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Essentials of the Living World
Second Edition
George B. Johnson
Jonathan B. Losos
Chapter 5
Cells
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
5.1 Cells
• All cells are small
 most cells can only be observed under a
microscope
• Robert Hooke first described cells in 1665
• Cells are fundamentally important
• Matthias Schleiden in 1838 recognized that cells
were fundamental to plant composition
• Theodor Schwann in 1839 reported that all animal
tissues also are comprised of cells
5.1 Cells
Figure 5.1
• Cells are not all the
same size
5.1 Cells
•
Cell theory states the importance of cells to life
1. All organisms are composed of one or more cells
2. Cell are the smallest living things
3. Cells arise only by division of previously existing
cells
5.1 Cells
• Most cells are small because larger cells do not
function as efficiently
 larger cells are more difficult to control because of the
distances involved from the command center at the
core to the peripheral regions
 organisms that are comprised of many, small cells are
at an efficiency advantage over organisms comprised
of few, larger cells
5.1 Cells
• Cell size is also limited by the surface-tovolume ratio
 as cell size increases, the volume grows more
rapidly than surface area
• a cell’s surface provides the interior’s only
opportunity to interact with the environment
Figure 5.2 Surface-to-volume ratio
Large cells have far less surface for each unit of volume than do small cells.
5.1 Cells
• Some cells can be large because they have
special modifications to increase surface area
 some cells, such as neurons, are very long and thin
 other cells, such as the ones lining the digestive tract,
have microvilli—finger-like projections that make
more surface area
• Otherwise, in order for organisms to become
larger, they must have many cells
5.1 Cells
• Cellular structure is organized
 plasma membrane forms the boundary of the
cell
• also controls the permeability of the cell to water
and dissolved substances
 cytoplasm fills the interior of the cell
5.1 Cells
• Most cells are too small to be viewed by the
naked eye, which has limited resolution
• resolution refers to the minimum distance that two points
can be apart and still be distinguished as two separated
points
• the limit of resolution of the human eye is about 100
micrometers
• One way to increase resolution is to increase
magnification, such as by using a microscope
5.1 Cells
• Different types of microscopes are used to
increase magnification for viewing
 compound light microscope uses sets of magnifying
glass lenses to resolve structures that are separated
by more than 200 nanometers
 electron microscopes have 1000 times the resolving
power of light microscopes and can resolve objects
as close as 0.2 nanometers apart
Figure 5.3 A scale of visibility
5.1 Cells
• Stains are used to analyze otherwise
transparent or hidden cellular components
 histology is the analysis of tissues using
microscopy
• specific stains can be applied that bind to target
molecules for studying structure and function
Table 5.1 Types of Microscopes
5.2 The Plasma Membrane
• The plasma membrane is conceptualized
by the fluid mosaic model
 a sheet of lipids with embedded proteins
• the lipid layer forms the foundation of the
membrane
• the fat molecules comprising the lipid layers are
called phospholipids
5.2 The Plasma Membrane
• A phospholipid has a
polar head and two nonpolar tails
• The polar region is
comprised of a phosphate
chemical group and is
water-soluble
• The non-polar region is
comprised of fatty acids
and is water-insoluble
Figure 5.4(a) Phospholipid structure
5.2 The Plasma Membrane
• A lipid bilayer forms spontaneously whenever a
collection of phospholipids is placed in water
Figure 5.5 The lipid bilayer.
5.2 The Plasma Membrane
• The interior of the lipid bilayer is
completely nonpolar
 no water-soluble molecules can cross through
it
 cholesterol is also found in the interior
• it affects the fluid nature of the membrane
• its accumulation in the walls of blood vessels can
cause plaques
• plaques lead to cardiovascular disease
5.2 The Plasma Membrane
• Another major component of the membrane is a
collection of membrane proteins
 some proteins form channels that span the
membrane
• these are called transmembrane proteins
 other proteins are integrated into the structure of the
membrane
• for example, cell surface proteins are attached to the outer
surface of the membrane and act as markers
Figure 5.6 Proteins are embedded
within the lipid bilayer
5.3 Prokaryotic Cells
• There are two major types of cells
 prokaryotic
• lacks a nucleus and does not have an extensive
system of internal membranes
• all bacteria and archae have this cell type
 eukaryotic
• has a nucleus and has internal membrane-bound
compartments
• all organisms other than bacteria or archae have
this cell type
5.3 Prokaryotic Cells
• Prokaryotes are the simplest cellular
organisms
 have a plasma membrane surrounding a
cytoplasm without interior compartments
• some bacteria have additional outer layers to the
plasma membrane
– cell wall comprised of carbohydrates to confer rigid
structure
– capsule may surround the cell wall
5.3 Prokaryotic Cells
• The interior of the prokaryotic cell shows simple
organization
 cytoplasm is uniform with little or no internal support
framework
 ribosomes (sites for protein synthesis) are scattered
throughout the cytoplasm
 nucleoid region (an area of the cell where DNA is
localized)
• not membrane-bound, so not a true nucleus
5.3 Prokaryotic Cells
• Other structures sometimes found in prokaryotes
relate to locomotion, feeding, or genetic
exchange
 flagellum (plural, flagellae) is a collection of protein
fibers that extends from the cell surface
• may be one or many
• aids in locomotion and feeding
 pilus (plural, pili) is a short flagellum
• aids in attaching to substrates and in exchanging genetic
information between cells
Figure 5.9 Organization of a
prokaryotic cell
5.4 Eukaryotic Cells
• Eukaryotic cells are larger and more
complex than prokaryotic cells
 have a plasma membrane encasing a
cytoplasm
• internal membranes form compartments called
organelles
• the cytoplasm is semi-fluid and contains an
network of protein fibers that form a scaffold called
a cytoskeleton
5.4 Eukaryotic Cells
• many organelles are immediately
conspicuous under the microscope
 nucleus
• a membrane-bound compartment for DNA that
gives eukaryotes (literally, “true-nut”) their name
 endomembrane system
• gives rise to the internal membranes found in the
cell
• each compartment can provide specific conditions
favoring a particular process
5.4 Eukaryotic Cells
• not all eukaryotic cells are alike
 the cells of plants, fungi, and many protists
have a cell wall beyond the plasma
membrane
 all plants and many protists contain
organelles called chloroplasts
 plants contain a central vacuole
 only animal cells contain centrioles
Animal versus Plant Cell
Figure 5.10 Structure of animal cell
Figure 5.11 Structure of plant cell
5.5 The Nucleus: The Cell’s Control
Center
• The nucleus is the command and control center
of the cell
 it also stores hereditary information
• The nuclear surface is bounded by a doublemembrane called the nuclear envelope
 groups of proteins form openings called nuclear
pores that permit proteins and RNA to pass in and
out of the nucleus
5.5 The Nucleus: The Cell’s Control
Center
• The DNA of eukaryotes is packaged into
segments and associated with a protein
 this complex is called a chromosome
• the proteins enable the DNA to be wound tightly so
it appears condensed
– the condensed or chromosome form of DNA occurs
during cell division
• the DNA is uncoiled into strands called chromatin
that are no longer visible as segments
– protein synthesis occurs when the DNA is in the
chromatin form
5.5 The Nucleus: The Cell’s Control
Center
• The nucleus is the site for the subunits of
the ribosome to be synthesized
 the nucleolus is a dark-staining region of the
nucleus
• it contains the genes that code for the rRNA
(ribosomal RNA) that makes up the ribosomal
subunits
• the subunits leave the nucleus via the nuclear
pores and the final ribosome is assembled in the
cytoplasm
Figure 5.12 The nucleus
5.6 The Endomembrane System
• The endoplasmic reticulum (ER) is an
extensive system of internal membranes
 some of the membranes form channels and
interconnections
 other portions become isolated spaces
enclosed by membranes
• these spaces are known as vesicles
5.6 The Endomembrane System
• The segment of the ER dedicated to protein
synthesis is called the rough ER
 the surface of this region looks pebbly
 the rough spots are due to embedded ribosomes
• The segment of the ER that aids in the
manufacture of carbohydrates and lipids is
called the smooth ER
 the surface of this region looks smooth because it
contains no embedded ribosomes
Figure 5.13 The endoplasmic
reticulum (ER)
5.6 The Endomembrane System
• After synthesis in the ER, the newly-made
molecules are passed to the Golgi bodies
 Golgi bodies are flattened membranes that
that form collective stacks called the Golgi
complex
 their numbers vary depending on the cell
 their function is to collect, package, and
distribute molecules manufactured in the cell
Figure 5.14 Golgi complex
5.6 The Endomembrane System
• The ER and Golgi complex function together as
a transport system in the cell
Figure 5.15 How the endomembrane system works
5.6 The Endomembrane System
• The Golgi complex also gives rise to
lysosomes
 these membrane-bound structures contain
enzymes that the cell uses to break down
macromolecules
• worn-out cell parts are broken down and their
components recycled to form new parts
• particles that the cell has ingested are also
digested
5.6 The Endomembrane System
•
Peroxisomes are different components
of the endomembrane system that are
also found in the cell

the reactions that are confined to these
organelles function to
1. detoxify harmful byproducts of metabolism
2. convert fats to carbohydrates in plants seeds for
growth
5.7 Organelles That Contain DNA
• Eukaryotic cells contain cell-like organelles
that, besides the nucleus, also contain
DNA
 these organelles appear to have been derived
from ancient bacteria that were then
assimilated by the eukaryotic cell
 they include the following organelles:
mitochondria and chloroplasts
5.7 Organelles That Contain DNA
• Mitochondria are cellular
powerhouses
• Sites for chemical
reactions called
oxidative metabolism
• The organelle is
surrounded by two
membranes
Figure 5.16(a) Mitochondria
5.7 Organelles That Contain DNA
• Chloroplasts are the
location for
photosynthesis
• The organelle is also
surrounded by two
membranes
Figure 5.17 A chloroplast
5.7 Organelles That Contain DNA
• Both mitochondria and chloroplasts
possess circular DNA that is not found
elsewhere in the cell
• They cannot be grown free of the cell
 they are totally dependent on the cells within
which they occur
5.7 Organelles That Contain DNA
• The theory of endosymbiosis
 states that some organelles evolved from a symbiosis
in which one cell of a prokaryotic species was
engulfed by and lived inside of a cell of another
species of prokaryote
 the engulfed species provided their hosts with
advantages because of special metabolic activities
 the modern organelles of mitochondria and
chloroplasts are believed to be found in the eukaryotic
descendants of these endosymbiotic prokaryotes
Figure 5.18 Endosymbiosis
5.7 Organelles That Contain DNA
• In addition to the double membranes and
circular DNA found in mitochondria and
chloroplasts, there is a lot of other evidence
supporting endosymbiotic theory
 mitochondria are about the same size as modern
bacteria
 the cristae in mitochondria resemble folded
membranes in modern bacteria
 mitochondrial ribosomes are similar to modern,
bacterial ribosomes in size and structure
 mitochondria divide by fission, just like modern
bacteria
5.8 The Cytoskeleton: Interior
Framework of the Cell
• The cytoskeleton is comprised of an internal
framework of protein fibers that
 anchor organelles to fixed locations
 support the shape of the cell
 helps organize ribosomes and enzymes needed for
synthesis activities
• The cytoskeleton is dynamic and its components
are continually being rearranged
5.8 The Cytoskeleton: Interior
Framework of the Cell
• Three different types of protein fibers
comprise the cytoskeleton
 intermediate filaments
• thick ropes of intertwined protein
 microtubules
• hollow tubes made up of the protein tubulin
 microfilaments
• long, slender microfilaments made up of the
protein actin
Figure 5.19 The three protein fibers
of the cytoskeleton
5.8 The Cytoskeleton: Interior
Framework of the Cell
• Centrioles are complex structures that
assemble microtubules in animal cells and
the cells of most protists
 they anchor locomotory structures, such as
flagella or cilia
 they assemble microtubules near the nuclear
envelope
 they might also have an endosymbiotic origin
5.8 The Cytoskeleton: Interior
Framework of the Cell
• Cellular motion is associated with the
movement of actin microfilaments and/or
microtubules
 some cells “crawl” by coordinating the
rearrangement of actin microfilaments
 some cells swim by coordinating the beating
of microtubules grouped together to form
flagella or cilia
Figure 5.21 Flagella and cilia
5.8 The Cytoskeleton: Interior
Framework of the Cell
• Microtubules provide a means to transport
material inside the cell efficiently over long
distances
Figure 5.22 Molecular motors
5.8 The Cytoskeleton: Interior
Framework of the Cell
• The cytoskeleton also anchors storage
compartments
 vacuoles are membrane-bound storage
centers
• central vacuole appears to be a large empty
space inside a plant cell but is actually filled with
water and dissolved substances
• contractile vacuole is found near the cell surface
of some protists and accumulates excess water
from inside the cell that it then pumps out
5.9 Outside the Plasma Membrane
 Cell walls
• found in plants, fungi,
and many protists
• comprised of different
components than
prokaryotic cell walls
• function in providing
protection, maintaining
cell shape, and
preventing excessive
water loss/uptake
Figure 2.24 Cell walls in plants
5.9 Outside the Plasma Membrane
 Extracellular matrix (ECM)
• takes the place of the cell wall in animal cells and is comprised by a
mixture of proteins secreted by the cell
• collagen and elastin proteins form a protective layer over the cell
surface
• fibronectin protein connects the ECM to the plasma membrane
• the fibronectin molecules also connect to integrins, proteins that
extend into the cytoplasm of the cell
– this extracellular-intracellular connection allows the ECM to influence
cellular behavior and to coordinate groups of cells functioning as
tissues
Figure 5.25 The extracellular matrix
5.10 Diffusion and Osmosis
• Movement of water and nutrients into a
cell or elimination of wastes out of cell is
is essential for survival
• This movement occurs across a biological
membrane in one of three ways
• diffusion
• membrane folding
• protein transport
5.10 Diffusion and Osmosis
• Molecules move in a random fashion but there is
a tendency to produce uniform mixtures
• The net movement of molecules from an area of
higher concentration to an area of lower
concentration is termed diffusion
• Molecules diffuse down a concentration gradient
from higher to lower concentrations
 diffusion ends when equilibrium is reached
Figure 5.26 How diffusion works
5.10 Diffusion and Osmosis
• Only certain substances undergo diffusion
across the plasma membrane
 molecules like oxygen, carbon dioxide, and nonpolar
lipids
 ions and polar molecules cannot cross the interior of
the membrane
• Water, although polar, is able to diffuse freely
across the plasma membrane
 aquaporins are selective channels that permit water
to cross
5.10 Diffusion and Osmosis
• Water moves down its concentration
gradient in moving into or out of a cell
through a process called osmosis
 the movement of water is dependent on the
concentration of other substances in a
solution
 the greater the amount of solutes that are
dissolved in a solution, then the lesser the
amount of water molecules that are free to
move
5.10 Diffusion and Osmosis
• The concentration of all molecules
dissolved in a solution is called the
osmotic concentration of the solution
• Osmotic concentrations of different
solutions can be compared relative to
each other
5.10 Diffusion and Osmosis
• Consider two solutions
with unequal osmotic
concentrations
• the solution with the
higher concentration is
called hypertonic
• The solution with the
lower concentration is
called hypotonic
Figure 5.27(2)
5.10 Diffusion and Osmosis
• Consider two
solutions with equal
osmotic
concentrations
• The solutions are
each called isotonic
Figure 5.27(1)
5.10 Diffusion and Osmosis
• Movement of water by osmosis into a cell
causes pressure called osmotic pressure
 enough pressure may cause a cell to swell
and burst
 osmotic pressure explains why so many cell
types are reinforced by cell walls
5.11 Bulk Passage into and out of
Cells
• Bulky substances are contained within
vesicles as they are moved into and out of
a cell
 endocytosis is the engulfing of substances
outside of the cell in order to form a vesicle
that is brought inside the cell
 exocytosis is the discharge of substances
from vesicles at the inner surface of the cell
Forms of Endosymbiosis
• Phagocytosis is
endocytosis of
particulate (solid)
matter
Figure 5.28(a) endocytosis
• Pinocytosis is
endocytosis of liquid
matter
Figure 5.28(b) endocytosis
Figure 5.29(a) Exocytosis
5.11 Passage into and out of Cells
• Transport of specific molecules into the cell
involves receptor-mediated endocytosis
 molecules to be transported must first bind to specific
receptors in the plasma membrane
 a portion of the receptor extends into the membrane
in an indented pit coated with the protein clathrin
 when a molecule binds to its specific receptor, the cell
reacts immediately by initiating endocytosis of a now
clathrin-coated vesicle
Figure 5.30 Receptor-mediated
endocytosis
5.12 Selective Permeability
• Selective permeability allows cells to
control specifically what enters and leaves
 involves using proteins in the membrane for
transporting substances across
 transport can be down a concentration
gradient (i.e., diffusion) or against a
concentration gradient (i.e., active transport)
5.12 Selective Permeability
 Selective diffusion
• proteins act as open channels for whatever is
small enough to fit inside the channel
• this form of diffusion is common in ion transport
 Facilitated diffusion
• proteins act as carriers that can bind only to
specific molecules to transport
• transport is limited by the availability of carriers
• if there are not enough carriers, then the transport
is saturated
5.12 Selective Permeability
• Active transport
 utilizes protein channels that open only when energy is supplied
 energy is used to pump substances against or up their
concentration gradients
 allows cells to maintain high or low concentration of certain
molecules
• recall that diffusion always ends in equilibrium
• There are two kinds of channels that perform active
transport in cells
 sodium-potassium pump
 proton pump
5.12 Selective Permeability
• Sodium-potassium (Na+-K+) pump
 uses energy, in the form of ATP, to pump three
Na+ out of the cell and to pump two K+ into
the cell
 nearly 1/3 of the energy expended by the
body’s cells is given over to driving these
pumps
Figure 5.32 How the sodiumpotassium pump works
5.12 Selective Permeability
• The result of the Na+-K+ pump is to
generate a concentration gradient with
more Na+ outside of the cell than inside
• Cells exploit this gradient in key ways
 for the conduction of signals along nerve cells
 for the transportation of important molecules
into the cell against their concentration
gradient
5.12 Selective Permeability
• the cell membrane has many facilitated diffusion
channels for Na+ but it is only transported if
partnered with another substance
 this is called coupled transport
• The concentration gradient favoring the entry of
Na+ into the cell is so strong that a coupled
substance will be transported even if it is against
the concentration gradient
 coupled transport is a common way for cells to
accumulate sugars and amino acids
Figure 5.33 A coupled channel
5.12 Selective Permeability
• Proton pump
 expends energy to pump protons across
membranes
 the result is a concentration gradient favoring
the re-entry of protons back into the cell
 the only way that protons can cross back into
the cell is through channels that generate ATP
• this process, known as chemiosmosis, is
essential to energy metabolism
Figure 5.34 How the proton pump
works
Inquiry & Analysis
• Why does the rate of
glucose uptake by red
blood cells in the
experiment never
exceed 500
mM/ml/hr?
• At what glucose
concentration are half
of the transport
channels occupied?
Figure p. 106 graph