Transcript Cell Surfaces and Junctions

Cell Notes Part One
The Nucleus
Nucleolus
Ribosomes
Endomembrane System
Outer Nucleus
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The outer membrane of the nucleus is
composed of TWO lipid bilayers
Attached outside this layer are ribosomes
Enbedded within this layer are pore complexes
which control what enter or leave the nucleus
Inner Nucleus
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The inner wall of the nuclear membrane is lined with a netlike
complex of protein filaments called the Nuclear Lamina that
aids in maintaining the shape of the nucleus
Chromatin is a substance composed of DNA and proteins that
appears as a gray, grainy diffuse mass in a non-dividing cell.
(the only other areas where DNA can be found are
mitochondria and some chloroplasts)
However, when the cell divides, this coils and condenses to
form chromosomes
The Nucleolus is a mass of dense granules and fibers which
produces ribosomes out of ribosomal RNA and proteins(There
may be more than one in a cell)
Ribosomes
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Ribosomes participate in and are the site of protein
synthesis
Ribosomes have a small and large subunit
Ribosomes may be free (in cytoplasm) or bound ( to
outside of nucleus or Endoplasmic Reticulum)
Free ribosomes make proteins used in the cytosol of
the cell
Bound ribosomes usually make those destined for
use outside the cell or inside membranes within the
cell
The Endomembrane System
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Membranes related by physical continuity or
by passing vesicles between one another
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Includes:
Nuclear envelope
Endoplasmic Reticulum
Golgi Apparatus
Lysosomes
Vacuoles
Plasma membrane
Notice the differences between the
smooth and rough ER as well as the
cisternae and cisternal space
Functions of the Smooth ER
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Synthesis of lipids (oils, phospholipids, steroids)
Example: sex hormones and steroids in
vertebrates in Smooth ER of testicle,
ovarian & adrenal cells
Metabolism of carbohydrates
Example: hydrolysis of glycogen in
the liver produces glucose phosphate
which enzymes made in Smooth ER remove
so glucose can enter the bloodstream
Continued……
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Detoxification of Drugs and Poisons
Example: Smooth ER adds hydroxyl groups
to drugs like alcohol and barbiturates to
aid the body in flushing them out
Contraction of Muscle Cells
Example: ER membrane pumps ions into the
ER against the concentration gradient. When
a nerve stimulation occurs, calcium ions go
rushing back across the membrane and contraction begins.
Functions of the Rough ER
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Synthesis of secretory proteins
Example: bound ribosomes make proteins
like insulin and the ER attaches small
carbohydrates called oligosaccharides to them.
ER wraps membranes of transport vesicles
around them and they bud from transitional
surface of ER and go where they are needed.
Continued……….
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Membrane production
Rough ER makes phospholipids and
membrane proteins, causing the membrane
itself to grow or new parts to be transported to
where they are needed.
The Golgi Apparatus
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Flattened membranous sacs called cisternae
In stacks
Has a receiving side or cis face and a shipping
side or trans face
Cis face is usually facing ER and trans face
gives rise to vesicles which travel to other sites
“Teams” of enzymes needed for specific
modifications are located in each cisternae
Functions of The Golgi
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Modification of products of the ER by
removing or adding monosaccharides
Production of macromolecules such as pectins
Transport of products to and fusion with the
plasma membrane
Modifies,Produces, Sorts and Ships
Lysosomes
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Membranous sacs of digestive enzymes
Made by Rough ER and Golgi
Inside an acidic pH (5) is maintained by the lysosome
because hydrogen ions are pumped into it from the
cytosol (cytoplasm)
Can aid in digestion by breaking down (by
Hydrolysis) macromolecules (polysaccharides like
starch, etc.)
Can aid in the recycling of nutrients within the cell by
breaking them down and releasing them into the
cytoplasm for use again by the cell.
Continued…….
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Lysosomes can also be involved in geneticallyprogrammed tasks, like making fingers in
unborn humans, reducing a tail in a tadpole as
it turns into a frog, etc.
In some one-celled animals and in
macrophages, lysosomes fuse with vacuoles to
digest prey.
Vacuoles
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Larger than vesicles
Membranous sacs
Can function as:
storage sites for food (food vacuoles)
water removal machines (contractile
vacuoles)
Plant Vacuoles
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Most plant cells have a large central vacuole
The membrane that separates the vacuole from
the rest of the cell is called the tonoplast.
The tonoplast is part of the endomembrane
system and selectively transports substances
across its membrane.
Functions of Plant Vacuoles
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Storage sites for substances dangerous to the
plant
Storage sites for pigments in flowers
Storage sites for organic compounds needed by
the plant cell
May act as plant defenders by storing
substances that make animals reluctant to eat
them
The Dynamic Cell
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Cells can vary the numbers of free and bound
ribosomes according to its metabolic needs
They can program their own deaths
Modify their membranes
Cell Notes Part Two
Mitochondria and Chloroplasts
Cytoskeleton
Extracellular Matrix
Cell Surfaces and Junctions
Mitochondria
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Found in nearly all eukaryotic cells
Site of cellular respiration
Have DNA that makes specific proteins on its
own ribosomes
Number within a particular cell will depend on
that cell’s metabolic activity
Move and replicate themselves
Enclosed by 2 lipid bilayers embedded with
proteins
Continued………
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Narrow area between inner and outer membranes is
called the intermembrane space
Inner membrane curved upon itself with infoldings
called cristae
Within the inner membrane is the mitochondrial
matrix, the area where DNA and enzymes and
ribosomes are found
Cristae gives more surface area for chemical reactions
needed in cellular respiration like the Krebs Cycle
Chloroplast
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A plastid (membranous sacs with various functions in
plants)
Two membranes enclose it with a narrow
intermembrane space
Within the inner membrane are stacks of
thylakoids(thylakoid stacks are called grana),that
have inner spaces of their own as well as the stroma,
or fluid area where DNA, ribosomes and enzymes
are involved in the chemical reactions of
photosynthesis like the light reactions and the Calvin
Cycle.
Peroxisomes
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Specialized compartments
Enclosed by one membrane
Transfer hydrogen to oxygen
Both makes and degrades hydrogen peroxide
Can split in two when more are needed
Can break down fatty acids and convert them to
sugars in plant seeds
Cooperate with mitochondria and chloroplasts in
certain metabolic functions
The Cytoskeleton
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Network of fibers that:
support
allow movement
regulation of biochemical activities
Three types of fibers:
microtubules
microfilaments
intermediate filaments
Microtubules
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Hollow rods
Largest of all 3 types of fibers
Wall of tubulin which is made of two
polypeptide subunits (alpha and beta)
Separate chromosomes during cell division
Provide “tracks “ for organelles with motor
molecules(ex: vesicles)
Bear compression forces
Centrosomes
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Microtubules grow out from a pair of
centrioles within centrosomes
Active during cell division
Allow movement of chromosomes
Each centriole composed of a ring of 9 triplet
microtubules
Cilia and Flagella
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Specialized arrangements of microtubules
make up these organelles
They are a core of microtubules surrounded by
the extension of the plasma membrane.
Ring of nine double microtubules with a pair at
their center
Anchored to a basal body, which is structurally
identical to a centriole.
Movement of Cilia & Flagella
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Pairs of microtubules have extensions made of
dynein, a protein that can change shape(with the help
of ATP)
These shape changes allow these extensions to grip
other doublets and “walk” along them which results
in bending the cilia or flagella
This coordinated bending allows them to move which
allows them to function in a number of important
ways. (sperm to swim to the egg, cilia to clean air
passages, etc. )
Microfilaments
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Solid rods built from intertwined strands of actin ( a
globular protein)
Bear tension forces
Examples of functions:
act to contract muscles with myosin(protein)
pinch off cytoplasm in dividing cells
contract and push cytoplasm forward in
amoebae
allow cytoplasmic streaming in plants
Intermediate Filaments
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Bear tension forces
There are many types that are specialized
depending on the cell
All are proteins called keratins
More permanent than microtubules and
microfilaments
Especially important in reinforcing the shape
of the cell and the fixed position of some
organelles like the nucleus
Cell Surfaces and Junctions
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Plant cells have complex structures called cell
walls composed of polysaccharides (like
cellulose) and proteins
They construct a primary cell wall and then a
secondary cell wall
Between them lies a sticky polysaccharide
structure called the middle lamella
Plant cells connect via channels through the
walls called plasmodesmata
Extracellular Matrix
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Outside animal cells lie a complex of structures called
the ECM
The ECM consists of glycoproteins like collagen and
proteoglycans and fibronectins.
Collagen fibers are embedded in a network of
proteoglycans
Some cells are attached to the ECM by fibronectins
which attach to receptor proteins (integrins)
Some communication between the cell and the ECM
occurs through Integrins, receptor proteins
Intercellular Junctions
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Plant cells are connected to and continuous
with the membranes of other cells through the
plasmodesmata.
It allows water and solutes and sometimes
nucleic acids to pass from one cell to another.
Macromolecules can move via fibers in the
cytoskeleton through the same passage.
Animal Cell Junctions
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Tight junctions: membranes of neighboring
cells are fused preventing leakage between
membranes
Desmosomes: fasten cells together into strong
sheets (act like nails or rivets)
Gap Junctions: provide channels for
cytoplasm between neighboring cells (in the
heart this allows close coordination for
heartbeat (contractions of the heart muscle)