4 Cellular Form and Function

8/12/2019 4 Cellular Form and Function

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Development of the cell theory:

Hooke in 1663, observed cork

(plant): named the cell Schwann in 1800s states:

all animals are made of cells

Pasteurs work with bacteria ~ 1860 disprovedidea of spontaneous generation (living things

from nonliving)

Modern cell theory emerged by 1900

Chapter 4:

Cellular Form and Function


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Modern Cell Theory All organisms composed of cells and cell products.

A cell is the simplest structural and functional unit of life. Thereare no smaller subdivisions of a cell or organism that, in themselves,

are alive.

An organisms structure and all of its functions are ultimately due to

the activities of its cells. Cells come only from preexisting cells, not from nonliving

matter. All life, therefore, traces its ancestry to the same original

cells.

Because of this common ancestry, the cells of all species have manyfundamental similarities in their chemical composition and

metabolic mechanisms.

Cells carry genetic information in the form of DNA, which is

passed from the parent to the daughter cell.


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Methods and tools

MicroscopyCompound light microscope

Magn. = eyepiecex objective, eg. 10x times 4x = 40x

Mostly for non-living specimen (dyes used)

Phase contrast microscope Allows visualization of living cells

Electron microscope Uses a beam of e-, resolution is thus not limited to light wavelength

Order of pm

Autoradiography Label cells with radioactive atoms or molecules and follow their

location

Centrifugation Sedimentation coefficient:

Depends on mass and particles size (radius)


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Prokaryotes

bacteria, cyanobacteria (blue-green algae) and archaeaabsent nucleus, no mitochondria or chloroplasts, no ER and

Golgi

size of ribosomes similar to mitochondrial ribosomes ineukaryotes

circular DNA is held within irregular nucleoid

oxidative phosphorylation takes place directly on plasmamembrane

do have ribosomes (30,50,70) and cytoskeletons

contain plasmidssome may organize into cellular communities (biofilms)

asexual reproduction (binary fission or budding) andconjugation

have flagellum and cell walls (except gen. mycoplasma)


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Cell Shapes

thin, flat, angular contours

irregular angular shapes,

> 4 sides

round to oval

disc shaped


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Cell Shapes 2

squarish thick middle, tapered ends

taller than wide long, slender

Stellate

nerve cells have extensions,look starlike


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Epithelial Cell Surfaces

Epithelial cells line organ surfaces andcavities

Basal surface

cell rests on this lower surface

Lateral surface

the sides of the cell

Apical surface

exposed upper surface


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Cell Size

Human cell sizemost range from 10 - 15 m

egg cells (very large)100 m diameter, visible to

naked eyenerve cell over 1 meter, muscle cell up to 30 cm,

(too slender to be seen)

Limitations on cell size

as cell enlarges, volume increases faster than

surface area so the need for increased nutrients

and waste removal exceeds ability of membrane

surface to exchange


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Cell Surface Area and Volume


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Evolving Perspective on Cells

Early study with light microscope revealedsurface membrane, nucleus and cytoplasm

Electron microscopes have much higher resolution

and revealed much greater details, such as the cellultrastructure of the cytoplasm

fibers, passageways and compartments, and organelles

surrounded by cytosol (a clear gelatinous component

also called intracellular fluid)


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Cell Structure


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Cell Structure 2


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Defines cell boundaries Controls interactions with other cells

Controls passage of materials in and out of cell

Appears as pair of dark parallel lines around cell(viewed with the electron microscope)

intracellular face - side faces cytoplasm

extracellular face - side faces outwards

Structure described by fluid-mosaic theory

arrangement of mobile globular proteins embedded in

an oily film of phospholipids

Plasma Membrane


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Plasma Membrane Preview


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Membrane Lipids

Lipids constitute90 to 99% of the

plasma membrane

Glycolipids5% of the lipids, found

only on extracellularface, contribute to

glycocalyx


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Cholesterol20% of the lipids, affects membrane

fluidity (low conc.. more rigid, highconc.. more fluid)

Phospholipid bilayer75% of the lipids, with hydrophilic heads

(phosphate) on each side andhydrophobic tails in the center

motion of these molecules createsmembrane fluidity, an important qualitythat allows self repair

Membrane Lipids 2


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Membrane Proteins

Proteins constituteonly 1 to 10% of the plasma

membrane, but they are larger andaccount for half its weight

Integral (transmembrane) proteinspass through membrane, have

hydrophobic regions embedded inphospholipid bilayer and hydrophilicregions extending into intra- andextracellular fluids

most are glycoproteins, conjugatedwith oligosaccharides on theextracellularside of membrane


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Integral proteins (cont.)may cross the plasma

membrane once or multiple

times

Peripheral proteins

adhere to intracellular

surface of membrane

anchors integral proteins to

cytoskeleton

Membrane Proteins 2


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Membrane Protein Functions

Receptors Second messenger systems

Enzymes

Channel proteins

Carriers and pumps

Motor molecules

Cell-identity markers

Cell-adhesion molecules


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Protein Functions - Receptors

Cells communicate with chemicalsignals that cannot enter target

cells

Receptors bind these messengers(hormones, neurotransmitters)

Each receptor is usually specific

for one messenger


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Second Messenger System

A messenger (epinephrine)binds to a receptor 1

Receptor releases a Gprotein

2

G protein binds to anenzyme, adenylate cyclase,which converts ATP to

cAMP, the 2ndmessenger 3 cAMP activates a kinase 4

Kinases add Pi, activates or

inactivates other enzymes


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Enzymes in Plasma Membrane

Break down chemicalmessengers to stop their effects

Final stages of starch and

protein digestion in smallintestine

Involved in producing second

messengers (cAMP)


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Protein Functions - Channel Proteins

Formed by integral proteins

Channels are constantly open,

allow water and hydrophilic

solutes in and out


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Gates open to three type of stimulantsligand-regulated gates: bind to chemicalmessenger

voltage-regulated gates: potential changes

across plasma membranemechanically regulated gates:physicalstress such as stretch and pressure

Gates control passage of electrolytes so

are important in nerve signals andmuscle contraction

Protein Functions - Channel Proteins 2


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Protein Functions - Motor Molecules

A filamentous protein that arisesdeep in the cytoplasm and pulls

on membrane proteins causing

movement:within a cell (organelles)

of a cell (WBCs)

shape of cell (cell division,

phagocytosis)


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Protein Functions - Carriers

Integral proteins that bind to solutes andtransfer them across membrane

Carriers that consume ATP are called

pumps


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Protein Functions - Cell-identity Markers

Glycoproteins contribute tothe glycocalyx, a surface

coating that acts as a cells

identity tag


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Protein Functions - Cell-adhesion Molecules

Membrane proteins thatadhere cells together and

to extracellular material


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Glycocalyx

Surface of animal cellsCHO moieties of membrane glycoproteins

and glycolipids that retains a film of water

Functionsimmune response to infection and cancer

basis of tissue transplant compatibility

cellular uptake of water, dissolved solutesassists in cell adhesion, fertilization and

embryonic development


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Structure extensions of plasma membrane (1-2m) that

increase surface area for absorptive cells (by 15-

40x in intestine, kidney)

Brush border

on some cells, they are very dense and appear as

a fringe on apical cell surface

Milking actionprotein filaments (actin) attach from the tip of

microvillus to its base, anchors to a protein mesh

in the cytoplasm called the terminal web and can

shorten pushing absorbed contents into cell

Microvilli

Cross Section of a Microvillus reveals


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Cross Section of a Microvillus reveals

actin filaments


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Cilia

Hairlike processes 7-10m long, 50-200 on cell surfacemove mucus, egg cells note that microvilli, unlike cilia, are extensions of the plasma

membrane

Covered by saline layer created by chloride pumps

Cilia beat in waves, sequential power strokes followed byrecovery strokes

Cross Section of a Cilium reveals


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Cross Section of a Cilium reveals

microtubules


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Cilia 2

Axonemehas a 9+2 structure of microtubules2 central microtubules stop at cell surface

9 pairs of peripheral microtubules continue into cell as

abasal bodythat acts as an anchor

dynein (motor protein) arms on one pair of peripheral

microtubules crawls up adjacent pair bending cilia

Sensory cells

some cilia lose motility and are involved in vision,

smell, hearing and balance


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Cilium At Cell Surface


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Flagella

Long whiplike structure that has an axonemeidentical to that of a cilium

the only functional flagellum in humans is the tail

of the sperm


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Nucleus

Largest organelle Nuclear envelope surrounds nucleus with a double

membrane punctures with nuclear pores

Contains DNA, the genetic program for a cellsstructure and function

nucleolus = a subsection within the nucleus, where

rRNA is synthesized


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Cell Structure


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Rough ERextensive sheets of parallel unit membranes with cisternae

between them and covered with ribosomes, continuous withnuclear envelope

protein synthesis, in particular secretory proteins,production of cell membranes

protein folding environment

Ca2+ homeostasis

Smooth ER lack ribosomes, cisternae more tubular and branch more

extensively, continuous with rough ER

function in lipid synthesis, detoxification, Ca2+storage and

signaling

Endoplasmic Reticulum


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Endoplasmic Reticulum Diagram


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Ribosomes

Small dark granules of protein and RNA free incytosol or on surface of rough ER

Interpret the genetic code and synthesize

polypeptides


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Golgi Complex

Synthesizes CHOs, processes proteins from RERand packages them into golgi vesicles

Golgi vesiclesirregular sacs near golgi complex that bud off cisternae

some become lysosomes, some fuse with plasmamembrane and some become secretory vesicles

Secretory vesiclesstore a cell product

for later release Note: in plants

vesicles are larger and

are called vacuoles


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Lysosomes

Package of enzymes in a single unit membrane,variable in shape

garbage brought by endosomes

Functions

intracellular digestion - hydrolyze proteins, nucleic acids,complex carbohydrates, phospholipids and other substrates

hydrolytic enzymes work at pH = 5

autophagy - the digestion of worn out organelles andmitochondrion

autolysis/apoptosis - programmed cell death

glucose mobilization - lysosomes in liver cells break downglycogen


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Microbodies

Peroxisome Appear similar to lysosomes, lighter in color

Abundant in liver and kidney

Function

neutralize free radicalsproduce H2O2in process of alcohol detoxification in the

liver, and in killing bacteria

break down excess H2O2 with the enzyme catalase

break down fatty acids into acetyl groupsGlyosome (in plants)

important during germination

allows plants to break down fats into carbohydrates for

energy


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Mitochondrion

Double unit membrane Inner membrane contains folds called cristae

ATP synthesized by enzymes on cristae from energy extractedfrom organic compounds

Space between cristae called the matrix contains ribosomes and small, circular DNA (mitochondrial

DNA) are semiautonomous: mDNA is replicated independently,

encodes for many proteins others proteins (e.g. ribosomes) are encoded by nuclear DNA

Reproduce independentlyof cell and live for 10 days

capable of triggering apoptosis in plants:

chloroplasts contain their own DNA

carry out photosynthesis


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Mitochondrion, Electron Micrograph

ll ll


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Cell wall

present in all procaryotes and some eukaryotes:fungi, made of chitin

plants composed of cellulose

protection, structural support notpresent in animals

C i l


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Centrioles

not present in plants Short cylindrical assembly of microtubules,

arranged in nine groups of three microtubules each

Two centrioles, perpendicular to each other, lienear the nucleus in an area called the centrosome

these play a role in cell division

Other single centrioles migrate to plasmamembrane formingbasal bodiesof cilia or flagella

two microtubules of each triplet elongate to form the

nine pairs of peripheral microtubules of the axoneme

P di l C i l Di


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Perpendicular Centrioles Diagram

C k l


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Cytoskeleton

Microfilaments (actin filaments)made of protein actin, form network on cytoplasmic

side of plasma membrane called the membrane skeleton

supports phospholipids of plasma membrane, supports

microvilli and produces cell movement,

with myosin causes muscle contraction

Intermediate fibers

in junctions that hold epithelial cells together and resist

stresses on a cell

Microtubules

Mi b l


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Microtubules

Hollow cylinder of 13 parallel strands calledprotofilaments

(a long chain of globular protein units called tubulin)

Hold organelles in place and maintain cell shape Form tracks to guide organelles and molecules to

specific destinations in a cell

Form axonemes of cilia and flagella, centrioles,basal bodies and mitotic spindle

Not all are permanent structures and can be

disassembled and reassembled where needed

Mi b l Di


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Microtubule Diagram

C t k l t Di


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Cytoskeleton Diagram

I l i


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Inclusions

Highly variable appearance, no unit membrane Stored cellular products

glycogen granules,pigments and fat droplets

in plants: water vacuoles Foreign bodies

dust particles, viruses and intracellular bacteria

M b T S l i P bili


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Membrane Transport: Selective Permeability

Plasma membrane allows passage of some things

between cytoplasm and ECF but not others

Passive transport requires no ATP, movement of

particles across selectively permeable membrane,

down concentration gradient filtration

simple diffusion

facilitated diffusion through channels Active transport requires ATP, transports particles

up concentration gradient

carrier mediated (active transport) and bulk transport

M b T t Filt ti


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Membrane Transport: Filtration

Movement of particles through a selectivelypermeable membrane by hydrostatic pressure

Hydrostatic pressure - the force exerted on the

membrane by water

In capillaries, blood pressure forces water, salts,

nutrients and solutes into tissue fluid, while larger

particles like blood cells and protein are held back

Si l Diff i


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Simple Diffusion

Simple diffusion is the movement of particles as aresult of their constant, random motion

Net diffusion is the movement of particles from an

area of high concentration to an area of lowconcentration (down or with the concentration

gradient)

Diff i R t


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Diffusion Rates

Factors that affect rate of diffusion through amembrane

Temperature - temp., motion of particles

Molecular weight - larger molecules move slower

Steepness of conc.gradient - difference, rate

Membrane surface area - area, rate

Membrane permeability - permeability, rate

O i


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Osmosis

Net diffusion of waterthrough a selectively

permeable membrane from

an area of more water, sideB (less dissolved solute) to

an area of less water, side

A (more dissolved solute)

O ti P


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Osmotic Pressure

Osmosis opposed byfiltration of water

back across

membrane due to hydrostatic pressure

Amount of

hydrostatic pressure

required to stop

osmosis is called

osmotic pressure

O l it


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Osmolarity

One osmole is 1 mole of dissolved particles1M NaCl contains 1 mole Na+ions and 1 mole Cl-

ions/L, both affect osmosis, thus 1M NaCl = 2 osm/L

Osmolarity = # osmoles/liter solution

Tonicit


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Tonicity

Tonicity - ability of a solution to affect fluidvolume and pressure within a celldepends on concentration and permeability of solute

Hypotonic solution

has low concentration of nonpermeating solutes (highwater concentration)

cells in this solution would absorb water, swell and mayburst (lyse)

Hypertonic solutionhas high concentration of nonpermeating solutes (low

water concentration)cells in this solution would lose water +shrivel (crenate)

Isotonic: no net movement

M b T C i M di d T


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Membrane Transport: Carrier Mediated Transport

Proteins in cell membrane carry solutes through it Specificity

solute binds to a receptor site on carrier protein that is

specific for that solute Two types of carrier mediated transport are

facilitated diffusion and active transport

Exhibits saturation (see next slide)

Carrier Saturation


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Carrier Saturation

As concentration of solute , rate of transport upto the point when all carriers are occupied and rate

of transport levels off at the transport maximum

Membrane Transport: Facilitated Diffusion


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Membrane Transport: Facilitated Diffusion

Passivetransport of solute downits concentrationgradient, across membrane, with aid of a carrier

Solute binds to carrier, carrier changes shape andreleases solute on other side of membrane

Active Transport


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Active Transport

Activetransport of solute upits concentrationgradient, across membrane, carrier requires ATP

Carrier binds to ligand ATP phosphorylates carrier

Carrier changes conformation

Carrier releases ligand on other side

Prominent example is the sodium-potassium pump

Sodium Potassium Pump


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Sodium-Potassium Pump

3Na+

bind to receptor, carrier phosphorylated,changes conformation, releases Na+in ECF, binds

2K+, releases Pi, resumes conformation, releases K+

Na+ = naverx,

3 signs

K+ = 2 signs

Functions of Sodium Potassium Pump


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Functions of Sodium-Potassium Pump

Regulation of cell volumecell swelling stimulates the Na+-K+pump:

ion concentration, osmolarity and cell swelling

Heat production

Maintenance of a membrane potential

Na+-K+pump keeps inside of membrane negative,

outside of membrane positive

Secondary active transport

transport of solute particles by carrier that does not

need ATP, but depends on the concentration gradient

provided by active transport pumps ...

Secondary Active Transport


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Secondary Active Transport

Transport of glucose byfacilitated diffusion, along with

Na+by SGLT carrier (no ATP),

depends on Na+

-

K+

pump (usesATP)

Cotransport


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Cotransport

When carrier transports 2 different solutessimultaneously, or within one transport cycle

Symport - a carrier that transports both solutes in

the same direction Antiport - a carrier that transports solutes in

opposite directions

Bulk Transport


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Bulk Transport

Transport of large particles and fluid dropletsthrough membrane, using vacuoles or vesicles of

plasma membrane, uses a lot of ATP

Endocytosis - bulk transport intocell Exocytosis - bulk transport out of cell

Endocytosis has three forms

phagocytosis- engulfing large particles by pseudopodsfluid phase pinocytosis

receptor mediated endocytosis

Phagocytosis


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Phagocytosis

Fluid phase Pinocytosis


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Fluid-phase Pinocytosis

Cell takes in droplets of ECF (along with dissolvedor broken down particles)

Plasma membrane dimples, then pinches off as

pinocytotic vesicle Occurs in all human cells

Receptor Mediated Endocytosis


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Receptors on membrane bind to specific moleculesin ECF, cluster together, then sink in, become

coated with a peripheral protein, clathrin, and

pinch off into cell as clathrin-coated vesicle

This occurs in the uptake of LDLs by endothelium

of blood vessels

Transcytosis uses this process to move a substance

across a cell

insulin absorbed into endothelial cell from blood by

RME, then transported out into tissues



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Receptor Mediated Endocytosis EM


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Receptor Mediated Endocytosis EM

Exocytosis


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Exocytosis

Eliminating or secreting material from cell andreplacement of plasma membrane

Exocytosis EM


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Exocytosis EM

Viruses are not living organisms


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Viruses are not living organisms

cells are 1-10 um viruses are about two orders of magnitude smaller

main features:

protein coat = capsidDNA or RNA, single or double stranded

obligate intracellulare parasites

bacteriophages target bacteria

4 Cellular Form and Function

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