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Have properties that come from interaction between their
components
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Some live together in colonies
Eg: Volvox Aureus
Each colony of volvox aureus has ball of protein gel with 500 or
more identical cells attached and daughter colonies being formed
inside
They are not fused- not single organisms
Organisms- single mass of cells, fused, multicellular
Caenorhabditis elegans- most intensively researched multicellular
organism
o 1 mm long (adult) and made of 959 cells exactly
o no common name for it
o Lives unseen in decomposing organic matter; feeds on
bacteria that cause decomp
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if you look at
them all at once including their interactions

Gene Expression and Cell Differentiation

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Specialized tissues can develop by cell differentiation in
multicellular organisms
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Diff
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functions aka division of labor
Many times cells specialize in same way for same function- aka
tissue
More efficient to be specialized
Can develop ideal structure with necessary enzymes
Differentiation- development of cells in different ways to carry
out special function
Humans have 220 distinctive and highly specialized cell types,
come from differentiation
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25000 genes in human genome; all present in
every body cell
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Most cells less than half of the genes will ever be required
When gene is being used, it’s referred to as “expressed”
Cell development includes the switching on of specific genes and
expressing them
Differentiation occurs due to the different sequence of genes
being expressed in cells
Gene expression controlling is key to development

Stem Cells

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Ability of stem cells to differentiate and divide to different paths is
important for embryonic development; good use for therapeutics
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Embryo cells are versatile and can develop along any pathway
possible
19th century, zygote and embryo cells were given the name stem
cell (all tissues from adult stem from them)
2 properties making them most active areas of research in bio

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and med
o Divide again and again for a huge quantity; useful for
growth of tissues and cell replacement
o Not fully differentiated and can be manipulated
They are therapeutic because they can possibly provide therapies

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or fix diseases and health problems
Non-therapeutic uses: Can possibly produce more striated
muscle aka meat; future meat could be grown from stem cells
Once a cell commits to a path, it can still divide but the resulting
cells will be committed to the same path
Stem cells are still present in adult human body (bone marrow,
skin, and liver)
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o Probably Leukemia: Over 30,000 mm cubed
o Acute Leukemia: Over 100,000 mm cubed
Cure: destroy cancer cells in bone marrow that are making more
white blood cells
o Patients can be given chemicals that kill dividing cells aka
chemotherapy
o But it kills stem cells that make blood cells
o So, they do this:
 Needle into bone marrow to remove fluid
 Stem cells are extracted and frozen (adult ones that
can only make blood cells)
 Chemotherapy drugs are taken to kill cancer cells

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Stem cells are returned and blood cells are able to be
produced again
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2micrometers
Electron microscopes (made in 1930s Germany) and used in 40s
and 50s
Allowed to see things as small as 0
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1 millimeters as separate objects but no smaller
 Light microscope shows 0
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for light magnification is x400
 Being able to separate parts so they’re distinguishable is called
resolution
 Electron beams have shorter wavelength, leading to higher
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resolution
Modern ones have resolutions up till 0
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of Structure can be done
by using a circle for the phosphate
group and two lines for the
hydrocarbon chains
2 parts of the structure are called

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phosphate heads and 4 tails
When mixed with water they form a
double structure with the heads

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facing the water and the tails facing inwards to each other
This double layer formation is called a phospholipid bilayer
They are stable and form the structure of a cell membrane

Models of Membrane Structure

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Gorter and Grendel
 1920s, calculated that area that phospholipids occupied in a
monolayer was twice the area of a plasma membrane
 deduce that there was a bilayer
 the model didn’t show the proteins that are in the membrane
Davson and Danielli
 1930s, proposed layers of protein adjacent to the phospholipid
on both sides of the membrane
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Proposed this sandwich method to describe why the membrane
are such effective barriers
1950s, electron micrographs showed that their suggestion was
true as there was a railroad track figure, the dark being the
proteins and the light being the phospholipids

Singer and Nicolso—Fluid Mosaic Model
 1966, suggested that proteins occupy a variety of positions in
the membrane
 peripheral proteins are at the inner and outer surface; integral
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proteins are embedded in the bilayer (in some cases protruding)
The proteins are like tiles in a mosaic
Since the phospholipid molecules can move in both layers, the
proteins can also move; thus giving the name
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Receive
neurotransmitters at synapses)

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o Channels for passive transportation allowing for hydrophilic
particles to go across using facilitated diffusion
o Also act as pumps for active transport which use ATP to
move particles across membrane
Due to diverse functions they have diverse structures
o Integral proteins
 hydrophobic (atleast part of their surface) and are
embedded in the hydrocarbon chains in the center of
the membrane
 transmembrane: they extend across membrane with
the hydrophilic parts projecting thru the region
where the hydrophilic phosphate heads are
o Peripheral proteins
 They are hydrophilic on their surface, so not

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embedded in the membrane
 Most of them attached to surface of integral proteins
and this attachment can be reversible
 Some have a single hydrocarbon chain inserted into
the membrane, anchoring the protein to the
membrane surface
Membranes have inner and outer faces and the proteins are in a
way so that they carry out their function correctly

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o Pump proteins in membranes of root cells of plants are
orientated so they pick up potassium ions from the soil and
pump them into the cell
The more active a membrane, the higher its protein content
o The myelin sheath around nerve fibers just act as
insulators so they have 18% protein content
o Most membranes around cells have 50% protein content
o Cholorplasts and Mitochondria have membranes that have
a 75% protein content because they are so active

Endocytosis and Vesicle Movement in Cells

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Cholesterol is a component of animal cell membranes

Cholesterol is a type of lipid, but it’s not a fat or oil; it belongs to a group
called steroids
Most of the molecule is hydrophobic so it attracted to the hydrophobic
hydrocarbon tails, but one end of the molecule has a hydroxyl (-OH)
group which is hydrophilic
The hydroxyl is attracted to the phosphate heads; therefore, these
molecules are found between phospholipids in the membrane
Amount of cholesterol in animal cell membranes varies; vesicles that have
neurotransmitters at synapses as much of 30% of the lipid in the
membrane is cholesterol
Cholesterol in mammalian membranes reduces membrane fluidity
and permeability to some solutes
The hydrophobic hydrocarbon tails behave like liquid,
hydrophilic phosphate heads act solid
too fluid membrane would allow too many substances in, too not fluid
would restrict movement of the cell and substances
Cholesterol
 Disrupts packing of hydrocarbon tail; prevents their
crystallization and acting as solid
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Also restricts molecular motionaffects fluidity of membrane
(less fluid)
Reduces permeability to hydrophilic particles like Na ion and H
ion
Its shape helps membranes curve into a concave shape; helps in
the formation of vesicles during endocytosis (when a cell
invaginates to take in substances by making a vacuole)

The fluidity of membranes allows materials to be taken into cells by
endocytosis

Vesicle- small sac of membrane with droplet of fluid inside; spherical and
present in eukaryotes
 Very dynamic; they are constructed, moved, then deconstructed
 Happens because of fluidity of membranes allowing room for
shape change and movement
To form a vesicle, small region of membrane is pulled and pinced off from
membrane; proteins in the membrane carry out this process using ATP
Vesicle is then formed inside the cell; contains stuff from outside of the
cell; it is a method of in-taking material and is called endocytosis
These vesicles generally contains water and solutes from outside the cell
but also contain larger molecules needed that can’t pass through the
membrane
 Eg: in the placenta, proteins including antibodies are taken in
through endocytosis
 Some cells take undigested food particles through endocytosis
o Eg: Amoebas and Paramecium use this method
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White blood cells take in pathogens through endocytosis and kill
them

Vesicles move materials within cells
Sometimes it is the contents of the vesicles that need to be moved
 Eg: Secretory cells; proteins is synthesized from ribosomes and
is collected in the rER
 The vesicles containing the protein bud off and take it to the GA,
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where they fuse to allow for the protein to be processed
Then the vesicles take the processed proteins to the membrane
for secretion

Other times it is proteins in the membrane of the vesicle that cause
vesicle movement
 When a cell grows, it requires an increase in the size of the
membrane
 Phospholipids are synthesized in the rER and the the proteins for
the future membrane are made using the rER ribosomes

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Vesicles bud off the rER and fuse with the membrane to increase
the area of the membrane
This method also increases the size of organelles

The fluidity of membranes allows materials to be taken into cells
by exocytosis
If a vesicle fuses with the plasma membrane, the contents are then
outside the membrane and cell; this is called exocytosis
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Used for releasing digestive enzymes from gland cells
Polypeptides in the enzymes are synthesized by the rER,
processed in the GA, then carried out to the membrane using
vesicles for release
In this case it is secretion as useful substance is being released
not waste product (so not excretion)
Can be used for excretion (eg: removal of excess water from
unicellular organs)
o The water is loaded into a vesicle aka contractile vacuole
which then moves to the plasma membrane for expulsion
o Happens with paramecium

4 Types of Membrane Movement

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Simple Diffusion Diffusion- spreading out of particles in liquids and gases because
of continuous random motion
 This sort of diffusion occurs when the substance is allowed to
pass through the phospholipid molecules of the cell membrane
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 Simple diffusion is passive in the sense that it doesn’t require

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energy, and can only occur if the membrane allows for the
particles to diffuse
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Ions can’t easily pass since the center of the membrane is
hydrophobic
Polar molecules can diffuse at low rates, especially smaller polar
particles
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Cells usually move from a region of higher concentration to lower
concentration
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Osmosis
 The net movement of water molecules moving in and out is
called osmosis
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It occurs due to the different in solute concentration
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Further, this allows for the movement of high concentration free
water to low concentration free water
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Some cells even have aquaporins that increase

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permeability to water (i
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kidney)
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Less commonly, cells can pump out into the higher
concentration
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This process is carried out by the globular membrane proteins
aka pump proteins; the cell has full control over these proteins
so they can control the content of the cytoplasm very well
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T
The protein then returns to its original state
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3 nm in width at narrowest
Potassium ions are smaller than 0
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Interior of pump is open to the inside of the axon and 3Na
ions enter and attach to binding sites
ii
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The interior of the pump opens up to the outside of the axon
and the 3Na ions are released
iv
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The binding causes the phosphate group to be released and
the pump changes shape again

The pump opens up to the interior and the potassium ions are released
inside; repeat

Origin of Eukaryotes

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Tissues or organs to be used in medical procedures must be
bathed in a solution with the same osmolarity as the cytoplasm to
prevent osmosis
 Animal cells can be damaged if osmosis occurs
 When a hypertonic solution is near a cell, then the area of the
cytoplasm shrinks
 the plasma membrane’s area doesn’t change, it just develops
indentations which are also called crenellations
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in a hypotonic solution, the cells can swell and then burst which
can leave behind ghost cells (ruptured membranes)
Both types of solutions can harm human cells, but in an isotonic
solution, water molecules move in and out at the same rate, so
they remain healthy
In medical processes, organs are bathed in isotonic solutions
Usually, isotonic sodium chloride is used aka normal saline: has
an osmolarity of about 300mOsm (milliOsomles)
Uses of normal saline
o Can be introduced into system using IV drip
o Can be used for cleaning wounds and skin abrasions
o Can be used to keep area of damaged skin moist and
prevent skin grafts
o Used as a basis for eye drops
o Can be frozen to pack donor organs

Cyanobacteria
 Type of prokaryote with a lot of in folding membrane
 Can perform photosynthesis and releases oxygen into air
Oxygen
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Atmosphere
Oxygen began to accumulate 2
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7 bya
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Evidence found through banded iron in rocks (rusting)
Made aerobic respiration possible

Eukaryotes
 Depend on free oxygen to carry out metabolic processes
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Couldn’t have evolved before oxygen atmosphere

Two processes though to have led to eukaryotes
 Infolding of prokaryotic cell membrane
o Created internal microenvironments
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o Led to efficiency
Endosymbiosis- symbiotic relationship while one lives inside
another
o Early prokaryotes engulfed aerobic bacteria but didn’t
digest/ photosynthetic bacteria
o Led to creation or evolving to mitochondria/chloroplasts;
mutually beneficial relationship
o Evidence found in both mitochondria and chloroplasts
 Have bacterial structure
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Have cell membranes
Same size as prokaryotes
Have 70S ribosomes
Have circular DNA
DNA shares same common sequences with modern
prokaryotes

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