The Cell The Basic Unit of Life. The Importance of Cells All organisms are made of cells The cell is the simplest collection of matter that can live

The CellThe Basic Unit of Life

The Importance of Cells

• All organisms are made of cells•The cell is the simplest collection of matter

that can live•Cell structure is correlated to cellular

function•All cells are related by their descent from

earlier cells

Microscopy•light microscope (LM)

▫visible light passes through a specimen and then through glass lenses, which magnify the image

▫Magnify up to 1000x

LE 6-2

Measurements1 centimeter (cm) = 10–2 meter (m) = 0.4 inch1 millimeter (mm) = 10–3 m1 micrometer (µm) = 10–3 mm = 10–6 m1 nanometer (nm) = 10–3 µm = 10–9 m

10 m

1 mHuman height

Length of somenerve andmuscle cells

Chicken egg

0.1 m

1 cm

Frog egg1 mm

100 µm

Most plant andanimal cells

10 µmNucleus

1 µm

Most bacteria

Mitochondrion

Smallest bacteria

Viruses100 nm

10 nmRibosomes

Proteins

Lipids

1 nmSmall molecules

Atoms0.1 nmU

na

ide

d e

ye

Lig

ht

mic

rosc

op

e

Ele

ctr

on

mic

ros

co

pe

LE 6-3a

Brightfield (unstained specimen)

50 µmBrightfield (stained specimen)

Phase-contrast

LE 6-3b

50 µm

50 µm

Confocal

Differential-interference-contrast (Nomarski)

Fluorescence

•electron microscopes (EMs)▫ are used to study subcellular structures ▫Two types

Scanning electron microscopes (SEMs) focus a beam of electrons onto the surface of a

specimen providing images that look 3D

Transmission electron microscopes (TEMs) focus a beam of electrons through a specimen to study the internal structures of cells

LE 6-41 µm

1 µm

Scanning electronmicroscopy (SEM) Cilia

Longitudinalsection ofcilium

Transmission electronmicroscopy (TEM)

Cross sectionof cilium

•Basic features of all cells: ▫Plasma membrane

Selectively permeable double layer of phospholipids

▫Semifluid substance called the cytosol Includes cytoplasm & organelles

▫Chromosomes (carry genes)▫Ribosomes (make proteins)

LE 6-8

Hydrophilicregion

Hydrophobicregion

Carbohydrate side chain

Structure of the plasma membrane

Hydrophilicregion

Phospholipid Proteins

Outside of cell

Inside of cell 0.1 µm

TEM of a plasma membrane

Prokaryotic Cells Eukaryotic Cells

•no nucleus•DNA is in an

unbound region called the nucleoid

•No membrane-bound organelles

•Include bacteria and Achaea

• Have nucleus• DNA in nucleus• Membrane-bound

organelles• Usually larger than

prokaryotic cells• Cell size limited by

metabolic activities• Include plants,

animals, and fungi

LE 6-6

A typicalrod-shapedbacterium

A thin section through thebacterium Bacilluscoagulans (TEM)

0.5 µm

Pili

Nucleoid

Ribosomes

Plasmamembrane

Cell wall

Capsule

Flagella

Bacterialchromosome

LE 6-7

Total surface area(height x width xnumber of sides xnumber of boxes)

6

125 125

150 750

1

11

5

1.2 66

Total volume(height x width x lengthX number of boxes)

Surface-to-volumeratio(surface area volume)

Surface area increases whileTotal volume remains constant

LE 6-9a

Flagellum

Centrosome

CYTOSKELETON

Microfilaments

Intermediate filaments

Microtubules

Peroxisome

Microvilli

ENDOPLASMIC RETICULUM (ER

Rough ER Smooth ER

MitochondrionLysosome

Golgi apparatus

Ribosomes:

Plasma membrane

Nuclear envelope

NUCLEUS

In animal cells but not plant cells: LysosomesCentriolesFlagella (in some plant sperm)

Nucleolus

Chromatin

LE 6-9b

Roughendoplasmicreticulum

In plant cells but not animal cells: ChloroplastsCentral vacuole and tonoplastCell wallPlasmodesmata

Smoothendoplasmicreticulum

Ribosomes(small brown dots)

Central vacuole

Microfilaments

IntermediatefilamentsMicrotubules

CYTOSKELETON

Chloroplast

Plasmodesmata

Wall of adjacent cell

Cell wall

Nuclearenvelope

Nucleolus

Chromatin

NUCLEUS

Centrosome

Golgiapparatus

Mitochondrion

Peroxisome

Plasmamembrane

The Nucleus•contains most of the cell’s genes •usually the most conspicuous organelle•Enclosed by nuclear envelope

LE 6-10

Close-up of nuclearenvelope

Nucleus

Nucleolus

Chromatin

Nuclear envelope:Inner membraneOuter membrane

Nuclear pore

Porecomplex

Ribosome

Pore complexes (TEM) Nuclear lamina (TEM)

1 µm

Rough ER

Nucleus

1 µm

0.25 µm

Surface of nuclear envelope

Ribosomes•made of ribosomal RNA and protein•protein synthesis

▫In the cytosol (free ribosomes)▫On the outside of the endoplasmic reticulum

(ER) or the nuclear envelope (bound ribosomes)

LE 6-11

Ribosomes

0.5 µm

ER Cytosol

Endoplasmicreticulum (ER)

Free ribosomes

Bound ribosomes

Largesubunit

Smallsubunit

Diagram ofa ribosome

TEM showing ERand ribosomes

Endomembrane System

•Regulate protein traffic•Perform metabolic functions•Components of the endomembrane system:

▫Nuclear envelope▫Endoplasmic reticulum▫Golgi apparatus▫Lysosomes▫Vacuoles▫Plasma membrane

• components are either continuous or connected via transfer by vesicles

The Endoplasmic Reticulum (ER)

•is continuous with the nuclear envelope•two distinct regions of ER:

▫Smooth ER, lacks ribosomes

▫Rough ER with ribosomes studding its surface

LE 6-12

Ribosomes

Smooth ER

Rough ER

ER lumen

Cisternae

Transport vesicle

Smooth ER Rough ER

Transitional ER

200 nm

Nuclearenvelope

Functions of Smooth ER•Synthesizes lipids•Metabolizes carbohydrates•Stores calcium•Detoxifies poison

Functions of Rough ER•bound ribosomes•Produces proteins and membranes

▫distributed by transport vesicles

The Golgi Apparatus

•consists of flattened membranous sacs called cisternae

•Functions of the Golgi apparatus:▫Modifies products of the ER▫Manufactures certain macromolecules▫Sorts and packages materials into

transport vesicles

LE 6-13

trans face(“shipping” side ofGolgi apparatus) TEM of Golgi apparatus

0.1 µm

Golgi apparatus

cis face(“receiving” side ofGolgi apparatus)

Vesicles coalesce toform new cis Golgi cisternae Vesicles also

transport certainproteins back to ER

Vesicles movefrom ER to Golgi

Vesicles transport specificproteins backward to newerGolgi cisternae

Cisternalmaturation:Golgi cisternaemove in a cis-to-transdirection

Vesicles form andleave Golgi, carryingspecific proteins toother locations or tothe plasma mem-brane for secretion

Cisternae

Lysosomes•membranous sac of hydrolytic enzymes

▫hydrolyze proteins, fats, polysaccharides, and nucleic acids

▫use enzymes to recycle organelles and macromolecules a process called autophagy

LE 6-14a

Phagocytosis: lysosome digesting food

1 µm

Plasmamembrane

Food vacuole

Lysosome

Nucleus

Digestiveenzymes

Digestion

Lysosome

Lysosome containsactive hydrolyticenzymes

Food vacuolefuses withlysosome

Hydrolyticenzymes digestfood particles

LE 6-14b

Autophagy: lysosome breaking down damaged organelle

1 µm

Vesicle containingdamaged mitochondrion

Mitochondrionfragment

Lysosome containingtwo damaged organelles

Digestion

Lysosome

Lysosome fuses withvesicle containingdamaged organelle

Peroxisomefragment

Hydrolytic enzymesdigest organellecomponents

Vacuoles & Vesicles•membrane-bound sacs with varied functions

▫Food vacuoles formed by phagocytosis

▫Contractile vacuoles found in many freshwater protists pump excess water out of cells

▫Central vacuoles found in many mature plant cells hold organic compounds and water

LE 6-15

5 µm

Central vacuole

Cytosol

Tonoplast

Central vacuole

Nucleus

Cell wall

Chloroplast

LE 6-16-1

Nuclear envelope

Nucleus

Rough ER

Smooth ER

LE 6-16-2

Nuclear envelope

Nucleus

Rough ER

Smooth ER

Transport vesicle

cis Golgi

trans Golgi

LE 6-16-3

Nuclear envelope

Nucleus

Rough ER

Smooth ER

Transport vesicle

cis Golgi

trans Golgi

Plasma membrane

Mitochondria & ChloroplastsMitochondria Chloroplasts

• sites of cellular respiration• not part of the

endomembrane system

• found only in plants and algae

• the sites of photosynthesis• not part of the

endomembrane system

Mitochondria•in nearly all eukaryotic cells•smooth outer membrane•inner membrane folded into cristae

▫creates two compartments: intermembrane space mitochondrial matrix

▫Folding creates more surface area for enzymes that synthesize ATP

LE 6-17

Mitochondrion

Intermembrane space

Outer membrane

Inner membrane

Cristae

Matrix

100 nmMitochondrialDNA

Freeribosomes in themitochondrialmatrix

Chloroplasts•Type of plastid•contain the green pigment chlorophyll•contains enzymes and other molecules that

function in photosynthesis•found in leaves and other green organs of

plants and in algae•Chloroplast structure includes:

▫Thylakoids membranous sacs

▫Stroma the internal fluid

LE 6-18

Chloroplast

ChloroplastDNA

RibosomesStroma

Inner and outermembranes

Granum

Thylakoid1 µm

Plastids

•Responsible for photosynthesis•Storage of products (ie. Starch)•Synthesis of molecules (ie. Fatty acids)

Peroxisomes

•specialized metabolic compartments •single membrane•produce hydrogen peroxide and convert it to water•are oxidative organelles•Aid in breakdown of lipids

LE 6-19

Chloroplast

Peroxisome

Mitochondrion

1 µm

Cytoskeleton•network of fibers extending throughout the

cytoplasm•organizes the cell’s structures and activities•anchors many organelles•composed of:

▫Microtubules▫Microfilaments▫Intermediate filaments

LE 6-20

Microtubule

Microfilaments0.25 µm

Roles of the Cytoskeleton•support the cell •maintain cell shape•interacts with motor proteins to produce

motility•Inside the cell, vesicles can travel along

“monorails” provided by the cytoskeleton•may help regulate biochemical activities

Centrosomes and Centrioles•microtubules grow out from a centrosome

near the nucleus•“microtubule-organizing center”•In animal cells, the centrosome has a pair

of centrioles

LE 6-22

0.25 µm

Microtubule

Centrosome

Centrioles

Longitudinal sectionof one centriole

Microtubules Cross sectionof the other centriole

Cilia and Flagella•Beating controlled by microtubules•sheathed by the plasma membrane•Dynein

▫Motor protein▫drives the bending movements of a cilium or

flagellum

LE 6-23a

5 µm

Direction of swimming

Motion of flagella

LE 6-23b

15 µm

Direction of organism’s movement

Motion of cilia

Direction ofactive stroke

Direction ofrecovery stroke

Microfilaments (Actin Filaments)•twisted double chain of actin subunits

▫Also have myosin if the microfilaments are used for movement

•bear tension, resisting pulling forces within the cell

• form a 3D network just inside the plasma membrane to help support the cell’s shape

•Bundles of microfilaments make up the core of microvilli

LE 6-26

Microfilaments (actinfilaments)

Microvillus

Plasma membrane

Intermediate filaments

0.25 µm

LE 6-27b

Cortex (outer cytoplasm):gel with actin network

Amoeboid movement

Inner cytoplasm: solwith actin subunits

Extendingpseudopodium

•Cytoplasmic streaming ▫circular flow of cytoplasm within cells▫speeds distribution of materials within the

cell▫In plant cells, actin-myosin interactions and

sol-gel transformations drive cytoplasmic streaming

LE 6-27c

Nonmovingcytoplasm (gel)

Cytoplasmic streaming in plant cells

Chloroplast

Streamingcytoplasm(sol)

Cell wall

Parallel actinfilaments

Vacuole

Intermediate Filaments•range in diameter from 8–12 nanometers

▫ larger than microfilaments▫smaller than microtubules

•support cell shape•fix organelles in place

Extracellular Structures

•These extracellular structures include:▫ Cell walls of plants▫The extracellular matrix (ECM) of animal

cells▫Intercellular junctions

Cell Walls of Plants•distinguishes plant cells from animal cells•protects the plant cell•maintains cell shape•prevents excessive uptake of water•made of cellulose fibers embedded in other

polysaccharides and protein

Cell Walls of Plants•Plant cell walls may have multiple layers:

▫Primary cell wall relatively thin and flexible

▫Middle lamella thin layer between primary walls of adjacent

cells▫Secondary cell wall (in some cells)

added between the plasma membrane and the primary cell wall

• Plasmodesmata are channels between adjacent plant cells

LE 6-28

Centralvacuole of cell

PlasmamembraneSecondarycell wall

Primarycell wall

Middlelamella

1 µm

Centralvacuole of cell

Central vacuoleCytosol

Plasma membrane

Plant cell walls

Plasmodesmata

The Extracellular Matrix (ECM) of Animal Cells•made up of glycoproteins and other

macromolecules•Functions of the ECM:

▫Support▫Adhesion▫Movement▫Regulation

Intercellular Junctions•facilitate contact between cells

▫Adhesion▫Interaction▫Communication

Plants: Plasmodesmata•channels that perforate plant cell walls

▫water and small solutes (and sometimes proteins and RNA) can pass from cell to cell

LE 6-30

Interiorof cell

Interiorof cell

0.5 µm Plasmodesmata Plasma membranes

Cell walls

Animals: Tight Junctions, Desmosomes, and Gap Junctions

• tight junctions▫membranes of neighboring cells are pressed

together▫prevents leakage of extracellular fluid

• Desmosomes (anchoring junctions) ▫fasten cells together into strong sheets

• Gap junctions (communicating junctions) ▫provide cytoplasmic channels between adjacent

cells

LE 6-31

Tight junctions preventfluid from moving across a layer of cells

Tight junction

0.5 µm

1 µm

0.1 µm

Gap junction

Extracellularmatrix

Spacebetweencells

Plasma membranesof adjacent cells

Intermediatefilaments

Tight junction

Desmosome

Gapjunctions

Fluid Mosaic Model

•Phospholipids ▫hydrophobic fatty acid tails▫hydrophilic phosphate heads

LE 7-2

Hydrophilichead

Hydrophobictail

WATER

WATER

Fluidity of Membranes

•Phospholipids move within the bilayer•Most of the lipids, and some proteins, drift

laterally

•Cool temperatures▫membranes switch from a fluid state to a solid

state

•Membranes must be fluid to work properly▫Consistency of oil

•Cholesterol▫Steroid present in cell membrane▫Maintains fluidity

At warm temperatures (such as 37°C) restrains movement

At cool temperatures prevents tight packing

LE 7-5c

Cholesterol

Cholesterol within the animal cell membrane

Membrane Proteins

•Peripheral proteins▫not embedded

•Integral proteins ▫penetrate the hydrophobic core ▫often span the membrane

transmembrane proteins

LE 7-7

Fibers ofextracellular

matrix (ECM)

Glycoprotein

Carbohydrate

Microfilamentsof cytoskeleton

Cholesterol

Integralprotein

Peripheralproteins

CYTOPLASMIC SIDEOF MEMBRANE

EXTRACELLULARSIDE OF

MEMBRANE

Glycolipid

•Six major functions of membrane proteins:▫Transport▫Enzymatic activity▫Signal transduction▫Cell-cell recognition▫Intercellular joining▫Attachment to the cytoskeleton and

extracellular matrix (ECM)

LE 7-9a

EnzymesSignal

ReceptorATP

Transport Enzymatic activity Signal transduction

LE 7-9b

Glyco-protein

Cell-cell recognition Intercellular joining Attachment to thecytoskeleton and extra-

cellular matrix (ECM)

• Carbohydrates • Cell to cell recognition

• Bonded to lipids = glycolipids• Bonded to proteins = glycoproteins (more

common)

Selective Permeability

•Permeability factors▫Molecular size

▫Solubility in lipids ex. Oxygen, carbon dioxide, steroid hormones

▫Charge of ions

▫Presence of carrier molecules

Carrier Molecules

•transport proteins▫channel proteins• carrier proteins• bind to molecules• change shape to shuttle them across the

membrane

•Diffusion▫Net direction of movement from areas of

high to low concentration▫Continues until equilibrium has been met

Movement continues at equal rates in both directions

Animation: Membrane Selectivity Animation: Diffusion

LE 7-11a

Molecules of dye Membrane (cross section)

WATER

Net diffusion Net diffusion Equilibrium

Diffusion of one solute

•Osmosis ▫diffusion of water▫direction of osmosis is determined by a

difference in total solute concentration Movement from region of lower solute

concentration to the region of higher solute concentration

•Tonicity ▫the ability of a solution to cause a cell to gain or lose water

• Isotonic solution:▫ solute concentration is the same as that inside the cell▫no net water movement across the plasma membrane

•Hypertonic solution▫solute concentration is greater than that inside the cell▫cell loses water

•Hypotonic solution▫solute concentration is less than that inside the cell▫cell gains water

Cells without Cell Walls•Special adaptations for osmoregulation

▫Control of water balance Ex. Contractile vacuole in Paramecium

LE 7-14

Filling vacuole50 µm

50 µmContracting vacuole

Cells with Cell Walls•Cell walls help maintain water balance

▫hypotonic solution swells until the wall opposes uptake the cell is now turgid (firm)

▫Isotonic is no net movement of water into the cell the cell becomes flaccid (limp), and the plant may wilt

▫hypertonic environment plant cells lose water the membrane pulls away from the wall

a usually lethal effect called plasmolysis

LE 7-13

Animalcell

Lysed

H2O H2O H2O

Normal

Hypotonic solution Isotonic solution Hypertonic solution

H2O

Shriveled

H2OH2OH2OH2OPlantcell

Turgid (normal) Flaccid Plasmolyzed

•Facilitated diffusion▫passive

solute moves down its concentration gradient▫Transport proteins

Channel proteins Carrier proteins

LE 7-15a

EXTRACELLULARFLUID

Channel protein Solute

CYTOPLASM

LE 7-15b

Carrier protein Solute

•Active transport ▫moves substances against their

concentration gradient▫requires energy, usually in the form of ATP▫Integral proteins

Ex. sodium-potassium pump

Animation: Active Transport

LE 7-17

Diffusion Facilitated diffusion

Passive transport

ATP

Active transport

•Membrane potential ▫voltage difference across a membrane

•Electrochemical gradient▫drives the diffusion of ions across a membrane▫Includes :

A chemical force the ion’s concentration gradient

An electrical force the effect of the membrane potential on the ion’s

movement

• electrogenic pump• transport protein• generates the voltage across a membrane

main electrogenic pump of plants, fungi, and bacteria is a proton pump

LE 7-18

H+

ATP

CYTOPLASM

EXTRACELLULARFLUID

Proton pump

H+

H+

H+

H+

H+

+

+

+

+

+

–

–

–

–

–

•Cotransport ▫active transport of a solute indirectly drives

transport of another solute Ex. Plants - gradient of hydrogen ions

generated by proton pumps drives active transport of nutrients into the cell

LE 7-19

H+

ATP

Proton pump

Sucrose-H+

cotransporter

Diffusionof H+

Sucrose

H+

H+

H+

H+

H+

H+

+

+

+

+

+

+

–

–

–

–

–

–

•Exocytosis▫Type of active transport▫transport vesicles migrate to the

membrane, fuse with it, and release their contents

•Endocytosis▫cell takes in macromolecules by forming vesicles from the

plasma membrane▫reversal of exocytosis▫involves different proteins

• Three types of endocytosis:▫Phagocytosis

“cellular eating” Cell engulfs particle in a vacuole

▫Pinocytosis “cellular drinking” Cell creates vesicle around fluid

▫Receptor-mediated endocytosis Binding of ligands to receptors triggers vesicle formation

Figure 3-11

2 of 8Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

Phagocytosis

A phagocytic cell comes in contact with the foreign object and sends

pseudopodia (cytoplasmic extensions) around it.

Pseudopodium(cytoplasmic

extension)

EXTRACELLULAR FLUID

CYTOPLASM

Foreignobject

Cell membraneof phagocytic

cell

Figure 3-11



cell

Phagocytosis




extension)

EXTRACELLULAR FLUID

CYTOPLASM

Foreignobject

The pseudopodia approach one another and fuse to trap the

material within the vesicle.

Figure 3-11



cell

Phagocytosis




extension)

EXTRACELLULAR FLUID

CYTOPLASM

Foreignobject

Vesicle



The vesicle moves into the cytoplasm.

Figure 3-11



cell

Phagocytosis




extension)

EXTRACELLULAR FLUID

CYTOPLASM

Foreignobject

Vesicle

Lysosomes




Lysosomes fuse with the vesicle.

Figure 3-11



cell

Phagocytosis




extension)

EXTRACELLULAR FLUID

CYTOPLASM

Foreignobject

Vesicle

Lysosomes





This fusion activates digestive enzymes.

Figure 3-11



cell

Phagocytosis




extension)

EXTRACELLULAR FLUID

CYTOPLASM

Foreignobject

Vesicle

Lysosomes






The enzymes break down the structure of the phagocytized

material.

Figure 3-11



cell

Phagocytosis




extension)

EXTRACELLULAR FLUID

CYTOPLASM

Foreignobject

Vesicle

Lysosomes

Undissolvedresidue






The enzymes break down the structure of the phagocytized

material.

Residue is then ejected from the cell by exocytosis.

LE 7-20c

Receptor

RECEPTOR-MEDIATED ENDOCYTOSIS

Ligand

Coatedpit

Coatedvesicle

Coat protein

Coat protein

Plasmamembrane

0.25 µm

A coated pitand a coated

vesicle formedduring

receptor-mediated

endocytosis(TEMs).

Figure 3-10


EXTRACELLULARFLUID Ligands binding

to receptors

Ligandreceptors

CYTOPLASM

Ligands Receptor-Mediated Endocytosis

Target molecules (ligands) bind to receptors in cell membrane.

Figure 3-10



to receptors

Ligandreceptors

CYTOPLASM

Ligands

Endocytosis

Receptor-Mediated Endocytosis


Areas coated with ligands form deep pockets in membrane surface.

Figure 3-10



to receptors

Ligandreceptors

CYTOPLASM

Coatedvesicle

Ligands

Endocytosis




Pockets pinch off, forming vesicles.

Figure 3-10



to receptors

Ligandreceptors

CYTOPLASM

Coatedvesicle

Ligands

Endocytosis

Lysosome

Fused vesicleand lysosome

Fusion





Vesicles fuse with lysosomes.

Figure 3-10



to receptors

Ligandreceptors

CYTOPLASM

Coatedvesicle

Ligands

Endocytosis

Lysosome


Fusion






Ligands are removed and absorbed into the cytoplasm.

Figure 3-10



to receptors

Ligandreceptors

CYTOPLASM

Coatedvesicle

Ligands

Endocytosis

Lysosome


Ligandsremoved

FusionDetachment







The membrane containing the receptor molecules separates from

the lysosome.

Figure 3-10



to receptors

Exocytosis

Ligandreceptors

CYTOPLASM

Coatedvesicle

Ligands

Endocytosis

Lysosome


Ligandsremoved

FusionDetachment







The membrane containing the receptor molecules separates from

the lysosome.

The vesicle returns to the surface.

Cell to Cell Communication•signal-transduction pathway

▫A signal on a cell’s surface is converted into a specific cellular response

•chemical messengers▫cell junctions

directly connect the cytoplasm of adjacent cells

• local signaling▫communicate by direct contact

Some animal cells

LE 11-3Plasma membranes

Gap junctionsbetween animal cells

Cell junctions

Cell-cell recognition

Plasmodesmatabetween plant cells

•local regulators▫messenger molecules that travel only short

distances Some animal cells

•long-distance signaling▫Hormones

plants and animals

LE 11-4

Paracrine signaling

Local regulatordiffuses throughextracellular fluid

Secretoryvesicle

Secretingcell

Target cell

Local signaling

Electrical signalalong nerve celltriggers release ofneurotransmitter

Neurotransmitter diffuses across synapse

Endocrine cell Bloodvessel

Long-distance signaling

Hormone travelsin bloodstreamto target cells

Synaptic signaling

Target cellis stimulated

Hormonal signaling

Target cell

Cell Signaling

•Cells receiving signals went through three processes:▫Reception▫Transduction▫Response

LE 11-5_1

EXTRACELLULARFLUID

Reception

Plasma membrane

Transduction

CYTOPLASM

Receptor

Signalmolecule

LE 11-5_2

EXTRACELLULARFLUID

Reception

Plasma membrane

Transduction

CYTOPLASM

Receptor

Signalmolecule

Relay molecules in a signal transductionpathway

LE 11-5_3

EXTRACELLULARFLUID

Reception

Plasma membrane

Transduction

CYTOPLASM

Receptor

Signalmolecule

Relay molecules in a signal transductionpathway

Response

Activationof cellularresponse

•Reception ▫binding between a signal molecule (ligand)

and receptor highly specific

▫conformational change in a receptor Often the initial transduction of the signal

▫Most signal receptors are plasma membrane proteins

LE 11-7c

Signalmolecule(ligand)

Gateclosed Ions

Ligand-gatedion channel receptor

Plasmamembrane

Gate closed

Gate open

Cellularresponse

•Transduction▫Multistep pathways ▫can amplify a signal▫opportunities for coordination and

regulation

•Multistep pathways have two important benefits:▫Amplifying the signal (and thus the

response)▫Contributing to the specificity of the

response

LE 11-8Signal molecule

Activated relaymolecule

Receptor

Inactiveprotein kinase

1 Activeprotein kinase

1



2



3

ADP

Inactiveprotein

Activeprotein

Cellularresponse

Phosphorylation cascade

ATP

PPP i

ADPATP

PPP i

ADPATP

PPP i

P

P

P

• Cyclic AMP (cAMP) • one of the most widely used second

messengers• Adenylyl cyclase• enzyme in the plasma membrane• converts ATP to cAMP in response to an

extracellular signal

•Terminating the Signal▫signal molecules leave the receptor

receptor reverts to its inactive state

Documents

The Cell The Basic Unit of Life. The Importance of Cells All organisms are made of cells The cell is the simplest collection of matter that can live