PowerLecture:%20Chapter%203

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PowerLectureChapter 3

• Cells and
• How They Work

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Learning Objectives

• Understand the basic parts of eukaryotic cells.
• Understand the essential structure and function
  of the cell membrane.
• Know the forces that cause water and solutes to
  move across membranes passively and by active
  transport.
• Understand how material can be imported into or
  exported from a cell by being wrapped in
  membranes.

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Learning Objectives (contd)

• Describe the nucleus of eukaryotes with respect
  to structure and function.
• Describe the organelles associated with the
  endomembrane system, and tell the general
  function of each.
• Describe the cytoskeleton of eukaryotes and
  distinguish it from the endomembrane system.
• Define a metabolic pathway and the types of
  substances that participate in it.

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Learning Objectives (contd)

• Characterize an enzyme and what type of cofactors
  may be needed for its functioning.
• Define ATP and describe the pathways for its
  formation within the cell.
• Describe the process of cellular respiration with
  special reference to the quantity of ATP
  produced.

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Impacts/Issues

• When Mitochondria Spin Their Wheels

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When Mitochondria Spin Their Wheels

• Mitochondria are specialized compartments in the
  cell that produce energy.
• Mitochondrial disorders can cause
• reduced energy for cell use.
• Lufts syndrome is a rare disorder in which
• the mitochondria are misshapen and do
• not produce enough ATP.
• Many mitochondrial disorders exist, but are rare
  this means that pharmaceutical companies have
  little financial incentive to develop drugs for
  treatment.

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How Would You Vote?

• To conduct an instant in-class survey using a
  classroom response system, access JoinIn Clicker
  Content from the PowerLecture main menu.
• Should pharmaceutical companies receive financial
  incentives (such as tax breaks) to search for
  cures for diseases that affect only a small
  number of people?
• a. Yes, those that suffer from any disease, even
  if it is rare, deserve treatment.
• b. No, the public shouldn't subsidize this
  research - let market forces take their course.

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Section 1

• What Is a Cell?

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What is a Cell?

• The cell theory has three generalizations
• All organisms are composed of one or more cells.
• The cell is the smallest unit having the
  properties of life.
• All cells come from pre-existing cells.

Figure 3.3
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What is a Cell?

• All cells are alike in three ways.
• A plasma membrane separates each cell from the
  environment, but also allows the flow of
  molecules across the membrane.
• DNA carries the hereditary instructions.
• The cytoplasm containing a semifluid matrix
  (cytosol) and organelles is located between the
  plasma membrane and the region of DNA.

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What is a Cell?

• There are two basic kinds of cells.
• Prokaryotic cells (bacteria)
• do not have a separation of
• the DNA from the remainder
• of the cell parts.
• Eukaryotic cells have a
• definite nucleus and
• membrane-bound organelles.

cytoplasm
DNA
plasma membrane
Figure 3.1
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What is a Cell?

• Why are cells small?
• Most cells are so small they can only be seen by
  using light and electron microscopes.
• Cells are necessarily small so that the
  surface-to-volume ratio remains low this means
  that the interior will not be so
• extensive that it cannot
• exchange materials
• efficiently through the
• plasma membrane.

Figure 3.2
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What is a Cell?

• Membranes enclose cells and organelles.
• A large portion of the cell membrane is composed
  of phospholipids, each of which possesses a
  hydrophilic head and two hydrophobic tails.
• If phospholipid molecules are surrounded by
  water, their hydrophobic fatty acid tails cluster
  and a lipid bilayer results hydrophilic heads
  are at the outer faces of a two-layer sheet with
  the hydrophobic tails shielded inside.

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fluid
fluid
one layer of lipids
cross-section through lipid bilayer
one layer of lipids
Figure 3.4
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Section 2

• The Parts of an Eukaryotic Cell

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The Parts of a Eukaryotic Cell

• All eukaryotic cells contain organelles.
• Organelles form compartmentalized portions of the
  cytoplasm.
• Organelles separate reactions with respect to
  time (allowing proper sequencing) and space
  (allowing incompatible reactions to occur in
  close proximity).

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Fig. 3.5, p. 44
nuclear envelope
CYTOSKELTON
nucleolus
NUCLEUS
DNA in nucleoplasm
microtubules

RIBOSOMES
microfilaments
intermediate filaments

ROUGH ER
MITOCHONDRION
SMOOTH ER
CENTRIOLES
PLASMA MEMBRANE
GOLGI BODY
LYSOSOME
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PLASMA MEMBRANE
ENDOPLASMIC RETICULUM (ER)
GOLGI BODY
LYSOSOME
nuclear envelope
nucleolus
MITOCHONDRION
NUCLEUS
Fig. 3.6, p. 45
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Section 3

• The Plasma Membrane A Double Layer of Lipids

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The Plasma Membrane

• The plasma membrane is a mix of lipids and
  proteins.
• Bilayers of phospholipids, interspersed with
  glycolipids and cholesterol, are the structural
  foundation of cell membranes.
• Within a bilayer, phospholipids show quite a bit
  of movement they diffuse sideways, spin, and
  flex their tails to prevent close packing and
  promote fluidity, which also results from
  short-tailed lipids and unsaturated tails (kinks
  at double bonds).

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The Plasma Membrane

• Proteins perform most of the functions of cell
  membranes.
• The scattered islands of protein in the sea of
  lipids create a mosaic effect.
• Membrane proteins (most are glycoproteins) serve
  as enzymes, transport proteins, receptor
  proteins, and recognition proteins.

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Fig. 3.7, p. 46
EXTRACELLULAR FLUID
receptor protein
adhesion protein
recognition protein
cholesterol
phospholipid
LIPID BILAYER
cytoskeletal proteins just beneath the plasma
membrane
CYTOPLASM
transport proteins
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Section 4

• How Do We See Cells?

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How Do We See Cells?

• Microscopy allows us to see cells and their
  pieces.
• Many types of microscopes exist, which can
  produce many types of pictures (micrographs)
• Light microscopes use
• light to see samples
• specimens usually must
• be thin and colored with
• dyes to be seen.

Figure 3.8a
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How Do We See Cells?

• Electron microscopes use beams of electrons
  rather than light to see details transmission
  and scanning electron microscopy can magnify
  (enlarge) specimens far beyond the limits of the
  light microscope.

Figure 3.8b-c
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Animation How an Electron Microscope Works
CLICKTO PLAY
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Section 5

• The Nucleus

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The Nucleus

• The nucleus encloses DNA, the building code for
  cellular proteins.
• Its membrane isolates DNA from the sites
  (ribosomes in the cytoplasm) where proteins will
  be assembled.
• The nuclear membrane helps regulate the exchange
  of signals between the nucleus and the cytoplasm.

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The Nucleus

• A nuclear envelope encloses the nucleus.
• The nuclear envelope consists of two lipid
  bilayers with pores.
• The envelope membranes are continuous with the
  endoplasmic reticulum (ER).

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nuclear pore (protein complex that spans both
lipid bilayers)
one of two lipid bilayers (facing cytoplasm)
NUCLEAR ENVELOPE
one of two lipid bilayers (facing nucleoplasm)
Fig. 3.10, p. 49
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The Nucleus

• The nucleolus is where cells make the units of
  ribosomes.
• The nucleolus appears as a dense mass inside the
  nucleus.
• In this region, subunits of ribosomes are
  prefabricated before shipment out of the
  nucleus.

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The Nucleus

• DNA is organized in chromosomes.
• Chromatin describes the cells collection of DNA
  plus the proteins associated with it.
• Each chromosome is one DNA molecule and its
  associated proteins.

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The Nucleus

• Events that begin in the nucleus continue to
  unfold in the cell cytoplasm.
• Outside the nucleus, new polypeptide chains for
  proteins are assembled on ribosomes.
• Some proteins are stockpiled others enter the
  endomembrane system.

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Nucleus of an Animal Cell
Figure 3.9
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Section 6

• The Endomembrane System

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The Endomembrane System

• ER is a protein and lipid assembly line.
• The endoplasmic reticulum is a collection of
  interconnected tubes and flattened sacs,
  continuous with the nuclear membrane.
• Rough ER consists of stacked, flattened sacs with
  many ribosomes attached oligosaccharide groups
  are attached to polypeptides as they pass through
  on their way to other organelles, membranes, or
  to be secreted from the cell.

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The Endomembrane System

• Smooth ER has no ribosomes it is the area from
  which vesicles carrying proteins and lipids are
  budded it also inactivates harmful chemicals and
  aids in muscle contraction.
• Golgi bodies finish, pack, and ship.
• In the Golgi body, proteins and lipids undergo
  final processing, sorting, and packaging.
• The Golgi bodies resemble stacks of flattened
  sacs whose edges break away as vesicles.

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The Endomembrane System

• A variety of vesicles move substances into and
  through cells.
• Lysosomes are vesicles that bud from Golgi
  bodies they carry powerful enzymes that can
  digest the contents of other vesicles, worn-out
  cell parts, or bacteria and foreign particles.
• Peroxisomes are membrane-bound sacs of enzymes
  that break down fatty acids and amino acids.

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Fig. 3.11ab, p. 50
RNA messages from the nucleus
vesicle
cytoplasm
ribosome
vesicle
inside nucleus
rough ER
nuclear envelope
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Fig. 3.11c-g, p. 51
Secretory pathway ends
endocytic pathway begins
smooth ER channel, cross-section
budding vesicle
plasma membrane
smooth ER
Golgi body
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Section 7

• Mitochondria The Cells Energy Factories

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Mitochondria

• Mitochondria make ATP.
• Mitochondria are the primary organelles for
  transferring the energy in carbohydrates to ATP
  they are found only in eukaryotic cells.
• Oxygen is required for the release of this
  energy.

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Mitochondria

• ATP forms in an inner compartment of the
  mitochondrion.
• Each mitochondrion has compartments formed by
  inner folded membranes (cristae) surrounded by a
  smooth outer membrane.
• Mitochondria have their own DNA and some
• ribosomes, which leads scientists to believe
  they
• may have evolved from ancient bacteria.

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Fig. 3.12, p. 52
cristae
outer compartment
inner compartment
outer mitochondrial membrane
inner membrane
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Section 8

• The Cells Skeleton

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The Cells Skeleton

• The cytoskeleton is an interconnected system of
  bundled fibers, slender threads, and lattices
  extending from the nucleus to the plasma membrane
  in the cytosol.
• The main components are microtubules,
  microfilaments, and intermediate filamentsall
  assembled from protein subunits.
• The skeleton helps organize and reinforce the
  cell and serves in some cell functions.

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microtubules
microfilaments
intermediate filaments
Figure 3.13
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The Cells Skeleton

• Movement is one function of the cytoskeleton.
• Microtubular extensions of the plasma membrane
  display a 9 2 cross-sectional array and are
  useful in propulsion.
• Flagella are quite long, whiplike, and are found
  on animal sperm cells.
• Cilia are shorter, more numerous, and may
  function as sweeps to clear, as an example, the
  respiratory tract of dust or other materials.
• The microtubules of flagella and cilia arise from
  centrioles, which play a role in cell division.

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Fig. 3.13, p. 53
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Fig. 3.14, p. 53
one of nine pairs of microtubules
microtubules near base of flagellum or cilium
plasma membrane
basal body in cytoplasm
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Section 9
How Diffusion and Osmosis Move Substances Across
Membranes
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Diffusion and Osmosis

• The plasma membrane is selective.
• Lipid-soluble molecules and small, electrically
  neutral molecules (for example, oxygen, carbon
  dioxide, and ethanol) cross easily through the
  lipid bilayer.
• Larger molecules (such as glucose) and charged
  ions (such as Na, Ca, HCO3-) must be moved by
  membrane transport proteins.
• Because some molecules pass through on their own
  and others must be transported, the plasma
  membrane is said to have the property of
  selective permeability.

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Selective Permeability
Figure 3.15
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Diffusion and Osmosis

• In diffusion, a solute moves down a concentration
  gradient.
• A concentration gradient is established when
  there is a difference in the number of molecules
  or ions of a given substance between two adjacent
  regions.
• Molecules constantly collide and tend to move
  from areas of high concentration to areas of low
  concentration.
• The net movement of like molecules down a
  concentration gradient (high to low) is called
  diffusion when this occurs across a plasma
  membrane, it is called passive transport.

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Diffusion and Osmosis

• Molecules move faster
• when gradients are
• steep, and different
• solutes move
• independently according
• to their respective
• gradients.
• Electric gradients
• (gradients of electrical
• charge) are important
• to nerve function

dye
dye
water
Figure 3.16
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Diffusion and Osmosis

• Water crosses membranes by osmosis.
• Osmosis is the passive diffusion of water across
  a differentially permeable membrane in response
  to solute concentration gradients.

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Fig. 3.17, p. 55
selectively permeable membrane between two
compartments
protein molecule
water molecule
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Diffusion and Osmosis

• Osmotic movements are affected by the relative
  concentrations of solutes in the fluids inside
  and outside the cell (tonicity).
• An isotonic fluid has the same concentration of
  solutes as the fluid in the cell immersion in it
  causes no net movement of water.
• A hypotonic fluid has a lower concentration of
  solutes than does the fluid in the cell cells
  immersed in it may swell as water moves into the
  cell down its gradient.
• A hypertonic fluid has a greater concentration of
  solutes than does the fluid in the cell cells in
  it may shrivel as water moves out of the cell,
  again down its gradient.

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Fig. 3.18, p. 55
98 water 2 sucrose
90 water 10 sucrose
98 water 2 sucrose
100 water (distilled)
HYPOTONICCONDITIONS
HYPERTONICCONDITIONS
ISOTONICCONDITIONS
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Section 10
Other Ways Substances Cross Cell Membranes
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Crossing Cell Membranes

• Many solutes cross membranes through transport
  proteins.
• In facilitated diffusion, solutes pass through
  channel proteins in accordance with the
  concentration gradient this process requires no
  input of energy.
• Channel proteins are open to both sides of the
  membrane and undergo changes in shape during the
  movement of solutes.
• The transport proteins are selective for what
  they allow through the membrane.

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glucose, more concentrated outside cell than
inside
transport protein for glucose
When the glucose binding site is again
vacant, the protein resumes its original shape.
Glucose binds to a vacant site inside the
channel through the transport protein.
Glucose detaches from the binding site and
diffuses out of the channel.
Now the protein changes shape. Part of the
channel closes behind the solute. Another part
opens in front of it.
Fig. 3.20, p. 56
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d When the glucose binding site is again
vacant, the protein resumes its original shape.
a Glucose binds to a vacant site inside the
channel through the transport protein.
c Glucose detaches from the binding site and
diffuses out of the channel.
b Now the protein changes shape. Part of the
channel closes behind the solute. Another part
opens in front of it.
Stepped Art
Fig. 3.20, p. 56
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Crossing Cell Membranes

• In active transport, solutes move against their
  concentration gradients with the assistance of
  transport proteins that change their shape with
  the energy supplied by ATP.

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higher concentration of calcium outside cell
lower concentration of calcium inside cell
The pump returns toits resting shape.
ATP binds to a calcium pump.
Shape change permits calcium release at
opposite side of membrane. Phosphate group and
ADP are released.
Calcium enters tunnel through pump.
ATP transfers a phosphate group to pump. This
energy input will cause pumps shape to change.
Fig. 3.21, p. 57
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e The pump returns toits resting shape.
a ATP binds to a calcium pump.
d Shape change permits calcium release at
opposite side of membrane. Phosphate group and
ADP are released.
b Calcium enters tunnel through pump.
c ATP transfers a phosphate group to pump. This
energy input will cause pumps shape to change.
Stepped Art
Fig. 3.21, p. 57
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High
Concentration gradient
ATP
Low
Passive transport of water-soluble
substances through channel protein no energy
input needed
Active transport through ATPase requires
energy input from ATP
Diffusion of lipid-soluble substances across
bilayer
Fig. 3.19, p. 56
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Crossing Cell Membranes

• Vesicles transport large solutes.
• Exocytosis moves substances from the cytoplasm to
  the plasma membrane during secretion, moving
  materials out of the cell.
• Endocytosis encloses particles in small portions
  of plasma membrane to form vesicles that then
  move into the cytoplasm if this process brings
  organic material into the cell, it is called
  phagocytosis.

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Fig. 3.22, p. 57
plasma membrane
exocytic vesicle leaving cytoplasm
endocytic vesicle forming
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Section 11
Metabolism Doing Cellular Work
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Metabolism Doing Cellular Work

• ATP is the cells energy currency.
• Metabolism refers to all of the chemical
  reactions that occur in cells ATP links the
  whole of these reactions together.
• ATP is composed of adenine, ribose, and three
  phosphate groups.
• ATP transfers energy in many different chemical
  reactions almost all metabolic pathways directly
  or indirectly run on energy supplied by ATP.
• ATP can donate a phosphate group
  (phosphorylation) to another molecule, which then
  becomes primed and energized for specific
  reactions.

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Metabolism Doing Cellular Work

• The ATP/ADP cycle is a method for renewing the
  supply of ATP that is constantly being used up in
  the cell it couples inorganic phosphate to ADP
  to form energized ATP.

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Fig. 3.23, p. 58
base
ATP
three phosphate groups
cellular work (e.g., synthesis, breakdown, or
rearrangement of substances contraction of
muscle cells active transport across a cell
membrane)
sugar
ATP
reactions that require energy
reactions that release energy
ADP Pi
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Metabolism Doing Cellular Work

• There are two main types of metabolic pathways.
• Metabolic pathways form series of interconnected
  reactions that regulate the concentration of
  substances within cells.
• In anabolism, small molecules are assembled into
  large moleculesfor example, simple sugars are
  assembled into complex carbohydrates.
• In catabolism, large molecules such as
  carbohydrates, lipids, and proteins are broken
  down to form products of lower energy, releasing
  energy for cellular work.

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Metabolism Doing Cellular Work

• Pathways exist as enzyme-mediated linear or
  circular sequences of reactions involving the
  following
• Reactants are the substances that enter a
  reaction.
• Intermediates are substances that form between
  the start and conclusion of a metabolic pathway.
• End products are the substances present at the
  conclusion of the pathway.

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Stepped Art
80
enzyme 1
end product
enzyme 2
enzyme 3
Stepped Art
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Metabolism Doing Cellular Work

• Enzymes play a vital role in metabolism.
• Enzymes are proteins that serve as catalysts
  they speed up reactions.
• Enzymes have several features in common
• Enzymes do not make anything happen that could
  not happen on its own they just make it happen
  faster.
• Enzymes can be reused.
• Enzymes act upon specific substrates, molecules
  which are recognized and bound at the enzymes
  active site.

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two substrate molecules
substrates contacting active site of enzyme
active site

substrates briefly bind tightly to enzyme
active site
product molecule
enzyme unchanged by the reaction
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two substrate molecules
substrates contacting active site of enzyme

substrates briefly bind tightly to enzyme
active site
enzyme unchanged by the reaction
Stepped Art
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Metabolism Doing Cellular Work

• Because enzymes operate best within defined
  temperature ranges, high temperatures decrease
  reaction rate by disrupting the bonds that
  maintain three-dimensional shape (denaturation
  occurs).
• Most enzymes function best at a pH near 7 higher
  or lower values disrupt enzyme shape and halt
  function.

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Metabolism Doing Cellular Work

• Coenzymes are large organic molecules such as
  NAD and FAD (both derived from vitamins), which
  transfer protons and electrons from one
  substrate to another to assist with many chemical
  reactions.

Figure 3.29
86
Section 12
How Cells Make ATP
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How Cells Make ATP

• Cellular respiration makes ATP.
• Electrons acquired by the breakdown of
  carbohydrates, lipids, and proteins are used to
  form ATP.
• Overall, the formation of ATP occurs by cellular
  respiration in humans this is an aerobic process
  meaning it requires oxygen.

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How Cells Make ATP

• Step 1 Glycolysis breaks glucose down to
  pyruvate.
• Glycolysis reactions occur in the cytoplasm and
  result in the breakdown of glucose to pyruvate,
  generating small amounts of ATP.
• Glucose is first phosphorylated in
  energy-requiring steps, then split to form two
  molecules of PGAL.
• Four ATP are produced by phosphorylation in
  subsequent reactions but because two ATP were
  used previously, there is a net gain of only two
  ATP by the end of glycolysis.
• Glycolysis does not use oxygen.

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GLUCOSE
ATP
ADP
P
Energy in(2 ATP)
ATP
ADP
P
P
PGAL
P
P
INTERMEDIATES DONATEPHOSPHATE TO ADP, MAKING 4
ATP
To second set of reactions
Pyruvate
ATP
NET ENERGY YIELD 2
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How Cells Make ATP

• Step 2 The Krebs cycle produces energy-rich
  transport molecules.
• Pyruvate (produced in the cytoplasm) enters the
  mitochondria for the oxygen requiring steps of
  cellular respiration.
• The pyruvate is converted to acetyl-CoA, which
  enters the Krebs cycle to eventually be converted
  to CO2.

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How Cells Make ATP

• Reactions within the mitochondria and the Krebs
  cycle serve three important functions
• Two molecules of ATP are produced by
  substrate-level phosphorylation.
• Intermediate compounds are regenerated to keep
  the Krebs cycle going.
• H and e- are transferred to NAD and FAD,
  generating NADH and FADH2.

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How Cells Make ATP

• Step 3 Electron transport produces many ATP
  molecules.
• The final stage of cellular
• respiration occurs in the
• electron transport
• systems embedded in
• the inner membranes
• (cristae) of the mitochondrion.

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How Cells Make ATP

• NADH and FADH2 from previous reactions give up
  their electrons to transport (enzyme) systems
  embedded in the mitochondrial inner membrane.
• Electrons flow through the system eventually to
  oxygen, forming water as they flow, H are
  pumped into the outer compartment of the
  mitochondrion to create a proton gradient.
• H ions move down their gradient, through a
  channel protein called ATP synthase, in the
  process driving the synthesis of ATP.

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How Electron Transport Forms ATP
Figure 3.26
95
Section 13
Summary of Cellular Respiration
96
Summary of Cellular Respiration

• In total, glycolysis, the Krebs cycle, and the
  electron transport system can yield a maximum of
  36 ATP per glucose molecule.

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Fig. 3.27, p. 62
CYTOPLASM
glucose
4
ATP
2
ATP
GLYCOLYSIS
energy input to start reactions
(2 ATP net)
e- H
2 pyruvate
2 NADH
MITOCHONDRION
e- H
2 CO2
2 NADH
e- H
4 CO2
8 NADH
KREBS CYCLE
2
e- H
ATP
2 FADH2
ELECTRON TRANSPORT SYSTEM
e-
ATP
32
water
H
e- oxygen
TYPICAL ENERGY YIELD 36 ATP
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Section 14
Alternative Energy Sources in the Body
99
Alternative Energy Sources

• How the body uses carbohydrates as fuel.
• Excess carbohydrate intake is stored as glycogen
  in liver and muscle for future use.
• Free glucose is used until it runs low then
  glycogen reserves are tapped.
• Under some conditions a process called lactate
  fermentation can be used to produce ATP here,
  pyruvate is converted
• directly to lactic acid with
• production of quick, but
• limited, energy.

Figure 3.28
100
Alternative Energy Sources

• Fats and proteins also provide energy.
• Lipids are used when carbohydrate supplies run
  low.
• Excess fats are stored away in cells of adipose
  tissue.
• Fats are digested into glycerol (which enters
  glycolysis) and fatty acids, which enter the
  Krebs cycle.
• Because fatty acids have many more carbon and
  hydrogen atoms, they are degraded more slowly and
  yield greater amounts of ATP.

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Alternative Energy Sources

• Proteins are used as the last resort for
  supplying energy to the body.
• Amino acids are released by enzymatic digestion
  of proteins protein is never stored by the body.
• After the amino group is removed, the amino acid
  remnant is fed into the Krebs cycle to produce
  energy (ATP), or is used to make fats and
  carbohydrates.
• Ammonia (from the amino group) is excreted as
  waste.

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