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TRANSCRIPT
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Cell Structure and Function
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Cell Structure
In 1655, the English scientist Robert Hooke coined the
term cellulae for the small box-like structures he saw
while examining a thin slice of cork under a
microscope.
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Basic Cell Structure All cells have the following basic structure:
A thin, flexibleplasma membrane
surrounds the entire cell.
The interior is filled with a semi-fluid
material called thecytoplasm.
Also inside are specialized structures
called organelles and the cells genetic
material.
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Generalized Eukaryotic Cell
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Visualizing Cells
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Prokaryotic Cells
Simplest organisms
Cytoplasm is surrounded by plasma membrane and
encased in a rigid cell wall composed of peptidoglycan.
No distinct interior compartments
Some use flagellum for locomotion, threadlike structuresprotruding from cell surface
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Eukaryotic Cells
Characterized by compartmentalization byan endomembrane system, and the
presence of membrane-bound organelles.
central vacuole vesicles
chromosomes
cytoskeleton cell walls
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Animal cell anatomyAnimal Cell
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Membrane Function
All cells are surroundedby a plasma membrane.
Cell membranes arecomposed of a lipidbilayer with globularproteins embedded in thebilayer.
On the external surface,carbohydrate groups joinwith lipids to formglycolipids, and withproteins to form
glycoproteins. Thesefunction as cell identitymarkers.
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Fluid Mosaic Model
In 1972, S. Singer and G. Nicolson proposed the Fluid
Mosaic Model of membrane structureExtracellular fluid
CarbohydrateGlycolipid
Transmembraneproteins
Glycoprotein
Peripheralprotein
Cholesterol
Filaments ofcytoskeleton
Cytoplasm
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Phospholipids
Glycerol
Two fatty acids
Phosphate group
Hydrophilic
heads
Hydrophobic
tails
ECF WATER
ICF WATER
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Phospholipid Bilayer
Mainly 2 layers of phospholipids; the non-polar tails
point inward and the polar heads are on the surface.
Contains cholesterol in animal cells.
Is fluid, allowing proteins to move around within the
bilayer.
Polarhydro-philicheads
Nonpolarhydro-phobic
tails
Polarhydro-philicheads
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Steroid Cholesterol
Effects on membrane fluidity within
the animal cell membrane
Cholesterol
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Functions of Cell Membranes
Regulate the passage of substance
into and out of cells and between cellorganelles and cytosol
Detect chemical messengers arriving
at the surface
Link adjacent cells together bymembrane junctions
Anchor cells to the extracellularmatrix
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Outside
Plasmamembrane
Inside
Transporter Cell surfacereceptorEnzyme
Cell surface identity
marker
Attachment to the
cytoskeletonCell adhesion
Functions of Plasma Membrane Proteins
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Membrane Transport
The plasma membrane exhibits
selective permeability - It allows somesubstances to cross it more easily
than others
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Passive Transport
Passive transport is diffusion of a
substance across a membrane withno energy investment
4 types
Simple diffusion
Dialysis
Osmosis
Facilitated diffusion
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Solutions and Transport
Solution homogeneous mixture of
two or more components Solvent dissolving medium
Solutes components in smaller quantitieswithin a solution
Intracellular fluid nucleoplasm andcytosol
Extracellular fluid Interstitial fluid fluid on the exterior of thecell within tissues
Plasma fluid component of blood
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Diffusion Across a Membrane
The membrane has pores large enough for the molecules to pass
through.
Random movement of the molecules will cause some to passthrough the pores; this will happen more often on the side with more
molecules. The dye diffuses from where it is more concentrated to
where it is less concentrated
This leads to a dynamic equilibrium: The solute molecules continue
to cross the membrane, but at equal rates in both directions.
Net diffusion Net diffusion Equilibrium
Diff i A M b
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Diffusion Across a Membrane
Two different solutes are separated by a membrane that is
permeable to both
Each solute diffuses down its own concentration gradient.
There will be a net diffusion of the purple molecules toward the left,
even though the total solute concentration was initially greater on
the left side
Net diffusion
Net diffusion
Net diffusion
Net diffusion Equilibrium
Equilibrium
Th P bilit f th Li id Bil
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The Permeability of the Lipid Bilayer
Permeability Factors
Lipid solubility Size
Charge
Presence of channels and transporters
Hydrophobic molecules are lipid soluble and canpass through the membrane rapidly
Polar molecules do not cross the membranerapidly
Transport proteins allow passage of hydrophilicsubstances across the membrane
P i T t P
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Passive Transport Processes
3 special types of diffusionthat involve movement of
materials across asemipermeable membrane
Dialysis/selective diffusionof solutes
Lipid-soluble materials
Small molecules that
can pass throughmembrane poresunassisted
Facilitated diffusion -substances require aprotein carrier for passive
transport Osmosis simple diffusion
of water
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Osmosis
Diffusion of the solvent across asemipermeable membrane.
In living systems the solvent is
always water, so biologistsgenerally define osmosis as the
diffusion of water across asemipermeable membrane:
O i
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Lower
concentration
of solute (sugar)
Higher
concentration
of sugar
Same concentration
of sugar
Selectively
permeable mem-
brane: sugar mole-
cules cannot pass
through pores, but
water molecules canMore free water
molecules (higher
concentration)
Water molecules
cluster around
sugar molecules
Fewer free water
molecules (lower
concentration)
Water moves from an area of higher
free water concentration to an area
of lower free water concentration
Osmosis
Osmosis
O ti P
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Osmotic Pressure
Osmotic pressure of a solution is the
pressure needed to keep it inequilibrium with pure H20.
The higher the concentration of
solutes in a solution, the higher its
osmotic pressure.
Tonicity is the ability of a solution tocause a cell to gain or lose water
based on the concentration of solutes
T i it
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Tonicity
If 2 solutions have equal [solutes], they are called
isotonic
If one has a higher [solute], and lower [solvent], is
hypertonic
The one with a lower [solute], and higher [solvent], is
hypotonic
Hypotonic solution Isotonic solution Hypertonic solution
H2OH2O H2O H2O
Lysed Normal Shriveled
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Water Balance In Cells With Walls
Plant cell. Plant cells
are turgid (firm) and
generally healthiest in
a hypotonic environ-
ment, where the
uptake of water is
eventually balanced
by the elastic wallpushing back on the
cell.
(b)
H2OH2OH2OH2O
Turgid (normal) Flaccid Plasmolyzed
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My definition of Osmosis
Osmosis is the diffusion of wateracross a semi-permeable membrane
from a hypotonic solution to a
hypertonic solution
F ilit t d Diff i
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Facilitated Diffusion
Diffusion of solutes through a semipermeable membrane with the
help of special transport proteins i.e. large polar molecules and ions
that cannot pass through phospholipid bilayer. Two types of transport proteins can help ions and large polar
molecules diffuse through cell membranes:
Channel proteins provide a narrow channel for the substance to pass
through.
Carrier proteins physically bind to the substance on one side ofmembrane and release it on the other.
EXTRACELLULAR
FLUID
Channel protein Solute
CYTOPLASM
Carrier proteinSolute
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Facilitated Diffusion
Specific each channel or carrier
transports certain ions or molecules
only
Passive direction of net movementis always down the concentration
gradient
Saturates once all transportproteins are in use, rate of diffusion
cannot be increased further
Active Transport
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Active Transport
Uses energy (from ATP) to move a
substance against its naturaltendency e.g. up a concentration
gradient.
Requires the use of carrier proteins
(transport proteins that physically bind
to the substance being transported).
2 types: Membrane pump (protein-mediated active
transport)
Membrane Pump
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Membrane Pump
A carrier protein uses energy from
ATP to move a substance across amembrane, up its concentration
gradient:
The Sodium potassium Pump
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One type of active transport system
The Sodium-potassium Pump
2. Na+ binding stimulates
phosphorylation by ATP.
1. Cytoplasmic Na+ binds
to the sodium-potassium
pump.
6. K+ is released and Na+
sites are receptive again;
the cycle repeats.
3. Phosphorylation causes the
protein to change its conformation,
expelling Na+ to the outside.
4. Extracellular K+ binds to the
protein, triggering release of the
Phosphate group.
5. Loss of the phosphate
restores the proteins
original conformation.
P
EXTRACELLULAR
FLUID
Na+
CYTOPLASM
[Na+] low
[K+] high
Na+
Na+
Na+
Na+
Na+
P ATP
Na+Na+
Na+
P
ADP
K+
K+
K+
K+K+
K+
[Na+] high[K+] low
P iP i
Coupled transport
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Coupled transport
2 stages:
Carrier protein uses ATP to move a substance across the
membrane against its concentration gradient. Storing energy.
Coupled transport protein allows the substance to move down its
concentration gradient using the stored energy to move a
second substance up its concentration gradient:
Review: Passive And Active Transport Compared
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Review: Passive And Active Transport Compared
Passive transport. Substances diffuse spontaneously
down their concentration gradients, crossing a
membrane with no expenditure of energy by the cell.
The rate of diffusion can be greatly increased by transport
proteins in the membrane.
Active transport. Some transport proteins act
as pumps, moving substances across a
membrane against their concentrationgradients. Energy for this work is usually
supplied by ATP.
Diffusion. Hydrophobic
molecules and (at a slow
rate) very small uncharged
polar molecules can diffuse
through the lipid bilayer.
Facilitated diffusion. Many
hydrophilic substances diffuse
through membranes with the
assistance of transport proteins,
either channel or carrier proteins.
ATP
Bulk Transport
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Bulk Transport
Allows small particles, or groups of
molecules to enter or leave a cellwithout actually passing through the
membrane.
2 mechanisms of bulk transport:
endocytosis and exocytosis.
E d t i
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Endocytosis
The plasma membrane envelops
small particles or fluid, then seals onitself to form a vesicle or vacuole
which enters the cell:
Phagocytosis
Pinocytosis
Receptor-Mediated Endocytosis -
Three Types Of Endocytosis
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Three Types Of Endocytosis
EXTRACELLULAR
FLUID
Pseudopodium
CYTOPLASM
Food or
other particle
Food
vacuole
1 m
Pseudopodium
of amoeba
Bacterium
Food vacuole
An amoeba engulfing a bacterium via
phagocytosis (TEM).PINOCYTOSIS
Pinocytosis vesicles
forming (arrows) ina cell lining a smallblood vessel (TEM).
0.5 m
In pinocytosis, the cell
gulps droplets of
extracellular fluid into tiny
vesicles. It is not the fluid
itself that is needed by thecell, but the molecules
dissolved in the droplet.
Because any and all
included solutes are taken
into the cell, pinocytosis
is nonspecific in the
substances it transports.
Plasma
membrane
Vesicle
In phagocytosis, a cell
engulfs a particle by
Wrapping pseudopodiaaround it and packaging
it within a membrane-
enclosed sac large
enough to be classified
as a vacuole. The
particle is digested after
the vacuole fuses with a
lysosome containinghydrolytic enzymes.
PHAGOCYTOSIS
Process of Phagocytosis
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Process of Phagocytosis
Receptor mediated Endocytosis
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0.25 m
Ligand
Coat protein
Coated
pit
Coatedvesicle
Receptor
A coated pit
and a coated
vesicle formedduring
receptor-
mediated
endocytosis
(TEMs).Plasma
membrane
Coat
protein
Receptor-mediated endocytosis
enables the cell to acquire bulk quantities
of specific substances, even though those
substances may not be very concentrated
in the extracellular fluid. Embedded in the
membrane are proteins with specific
receptor sites exposed to the extracellular
fluid. The receptor proteins are usually
already clustered in regions of themembrane called coated pits, which are
lined on their cytoplasmic side by a fuzzy
layer of coat proteins.
Extracellular substances (ligands) bind
to these receptors. When binding occurs,
the coated pit forms a vesicle containing
the ligand molecules. Notice that there are
relatively more bound molecules (purple)
inside the vesicle, other molecules
(green) are also present. After this
ingested material is liberated from the
vesicle, the receptors are recycled to the
plasma membrane by the same vesicle.
Receptor-mediated Endocytosis
Exocytosis
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Exocytosis
The reverse of endocytosis
During this process, the membrane of a vesiclefuses with the plasma membrane and its
contents are released outside the cell:
Cell Junctions
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Cell Junctions
Long-lasting or permanent connections betweenadjacent cells, 3 types of cell junctions:
Tight junctions prevent
fluid from moving
across a layer of cells
Tight junction
0.5 m
1 m
Space
between
cellsPlasma membranes
of adjacent cells
Extracellular
matrix
Gap junction
Tight junctions
0.1 m
Intermediate
filamentsDesmosome
Gap
junctions
At tight junctions, the membranes of
neighboring cells are very tightly pressed
against each other, bound together by
specific proteins (purple). Forming continu-
ous seals around the cells, tight junctions
prevent leakage of extracellular fluid across
A layer of epithelial cells.
Desmosomes (also called anchoring
junctions) function like rivets, fastening cells
Together into strong sheets. Intermediate
Filaments made of sturdy keratin proteins
Anchor desmosomes in the cytoplasm.
Gap junctions (also called communicating
junctions) provide cytoplasmic channels from
one cell to an adjacent cell. Gap junctions
consist of special membrane proteins that
surround a pore through which ions, sugars,
amino acids, and other small molecules may
pass. Gap junctions are necessary for commu-
nication between cells in many types of tissues,
including heart muscle and animal embryos.
TIGHT JUNCTIONS
DESMOSOMES
GAP JUNCTIONS
The Nucleus And The Nuclear Envelope
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Nucleus
NucleusNucleolusChromatin
Nuclear envelope:Inner membraneOuter membrane
Nuclear pore
Rough ER
Pore
complex
Surface of nuclear
envelope.
Pore complexes (TEM). Nuclear lamina (TEM).
Close-up of
nuclear
envelope
Ribosome
1 m
1 m
0.25 m
The Nucleus And The Nuclear Envelope Repository for genetic material called chromatin - DNA and proteins
Nucleolus: holds chromatin and ribosomal subunits- region of intensiveribosomal RNA synthesis
Nuclear envelope: Surface of nucleus bound by two phospholipid bilayermembranes - Double membrane with pores
Nucleoplasm: semifluid medium inside the nucleus
Ch
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Chromosomes DNA of eukaryotes is divided into linear
chromosomes. Exist as strands of chromatin, except
during cell division
Histones associated packaging proteins
Ribosomes
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Ribosomes Ribosomes are RNA-protein complexes composed of two
subunits that join and attach to messenger RNA.
Site of protein synthesis
Assembled in nucleoli
ERRibosomes Cytosol
Free ribosomes
Bound ribosomes
Largesubunit
Small
subunit
TEM showing ER and ribosomes Diagram of a ribosome
0.5 m
Endoplasmic reticulum (ER)
E d b S t
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Endomembrane System
Compartmentalizes cell, channeling passage
of molecules through cells interior. Endoplasmic reticulum
Rough ER - studded with ribosomes
Smooth ER - few ribosomes
R h ER
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Rough ER Rough ER is especially abundant in cells that secrete proteins.
As a polypeptide is synthesized on a ribosome attached to rough ER, it is threaded into the
cisternal space through a pore formed by a protein complex in the ER membrane.
As it enters the cisternal space, the new protein folds into its native conformation. Most secretory polypeptides are glycoproteins, proteins to which a carbohydrate is
attached.
Secretory proteins are packaged in transport vesicles that carry them to their next stage.
Rough ER is also a membrane factory.
Membrane-bound proteins are synthesized directly into the membrane.
Enzymes in the rough ER also synthesize phospholipids from precursors in the cytosol.
As the ER membrane expands, membrane can be transferred as transport vesicles to other
components of the endomembrane system.
S th ER
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Smooth ER The 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.
These include the sex hormones of vertebrates and adrenal steroids.
In the smooth ER of the liver, enzymes help detoxify poisons and drugs such as
alcohol and barbiturates.
Smooth ER stores calcium ions.
Muscle cells have a specialized smooth ER that pumps calcium ions from
the cytosol and stores them in its cisternal space.
When a nerve impulse stimulates a muscle cell, calcium ions rush from
the ER into the cytosol, triggering contraction.
Th G l i t
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The Golgi apparatus The Golgi apparatus is the shipping and receiving center for cell
products.
Many transport vesicles from the ER travel to the Golgi apparatusfor modification of their contents.
The Golgi is a center of manufacturing, warehousing, sorting, andshipping.
The Golgi apparatus consists of flattened membranous sacscisternaelooking like a stack of pita bread.
The Golgi sorts and packages materials into transport vesicles.
F ti Of Th G l i A t
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Functions Of The Golgi Apparatus
TEM of Golgi apparatus
cis face
(receiving side of
Golgi apparatus)
Vesicles move
from ER to GolgiVesicles also
transport certainproteins back to ER
Vesicles coalesce to
form new cis Golgi cisternae
Cisternal
maturation:
Golgi cisternae
move in a cis-
to-trans
direction
Vesicles form and
leave Golgi, carrying
specific proteins toother locations or to
the plasma mem-
brane for secretionVesicles transport specificproteins backward to newer
Golgi cisternae
Cisternae
trans face
(shipping side ofGolgi apparatus)
0.1 0 m1
6
5
2
3
4
Golgi
apparatus
Membrane Bound Organelles
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Membrane Bound Organelles
Lysosomes vesiclecontaining digestiveenzymes that break downfood/foreign particles
Vacuoles food storageand water regulation
Peroxisomes - containenzymes that catalyze the
removal of electrons andassociated hydrogenatoms
(a) Phagocytosis: lysosome digesting food
1 m
Lysosome contains
active hydrolytic
enzymes
Food vacuole
fuses with
lysosome
Hydrolytic
enzymes digest
food particles
Digestion
Food vacuole
Plasma membrane
Lysosome
Digestive
enzymes
Lysosome
Nucleus
Mitochondria
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Mitochondria Sites of cellular respiration, ATP synthesis
Bound by a double membrane surrounding fluid-filled matrix.
The inner membranes of mitochondria are cristae The matrixcontains enzymes that break down carbohydrates and
the cristae house protein complexes that produce ATP
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Cytoskeleton The eukaryotic cytoskeleton is a network of
filaments and tubules that extends from thenucleus to the plasma membrane that supportcell shape and anchor organelles.
Protein fibers
Actin filaments
cell movement
Intermediate filaments Microtubules
centrioles
Centrioles
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Centrioles
Centrioles are shortcylinders with a 9 + 0
pattern of microtubule
triplets.
Centrioles may beinvolved in microtubule
formation and
disassembly during cell
division and in the
organization of cilia and
flagella.
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Cilia and Flagella
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Cilia and Flagella
Cilia (small and numerous) and flagella (large and single)have a 9 + 2 pattern of microtubules and are involved in
cell movement. Cilia and flagella move when the microtubule doublets
slide past one another.
Each cilium and flagellum has a basalbodyat its base.
Cilia and Flagella
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(a) Motion of flagella. A flagellumusually undulates, its snakelike
motion driving a cell in the same
direction as the axis of the
flagellum. Propulsion of a human
sperm cell is an example of
flagellatelocomotion (LM).
1 m
Direction of swimming
Cilia and Flagella
(b) Motion of cilia. Cilia have a back-
and-forth motion that moves the
cell in a direction perpendicular
to the axis of the cilium. A dense
nap of cilia, beating at a rate ofabout 40 to 60 strokes a second,
covers this Colpidium, a
freshwater protozoan (SEM).