Cell Membrane Structure

and Cellular

Transport

Accel Bio 2014

Biological Membranes are Phospholipid Bilayers

Phospholipid Structure

• hydrophobic “tails” of fatty acids

• hydrophilic “head” with a glycerol & a phosphate group (PO4)

Hydrophobic = “afraid” of water

Hydrophilic = “_______” of waterfriend

Biological membranes are selectively permeable barriers

Lipid bilayer membranes are permeable to:• hydrophobic lipids• small, uncharged molecules such O2 and CO2

• small molecules such as water

Lipid bilayers are not permeable to:• ions (charged atoms)• large hydrophilic molecules (like glucose)

These substances cannot pass through the lipid bilayer easily. They must find another (more favorable) way across…

A more complete picture of a Cell Membrane

• Embedded proteins “float” in fluid phospholipid “sea”

A model of the FLUID membrane: http://www.susanahalpine.com/anim/Life/memb.htm

Componentsof the Cell Membrane

• Phospholipids: make up selectively-permeable barrier

• Proteins: *many functions*, including transport channels - selectively allow some larger molecules and charged atoms (ions) through

• Carbohydrates: chemical ID tags, found on outside of cell

• Cholesterol: keeps membrane fluid

Cell Membrane Proteins: a slightly different representation

http://www2.warwick.ac.uk/fac/sci/chemistry/research/dixon/dixongroup/members/msrhar/research/background/

Membrane Protein Functions are Varied

Transport Across the Cell Membrane Occurs Various Ways

• Passive Processes1. Diffusion2. Osmosis3. Facilitated Diffusion

• Active Processes4. Active Transport (protein pump)5. Endocytosis

• Phagocytosis• Pinocytosis• Receptor-Mediated Endocytosis

6. Exocytosis

Movement across a membrane can occur by diffusion

Molecules are in constant motion. Due to their random movement, molecules have a natural tendency to exhibit net movement from areas of _______ concentration to areas of _____ concentration. This movement is called diffusion.

high low

Molecular motion never ceases, but we can reach equilibrium

Eventually, diffusion leads to a state where there are equal concentrations of molecules across a membrane. This state is called equilibrium. In equilibrium, the movement of molecules in a given direction (left or right) across a membrane is equally probable. Thus, unless disturbed, a system in equilibrium will tend to stay in equilibrium. DYNAMIC

Diffusion occurs down a concentration gradient

A difference in concentrations of an atom/molecule across a membrane is called a concentration gradient.

Diffusion of molecules to an area of low concentration is described as movement down the concentration gradient.

(Active transport to an area of higher concentration is described as movement against the conc gradient)

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane, from areas of ____ concentration to areas of ____ concentration.

Cells contain and are bathed in solutions that contain dissolved substances like ions, sugars, proteins, and vitamins in water.

A solution is a mixture of a liquid and at least one type of dissolved solid substance. The substance that is dissolved is called the solute. The liquid that it is dissolved in is called the solvent.

lowhigh

What drives

osmosis?Setup: U-tube with sel. perm. membrane, pour two different concentration salt solutions on either side of tube.Membrane is permeable to H2O but NOT to salt. What happens???

WHY?

What drives

osmosis?Setup: U-tube with sel. perm. membrane, pour two different concentration salt solutions on either side of tube.Membrane is permeable to H2O but NOT to salt. What happens???

WHY?

A little solution vocabulary

Hypo = Hyper = Iso =

Hypotonic:refers to a solution that has a solute content than the solution you are comparing it to

Hypertonic:refers to a solution that has a solute content than the solution you are comparing it to

Isotonic: Guess what this means!

lower

higher

belowabove / over

same

Movement of Water Across a Membrane: Which way net flow?

Movement of Water Across a Membrane: Which way net flow?

Red Blood Cells in Different Solutions

http://arbl.cvmbs.colostate.edu/hbooks/cmb/cells/pmemb/osmosis.html

Isotonic HypertonicHypotonic

Facilitated Diffusion: Movement across a membrane, from high to low

concentration, using a transport protein

Water also crosses membrane through Aquaporin protein channels

http://en.wikipedia.org/wiki/File:AQP-channel.png

http://plantphys.info/plant_physiology/osmosis.shtml

Not all cell transport is passive!

Passive Processes• Do not require the use of cellular energy.• Always involve net movement from areas of high

concentration to areas of low concentration.Ex) Diffusion, Osmosis, & Facilitated Diffusion

Active Processes• Require use of cellular energy.• Allow net movement from areas of low to areas

of high concentration.Ex) Active Transport, Endocytosis, & Exocytosis

How do cells “use” energy?

ATP (Adenosine Tri-phosphate)Energy “currency” of the cell. Transfers energy by transferring PO4

- groups.

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The bonds between the phosphate (PO4

-) groups are less stable and have high (potential) energy.

Adenine

Hydrolysis of ATP• releases energy and produces ADP + Pi

Where does the energy in ATP come from?

Cell Respiration

C6H12O6 + O2 + ADP + Pi ATP + CO2 + H2O

http://gleesonbiology.pbworks.com/f/1221960382/03_32_ATP_and_ADP_cycle.jpg

How does ATP transfer energy to do work?

Phosphorylation: transfer of a phosphate group

Active Transport: Movement across a membrane from low to high concentration which requires the use of energy (ATP) and a protein “pump”

Another Active Process: Exocytosis

• a transport vesicle fuses with cell membrane, releasing vesicle contents to the outside

Yet another Active Process: Endocytosis

http://bioap.wikispaces.com/file/view/endocytosis_types.png/177216843/735x365/endocytosis_types.png

Again, motor proteins use ATP, as does the process by which the cell cytoskeleton proteins are re-arranged to re-shape the membrane.

Receptor-Mediated Endocytosis

Rec-Med Endocytosis of LDL particles (Low-Density Lipoproteins):http://www.susanahalpine.com/anim/Life/endo.htm

Specific Examples of Cell TransportPassive Processes• Diffusion: movement of oxygen into cells• Osmosis: movement of water into cells placed in

pure water• Facilitated Diffusion: movement of glucose into cells

through specialized glucose protein channels

Active Processes• Active Transport: sodium-potassium (Na+/K+) pump• Endocytosis

• Phagocytosis: ingestion of bacteria by white blood cells• Pinocytosis: “nursing” of human egg cells• Receptor-Mediated Endocytosis: uptake of LDL particles

• Exocytosis: Ex1) secretion of mucus Ex2) secretion of insulin protein

Transport AnimationsFluid Mosaic Model of the Membranehttp://www.susanahalpine.com/anim/Life/memb.htm

Sodium-Potassium Pump (narrated)http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html

Endo/Exo-cytosis (narrated)http://local.brookings.k12.sd.us/biology/PHVideos/Chapter%2007E.mpg

Receptor-Mediated Endocytosis of an LDL particlehttp://www.susanahalpine.com/anim/Life/endo.htm

Overview of Membranes & Transport, including Action Potentialhttp://www.wiley.com/legacy/college/boyer/0470003790/animations/membrane_transport/membrane_transport.htm

Microscope Videos of various types of Cell Transporthttp://www.linkpublishing.com/video-transport.htm