Chapter 7. Tour of the Cell.
structural support, motility and regulation.
Surfaces and Junctions.
cells are encased in cell walls.
cell is a living unit greater than the sum of its parts.
cell is as fundamental to biology as the atom is to chemistry.
We Study Cells.
provide a window to the world of the cell.
the understanding of cells have been aided with the increase in the resolution
(resolving power) of microscopes.
is the smallest distance between two objects that allows them to be seen as
Most people see two fine parallel lines as distinct at
a distance of 0.1 mm apart.
Any closer and they seem
a single line.
microscope uses glass lenses and visible light to form a
magnified image of an object.
Resolving power of about 200 nm (0.2 mm or 0.0002 mm).
Gives a useful view of cells and some organelles
Ribosomes are 20 nm or less in size and cannot be visualized
with a light microscope.
microscopes use powerful magnets (lenses) to focus an electron
The electrons are directed at a fluorescent screen or
photographic film to create the image, called electron micrographs.
and transmission electron microscopy.
Scanning electron microscopy (SEM) Electrons
directed at the surface of a sample (3D).
Transmission electron microscopy (TEM) electrons are
passed through the sample which must be fixed and sliced very thinly.
are one of the most important tool in cytology especially now that we
have scopes powerful enough to visualize organelles.
biologists can isolate organelles to study their functions.
fractionation we can take cells apart, separating out organelles so we can
answer questions about function or composition of the organelles.
Cell fractionation is done by rupturing cells followed
by systematic centrifugation of these cells to separate the components.
Cells must first be ruptured by some method: mortar
and pestle, glass homogenizer or blender.
These methods are done
with care to not break the internal membrane bound compartments.
Isotonic solutions are used and the reactions are done on ice
to further protect the organelles.
The cell suspension is placed in a centrifuge
to create large forces causing components of the suspension to pellet at the
bottom of the tube, differential centrifugation.
Ultracentrifuges can spin up to 130,000 revolutions per minute (1
million times the force of gravity)
Panoramic View of the Cell.
organism is composed of one or more prokaryotic or eukaryotic cells.
And cells come from preexisting cells this constitutes the cell theory.
and eukaryotic cells differ in size and complexity.
cell is from Greek meaning pre-nucleus (nucleoid) and eukaryote means
have certain features in common: Plasma membrane (Fig 7.6), cytosol,
chromosomes and ribosomes.
small with a volume of 1-1000 mm3.
They must maintain a surface area-to-volume ratio
(SA/V) that is beneficial for metabolic processes to occur (Fig 7.5).
As a cells volume increases so does its surface area,
however not to the same extent.
Surface area indicates
its ability to exchange nutrients and waste products with the environment.
Therefore, if a cell was larger its waste production
would outwork its ability to deal with that waste.
Cells maintain a large SA/V ratio by being small in
The large amount of surface area is important for many
cells differ from eukaryotic cells in more than just the nucleus (Fig 7.4).
They are generally smaller in size, have a nucleoid,
have no membrane bound organelles, have cell walls, have smaller ribosomes.
Prokaryotic cells include cells of the kingdoms
Eubacteria and Archaebacteria.
Prokaryotes are generally single celled organisms,
however they are often seen in chains, clusters or colonies.
Prokaryotic Cells- A metabolically diverse group of
cells that can inhabit nearly all environmental extremes.
Some can use light energy to generate needed materials
others can generate needed materials from inorganic compounds.
Some prokaryotes move about with the aid of flagella,
while Pili can be used for attachment to substrates or used
membranes compartmentalize the functions of the eukaryotic cell (Fig 7.7).
have membrane enclosed compartments which isolate certain molecules and
Some reactions (proteins) are built into the walls of
membranes (Fig 7.6) are normally a phospholipid bilayer, each different
membrane has a special set of proteins and lipids for specific functions.
Animals, plants, fungi and protists have larger more
Like prokaryotic cells
eukaryotic cells have a plasma membrane, cytosol and ribosomes.
Unlike a prokaryotic
cell eukaryotic cells have an internal cytoskeleton (gives shape and aids in
cell cargo movement) and membranous compartments.
specialized compartments in the cell are organelles (also ribosomes).
Not all organelles are equally abundant in all cells
and components between plants and animal cells may also differ (Fig 7.8).
cell wall and plasmodesmata vs.
lysosomes, centrioles and flagella.
Nucleus and Ribosomes.
resides in the nucleus this information maybe translated into proteins on the surface
of ribosomes (genetic control in the cell).
nucleus contains a eukaryotic cells genetic library (Fig7.9).
largest organelle ~5 mm which is larger
than many prokaryotic cells.
before, a membrane enclosed nucleus is the defining feature of a eukaryotic
cell (true nucleus).
membranes are very close together and are perforated by nuclear pores.
These pores are approximately 9 nm in diameter and
connect the interior of the nucleus (nucleoplasm) with the cytoplasm.
Each pore is surrounded by eight protein granules to
allow material to pass in and out of the cell.
under an electron microscope you can visualize the two membranes that make up
the nuclear envelope (double membrane).
The nuclear envelope is normally a stable structure
except during mitosis or meiosis when it breaks down into small vesicles.
nucleus DNA combines with proteins to form a fibrous complex called chromatin.
surrounded by the aqueous nucleoplasm (liquid and dissolved particles.
periphery of the nucleus the chromatin interacts with the nuclear
lamina which is formed by specialized proteins called lamins.
A net like array of protein filaments that maintains
the shape of the nucleus.
The nuclear lamina depolymerizes when the
nucleus breaks down for cellular reproduction.
Most of the
time DNA is in the tangled mess called chromatin only when the cell is
going to divide will it form the readily visible chromosomes (bowling
Each contains one long molecule of DNA that carries
the hereditary information and directs protein synthesis.
the cells RNA is located in a dark circle in the nucleus called the nucleolus
this is where rRNA and proteins come together to form ribosomal subunits.
These subunits leave the nucleus and are assembled
into functional ribosomes in the cytoplasm.
Nucleoli disappear as nuclear/cellular division
approaches and reappear after cell division.
build a cells proteins (Fig 7.10).
are found in three places in the cell: Cytoplasm, attached to the endoplasmic
reticulum and in the mitochondria (chloroplast).
and eukaryotic ribosomes are similar in that both have two different sized
Eukaryotic ribosomes are
larger than prokaryotes.
makeup of ribosomes include rRNA (ribosomal RNA) and proteins
bind mRNA and tRNA in order to translate hereditary information (DNA) into 1o
Ribosomes are found free in the cytoplasm, attached to
the endoplasmic reticulum and in the mitochondria.
Much of the volume of a
cell is taken up by its extensive internal membrane system.
EM has shown that there
are connections between many of the membranes suggesting that there may be a
single endomembrane system.
(sacs of membrane) are involved in transfer between components of the
reticulum (ER) manufactures membranes and performs many other biosynthetic
functions (Fig 7.11).
it accounts for over half of all cellular membranes in a a eukaryotic cell
Parts are continuous with the outer nuclear membrane.
a network of membranous tubules and sacs called cisternae (cisternae is
Latin liquid reservoir)
the ER have ribosomes attached these segments are called the rough ER.
The ribosomes are sites of protein synthesis.
For proteins that are used outside the cytosol: secreted
proteins, placed in membranes or moved into certain membrane organelles.
These proteins enter the lumen of the ER of the ER due
to a special amino acid signal sequence.
The proteins mature into their tertiary structure and
some have carbohydrates attached.
Makes these proteins glycoproteins
which helps in directing the protein to the right part of the cell and involved
in some proteins function.
Transport vesicles bud like bubbles from the ER to
take cargo to other parts of the cell (Golgi).
The ER with
no ribosomes is called the smooth ER.
Diverse metabolic processes occur here:
This is the site of
carbohydrate metabolism, phospholipid, steroid, fatty acid biosynthesis and
toxic substances can also be detoxified here.
If a cell is
involved in a lot of protein production for export then they will be packed
Ex. Gland cells and antibody producing cells.
Golgi apparatus finishes, sorts and ships cell products (Fig 7.12).
most eukaryotic cells; discovered in 1898 by Camillo Golgi.
flattened sacs called cisternae and small membrane enclosed vesicles.
Sacs lay together like a stack of saucers.
and oligosaccharides are often altered in the passage through the cisternae of
cells (vertebrates) individual stacks form a network.
The cis Golgi is the bottom stack that lies nearest
the nucleus or the rough ER.
Golgi is the top portion of stacks and is nearest the plasma membrane.
While the medial Golgi lies in between.
Each of these three parts of the Golgi have different enzymes
Vesicles from the Rough ER move to and fuse with the
cis Golgi to empty their contents into the lumen of the organelle.
Vesicles move from the cis to the medial and finally
to the trans Golgi.
Vesicles then leave the Golgi and travel to other
membranes in the cell.
membranes of two vesicles may fuse resulting in a larger vesicle with mixed
How does the
Golgi sort, package and send proteins to their correct destinations?
Proteins are tagged with specific molecules that direct
them to their destination.
Proteins in the trans
Golgi recognize these signals and package the protein and ship it to its final
are digestive compartments (Fig 7.13).
that originate in part from the Golgi apparatus contain digestive enzymes that
accelerate macromolecule breakdown.
contain enzymes that can digest all the major macromolecules: protein,
polysaccharide, nucleic acid and lipid breakdown.
Low internal pH of the lysosome is necessary for
normal enzyme function.
the importance of separate compartments (How?).
organelle is the site of breakdown for food or foreign material taken up by
is the process that a cell uses to bring in materials from outside the cell inside
a membrane bound vesicle called a phagosome (Fig 7.14).
This may fuse with a lysosome from the Golgi to form
the secondary lysosome.
Digestion then occurs.
digestion and phagocytosis are extremely important to white blood cell function
(White blood cells identify and attack foreign objects).
have diverse functions in cell maintenance (Fig 7.15).
an aqueous solution containing many dissolved particles.
structure vacuoles have diverse functions.
Plants (central vacuole) cannot excrete all
wastes so many waste products stored here also for storage of ions.
Can give turgor, stiffness, to the cell.
Some color pigments used for plant reproduction are
stored here (petal color).
Contractile vacuole in fresh water protists is used to get rid of excess water.
and chloroplast are the main energy transformers of cells.
energy to transform raw materials into cell specific materials that are
involved in: growth, reproduction and movement.
in the mitochondria (mitochondrion) of all eukaryotic cells and the chloroplast
of cells that can harvest energy from sunlight.
organelles have their own ribosomes and DNA.
These are responsible for making proteins important
for their function as energy powerhouses.
Utilization of glucose as an energy source begins in
the cytosol then moves to the mitochondria (mitochondrion)
Takes partially degraded fuel molecules and changes them
from potential chemical energy into the usable form ATP (adenosine
ATP is not a storage form of energy but a
participatory form that can be used to run many cellular processes.
Production of ATP in the mitochondria using fuel
molecules and O2 is aerobic cellular respiration.
Mitochondria are about the size of bacteria 2-8 mm in length and 1.5 mm in diameter.
Electron microscopy has shown us that mitochondria
have two membranes; an outer and inner membrane.
membrane is protective offering little resistance to substance moving into
and out of the mitochondria.
Give greater surface area for energy producing
reactions to occur.
membrane has large proteins involved in the cellular respiration.
inner membrane is the mitochondrial matrix which contains enzymes, ribosomes
and mitochondrial DNA.
Mitochondrial DNA encodes for some mitochondrial
Function almost like separate small organisms.
The number of mitochondria per cell vary depending on
If the cell requires
more energy they have more mitochondria.
photosynthesize or store materials (Fig 7.18).
Plastids are only found only in plants and certain
A familiar plastid is the chloroplast which
contains the green pigment chlorophyll and is the site of
Light is converted to energy of chemical bonds,
provides food for the plant and for other organisms.
Chloroplast contain membrane structures that look like
stacks of pancakes called grana.
The circular stacks that make up grana are called thylakoids.
Thylakoids are single membraned sack made of
phospholipids and proteins.
Added are chlorophyll
Used to harvest light
energy and convert it to glucose from CO2 and H2O.
Thylakoids from one
grana may be attached to others making the chloroplast a complex membrane
the chloroplast is the stroma and contains the grana, ribosomes, enzymes
of plastids include the chromoplasts and luecoplasts.
Chromoplasts (tomato color from carotenoids) may be
important just for their color.
Leucoplasts are involved in storage of starch and
The fact that
chloroplast and mitochondria have their own DNA and Ribosomes led in part to
the theory of endosymbiosis.
proposes that larger prokaryotes engulfed smaller ones and these may have
become the first eukaryotic cells (with organelles).
and Chloroplasts also have membranes and ribosomes that are similar to
Peroxisome degrades H2O2.
are small organelles with a granular interior and a single outer membrane.
Within Peroxisomes are found highly reactive and toxic
peroxides (H2O2)that result from chemical reactions in
These peroxides are
detoxified in the peroxisome (catalase).
and electron microscopy have elucidated the cytoskeleton, a network of
fibers extending throughout the cytoplasm (Fig 7.20).
structural support to the cell, the cytoskeleton also functions in cell motility
obviously involved in support for the cell (scaffolding).
involved in several types of motility: Cell movement includes movement of the
cell and movement of of objects inside the cell.
involves the interaction of cytoskeletal components and motor molecules (Fig
Dynein and kinesin.
of cytoskeletal components: Microtubules, microfilaments and
are long hollow cylinders that radiate from the microtubule organizing
center and are made of the globular protein tubulin.
Microtubule organizing center in many cells (animal)
is the centrosome which contains two centrioles.
Tubulin is a dimer made of two subunits, a- and b- tubulin.
Microtubules grow by adding tubulin dimers to one end.
Thirteen rows, or
protofilaments, of tubulin dimers surround the central cavity of the
There is a positive and
negative end to microtubules giving them polarity and making them dynamic
Function of microtubules include:
Control the arrangement
of cell walls.
Areas of a cell changing
Tracks for cargo
movement in the cell (dynein and kinesin)
chromatids to daughter cells during mitosis and meiosis.
Used for cilia and
Many eukaryotic cells possess whip like appendages,
the cilia and/or flagella (Fig 7.23 and 7.24).
longer than cilia and usually found singly or in pairs.
Whip like undulation
from one end to another.
Beat stiffly in one
direction and recover flexibly, like a swimmers arm.
Flagella and cilia have a 9+2 arrangement of
Actually 9 fused pairs
of microtubules (doublets) around two single microtubules.
The movement of flagella
and cilia is due too sliding of the microtubules along one another.
Sliding is due to a
motor protein called dynein (towards positive end) which attaches to both
microtubules and marches them past one another.
Kinesin is another motor
protein used in cargo transport ( in vesicles) along microtubules towards the negative
At the end of each microtubule is a basal body the 9
doublets exist in the basal body but there are no central microtubules (2).
doublet is accompanied by another microtubule making nine sets of three.
assembled into a long chain of globular actin (G actin) subunits.
G actin has a distinct head and tail for formation of filamentous
actin (F actin).
Two chains of filamentous actin form a double helical
structure called a microfilament.
Formation of filamentous actin is reversible a process
Microfilaments can be very stable as well give cells
specific shapes when interacting with specific actin binding proteins
Forms beneath the P.M.
are well known for their role in motility: amoeboid movement, muscle
cell contraction and cytoplasmic streaming (Fig 7.27).
Help extend pseudopodia in amoeboid movement.
Actin interlaced with myosin, myosin acts as a motor protein
to walk along actin filaments causing muscle cell contraction.
Actin and myosin interactions in plant cells cause a
circular movement of cytoplasm, cytoplasmic streaming.
filaments stabilize cell structure and resist tension.
8-12 nm in diameter.
5 types that have similar structures and are composed
of the keratin family of
May help to hold
organelles in position.
tissue rigidity by forming desmosomes between cells (spot welds).
these components associated with the cytoskeleton can result in genetic
Loss of functional spectrin and ankyrin (actin
associated proteins) results in hemolytic anemia (spherocytosis).
The most common form of muscular dystrophy (1
in 3,500 male children), progressive loss of muscle cells, is caused by a
mutation in the dystrophin gene which encodes for a protein that
attaches actin filaments to the muscle cell P.M.
Surfaces and Junctions.
cells are encased by cell walls.
is a semirigid structure outside of the P.M. consisting primarily of
There are connections
between plant cells called plasmodesmata.
nm in diameter, link plant cells allowing diffusion of water, ions, small
molecules and many proteins between the cells ensuring their uniform
extracellular matrix (ECM) of animal cells function in support, adhesion,
movement and regulation (Fig 7.29).
animals have a complex extracellular matrix.
Composed of collagen (most abundant protein in
mammals) and other glycoproteins.
Some cells are attached to the ECM by fibronectins
Secreted by cells, some have large amounts of
extracellular matrix (bone and cartilage) others have little (brain).
Cells embedded in bone
secrete collagen and the ionic solid calcium phosphate that gives bone its
Epithelial cells that line body cavities spread as a
sheet over the basal lamina (basement membrane), a form of extracellular
Some extracellular matrix proteins can be spectacular.
Proteoglycan is made of
approximately one hundred proteins attached to an enormous polysaccharide.
M.W. over 100 million
and it is about the size of a prokaryotic cell.
Functions of the extracellular matrix:
skin, cartilage and other tissues.
Filter materials (ex:
between the blood and urine).
Orientation of cell
migration during embryonic development and during tissue regeneration
junctions help integrate cells into higher levels of structure and function.
to be organized into tissues, organs and organ systems.
allow cells to adhere to one another, interact and communicate all through
direct contact and special protein patches (cell adhesion proteins).
these cell surface junctions include: Tight junctions, desmosomes
and gap junctions.
junctions are specialized structures that link adjacent epithelial cells
lining a lumen or cavity.
Limit the passage of materials from the lumen and
inhibit the movement of membrane proteins.
Thus membrane proteins on the apical surface
(faces the lumen) differ from those on the basolateral.
Specialized structures associated with the plasma membrane that hold adjacent
cells together much like spot welds or rivets.
Consists of a plaque on the cytoplasmic surface of the
P.M. attached to keratin fibers of the cytoplasm and adhesion proteins in the
The adhesion proteins pass from the plaque of one cell
to the plaque of another.
junctions: facilitate communication between cells.
Made of specialized protein channels called connexons
that span the P.M. of adjacent cells.
Small molecules and proteins may pass through these
channels but not proteins, nucleic acids or organelles.
In some nerve cells and in the vertebrate heart gap
junctions allow for the direct passage of electrical signals.
cell is more than just a sum of its parts.
correlation of structure to function is illustrated throughout the cell.
despite the fact that many cells share these same structures they have very
From the detoxification of the blood done by a
hepatocyte to the phagocytosis of invader microorganisms by the macrophage.