Chapter 3The plasma membrane and membrane potential

• Explain how cell membrane constituents function in creating membrane potentials. This will be measured by quiz and exam scores.

Review

• Membrane structure and composition

• Cell to cell adhesions

• Membrane transport

New• Membrane potentials

Plasma Membrane

• Forms outer boundary of every cell

• Controls movement of molecules between the cell and its environment

• Joins cells to form tissues and organs

• Plays important role in the ability of a cell to respond to changes in the cell’s environment

Plasma Membrane Structure• Fluid lipid bilayer embedded with proteins

– Most abundant lipids are phospholipids

• Also has small amount of carbohydrates

– On outer surface only

• Cholesterol

– Tucked between phospholipid molecules

– Contributes to fluidity and stability of cell membrane

• Proteins

– Attached to or inserted within lipid bilayer

Gated channelprotein

Lipidbilayer

Cholesterolmolecule

Leak channelprotein

ICF

Cell adhesion molecule (linking microtubule tomembrane)

Carrierprotein

Microfilamentof cytoskeleton

Phospholipidmolecule

Carbohydratechain

ECF

Appearanceusing an electronmicroscope

Dark line

Light space

Dark lineIntegralproteins

Receptorprotein

Receptorprotein

Proteins

Glycolipid Glycolipid

Fig. 3-3, p. 59

Plasma Membrane Structure

Cell-To-Cell Adhesions– Extracellular matrix

• Serves as biological “glue”

• Major types of protein fibers interwoven in matrix– Collagen, elastin, fibronectin

– CAMs in cells’ plasma membranes

– Specialized cell junctions• Desmosomes

• Tight junctions (impermeable junctions)

• Gap junctions (communicating junctions

Specialized Cell Junctions

Gap junctions• Small connecting tunnels formed by

connexons• Especially abundant in cardiac and smooth

muscle• In nonmuscle tissues permit unrestricted

passage of small nutrient molecules between cells

• Also serve as method for direct transfer of small signaling molecules from one cell to the next

Desmosomes• Act like “spot rivets” that anchor two

closely adjacent nontouching cells• Most abundant in tissues that are

subject to considerable stretching

Tight junctions• Firmly bond adjacent cells together• Seal off the passageway between the

two cells• Found primarily in sheets of epithelial

tissue• Prevent undesirable leaks within

epithelial sheets• C

Lumen (contains undigested foodand potent digestive enzymes)

SELECTIVE PASSAGETHROUGH CELLS

Luminalmembrane

NO PASSAGEBETWEEN CELLS

Bloodvessel

Epithelialcell liningintestine

Basolateralmembrane

Cell 2

Cell 1

Lateralmembrane

Tightjunction

Fig. 3-5a, p. 63

Membrane Transport

• Unassisted membrane transport

– Diffusion

– Osmosis

• Assisted membrane transport

– Carrier-mediated transport

– Facilitated transport

– Active transport

If a substance canpermeate the membrane

If the membrane isimpermeable to a substance

Membrane

(a) Diffusion occurs (b) No diffusion occurs

KEY

= Penetrating solute

= Nonpenetrating solute

Fig. 3-8, p. 66

Area A Area AArea B Area B

Diffusion from area Ato area B

Diffusion from area Ato area B

Diffusion from area Bto area A

Diffusion from area Bto area A

Net diffusion

(a) Diffusion (b) Equilibrium

No net diffusion

KEY

Differences in arrow length, thickness, and direction represent the relative magnitude of molecular movement in a given direction.

Net diffusion = Diffusion from area A to area B minus diffusion from area B to area A

= Solute molecule

Fig. 3-7, p. 65

Membrane Transport

Factors affecting rate of diffusion collectively make up Fick’s law of diffusion:

• Magnitude (or steepness) of the concentration gradient

• Permeability of the membrane to the substance– Charge?

• Surface area of the membrane across which diffusion is taking place

• Molecular weight of the substance

• Distance through which diffusion takes place

Membrane Transport

• Osmosis

– Net diffusion of

water down its

own concentration

gradient

100% water concentration 0% solute concentration

90% water concentration10% solute concentration

(a) Pure water (b) Solution

KEY

= Water molecule

= Solute moleculeFig. 3-9, p. 67

Normal cell volumeIntracellular fluid 300 mOsm/L

nonpenetrating solutes

300 mOsm/Lnonpenetrating solutes

200 mOsm/Lnonpenetrating solutes

400 mOsm/Lnonpenetrating solutes

H2O H2O

No net movement of water; no change in cell volume.

Water diffuses intocells; cells swell.

Water diffuses out ofcells; cells shrink.

(a) Isotonicconditions

(b) Hypotonicconditions

(c) Hypertonicconditions Fig. 3-13, p. 71

Membrane TransportUnassisted membrane transportAssisted membrane transport• Carrier-mediated transport

– Accomplished by membrane carrier flipping its shape

– Can be active or passive– Characteristics that determine the kind and

amount of material that can be transferred across the membrane

• Specificity• Saturation• Competition

Membrane Transport

Types of assisted membrane transport• Facilitated diffusion

• Active transport

• Vesicular transport

Direction oftransport

Transportedsolute is released

and carrier proteinreturns to

conformation instep 1.

Plasmamembrane

Carrier protein takesconformation in which solutebinding site is exposed toregion of higher concentration.

Carrier protein changesconformation so that bindingsite is exposed to region oflower concentration.

Solutemolecule binds to carrier protein.

(Low)

(High)

ConcentrationgradientECF

ICF

Binding siteCarrier protein

Solute moleculeto be transported

1

2

3

4

Fig. 3-14, p. 72

Facilitated diffusion• Substances move from a higher

concentration to a lower concentration

• Requires carrier molecule• Means by which glucose is

transported into cells

Membrane Transport

Active transport• Moves a substance against its concentration

gradient• Requires a carrier molecule• Primary active transport

– Requires direct use of ATP• Secondary active transport

– Driven by an ion concentration gradient established by a primary active transport system

K+ concentrationgradient

2

High-affinity bindingsite for K+

Low-affinity bindingsite for Na+

3 Na+

Direction ofNa+ transport

3

2 K+

4

5

Direction ofK+ transport

Na+ concentrationgradient

2 K+

6

Stepped Art

Low-affinity bindingsite for K+

High-affinity bindingsite for Na+

Na+–K+ pump

3 Na+

Low K+High Na+

ECF

ICFHigh K+Low Na+

1

Plasmamembrane

Fig. 3-16, p. 75

Active TransportSodium Potassium PumpWhen open to the ECF, the carrier drops off Na+ on its high-concentration side and picks up K+ from its low-concentration side

Active Transport• Moves a substance against its

concentration gradient.

• Primary active transport:

– Requires direct use of ATP

• Secondary active transport:

– Driven by an ion concentration gradient established by a primary active transport

– Two types, symport and antiport

Transportedsolute in highconcentration

(a) Symport

Driving ionin highconcentration

Transportedsolute in lowconcentration

Driving ionin lowconcentration

Fig. 3-17a, p. 77

Transportedsolute in lowconcentration

(b) Antiport

Driving ionin highconcentration

Transportedsolute in highconcentration

Driving ionin lowconcentration

Fig. 3-17b, p. 77

Secondary Activetransport

Active Transport• Moves a substance against its

concentration gradient.

• Primary active transport:

– Requires direct use of ATP

• Secondary active transport:

– Driven by an ion concentration gradient established by a primary active transport

– Two types, symport and antiport

Carrier-mediated Transport Characteristics

• Specificity: Each carrier transports a specific substance or a few closely related compounds.

• Saturation: A limited number of carrier binding sites are available.

• Transport maximum (Tm): The amount of a substance transported in a given time.

• Competition: Several closely related compounds may compete for transport on the same carrier.

Simple diffusiondown concentrationgradient

Carrier-mediatedtransport downconcentration gradient(facilitated diffusion)

Rat

e o

f tr

ansp

ort

of

mo

lecu

le i

nto

cel

l

Low High

Concentration of transported molecules in ECFFig. 3-15, p. 73

Membrane Transport• Vesicular transport

– Material is moved into or out of the cell wrapped in membrane

– Active method of membrane transport– Two types of vesicular transport

• Endocytosis– Process by which substances move into cell– Pinocytosis – nonselective uptake of ECF– Phagocytosis – selective uptake of multimolecular

particle

• Exocytosis – Provides mechanism for secreting large polar molecules– Enables cell to add specific components to membrane

Table 3-2b p80

What is an excitable cell?

Membranes and their potentials are what make cells excitable.

Membrane Potential

• Plasma membrane of all living cells has a membrane potential (polarized electrically)

• Separation of opposite charges across plasma membrane

• Due to differences in concentration and permeability of key ions

• Separated charges create the ability to do work like electrons in a battery.

• millivolt- 1/1000 volt

Basic Physics

• Brownian motion

• Electrons protons neutrons

• Ohms law E=I*P

• Opposites attract, likes repel (hydrophobic/hydrophyllic)

• Potential and kinetic energy

• Velocity and force, F= MA

– Larger mass requires more force to move

– Objects in motion stay in motion unless there is friction and drag.

Basic measurements

• Volt – unit of charge

– mv – 1/1000 volt

– Car battery =12V, Cell = -70 mv

• Watt = unit of power

– Kw = 1000 watts, light bulb = 60 wattsShearon Harris = 900 MW

• Ampere

– Unit of current• ma = 1/1000 ampere

• http://www.osha.gov/SLTC/etools/construction/electrical_incidents/eleccurrent.html

Membrane Potential

Which has the greatest membrane potential?

B>A B<C

Membrane Potential

• Nerve and muscle cells– Excitable cells– Have ability to produce rapid, transient

changes in their membrane potential when excited

• Resting membrane potential– Constant membrane potential present in cells

of nonexcitable tissues and those of excitable tissues when they are at rest

– Na+, K+, A-

Membrane Potential

• Effect of sodium-potassium pump on membrane potential

– Makes only a small direct contribution to membrane potential through its unequal transport of positive ions

– The movement of ions and the large negatively charged proteins (A-) generate the potential difference.

High-affinity bindingsite for K+

Low-affinity bindingsite for Na+

3 Na+

K+ concentrationgradient

Direction ofNa+ transport

2 K+

Direction ofK+ transport

Na+ concentrationgradient

Low-affinity bindingsite for K+

High-affinity bindingsite for Na+

Na+–K+ pump

2 K+

3 Na+

Low K+High Na+

ECF

ICF

High K+Low Na+

1

2

3

4

5

6

Fig. 3-16, p. 75

ICF

ECF

(Passive)Na+–K+

pump (Active)

(Active)(Passive)K+ channelNa+ channel

Fig. 3-23, p. 79

Table 3-3 p82

• Rp + -70mv• Variable from one cell to another• Poison eliminates this potential• Generated by the imbalance of ions in the intracellular and extracellular spaces.

Nernst Equation E=(61) log Co/Ci

60mv

-90mv

-70mv

Nernst equation

• E=(61) log Co/Ci

• For Potassium Ek=(61) log 5mM/150mM

• For sodium ENa=(61) log 150mM/15mM

• Co concentration in the ECF• Ci concentration in the ICF• Used to calculate the contribution of ions to the resting potential of -70mv

Resting potential

• EK = -90mv

• ENa = 60mv

• ECl = -70mv

• K and Na drive Cl gradient

Usefulness?

• Neurons and muscle fibers can alter membrane potential to send signals and create motion.

Plasma membrane

ECF ICF

Concentrationgradient for K+

Electricalgradient for K+

EK+ = –90 mVFig. 3-20, p. 83

Plasma membrane

ECF ICF

Concentrationgradient for Na+

Electricalgradient for Na+

mostly

ENa+ = +60 mV

ECFanions,

Fig. 3-21, p. 84

Plasma membrane

ECF ICF

and associated

Resting membrane potential = –70 mV

Relatively small net diffusion of Na+ inward neutralizessome of thepotential created by K+ alone

No diffusion of A– across membrane

Relatively large net diffusion of K+ outward establishesan EK+ of –90 mV

Fig. 3-22, p. 85

ANIMATION: Resting Potential