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Chapter 3
The Molecules of Life
PowerPoint® Lectures for
Campbell Essential Biology, Fourth Edition
– Eric Simon, Jane Reece, and Jean Dickey
Campbell Essential Biology with Physiology, Third Edition
– Eric Simon, Jane Reece, and Jean Dickey
Lectures by Chris C. Romero, updated by Edward J. Zalisko
© 2010 Pearson Education, Inc.
Biology and Society:
Got Lactose?
• Lactose is the main sugar found in milk.
• Some adults exhibit lactose intolerance, the inability to properly
digest lactose.
• Lactose-intolerant individuals are unable to digest lactose
properly.
– Lactose is broken down by bacteria in the large intestine producing
gas and discomfort.
© 2010 Pearson Education, Inc.
• There is no treatment for the underlying cause of lactose
intolerance.
• Affected people must
– Avoid lactose-containing foods or
– Take the enzyme lactase when eating dairy products
© 2010 Pearson Education, Inc.
ORGANIC COMPOUNDS
• Carbon forms large, complex, and diverse molecules necessary
for life’s functions.
• Organic compounds are carbon-based molecules.
• Carbon is a versatile atom.
– It has four electrons in an outer shell that holds eight.
– Carbon can share its electrons with other atoms to form up to four
covalent bonds.
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Carbon skeletons vary in length
Double bond
Carbon skeletons may have double bonds,
which can vary in location
Carbon skeletons may be unbranched or branched
Carbon skeletons may be arranged in rings
Figure 3.1
Carbon skeletons vary in length
Figure 3.1a
Double bond
Carbon skeletons may have double bonds,
which can vary in location
Figure 3.1b
Carbon skeletons may be unbranched or branched
Figure 3.1c
Carbon skeletons may be arranged in rings
Figure 3.1d
Giant Molecules from Smaller Building Blocks
• On a molecular scale, many of life’s molecules are gigantic,
earning the name macromolecules.
• Three categories of macromolecules are
– Carbohydrates
– Proteins
– Nucleic acids
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• Most macromolecules are polymers.
• Polymers are made by stringing together many smaller molecules
called monomers.
• A dehydration reaction
– Links two monomers together
– Removes a molecule of water
• Organisms also have to break down macromolecules.
• Hydrolysis
– Breaks bonds between monomers
– Adds a molecule of water
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Short polymer
Monomer
Dehydration
reaction
Hydrolysis
Longer polymer
a Building a polymer chain
b Breaking a polymer chain
Figure 3.4
Short polymer
Monomer
Dehydration
reaction
Longer polymer
a Building a polymer chain
Figure 3.4a
Hydrolysis
b Breaking a polymer chain
Figure 3.4b
LARGE BIOLOGICAL MOLECULES
• There are four categories of large molecules in cells:
– Carbohydrates
– Lipids
– Proteins
– Nucleic acids
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Carbohydrates
• Carbohydrates are sugars or sugar polymers. They include
– Small sugar molecules in soft drinks
– Long starch molecules in pasta and potatoes
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Monosaccharides
• Monosaccharides are simple sugars that cannot be broken down
by hydrolysis into smaller sugars.
• Common examples are
– Glucose in sports drinks
– Fructose found in fruit
• Glucose and fructose are isomers, molecules that have the same
molecular formula but different structures.
• Monosaccharides are the main fuels for cellular work.
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Glucose
Fructose
C6H12O6
C6H12O6
Isomers
Figure 3.5a
b Abbreviated
ring structure
a Linear and ring structures
Figure 3.6
Disaccharides
• A disaccharide is
– A double sugar
– Constructed from two monosaccharides
– Formed by a dehydration reaction
• Disaccharides include
– Lactose in milk
– Maltose in beer, malted milk shakes, and malted milk ball candy
– Sucrose in table sugar
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Galactose
Glucose
Lactose
Figure 3.7
• Sucrose is
– The main carbohydrate in plant sap
– Rarely used as a sweetener in processed foods
• High-fructose corn syrup is made by a commercial process that
converts natural glucose in corn syrup to much sweeter fructose.
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processed to extract
Starch
broken down into
Glucose
converted to sweeter
Fructose
added to foods as
high-fructose corn syrup
Ingredients: carbonated water,
high-fructose corn syrup,
caramel color, phosphoric acid,
natural flavors
Figure 3.8
Polysaccharides
• Polysaccharides are
– Complex carbohydrates
– Made of long chains of sugar units and polymers of
monosaccharides
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Glucose
monomer
Starch granules
a Starch
Glycogen
granules
b Glycogen
Cellulose fibril
Cellulose
molecules
c Cellulose
Figure 3.9
• Starch is
– Used by plant cells to store energy
– Potatoes and grains are major sources.
• Glycogen is
– Used by animals cells to store energy
– Converted to glucose when it is needed
• Cellulose
– Is the most abundant organic compound on Earth
– Forms cable-like fibrils in the tough walls that enclose plants
– Cannot be broken apart by most animals
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• Monosaccharides and disaccharides dissolve readily in water.
• Cellulose does not dissolve readily in water.
• Almost all carbohydrates are hydrophilic, or “water-loving,”
adhering water to their surface.
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Lipids
• Lipids are
–
Neither macromolecules nor polymers
–
Hydrophobic, unable to mix with water
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Oil (hydrophobic)
Vinegar (hydrophilic)
Figure 3.10
Fats
• A typical fat, or triglyceride, consists of a glycerol molecule
joined with three fatty acid molecules via a dehydration reaction.
• Fats perform essential functions in the human body including
– Energy storage
– Cushioning
– Insulation
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Fatty acid
Glycerol
(a) A dehydration reaction linking a fatty acid to glycerol
(b) A fat molecule with a glycerol “head” and three
energy-rich hydrocarbon fatty acid “tails”
Figure 3.11
• If the carbon skeleton of a fatty acid has
– Fewer than the maximum number of hydrogens, it is unsaturated
– The maximum number of hydrogens, then it is saturated
• A saturated fat has no double bonds, and all three of its fatty acids
are saturated.
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• Most animal fats
– Have a high proportion of saturated fatty acids
– Can easily stack, tending to be solid at room temperature
– Contribute to atherosclerosis, a condition in which lipidcontaining plaques build up within the walls of blood vessels
– Most plant oils tend to be low in saturated fatty acids and liquid at
room temperature.
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• Hydrogenation
– Adds hydrogen
– Converts unsaturated fats to saturated fats
– Makes liquid fats solid at room temperature
– Creates trans fat, a type of unsaturated fat that is even less healthy
than saturated fats
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TYPES OF FATS
Saturated Fats
Unsaturated Fats
Margarine
INGREDIENTS: SOYBEAN OIL, FULLY HYDROGENATED
COTTONSEED OIL, PARTIALLY HYDROGENATED
COTTONSEED OIL AND SOYBEAN OILS, MONO AND
DIGLYCERIDES, TBHO AND CITRIC ACID
Plant oils
Trans fats
ANTIOXIDANTS
Omega-3 fats
Figure 3.12
Steroids
• Steroids are very different from fats in structure and function.
– The carbon skeleton is bent to form four fused rings.
• Cholesterol is
– A key component of cell membranes
– The “base steroid” from which your body produces other steroids,
such as estrogen and testosterone
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Cholesterol
Testosterone
A type of estrogen
Figure 3.13
Proteins
• Proteins
– Are polymers constructed from amino acid monomers
– Perform most of the tasks the body needs to function
– Form enzymes, chemicals that change the rate of a chemical
reaction without being changed in the process
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MAJOR TYPES OF PROTEINS
Structural Proteins
Storage Proteins
Contractile Proteins
Transport Proteins
Enzymes
Figure 3.15
The Monomers of Proteins: Amino Acids
• All proteins are constructed from a common set of 20 kinds of
amino acids.
• Each amino acid consists of a central carbon atom bonded to four
covalent partners in which three of those attachment groups are
common to all amino acids.
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Amino
group
Carboxyl
group
Side
group
a The general structure of an amino acid
Hydrophobic
side group
Hydrophilic
side group
Leucine
Serine
b Examples of amino acids with hydrophobic and hydrophilic
side groups
Figure 3.16
Proteins as Polymers
• Cells link amino acids together by dehydration reactions, forming
peptide bonds and creating long chains of amino acids called
polypeptides.
© 2010 Pearson Education, Inc.
Carboxyl
group
Amino
group
Side
group
Side
group
Amino acid
Amino acid
Dehydration reaction
Side
group
Side
group
Peptide bond
Figure 3.17-2
• Your body has tens of thousands of different kinds of protein.
• Proteins differ in their arrangement of amino acids.
• The specific sequence of amino acids in a protein is its primary
structure.
© 2010 Pearson Education, Inc.
15
5
1
10
30
35
20
25
45
40
50
55
65
60
70
75
Amino acid
85
80
95
100
90
110
115
105
125
120
129
Figure 3.18
SEM
1
2
Normal red blood cell
3
4
5
6
7. . . 146
Normal hemoglobin
SEM
a Normal hemoglobin
1
Sickled red blood cell
2
3
4
5
6
7. . . 146
Sickle-cell hemoglobin
b Sickle-cell hemoglobin
Figure 3.19
Protein Shape
• A functional protein consists of one or more polypeptide chains,
precisely folded and coiled into a molecule of unique shape.
• A protein’s three-dimensional shape
– Recognizes and binds to another molecule
– Enables the protein to carry out its specific function in a cell
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Target
Protein
Figure 3.21
What Determines Protein Shape?
• A protein’s shape is sensitive to the surrounding environment.
• Unfavorable temperature and pH changes can cause
denaturation of a protein, in which it unravels and loses its
shape.
• High fevers (above 104º F) in humans can cause some proteins to
denature.
• Misfolded proteins are associated with
– Alzheimer’s disease
– Mad cow disease
– Parkinson’s disease
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Nucleic Acids
• Nucleic acids
– Are macromolecules that provide the directions for building
proteins
– Include DNA and RNA
– Are the genetic material that organisms inherit from their parents
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• DNA resides in cells in long fibers called chromosomes.
• A gene is a specific stretch of DNA that programs the amino acid
sequence of a polypeptide.
• The chemical code of DNA must be translated from “nucleic acid
language” to “protein language.”
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Gene
DNA
Nucleic acids
RNA
Amino acid
Protein
Figure 3.22
•
Nucleic acids are polymers of nucleotides.
•
Each nucleotide has three parts:
–
A five-carbon sugar
–
A phosphate group
–
A nitrogenous base
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Nitrogenous base
A, G, C, or T
Thymine T
Phosphate
group
Phosphate
Base
Sugar
deoxyribose
a Atomic structure
Sugar
b Symbol used in this book
Figure 3.23
• Each DNA nucleotide has one of the following bases:
– Adenine (A)
– Guanine (G)
– Thymine (T)
– Cytosine (C)
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Adenine A
Thymine T
Guanine G
Cytosine C
Figure 3.24a
Adenine A
Guanine G
Thymine T Cytosine C
Space-filling model of DNA
Figure 3.24b
Sugar-phosphate
backbone
Base
Nucleotide
pair
Hydrogen
bond
Bases
a DNA strand
polynucleotide
b Double helix
two polynucleotide strands
Figure 3.25
• Two strands of DNA join together to form a double helix.
• Bases along one DNA strand hydrogen-bond to bases along the
other strand.
• The functional groups hanging off the base determine which bases
pair up:
– A only pairs with T.
– G can only pair with C.
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• RNA, ribonucleic acid, is different from DNA.
– RNA is usually single-stranded but DNA usually exists as a double
helix.
– RNA uses the sugar ribose and the base uracil (U) instead of
thymine (T).
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Nitrogenous base
A, G, C, or U
Uracil U
Phosphate
group
Sugar ribose
Figure 3.26
Large biological
molecules
Carbohydrates
Functions
Components
Examples
Monosaccharides:
glucose, fructose
Disaccharides:
lactose, sucrose
Polysaccharides:
starch, cellulose
Dietary energy;
storage; plant
structure
Monosaccharide
Lipids
Long-term
energy storage
fats;
hormones
steroids
Fatty acid
Glycerol
Components of
a triglyceride
Amino
group
Proteins
Enzymes, structure,
storage, contraction,
transport, and others
Fats triglycerides;
Steroids
testosterone,
estrogen
Carboxyl
group
Side
group
Lactase
an enzyme,
hemoglobin
a transport protein
Amino acid
Phosphate
Base
Nucleic acids
Information
storage
DNA, RNA
Sugar
Nucleotide
Figure UN3-2
Carbohydrates
Functions
Components
Examples
Monosaccharides:
glucose, fructose
Disaccharides:
lactose, sucrose
Polysaccharides:
starch, cellulose
Dietary energy;
storage; plant
structure
Monosaccharide
Figure UN3-2a
Lipids
Functions
Long-term
energy storage
fats;
hormones
steroids
Components
Fatty acid
Glycerol
Components of
a triglyceride
Examples
Fats triglycerides;
Steroids
testosterone,
estrogen
Figure UN3-2b
Proteins
Functions
Components
Amino
group
Enzymes, structure,
storage, contraction,
transport, and others
Examples
Carboxyl
group
Side
group
Lactase
an enzyme,
hemoglobin
a transport protein
Amino acid
Figure UN3-2c
Nucleic acids
Functions
Components
Examples
Phosphate
Base
Information
storage
DNA, RNA
Sugar
Nucleotide
Figure UN3-2d
Chapter 4
A Tour of the Cell
PowerPoint® Lectures for
Campbell Essential Biology, Fourth Edition
– Eric Simon, Jane Reece, and Jean Dickey
Campbell Essential Biology with Physiology, Third Edition
– Eric Simon, Jane Reece, and Jean Dickey
Lectures by Chris C. Romero, updated by Edward J. Zalisko
© 2010 Pearson Education, Inc.
Biology and Society:
Drugs That Target Bacterial Cells
• Antibiotics were first isolated from mold in 1928.
• The widespread use of antibiotics drastically decreased deaths
from bacterial infections.
• Most antibiotics kill bacteria while minimally harming the human
host by binding to structures found only on bacterial cells.
• Some antibiotics bind to the bacterial ribosome, leaving human
ribosomes unaffected.
• Other antibiotics target enzymes found only in the bacterial cells.
© 2010 Pearson Education, Inc.
Colorized TEM
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Figure 4.00
THE MICROSCOPIC WORLD OF CELLS
• Organisms are either
– Single-celled, such as most prokaryotes and protists or
– Multicelled, such as plants, animals, and most fungi
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LM
Light Micrograph (LM)
(for viewing living cells)
Light micrograph of a protist, Paramecium
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Figure 4.1a
Colorized SEM
Scanning Electron Micrograph (SEM)
(for viewing surface features)
Scanning electron micrograph of Paramecium
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Figure 4.1b
Colorized TEM
Transmission Electron Micrograph (TEM)
(for viewing internal structures)
Transmission electron micrograph of Paramecium
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Figure 4.1c
10 m
1m
Length of some
nerve and
muscle cells
10 cm
Chicken egg
Unaided eye
Human height
1 cm
Frog eggs
Plant and
animal cells
10 mm
1 mm
100 nm
Nucleus
Most bacteria
Mitochondrion
Smallest bacteria
Viruses
Ribosomes
10 nm
Electron microscope
100 mm
Light microscope
1 mm
Proteins
Lipids
1 nm
0.1 nm
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Small molecules
Atoms
Figure 4.3
The Two Major Categories of Cells
• The countless cells on earth fall into two categories:
– Prokaryotic cells — Bacteria and Archaea
– Eukaryotic cells — plants, fungi, and animals
• All cells have several basic features.
– They are all bound by a thin plasma membrane.
– All cells have DNA and ribosomes, tiny structures that build
proteins.
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• Prokaryotic cells are older than eukaryotic cells.
– Prokaryotes appeared about 3.5 billion years ago.
– Eukaryotes appeared about 2.1 billion years ago.
• Prokaryotic and eukaryotic cells have important differences.
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• Prokaryotes
– Are smaller than eukaryotic cells
– Lack internal structures surrounded by membranes
– Lack a nucleus
– Have a rigid cell wall
• Eukaryotes
– Only eukaryotic cells have organelles, membrane-bound
structures that perform specific functions.
– The most important organelle is the nucleus, which houses most of
a eukaryotic cell’s DNA.
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Plasma membrane
(encloses cytoplasm)
Cell wall (provides
Rigidity)
Capsule (sticky
coating)
Colorized TEM
Prokaryotic
flagellum
(for propulsion)
Ribosomes
(synthesize
proteins)
Nucleoid
(contains DNA)
Pili (attachment structures)
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Figure 4.4
Plasma membrane
(encloses cytoplasm)
Cell wall (provides rigidity)
Capsule (sticky coating)
Prokaryotic
flagellum
(for propulsion)
Ribosomes
(synthesize
proteins)
Nucleoid
(contains DNA)
Pili (attachment structures)
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Figure 4.4a
Colorized TEM
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Figure 4.4b
An Overview of Eukaryotic Cells
• Eukaryotic cells are fundamentally similar.
• The region between the nucleus and plasma membrane is the
cytoplasm.
• The cytoplasm consists of various organelles suspended in fluid.
• Unlike animal cells, plant cells have
– Protective cell walls
– Chloroplasts, which convert light energy to the chemical energy of
food
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Ribosomes
Cytoskeleton
Centriole
Lysosome
Not in most
plant cells
Flagellum
Plasma
membrane
Nucleus
Mitochondrion
Rough
endoplasmic
reticulum (ER)
Golgi
apparatus
Idealized animal cell
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Smooth
endoplasmic
reticulum (ER)
Figure 4.5a
Cytoskeleton
Central
vacuole
Cell wall
Chloroplast
Mitochondrion
Nucleus
Not in
animal cells
Rough endoplasmic
reticulum (ER)
Ribosomes
Plasma
membrane
Smooth
endoplasmic
reticulum (ER)
Channels between
cells
Golgi apparatus
Idealized plant cell
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Figure 4.5b
MEMBRANE STRUCTURE
• The plasma membrane separates the living cell from its nonliving
surroundings.
• The membranes of cells are composed mostly of
– Lipids
– Proteins
• The lipids belong to a special category called phospholipids.
• Phospholipids form a two-layered membrane, the phospholipid
bilayer.
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Outside of cell
Hydrophilic
head
Hydrophobic
tail
Phospholipid
Cytoplasm (inside of cell)
(a) Phospholipid bilayer of membrane
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Figure 4.6a
Outside of cell
Proteins
Hydrophilic
region of
protein
Phospholipid
bilayer
Hydrophilic
head
Hydrophobic
tail
Hydrophobic
regions of
protein
Cytoplasm (inside of cell)
(b) Fluid mosaic model of membrane
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Figure 4.6b
• The plasma membrane is a fluid mosaic:
– Fluid because molecules can move freely past one another
– A mosaic because of the diversity of proteins in the membrane
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Cell Surfaces
• Plant cells have rigid cell walls surrounding the membrane.
• Plant cell walls
– Are made of cellulose
– Protect the cells
– Maintain cell shape
– Keep the cells from absorbing too much water
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THE NUCLEUS AND RIBOSOMES:
GENETIC CONTROL OF THE CELL
• The nucleus is the chief executive of the cell.
– Genes in the nucleus store information necessary to produce
proteins.
– Proteins do most of the work of the cell.
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Structure and Function of the Nucleus
• The nucleus is bordered by a double membrane called the nuclear
envelope.
• Pores in the envelope allow materials to move between the
nucleus and cytoplasm.
• The nucleus contains a nucleolus where ribosomes are made.
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Nuclear
envelope
Nucleolus
Surface of nuclear envelope
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Pore
TEM
Chromatin
TEM
Ribosomes
Nuclear pores
Figure 4.8
Ribosomes
• Ribosomes are responsible for protein synthesis.
• Ribosome components are made in the nucleolus but assembled in
the cytoplasm.
• Ribosomes may assemble proteins:
– Suspended in the fluid of the cytoplasm or
– Attached to the outside of an organelle called the endoplasmic
reticulum
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Ribosome
mRNA
Protein
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Figure 4.10
The Endoplasmic Reticulum
• The endoplasmic reticulum (ER) is one of the main
manufacturing facilities in a cell.
• The ER
– Produces an enormous variety of molecules
– Is composed of smooth and rough ER
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Nuclear
envelope
Ribosomes
TEM
Rough ER
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Smooth ER
Ribosomes
Figure 4.13
Rough ER
• The “rough” in the rough ER is due to ribosomes that stud the
outside of the ER membrane.
• These ribosomes produce membrane proteins and secretory
proteins.
• After the rough ER synthesizes a molecule, it packages the
molecule into transport vesicles.
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Proteins are
often modified in
the ER.
Secretory
proteins depart in
transport vesicles.
Ribosome
Vesicles bud off
from the ER.
Transport
vesicle
Protein
A ribosome
links amino acids
into a
polypeptide.
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Rough ER
Polypeptide
Figure 4.14
Smooth ER
• The smooth ER
– Lacks surface ribosomes
– Produces lipids, including steroids
– Helps liver cells detoxify circulating drugs
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The Golgi Apparatus
• The Golgi apparatus
– Works in partnership with the ER
– Receives, refines, stores, and distributes chemical products of the
cell
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Transport vesicle
from rough ER
“Receiving” side of
Golgi apparatus
New
vesicle
forming
Transport
vesicle
from the
Golgi
“Shipping” side of
Golgi apparatus
Plasma
membrane
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Figure 4.15a
Colorized SEM
“Receiving” side of Golgi apparatus
New vesicle forming
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Figure 4.15b
Lysosomes
• A lysosome is a sac of digestive enzymes found in animal cells.
• Enzymes in a lysosome can break down large molecules such as
– Proteins
– Polysaccharides
– Fats
– Nucleic acids
• Lysosomes can also
– Destroy harmful bacteria
– Break down damaged organelles
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• Lysosomes have several types of digestive functions.
– Many cells engulf nutrients in tiny cytoplasmic sacs called food
vacuoles.
– These food vacuoles fuse with lysosomes, exposing food to
enzymes to digest the food.
– Small molecules from digestion leave the lysosome and nourish
the cell.
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Plasma membrane
Digestive enzymes
Lysosome
Lysosome
Digestion
Food vacuole
Vesicle containing
damaged organelle
(a) Lysosome digesting food
Digestion
(b) Lysosome breaking down the molecules of damaged
organelles
Organelle fragment
Vesicle containing two
damaged organelles
TEM
Organelle fragment
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Figure 4.16
• Central vacuoles of plants
– Store nutrients
– Absorb water
– May contain pigments or poisons
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TEM
Vacuole filling with water
TEM
Vacuole contracting
(a) Contractile vacuole in Paramecium
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Figure 4.17a
Colorized TEM
Central vacuole
(b) Central vacuole in a plant cell
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Figure 4.17b
Golgi
apparatus
Rough ER
Transport
vesicle
Transport vesicles
carry enzymes and
other proteins from
the rough ER to the
Golgi for processing.
Transport
vesicle
Plasma
membrane
Secretory
protein
Some products
are secreted
from the cell.
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Vacuole
Lysosomes carrying
digestive enzymes can
Lysosome fuse with other vesicles.
Vacuoles store some
cell products.
Figure 4.18a
CHLOROPLASTS AND MITOCHONDRIA:
ENERGY CONVERSION
• Cells require a constant energy supply to perform the work of life.
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Chloroplasts
• Most of the living world runs on the energy provided by
photosynthesis.
• Photosynthesis is the conversion of light energy from the sun to
the chemical energy of sugar.
• Chloroplasts are the organelles that perform photosynthesis.
– Grana, stroma, thylakoids, etc…
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Inner and outer
membranes
Space between
membranes
Granum
TEM
Stroma (fluid in chloroplast)
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Figure 4.19
Mitochondria
• Mitochondria are the sites of cellular respiration, which produce
ATP from the energy of food molecules.
• Mitochondria are found in almost all eukaryotic cells.
• An envelope of two membranes encloses the mitochondrion.
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TEM
Outer
membrane
Inner
membrane
Cristae
Matrix
Space between
membranes
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Figure 4.20
THE CYTOSKELETON: CELL SHAPE AND
MOVEMENT
• The cytoskeleton is a network of fibers extending throughout the
cytoplasm.
• The cytoskeleton
– Provides mechanical support to the cell
– Maintains its shape
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LM
(a) Microtubules
in the cytoskeleton
LM
(b) Microtubules
and movement
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Figure 4.21
Cilia and Flagella
• Cilia and flagella aid in movement.
– Flagella propel the cell in a whiplike motion.
– Cilia move in a coordinated back-and-forth motion.
– Cilia and flagella have the same basic architecture.
• Cilia may extend from nonmoving cells.
• On cells lining the human trachea, cilia help sweep mucus out of
the lungs.
© 2010 Pearson Education, Inc.
Colorized SEM
(a) Flagellum of a human sperm cell
© 2010 Pearson Education, Inc.
Colorized SEM
Colorized SEM
(b) Cilia on a protist
(c) Cilia lining the
respiratory tract
Figure 4.22
Evolution Connection:
The Evolution of Antibiotic Resistance
• Many antibiotics disrupt cellular structures of invading
microorganisms.
• Introduced in the 1940s, penicillin worked well against such
infections.
• But over time, bacteria that were resistant to antibiotics were
favored.
• The widespread use and abuse of antibiotics continues to favor
bacteria that resist antibiotics.
© 2010 Pearson Education, Inc.