The Cell
Chapter 6
How do we know about
cells?
1.
Microscopes: windows to the world of the cell
• The discovery and early
study of cells progressed
with the invention and
improvement of
microscopes in the 17th
century.
• In a light microscope (LM)
visible light passes through
the specimen and then
through glass lenses
• Microscopes vary in magnification and resolving power.
Resolving power is a measure of image clarity.
• It is the minimum distance two points can be separated by
and still be viewed as two separate points.
Robert Hooke 1665
• The minimum resolution of a
light microscope is about 2
microns, the size of a small
bacterium
• Light microscopes can
magnify effectively to about
1,000 times the size of the
actual specimen.
• Techniques developed in the 20th century have enhanced contrast
and enabled cell components to be labeled so that they stand out.
To resolve cell organelles we use an
electron microscope (EM), which
focuses a beam of electrons through the
specimen or onto its surface.
• Electron microscopes have finer
resolution than light microscopes
• Transmission electron microscopes (TEMs) are used mainly
to study the internal ultrastructure of cells.
• A TEM aims an electron beam through a thin section of the
specimen.
Cucumber cotyledon
• Scanning electron microscopes (SEMs) are useful for
studying surface structures.
• The image is focused on a screen
• Three dimensional
• The SEM has great
depth of field,
resulting in an
image that seems
three-dimensional.
Rabbit trachea cells (SEM)
• Electron microscopes reveal organelles, but they can only be
used on dead cells.
• Light microscopes do not have as high a resolution, but they can
be used to study live cells.
2. Cell biologists can isolate organelles to study their
functions and separate chemical components
• Cell fractionation separates the major organelles of the cells so
that their individual functions can be studied.
• This process is driven by an ultracentrifuge, a machine that
can spin at up to 130,000 revolutions per minute and apply
forces more than 1 million times gravity (1,000,000 g).
• Microcentrifuge is standard equipment in biotechnology labs
activities.
Equipment used to study cells at the genetic and protein level.
Paper chromatography separates leaf pigments
The Cell Theory
Understanding the cellular nature of life followed the development of
tools and techniques:
In 1665, Robert Hooke observed
"compartments" in a thin slice of cork (oak
bark) using a light microscope. Used the
term “Cell.”
By 1700, Anton van Leeuwenhoek developed simple light
microscopes with high-quality lenses to observe tiny living
organisms, such as those in pond water.
"animalcules"
The Cell Theory
Generalization that all living things are composed of cells.
Cells are the basic unit of structure and function in living things
Cells come from pre-existing cells
3. Two Major Classes of Cells:
Prokaryotic and Eukaryotic
• All cells are surrounded by a plasma membrane.
• All cells contain chromosomes which have genes in the form of DNA.
• All cells also have ribosomes
Prokaryotic cell movie
•Prokaryotic and eukaryotic cells differ in the location of chromosomes.
•Eukaryotic cell chromosomes are in a nucleus.
•In a prokaryotic cell, the DNA is concentrated in the nucleoid without
a membrane separating it from the rest of the cell.
Eukaryotic cell movie
The prokaryotic cell is much simpler in structure, lacking a nucleus and the other membraneenclosed organelles of the eukaryotic cell.
• What limits cell size?
• As a cell increases in size its volume
increases faster than its surface area.
• Smaller objects have a greater
ratio of surface area to volume.
• Square/Cube Law
• What cell organelle is critical in
maintaining this ratio?
• The plasma membrane functions as a selective barrier that
allows passage of oxygen, nutrients, and wastes for the whole
volume of the cell.
• The volume of cytoplasm determines the need for this
exchange.
• Rates of chemical exchange may be inadequate to maintain
a cell with a very large cytoplasm.
• The need for a surface sufficiently large to accommodate
the volume explains the microscopic size of most cells.
• Larger organisms do not generally have larger cells than
smaller organisms - simply more cells.
4. Internal membranes compartmentalize the functions
of a eukaryotic cell
• A eukaryotic cell has extensive and elaborate internal membranes,
which partition the cell into compartments.
• Many enzymes are built into membranes.
• Membranes provide different local environments for specific
metabolic functions.
• Each type of membrane has a unique combination of lipids and
proteins for its specific functions.
5. The nucleus contains a eukaryotic cell’s genetic
library
• The nucleus contains most of
the genes in a eukaryotic cell.
• Some genes are located in
mitochondria and
chloroplasts.
• The nucleus is separated from
the cytoplasm by a double
membrane.
• Pores allows large
macromolecules and particles to
pass through.
• The nuclear side of
the envelope is lined
by a network of
filaments that
maintain the shape
of the nucleus.
• Within the nucleus, the DNA and associated proteins are
organized into chromatin.
• In a normal cell they appear as a diffuse mass.
•
When the cell prepares to divide, the chromatin fibers coil up to
be seen as separate structures, chromosomes.
• What is special about chromosome numbers?
• In the nucleus is the nucleolus.
• In the nucleolus, ribosomal RNA is synthesized and
assembled with proteins to form ribosomal subunits.
• The subunits pass from the nuclear pores to the cytoplasm
where they combine to form ribosomes.
Trace the path from
gene to the protein
product.
6. Ribosomes build a cell’s proteins
• Ribosomes contain rRNA and protein.
• A ribosome is composed of two subunits that combine to carry out
protein synthesis.
• What is implied if a cell type has large numbers of ribosomes
and prominent nuclei. (e.g., pancreas)
• Free ribosomes, are suspended in the cytoplasm and synthesize
proteins that function within the cytoplasm.
• Bound ribosomes, are attached to the outside of the
endoplasmic reticulum.
The Endomembrane System
•Many internal membranes in a eukaryotic cell are part of the
endomembrane system.
•The endomembrane system includes the nuclear envelope,
endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and the
plasma membrane.
What is the adaptive
value of this system?
7. The endoplasmic reticulum manufactures membranes
and modifies proteins
• The endoplasmic reticulum
(ER) accounts for half the
membranes in a eukaryotic
cell.
• The ER includes membranous
tubules and internal, fluidfilled spaces, the cisternae.
• There are two regions of ER that
differ in structure and function.
• Smooth ER looks smooth
because it lacks ribosomes.
• Rough ER looks rough
because ribosomes (bound
ribosomes) are attached to the
outside, including the outside
of the nuclear envelope.
• Smooth ER is rich in enzymes
and plays a role in a variety of
metabolic processes.
• Enzymes of smooth ER
synthesize lipids, including oils,
phospholipids, and steroids.
• The smooth ER helps catalyze
conversion of glucose from stored
glycogen in the liver.
• Smooth ER of the liver help
detoxify drugs and poisons.
(proliferation of smooth ER
increases tolerance to the target
and other drugs)
• Rough ER is especially abundant in those cells that secrete proteins.
• As a polypeptide is synthesized by the ribosome, it is threaded
into the cisternal space through a pore formed by a protein in the
ER membrane.
• The protein is modified in the ER
• These secretory proteins are packaged in transport vesicles that
carry them to their next stage.
8. The Golgi apparatus finishes, sorts, and ships cell
products
• Many transport vesicles from the ER travel to the Golgi apparatus
for modification of their contents.
• The Golgi is a center of manufacturing, warehousing, sorting, and
shipping.
• Which cells would have extensive Golgi apparatus?
DR. CAMILLO GOLGI
(1843-1926)
• The Golgi apparatus consists of flattened membranous sacs cisternae - looking like a stack of pita bread.
9. Lysosomes are digestive compartments
• The lysosome is a membrane-bounded sac of hydrolytic enzymes
that digests macromolecules.
• Lysosomal enzymes can hydrolyze proteins, fats, polysaccharides,
and nucleic acids.
• These enzymes work best at pH 5.
• What is the value of this compartmentalization?
• The lysosomal enzymes and membrane are synthesized by
rough ER and then transferred to the Golgi.
• At least some
lysosomes
bud from
the trans
face of
the Golgi.
• Lysosomes can fuse with food vacuoles, formed when a food
item is brought into the cell by phagocytosis.
• Lysosomes can also
fuse with another
organelle or part
of the cytosol.
• This recycling,
or
autophagy,
renews the cell.
Lysosome Movie
10. Vacuoles have diverse functions in cell maintenance
• Vesicles and vacuoles (larger versions) are membrane-bound sacs
with varied functions.
• Food vacuoles, from phagocytosis, fuse with lysosomes.
• Contractile vacuoles, found in freshwater protists, pump
excess water out of the cell.
• Central vacuoles are found in many mature plant cells.
• What is the adaptive role of the endomembrane system?
• The endomembrane system plays a key role in the synthesis
(and hydrolysis) of macromolecules in the cell.
• The various
components
modify
macromolecules
for their various
functions.
11. Mitochondria and chloroplasts are the main energy
transformers of cells
• Mitochondria and chloroplasts are the organelles that convert
energy to forms that cells can use for work.
• Mitochondria are the sites of cellular respiration, generating ATP
from the breakdown of sugars, fats, and other fuels in the presence
of oxygen.
• Chloroplasts, found in plants and eukaryotic algae, are the sites of
photosynthesis.
• They convert solar energy to chemical energy and synthesize
new organic compounds from CO2 and H2O.
• Mitochondria and chloroplasts are not part of the endomembrane
system.
• Their proteins come primarily from free ribosomes in the cytosol and
a few from their own ribosomes.
• Both organelles have small quantities of DNA that direct the synthesis
of the polypeptides produced by these internal ribosomes.
• Mitochondria and chloroplasts grow and reproduce as semiindependent organelles.
• Mitochondria have a smooth outer
membrane and a highly folded
inner membrane, the cristae.
• This creates a fluid-filled space
between them.
• The cristae present ample
surface area for the enzymes
that synthesize ATP.
• The inner membrane encloses the
mitochondrial matrix, a fluidfilled space with DNA, ribosomes,
and enzymes.
• The chloroplast is one of several members of a generalized class
of plant structures called plastids.
• The chloroplast produces sugar via photosynthesis.
• Chloroplasts gain their color from high levels of the green
pigment chlorophyll.
• Inside the innermost membrane is
a fluid-filled space, the stroma, in
which float membranous sacs, the
thylakoids.
• The stroma contains DNA,
ribosomes, and enzymes for
part of photosynthesis.
• The thylakoids, flattened sacs,
are stacked into grana and are
critical for converting light to
chemical energy.
• Like mitochondria, chloroplasts are dynamic structures.
• Their shape is plastic and they can reproduce themselves by
pinching in two.
• Mitochondria and chloroplasts are mobile and move around the
cell along tracks in the cytoskeleton.
12. Providing structural support to the cell, the
cytoskeleton also functions in cell motility and regulation
• The cytoskeleton is a network
of fibers that provide
mechanical support and
maintains shape of the cell.
• The cytoskeleton provides
anchorage for many
organelles, enzymes, and
organizes cell structures and
activities.
• The cytoskeleton also plays a major role in cell motility.
• The cytoskeleton interacts with motor proteins.
• In cilia and flagella motor proteins pull components
of the cytoskeleton past each other.
• This is also true in muscle cells.
• Motor molecules also carry vesicles or organelles to various
destinations along “monorails’ provided by the cytoskeleton.
• Interactions of motor proteins and the cytoskeleton circulate
materials within a cell by cytoplasmic streaming.
•There are three main types of fibers in the cytoskeleton: microtubules,
microfilaments, and intermediate filaments.
• Microtubules, the thickest fibers, are constructed of the
globular protein, and they grow or shrink as more molecules are
added or removed.
• They move chromosomes during cell division.
• Another function is
as tracks that guide
motor proteins
carrying organelles
to their destination.
•In many cells, microtubules grow out from a centrosome near the
nucleus.
•In animal cells, the centrosome has a pair of centrioles, each with
nine triplets of microtubules arranged in a ring.
•During cell division the centrioles replicate.
• Microtubules are the central structural supports in cilia and flagella.
• In spite of their differences, both cilia and flagella have the
same ultrastructure.
• Microtubules arranged in the “9 + 2” pattern.
• The bending of cilia and flagella is driven by the arms of a motor
protein, dynein.
• Addition to dynein of a phosphate group from ATP and its
removal causes changes in the protein.
• Dynein arms alternately
grab, move, and release
the outer microtubules.
• Protein cross-links limit
sliding and the force is
expressed as bending.
Cilia and Flagella Movie
• Microfilaments, the thinnest
class of the cytoskeletal
fibers, are solid rods of the
globular protein actin.
• With other proteins, they
form a three-dimensional
network just inside the
plasma membrane.
The shape of the microvilli in this
intestinal cell are supported by
microfilaments, anchored to a network
of intermediate filaments.
• In muscle cells, thousands of actin filaments are arranged parallel
to one another.
• Thicker filaments composed of a motor protein, myosin,
interdigitate with the thinner actin fibers.
• Myosin molecules walk along the actin filament, pulling
stacks of actin fibers together and shortening
the cell.
• In other cells, these actin-myosin clusters still cause localized
contraction.
• A contracting belt of microfilaments divides the cytoplasm of
animal cells during cell division.
• Localized contraction also drives amoeboid movement.
• In plant cells (and others), actin-myosin interactions and sol-gel
transformations drive cytoplasmic streaming.
• This creates a circular flow of cytoplasm in the cell.
• This speeds the distribution of materials within the cell.
• Intermediate filaments are
specialized for bearing tension.
• Intermediate filaments are built
of proteins called keratins.
• Intermediate filaments are more
permanent fixtures of the
cytoskeleton than are the other two
classes.
• They reinforce cell shape and fix
organelle location.
13. Plant cells are encased by cell walls
• The cell wall, found in prokaryotes, fungi, and some protists, has
multiple functions.
• In plants, the cell wall protects the cell, maintains its shape, and
prevents excessive uptake of water.
• The thickness and chemical composition of cell walls differs from
species to species and among cell types.
• Consists of microfibrils of cellulose embedded in a matrix of
proteins and other polysaccharides.
• A mature cell wall consists of a primary cell wall, a middle
lamella with sticky polysaccharides that holds cell together, and
layers of secondary cell wall.
14. Animal cells have an extracellular matrix functions
in support, adhesion, movement, and regulation
• Lacking cell walls,
animals cells have an
elaborate extracellular
matrix (ECM).
15. Intercellular junctions help cells transport and
communicate
• Neighboring cells in tissues, organs, or organ systems often
adhere, interact, and communicate through direct physical contact.
• Plant cells are perforated with plasmodesmata, channels
allowing cysotol to pass between cells.
MEMBRANE STUCTURE AND FUNCTION
1. Membranes are mosaics of structure and function
• A membrane is a collage of different proteins embedded in the
fluid matrix of the lipid bilayer.
2. Membrane Structure
It is the boundary that separates the interior of a living cell from its
surroundings.
The membrane is a remarkable film so thin that you would have to stack
8,000 of them to equal the thickness of a sheet of paper.
Membranes are composed
mostly of proteins and a type
of lipid called phospholipids.
Phospholipids are in two layers (bilipid)
3. Membranes are fluid
• Membrane molecules are held in place by relatively weak
hydrophobic interactions.
• Most of the lipids and some proteins can drift laterally in the plane
of the membrane, but rarely flip-flop from one layer to the other.
• The lateral movements of phospholipids are rapid, about 2
microns per second.
• Many larger membrane proteins move more slowly but do drift.
Proteins are important to membrane functions
Cell membranes have many functions beyond serving as a
boundary!
• The proteins in the plasma membrane may provide a variety of
major cell functions.
• The proteins determine most of the membrane’s specific
functions.
• Surface of the protein often connect to the other membrane
proteins.
• Integral proteins penetrate and may span the hydrophobic core
of the lipid bilayer.
How do you think the amino
acids differ in the integral
proteins?
Membrane Structure Movie
Membrane carbohydrates are important for cell-cell
recognition
• Membrane carbohydrates are usually branched saccharides with
fewer than 15 sugar units.
• They may be covalently bonded either to lipids or proteins.
The saccharides on the
membrane may be unique and
serve for cell recognition.
Human blood groups (A,
B, AB, and O) differ in
the external carbohydrates
on red blood cells.
4. A membrane’s molecular organization results in
selective permeability
• A steady traffic of small molecules and ions moves across the
plasma membrane in both directions.
• Sugars, amino acids, and other nutrients enter a cell and
metabolic waste products leave.
• The cell absorbs oxygen and excretes carbon dioxide.
• It also regulates concentrations of inorganic ions, like Na+,
K+, Ca2+, and Cl-, by shuttling them across the membrane.
• However, substances do not move across the barrier
indiscriminately; membranes are selectively permeable.
• What determines whether materials pass through
membranes?
• Permeability of a molecule depends on the interaction of that
molecule with the hydrophobic core of the membrane.
• Hydrophobic molecules, like hydrocarbons, CO2, and O2 can and
cross easily.
• Ions and polar molecules pass through with difficulty.
• This includes small molecules, like water, and larger critical
molecules, like glucose and other sugars.
• Ions, whether atoms or molecules, and their surrounding shell of
water also have difficulties penetrating the hydrophobic core.
• Specific ions and polar molecules can cross the lipid bilayer by
passing through transport proteins that span the membrane.
• Each transport protein is specific as to the substances that it will
translocate (move).
5. Passive transport is diffusion across a membrane
• Diffusion is the tendency of molecules of any substance to spread
out in the available space
• Diffusion is driven by energy (thermal motion or heat) of
molecules.
• Movements of individual molecules are random.
• However, movement of a population of molecules may be
directional.
• A substance will diffuse from where it is more concentrated to
where it is less concentrated, down its concentration gradient.
• Each substance diffuses down its own concentration gradient,
independent of the concentration gradients of other substances.
• The concentration gradient represents potential energy
and drives diffusion.
Diffusion Movie
6. Osmosis is the passive transport of water
• Differences in the relative concentration of dissolved materials in
two solutions can lead to the movement of ions from one to the
other.
• The solution with the higher concentration of solutes is
hypertonic.
• The solution with the lower concentration of solutes is
hypotonic.
• These are comparative terms.
• The hypertonic solution has a lower water concentration
than the hypotonic solution.
• Solutions with equal solute concentrations are isotonic.
• Water molecules will move from the hypotonic solution to the
hypertonic solution.
• This diffusion of water across a selectively permeable membrane
is a special case of passive transport called osmosis.
• Osmosis continues
until the solutions
are isotonic.
7. Cell survival depends on balancing water
• Organisms without rigid walls have osmotic problems in either
a hypertonic or hypotonic environment and must have
adaptations for osmoregulation to maintain their internal
environment.
• Paramecium, a freshwater protist, is hypertonic when compared
to the pond water in which it lives.
• So, even with a less permeable membrane water still continually
enters the Paramecium cell.
• Paramecium have a
specialized organelle,
the contractile vacuole,
that functions as a bilge
pump to force water out
of the cell.
• A cell with a cell wall in a hypotonic solution will swell until
the elastic wall opposes further uptake.
• At this point the cell is turgid, a healthy state for most plant
cells.
• Turgid cells contribute to the mechanical support of the plant.
Tonicity movie
• In a hypertonic solution, the plant cell loses water, and the
plasma membrane pulls away from the wall.
• This plasmolysis is usually lethal.
8. Specific proteins facilitate passive transport of water
and selected solutes
• The passive movement of molecules down its concentration
gradient via a transport protein is called facilitated diffusion.
Transport proteins provide corridors for specific molecule or ion
to cross the membrane.
• These channel proteins allow fast transport.
• For example, water channel proteins, aquaprorins, facilitate
massive amounts of diffusion.
• Some channel proteins, gated channels, open or close
depending on the presence or absence of a physical or chemical
stimulus.
• Some transport proteins actually translocate the solute across
the membrane as the protein changes shape.
• These shape changes could be triggered by the binding and
release of the transported molecule.
9. Active transport is the pumping of molecules against
their gradients
• Active transport requires the cell to use its own metabolic energy.
• Active transport is performed by specific proteins embedded in the
membranes.
• ATP supplies the energy for most active transport
The sodium-potassium pump actively maintains the gradient of
sodium (Na+) and potassium ions (K+) across the membrane.
Both diffusion and facilitated diffusion are forms of passive transport of molecules down their
concentration gradient, while active transport requires an investment of energy to move molecules
against their concentration gradient.
12. Exocytosis and endocytosis transport large
molecules
• Large molecules, such as
polysaccharides and proteins,
cross the membrane via vesicles.
• During exocytosis, a transport
vesicle budded from the Golgi
apparatus is moved by the
cytoskeleton to the plasma
membrane.
• When the two membranes come
in contact, the bilayers fuse and
spill the contents to the outside.
• During endocytosis, a cell brings in macromolecules and
particulate matter by forming new vesicles from the plasma
membrane.
• Three types of endocytosis: phagocytosis, pinocytosis, and
receptor-mediated endocytosis
• In phagocytosis, the cell engulfs a particle by extending
pseudopodia around it and packaging it in a large vacuole.
• The contents of the vacuole are digested when the vacuole fuses
with a lysosome.
Electron Micrograph of a
Macrophage Phagocytosis of E. coli
In pinocytosis, “cellular drinking,” a cell creates a vesicle around
a droplet of extracellular fluid.
This is a non-specific process.
Pinocytosis smooth muscle
(Guinea pig).