PowerLecture:
Chapter 3
Cells and
How They Work
Learning Objectives




Understand the basic parts of eukaryotic
cells.
Understand the essential structure and
function of the cell membrane.
Know the forces that cause water and
solutes to move across membranes
passively and by active transport.
Understand how material can be imported
into or exported from a cell by being
wrapped in membranes.
Learning Objectives (cont’d)




Describe the nucleus of eukaryotes with
respect to structure and function.
Describe the organelles associated with the
endomembrane system, and tell the general
function of each.
Describe the cytoskeleton of eukaryotes
and distinguish it from the endomembrane
system.
Define a metabolic pathway and the types
of substances that participate in it.
Learning Objectives (cont’d)
 Characterize
an enzyme and what type of
cofactors may be needed for its functioning.
 Define ATP and describe the pathways for its
formation within the cell.
 Describe the process of cellular respiration
with special reference to the quantity of ATP
produced.
Impacts/Issues
When Mitochondria Spin
Their Wheels
When Mitochondria Spin Their Wheels
Mitochondria are specialized compartments
in the cell that produce energy.




Mitochondrial disorders can cause
reduced energy for cell use.
Luft’s syndrome is a rare disorder in which
the mitochondria are misshapen and do
not produce enough ATP.
Many mitochondrial disorders exist, but are
rare; this means that pharmaceutical
companies have little financial incentive to
develop drugs for treatment.
How Would You Vote?
To conduct an instant in-class survey using a classroom response
system, access “JoinIn Clicker Content” from the PowerLecture main
menu.
 Should
pharmaceutical companies receive
financial incentives (such as tax breaks) to
search for cures for diseases that affect only
a small number of people?


a. Yes, those that suffer from any disease, even if
it is rare, deserve treatment.
b. No, the public shouldn't subsidize this
research - let market forces take their course.
Section 1
What Is a Cell?
What is a Cell?
The cell theory has three generalizations:




All organisms are composed of one or more
cells.
The cell is the smallest unit having the
properties of life.
All cells come from pre-existing cells.
Figure 3.3
What is a Cell?
All cells are alike in three ways.




A plasma membrane separates each cell from
the environment, but also allows the flow of
molecules across the membrane.
DNA carries the hereditary instructions.
The cytoplasm containing a semifluid matrix
(cytosol) and organelles is located between the
plasma membrane and the region of DNA.
What is a Cell?

There are two basic kinds of cells.

Prokaryotic cells (bacteria)
do not have a separation of
the DNA from the remainder
of the cell parts.
cytoplasm
DNA
plasma
membrane

Eukaryotic cells have a
definite nucleus and
membrane-bound organelles.
Figure 3.1
What is a Cell?
Why are cells small?



Most cells are so small they can only be seen
by using light and electron microscopes.
Cells are necessarily small so that the surfaceto-volume ratio remains low; this means that
the interior will not be so
extensive that it cannot
exchange materials
efficiently through the
plasma membrane.
Figure 3.2
What is a Cell?
Membranes enclose cells and organelles.



A large portion of the cell membrane is
composed of phospholipids, each of which
possesses a hydrophilic head and two
hydrophobic tails.
If phospholipid molecules are surrounded by
water, their hydrophobic fatty acid tails cluster
and a lipid bilayer results; hydrophilic heads
are at the outer faces of a two-layer sheet with
the hydrophobic tails shielded inside.
fluid
fluid
one layer of lipids
one layer of lipids
cross-section
through lipid
bilayer
Figure 3.4
Section 2
The Parts of an
Eukaryotic Cell
The Parts of a Eukaryotic Cell
All eukaryotic cells contain organelles.



Organelles form compartmentalized portions of
the cytoplasm.
Organelles separate reactions with respect to
time (allowing proper sequencing) and space
(allowing incompatible reactions to occur in
close proximity).
CYTOSKELTON
microtubules
microfilaments
intermediate
filaments
•
•
nuclear envelope
nucleolus
DNA in
nucleoplasm
NUCLEUS
RIBOSOMES
ROUGH ER
MITOCHONDRION
SMOOTH ER
CENTRIOLES
PLASMA
MEMBRANE
GOLGI BODY
LYSOSOME
Fig. 3.5,
PLASMA
MEMBRANE
GOLGI BODY
LYSOSOME
ENDOPLASMIC
RETICULUM (ER)
nuclear
envelope
NUCLEUS
nucleolus
MITOCHONDRION
Fig. 3.6, p. 45
Section 3
The Plasma Membrane:
A Double Layer of Lipids
The Plasma Membrane
The plasma membrane is a mix of lipids and
proteins.



Bilayers of phospholipids, interspersed with
glycolipids and cholesterol, are the structural
foundation of cell membranes.
Within a bilayer, phospholipids show quite a bit
of movement; they diffuse sideways, spin, and
flex their tails to prevent close packing and
promote fluidity, which also results from shorttailed lipids and unsaturated tails (kinks at
double bonds).
The Plasma Membrane
Proteins perform most of the functions of
cell membranes.



The scattered islands of protein in the sea of
lipids create a “mosaic” effect.
Membrane proteins (most are glycoproteins)
serve as enzymes, transport proteins, receptor
proteins, and recognition proteins.
EXTRACELLULAR
FLUID
receptor
protein
adhesion protein
recognition
protein
cytoskeletal proteins
just beneath the
plasma membrane
transport proteins
phospholipid cholesterol
LIPID
BILAYER
CYTOPLASM
Fig. 3.7, p. 46
Section 4
How Do We See Cells?
How Do We See Cells?
Microscopy allows us to see cells and their
pieces.
Many types of microscopes exist, which can
produce many types of pictures
(micrographs):



Light microscopes use
light to see samples;
specimens usually must
be thin and colored with
dyes to be seen.
Figure 3.8a
How Do We See Cells?

Electron microscopes use beams of electrons
rather than light to see details; transmission and
scanning electron microscopy can magnify
(enlarge) specimens far beyond the limits of the
light microscope.
Figure 3.8b-c
Animation: How an
Electron Microscope Works
CLICK
TO PLAY
Section 5
The Nucleus
The Nucleus
The nucleus encloses DNA, the building
code for cellular proteins.



Its membrane isolates DNA from the sites
(ribosomes in the cytoplasm) where proteins
will be assembled.
The nuclear membrane helps regulate the
exchange of signals between the nucleus and
the cytoplasm.
The Nucleus
A nuclear envelope encloses the nucleus.



The nuclear envelope consists of two lipid
bilayers with pores.
The envelope membranes are continuous with
the endoplasmic reticulum (ER).
nuclear pore
(protein complex
that spans both
lipid bilayers)
one of two
lipid bilayers
(facing
cytoplasm)
NUCLEAR
ENVELOPE
one of two lipid bilayers
(facing nucleoplasm)
Fig. 3.10, p. 49
The Nucleus
 The
nucleolus is where cells make the units
of ribosomes.


The nucleolus appears as a dense mass inside
the nucleus.
In this region, subunits of ribosomes are
prefabricated before shipment out of the nucleus.
The Nucleus
DNA is organized in chromosomes.



Chromatin describes the cell’s collection of
DNA plus the proteins associated with it.
Each chromosome is one DNA molecule and
its associated proteins.
The Nucleus
 Events
that begin in the nucleus continue to
unfold in the cell cytoplasm.


Outside the nucleus, new polypeptide chains for
proteins are assembled on ribosomes.
Some proteins are stockpiled; others enter the
endomembrane system.
Nucleus of an
Animal Cell
Figure 3.9
Section 6
The Endomembrane
System
The Endomembrane System
ER is a protein and lipid assembly line.



The endoplasmic reticulum is a collection of
interconnected tubes and flattened sacs,
continuous with the nuclear membrane.
Rough ER consists of stacked, flattened sacs
with many ribosomes attached; oligosaccharide
groups are attached to polypeptides as they
pass through on their way to other organelles,
membranes, or to be secreted from the cell.
The Endomembrane System

Smooth ER has no ribosomes; it is the area
from which vesicles carrying proteins and lipids
are budded; it also inactivates harmful
chemicals and aids in muscle contraction.
Golgi bodies “finish, pack, and ship.”



In the Golgi body, proteins and lipids undergo
final processing, sorting, and packaging.
The Golgi bodies resemble stacks of flattened
sacs whose edges break away as vesicles.
The Endomembrane System
A variety of vesicles move substances into
and through cells.



Lysosomes are vesicles that bud from Golgi
bodies; they carry powerful enzymes that can
digest the contents of other vesicles, worn-out
cell parts, or bacteria and foreign particles.
Peroxisomes are membrane-bound sacs of
enzymes that break down fatty acids and amino
acids.
RNA messages from the nucleus
vesicle
cytoplasm
ribosome
vesicle
inside nucleus
rough ER
nuclear envelope
Fig. 3.11ab, p. 50
Secretory pathway ends
endocytic pathway begins
smooth ER channel,
cross-section
smooth ER
budding vesicle
plasma membrane
Golgi body
Fig. 3.11c-g, p. 51
Section 7
Mitochondria: The Cell’s
Energy Factories
Mitochondria
Mitochondria make ATP.



Mitochondria are the primary organelles for
transferring the energy in carbohydrates to ATP;
they are found only in eukaryotic cells.
Oxygen is required for the release of this
energy.
Mitochondria
 ATP
forms in an inner compartment of the
mitochondrion.


Each mitochondrion has compartments formed
by inner folded membranes (cristae) surrounded
by a smooth outer membrane.
Mitochondria have their own DNA and some
ribosomes, which leads scientists to believe they
may have evolved from ancient bacteria.
cristae
outer compartment
inner compartment
outer mitochondrial membrane
inner membrane
Fig. 3.12, p. 52
Section 8
The Cell’s Skeleton
The Cell’s Skeleton
The cytoskeleton is an interconnected
system of bundled fibers, slender threads,
and lattices extending from the nucleus to
the plasma membrane in the cytosol.



The main components are microtubules,
microfilaments, and intermediate filaments—
all assembled from protein subunits.
The skeleton helps organize and reinforce the
cell and serves in some cell functions.
microtubules
microfilaments
intermediate
filaments
Figure 3.13
The Cell’s Skeleton
Movement is one function of the
cytoskeleton.


Microtubular extensions of the plasma
membrane display a 9 + 2 cross-sectional array
and are useful in propulsion.
•
•

Flagella are quite long, whiplike, and are found on
animal sperm cells.
Cilia are shorter, more numerous, and may function
as “sweeps” to clear, as an example, the respiratory
tract of dust or other materials.
The microtubules of flagella and cilia arise from
centrioles, which play a role in cell division.
Fig. 3.13, p. 53
one of nine pairs of microtubules
plasma
membrane
microtubules
near base of
flagellum or
cilium
basal body in
cytoplasm
Fig. 3.14, p. 53
Section 9
How Diffusion and
Osmosis Move
Substances Across
Membranes
Diffusion and Osmosis
The plasma membrane is “selective.”




Lipid-soluble molecules and small, electrically
neutral molecules (for example, oxygen, carbon
dioxide, and ethanol) cross easily through the
lipid bilayer.
Larger molecules (such as glucose) and
charged ions (such as Na+, Ca+, HCO3-) must
be moved by membrane transport proteins.
Because some molecules pass through on their
own and others must be transported, the
plasma membrane is said to have the property
of selective permeability.
Selective Permeability
Figure 3.15
Diffusion and Osmosis
In diffusion, a solute moves down a
concentration gradient.


A concentration gradient is established when
there is a difference in the number of molecules
or ions of a given substance between two
adjacent regions.
•
•
Molecules constantly collide and tend to move from
areas of high concentration to areas of low
concentration.
The net movement of like molecules down a
concentration gradient (high to low) is called
diffusion; when this occurs across a plasma
membrane, it is called passive transport.
Diffusion and Osmosis


Molecules move faster
when gradients are
steep, and different
solutes move
independently according
to their respective
gradients.
Electric gradients
(gradients of electrical
charge) are important
to nerve function
dye
dye water
Figure 3.16
Diffusion and Osmosis
Water crosses membranes by osmosis.


Osmosis is the passive diffusion of water
across a differentially permeable membrane in
response to solute concentration gradients.
selectively permeable membrane
between two compartments
water
molecule
protein
molecule
Fig. 3.17, p. 55
Diffusion and Osmosis

Osmotic movements are affected by the relative
concentrations of solutes in the fluids inside
and outside the cell (tonicity).
•
•
•
An isotonic fluid has the same concentration of
solutes as the fluid in the cell; immersion in it causes
no net movement of water.
A hypotonic fluid has a lower concentration of
solutes than does the fluid in the cell; cells immersed
in it may swell as water moves into the cell down its
gradient.
A hypertonic fluid has a greater concentration of
solutes than does the fluid in the cell; cells in it may
shrivel as water moves out of the cell, again down its
gradient.
98% water
2% sucrose
100% water
(distilled)
HYPOTONIC
CONDITIONS
90% water
10% sucrose
HYPERTONIC
CONDITIONS
98% water
2% sucrose
ISOTONIC
CONDITIONS
Fig. 3.18, p. 55
Section 10
Other Ways Substances
Cross Cell Membranes
Crossing Cell Membranes
Many solutes cross membranes through
transport proteins.


In facilitated diffusion, solutes pass through
channel proteins in accordance with the
concentration gradient; this process requires no
input of energy.
•
•
Channel proteins are open to both sides of the
membrane and undergo changes in shape during the
movement of solutes.
The transport proteins are selective for what they
allow through the membrane.
glucose, more
concentrated
outside cell
than inside
When the glucose binding
site is again vacant, the protein
resumes its original shape.
Glucose detaches from the
binding site and diffuses out of
the channel.
transport protein
for glucose
Glucose binds to a vacant
site inside the channel through
the transport protein.
Now the protein changes shape. Part
of the channel closes behind the solute.
Another part opens in front of it.
Fig. 3.20, p. 56
glucose, more
concentrated
outside cell
than inside
transport protein
for glucose
d When the glucose binding
site is again vacant, the protein
resumes its original shape.
c Glucose detaches from the
binding site and diffuses out of
the channel.
a Glucose binds to a vacant
site inside the channel through
the transport protein.
b Now the protein changes shape. Part
of the channel closes behind the solute.
Another part opens in front of it.
Stepped Art
Crossing Cell Membranes

In active transport, solutes move against their
concentration gradients with the assistance of
transport proteins that change their shape with
the energy supplied by ATP.
higher concentration of
calcium outside cell
lower concentration of
calcium inside cell
The pump returns to
its resting shape.
ATP binds to
a calcium pump.
Shape change permits
Calcium enters tunnel
calcium release at opposite
side of membrane. Phosphate through pump.
group and ADP are released.
ATP transfers a phosphate
group to pump. This energy
input will cause pump’s shape to
change.
Fig. 3.21, p. 57
higher concentration of
calcium outside cell
lower concentration of
calcium inside cell
e The pump returns to
its resting shape.
a ATP binds to
a calcium pump.
d Shape change permits
b Calcium enters tunnel
calcium release at opposite
side of membrane. Phosphate through pump.
group and ADP are released.
c ATP transfers a phosphate
group to pump. This energy
input will cause pump’s shape to
change.
Stepped Art
High
Concentration
gradient
ATP
Low
Diffusion of
lipid-soluble
substances
across bilayer
Passive transport of watersoluble substances
through channel protein;
no energy input needed
Active transport
through ATPase;
requires energy
input from ATP
Fig. 3.19, p. 56
Crossing Cell Membranes
 Vesicles


transport large solutes.
Exocytosis moves substances from the
cytoplasm to the plasma membrane during
secretion, moving materials out of the cell.
Endocytosis encloses particles in small portions
of plasma membrane to form vesicles that then
move into the cytoplasm; if this process brings
organic material into the cell, it is called
phagocytosis.
plasma membrane
exocytic vesicle leaving cytoplasm
endocytic
vesicle
forming
Fig. 3.22, p. 57
Section 11
Metabolism: Doing
Cellular Work
Metabolism: Doing Cellular Work
ATP is the cell’s energy currency.



Metabolism refers to all of the chemical
reactions that occur in cells; ATP links the
whole of these reactions together.
ATP is composed of adenine, ribose, and three
phosphate groups.
•
•
ATP transfers energy in many different chemical
reactions; almost all metabolic pathways directly or
indirectly run on energy supplied by ATP.
ATP can donate a phosphate group
(phosphorylation) to another molecule, which then
becomes primed and energized for specific
reactions.
Metabolism: Doing Cellular Work

The ATP/ADP cycle is a method for renewing
the supply of ATP that is constantly being used
up in the cell; it couples inorganic phosphate to
ADP to form energized ATP.
base
ATP
three phosphate groups
cellular work
sugar
reactions
that release
energy
ATP
reactions
that
require
energy
(e.g., synthesis,
breakdown, or
rearrangement
of substances;
contraction of
muscle cells;
active transport
across a cell
membrane)
ADP + Pi
Fig. 3.23, p. 58
Metabolism: Doing Cellular Work
There are two main types of metabolic
pathways.


Metabolic pathways form series of
interconnected reactions that regulate the
concentration of substances within cells.
•
•
In anabolism, small molecules are assembled into
large molecules—for example, simple sugars are
assembled into complex carbohydrates.
In catabolism, large molecules such as
carbohydrates, lipids, and proteins are broken down
to form products of lower energy, releasing energy
for cellular work.
Metabolism: Doing Cellular Work

Pathways exist as enzyme-mediated linear or
circular sequences of reactions involving the
following:
•
•
•
Reactants are the substances that enter a reaction.
Intermediates are substances that form between the
start and conclusion of a metabolic pathway.
End products are the substances present at the
conclusion of the pathway.
enzyme
enzyme
A
B
enzyme
C
D
end product
Stepped Art
A
enzyme 1
end product
B
D
enzyme 2
enzyme 3
C
Stepped Art
Metabolism: Doing Cellular Work
 Enzymes


play a vital role in metabolism.
Enzymes are proteins that serve as catalysts;
they speed up reactions.
Enzymes have several features in common:
• Enzymes do not make anything happen that could not
happen on its own; they just make it happen faster.
• Enzymes can be reused.
• Enzymes act upon specific substrates, molecules
which are recognized and bound at the enzyme’s
active site.
two substrate molecules
substrates
contacting
active site
of enzyme
substrates
briefly bind
tightly to
enzyme
active site
product
molecule
enzyme unchanged
by the reaction
two substrate molecules
substrates
contacting
active site
of enzyme
substrates
briefly bind
tightly to
enzyme
active site
product
molecule
enzyme unchanged
by the reaction
Stepped Art
Metabolism: Doing Cellular Work


Because enzymes operate best within defined
temperature ranges, high temperatures
decrease reaction rate by disrupting the bonds
that maintain three-dimensional shape
(denaturation occurs).
Most enzymes function best at a pH near 7;
higher or lower values disrupt enzyme shape
and halt function.
Metabolism: Doing Cellular Work

Coenzymes are large organic molecules such
as NAD+ and FAD (both derived from vitamins),
which transfer protons and electrons from one
substrate to another to assist with many
chemical reactions.
Figure 3.29
Section 12
How Cells Make ATP
How Cells Make ATP
Cellular respiration makes ATP.



Electrons acquired by the breakdown of
carbohydrates, lipids, and proteins are used to
form ATP.
Overall, the formation of ATP occurs by cellular
respiration; in humans this is an aerobic
process meaning it requires oxygen.
How Cells Make ATP
Step 1: Glycolysis breaks glucose down to
pyruvate.


Glycolysis reactions occur in the cytoplasm
and result in the breakdown of glucose to
pyruvate, generating small amounts of ATP.
•
•

Glucose is first phosphorylated in energy-requiring
steps, then split to form two molecules of PGAL.
Four ATP are produced by phosphorylation in
subsequent reactions; but because two ATP were
used previously, there is a net gain of only two ATP
by the end of glycolysis.
Glycolysis does not use oxygen.
GLUCOSE
ATP
ADP
P
Energy
in
(2 ATP)
ATP
ADP
P
PGAL: P
P
P
INTERMEDIATES DONATE
PHOSPHATE TO ADP, MAKING 4
Pyruvate
ATP
To second
set of
reactions
NET ENERGY YIELD: 2 ATP
How Cells Make ATP
Step 2: The Krebs cycle produces energyrich transport molecules.



Pyruvate (produced in the cytoplasm) enters
the mitochondria for the oxygen requiring steps
of cellular respiration.
The pyruvate is converted to acetyl-CoA,
which enters the Krebs cycle to eventually be
converted to CO2.
How Cells Make ATP

Reactions within the mitochondria and the
Krebs cycle serve three important functions:
•
•
•
Two molecules of ATP are produced by substratelevel phosphorylation.
Intermediate compounds are regenerated to keep
the Krebs cycle going.
H+ and e- are transferred to NAD+ and FAD,
generating NADH and FADH2.
How Cells Make ATP
Step 3: Electron transport produces many
ATP molecules.


The final stage of cellular
respiration occurs in the
electron transport
systems embedded in
the inner membranes
(cristae) of the mitochondrion.
How Cells Make ATP

NADH and FADH2 from previous reactions give
up their electrons to transport (enzyme)
systems embedded in the mitochondrial inner
membrane.
•
•
Electrons flow through the system eventually to
oxygen, forming water; as they flow, H+ are pumped
into the outer compartment of the mitochondrion to
create a proton gradient.
H+ ions move down their gradient, through a channel
protein called ATP synthase, in the process driving
the synthesis of ATP.
How Electron Transport Forms ATP
Figure 3.26
Section 13
Summary of Cellular
Respiration
Summary of Cellular Respiration

In total, glycolysis, the Krebs cycle, and the
electron transport system can yield a
maximum of 36 ATP per glucose molecule.
CYTOPLASM
2
glucose
ATP
4 ATP
GLYCOLYSIS
energy input to
start reactions
e- + H+
(2 ATP net)
2 pyruvate
2 NADH
MITOCHONDRION
e- + H+
2
CO2
2 NADH
8 NADH
2 FADH2
e-
e- + H+
KREBS
CYCLE
e- + H+
ELECTRON
TRANSPORT
SYSTEM
H+
4
CO2
2
ATP
32 ATP
water
e- + oxygen
TYPICAL ENERGY YIELD: 36 ATP
Fig. 3.27, p. 62
Section 14
Alternative Energy
Sources in the Body
Alternative Energy Sources
How the body uses carbohydrates as fuel.




Excess carbohydrate intake is stored as
glycogen in liver and muscle for future use.
Free glucose is used until it runs low; then
glycogen reserves are tapped.
Under some conditions a process called lactate
fermentation can be used to produce ATP;
here, pyruvate is converted
directly to lactic acid with
production of quick, but
limited, energy.
Figure 3.28
Alternative Energy Sources
Fats and proteins also provide energy.


Lipids are used when carbohydrate supplies
run low.
•
•
•
Excess fats are stored away in cells of adipose
tissue.
Fats are digested into glycerol (which enters
glycolysis) and fatty acids, which enter the Krebs
cycle.
Because fatty acids have many more carbon and
hydrogen atoms, they are degraded more slowly and
yield greater amounts of ATP.
Alternative Energy Sources

Proteins are used as the last resort for
supplying energy to the body.
•
•
•
Amino acids are released by enzymatic digestion of
proteins; protein is never stored by the body.
After the amino group is removed, the amino acid
remnant is fed into the Krebs cycle to produce
energy (ATP), or is used to make fats and
carbohydrates.
Ammonia (from the amino group) is excreted as
waste.