Dawson Bio 11 Fall 2012 Week 3

© 2013 Pearson Education, Inc.

Cell Theory

• Cell - structural and functional unit of life

• Organismal functions depend on individual and collective cell functions

• Biochemical activities of cells dictated by their shapes or forms, and specific subcellular structures

• Continuity of life has cellular basis


Cell Diversity

• Over 200 different types of human cells

• Types differ in size, shape, subcellular components, and functions


Figure 3.1 Cell diversity.

Erythrocytes

Fibroblasts

Epithelial cells

Cells that connect body parts, form linings, or transport gases

Skeletalmusclecell

Smoothmuscle cells

Cells that move organs and body parts

Fat cell

Macrophage

Cell that stores nutrients Cell that fights disease

Nerve cell

Cell that gathers information and controls body functions

Cell of reproduction

Sperm


Generalized Cell

• All cells have some common structures and functions

• Human cells have three basic parts:– Plasma membrane—flexible outer boundary– Cytoplasm—intracellular fluid containing

organelles– Nucleus—control center


Figure 3.2 Structure of the generalized cell.

Chromatin

Nucleolus

Smooth endoplasmicreticulum

Cytosol

Mitochon-drion

Lysosome

Centrioles

Centro-somematrix

Cytoskeletalelements• Microtubule• Intermediate filaments

Nuclear envelope

Nucleus

Plasmamembrane

Roughendoplasmicreticulum

Ribosomes

Golgi apparatus

Secretion being releasedfrom cell by exocytosis

Peroxisome


Plasma Membrane

• Lipid bilayer and proteins in constantly changing fluid mosaic

• Plays dynamic role in cellular activity

• Separates intracellular fluid (ICF) from extracellular fluid (ECF)– Interstitial fluid (IF) = ECF that surrounds

cells

PLAYPLAY Animation: Membrane Structure


Figure 3.3 The plasma membrane.

Extracellular fluid(watery environmentoutside cell)

Polar head of phospholipid molecule

Cholesterol GlycolipidGlyco-protein

Nonpolar tail of phospholipid molecule

Glycocalyx(carbohydrates)

Lipid bilayercontaining proteins

Outward-facinglayer ofphospholipids

Inward-facinglayer of phospholipids

Cytoplasm (watery environmentinside cell)

Integral proteins

Filament of cytoskeleton

Peripheral proteins


Membrane Lipids

• 75% phospholipids (lipid bilayer)– Phosphate heads: polar and hydrophilic– Fatty acid tails: nonpolar and hydrophobic

(Review Fig. 2.16b)

• 5% glycolipids– Lipids with polar sugar groups on outer

membrane surface

• 20% cholesterol– Increases membrane stability


Membrane Proteins

• Allow communication with environment

• ½ mass of plasma membrane

• Most specialized membrane functions

• Some float freely

• Some tethered to intracellular structures

• Two types:– Integral proteins; peripheral proteins


PLAYPLAY Animation: Transport Proteins

Membrane Proteins

• Integral proteins– Firmly inserted into membrane (most are

transmembrane)– Have hydrophobic and hydrophilic regions

• Can interact with lipid tails and water

– Function as transport proteins (channels and carriers), enzymes, or receptors


Animation: Structural ProteinsPLAYPLAY

Animation: Receptor ProteinsPLAYPLAY

Membrane Proteins

• Peripheral proteins– Loosely attached to integral proteins – Include filaments on intracellular surface for

membrane support– Function as enzymes; motor proteins for

shape change during cell division and muscle contraction; cell-to-cell connections


Six Functions of Membrane Proteins

1. Transport

2. Receptors for signal transduction

3. Attachment to cytoskeleton and extracellular matrix


PLAYPLAY Animation: Transport Proteins

Figure 3.4a Membrane proteins perform many tasks.

• A protein (left) that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. • Some transport proteins (right) hydrolyze ATP as an energy source to actively pump substances across the membrane.

Transport


Animation: Receptor ProteinsPLAYPLAY

Figure 3.4b Membrane proteins perform many tasks.

• A membrane protein exposed to the outside of the cell may have a binding site that fits the shape of a specific chemical messenger, such as a hormone. • When bound, the chemical messenger may cause a change in shape in the protein that initiates a chain of chemical reactions in the cell.

Receptors for signal transductionSignal

Receptor


Figure 3.4c Membrane proteins perform many tasks.

Attachment to the cytoskeleton andextracellular matrix

• Elements of the cytoskeleton (cell's internal supports) and the extracellular matrix (fibers and other substances outside the cell) may anchor to membrane proteins, which helps maintain cell shape and fix the location of certain membrane proteins. • Others play a role in cell movement or bind adjacent cells together.

Animation: Structural ProteinsPLAYPLAY


Six Functions of Membrane Proteins

4. Enzymatic activity

5. Intercellular joining

6. Cell-cell recognition

© 2013 Pearson Education, Inc.Figure 3.4d

Figure 3.4d Membrane proteins perform many tasks.

Enzymatic activity

• A membrane protein may be an enzyme with its active site exposed to substances in the adjacent solution. • A team of several enzymes in a membrane may catalyze sequential steps of a metabolic pathway as indicated (left to right) here.

Enzymes

Animation: EnzymesPLAYPLAY


Figure 3.4e Membrane proteins perform many tasks.

Intercellular joining

• Membrane proteins of adjacent cells may be hooked together in various kinds of intercellular junctions. • Some membrane proteins (cell adhesion molecules or CAMs) of this group provide temporary binding sites that guide cell migration and other cell-to-cell interactions.

CAMs


Figure 3.4f Membrane proteins perform many tasks.

• Some glycoproteins (proteins bonded to short chains of sugars) serve as identification tags that are specifically recognized by other cells.

Cell-cell recognition

Glycoprotein


Lipid Rafts

• ~20% of outer membrane surface

• Contain phospholipids, sphingolipids, and cholesterol

• More stable; less fluid than rest of membrane– May function as stable platforms for cell-

signaling molecules, membrane invagination, or other functions


The Glycocalyx

• "Sugar covering" at cell surface– Lipids and proteins with attached

carbohydrates (sugar groups)

• Every cell type has different pattern of sugars– Specific biological markers for cell to cell

recognition– Allows immune system to recognize "self" and

"non self"– Cancerous cells change it continuously


Cell Junctions

• Some cells "free"– e.g., blood cells, sperm cells

• Some bound into communities– Three ways cells are bound:

• Tight junctions • Desmosomes • Gap junctions


Cell Junctions: Tight Junctions

• Adjacent integral proteins fuse form impermeable junction encircling cell– Prevent fluids and most molecules from

moving between cells

• Where might these be useful in body?


Plasma membranesof adjacent cells

Microvilli

Intercellularspace

Basement membrane

Interlockingjunctionalproteins

Intercellularspace

Tight junctions: Impermeable junctionsprevent molecules from passing throughthe intercellular space.

Figure 3.5a Cell junctions.


Cell Junctions: Desmosomes

• "Rivets" or "spot-welds" that anchor cells together at plaques (thickenings on plasma membrane)– Linker proteins between cells connect plaques– Keratin filaments extend through cytosol to

opposite plaque giving stability to cell – Reduces possibility of tearing

• Where might these be useful in body?


Intercellularspace

Linkerproteins(cadherins)Intermediate

filament(keratin)

Plaque

Desmosomes: Anchoring junctions bind adjacent cells together like a molecular “Velcro” and help form an internal tension-reducing network of fibers.

Microvilli

Intercellularspace

Basement membrane


Figure 3.5b Cell junctions.


Cell Junctions: Gap Junctions

• Transmembrane proteins form pores (connexons) that allow small molecules to pass from cell to cell– For spread of ions, simple sugars, and other

small molecules between cardiac or smooth muscle cells


Figure 3.5c Cell junctions.


Microvilli

Intercellularspace

Basement membrane

Intercellularspace

Channelbetween cells(formed byconnexons)

Gap junctions: Communicating junctionsallow ions and small molecules to passfor intercellular communication.


Plasma Membrane

• Cells surrounded by interstitial fluid (IF)– Contains thousands of substances, e.g.,

amino acids, sugars, fatty acids, vitamins, hormones, salts, waste products

• Plasma membrane allows cell to– Obtain from IF exactly what it needs, exactly

when it is needed– Keep out what it does not need


Cytoplasm

• Located between plasma membrane and nucleus– Composed of

• Cytosol– Water with solutes (protein, salts, sugars, etc.)

• Organelles– Metabolic machinery of cell; each with specialized

function; either membranous or nonmembranous

• Inclusions– Vary with cell type; e.g., glycogen granules, pigments,

lipid droplets, vacuoles, crystals


• Nonmembranous– Cytoskeleton – Centrioles – Ribosomes

• Membranes allow crucial compartmentalization

Cytoplasmic Organelles

• Membranous– Mitochondria– Peroxisomes– Lysosomes– Endoplasmic reticulum– Golgi apparatus


Mitochondria

• Double-membrane structure with inner shelflike cristae

• Provide most of cell's ATP via aerobic cellular respiration– Requires oxygen

• Contain their own DNA, RNA, ribosomes

• Similar to bacteria; capable of cell division called fission


Figure 3.17 Mitochondrion.

Outer mitochondrial

membrane

Ribosome

Mitochondrial DNA

Innermitochondrial

membrane

Cristae

Matrix

Enzymes


Ribosomes

• Granules containing protein and rRNA

• Site of protein synthesis

• Free ribosomes synthesize soluble proteins that function in cytosol or other organelles

• Membrane-bound ribosomes (forming rough ER) synthesize proteins to be incorporated into membranes, lysosomes, or exported from cell


Endoplasmic Reticulum (ER)

• Interconnected tubes and parallel membranes enclosing cisterns

• Continuous with outer nuclear membrane

• Two varieties:– Rough ER– Smooth ER


Rough ER

• External surface studded with ribosomes

• Manufactures all secreted proteins

• Synthesizes membrane integral proteins and phospholipids

• Assembled proteins move to ER interior, enclosed in vesicle, go to Golgi apparatus


Smooth ER

• Network of tubules continuous with rough ER

• Its enzymes (integral proteins) function in– Lipid metabolism; cholesterol and steroid-

based hormone synthesis; making lipids of lipoproteins

– Absorption, synthesis, and transport of fats– Detoxification of drugs, some pesticides,

carcinogenic chemicals– Converting glycogen to free glucose– Storage and release of calcium


Figure 3.18 The endoplasmic reticulum.

Nucleus

Smooth ER

Nuclearenvelope

Rough ER

Ribosomes

Electron micrograph of smooth and roughER (25,000x)

Diagrammatic view of smooth and rough ER


Golgi Apparatus

• Stacked and flattened membranous sacs

• Modifies, concentrates, and packages proteins and lipids from rough ER

• Transport vessels from ER fuse with convex cis face; proteins modified, tagged for delivery, sorted, packaged in vesicles


Golgi Apparatus

• Three types of vesicles bud from concave trans face– Secretory vesicles (granules)

• To trans face; release export proteins by exocytosis

– Vesicles of lipids and transmembrane proteins for plasma membrane or organelles

– Lysosomes containing digestive enzymes; remain in cell


Figure 3.19a Golgi apparatus.

Transport vesiclefrom rough ER

Secretoryvesicle

Cis face—“receiving” side of Golgi apparatus

Cisterns

New vesiclesforming

Transportvesiclefromtrans face

Trans face—“shipping” side ofGolgi apparatus

Many vesicles in the process of pinching offfrom the Golgi apparatus.


Figure 3.19b Golgi apparatus.New vesicles forming

Golgiapparatus

Transport vesicle at the trans face

Electron micrograph of the Golgi apparatus(90,000x)


Figure 3.20 The sequence of events from protein synthesis on the rough ER to the final distribution of those proteins.

Slide 1

Protein-conta-ining vesiclespinch off roughER and migrate to fuse withmembranes ofGolgi apparatus.

Proteins aremodified withinthe Golgi compartments.

Proteins are then packagedwithin differentvesicle types,depending ontheir ultimatedestination.

Rough ER ER membrane

Phagosome

Proteins incisterns

Plasmamembra-ne

Pathway C:Lysosomecontaining acidhydrolaseenzymesVesicle

becomeslysosome

Pathway B:Vesicle membraneto be incorporatedinto plasmamembrane

Extracellular fluidSecretion byexocytosis

Pathway A:Vesicle contentsdestined forexocytosis

Golgi apparatus

Secretoryvesicle

1

2

3


Peroxisomes

• Membranous sacs containing powerful oxidases and catalases

• Detoxify harmful or toxic substances

• Catalysis and synthesis of fatty acids

• Neutralize dangerous free radicals (highly reactive chemicals with unpaired electrons)– Oxidases convert to H2O2 (also toxic)

– Catalases convert H2O2 to water and oxygen


Lysosomes

• Spherical membranous bags containing digestive enzymes (acid hydrolases)– "Safe" sites for intracellular digestion

• Digest ingested bacteria, viruses, and toxins• Degrade nonfunctional organelles• Metabolic functions, e.g., break down and

release glycogen• Destroy cells in injured or nonuseful tissue

(autolysis)• Break down bone to release Ca2+


Figure 3.21 Electron micrograph of lysosomes (20,000x).

Lysosomes

Light green areas are regionswhere materials are being digested.


Endomembrane System

• Overall function– Produce, degrade, store, and export biological

molecules– Degrade potentially harmful substances

• Includes ER, golgi apparatus, secretory vesicles, lysosomes, nuclear and plasma membranes


Figure 3.22 The endomembrane system.Nuclearenvelope

Nucleus

Smooth ER

Rough ER

Golgiapparatus

Transportvesicle

Secretoryvesicle

Plasmamembrane

Lysosome


PLAYPLAY Animation: Endomembrane System

Endomembrane System


Cytoskeleton

• Elaborate series of rods throughout cytosol; proteins link rods to other cell structures– Three types

• Microfilaments • Intermediate filaments• Microtubules


Microfilaments

• Thinnest of cytoskeletal elements

• Dynamic strands of protein actin

• Each cell-unique arrangement of strands

• Dense web attached to cytoplasmic side of plasma membrane-terminal web– Gives strength, compression resistance

• Involved in cell motility, change in shape, endocytosis and exocytosis


Figure 3.23a Cytoskeletal elements support the cell and help to generate movement.

Microfilaments

Strands made of spherical proteinsubunits called actins

Actin subunit

7 nm

Microfilaments form the blue networksurrounding the pink nucleusin this photo.


Intermediate Filaments

• Tough, insoluble, ropelike protein fibers

• Composed of tetramer fibrils

• Resist pulling forces on cell; attach to desmosomes

• E.g., neurofilaments in nerve cells; keratin filaments in epithelial cells


Figure 3.23b Cytoskeletal elements support the cell and help to generate movement.

Tough, insoluble protein fibersconstructed like woven ropes

composed of tetramer (4) fibrils

Tetramer subunits

10 nm

Intermediate filaments form the purplebatlike network in this photo.

Intermediate filaments


Microtubules

• Largest of cytoskeletal elements; dynamic hollow tubes; most radiate from centrosome

• Composed of protein subunits called tubulins

• Determine overall shape of cell and distribution of organelles

• Mitochondria, lysosomes, secretory vesicles attach to microtubules; moved throughout cell by motor proteins


Figure 3.23c Cytoskeletal elements support the cell and help to generate movement.

Hollow tubes of spherical proteinsubunits called tubulins

Tubulin subunits

25 nm

Microtubules appear as gold networkssurrounding the cells’ pink nuclei inthis photo.

Microtubules


Motor Proteins

• Protein complexes that function in motility (e.g., movement of organelles and contraction)

• Powered by ATP


Figure 3.24 Microtubules and microfilaments function in cell motility by interacting with motor molecules powered by ATP.

Receptor formotor molecule

Vesicle

Microtubuleof cytoskeleton

Motor molecule(ATP powered)

Motor molecules can attach to receptors onvesicles or organelles, and carry the organellesalong the microtubule “tracks” of the cytoskeleton.

Motor molecule(ATP powered)

Cytoskeletal elements(microtubules or microfilaments)

In some types of cell motility, motor molecules attached to oneelement of the cytoskeleton can cause it to slide over anotherelement, which the motor molecules grip, release, and grip at anew site. Muscle contraction and cilia movement work this way.


Centrosome and Centrioles

• "Cell center" near nucleus

• Generates microtubules; organizes mitotic spindle

• Contains paired centrioles– Organelles; small tubes formed by

microtubules

• Centrioles form basis of cilia and flagella


Figure 3.25a Centrioles.

Centrosome matrix

Centrioles

Microtubules


Figure 3.25b Centrioles.


• Cilia and flagella– Whiplike, motile extensions on surfaces of

certain cells– Contain microtubules and motor molecules– Cilia move substances across cell surfaces– Longer flagella propel whole cells (tail of

sperm)

Animation: Cilia and Flagella

Cellular Extensions

PLAYPLAY


Cilia and Flagella

• Centrioles forming base called basal bodies

• Cilia movements alternate between power stroke and recovery stroke current at cell surface

• Primary cilia– Single, nonmotile projection on most cells– Probe environment for molecules receptors

can recognize; coordinate intracellular pathways


Figure 3.26 Structure of a cilium.

A longitudinal section of acilium shows microtubulesrunning the length of thestructure.

A cross section through thebasal body. The nine outerdoublets of a cilium extend intoa basal body where eachdoublet joins anothermicrotubule to form a ring ofnine triplets.

A cross section through thecilium shows the “9 + 2”arrangement of microtubules.

Microtubules

Plasmamembrane

Basal body

TEM TEM

TEM

Triplet

Plasmamembrane

Radial spoke

Cross-linkingproteins betweenouter doublets

Radial spoke

Cross-linkingproteins betweenouter doublets

Centralmicrotubule

Dynein arms

Outer microtubuledoublet

The doublets also have Attached motor proteins, the dynein arms.

The outermicrotubuledoublets and thetwo centralmicrotubules areheld together bycross-linkingproteins andradial spokes.

Cilium

Basal body(centriole)


Figure 3.27 Ciliary function. Recovery stroke, when ciliumis returning to its initial position

Layer of mucus

Cell surface

Phases of ciliary motion.

Traveling wave created by the activity ofmany cilia acting together propels mucusacross cell surfaces.

1 2 3 4 5 6 7

Power, orpropulsive, stroke


Cellular Extensions

• Microvilli– Minute, fingerlike extensions of plasma

membrane– Increase surface area for absorption– Core of actin filaments for stiffening


Figure 3.28 Microvilli.

Microvillus

Actinfilaments

Terminalweb


Nucleus

• Largest organelle; genetic library with blueprints for nearly all cellular proteins

• Responds to signals; dictates kinds and amounts of proteins synthesized

• Most cells uninucleate; skeletal muscle cells, bone destruction cells, and some liver cells are multinucleate; red blood cells are anucleate

• Three regions/structures


Figure 3.29a The nucleus.

NuclearenvelopeChromatin(condensed)Nucleolus

Cisterns ofrough ER

NuclearporesNucleus


The Nuclear Envelope

• Double-membrane barrier; encloses nucleoplasm

• Outer layer continuous with rough ER and bears ribosomes

• Inner lining (nuclear lamina) maintains shape of nucleus; scaffold to organize DNA

• Pores allow substances to pass; nuclear pore complex line pores; regulates transport of large molecules into and out of nucleus


Figure 3.29b The nucleus.

Fractureline of outermembrane

NuclearporesNucleus

Surface of nuclear envelope.

Nuclear pore complexes. Eachpore is ringed by protein particles.

Nuclear lamina. The netlike laminacomposed of intermediate filamentsformed by lamins lines the inner surfaceof the nuclear envelope.


Nucleoli

• Dark-staining spherical bodies within nucleus

• Involved in rRNA synthesis and ribosome subunit assembly

• Associated with nucleolar organizer regions– Contains DNA coding for rRNA

• Usually one or two per cell


Chromatin

• Threadlike strands of DNA (30%), histone proteins (60%), and RNA (10%)

• Arranged in fundamental units called nucleosomes

• Histones pack long DNA molecules; involved in gene regulation

• Condense into barlike bodies called chromosomes when cell starts to divide


Figure 3.30 Chromatin and chromosome structure.

1 DNA double helix (2-nm diameter)

Histones

2 Chromatin (“beads on a string”) structure with nucleosomes

Linker DNA

Nucleosome (10-nm diameter; eight histone proteins wrappedby two winds of the DNA double helix)

3 Tight helical fiber (30-nm diameter) 4 Looped domain

structure (300-nmdiameter) 5 Chromatid

(700-nm diameter) 6 Metaphase chromosome (at midpoint of cell division) consists of two sister chromatids


Membrane Transport: Active Processes

• Two types of active processes– Active transport– Vesicular transport

• Both require ATP to move solutes across a living plasma membrane because– Solute too large for channels– Solute not lipid soluble– Solute not able to move down concentration

gradient


Active Transport

• Requires carrier proteins (solute pumps)– Bind specifically and reversibly with

substance

• Moves solutes against concentration gradient– Requires energy


Active Transport: Two Types

• Primary active transport– Required energy directly from ATP hydrolysis

• Secondary active transport– Required energy indirectly from ionic

gradients created by primary active transport


Primary Active Transport

• Energy from hydrolysis of ATP causes shape change in transport protein that "pumps" solutes (ions) across membrane

• E.g., calcium, hydrogen, Na+-K+ pumps


Primary Active Transport

• Sodium-potassium pump– Most well-studied– Carrier (pump) called Na+-K+ ATPase– Located in all plasma membranes– Involved in primary and secondary active

transport of nutrients and ions


Sodium-Potassium Pump

• Na+ and K+ channels allow slow leakage down concentration gradients

• Na+-K+ pump works as antiporter– Pumps against Na+ and K+ gradients to

maintain high intracellular K+ concentration and high extracellular Na+ concentration

• Maintains electrochemical gradients essential for functions of muscle and nerve tissues

• Allows all cells to maintain fluid volume


Figure 3.10 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP.

Slide 1

Extracellular fluid Na+

Na+–K+ pump

K+

ATP-binding site

Cytoplasm

1 Three cytoplasmic Na+ bind to pumpprotein.

K+ released

6 Pump protein binds ATP; releases K+ to the inside, and Na+ sites are ready to bind Na+ again. The cycle repeats.

2 Na+ binding promotes hydrolysis of ATP.The energy released during this reactionphosphorylates the pump.

K+ bound

5 K+ binding triggers release of thephosphate. The dephosphorylated pumpresumes its original conformation.

K+

4 Two extracellular K+ bind to pump.

3 Phosphorylation causes the pump tochange shape, expelling Na+ to the outside.

Na+ bound

Na+ released

P

P

P

Pi


Secondary Active Transport

• Depends on ion gradient created by primary active transport

• Energy stored in ionic gradients used indirectly to drive transport of other solutes


Secondary Active Transport

• Cotransport—always transports more than one substance at a time– Symport system: Substances transported in

same direction– Antiport system: Substances transported in

opposite directions


Figure 3.11 Secondary active transport is driven by the concentration gradient created by primary activetransport.

Extracellular fluid

Na+-glucosesymporttransporterloads glucosefrom extracellularfluid

Na+-glucosesymport transporterreleases glucoseinto the cytoplasm

Glucose

Na+-K+

pump

Cytoplasm

Primary active transport

The ATP-driven Na+-K+ pump stores energy by creating a steep concentration gradient

for Na+ entry into the cell.

Secondary active transport

As Na+ diffuses back across the membranethrough a membrane cotransporter protein, itdrives glucose against its concentration gradientinto the cell.

1 2

Slide 1


Vesicular Transport

• Transport of large particles, macromolecules, and fluids across membrane in membranous sacs called vesicles

• Requires cellular energy (e.g., ATP)


Vesicular Transport

• Functions:– Exocytosis—transport out of cell – Endocytosis—transport into cell

• Phagocytosis, pinocytosis, receptor-mediated endocytosis

– Transcytosis—transport into, across, and then out of cell

– Vesicular trafficking—transport from one area or organelle in cell to another


Endocytosis and Transcytosis

• Involve formation of protein-coated vesicles

• Often receptor mediated, therefore very selective

• Some pathogens also hijack for transport into cell

• Once vesicle is inside cell it may– Fuse with lysosome– Undergo transcytosis


Figure 3.12 Events of endocytosis mediated by protein-coated pits. Slide 1 Coated pit ingests substance.

Coat proteins are recycled to plasma membrane.

1

Protein coat(typicallyclathrin)

Protein-coated vesicle deta- ches.

Transportvesicle

EndosomeUncoated endocyticvesicle

Transport vesicle containing

Uncoated vesicle fuses with a sorting vesicle called an endosome.

Fused vesicle may (a) fuse with lysosome for digestion of its contents, or (b) deliver its contents to the plasma membrane on the opposite side of the cell (transcytosis).

Extracellular fluid

Plasma membrane

Cytoplasm

Lysosome

2

membrane compone-nts moves to the plasmamembrane for recycling.

3

5 4

6


Endocytosis

• Phagocytosis– Pseudopods engulf solids and bring them into

cell's interior– Form vesicle called phagosome

• Used by macrophages and some white blood cells– Move by amoeboid motion

• Cytoplasm flows into temporary extensions• Allows creeping


Figure 3.13a Comparison of three types of endocytosis.

Receptors

Phagosome

PhagocytosisThe cell engulfs a large particleby forming projecting pseudopods ("false feet") around it and enclosing it within a membrane sac called a phagosome. The phagosome is combined with a lysosome. Undigested contents remain inthe vesicle (now called a residual body) or are ejected by exocytosis. Vesicle may or may not be protein coated but has receptors capable of binding to microorganisms or solid particles.


Endocytosis

• Pinocytosis (fluid-phase endocytosis)– Plasma membrane infolds, bringing

extracellular fluid and dissolved solutes inside cell

• Fuses with endosome

– Most cells utilize to "sample" environment – Nutrient absorption in the small intestine – Membrane components recycled back to

membrane


Figure 3.13b Comparison of three types of endocytosis.

Vesicle

PinocytosisThe cell "gulps" a drop of extracellular fluid containing solutes into tiny vesicles. No receptors are used, so the process is nonspecific. Most vesicles are protein-coated.


Endocytosis

• Receptor-mediated endocytosis– Allows specific endocytosis and transcytosis

• Cells use to concentrate materials in limited supply

– Clathrin-coated pits provide main route for endocytosis and transcytosis

• Uptake of enzymes, low-density lipoproteins, iron, insulin, and, unfortunately, viruses, diphtheria, and cholera toxins


Receptor-Mediated Endocytosis

• Different coat proteins– Caveolae

• Capture specific molecules (folic acid, tetanus toxin) and use transcytosis

• Involved in cell signaling but exact function unknown

– Coatomer• Function in vesicular trafficking


Figure 3.13c Comparison of three types of endocytosis.

Vesicle

Receptor-mediated endocytosisExtracellular substances bind to specific receptor proteins, enabling the cell to ingest and concentrate specific substances (ligands) in protein-coated vesicles. Ligands may simply be released inside the cell, or combined with a lysosome to digest contents. Receptors are recycled to the plasma membrane in vesicles.


Exocytosis

• Usually activated by cell-surface signal or change in membrane voltage

• Substance enclosed in secretory vesicle

• v-SNAREs ("v" = vesicle) on vesicle findt-SNAREs ("t" = target) on membrane and bind

• Functions– Hormone secretion, neurotransmitter release,

mucus secretion, ejection of wastes


Figure 3.14 Exocytosis. Slide 1

Extracellularfluid

Plasma membraneSNARE (t-SNARE)

The process of exocytosis

Secretoryvesicle Vesicle

SNARE(v-SNARE)

Molecule tobe secretedCytoplasm

1 The membrane-bound vesiclemigrates to theplasma membrane.

2 There, proteins at the vesicle surface (v-SNAREs) bind with t-SNAREs (plasma membrane proteins).

Fusedv- and

t-SNAREs

3 The vesicle and plasma membranefuse and a poreopens up.

Fusion pore formed

4 Vesicle contents are released to thecell exterior.


Figure 3.14b Exocytosis.

Photomicrograph of a secretoryvesicle releasingits contents by exocytosis (100,000x)


Table 3.2 Active Membrane Transport Processes (1 of 2)


Table 3.2 Active Membrane Transport Processes (2 of 2)

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Dawson Bio 11 Fall 2012 Week 3