Cell Membrane & Cell Transport

About Cell Membranes*

1.All cells have a cell membrane

2.Functions: a.Controls what enters

and exits the cell to maintain an internal balance called homeostasis

b.Provides protection and support for the cell

TEM picture of a real cell membrane.

Cell Membranes are made of aPHOSPHOLIPID bilayer which isAmphipathic –

Molecules with both hydrophilic

and hydrophobic regionsphosphate

lipid

hydrophilic

hydrophobic

PHOBIC TAILS in center determine what can pass through

*More than just a barrier…

• Expanding our view of cell membrane beyond just a phospholipid bilayer barrier– phospholipids plus…

*A membrane is a collage of different proteins & carbohydrates & cholesterol embedded in the fluid matrix of the lipid bilayer

Proteins

Membrane movement animation

Polar heads love water & dissolve.

Non-polar tails hide from water.

Carbohydrate cell markers

*Fluid Mosaic Model of the

cell membrane

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

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

Types of Cellular Transport

• Passive Transport

cell doesn’t use energy1. Diffusion

2. Facilitated Diffusion

3. Osmosis

• Active Transport

cell does use energy1. Protein Pumps

2. Endocytosis

3. Exocytosis

high

low

This is gonna be

hard work!!

high

low

Wee!!!

*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

*Diffusion: movement directly through membrane. What molecules can get through directly?

inside cell

outside cell

Small non-polar molecules (O2 & CO2) and hydrophobic molecules (fats & other lipids)

can slip directlythrough the phospholipid cell membrane, but…

lipid

salt

aa H2Osugar

NH3

O2

What about other stuff?

Diffusion

• Passive transport– No energy needed

• High to low concentration

• Small, nonpolar molecules

• Continues until equilibrium and then moves randomly

*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 until equilibrium and then moves randomly

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

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

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?

*How Organisms Deal with Osmotic Pressure

•Bacteria and plants have cell walls that prevent them from over-expanding. In plants the pressure exerted on the cell wall is called tugor pressure.

•A protist like paramecium has contractile vacuoles that collect water flowing in and pump it out to prevent them from over-expanding.

•Salt water fish pump salt out of their specialized gills so they do not dehydrate.

•Animal cells are bathed in blood. Kidneys keep the blood isotonic by remove excess salt and water.

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

http://www.painetworks.com/photos/gt/gt0461.JPG

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

= PLASMOLYSISCell wall keepsplant cell from bursting

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

*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

Facilitated diffusion• Move from HIGH to LOW concentration with

aid of membrane transport proteins – passive transport

• no energy needed

– facilitated = with help– Medium size molecules;

monomers

Ion Channels

Move from HIGH to LOW concentration with aid of membrane transport proteins

- passive transport- no energy needed

-facilitated = with help-Small, charged molecules

*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

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

• ACTIVE transport– Requires energy (ATP)

• Uses vesicles

*BULK TRANSPORT

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

Endocytosis

phagocytosis

pinocytosis

receptor-mediated endocytosis

“Cell eating” large molecules;whole cells

“Cell drinking”Fluids;Small molecules

triggered byligand signal

*PHAGOCYTOSISAnimation from: http://academic.brooklyn.cuny.edu/biology/bio4fv/page/cell-movement.html

http://www.accs.net/users/kriel/chapter%20nine/

“Cell eating” takes in large molecules;whole cells

See phagocytosis in action:

*Example in cells:WHITE BLOOD CELL ENGULFING BACTERIA using Phagocytosis

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

*PINOCYTOSIS

“Cell drinking”

Takes in fluids;

Small molecules

http://student.ccbcmd.edu/~gkaiser/biotutorials/eustruct/pinocyt.html

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

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

*Transport summary

*PROBLEM for Cells?

Cell membranes areSELECTIVELY PERMEABLE