Transcript chapter5_Sections 5-8.ppt

Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/biology/starr
Chapter 5
Ground Rules of Metabolism
(Sections 5.5 - 5.8)
Albia Dugger • Miami Dade College
5.5 Metabolism: Organized,
Enzyme-Mediated Reactions
• Cells build, convert, and dispose of most substances in
metabolic pathways
• metabolic pathway
• Series of enzyme-mediated reactions by which cells build,
remodel, or break down an organic molecule
• Some metabolic pathways are linear, some are cyclic
Types of Metabolic Pathways
Controls Over Metabolism
• Controls over enzymes allow cells to conserve energy and
resources by producing only what they require
• Feedback inhibition is an example of enzyme control
• Allosteric sites are points of control by which a cell adjusts
the types and amounts of substances it makes
Key Terms
• feedback inhibition
• Mechanism by which a change that results from some
activity decreases or stops the activity.
• allosteric
• Describes a region of an enzyme other than the active site
that can bind regulatory molecules
• Binding of an allosteric regulator alters the shape of the
enzyme in a way that enhances or inhibits its function
Feedback Inhibition
• Three kinds of enzymes
act in sequence to
convert a substrate to a
product, which inhibits
the activity of the first
enzyme
Feedback
Inhibition
reactant
X
enzyme 1
intermediate
enzyme 2
intermediate
enzyme 3
product
Stepped Art
Fig. 5.14, p. 82
ANIMATION: Feedback inhibition
Allosteric Effects
Allosteric Effects
active site
A inactive form
Fig. 5.15a, p. 82
Allosteric Effects
regulatory
molecules
substrate in
active site
B active form
Fig. 5.15b, p. 82
ANIMATION: Allosteric activation
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Redox Reactions
• Cells use oxygen to break the bonds of organic molecules –
but have no way to harvest an explosive burst of energy
• Oxidation–reduction (redox) reactions in electron transfer
chains allow cells to harvest energy in manageable
increments
• Coenzymes deliver electrons to electron transfer chains in
photosynthesis and aerobic respiration
Key Terms
• redox reaction (electron transfer)
• Oxidation–reduction reaction in which one molecule
accepts electrons (it becomes reduced) from another
molecule (which becomes oxidized)
• electron transfer chain
• Array of enzymes and other molecules that accept and
give up electrons in sequence, thus releasing the energy
of the electrons in usable increments
Energy Release:
Controlled and Uncontrolled
Energy Release:
Controlled and Uncontrolled
glucose
+
oxygen
A Glucose and
oxygen react
(burn) when
exposed to a
spark. Energy is
released all at
once as light and
heat when CO2
and water form.
carbon dioxide
+
water
spark
Fig. 5.16a, p. 83
Energy Release:
Controlled and Uncontrolled
carbon dioxide
Energy input splits glucose into carbon
1 dioxide, electrons, and hydrogen ions
(H+).
glucose
oxygen
2 Electrons lose energy as they move
H+
through an electron transfer chain.
3 Energy released by electrons is
harnessed for cellular work.
4 Electrons, hydrogen ions, and
oxygen combine to form water.
water
B The same overall reaction occurs in small steps
with an electron transfer chain. Energy is released in
amounts that cells can harness for cellular work.
Fig. 5.16b, p. 83
ANIMATION: Controlling energy release
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Key Concepts
• The Nature of Metabolism
• Metabolic pathways are energy-driven sequences of
enzyme-mediated reactions
• They build, convert, and dispose of materials in cells
• Controls that govern steps in metabolic pathways can
quickly shift cell activities
ANIMATION: Allosteric Inhibition
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ANIMATION: Chemical Equilibrium
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5.6 Movement of
Ions and Molecules
• Understanding how metabolism works in cells begins with the
behavior of solutions
• Solute molecules or ions tend to move “down” their
concentration gradient, from a region of higher
concentration to one of lower concentration
Key Terms
• concentration
• Number of solute molecules or ions per unit volume of a
solution
• concentration gradient
• Difference in solute concentration between adjoining
regions of a solution
Diffusion
• diffusion
• Net movement of
molecules or ions from a
region where they are
more concentrated to a
region where they are
less so
Factors in Rate of Diffusion
1. Size: Smaller molecules diffuse faster than larger ones
2. Temperature: Higher temperature causes faster diffusion
3. Steepness of concentration gradient: Rate of diffusion is
higher with steeper gradients
4. Charge: Difference in charge between two regions affects
the rate and direction of diffusion between them
5. Pressure: Diffusion occurs faster at higher pressures
Diffusion Across Membranes
• Tonicity refers to the total concentration of solutes in two
fluids separated by a selectively permeable membrane
• When the solute concentrations of two fluids differ, the one
with the lower concentration of solutes is hypotonic and the
one with the higher solute concentration is hypertonic
• Two fluids with identical solute concentrations are isotonic
Key Terms
• hypertonic
• hyper– (over): Describes a fluid with a high overall solute
concentration relative to another fluid
• hypotonic
• hypo– (under): Describes a fluid with a low overall solute
concentration relative to another fluid
• isotonic
• Describes two fluids with identical solute concentrations
Osmosis
• In osmosis, water diffuses across a selectively permeable
membrane, from a region with lower solute concentration
(hypotonic) toward a region with higher solute concentration
(hypertonic)
• There is no net movement of water between isotonic
solutions
• osmosis
• Diffusion of water across a selectively permeable
membrane in response to a differing overall solute
concentration
Osmosis
• Fluid volume changes in the two compartments as water
follows its gradient and diffuses across the membrane
selectively permeable
membrane
Fig. 5.17, p. 84
3D ANIMATION: Osmosis Experiment
Selective Permeability
• A lipid bilayer is a selectively permeable membrane
• Hydrophobic molecules, gases, and water molecules can
cross a lipid bilayer on their own
• Ions and most polar molecules such as glucose can’t cross a
lipid bilayer on their own – they cross cell membranes only
with the help of transport proteins in the bilayer
Selective Permeability
ANIMATION: Selective permeability
Tonicity
• If cells can’t compensate for differences in tonicity between
cytoplasm and external fluid, volume and solute concentration
of cytoplasm changes as water diffuses into or out of the cell
• Cells in a hypertonic solution shrink as water moves out
• Cells in a hypotonic solution swell as water moves in
Tonicity
Tonicity
2%
sucrose
A
2% sucrose
10% sucrose
water
Fig. 5.19a, p. 85
Red Blood Cells
• In isotonic, hypertonic, and hypotonic solutions
Red Blood Cells
B Red blood cells
immersed in an isotonic
solution do not change in
volume. The fluid portion
of blood is typically
isotonic with cytoplasm.
C Red blood cells
immersed in a hypertonic
solution shrivel up
because more water
diffuses out of the cells
than into them.
D Red blood cells
immersed in a
hypotonic solution
swell up because more
water diffuses into the
cells than out of them.
Fig. 5.19b-d, p. 85
ANIMATION: Tonicity and water
movement
Turgor
• Cell walls of plants and many protists, fungi, and bacteria can
resist an increase in the volume of cytoplasm even in
hypotonic environments
• Osmotic pressure is the amount of turgor (fluid pressure
against a cell membrane or wall) that stops osmosis
• If turgor inside plant cells decreases, the plant wilts
Key Terms
• turgor
• Pressure that a fluid exerts against a wall, membrane, or
other structure that contains it
• osmotic pressure
• Amount of turgor that prevents osmosis into cytoplasm or
other hypertonic fluid
Key Concepts
• Movement of Fluids
• Gradients drive the directional movements of substances
across membranes
• Water tends to diffuse across selectively permeable
membranes, including cell membranes, to regions where
solute concentration is higher
ANIMATION: Plasma Membranes: Simple
Diffusion
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5.7 Membrane-Crossing Mechanisms
• Gases, water, and small nonpolar molecules can diffuse
across a lipid bilayer
• Most other molecules, and ions, cross only with the help of
transport proteins, which allow a cell or membrane-enclosed
organelle to control which substances enter and exit
• Each type of transport protein can move a specific ion or
molecule
Passive Transport
• Passive transport proteins work without an energy input;
solute movement is driven by the concentration gradient
• passive transport
• A concentration gradient drives movement of a solute
across a cell membrane through a transport protein
• Requires no energy input
• Example: glucose transporter
Passive Transport
1. Glucose binds to a
glucose transporter
2. Binding causes the
transport protein to
change shape
3. Glucose detaches
from transport protein,
and protein resumes
its original shape
Passive Transport
Fig. 5.20.1, p. 86
Passive Transport
2
Fig. 5.20.2, p. 86
Passive Transport
3
Fig. 5.20.3, p. 86
ANIMATION: Passive transport
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Active Transport
• Active transport proteins such as calcium pumps use energy
(ATP) to pump a solute against its concentration gradient
• active transport
• Energy-requiring mechanism by which a transport protein
pumps a solute across a cell membrane against its
concentration gradient
Active Transport: Calcium Pump
(A) Two calcium ions
bind to the transport
protein
Active Transport: Calcium Pump
Sarcoplasmic
Reticulum
Cytoplasm
calcium
A
Fig. 5.21a, p. 87
Active Transport: Calcium Pump
(B) Energy (phosphate
group) is transferred
from ATP to the protein
Transfer causes protein
to change shape and
eject calcium ions to the
opposite side of the
membrane
Active Transport: Calcium Pump
(C) After it loses the
calcium ions, the
transport protein
resumes its original
shape
ANIMATION: Active transport
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Cotransport
• Cotransporters are active transport proteins that move two
substances at the same time, in the same or opposite
directions across a membrane
• Example: sodium–potassium pumps
• Sodium ions (Na+) in the cytoplasm are pumped to
extracellular fluid by active transport
• Potassium ions (K+) from extracellular fluid bind to the
channel are released into the cytoplasm
Sodium-Potassium Pump
Sodium-Potassium Pump
Extracellular
Fluid
Cytoplasm
Stepped Art
Fig. 5.22, p. 87
ANIMATION: Plasma Membranes:
Facilitated Diffusion
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ANIMATION: Plasma Membranes: Active
Transport
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ANIMATION: Active transport
ANIMATION: Passive transport
5.8 Membrane Trafficking
• Bulk substances and large particles move across plasma
membranes by endocytosis and exocytosis
• In exocytosis, a cytoplasmic vesicle fuses with the plasma
membrane, and contents are released outside of the cell
• In phagocytosis and other processes of endocytosis, a
patch of plasma membrane balloons into the cell, and forms a
vesicle that sinks into the cytoplasm
Key Terms
• exocytosis
• Process by which a cell expels a vesicle’s contents to
extracellular fluid
• endocytosis
• Process by which a cell takes in extracellular fluid by
inward ballooning of plasma membrane
• phagocytosis
• “Cell eating;” an endocytic pathway by which a cell engulfs
particles such as microbes or cellular debris
Endocytosis and Exocytosis
Endocytosis
Exocytosis
A Molecules or
particles enter pits
in the plasma
membrane.
pit
B The pits sink
inward and
become
endocytic
vesicles.
C Vesicle
contents are
sorted.
D Many of
the sorted
molecules
cycle back to
the plasma
membrane.
E Some vesicles
are routed to the
nuclear envelope
or ER membrane.
Others fuse with
Golgi bodies.
F Some vesicles
and their contents
are delivered to
lysosomes.
Endocytosis
and Exocytosis
Fig. 5.23, p. 88
Endocytosis
Exocytosis
A Molecules get
concentrated
inside coated pits
at the plasma
membrane.
D Many of
the sorted
molecules
cycle to the
plasma
membrane.
coated pit
B The pits
sink inward
and become
endocytic
vesicles.
E Some vesicles
are routed to the
nuclear envelope
or ER membrane.
Others fuse with
Golgi bodies.
C Vesicle
contents
are sorted.
F Some vesicles
and their
contents are
delivered to
lysosomes.
Endocytosis
and Exocytosis
lysosome
Golgi
Stepped Art
Fig. 5.23, p. 88
ANIMATION: Membrane cycling
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Endocytosis of Lipoproteins
Endocytosis of Lipoproteins
plasma
membrane
Fig. 5.24a, p. 88
Endocytosis of Lipoproteins
aggregates of
lipoproteins
Fig. 5.24b, p. 88
Phagocytosis
Phagocytosis
A Pseudopods surround a
pathogen (brown).
B Endocytic
vesicle forms.
C Lysosome fuses with
vesicle; enzymes digest
pathogen.
D Cell uses the
digested material
or expels it.
Stepped Art
Fig. 5.25, p. 89
ANIMATION: Phagocytosis
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Plasma Membrane
• Membrane proteins and lipids are made and modified in the
ER – both become part of vesicles that transport them to
Golgi bodies for final modification
• Finished proteins and lipids are repackaged as new vesicles
that travel to the plasma membrane, fuse with it, and become
part of the plasma membrane
Key Concepts
• Membrane Trafficking
• Transport proteins that work with or against gradients
adjust or maintain solute concentrations
• Large packets of substances move across the plasma
membrane by processes of endocytosis and exocytosis
A Toast to Alcohol Dehydrogenase (revisited)
• Alcohol dehydrogenase (ADH) converts ethanol to
acetaldehyde, an organic molecule even more toxic than
ethanol
• A different enzyme, ALDH, very quickly converts
acetaldehyde to nontoxic acetate
• Defects in ADH or ALDH affect alcohol metabolism
ANIMATION: Endocytosis and exocytosis