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 a PHOSPHOLIPID bilayer which is Amphipathic – Molecules with both hydrophilic and hydrophobic regions phosphate hydrophilic lipid 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

Polar heads

love water

& dissolve.

* Fluid Mosaic Model of the cell membrane

Non-polar tails

hide from water

.

Carbohydrate cell markers

Membrane movement animation

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 energy 1. Diffusion 2. Facilitated Diffusion 3. Osmosis •

Active Transport

cell does use energy 1. Protein Pumps 2. Endocytosis 3. Exocytosis high high Wee !!!

low low This is gonna be hard work!!

*Molecules need to move across membranes in cells OUT waste ammonia salts CO 2 H 2 O products IN food carbohydrates sugars, proteins amino acids lipids salts, O 2 , H 2 O

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

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

Small non-polar molecules (O 2 & CO 2 ) and hydrophobic molecules (fats & other lipids) can slip directly inside cell lipid O 2 NH 3 salt through the phospholipid cell membrane, but… outside cell sugar aa H 2 O 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

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

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

*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

water hypotonic hypertonic net movement of water

Solute concentration Lower outside than inside HYPOTONIC Equal outside and inside ISOTONIC Greater outside than inside 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

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

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

Plant cells

Cell wall keeps plant cell from bursting = PLASMOLYSIS

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 firm Flaccid = 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 H 2 O aa sugar NH 3 outside cell salt

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

Active transport

Uses 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.html

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

Active transport

SODIUM-POTASSIUM PUMP Sets up difference in charge across membranes Pumps 3 Na + and 2 K + in out Makes cells more + outside more - inside

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

*BULK TRANSPORT

• ACTIVE transport – Requires energy (ATP) • Uses vesicles

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

phagocytosis

Endocytosis

pinocytosis “Cell eating” large molecules; whole cells “Cell drinking” Fluids; Small molecules receptor-mediated endocytosis triggered by ligand signal

*PHAGOCYTOSIS

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

“Cell eating” takes in large molecules; whole cells See phagocytosis in action : http://www.accs.net/users/kriel/chapter%20nine/

*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

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

*GOLGI BODIES USE EXOCYTOSIS

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

*Transport summary

*PROBLEM for Cells?

Cell membranes are SELECTIVELY PERMEABLE