Download Chapter 4 – A Tour of the Cell

What is a cell?
• All living things possess levels of organization.
Organism
Organ System
Organ
Tissue
Cell
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What is a Cell?
• Cells are the building blocks of all organisms.
– They are the smallest living parts of an
organism.
– They are the smallest structures that can be
classified as living.
• An organism can be unicellular, consisting of a
single cell, or multicellular, consisting of many
cells.
The Cell Theory
• Cells are very small, so small that they were
not discovered until microscopes were
invented in the mid-seventeenth century.
• In 1665, Robert Hooke examined a piece of
cork using a basic, homemade microscope.
• Hooke observed a
matrix of tiny rooms,
which he called cells.
• Another scientist,
Antoni van
Leeuwenhoek, was the
first to observe and
describe unicellular
organisms.
Drawing of cork (dead plant tissue) by Robert
Hooke that appeared in Micrographia
The Cell Theory
• Advances in microscopy led to the Cell Theory
• The theory states that:
– A cell is the most basic unit of life
– All living things are composed of cells
– All cells arise from pre-existing cells
Cell
Organism
Organs
Tissues
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The Cell Theory
• Advances in microscopy led to the Cell Theory
• The theory states that:
– A cell is the most basic unit of life
– All living things are composed of cells
– All cells arise from pre-existing cells
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Why are cells so small?
• The maximum size of a cell is limited by the
amount of surface area it needs to obtain
nutrients and to exchange gases with its
environment.
• In the center of every cell
is the machinery (e.g. DNA,
enzymes, ribosomes) that
the cell needs to function.
• All of these components must
communicate with one another.
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Surface Area to Volume Ratio
• A cell’s surface provides the
only way to pass substances
into and out of the cell.
• As a cell’s size increases, both
its surface area and volume
increase, as well.
• However, the volume increases
faster than the surface area,
making exchange of
substances less efficient the
larger the cell becomes.
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Surface Area to Volume Ratio
• A cell’s surface area is
determined according to the
following calculation:
Surface Area =
4 x π x (radius)2
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Surface Area to Volume Ratio
• A cell’s volume is
determined according to the
following calculation:
Volume =
4/3 x π x (radius)3
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Surface Area to Volume Ratio
• As a cell’s size increases, its
radius increases.
• Surface area is calculated
by squaring the radius.
• Volume is calculated by
cubing the radius.
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Surface Area to Volume Ratio
• Both surface area and volume
increase as cell size increases
but volume increases faster
than surface area.
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Surface Area to Volume Ratio
• For a small cell, the ratio
between its surface area
and its volume is large
enough to permit efficient
transfer of materials.
• For a large cell, however,
the surface area to volume
ratio is much smaller.
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Surface Area to Volume Ratio
• In a small cell, surface
area exceeds volume
(SA:V is >1).
• In a large cell, volume
exceeds surface area
(SA:V is <1).
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• By remaining small, a cell has enough surface
area to accommodate its volume, and thus all
of the machinery contained within it.
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Prokaryotes vs. Eukaryotes
• All organisms can be divided into two groups,
based on their cell type:
– Prokaryotes
– Eukaryotes
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Prokaryotes vs. Eukaryotes
• Prokaryotes are the simplest organisms and
have the simplest cells.
• All Prokaryotes are unicellular and belong to
the domain
– Archaea
or
– Bacteria
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Prokaryotes vs. Eukaryotes
• Eukaryotes include unicellular and multi-cellular
organisms.
• Eukaryotes belong to
the domain Eukarya
– Kingdom Plantae
– Kingdom Fungi
– Kingdom Animalia
– Kingdom Protista
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Eukaryotes: A Quick Review
• Kingdom Protista: mostly unicellular, includes
heterotrophic and autotrophic organisms
• Kingdom Fungi: unicellular and multicellular
organisms, all heterotrophic (absorption)
• Kingdom Plantae: all multicellular and
autotrophic organisms
• Kingdom Animalia: all multicellular and
heterotrophic organisms
Prokaryotic Cells
• Prokaryotic cells are structurally simpler, and
smaller than eukaryotic cells.
• A prokaryotic cell lacks a nucleus,
the membrane-bound organelle
where DNA is stored.
• A prokaryotic cell is enclosed
by a plasma membrane,
but has no distinct
interior components.
– No membrane-bound organelles
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Prokaryotic Cells
• Prokaryotic cells contain ribosomes.
• Ribosomes are the sites of protein
synthesis, but are not considered
to be organelles because they are
not bound by a membrane.
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Eukaryotic Cells
• Eukaryotic cells are larger and structurally
more complex than prokaryotic cells.
• Every eukaryotic cell has a
nucleus, a membrane-bound
organelle where the cell’s
DNA is stored.
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Eukaryotic Cells
• The interior of a eukaryotic cell is divided into
functional compartments.
• Membrane-bound organelles
perform specialized functions
within the cell and are
connected via an extensive
internal membrane system.
• The organelles are anchored
in place by a cytoskeleton.
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Eukaryotic Cells
• Eukaryotic cells are further divided into:
– Plant cells
– Animal cells
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Plant Cells
• Plant cells, as well as those of many protists,
have a sturdy cell wall that surrounds
their plasma membrane.
• The cell wall is made of
cellulose.
• Animal cells do not
possess cell walls.
Plasma membrane
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Plant Cells
• Plant cells (and photosynthetic protists) also
possess chloroplasts, the organelle responsible
for photosynthesis.
• Animal and fungal cells
do not contain
chloroplasts, and are
therefore not capable of
performing photosynthesis.
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Plant Cells
• Plant cells (and some protist cells) also possess
a large central vacuole.
• The central vacuole may store
water, chemicals), or
waste products.
• The central vacuole usually
occupies ~30% of the cell’s
volume, but can occupy as
much as 80% in specialized cells.
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Plant Cells
• Central vacuoles help maintain pressure against
the cell wall, which keeps the plant in an
upright position, and pushes
the chloroplasts closer to
the membrane, and so
closer to the light!
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Animal Cells
• In contrast, animal cells do not possess cell
walls, chloroplasts, plasmodesmata or central
vacuoles.
• Instead, animal cells contain
food vacuoles, which are
involved in the engulfing
of food particles, while
other vacuoles store and
transport lipids and proteins
out of the cell.
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Nucleus
Ribosomes
No:
• Food
vacuole
• Centrioles
Central vacuole
Chloroplast
Cell wall
Plasma membrane
A Plant Cell
Cell wall of
adjacent cell
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NUCLEUS:
An Animal Cell
Nuclear envelope
Nucleus
Lysosome
Centriole
Plasma membrane
Ribosomes
No:
• Central
Plasmavacuole
membrane
• Chloroplasts
Mitochondrion• Cell Wall
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The Cell: Organelles
Cell
Organelles
• Eukaryotic cells contain
organelles, membranebound components with
specialized functions.
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The Nucleus
• One of the most important organelles in a
eukaryotic cell is the nucleus.
• The nucleus is the cell’s control center.
• The nucleus contains most of
the cell’s DNA and controls the
cell’s activities by directing
protein synthesis.
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Ribosomes
• Ribosomes carry out protein synthesis via the
genetic instructions encoded in the DNA.
• A single cell can contain a few million
ribosomes!
• Ribosomes consist of a
large subunit and a
small subunit, made
up of ribosomal RNA
and protein.
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The Endoplasmic Reticulum
• The endoplasmic reticulum (ER) is an extensive
system of internal membranes surrounding
the nucleus.
• The ER is further differentiated into:
– Rough endoplasmic reticulum
– Smooth endoplasmic reticulum
• The surface of the rough ER is
studded with ribosomes, while
the smooth ER lacks ribosomes.
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The Endoplasmic Reticulum
Smooth ER
Nuclear
envelope
Ribosomes
Rough ER
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The Endoplasmic Reticulum
• The ribosomes bound to the
rough ER produce proteins which
enter the ER membrane and are
transported to other organelles
or secreted by the cell.
• The smooth ER is embedded
with enzymes that aid in the
production of lipids and
carbohydrates.
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Rough Endoplasmic Reticulum
• Insulin – the hormone from the
pancreas responsible for
converting glucose into
glycogen - is an example of a
protein produced by
ribosomes, and secreted by
each cell via the rough
endoplasmic reticulum.
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Smooth Endoplasmic Reticulum
• Cells in the ovaries and
testes are rich in smooth ER
for their role in the
production of steroid sex
hormones.
• Smooth ER contains
enzymes which process
drugs and other potentially
harmful substances.
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The ER creates a series of interconnected channels
but also isolates some spaces by creating
membrane-enclosed sacs known as vesicles.
Vesicle budding
off of the ER
Protein inside
transport
vesicle
Ribosome
Sugar chains
Polypeptid
e
Glycoprotein
Rough
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The Golgi Complex
• After biomolecules are produced in the ER,
they are passed along to flattened stacks of
membranes known as Golgi bodies.
• The number of Golgi bodies in a cell ranges
from one or a few in protists, to 20 or more in
animal cells, and several hundred in certain
plant cells!
• Collectively, these Golgi bodies are referred to
as the Golgi complex or Golgi apparatus.
• Proteins and lipids
manufactured on the
ER membranes are
transported through
ER channels and
packaged into
transport vesicles
that bud off the ER.
• The vesicles fuse with
the Golgi bodies,
releasing their
contents.
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• Within the Golgi
complex, many of the
proteins and lipids are
tagged with
carbohydrates.
• The molecules collect
at the end of the Golgi
complex, where they
pinch off into vesicles
and are transported to
other regions of the
cell or to the plasma
membrane for
secretion.
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• The Golgi complex receives, sorts, chemically
alters and packages important molecules
manufactured by the ER.
• One side of the Golgi stack serves as a
receiving dock, while the other serves as a
shipping facility, where vesicles bud off to
transport the prepared materials elsewhere.
Receiving side
Transport
vesicle from the
ER
Transport
vesicle from the
Golgi
Shipping side
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Lysosomes
• Organelles known as lysosomes emerge from
the Golgi complex containing a concentrated
mix of enzymes manufactured in the rough ER
to break down macromolecules within the cell.
• Lysosomes are the recycling centers of the cell,
digesting worn-out organelles and cellular
components.
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Mitochondria: ATP Production
• Eukaryotic organisms extract energy from
organic molecules in a process known as
cellular respiration, whereby the chemical
energy of food (e.g. glucose) is converted into
the chemical energy of ATP.
• ATP or adenosine triphosphate is the main
energy source for cellular work.
– the molecular currency by which energy is
transferred within cells for metabolism.
Mitochondria: ATP Production
• Small quantities of ATP are produced within
cellular cytoplasm in a process known as
glycolysis.
• However, much larger quantities
of ATP are produced in the
presence of oxygen within
specialized organelles known
as mitochondria.
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Mitochondria: ATP Production
• The mitochondria are sausage-shaped
organelles, bound by two membranes.
• Mitochondria contain their own DNA.
– Different from cellular DNA stored in the nucleus.
– Similar to bacterial DNA!
• Mitochondria reproduce in a manner similar
to the asexual reproduction
of prokaryotes.
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• The unique characteristics of the mitochondria
provide evidence for endosymbiosis.
• Mitochondria are believed to have once
existed as free-living prokaryotes which were
engulfed and retained by ancient eukaryotic
cells approximately 1.5 billion years ago.
• Chloroplasts, the organelles
responsible for
photosynthesis, are also
believed to have evolved
via endosymbiosis.
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• Rather than being broken down by digestive
enzymes, the engulfed prokaryotes provided their
hosts with advantages from their special
metabolic abilities: oxidative metabolism and
photosynthesis!
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The Cytoskeleton
• Organelles do not float freely within the cell.
• Rather, they are supported and transported by
a dense network of protein fibers called the
cytoskeleton.
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The Cytoskeleton
• The cytoskeleton:
– Anchors organelles to fixed locations in the cell.
– Provides a framework that supports the shape
of the cell.
– Organizes cellular activities by providing a
scaffold that anchors ribosomes and enzymes.
– Acts as tracks along which organelles can move.
Protein Fibers of the Cytoskeleton
• The protein fibers of the cytoskeleton are
constantly being formed and disassembled.
• There are three different types of protein fibers:
– Microfilaments
– Intermediate
filaments
– Microtubules
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Microfilaments
• Microfilaments are
long, slender rods of a
protein called actin.
• Microfilaments are
made of two actin
molecules twisted
together into a double
chain.
7 nanometers
thick
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Intermediate
Filaments
• Intermediate filaments
are composed of
overlapping proteins
that form a rope-like
structure.
• These structures serve
to reinforce cell shape
and to anchor certain
organelles in place.
10 nanometers
thick
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Microtubules
• Microtubules are
straight, hollow tubes
composed of the
protein tubulin.
• Microtubules shape
and support the cell,
and act as tracks along
which organelles can
travel.
25 nanometers
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Cell Movement
• Cellular movement is dependent upon the
movement of microfilaments, microtubules or
both.
• Microfilaments play a major role in determining
the shape of cells because they can form and
dissolve very rapidly.
• The ability of microfilaments to form and
dissolve quickly enables some cells to crawl.
• Other cells use flagella or cilia for movement.
Flagella and Cilia
• Flagella are long, singular appendages used
for locomotion.
• Cilia are short, numerous appendages that
enable cell movement, and generate currents.
• Both flagella and cilia are structurally similar,
consisting of a circle of nine microtubule pairs
surrounding two central microtubules.
– This 9+2 arrangement is a fundamental
feature of eukaryotic cells.
9+2 Arrangement of Microtubules
1
2
9
3
8
7
4
6
5
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Flagella
• The undulating flagellum on a human sperm
cell propels the cell in a swimming motion
toward the egg.
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Cilia
• Cilia on cells lining the human windpipe create
currents that sweep mucus containing foreign
debris out and away from the lungs.
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Review: All Cells Contain…
• Regardless of cell type (prokaryotic or
eukaryotic, plant or animal, etc), ALL cells
contain:
– DNA
– Ribosomes
– Plasma membrane