Chapter 3
Cell Structure
BIOLOGY: Today and Tomorrow, 4e
starr
evers starr
3.1 Food For Thought
 Bacteria in our intestines make vitamins and keep us healthy
– other bacteria make toxins that can contaminate foods and
even kill us
 Example: Each year, about 265,000 people in the United
States become infected with toxin–producing E. coli
Toxin-producing bacteria
3.2 What, Exactly, Is a Cell?
 Cell theory is the fundamental theory of biology
 Cell theory
 All organisms consist of one or more cells
 The cell is the smallest unit of life
 Each new cell arises from another cell
 A cell passes hereditary information to its offspring
Components of All Cells
 Plasma membrane
 Surrounds the cell and controls which substances move in
and out (selectively permeable)
 Proteins embedded in a lipid bilayer or attached to one of
its surfaces carry out membrane functions
 Cytoplasm
 Semifluid substance enclosed by cell’s plasma membrane
 Metabolic functions
Components of All Cells
 Organelle
 Structure that carries out a specialized metabolic function
inside a cell
 Membrane-enclosed organelles compartmentalize tasks
such as building, modifying, and storing substances
 All cells start out life with DNA
 Eukaryotic cells have a nucleus that contains DNA
Cells
cytoplasm
A) A bacterial cell.
DNA
plasma membrane
Cells
DNA
nucleus
cytoplasm
B) A eukaryotic (plant) cell. Only
eukaryotic cells have a nucleus.
plasma membrane
Surface-to-Volume Ratio
 Cells must be small to efficiently exchange materials with their
environment
 Surface-to-volume ratio limits cell size and influences cell
shape
 Surface-to-volume ratio
 Relationship in which the volume of an object increases
with the cube of the diameter, but the surface areas
increases with the square
Surface-to-Volume Ratio
Diameter (cm)
2
3
6
Surface area (cm2)
12.6
28.2
113
Volume (cm3)
4.2
14.1
113
Surface-to-volume ratio
3:1
2:1
1:1
How Do We See Cells?
 No one knew cells existed until microscopes were invented
 Different types of microscopes use light or electrons to reveal
different details of cells:
 Light microscope (phase contrast)
 Light microscope (reflected light)
 Fluorescence microscope
 Transmission electron microscope
 Scanning electron microscope
Common Units of Length
Relative Sizes
electron microscopes
small molecules
lipids carbohydrates proteins
0.1 nm
viruses
molecules of life
1 nm
mitochondria,
chloroplasts
DNA
10 nm
100 nm
1 µm
most
bacteria
Relative Sizes
light microscopes
mitochondria,
chloroplasts
1 µm
most
bacteria
most
eukaryotic
cells
10 µm
100 µm
Relative Sizes
human eye (no microscope)
frog eggs
100 µm
1 mm
largest organisms
small animals
1 cm
10 cm
1m
10 m
100 m
Same Organism, Different Microscopes
A) Phase-contrast light microscopes yield high-contrast images of transparent
specimens. Dark areas have taken up dye.
Same Organism, Different Microscopes
B) Reflected light microscopes capture light reflected from the surface of
specimens.
Same Organism, Different Microscopes
C) This fluorescence micrograph shows fluorescent light emitted by chlorophyll
molecules in the cells.
Same Organism, Different Microscopes
D) Transmission electron micrographs reveal fantastically detailed images
of internal structures.
Same Organism, Different Microscopes
E) Scanning electron micrographs show surface details. SEMs may be
artificially colored to highlight specific details.
3.4 The Structure of Cell Membranes
 A cell membrane is a mosaic of proteins and lipids (mainly
phospholipids) that functions as a selectively permeable
barrier that separates an internal environment from an
external one
 Fluid mosaic model
 A cell membrane can be considered a two-dimensional
fluid of mixed composition
Phospholipids
phosphate group
hydrophilic
head
two
hydrophobic
tails
A) Phospholipids are the most
abundant component of
eukaryotic cell membranes.
Each phospholipid molecule
has a hydrophilic head and
two hydrophobic tails.
Cell Membrane Structure
one layer
of lipids
one layer
of lipids
B) In a watery fluid, phospholipids spontaneously line up into two layers: the hydrophobic
tails cluster together, and the hydrophilic heads face outward, toward the fluid. This lipid
bilayer forms the framework of all cell membranes. Many types of proteins intermingle
among the lipids—a few that are typical of plasma membranes are shown opposite.
Membrane Proteins
 Proteins associated with a membrane carry out most
membrane functions
 Adhesion proteins help cells stick together
 Recognition proteins tag cells as “self”
 Receptor proteins bind to a particular substance outside
the cell
 Transport proteins passively or actively assist specific
ions or molecules across a membrane
Membrane Proteins
C) Recognition
proteins such as this
MHC molecule tag a
cell as belonging to
one’s own body.
D) Receptor proteins bind substances
outside of the cell. This one is a B cell
receptor. B cell receptors help the body
eliminate toxins and infectious agents
such as bacteria.
E) Transport proteins
bind to molecules on one
side of the membrane,
and release them on the
other side. This one
transports glucose.
F) This transport protein,
an ATP synthase, makes
ATP when hydrogen ions
flow through its interior.
Extracellular Fluid
Lipid
Bilayer
Cytoplasm
3.4 Introducing Prokaryotic Cells
 Domains Bacteria and Archaea make up the prokaryotes
 Prokaryotes are the smallest and most metabolically diverse
forms of life
 Prokaryotes inhabit nearly all regions of the biosphere – many
archaeans are adapted to extreme environments
 Prokaryotes are single-celled organisms with no nucleus, but
many have a cell wall and one or more flagella or pili
Prokaryote Diversity: Bacteria
A) Protein filaments, or pili, anchor
bacterial cells to one another and
to surfaces. Here, Salmonella
typhimurium cells (red ) use their
pili to invade human cells.
B) Ball-shaped Nostoc cells are a type of
freshwater photosynthetic bacteria. The
cells in each strand stick together in a
sheath of their own jellylike secretions.
Prokaryote Diversity: Archaea
C) The square archaea Haloquadratum
walsbyi thrives in brine pools saltier than
soy sauce. Gas-filled organelles (white
structures) buoy these highly motile cells,
which can aggregate into flat sheets
reminiscent of tile floors.
D) Ferroglobus placidus prefers
superheated water spewing from the ocean
floor. The durable composition of archaeal
lipid bilayers (note the gridlike texture)
keeps their membranes intact at extreme
heat and pH.
Prokaryote Body Plan
 The cytoplasm contains ribosomes, a circular DNA molecule
in a nucleoid region, and may contain additional genes as
plasmids
 Ribosome
 Organelle of protein synthesis
Prokaryote Body Plan
 Cell wall
 Semirigid but permeable structure that surrounds the
plasma membrane of some cells
 Consists of peptides and polysaccharides (in bacteria) or
proteins (in archaeans)
 In some bacteria, a sticky capsule of polysaccharides
surrounds the cell wall
Prokaryote Body Plan
 Surface extensions allow certain actions
 Pili
 Protein filaments used to help cells cling to or move
across surfaces, or for plasmid transfer
 Flagella
 Long, slender cellular structures used for mobility
1 cytoplasm,
with ribosomes
2 DNA in nucleoid
3 plasma membrane
4 cell wall
5 capsule
6 pilus
Prokaryote Body Plan
7 flagellum
Biofilms
 Biofilms are shared living arrangements among bacteria and
other microbial organisms that provide various advantages to
the community
 Biofilm
 Community of different types of microorganisms living
within a shared mass of slime
A biofilm: dental plaque
3.5 A Peek Inside a Eukaryotic Cell
 Protists, fungi, plants, and animals are eukaryotes
 All eukaryotic cells start life with a nucleus, ribosomes,
organelles of the endomembrane system (including
endoplasmic reticulum, vesicles, Golgi bodies), mitochondria,
and other organelles
The Nucleus
 Pores, receptors, and transport proteins in the nuclear
envelope control the movement of molecules into and out of
the nucleus
 Nuclear envelope
 A double membrane that constitutes the outer boundary of
the nucleus
Nucleus of a cell
nuclear envelope
mitochondrion DNA in nuclear rough
nucleus pore
ER
The Endomembrane System
 The endomembrane system includes rough and smooth
endoplasmic reticulum, vesicles, and Golgi bodies
 Endomembrane system
 Series of interacting organelles between the nucleus and
plasma membrane
 Makes and modifies lipids and proteins
 Recycles molecules and particles such as worn-out cell
parts, and inactivates toxins
The Endomembrane System
 Endoplasmic reticulum (ER)
 A continuous system of sacs and tubes that is an
extension of the nuclear envelope
 Rough ER is studded with ribosomes (for protein
production)
 Smooth ER has no ribosomes
The Endomembrane System
 Vesicle
 Small, membrane-enclosed, saclike organelle
 Stores, transports, or degrades its contents
 Vacuole
 A fluid-filled organelle that isolates or disposes of wastes,
debris, or toxic materials
 Lysosome
 Vesicle with enzymes for intracellular digestion
The Endomembrane System
 Peroxisome
 Enzyme-filled vesicle that breaks down amino acids, fatty
acids, and toxic substances
 Golgi body
 Organelle that modifies polypeptides and lipids
 Sorts and packages the finished products into transport
vesicles
Bacteria-Like Organelles:
Mitochondria and Chloroplasts
 Mitochondria and chloroplasts have their own DNA – they
resemble bacteria and may have evolved by endosymbiosis
 Mitochondrion
 Double-membraned organelle that produces ATP
 Chloroplast
 Organelle of photosynthesis
Mitochondrion
outer membrane
outer
compartment
inner compartment
inner membrane
Chloroplast
two outer
membranes
stroma
inner
membrane
The Cytoskeleton
 Cytoskeleton
 Dynamic network of protein filaments that support,
organize, and move eukaryotic cells and their internal
structures
 The cytoskeleton interacts with accessory proteins, such as
motor proteins
Cytoskeletal Elements
 Microtubules
 Cytoskeletal elements involved in movement
 Hollow filaments of tubulin subunits
 Microfilaments
 Reinforcing cytoskeletal elements
 Fibers of actin subunits
 Intermediate filaments
 Elements that lock cells and tissues together
Cytoskeletal Elements
Motor Proteins
 Motor proteins are the basis of movement – they interact with
microfilaments in pseudopods or microtubules in cilia and
eukaryotic flagella
 Motor proteins
 Energy-using proteins that interact with cytoskeletal
elements to move cells parts or the whole cell
 Example: Motor protein moves a vesicle along a microtubule
Motor Protein
Cilia
 Cilia
 Short, hairlike structures that project from the plasma
membrane of some eukaryotic cells
 Coordinated beating stirs fluid, propels motile cells
 Moved by organized arrays of microtubules
 Example: clears particles from airways
Flagella
 Eukaryotic flagella whip back and forth to propel cells such
as sperm through fluid
Pseudopods
 Temporary protrusion that
helps some eukaryotic cells
move and engulf prey
 Moved by motor proteins
attached to microfilaments
that drag plasma membrane
 Example: amoebas
Components of an Animal Cell
3D ANIMATION: Tour of an Animal Cell
ANIMATION: Eukaryotic evolution (formerly
"endosymbiont theory")
3.6 Cell Surface Specializations
 Extracellular matrix (ECM)
 Complex mixture of substances secreted by cells
 Supports cells and tissues
 Functions in cell signaling
 Cuticle
 Type of ECM secreted by cells at a body surface
 Found in plants and arthropods
 Cell wall
 ECM that protects, supports, and imparts shape
Plant ECM
cuticle
outer cell of leaf
photosynthetic
cell inside leaf
Animal Cell Junctions
 Cell junctions
 Connect a cell to another cell or to ECM
 Tight junction
 Array of fibrous proteins that joins epithelial cells and
prevents fluids from leaking between them
 Adhering junction
 Anchors cells to each other or to extracellular matrix
 Gap junction
 Channel across plasma membranes of adjoining cells
Animal Cell Junctions
tight
junctions
gap junction
basement membrane
Tight Junctions
Around Kidney
Cells
adhering junction
Cell Junctions in Plants
 In plants, plasmodesmata connect the cytoplasms of
adjoining cells
 Plasmodesmata
 Open channels that extend across the primary walls of
adjoining cells
 Allow materials such as water, nutrients, and signaling
molecules to flow through
3.7 The Nature of Life
 Six properties characterize living things as different from
nonliving things:
1. Make and use organic molecules of life
2. Consist of one or more cells
3. Self-sustaining biological processes such as metabolism
4. Change over their lifetime (develop, mature, age)
5. Use DNA as hereditary material when they reproduce
6. Have the collective capacity to change over successive
generations
Life
3.8 Food For Thought (revisited)
 Some think the safest way to protect consumers from food
poisoning is by sterilizing food to kill toxic bacteria
 Example:
 Recalled, contaminated ground beef is typically sterilized,
then processed into ready-to-eat products
Digging Into Data:
Organelles and Cystic Fibrosis