Plasma Membrane Structure and Function

Image from: http://www.biologie.uni-hamburg.de/b-online/ge22/03.gif

Cell Size Limitations• The size of cells is limited by the surface area

to volume ratio of cells– As cells grow the volume inside of the cell grows

faster than the surface area available to diffuse nutrients into the cell and expel wastes out of the cell

• Surface area determines rate molecules enter and leave cell

• Volume determines the demand for resources needed by the cell (increase in volume requires more resources to keep the cell functioning properly)

• Smaller cells have more favorable surface area to volume ratios for exchange with the environment

Root hairs Cells of alveoli: Lungs

Villi: Small Intestines

• You should be able to solve basic surface area and volume equations given to you on the AP Biology exam.

• In order for cells to grow, reproduce and maintain dynamic homeostasis, cells must create and maintain an internal environment that is different from the external environment– Cell membranes are selectively permeable to

accomplish this goal– Cell membranes separate the internal

environment from the external environment

FLUID MOSAIC MODEL

1972- S.J. Singer and G. Nicolson propose membrane is a “mosaic” of proteins and phospholipids that are constantly moving and changing

Click here to See Fluidity

Animation from: http://www.sp.uconn.edu/~terry/images/anim/fluidmem.gif

Click here to SeeFLUIDITY

More than just a barrier…

• The membranes selective permeability is a direct consequence of the membranes structure consisting of phospholipids, embedded proteins, cholesterol, glycoproteins and glycolipids

A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer

Molecules need to move across membranes in cells

Image modiified from: http://www.accessexcellence.org/AB/GG/importProt.html

INfoodcarbohydratessugars, proteinsamino acidslipidssalts, O2, H2O

OUTwasteammoniasaltsCO2

H2O products

Membranes provide a variety of cell functions

Cell Membranes are made ofPHOSPHOLIPIDS & PROTEINS

Amphipathic –Molecules with both hydrophilic and hydrophobic regions

phosphate

lipid

hydrophilic

hydrophobic

Membrane Proteins• Proteins determine most of membrane’s specific

functions– cell membrane & organelle membranes each have

unique collections of proteins

• Membrane proteins:– peripheral proteins = loosely

bound to surface of membrane– integral proteins = penetrate into

lipid bilayer, often completely spanning the membrane = transmembrane protein

HYDROPHILIC/HYDROPHOBIC areas determine positions of molecules in cell membranes

hydrophobic amino acids– Those with nonpolar side groups– stick in the lipid membrane– anchors the protein

in membrane

hydrophilic amino acids– Amino acids with charge

and polar side groups– stick out in the watery

fluid in or out of cell

Membrane Carbohydrates • Attached to proteins (glycoproteins)

or lipids (glycolipids)

• Play a key role in cell-cell recognition– ability of a cell to distinguish neighboring cells

from another– important in organ &

tissue development– basis for rejection of

foreign cells by immune system

http://faculty.southwest.tn.edu/rburkett/GB1-osmosis.htm

Semi-permeable membrane• Need to allow passage through the

membrane

• But need to control what gets in or out– membrane needs to be semi-permeable

aa H2Osugar lipid salt NH3

So what makes a membrane semi permeable?

See a movie

PHOBIC TAILS in center determine what can pass through

What molecules can get through directly?

inside cell

outside cell

Small non-polar (uncharged) molecules (O2, N2, & CO2) and hydrophobic molecules (fats & other lipids)can slip directly through the phospholipid cell membrane, but…

lipid

salt

aa H2Osugar

NH3

O2

What about other stuff?

Types of Cell Transport

Diffusion• 2nd Law of Thermodynamics

governs biological systems– Universe tends towards disorder

• Diffusion– movement from [higher] [lower] concentration

Animatioin from: http://www.biologycorner.com/resources/diffusion-animated.gif

Diffusion will also happen across a cell membrane as long as there is a difference in concentration and the membrane will let the molecule pass through.

http://www.le.ac.uk/pa/teach/va/anatomy/case2/2_2.html

Example: DIFFUSION IN CELLS O2 automatically moves from

HIGHER concentration (in lungs) to LOWER concentration (in blood)

CO2 automatically moves from HIGHER concentration (in blood) to LOWER concentration (in lungs)

http://facstaff.bloomu.edu/gdavis/links%20100.htm

Diffusion of 2 solutes • Each substance diffuses down its own

concentration gradient, independent of concentration gradients of other substances

What if cell needs to move a molecule _________ the CONCENTRATION GRADIENT?_______________

Cell example:Want to put MORE glucoseinto mitochondria when there

isalready glucose in there

(LOWER HIGHER)

Image from: http://www.biologyclass.net/mitochondria.jpg

AGAINST

What if a cell needs to move _____ or ______ moleculesthat can’t getthrough themembrane?

http://www.d.umn.edu/~sdowning/Membranes/membraneImages/jpegimages/diffusionmedium.jpg

LARGE POLAR

What if cell needs to move molecules really

_______? (can’t wait for it to diffuse)

Cell example:Movement of Na + & K+ ions required to send nerve signals

http://www.steve.gb.com/images/science/neuron.png

FAST

Cells need a ____ to ____ molecules across cell membranes that

_______ across by ___________

WAYHELP

can’t go

themselves

Facilitated diffusion• Move from HIGH to LOW concentration with

aid of membrane transport proteins – passive transport– no energy needed– facilitated = with help– Examples:

• Way glucose enters cells• Also how charged and polar molecules enter the cell

Facilitated DiffusionAnimation from: http://bio.winona.edu/berg/ANIMTNS/facdifan.gif

Carrier ProteinsGrab molecule, undergo conformational change, flip to other side

Channel ProteinsCreate passageway forsubstances to pass through

May be gated or not:open in response to chemical or electrical signals

Animation from: http://www2.uic.edu/~myilma1/ionchannel.gif

Gated channels• open only in presence of stimulus (signal)

– stimulus usually different from transported molecule

• ex: ion-gated channelswhen neurotransmitters bind to a specific gated channels on a neuron, these channels open = allows Na+ ions to enter nerve cell

• ex: voltage-gated channelschange in electrical charge across nerve cell membrane opens Na+ & K+ channels

Carriers and Channels are specific

inside cell

outside cell

sugaraaH2O

saltNH3

. . . BUT STILL MOVES FROM [HIGHER] to [LOWER]

Active transportUses energy from ATP to move molecules

against concentration gradient

– Moves from [lower] → [higher]

– Uses protein pumps

OR

vesicles

Animations from: http://academic.brooklyn.cuny.edu/biology/bio4fv/page/cell-movement.htmlhttp://www.cat.cc.md.us/courses/bio141/lecguide/unit1/eustruct/images/sppump.gif

PROTON PUMP

See a movie proton pump

Moves Protons (H+) across membrane

EXAMPLES: - Creates acidic condition inside lysosomes- Photosynthesis/respiration use H+ gradients togenerate ATP

Active transportSODIUM-POTASSIUM PUMPSets up difference in charge across membranes

Pumps 3 Na+ outand 2 K+ in

Makes cells more + outside more - inside

Animation from: http://www.cat.cc.md.us/courses/bio141/lecguide/unit1/eustruct/images/sppump.gif

See a movie about Na+ - K+ pump

All cells have voltages across their membranes

= membrane potential

• Cytoplasm inside cell is more negative

than extracellular fluid outside

• TWO FORCES drive diffusion• Concentration gradient• Electrical force

• Ions move DOWN the electrochemical gradientFavors the passage of cations (+) into cells

MEMBRANE POTENTIAL created by electrogenic pumps (proteins that generate

voltage by pumping ions)

http://fig.cox.miami.edu/~cmallery/150/memb/electrogenic.jpg

PROTON PUMPMain electrogenic pump in plants, fungi, and bacteria

Na+- K+ PUMPMain electrogenic pump

in animal cells

Animation from: http://www.lionden.com/cell_animations.htm

Electrogenic pumps can be coupled to actively transport other substances

http://faculty.southwest.tn.edu/rburkett/GB1-os29.jpg

= COTRANSPORT

Ex: Cells pump H+

out of cell then use the diffusion of H+ back into cell downits gradient to drivethe uptake of sucrose into cells

Na+ - K+ pump sets up MEMBRANE POTENTIAL

http://hyperphysics.phy-astr.gsu.edu/hbase/biology/nervecell.html#c2

• ACTIVE transport

• Requires energy (ATP)

• Uses vesicles

BULK TRANSPORT

Animation from: http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBooktransp.html

Watch two video clips aboutendo/exocytosis

endo/exocytosis

Endocytosis

phagocytosis

pinocytosis

receptor-mediated endocytosis

“Cell eating” large molecules;whole cells

“Cell drinking”Fluids;Small molecules

triggered byligand signal

Example in cells:WHITE BLOOD CELL ENGULFING BACTERIA

SEE PHAGOCYTOSIS MOVIE

http://fig.cox.miami.edu/~cmallery/255/255ion/fig14x28.jpg

EXOCYTOSIS• Active transport (requires ATP)• Uses vesicles• Releases substances to outside

INSULIN being released by pancreas cells using exocytosis

GOLGI BODIES USE EXOCYTOSIS

Animation from: http://www.franklincollege.edu/bioweb/A&Pfiles/week04.html

See a Golgi movie

Video: http://www.southtexascollege.edu/tdehne/BC_ShockwaveAnimations/07SWF-TourOfTheCell/07-16-EndomembraneSystem.swf

http://fig.cox.miami.edu/~cmallery/255/255ion/fig14x26.jpg

Transport summary

Water Transport

The Special Case of Water

The Special Case of Water

Movement of water across the cell membrane =

OSMOSIS

Osmosis is diffusion of water• Water is very important, so we talk about

water separately

• Diffusion of water from high concentration of water to low concentration of water– across a

semi-permeable membrane

– Passive– Uses no energy

Aquaporins• Transport proteins that move water rapidly

into & out of cells– evidence that there were water channels

1991 | 2003

Peter AgreJohn Hopkins

Roderick MacKinnonRockefeller

TONICITY- ability of a solution to cause a cell to lose or gain water

• Refers to the concentration of SOLUTES

• Is a RELATIVE term, comparing two different solutions

Solute-substance that is dissolvedSolvent- substance solute in dissolved inSolution = solute + solvent

What if there is a difference in concentration but solute

molecules can’t move across a membrane?

WATER will move until concentrationsreach equilibrium

 See an animation Osmosis1

http://faculty.etsu.edu/currie/images/osmosis1.jpg

Animation: http://www.ouhscphysio.org/humanphys/animations/osmosis1.swf

Animation

Concentration of water• Direction of osmosis is determined by

comparing total solute concentrations– Hypertonic - more solute, less water– Hypotonic - less solute, more water– Isotonic - equal solute, equal water

hypotonic hypertonic

water

net movement of water

• Pressure potential:– pressure exerted by the rigid cell wall

that limits further water uptake.

• Solute potential – The effect of solute concentration. – Pure water at atmospheric pressure

has a solute potential of zero. – As solute is added, the value for solute

potential becomes more negative. This causes water potential to decrease also.

– As solute is added, the water potential of a solution drops, and water will tend to move into the solution.

In beaker B, what is the water potential of the distilled water in the beaker, and of the beet core?– Since water potential = solute potential (-0.4) + pressure

potential (0.2), – water potential = -0.2 (water left the beet core)

Solute concentration

Lower outsidethan inside

Equal outsideand inside

Greater outsidethan inside

HYPOTONIC ISOTONIC HYPERTONIC

What will happen to an animal cell placed in different solutions?

OSMOSIS

HYPOTONIC: Concentration outside cell is ________________ inside the cell

More water enters than leaves cell so cellwill ___________________

LESS THAN

swell and possibly burst

See an animationOsmosis3

Animation from: http://www.ouhscphysio.org/humanphys/animations/osmosis3.swf

VideoChoose Blood Hypotonic link

OSMOSIS

HYPERTONIC: Concentration outside cell is ____________________ inside cell

More water leaves cell than enters so cell ____________

GREATER THAN

shrinks

See an animationOSMOSIS 4

Animation from: http://www.ouhscphysio.org/humanphys/animations/osmosis4.swf

Video Choose Blood Hypertonic link

Remember:Cells try to “maintain

stable internal conditions =

____________________HOMEOSTASIS

So an animal cell inISOTONIC conditionsstays same size

http://bioweb.wku.edu/courses/biol121/Osmosis/Osmosis.asp

Water entering = water leavingVideoChoose Blood Isotonic link

Animal cells

http://www.stchs.org/science/courses/sbioa/metenergy/bloodcells.gif

= CYTOLYSIS = CRENATION

Plant cellshttp://www.stchs.org/science/courses/sbioa/metenergy/aplantturgor.gif

Cell wall keeps plant cell from bursting made of cellulose rather than phospholipids and is external the cell membrane. Provides structural barrier for some substances to the internal environment.

Managing water balance• Isotonic

– animal cell immersed in isotonic solution• blood cells in blood• no net movement of water across

plasma membrane• water flows across membrane, at

same rate in both directions• volume of cell is stable

Cell (compared to beaker) hypertonic or hypotonic

Beaker (compared to cell) hypertonic or hypotonic

Which way does the water flow? in or out of cell

.05 M .03 M

Osmosis…

http://www.biology4kids.com/files/cell_vacuole.html

Loss of water from central vacuole= plants “wilt”

Vacuole full of water gives plant support (turgor pressure)

Turgid = very firmFlaccid = limp

HYPOTONICSitting in the bathtub makes your fingers plump up and get “pruny”

Grocery stores spray water on their veggies to “plump them up”

If cells can’t maintain “stable internal conditions” . . .damage can result and cells can die.

http://www.the-aps.org/education/lot/cell/Quiz.htm

http://www.microscopy-uk.org.uk/mag/imgjun99/vidjun1.gif

Cell survival depends on balancing water

uptake & loss = OSMOREGULATION

Paramecium vs. pond waterParamecium is hypertonicH2O continually enters cellto solve problem, specialized organelle, contractile vacuole

Gills ACTIVELY pump ions in;Urinate frequently

Drink salt water ACTIVELY pump ions OUT;Urinate less frequently

Kidspiration by: Riedell