toxic nitrogenous waste, such
as ammonia (NH3), which must be disposed of.
– The high solubility of ammonia makes it a safe
excretory product in freshwater and saltwater
protists and animals.
– Mammals, some fishes, and amphibians excrete
nitrogenous wastes chiefly as urea.
– Uric acid, an almost insoluble and nontoxic form
of nitrogenous waste, is an adaptation of birds
and many desert reptiles.
Exchange in Multicellular Organisms
3.8 Human Urinary System
• The human urinary system is an example of how
waste removal is critical to maintaining homeostasis.
• The excretory tubules of humans, the nephrons, are
collected into compact organs, the kidneys.
• The two kidneys are the major organs in mammals
responsible for processing the waste products of
metabolism.
• The urinary system is composed of the kidneys, the
blood vessels that serve them, and the plumbing that
carries fluid formed in the kidneys out of the body.
Exchange in Multicellular Organisms
3.8 Human Urinary System (cont.)
(a), The human urinary system. (b), A section through the human kidney.
(c), An enlarged view of one nephron with its surrounding capillaries.
Exchange in Multicellular Organisms
3.8 Human Urinary System (cont.)
• Blood to be filtered enters the kidneys via the renal
artery and leaves via the renal vein.
• The waste fluid, urine, leaves the kidneys through a
tube called the ureter.
• The ureter drains into a holding tank, the
urinary bladder.
• The urinary bladder is periodically drained when the
urine passes through a tube called the urethra
during urination.
Exchange in Multicellular Organisms
3.8 Human Urinary System (cont.)
• A nephron is a long, coiled tube with one cuplike end
that fits over a mass of capillaries. The other end of
the nephron opens into a duct that collects urine.
• The cup of the nephron is called the
glomerular capsule, or Bowman’s capsule.
• The ball of capillaries within the cup is called
a glomerulus.
• Collecting tubules from all the nephrons eventually
empty into the ureter.
Exchange in Multicellular Organisms
3.8 Human Urinary System (cont.)
• Nephrons have three functions:
filtration, reabsorption, and
secretion.
• Filtration occurs in the
glomerulus, where the fluid
portion of the blood is forced
into the glomerular capsule.
• The filtrate includes the blood
plasma, nitrogenous wastes
from cells, urea, salts, ions,
glucose, and amino acids.
Exchange in Multicellular Organisms
3.8 Human Urinary System (cont.)
• Reabsorption and secretion
take place in the tubule of
the nephron.
• Cells of the tubule walls
reabsorb substances needed
by the body from the filtrate
and return them to the blood.
Exchange in Multicellular Organisms
3.8 Human Urinary System (cont.)
• Secretion occurs as cells of
the tubule wall selectively
remove from the surrounding
capillaries substances that
were left in the plasma after
filtration or returned by
reabsorption.
• The cells then secrete these
substances into the filtrate.
Exchange in Multicellular Organisms
3.8 Human Urinary System (cont.)
• Reabsorption accounts for
85% of the salt, water, and
other substances processed
by the kidney.
• The remaining 15% is
regulated by hormones or
nervous-system controls.
Exchange in Multicellular Organisms
3.8 Human Urinary System (cont.)
• Excretion of sodium and
potassium is regulated by
aldosterone, a hormone
secreted by the adrenal
gland.
All of the potassium ions in the filtrate are
reabsorbed into the blood by the time the filtrate
passes the nephron loop. Under the influence of
aldosterone, potassium ions are secreted back
into the filtrate near the collecting duct, where
they are excreted with the urine.
Exchange in Multicellular Organisms
3.8 Human Urinary System (cont.)
• Feedback regulation is a process in which
substances (such as aldosterone) inhibit their own
formation and to maintain balance and stability
• The hypothalamus in the brain detects a drop in
blood pressure and stimulates the pituitary gland to
release antidiuretic hormone (ADH) into the
bloodstream.
Exchange in Multicellular Organisms
3.8 Human Urinary System (cont.)
Water content of the
blood is controlled
by ADH from the
hypothalamus and
pituitary gland.
Exchange in Multicellular Organisms
3.8 Human Urinary System (cont.)
• The kidneys can also remove excess salt from the
body, but only in small amounts.
• The kidneys remove nitrogenous wastes from the
blood as urea, help regulate blood pressure, regulate
water-salt balance, conserve blood glucose, and
excrete excess salt, within limits.
Summary
• A living system is a single or series of protected
compartments.
• The internal conditions are usually different from conditions
outside the organism.
• Internal conditions must be carefully balanced with regard to
nutrients and wastes, a condition known as homeostasis.
• The cell membrane is selectively permeable, which helps it
control an organism’s exchange of substances with the
environment.
• The physical processes of diffusion and osmosis are
responsible for movement of substances into and out of cells.
• Transport proteins in the membrane can help specific
substances cross the membrane barrier. Transport is either
passive or, if it requires energy, active.
Summary (cont.)
• Exocytosis and endocytosis are responsible for exporting or
importing large materials, respectively.
• Gas exchange is an essential aspect of living processes.
Exchange surfaces must be kept moist, and the ratio of
surface area to volume affects the efficiency of exchange by
diffusion.
• Land organisms must balance the need for large surface area
of the exchange membranes against the danger of drying out.
• Wastes must be expelled from all living systems. Nitrogenous
wastes are particularly toxic and may be excreted as
ammonia, urea, or uric acid.
• Contractile vacuoles in unicellular organisms force wastes out
of the cell.
Summary (cont.)
• In humans, the kidneys are the major organs for removing
waste products from the internal environment.
• The nephron is the functional unit of the kidney.
• Hormones assist the urinary system in regulating ion balance,
water levels in the blood, and blood pressure.
Reviewing Key Terms
Match the term on the left with the correct description.
___
diffusion
d
___
osmosis
a
___
turgor
e
___
endocytosis
b
___
cuticle
f
___
homeostasis
c
a. the movement of water through a
selectively permeable membrane
b. the cellular uptake of materials in
which the plasma membrane
surrounds and engulfs extracellular
materials
c. state of balance within an internal
environment
d. the movement of a substance down
its concentration gradient
e. a cell’s swelling against its cell wall
caused by internal pressure
f.
the waxy outer layer covering the
surfaces of most land-dwelling plants
Reviewing Ideas
1. What function to glycoproteins perform in a cell
membrane?
Glycoproteins act as antennae that receive
chemical messages from other cells.
Reviewing Ideas
2. What challenges do land-dwelling organisms
face in relation to gas-exchange?
Organisms living on land are constantly battling
the tendency to dry out. Land organisms must
also dissolve gases in water on their exchange
membrane.
Using Concepts
3. How does a concentration gradient represent
potential energy?
The barrier formed by a membrane can act like a
dam that holds back the water of a lake. In a
cellular compartment, the membrane may hold
back ions. A great amount of potential energy is
stored in this way, just as the water behind a dam
has the potential to rush out if the dam is opened.
Using Concepts
4. How is the function of a stomate
self-regulating?
Each stomate is surrounded by a specialized pair of
guard cells, which function as gates. When guard
cells are swollen with water, they bend apart,
opening the stomate. This opening allows carbon
dioxide to diffuse in and water vapor and oxygen to
exit. If a plant loses more water than it can take in
through its roots, the plant wilts. As osmosis results
in the loss of water from the guard cells, they shrink
and draw toward one another, closing the stomate
and minimizing further water loss.
Synthesize
5. If a person is dehydrated, why can’t pure water
simply be injected into them? Ringer’s solution
is commonly injected directly into the blood
stream to help fight dehydration. What allows
this to be safe?
Injecting pure water would cause a high water
concentration in the blood. This would cause the
blood cells to swell and possibly burst as water
rushes down its concentration gradient and into
the cells. Ringer’s solution is isometric, containing
the same concentration of dissolved material as
found inside the blood cells.
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Chapter Animations
A selectively permeable membrane
The fluid-mosaic model of a membrane’s structure
Passive and active transport
A selectively permeable membrane
The fluid-mosaic model of a membrane’s structure
Passive and active transport
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