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Unit 4: Cells
IB Biology
Cell Theory
• Three main principles:
– All organisms are composed of one or more cells
– Cells are the smallest units of life
– All cells come from pre-existing cells
Cell Theory
• This theory has taken several hundred years to develop
and has gained credibility through the development of
the microscope.
– Robert Hooke first described cells in 1665 by observing
cork cells with a microscope that he built.
– Several years later, Antonie Van Leeuwenhoek observed
the first living cells. He called them “animalcules” (little
animals).
– In 1838, Mathias Schleiden (botanist) stated that all plants
are made of ‘independent, separate beings’, or cells.
– A year later, Theodor Schwann (zoologist) made a similar
statement about animals.
Cell Theory
• The second principle in the theory, cells are the
smallest units of life, continues to gain support
because scientists have yet to find a living entity
that is not made of at least one cell.
• Louis Pasteur performed experiments in the
1860’s to support the last principle, all cells come
from pre-existing cells. He sterilized chicken
broth by boiling and showed that living things
would not “spontaneously” reappear.
Cell Theory
• Functions of Life
– All organisms exist as either unicellular or
multicellular. The functions they carry out are:
•
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•
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Metabolism
Growth
Reproduction
Response
Homeostasis
Nutrition
Cell Theory
• These functions are all tied together to produce a functional
living unit:
– Metabolism includes all the chemical reactions that occur
within an organism
– Growth may be limited but is always evident
– Reproduction involves hereditary molecules that can be
passed to offspring
– Response to the environment is imperative for survival
– Homeostasis refers to the maintenance of constant internal
conditions.
– Nutrition provides a source of compounds with many
chemical bonds that can be broken down to provide the
organism with the energy and nutrients it needs to stay alive.
Cells and sizes
• In most cases, microscopes are needed to observe
cells. These microscopes must have a high
magnification and resolution. Resolution refers to the
clarity of a viewed object.
• Light microscopes use light to form an image. The light
passes through the living or dead specimen. Stains
may be used to improve viewing of the parts.
• Electron microscopes provide the greatest
magnification (over 100,000x) and resolution.
– Electron microscopes use electrons passing through a
specimen to form an image.
Cells and sizes
• Cells are relatively large in comparison. In
decreasing size order:
– Cells (up to 100 µm)
– Organelles (10 µm)
– Bacteria (1 µm)
– Viruses (100 nm)
– Membranes (10 nm)
– Molecules (1 nm)
Cells and sizes
• In order to calculate magnification you would use this
formula:
Magnification = size of image/size of specimen
• In order to calculate the actual size of a specimen seen
with a microscope, you need to know the diameter of
the microscope’s field of vision.
– This may be calculated with a special micrometer or with a
simple ruler on a light microscope.
– The size of the specimen can then be calculated with the
field.
– Scale bars are often used with a micrograph or drawing so
that actual size can be determined.
Limiting cell size
• The surface area to volume ratio limits the size
of cells.
• In the cell, the rate of heat and waste
production and the rate of resource
consumption depend on the cells volume.
• Most of the chemical reactions (metabolism)
of a cell occur within the cell. The size of the
cell affects the rate of these reactions.
Limiting cell size
• The surface of the cell (the membrane)
controls what materials are allowed in or out
of the cell. Cells with more surface area per
unit volume are able to move more materials
in and out of the cell. (example, villus of the
small intestine)
• As the width of the cell increases, the surface
area also increases but at a slower rate.
Limiting cell size
• The following table shows that volume increases by a
factor calculated by cubing the radius; at the same
time, the surface area increases by a factor
calculated by squaring the radius.
Cell radius (r)
0.25 units
0.5 units
1.25 units
Surface area
0.79 units
3.14 units
7.07 units
Volume
0.06 units
0.52 units
1.77 units
Surface area:
13.17: 1
6.04: 1
3.99: 1
Volume
Cell reproduction and differentiation
• Many cells have the ability to reproduce
themselves.
• This allows the possibility of growth and
allows for the replacement of damaged or
dead cells.
Cell reproduction and differentiation
• Multicellular organisms, such as humans,
usual start out as a single cell after a form of
sexual reproduction.
– This single cell has the ability to reproduce at a
very rapid rate.
– The resulting cells undergo differentiation to
produce all the required cell types that are
necessary for the organism.
Cell reproduction and differentiation
• There can be a large number of different cell
types that result from this one cell.
• Differentiation is the result of the expression
of certain specific genes but not others.
• Genes, or segments of DNA on a
chromosome, , allow for the production of all
the different cell types in an organism.
Stem Cells
• Stem cells are a population of cells within an
organism that retain the ability to divide and
differentiate into different cell types.
• Plants contain stem cells in regions of
meristematic tissue, which is near root and stem
tips.
– They are composed of rapidly reproducing cells that
produce new cells that can become various types of
tissue within the root or stem.
– Farmers use these meristem cells when they take
cuttings from stems or roots to produce new plants.
Stem Cells
• In the early 1980’s, scientists found pluripotent or
embryonic stem cells in mice. These stem cells were
capable of forming any time or cell in the organism.
• When stem cells divide they produce some cells that
remain as stem cells.
– This allows for production of a particular type of tissue.
– Medical experts noted the possibilities of these cells in
treating certain human diseases.
– One issue is that stem cells cannot be distinguished from
other cells based on appearance.
– They can only be isolated from other cells based on their
behavior.
Stem Cells
• Stem cell research and treatments
– Some promising research has been directed
toward growing large numbers of embryonic stem
cells so that they can be used to replace
differentiated cells that are lost due to disease or
injury.
Stem Cells
• This process involves therapeutic cloning.
• Parkinson’s disease and Alzheimer’s disease
are caused by loss of brain cells. It is hoped
that stem cells can be implanted to replace
many of the lost brain cells.
• Certain forms of diabetes are caused by the
loss of pancreatic islet cells which are
essential in production of hormones that
regulate blood glucose levels.
Stem Cells
• One type of stem cell treatment that has been
successful in humans for many years is the use
of adult stem cells (tissue-specific stem cells).
– These stem cells are already within certain tissue
types and can only produce new cells of that
particular tissue.
– For example, stem cells have been introduced into
humans to replace damaged bone marrow in
leukemia patients.
Stem Cells
• There are some ethical issues that arise for
stem cell research.
– Embryonic stem cells or pluripotent stem cells are
highly controversial because these cells come
from human embryos.
– These embryos are obtained from in-vitro
fertilization clinics (IVF). These embryos are leftover or discarded embryos.
Stem Cells
– To gather stem cells from an embryo involves the
death of the embryo and opponents argue that
this represents the taking of a human life.
– On the other hand, proponents argue that this
research could result in the significant reduction
of human suffering and is therefore acceptable.
NOVA: Stem Cells, Early Research
Prokaryotic Cells
• What is a prokaryotic cell?
– Prokaryotic cells are smaller and simpler than
eukaryotic cells.
– Most are less than 1 µm in diameter.
– Prokaryotes are thought to have appeared on
Earth first.
– Bacteria are prokaryotes.
Prokaryotic Cells
• Features of prokaryotic cells
– Identify the following on the prokaryotic cell
below:
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•
•
•
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The cell wall
The plasma membrane
The flagella
Ribosomes
The nucleoid (region containing free DNA)
Prokaryotic Cells
Prokaryotic Cells
• Cell wall and plasma membrane
– The cell wall protects and maintains the shape of
the cell.
– In most prokaryotic cells the cell wall is composed
of a carbohydrate-protein complex called
peptidoglycan.
– Some bacteria have an extra layer of
polysaccharide outside the wall. This layer makes
it possible for some bacteria to adhere to teeth,
skin, and food.
Prokaryotic Cells
– The plasma membrane is inside the cell wall and
has a similar composition to the membranes of
eukaryotic cells.
– It controls the movement of materials into and
out of the cell and plays a role in binary fission of
prokaryotic cells.
– The cytoplasm occupies the interior of the cell.
There is no compartmentalization in the
cytoplasm, therefore, all cellular processes occur
inside the cytoplasm of prokaryotic cells.
Prokaryotic Cells
• Pili and flagella
– Some bacterial cells contain hair-like growths on
the outside of the cell wall called pili, which are
used for attachment. Their main function is in
joining bacterial cells in preparation for the
transfer of DNA.
– Some bacteria have flagella. The plural form is
flagella and the singular form is flagellum. They
are longer than pili and allow for motility.
Prokaryotic Cells
• Ribosomes
– Ribosomes occur in all prokaryotic cells and
function as sites of protein synthesis.
– They occur in very large numbers in cells with high
protein production.
– Electron micrographs of prokaryotic cells appear
granular which ribosomes occur in large numbers.
Prokaryotic Cells
• The nucleoid region
– The nucleoid is non-compartmentalized and
contains a single, long, circular thread of DNA.
– This region is involved with cell control and
reproduction.
– In addition to the bacterial chromosome, bacteria
may also contain plasmids.
Prokaryotic Cells
• Plasmids are small, circular, DNA molecules
are not connected to the main bacterial
chromosome.
– They replicate independently of the chromosomal
DNA.
– They are not required by the cell but may help the
cell adapt to unusual circumstances.
Prokaryotic Cells
• Binary Fission
– Prokaryotic cells divide by binary fission.
– DNA is copied in this process and the two daughter
chromosomes become attached to different regions
on the plasma membrane.
– The cell then divides into two genetically identical
daughter cells.
– This process includes an elongation of the cell and
partitioning of the newly produced DNA by fibers that
are similar to microtubules which are made of a
protein called FtsZ.
Eukaryotic Cells
• What is a eukaryotic cell?
– Eukaryotic cells occur in organisms such as algae,
protozoa, fungi, plants and animals.
– They range in diameter from 5 to 100 µm.
Eukaryotic Cells
• Different types of cells often have different
organelles.
– These organelles compartmentalize eukaryotic cells,
which is not a characteristic of prokaryotic cells.
– Compartmentalization allows chemical reactions to be
separated, which is important when chemical
reactions that occur near each other are not
compatible.
– Efficiency of chemical reactions also increases because
chemicals are isolated in organelles.
Eukaryotic Cells
• Organelles of eukaryotic cells
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Endoplasmic reticulum
Ribosomes
Lysosomes (not usually in plant cells)
Golgi apparatus
Mitochondria
Nucleus
Chloroplasts (found only in plant and algal cells)
Centrosomes (in all eukaryotic cells, however
centrioles are not found in higher plant cells)
– Vacuoles
Eukaryotic Cells
Eukaryotic Cells
Eukaryotic Cells
• Cytoplasm
– The cytoplasm occurs inside the plasma
membrane.
– It is the region of the cell where the organelles are
located.
– The fluid portion is referred to as the cytosol
Eukaryotic Cells
• Endoplasmic reticulum (ER)
– A network of tubules or channels that can extend
from the nucleus to the plasma membrane.
– Its function is to transport materials throughout
the internal region of the cell.
Eukaryotic Cells
• There are two types:
– Smooth ER: contains enzymes embedded on its
surface. Functions include:
• Production of membrane phospholipids and cellular lipids
• Production of sex hormones such as testosterone and
estrogen (in gonads)
• Detoxification of drugs in the liver
• Storage of calcium ions needed for contraction in muscle
cells
• Transportation of lipid-based compounds
• To aid the liver in releasing glucose into the bloodstream
Eukaryotic Cells
– Rough ER: has ribosomes on the exterior of the
channels that are involved in protein synthesis.
• This type is involved in protein development and
transport
• The proteins may become parts of membranes,
enzymes, or even messengers between cells.
– Most cells contain both rough and smooth ER,
with rough ER being closer to the nuclear
membrane.
Eukaryotic Cells
• Ribosomes
– Ribosomes are the site of protein synthesis and do
not contain an external membrane.
– They can either be found free floating in the cell
or attached to rough endoplasmic reticulum.
– They are always componsed of a type of RNA
(ribosomal RNA) and protein.
Eukaryotic Cells
• Prokaryotes also contain ribosomes, however
eukaryotic ribosomes are larger and more
dense than those in prokaryotic cells.
• Eukaryotic ribosomes are composed of two
subunits which together equal 80S.
• Prokaryotic ribosomes are also composed of
two subunits, but they only equal 70S.
Eukaryotic Cells
• Lysosomes
– Lysosomes are digestive centers inside the cell
that arise from the Golgi apparatus.
– They are sacs bound by a single external
membrane that contains as many as 40 different
enzymes.
– All of the enzymes within the lysosome are
hydrolytic and catalyze the breakdown of proteins,
nucleic acids, lipids and carbohydrates.
Eukaryotic Cells
• Lysosomes also fuse with old or damaged
organelles within the cell and break them down
so that their components can be recycled.
• Lysosomes are also involved with the breakdown
of materials that are brought into the cell during
phagocytosis.
• The interior of the lysosome is acidic and which is
necessary for the enzymes to hydrolyze large
molecules.
Eukaryotic Cells
• Golgi apparatus
– The golgi apparatus is composed of flattened sacs
called cisternae, which are stacked on top of each
other.
– The Golgi apparatus packages, modifies, and
distributes materials that are synthesized in the
cell.
– It receives products from the ER which arrive on
the cis side of the Golgi apparatus.
Eukaryotic Cells
• Products are then packaged and modified,
then sent out of the Golgi apparatus from the
trans side.
• Small sacs called vesicles carry modified
materials to where they are needed either
inside or outside the cell.
• This organelle is very common in glandular
cells such as those in the pancreas, which
manufacture and secrete substances.
Eukaryotic Cells
Eukaryotic Cells
• Mitochondria
– Mitochondria are rod shaped organelles that
appear throughout the cytoplasm.
– They are about the size of a bacterial cell (1 µm).
– Mitochondria have their own DNA and a double
membrane
– Their outer membrane is smooth but their inner
membrane is folded into cristae.
Eukaryotic Cells
– Inside the inner membrane is a semi-fluid
substance called matrix.
– Cristae provide a large amount of internal surface
area for cellular respiration to occur.
– Mitochondria produce usable energy, ATP,
therefore the mitochondria is referred to as the
powerhouse of the cell.
– Mitochondria also contain their own ribosomes,
which are of the 70S type.
Eukaryotic Cells
• Nucleus
– The nucleus of eukaryotic cells is the region where
DNA is found.
– It is bordered by a double membrane that is
referred to as the nuclear envelope.
– The nuclear membrane contains numerous pores
that allow it to communicate with the cell’s
cytoplasm.
Eukaryotic Cells
– The DNA of eukaryotic cells occurs in the form of
chromosomes, which vary in number depending
on species.
– Chromosomes carry all of the information
necessary for the cell to exist., which allows for
the survival of the organism.
– DNA is the genetic material of the cell.
– Most eukaryotic cells contain a single nucleus, but
some do not have a nucleus and others contain
multiple nuclei.
Eukaryotic Cells
– Cells cannot reproduce if they do not have a
nucleus. For example, human red blood cells do
not have nuclei; however, they are specialized to
transport respiratory gases.
– Most nuclei also include one or more dark areas
called nucleoli (singular form: nucleolus).
– Molecules of ribosomes are made within the
nucleolus and must pass through the nuclear
envelope before assembly as ribosomes.
Eukaryotic Cells
• Chloroplasts
– Chloroplasts occur only in algae and plant cells.
– It contains a double membrane and is
approximately the same size as a mitochondrion.
– Like mitochondrion, it contains its own DNA and
70S ribosomes. DNA in chloroplasts are in the
form of a ring.
– (Note that there are several characteristics of
chloroplasts and mitochondria that are similar to
prokaryotic cells.)
Eukaryotic Cells
• The interior of the chloroplast also includes grana (singular,
granum), thylakoids, and stroma.
• A granum is made of numerous thylakoids that are stacked
on top of each other.
• Thylakoids are flattened membrane sacs with components
that are needed for absorption of light for photosynthesis.
• Stroma is a fluid inside the chloroplast that is similar to the
cytosol.
• Stroma contains many enzymes and chemicals necessary
for photosynthesis.
• Chloroplasts are also capable of reproducing independent
of the cell.
Eukaryotic Cells
• Centrosomes
– A centrosome occurs in all eukaryotic cells and
consists of a pair of centrioles.
– The centrioles are at right angles to each other
and are responsible for assembling microtubules
for cell division.
– The centrosome is located close to the nucleus
Eukaryotic Cells
• Vacuoles
– Vacuoles are responsible for storage and are
usually formed from the Golgi apparatus.
– They are bound by a membrane and can function
to do the following:
• They may store substances such as food, metabolic
wastes, and toxins.
• They also enable cells to have a higher surface area to
volume ratio even at larger sizes.
• In plants, they allow an uptake of water to provide
rigidity for the organism.
Comparison of prokaryotic and
eukaryotic cells
Prokaryotic cells
Eukaryotic cells
DNA in a ring form without protein
DNA with proteins such as
chromosomes/chromatin
DNA free in the cytoplasm (nucleoid
DNA enclosed within a nuclear envelope
region)
(nucleus)
No mitochondria
Mitochondria present
70S ribosomes
80S ribosomes
No internal compartmentalization to
Internal compartmentalization present
form organelles
to form many types of organelles
Size less than 10 µm
Size more than 10 µm
Comparison of prokaryotic and
eukaryotic cells
• Similarities between prokaryotic and
eukaryotic cells:
– Both have an outside boundary and always have a
plasma membrane
– Both carry out all the functions of life
– DNA is present in both cell types
Comparison of plant and animal cells
Plant cells
Animal Cells
Exterior of cell includes an outer cell wall
Exterior of the cell only contains a
with a plasma membrane inside
plasma membrane
Chloroplasts are present in the
No chloroplasts present
cytoplasm
Possess large centrally located vacuoles
Vacuoles are usually not present or are
very small
Store carbohydrates such as starch
Store carbohydrates as glycogen
Do not contain centrioles within a
Contain centrioles within a centrosome
centrosome area
area
Because of the rigid cell wall, the cell
Cell is flexible and more likely to be
often has a fixed angular shape
rounded because of lack of cell wall
The outermost region of various cell types is often
unique. Those are described below:
Cell
Outermost part
Bacteria
Cell wall of peptidoglycan
Fungi
Cell wall of chitin
Yeasts
Cell wall of glucan and mannan
Algae
Cell wall of cellulose
Plants
Cell wall of cellulose
Animals
No cell wall; plasma membrane secretes a
mixture of sugar and proteins called
glycoproteins that forms the extracellular matrix
Comparison of plant and animal cells
• Cell walls are involved in maintaining cell
shape and regulating water uptake.
– They only allow a certain amount of water to
enter the cell
– When an adequate amount of water is present
inside the plant cell, there is pressure exerted on
the cell wall which supports the plant and allows it
to stand upright.
Comparison of plant and animal cells
• The extracellular matrix (ECM) of many animal
cells is composed of collagen fibers and
glycoproteins.
– The ECM strengthens the plasma membrane and
allows for attachment between cells.
– The ECM also allows for cell-to-cell interaction
which could possibly alter gene expression and
coordinate the action of cells within tissue.
– Researchers believe that the ECM may be involved
in directing stem cells to differentiate.
Extracellular matrix