Chapt 5

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Chapter 5
The Working Cell
PowerPoint Lectures
Campbell Biology: Concepts & Connections, Eighth Edition
REECE • TAYLOR • SIMON • DICKEY • HOGAN
© 2015 Pearson Education, Inc.
Lecture by Edward J. Zalisko
Introduction
• The plasma membrane and its proteins enable
cells to
• survive and
• function.
• Aquaporins are membrane proteins that function
as water channels.
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Introduction
• This chapter addresses how working cells use
• membranes,
• energy, and
• enzymes.
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MEMBRANE STRUCTURE AND FUNCTION
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Membranes are fluid mosaics of lipids and
proteins with many functions
• Biologists use the fluid mosaic model to describe
a membrane’s structure, a patchwork of diverse
protein molecules embedded in a phospholipid
bilayer.
• The plasma membrane exhibits selective
permeability.
• The proteins embedded in a membrane’s
phospholipid bilayer perform various functions.
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Figure 5.1
Cytoplasmic side
of membrane
Extracellular side
of membrane
O2
CO2
Fibers of
extracellular
matrices (ECM)
Phospholipid
Cholesterol
Membrane
proteins
Microfilaments
of cytoskeleton
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Figure 5.1-2
O2
CO2
Diffusion of small
nonpolar molecules
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Figure 5.1-3
Solute molecules
Active
transport
protein
Channel
protein
ATP
Transport Proteins
• Allow specific ions or molecules
to enter or exit the cell
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Figure 5.1-4
Initial
reactant
Enzymes
Product of
reaction
Enzymes
• Some membrane proteins are enzymes
• Enzymes may be grouped to carry out
sequential reactions
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Figure 5.1-5
Extracellular
matrix
Attachment
protein
Microfilaments
of cytoskeleton
Attachment Proteins
• Attach to the extracellular matrix and cytoskeleton
• Help support the membrane
• Can coordinate external and internal changes
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Figure 5.1-6
Signaling
molecule
Receptor
protein
Receptor Proteins
• Signaling molecules bind to receptor proteins
• Receptor proteins relay the message by
activating other molecules inside the cell
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Figure 5.1-7
Junction
protein
Junction
protein
Junction Proteins
• Form intercellular junctions that
attach adjacent cells
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Figure 5.1-8
Attached
sugars
Protein that
recognizes
neighboring
cell
Glycoprotein
Glycoproteins
• Serve as ID tags
• May be recognized by membrane
proteins of other cells
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Figure 5.1-9
Cytoplasmic side
of membrane
Extracellular side
of membrane
O2
Diffusion of small
nonpolar molecules
Fibers of
extracellular
matrices (ECM)
CO2
Enzyme
Enzyme
Phospholipid
Cholesterol
Attachment
protein
Receptor
protein
Channel
protein
Junction
protein
Active
transport
ATP
protein
Microfilaments
of cytoskeleton
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Junction
protein
Glycoprotein
The spontaneous formation of membranes
was a critical step in the origin of life
• Phospholipids, the key ingredient of biological
membranes, spontaneously self-assemble into
simple membranes.
• The formation of membrane-enclosed collections
of molecules was a critical step in the evolution of
the first cells.
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Figure 5.2
Water-filled
bubble made of
phospholipids
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Passive transport is diffusion across a
membrane with no energy investment
• Diffusion is the tendency of particles to spread out
evenly in an available space.
• Particles move from an area of more concentrated
particles to an area where they are less
concentrated.
• This means that particles diffuse down their
concentration gradient.
• Eventually, the particles reach dynamic equilibrium,
where there is no net change in concentration on
either side of the membrane.
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Figure 5.3a
Molecules of dye
Membrane
Pores
Net diffusion
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Net diffusion
Equilibrium
Figure 5.3b
Net diffusion
Net diffusion
Equilibrium
Net diffusion
Net diffusion
Equilibrium
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Passive transport is diffusion across a
membrane with no energy investment
• Diffusion across a cell membrane does not require
energy, so it is called passive transport.
• Diffusion down concentration gradients is the sole
means by which oxygen enters your cells and
carbon dioxide passes out of cells.
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Osmosis is the diffusion of water across a
membrane
• One of the most important substances that crosses
membranes by passive transport is water.
• The diffusion of water across a selectively
permeable membrane is called osmosis.
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Osmosis is the diffusion of water across a
membrane
• If a membrane, permeable to water but not to a
solute, separates two solutions with different
concentrations of solute, water will cross the
membrane, moving down its own concentration
gradient, until the solute concentration on both
sides is equal.
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Figure 5.4
Lower
Higher
concentration concentration
of solute
of solute
Solute
molecule
More equal
concentrations
of solute
H2O
Selectively
permeable
membrane
Water
molecule
Solute molecule
with cluster of
water molecules
Osmosis
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Water balance between cells and their
surroundings is crucial to organisms
• Tonicity is a term that describes the ability of a
surrounding solution to cause a cell to gain or lose
water.
• The tonicity of a solution mainly depends on its
concentration of solutes relative to the
concentration of solutes inside the cell.
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Water balance between cells and their
surroundings is crucial to organisms
• How will animal cells be affected when placed into
solutions of various tonicities?
• In an isotonic solution, the concentration of solute is
the same on both sides of a membrane, and the cell
volume will not change.
• In a hypotonic solution, the solute concentration is
lower outside the cell, water molecules move into the
cell, and the cell will expand and may burst.
• In a hypertonic solution, the solute concentration is
higher outside the cell, water molecules move out of the
cell, and the cell will shrink.
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Figure 5.5
Hypotonic solution
(lower solute levels)
H2O
Isotonic solution
(equal solute levels)
Hypertonic solution
(higher solute levels)
H2O
H2O
H2O
Animal
cell
Lysed
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Normal
Shriveled
Water balance between cells and their
surroundings is crucial to organisms
• For an animal cell to survive in a hypotonic or
hypertonic environment, it must engage in
osmoregulation, the control of water balance.
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• The cell walls of plant cells, prokaryotes, and fungi
make water balance issues somewhat different.
• The cell wall of a plant cell exerts pressure that
prevents the cell from taking in too much water and
bursting when placed in a hypotonic environment.
• But in a hypertonic environment, plant and animal
cells both shrivel.
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Figure 5.5
Hypotonic solution
(lower solute levels)
H2O
Isotonic solution
(equal solute levels)
H2O
Hypertonic solution
(higher solute levels)
Plasma
H2O
membrane
Plant
cell
Turgid (normal)
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Flaccid
Shriveled (plasmolyzed)
Transport proteins can facilitate diffusion
across membranes
• Hydrophobic substances easily diffuse across a
cell membrane.
• However, polar or charged substances do not
easily cross cell membranes and, instead, move
across membranes with the help of specific
transport proteins in a process called facilitated
diffusion, which
• does not require energy and
• relies on the concentration gradient.
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Figure 5.6
Solute
molecule
Transport
protein
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• Some proteins function by becoming a hydrophilic
tunnel for passage of ions or other molecules.
• Other proteins bind their passenger, change
shape, and release their passenger on the other
side.
• In both cases, the transport protein helps a specific
substance diffuse across the membrane down its
concentration gradient and thus requires no input
of energy.
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• Because water is polar, its diffusion through a
membrane’s hydrophobic interior is relatively slow.
• The very rapid diffusion of water into and out of
certain cells is made possible by a protein channel
called an aquaporin.
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Cells expend energy in the active transport of
a solute
• In active transport, a cell must expend energy to
move a solute against its concentration gradient.
• The energy molecule ATP supplies the energy for
most active transport.
• The following figures show the four main stages of
active transport.
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Figure 5.8-1
Transport
protein
Solute
1 Solute binds
to transport
protein.
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Figure 5.8-2
Transport
protein
ATP
Solute
1 Solute binds
to transport
protein.
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2 ATP provides
energy for
change in
protein shape.
Figure 5.8-3
Transport
protein
ATP
Solute
1 Solute binds
to transport
protein.
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2 ATP provides
energy for
change in
protein shape.
3 Protein returns
to original shape
and more solute
can bind.
Exocytosis and endocytosis transport large
molecules across membranes
• A cell uses two mechanisms to move large
molecules across membranes.
1. Exocytosis is used to export bulky molecules,
such as proteins or polysaccharides.
2. Endocytosis is used to take in large molecules.
• In both cases, material to be transported is
packaged within a vesicle that fuses with the
membrane.
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• There are two kinds of endocytosis.
1. Phagocytosis is the engulfment of a particle by
the cell wrapping cell membrane around it,
forming a vacuole.
2. Receptor-mediated endocytosis uses
membrane receptors for specific solutes. The
region of the membrane with receptors pinches
inward to form a vesicle.
• Receptor-mediated endocytosis is used to take in
cholesterol from the blood.
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Figure 5.9-0
Phagocytosis
EXTRACELLULAR
FLUID
CYTOPLASM
Pseudopodium
“Food” or
other particle
Food
vacuole
Receptor-mediated endocytosis
Coat protein
Coated
vesicle
Receptor
Specific
molecule
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Coated
pit
Figure 5.9-1
Phagocytosis
EXTRACELLULAR
FLUID
CYTOPLASM
Pseudopodium
“Food” or
other particle
Food
vacuole
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Figure 5.9-2
Receptor-mediated endocytosis
Coat protein
Coated
vesicle
Receptor
Specific
molecule
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Coated
pit
ENERGY AND THE CELL
Cells transform energy as they perform work
• Cells are miniature chemical factories, housing
thousands of chemical reactions.
• Some of these chemical reactions release energy,
and others require energy.
• Energy is the capacity to cause change or to
perform work.
• There are two basic forms of energy.
1. Kinetic energy is the energy of motion.
2. Potential energy is energy that matter
possesses as a result of its location or
structure.
• Thermal energy is a type of kinetic energy
associated with the random movement of atoms or
molecules.
• Thermal energy in transfer from one object to
another is called heat.
• Light is also a type of kinetic energy; it can be
harnessed to power photosynthesis.
• Chemical energy is the
• potential energy available for release in a chemical
reaction and
• the most important type of energy for living
organisms to power the work of the cell.
• Thermodynamics is the study of energy
transformations that occur in a collection of matter.
• The word system is used for the matter under study.
• The word surroundings is used for everything
outside the system; the rest of the universe.
• Two laws govern energy transformations in
organisms.
• Per the first law of thermodynamics (also known
as the law of energy conservation), energy in the
universe is constant.
• Per the second law of thermodynamics, energy
conversions increase the disorder of the universe.
• Entropy is the measure of disorder or
randomness.
• Automobile engines and cells use the same basic
process to make the chemical energy of their fuel
available for work.
• In the car and cells, the waste products are carbon
dioxide and water.
• Cells use oxygen in reactions that release energy
from fuel molecules.
• In cellular respiration, the chemical energy stored
in organic molecules is used to produce ATP,
which the cell can use to perform work.
Figure 5.10-0
Energy conversion
Fuel
Waste products
Heat
energy
Carbon dioxide
Gasoline
+
Combustion
Kinetic energy
of movement
+
Water
Oxygen
Energy conversion in a car
Heat
energy
Glucose
+
Oxygen
Cellular respiration
Carbon dioxide
ATP
ATP
Energy for cellular work
Energy conversion in a cell
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+
Water
Chemical reactions either release or store
energy
• Chemical reactions either
• release energy (exergonic reactions) or
• require an input of energy and store energy
(endergonic reactions).
• Exergonic reactions release energy.
• These reactions release the energy in covalent
bonds of the reactants.
• Burning wood releases the energy in glucose as
heat and light.
• Cellular respiration
• involves many steps,
• releases energy slowly, and
• uses some of the released energy to produce ATP.
Potential energy
Reactants
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Amount of
energy released
Energy
Products
• An endergonic reaction
• requires an input of energy and
• yields products rich in potential energy.
• Endergonic reactions
• start with reactant molecules that contain relatively
little potential energy but
• end with products that contain more chemical
energy.
Figure 5.11b
Potential energy
Products
Energy
Reactants
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Amount of
energy required
• Photosynthesis is a type of endergonic process.
• In photosynthesis,
• energy-poor reactants (carbon dioxide and water)
are used,
• energy is absorbed from sunlight, and
• energy-rich sugar molecules are produced.
Chemical reactions either release or store
energy
• A living organism carries out thousands of
endergonic and exergonic chemical reactions.
• The total of an organism’s chemical reactions is
called metabolism.
• A metabolic pathway is a series of chemical
reactions that either
• builds a complex molecule or
• breaks down a complex molecule into simpler
compounds.
• Energy coupling uses the energy released from
exergonic reactions to drive endergonic reactions,
typically using the energy stored in ATP molecules.
ATP drives cellular work by coupling
exergonic and endergonic reactions
• ATP, adenosine triphosphate, powers nearly all
forms of cellular work.
• ATP consists of
• adenosine and
• a triphosphate tail of three phosphate groups.
• Hydrolysis of ATP releases energy by transferring
its third phosphate from ATP to some other
molecule in a process called phosphorylation.
• Most cellular work depends on ATP energizing
molecules by phosphorylating them.
Triphosphate
Adenosine
P
ATP
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P
P
Triphosphate
Adenosine
P
P
P
ATP
Diphosphate
H 2O
Adenosine
ADP
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P
P
P
Phosphate
Energy
• There are three main types of cellular work:
1. chemical,
2. mechanical, and
3. transport.
• ATP drives all three of these types of work.
Chemical work
P
ATP
P
ADP + P
Reactants
Product formed
Transport work
ATP
ADP + P
P
P
Transport protein
Solute transported
Mechanical work
ATP
P
Motor protein
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P
Protein filament moved
ADP + P
• A cell uses and regenerates ATP continuously.
• In the ATP cycle, energy released in an exergonic
reaction, such as the breakdown of glucose during
cellular respiration, is used in an endergonic
reaction to generate ATP from ADP.
Figure 5.12c
ATP synthesis
is endergonic
Energy from
cellular respiration
(exergonic)
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ATP
ADP + P
ATP hydrolysis
is exergonic
Energy for
cellular work
(endergonic)
HOW ENZYMES FUNCTION
Enzymes speed up the cell’s chemical
reactions by lowering energy barriers
• Although biological molecules possess much
potential energy, it is not released spontaneously.
• An energy barrier must be overcome before a
chemical reaction can begin.
• This energy is called the activation energy
(because it activates the reactants).
• We can think of activation energy as the amount of
energy needed for a reactant molecule to move
“uphill” to a higher-energy but an unstable state so
that the “downhill” part of the reaction can begin.
• One way to speed up a reaction is to add heat,
which agitates atoms so that bonds break more
easily and reactions can proceed, but too much
heat will kill a cell.
• Enzymes
• function as biological catalysts,
• increase the rate of a reaction without being
consumed by the reaction, and
• are usually proteins (although some RNA molecules
can function as enzymes).
• Enzymes speed up a reaction by lowering the
activation energy needed for a reaction to begin.
Figure 5.13-2
Energy
Activation
energy
barrier
Reactant
Products
Without enzyme
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Figure 5.13-3
Enzyme
Activation
energy
barrier
reduced by
enzyme
Energy
Reactant
Products
With enzyme
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a
Energy
b
Reactants
c
Products
Progress of the reaction
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A specific enzyme catalyzes each cellular
reaction
• An enzyme
• is very selective in the reaction it catalyzes and
• has a shape that determines the enzyme’s
specificity.
• The specific reactant that an enzyme acts on is
called the enzyme’s substrate.
• A substrate fits into a region of the enzyme called
the active site.
• Enzymes are specific because only specific
substrate molecules fit into their active site.
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A specific enzyme catalyzes each cellular
reaction
• The following figure illustrates the catalytic cycle of
an enzyme.
Figure 5.14-1
1 The enzyme available
with an empty active site
Active site
Enzyme
(sucrase)
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Figure 5.14-2
1 The enzyme available
with an empty active site
Substrate
(sucrose)
Active site
2 Substrate
binds to
enzyme with
induced fit.
Enzyme
(sucrase)
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Figure 5.14-3
1 The enzyme available
with an empty active site
Substrate
(sucrose)
Active site
2 Substrate
binds to
enzyme with
induced fit.
Enzyme
(sucrase)
H2O
3 The substrate
is converted
to products
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Figure 5.14-4
1 The enzyme available
with an empty active site
Substrate
(sucrose)
Active site
2 Substrate
binds to
enzyme with
induced fit.
Glucose
Enzyme
(sucrase)
Fructose
H2O
4 The products
are released
3 The substrate
is converted
to products
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• For every enzyme, there are optimal conditions
under which it is most effective.
• Temperature affects molecular motion.
• An enzyme’s optimal temperature produces the
highest rate of contact between the reactants and
the enzyme’s active site.
• Most human enzymes work best at 35–40°C.
• The optimal pH for most enzymes is near
neutrality.
• Many enzymes require nonprotein helpers called
cofactors, which
• bind to the active site and
• function in catalysis.
• Some cofactors are inorganic, such as the ions of
zinc, iron, or copper.
• If a cofactor is an organic molecule, such as most
vitamins, it is called a coenzyme.
Enzyme inhibition can regulate enzyme
activity in a cell
• A chemical that interferes with an enzyme’s activity
is called an inhibitor.
• Competitive inhibitors
• block substrates from entering the active site and
• reduce an enzyme’s productivity.
• Noncompetitive inhibitors
• bind to the enzyme somewhere other than the
active site,
• change the shape of the active site, and
• prevent the substrate from binding.
Figure 5.15a
Substrate
Active site
Enzyme
Normal binding of substrate
Competitive
inhibitor
Noncompetitive
inhibitor
Enzyme inhibition
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PABA is necessary for
bacteria in producing
Folic Acid. Humans get
their Folic acid as a
dietary Vitamin B9.
Sulfanilamide blocks the
bacterial enzymes that
convert
PABA to folic acid thus
killing them.
The Medical use of
Sulfanilamide was
discovered in 1935 and
was awarded the Nobel
Prize in 1939 to Gerhard
Domagk
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Enzyme inhibition can regulate enzyme
activity in a cell
• Enzyme inhibitors are important in regulating cell
metabolism.
• In some reactions, the product may act as an
inhibitor of one of the enzymes in the pathway that
produced it. This is called feedback inhibition.
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Figure 5.15b
– Feedback inhibition
Enzyme 1
A
Reaction 1
Starting
molecule
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Enzyme 2
B
Enzyme 3
C
Reaction 2
D
Reaction 3
Product
Many drugs, pesticides, and poisons are
enzyme inhibitors
• Many beneficial drugs act as enzyme inhibitors,
including
• ibuprofen, which inhibits an enzyme involved in the
production of prostaglandins (messenger molecules
that increase the sensation of pain and
inflammation),
• some blood pressure medicines,
• some antidepressants,
• many antibiotics, and
• protease inhibitors used to fight HIV.
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• Enzyme inhibitors have also been developed as
• pesticides and
• deadly poisons for chemical warfare.
REVIEW
You should now be able to
1. Describe the fluid mosaic structure of cell
membranes.
2. Describe the diverse functions of membrane
proteins.
3. Relate the structure of phospholipid molecules to
the structure and properties of cell membranes.
4. Define diffusion and describe the process of
passive transport.
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You should now be able to
5. Explain how osmosis can be defined as the
diffusion of water across a membrane.
6. Distinguish between hypertonic, hypotonic, and
isotonic solutions.
7. Explain how transport proteins facilitate diffusion.
8. Distinguish between exocytosis, endocytosis,
phagocytosis, and receptor-mediated
endocytosis.
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You should now be able to
9. Define and compare kinetic energy, potential
energy, chemical energy, and heat.
10. Define the two laws of thermodynamics and
explain how they relate to biological systems.
11. Define and compare endergonic and exergonic
reactions.
12. Explain how cells use cellular respiration and
energy coupling to survive.
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You should now be able to
13. Explain how ATP functions as an energy shuttle.
14. Explain how enzymes speed up chemical
reactions.
15. Explain how competitive and noncompetitive
inhibitors alter an enzyme’s activity.
16. Explain how certain drugs, pesticides, and
poisons can affect enzymes.
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Passive transport
(requires no energy)
Diffusion
Facilitated
diffusion
Higher solute
concentration
Active transport
(requires energy)
Osmosis
Higher free water
concentration
Higher solute
concentration
Solute
Water
Lower solute
concentration
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Lower free water
concentration
ATP
Lower solute
concentration
Molecules cross
cell membranes
by
by
passive
transport
may be
(a)
moving
down
requires
moving
against
ATP
(b)
uses
diffusion
of
(c)
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(d)
uses
(e)
of
polar molecules
and ions
P
c.
b.
a.
d.
f.
e.
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