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Cell Structure
Chapter 3
3.1 Impacts/Issues
Food For Thought
 Bacteria in our intestines make vitamins and
keep us healthy – but other bacteria make toxins
that can contaminate foods and even kill us
Video: Food for thought
3.2 What, Exactly, Is a Cell?
 Cells are the fundamental units of all life
 All cells start life with a plasma membrane,
cytoplasm, and a region of DNA which, in
eukaryotic cells only, is enclosed by a nucleus
Examples of Cells
 Some single-celled organisms (protists)
Cell Structure
 A plasma membrane surrounds the cell and
controls which substances move in and out
 Plasma membrane
• A cell’s outermost membrane
 Lipid bilayer
• Structural foundation of cell membranes; mainly
phospholipids arranged tail-to-tail in a bilayer
A Lipid Bilayer
one layer
of lipids
one layer
of lipids
a lipid bilayer
p. 43
Cytoplasm
 An important part of homeostasis is maintaining
the composition of cytoplasm, which differs from
fluid outside the cell
 Cytoplasm
• Semifluid substance enclosed by a cell’s plasma
membrane
Organelles
 Cell metabolism occurs in cytoplasm and
internal compartments, including organelles
 Organelle
• Structure that carries out a specialized metabolic
function inside a cell
Prokaryotes and Eukaryotes
 Cells are classed as eukaryotes or prokaryotes
based on how DNA is housed in the cell
 Nucleus
• Organelle with two membranes that holds a
eukaryotic cell’s DNA
 Nucleoid
• Region of cytoplasm where DNA is concentrated
in a prokaryotic cell
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
• A 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
Animation: Surface-to-volume ratio
The Cell Theory
 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
The Cell Theory
Animation: Overview of cells
3.3 Measuring Cells
 Most cells are visible only with the help of
microscopes
 Different types of microscopes use light or
electrons to reveal different details of cells
Bacteria on the Tip of a Pin
 Bacteria are the smallest and simplest cells
Fig. 3-3a, p. 45
Fig. 3-3b, p. 45
Fig. 3-3c, p. 45
“Animalcules and Beasties”
 No one knew cells existed until microscopes
were invented
 1600s: van Leeuwenhoek’s microscope
sample holder
focusing knob
lens
Leeuwenhoek’s microscope
p. 45
Hooke, Schleiden, and Schwann
 1600s: Robert Hooke improved the microscope
and coined the term “cell”
 1839: Matthias Schleiden and Theodore
Schwann realized cells were alive and proposed
the cell theory
Modern Microscopes
 Different types of microscopes reveal different
aspects of cell structure
•
•
•
•
•
Light microscope (phase contrast)
Light microscope (reflected light)
Fluorescence microscope
Transmission electron microscope
Scanning electron microscope
Same Organism, Different Microscopes
A Light micrograph. B Light micrograph.
A phase-contrast
A refl ected light
microscope yields
microscope captures
high-contrast images light reflected from
of transparent
opaque specimens.
specimens, such
as cells.
C Fluorescence
micrograph. The
chlorophyll
molecules in these
cells emitted red
light (they
fluoresced)
naturally.
10μm
D A transmission E A scanning
electron
electron
micrograph
micrograph shows
reveals
surface details of
fantastically
cells and
detailed images of structures. SEMs
internal
may be artificially
structures.
colored to
highlight certain
details.
Fig. 3-4, p. 46
Relative Sizes
electron microscopes
viruses
molecules of life
complex carbohydrates
DNA
(width)
lipids
proteins
mitochondria,
chloroplasts
light microscopes
most
eukaryotic
most
cells
bacteria
small
molecules
0.1 nm
1 nm
10 nm
100 nm
1 µm
10 µm
Fig. 3-5a, p. 46
human eye (no microscope)
largest organisms
small animals
humans
frog eggs
100 µm
1 mm
1 cm
10 cm
1m
10 m
100 m
Fig. 3-5b, p. 47
Animation: How an electron microscope
works
3.4 The Structure of Cell Membranes
 The plasma membrane is basically a lipid bilayer
balloon filled with fluid
 The nonpolar tails of both layers are sandwiched
between the polar heads
fluid
p. 48
The Fluid Mosaic Model
 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 twodimensional fluid of mixed composition
Membrane Proteins
 Proteins associated with a membrane carry out
most membrane functions
• Transport proteins passively or actively assist
specific ions or molecules across a membrane
• Enzymes speed chemical processes
• Adhesion proteins help cells stick together
• Recognition proteins tag cells as “self”
• Receptor proteins bind to a particular substance
outside the cell
Cell Membrane Structure
A Phospholipids are the most abundant
component of eukaryotic cell membranes.
Each phospholipid molecule has a
hydrophilic head and two hydrophobic tails.
hydrophilic
head
two
hydrophobic
tails
Fig. 3-6a, p. 48
B In a watery fluid, phospholipids
spontaneously line up into two layers:
hydrophobic tails cluster together, and
hydrophilic heads face outward, toward
the fluid. This lipid bilayer forms the
framework of all cell membranes.
one layer
of lipids
one layer
of lipids
Fig. 3-6b, p. 48
Fig. 3-6c, p. 48
Animation: Lipid bilayer organization
Animation: Cell membranes
Animation: Fluid mosaic model
3.5 Introducing Prokaryotic Cells
 Domains Bacteria and Archaea make up the
prokaryotes
 Prokaryotes are single-celled organisms with no
nucleus, but many have a cell wall and one or
more flagella or pili
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
 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
 Surface extensions allow certain actions
 Flagellum
• Long, slender cellular structure used for mobility
 Pilus
• A protein filament used to help cells cling to or
move across surfaces, or for plasmid transfer
Prokaryote
Body Plan
flagellum
capsule
cell wall
plasma membrane
cytoplasm,
with ribosomes
DNA in nucleoid
pilus
Fig. 3-8, p. 50
Animation: Typical prokaryotic cell
Prokaryote Diversity
 As a group, 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
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.
Fig. 3-7a, p. 50
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.
Fig. 3-7b, p. 50
Prokaryote Diversity: Archaeans
C The archaean Pyrococcus furiosus was discovered in
ocean sediments near an active volcano. It lives best at
100°C (212°F), and it makes a rare kind of enzyme that
contains tungsten atoms.
Fig. 3-7c, p. 51
D Ferroglobus placidus prefers superheated water
spewing from the ocean floor. The durable composition of
archaean lipid bilayers (note the gridlike texture) keeps
their membranes intact at extreme heat and pH.
Fig. 3-7d, p. 51
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
3.6 A Peek Inside a Eukaryotic Cell
 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
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 wornout 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
 Peroxisome
• Enzyme-filled vesicle that breaks down amino
acids, fatty acids, and toxic substances
 Lysosome
• Vesicle with enzymes for intracellular digestion
The Endomembrane System
 Golgi body
• Organelle that modifies polypeptides and lipids
• Sorts and packages the finished products into
transport vesicles
 Vacuole
• A fluid-filled organelle that isolates or disposes of
wastes, debris, or toxic materials
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
Mitochondria and Chloroplasts:
Bacteria-Like Organelles
outer membrane
outer
compartment
inner compartment
inner membrane
Fig. 3-11a, p. 54
two outer
membranes
stroma
inner
membrane
Fig. 3-11b, p. 54
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
Fig. 3-12a, p. 55
tubulin subunit
Fig. 3-12a, p. 55
10 μm
Fig. 3-12b, p. 55
Motor Proteins
 Motor proteins are the basis of movement – they
interact with microfilaments in pseudopods or (in
cilia and eukaryotic flagella) microtubules
 Motor proteins
• Energy-using proteins that interact with
cytoskeletal elements to move cells parts or the
whole cell
Motor Proteins
 A motor protein moves a vesicle along a
microtubule
Cilia and False Feet
 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 are whiplike structures that
propel cells such as sperm through fluid
• Different internal structure and motion than
prokaryotic flagella
False Feet
 Pseudopod (false foot)
• Extendable lobe of membrane-enclosed
cytoplasm for movement or to engulf prey
• Moved by motor proteins attached to
microfilaments that drag the plasma membrane
• Example: amoebas
Components of an Animal Cell
An Animal Cell
nuclear mitochondrion DNA in nuclear
envelope
nucleus pore
rough ER with
attached ribosomes
Fig. 3-10, p. 53
3.7 Cell Surface Specializations
 Cell junctions
• Connect a cell structurally and functionally to
another cell or to extracellular matrix (ECM)
 Extracellular matrix (ECM)
• Complex mixture of substances secreted by cells
• Supports cells and tissues
• Functions in cell signaling
Types of Animal Cell Junctions
 Tight junction
• An 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
• Forms a channel across plasma membranes of
adjoining animal cells
Types of Animal Cell Junctions
1 Tight junctions
Rows of proteins
that run parallel
with the free
surface of a tissue;
stop leaks
between adjoining
cells.
2 Adhering junction
A mass of
interconnected
proteins that welds
one cell to another
or to ECM; anchored
under the plasma
membrane by
intermediate
filaments.
3 Gap junction
Cylindrical clusters
of proteins that span
the plasma
membrane of
adjoining cells;
clusters are often
paired as channels
that open and close.
Fig. 3-14, p. 56
Tight Junctions Around Kidney Cells
Cell Connections 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
3.8 Impacts/Issues Revisited
 Fresh foods marked with this symbol have been
irradiated to kill bacteria – potential health risks
from eating irradiated foods are unknown
Digging Into Data:
Organelles and Cystic Fibrosis
ATP
ATP
CF deletion
Fig. 3-16a, p. 59
Fig. 3-16b, p. 59