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EXPLORING LIFE & A TOUR
OF THE CELL
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Exploring Life
• Biology
– Is the scientific study of life
• We recognize life
– By what living things do
Figure 1.1
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Some properties of life
(a) Order
(b) Evolutionary
adaptation
(c) Response to the
environment
(d) Regulation
(e) Energy
processing
(f) Growth and
development
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(g) Reproduction
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From the biosphere to organisms
1 The biosphere
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From cells to molecules
9 Organelles
1 µm
Cell
8 Cells
Atoms
10 µm
10 Molecules
7 Tissues
50 µm
6 Organs and organ systems
Figure 1.3
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A Closer Look at Ecosystems
• Each organism
– Interacts with its environment
• Both organism and environment
– Are affected by the interactions between them
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A Closer Look at Cells
• The cell
– Is the lowest level of organization that can
perform all activities required for life
Figure 1.5
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25 µm
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The Cell’s Heritable Information
• Cells contain chromosomes made partly of
DNA, the substance of genes
– Which program the cells’ production of proteins
and transmit information from parents to
offspring
Sperm cell
Nuclei
containing
DNA
Egg cell
Fertilized egg
with DNA from
both parents
Figure 1.6
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Embyro’s cells
with copies of
inherited DNA
Offspring with traits
inherited from
both parents
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The molecular structure of DNA
• Accounts for it information-rich nature
Nucleus
DNA
Cell
Nucleotide
Figure 1.7
(a) DNA double helix. This model shows
each atom in a segment of DNA.Made
up of two long chains of building
blocks called nucleotides, a DNA
molecule takes the three-dimensional
form of a double helix.
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A
C
T
A
T
A
C
C
G
T
A
G
T
A
(b) Single strand of DNA. These geometric shapes and
letters are simple symbols for the nucleotides in a
small section of one chain of a DNA molecule.
Genetic information is encoded in specific sequences
of the four types of nucleotides (their names are
abbreviated here as A, T, C, and G).
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Two Main Forms of Cells
• All cells share certain characteristics
– They are all enclosed by a membrane
– They all use DNA as genetic information
• There are two main forms of cells
– Eukaryotic
– Prokaryotic
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• Eukaryotic cells
– Are subdivided by internal membranes into
various membrane-enclosed organelles
• Prokaryotic cells
– Lack the kinds of membrane-enclosed
organelles found in eukaryotic cells
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EUKARYOTIC CELL
PROKARYOTIC CELL
DNA
Membrane
(no nucleus)
Membrane
Cytoplasm
Organelles
Figure 1.8
Nucleus (contains DNA)
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1 µm
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Biologists explore life across its great diversity of species
• Diversity is a hallmark of life
Figure 1.13
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Grouping Species: The Basic Idea
• Taxonomy
– Is the branch of biology that names and
classifies species according to a system of
broader and broader groups
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Classifying life
Species Genus Family
Order
Class
Phylum
Kingdom
Domain
Ursus
americanus
(American
black bear)
Ursus
Ursidae
Carnivora
Mammalia
Chordata
Animalia
Figure 1.14
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Eukarya
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The Three Domains of Life
• At the highest level, life is classified into three
domains
– Bacteria
– Archaea
– Eukarya
• Domain Bacteria and domain Archaea
– Consist of prokaryotes
• Domain Eukarya, the eukaryotes
– Includes the various protist kingdoms and the
kingdoms Plantae, Fungi, and Animalia
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Life’s three domains
Bacteria are the most diverse
4 µm
and widespread prokaryotes
and are now divided among multiple
kingdoms. Each of the rod-shaped
structures in this photo is a bacterial cell.
DOMAIN ARCHAEA
Figure 1.15
Many of the prokaryotes known
0.5 µm
as archaea live in Earth‘s
extreme environments, such as salty lakes
and boiling hot springs. Domain Archaea
includes multiple kingdoms. The photo
shows a colony composed of many cells.
Protists (multiple kingdoms)
100 µm
are unicellular eukaryotes and
their relatively simple multicellular
relatives.Pictured here is an assortment of
protists inhabiting pond water. Scientists are
currently debating how to split the protists
into several kingdoms that better represent
evolution and diversity.
Kingdom Plantae consists of
multicellula eukaryotes that carry
out photosynthesis, the conversion
of light energy to food.
Kindom Fungi is defined in part by the
nutritional mode of its members, such
as this mushroom, which absorb
nutrientsafter decomposing organic
material.
Kindom Animalia consists of
multicellular eukaryotes that
ingest other organisms.
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Unity in the Diversity of Life
• As diverse as life is
– There is also evidence of remarkable unity
15 µm
1.0 µm
Cilia of Paramecium.
The cilia of Paramecium
propel the cell through
pond water.
5 µm
Cross section of cilium, as viewed
with an electron microscope
Cilia of windpipe cells. The cells that line the human windpipe
are equipped with cilia that help keep the lungs clean by moving
a film of debris-trapping mucus upward.
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A Tour of the Cell
Overview: The Importance of Cells
• All organisms are made of cells
• The cell is the simplest collection of matter
that can live
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Cell structure is correlated to cellular function
10 µm
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To study cells, biologists use microscopes and the
tools of biochemistry
1m
Human height
Length of some
nerve and
muscle cells
0.1 m
Chicken egg
1 cm
Frog egg
1 mm
Most plant
and Animal cells
10 µ m
Measurements
1 centimeter (cm) = 102 meter
(m) = 0.4 inch
1 millimeter (mm) = 10–3 m
1 micrometer (µm) = 10–3 mm =
10–6 m
1 nanometer (nm) = 10–3 mm =
10–9 m
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Nucleus
Most bacteria
Mitochondrion
1µm
Smallest bacteria
100 nm
Viruses
10 nm
Ribosomes
Electron microscope
100 µm
Electron microscope
– Can be used to visualize
different sized cellular
structures
Light microscope
• Different types of
microscopes
Unaided eye
10 m
Proteins
1 nm
Lipids
Small molecules
0.1 nm
Atoms
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Use different methods for enhancing visualization
of cellular structures
TECHNIQUE
RESULT
(a)
Brightfield (unstained specimen).
Passes light directly through specimen.
Unless cell is naturally pigmented or
artificially stained, image has little
contrast. [Parts (a)–(d) show a
human cheek epithelial cell.]
50 µm
(b)
Brightfield (stained specimen).
Staining with various dyes enhances
contrast, but most staining procedures
require that cells be fixed (preserved).
(c)
Phase-contrast. Enhances contrast
in unstained cells by amplifying
variations in density within specimen;
especially useful for examining living,
unpigmented cells.
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(d)
Differential-interference-contrast (Nomarski).
Like phase-contrast microscopy, it uses optical
modifications to exaggerate differences in
density, making the image appear almost 3D.
(e) Fluorescence. Shows the locations of specific
molecules in the cell by tagging the molecules
with fluorescent dyes or antibodies. These
fluorescent substances absorb ultraviolet
radiation and emit visible light, as shown
here in a cell from an artery.
50 µm
(f) Confocal. Uses lasers and special optics for
“optical sectioning” of fluorescently-stained
specimens. Only a single plane of focus is
illuminated; out-of-focus fluorescence above
and below the plane is subtracted by a computer.
A sharp image results, as seen in stained nervous
tissue (top), where nerve cells are green, support
cells are red, and regions of overlap are yellow. A
standard fluorescence micrograph (bottom) of this
relatively thick tissue is blurry.
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µm23
Electron microscopes (EMs)
• The scanning electron microscope (SEM)
– Provides for detailed study of the surface of a
specimen
TECHNIQUE
RESULTS
1 µm
Cilia
(a) Scanning electron microscopy (SEM). Micrographs taken
with a scanning electron microscope show a 3D image of the
surface of a specimen. This SEM
shows the surface of a cell from a
rabbit trachea (windpipe) covered
with motile organelles called cilia.
Beating of the cilia helps move
inhaled debris upward toward
the throat.
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The transmission electron microscope (TEM)
• Provides for detailed study of the internal ultrastructure of
cells
Longitudinal
section of
cilium
Cross section
of cilium
1 µm
(b) Transmission electron microscopy (TEM). A transmission electron
microscope profiles a thin section of a
specimen. Here we see a section through
a tracheal cell, revealing its ultrastructure.
In preparing the TEM, some cilia were cut
along their lengths, creating longitudinal
sections, while other cilia were cut straight
across, creating cross sections.
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Isolating Organelles by Cell Fractionation
Homogenization
Tissue
cells
1000 g
(1000 times the
force of gravity)
10 min
Homogenate
Differential centrifugation
Supernatant poured
into next tube
20,000 g
20 min
80,000 g
60 min
Pellet rich in
nuclei and
cellular debris
150,000 g
3 hr
Pellet rich in
mitochondria
(and chloroplasts if cells
are from a
plant)
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Pellet rich in
“microsomes”
(pieces of
plasma membranes and
cells’ internal
membranes)
Pellet rich in
ribosomes
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Comparing Prokaryotic and Eukaryotic Cells
• All cells have several basic features in common
– They are bounded by a plasma membrane
– They contain a semifluid substance called the
cytosol
– They contain chromosomes
– They all have ribosomes
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Prokaryotic cells: Do not contain a nucleus; Have their DNA
located in a region called the nucleoid.
Pili: attachment structures on
the surface of some prokaryotes
Nucleoid: region where the
cell’s DNA is located (not
enclosed by a membrane)
Ribosomes: organelles that
synthesize proteins
Plasma membrane: membrane
enclosing the cytoplasm
Cell wall: rigid structure outside
the plasma membrane
Bacterial
chromosome
(a) A typical
rod-shaped bacterium
Capsule: jelly-like outer coating
of many prokaryotes
0.5 µm
Flagella: locomotion
organelles of
some bacteria
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(b) A thin section through the
bacterium Bacillus coagulans
(TEM)
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Eukaryotic cells: Contain a true nucleus, bounded by a
membranous nuclear envelope; Are generally quite a bit
bigger than prokaryotic cells
•
The plasma membrane:
Functions as a selective barrier
Outside of cell
Allows sufficient passage
of nutrients and waste
Carbohydrate side chain
Hydrophilic
region
Inside of cell
0.1 µm
Hydrophobic
region
(a)
TEM of a plasma
membrane. The
plasma membrane,
here in a red blood
cell, appears as a
pair of dark bands
separated by a
light band.
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Hydrophilic
region
Phospholipid
Proteins
(b) Structure of the plasma membrane
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• A animal cell
ENDOPLASMIC RETICULUM (ER)
Rough ER
Smooth ER
Nuclear envelope
NUCLEUS
Nucleolus
Chromatin
Flagelium
Plasma membrane
Centrosome
CYTOSKELETON
Microfilaments
Intermediate filaments
Ribosomes
Microtubules
Microvilli
Golgi apparatus
Peroxisome
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publishing as Benjamin Cummings
Lysosome
In animal cells but not plant cells:
Lysosomes
Centrioles
Flagella (in some plant sperm)
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A plant cell
Nuclear envelope
Nucleolus
Chromatin
NUCLEUS
Centrosome
Rough
endoplasmic
reticulum Smooth
endoplasmic
reticulum
Ribosomes (small brwon dots)
Central vacuole
Tonoplast
Golgi apparatus
Microfilaments
Intermediate
filaments
CYTOSKELETON
Microtubules
Mitochondrion
Peroxisome
Plasma membrane
Chloroplast
Cell wall
Plasmodesmata
Wall of adjacent cell
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In plant cells but not animal cells:
Chloroplasts
Central vacuole and tonoplast
Cell wall
Plasmodesmata
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Concept : The eukaryotic cell’s genetic instructions
are housed in the nucleus and carried out by the
ribosomes
• The Nucleus: Genetic Library of the Cell
• The nucleus
– Contains most of the genes in the
eukaryotic cell
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The nuclear envelope
• Encloses the nucleus, separating its contents from the
Nucleus
cytoplasm
Nucleus
1 µm
Nucleolus
Chromatin
Nuclear envelope:
Inner membrane
Outer membrane
Nuclear pore
Pore
complex
Rough ER
Surface of nuclear
envelope.
1 µm
Ribosome
0.25 µm
Close-up of
nuclear
envelope
Pore complexes (TEM).
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Nuclear lamina (TEM).
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Ribosomes: Protein Factories in the Cell
– Are particles made of ribosomal RNA
and protein; Carry out protein synthesis
Ribosomes
ER
Cytosol
Endoplasmic reticulum (ER)
Free ribosomes
Bound ribosomes
Large
subunit
0.5 µm
TEM showing ER and ribosomes
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Small
subunit
Diagram of a ribosome
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Concept : The endomembrane system regulates
protein traffic and performs metabolic functions in
the cell
• The endomembrane system
– Includes many different structures
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The Endoplasmic Reticulum: Biosynthetic Factory
Accounts for more than half the total
membrane in many eukaryotic cells
• The ER membrane
Is continuous with the
nuclear envelope
Smooth ER
Rough ER
ER lumen
Cisternae
Ribosomes
Transport vesicle
Smooth ER
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Nuclear
envelope
Transitional ER
Rough ER
200 µm
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There are two distinct regions of ER
– Smooth ER, which lacks ribosomes
– Rough ER, which contains ribosomes
• The smooth ER: Synthesizes lipids;
Metabolizes carbohydrates; Stores calcium;
Detoxifies poison
• The rough ER: Has bound ribosomes;
Produces proteins and membranes, which are
distributed by transport vesicles
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The Golgi Apparatus: Shipping and
Receiving Center
• The Golgi apparatus
– Receives many of the transport vesicles
produced in the rough ER
– Consists of flattened membranous sacs called
cisternae
• Functions of the Golgi apparatus include
– Modification of the products of the rough ER
– Manufacture of certain macromolecules
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Functions of the Golgi apparatus
cis face
(“receiving” side of
Golgi apparatus)
Golgi
apparatus
6 Vesicles also
transport certain
proteins back to ER
1 Vesicles move
from ER to Golgi
5 Vesicles transport specific
proteins backward to newer
Golgi cisternae
2 Vesicles coalesce to
form new cis Golgi cisternae
0.1 0 µm
Cisternae
3 Cisternal
maturation:
Golgi cisternae
move in a cisto-trans
direction
4 Vesicles form and
leave Golgi, carrying
specific proteins to
other locations or to
the plasma membrane for secretion
trans face
(“shipping” side of
Golgi apparatus)
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TEM of Golgi apparatus
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Lysosomes: Digestive Compartments
Membranous sac of hydrolytic
enzymes; Can digest all
kinds of macromolecules
1 µm
Nucleus
• Carry out intracellular
• digestion by
• phagocytosis
Lysosome
Lysosome contains
active hydrolytic
enzymes
Food vacuole
fuses with
lysosome
Hydrolytic
enzymes digest
food particles
Digestive
enzymes
Lysosome
Plasma membrane
Digestion
Figure 6.14 A
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Food vacuole
(a) Phagocytosis: lysosome digesting food
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Vacuoles: Diverse Maintenance Compartments
• A plant or fungal cell
– May have one or several vacuoles
• Food vacuoles
– Are formed by phagocytosis
• Contractile vacuoles
– Pump excess water out of protist cells
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• Central vacuoles
– Are found in plant cells
– Hold reserves of important organic
compounds and water
Central vacuole
Cytosol
Tonoplast
Nucleus
Central
vacuole
Cell wall
Chloroplast
5 µm
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The Endomembrane System: A Review
• Relationships among organelles of the
endomembrane system
1
Nuclear envelope is
connected to rough ER,
which is also continuous
with smooth ER
Nucleus
Rough ER
2
3
Membranes and proteins
produced by the ER flow in
the form of transport vesicles
to the Golgi
Smooth ER
cis Golgi
Nuclear envelop
Golgi pinches off transport
Vesicles and other vesicles
that give rise to lysosomes and
Vacuoles
Plasma
membrane
trans Golgi
4
Lysosome available
for fusion with another
vesicle for digestion
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5 Transport vesicle carries
proteins to plasma
membrane for secretion
6
Plasma membrane expands
by fusion of vesicles; proteins
are secreted from cell
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Concept: Mitochondria and chloroplasts change
energy from one form to another
• Mitochondria
– Are the sites of cellular respiration
• Chloroplasts
– Found only in plants, are the sites of
photosynthesis
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I-1- 44
Mitochondria: Chemical Energy Conversion
• Are found in nearly all eukaryotic cells
• Mitochondria are enclosed by two membranes
– A smooth outer membrane; An inner membrane folded
into cristae
Mitochondrion
Intermembrane space
Outer
membrane
Free
ribosomes
in the
mitochondrial
matrix
Inner
membrane
Cristae
Matrix
Mitochondrial
DNA
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100 µm
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Chloroplasts: Capture of Light Energy
• The chloroplast is a specialized member of a family of
closely related plant organelles called plastids
– Contains chlorophyll; Are found in leaves and other
green organs of plants and in algae
Chloroplast
Ribosomes
Stroma
Chloroplast
DNA
Inner and outer
membranes
Granum
1 µm
Thylakoid
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Peroxisomes: Oxidation
– Produce hydrogen peroxide and convert it to water
Chloroplast
Peroxisome
Mitochondrion
1 µm
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Concept: The cytoskeleton is a network of fibers that
organizes structures and activities in the cell
• Cytoskeleton is a network of fibers extending throughout
the cytoplasm
Microtubule
0.25 µm
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Microfilaments
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Roles of the Cytoskeleton: Support, Motility, and
Regulation
– Is involved in cell motility, which utilizes motor proteins
ATP
Vesicle
Receptor for
motor protein
Motor protein
Microtubule
(ATP powered)
of cytoskeleton
(a) Motor proteins that attach to receptors on organelles can “walk”
the organelles along microtubules or, in some cases, microfilaments.
Vesicles
Microtubule
0.25 µm
(b) Vesicles containing neurotransmitters migrate to the tips of nerve cell
axons via the mechanism in (a). In this SEM of a squid giant axon, two
vesicles can be seen moving along a microtubule. (A separate part of the
experiment provided the evidence that they were in fact moving.)
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Components of the Cytoskeleton
• There are three main types of fibers that make up the
cytoskeleton
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Microtubules
– Shape the cell
– Guide movement of organelles
– Help separate the chromosome copies in
dividing cells
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Centrosomes and Centrioles
• The centrosome is considered to be a “microtubule-
organizing center”; Contains a pair of centrioles
Centrosome
Microtubule
Centrioles
0.25 µm
Figure 6.22
Longitudinal section
of one centriole
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Microtubules
Cross section
of the other centriole
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Cilia and Flagella
– Contain specialized arrangements of
microtubules
– Are locomotor appendages of some cells
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Flagella beating pattern
(a) Motion of flagella. A flagellum
usually undulates, its snakelike
motion driving a cell in the same
direction as the axis of the
flagellum. Propulsion of a human
sperm cell is an example of
flagellatelocomotion (LM).
Direction of swimming
1 µm
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Ciliary motion
(b) Motion of cilia. Cilia have a backand-forth motion that moves the
cell in a direction perpendicular
to the axis of the cilium. A dense
nap of cilia, beating at a rate of
about 40 to 60 strokes a second,
covers this Colpidium, a
freshwater protozoan (SEM).
15 µm
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Cilia and flagella share a common ultrastructure
Outer microtubule
doublet
Dynein arms
0.1 µm
Plasma
membrane
Central
microtubule
Outer doublets
cross-linking
proteins inside
Microtubules
Radial
spoke
Plasma
membrane
Basal body
(b)
0.5 µm
(a)
0.1 µm
Triplet
(c)
Cross section of basal body
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Microfilaments (Actin Filaments)
– Are built from molecules of the protein actin
– Are found in microvilli
Microvillus
Plasma membrane
Microfilaments (actin
filaments)
Intermediate filaments
0.25 µm
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Microfilaments that function in cellular motility
• Contain the protein myosin in addition to actin
Muscle cell
Actin filament
Myosin filament
Myosin arm
(a) Myosin motors in muscle cell contraction.
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Amoeboid movement
• Involves the contraction of actin and myosin filaments
Cortex (outer cytoplasm):
gel with actin network
Inner cytoplasm: sol
with actin subunits
Extending
pseudopodium
(b) Amoeboid movement
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Cell Walls of Plants
– Is an extracellular structure of plant cells that
distinguishes them from animal cells
– Are made of cellulose fibers embedded in other
polysaccharides and protein; May have multiple layers
Central
vacuole
of cell
Plasma
membrane
Secondary
cell wall
Primary
cell wall
Central
vacuole
of cell
Middle
lamella
1 µm
Central vacuole
Cytosol
Plasma membrane
Plant cell walls
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Plasmodesmata
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The Extracellular Matrix (ECM) of Animal Cells
• Animal cells lack cell walls and covered by an elaborate
matrix, the ECM.
– Is made up of glycoproteins and other macromolecules
EXTRACELLULAR FLUID
Collagen
A proteoglycan
complex
Polysaccharide
molecule
Carbohydrates
Core
protein
Fibronectin
Plasma
membrane
Integrin
Integrins
Microfilaments
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Proteoglycan
molecule
CYTOPLASM
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Functions of the ECM include
– Support
– Adhesion
– Movement
– Regulation
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Intercellular Junctions
– Plants: Plasmodesmata
– Are channels that perforate plant cell walls
Cell walls
Interior
of cell
Interior
of cell
0.5 µm
Plasmodesmata
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Plasma membranes
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Animals: Tight Junctions, Desmosomes, and Gap Junctions
• In animals, there are three types of intercellular
junctions
– Tight junctions
– Desmosomes
– Gap junctions
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I-1- 64
• Types of intercellular junctions in animals
TIGHT JUNCTIONS
Tight junction
Tight junctions prevent
fluid from moving
across a layer of cells
0.5 µm
At tight junctions, the membranes of
neighboring cells are very tightly pressed
against each other, bound together by
specific proteins (purple). Forming continuous seals around the cells, tight junctions
prevent leakage of extracellular fluid across
A layer of epithelial cells.
DESMOSOMES
Desmosomes (also called anchoring
junctions) function like rivets, fastening cells
Together into strong sheets. Intermediate
Filaments made of sturdy keratin proteins
Anchor desmosomes in the cytoplasm.
Tight junctions
Intermediate
filaments
Desmosome
Gap
junctions
Space
between Plasma membranes
cells
of adjacent cells
1 µm
Extracellular
matrix
Gap junction
0.1 µm
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GAP JUNCTIONS
Gap junctions (also called communicating
junctions) provide cytoplasmic channels from
one cell to an adjacent cell. Gap junctions
consist of special membrane proteins that
surround a pore through which ions, sugars,
amino acids, and other small molecules may
pass. Gap junctions are necessary for communication between cells in many types of tissues,
including heart muscle and animal embryos.
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The Cell: A Living Unit Greater Than the Sum of Its Parts
5 µm
• Cells rely on the integration of structures and
organelles in order to function
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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