Cell Membrane & Cell Transport

TEM picture of a real cell membrane. About Cell Membranes* All cells have a cell membrane Functions: Controls what enters and exits the cell to maintain an internal balance called homeostasis 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 The carbohydrates are not inserted into the membrane -- they are too hydrophilic for that. They are attached to embedded proteins -- glycoproteins.

*Fluid Mosaic Model of the cell membrane Polar heads love water & dissolve. Membrane movement animation Non-polar tails hide from water. Carbohydrate cell markers Proteins

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 The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells. http://faculty.southwest.tn.edu/rburkett/GB1-osmosis.htm

Types of Cellular Transport high low Wee!!! Passive Transport cell doesn’t use energy Diffusion Facilitated Diffusion Osmosis Active Transport cell does use energy Protein Pumps Endocytosis Exocytosis high low This is gonna be hard work!!

*Molecules need to move across membranes in cells OUT waste ammonia salts CO2 H2O products IN food carbohydrates sugars, proteins amino acids lipids salts, O2, H2O Image modiified from: http://www.accessexcellence.org/AB/GG/importProt.html

Small non-polar molecules (O2 & CO2) and hydrophobic molecules (fats & other lipids) can slip directly through the phospholipid cell membrane, but… *Diffusion: movement directly through membrane. What molecules can get through directly? inside cell outside cell lipid O2 salt NH3 What about other stuff? sugar aa H2O

Diffusion Passive transport High to low concentration 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 Movement from high concentration of that substance to low concentration of that substance. 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 hypotonic hypertonic water net movement of water

What will happen to an animal cell placed in different solutions? Solute concentration Greater outside than inside Lower outside than inside Equal outside and inside HYPERTONIC HYPOTONIC ISOTONIC 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

Plant cells = PLASMOLYSIS Cell wall keeps plant cell from bursting http://www.stchs.org/science/courses/sbioa/metenergy/aplantturgor.gif Plant cells Cell wall keeps plant cell from bursting = PLASMOLYSIS

*Loss of water from central vacuole= plants “wilt” 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 aa sugar H2O NH3 salt outside cell . . . 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 Plants have nitrate & phosphate pumps in their roots. Why? Nitrate for amino acids Phosphate for DNA & membranes Not coincidentally these are the main constituents of fertilizer. 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+ out and 2 K+ in Makes cells more + outside more - inside Plants have nitrate & phosphate pumps in their roots. Why? Nitrate for amino acids Phosphate for DNA & membranes Not coincidentally these are the main constituents of fertilizer. Animation from: http://www.cat.cc.md.us/courses/bio141/lecguide/unit1/eustruct/images/sppump.gif

*BULK TRANSPORT ACTIVE transport Uses vesicles Requires energy (ATP) Animation from: http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBooktransp.html

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

*PHAGOCYTOSIS “Cell eating” takes in large molecules; whole cells 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

*GOLGI BODIES USE 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