Cross Section of A Cell Membrane

Background…

• Cells can’t survive isolated from their environment.

• Cells need to take in nutrients/other materials (organic compounds, salts, minerals) and dispose of waste they produce (carbon dioxide, hydrogen peroxide).

• Nutrients and waste must pass the cell membrane.

• Plasma Membrane - boundary of a cell that encloses its contents and regulates which particles can enter and leave the cell. (shape, protection, gate keeper)

• Selective Permeability – only certain particles can pass through the membrane. (Ex. Tea bag)

Phospholipid Bilayer

• Two layers (phospholipid bilayer)

• Orientation due to polarity• Polar heads (face internal

and external environment because they like water)-hydrophilic

• Nonpolar tails (shielded from environments – do not like water)-hydrophobic

What is embedded in the Membrane?

Membrane Proteins

• Peripheral proteins – attached to surface of the membrane (interior and exterior)– Plays a role in holding adjoining cells

together.

• Integral proteins – embedded in the bilayer, exposed to both inside the cell and the outside environment.– Form channels or pores for certain molecules

to pass in active and passive transport.

Fluid Mosaic Model• Lipid bilayer is fluid rather than solid.

• Membrane proteins and lipids move laterally back and forth.

• Cholesterol is embedded in the bilayer to provide stability and regulate membrane fluidity.

Passive Transport

• Substances pass the cell membrane without any added energy from high to low concentration.

• Three types:– Simple Diffusion– Osmosis– Facilitated Diffusion

Diffusion• Movement of molecules from an area of

greater concentration to lower.

• No added energy is needed from the cell.

• Molecules move through kinetic energy (dye in a beaker p. 95)

Concentration Gradient

• Difference in concentration between a region of high concentration and a region of low concentration.

• Molecules move down the concentration gradient across the cell membrane in passive transport.

What Is Able to Diffuse straight through the lipid bilayer?

• Molecules that are small enough and lipid soluble.

• O2, CO2

What is the Goal of Diffusion?

• Equilibrium!!!!!! – concentration is close to equal on both sides of the membrane.

• Random movement continues.

Osmosis

Osmosis• Water diffusing from an area of more to

less.• Pass through aquaporins (proteins)• No added energy.

• http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htm

Concentration Terms for Osmosis

• Hypertonic – area of greater concentration of solute. (less solvent)

• Hypotonic – area of lesser concentration of solute. (more solvent)

• Isotonic – equal concentration of solute to solvent on both sides of the cell membrane.

As a result…

• Plasmolysis – Shrinking of cytoplasm by osmosis (cell loses water)

• Cytolysis – cell gains water and bursts• Turgor Pressure – Pressure exerted on the cell

wall. This gives plants their shape.

What is Happening to These Cells?

Plant vs. Animal Cells

• Can plasmolysis occur in both cells? Explain.

• Can cytolysis occur in both cells? Explain.

• When can animal cells burst?– Athletes– Dehydrated patient in hospital

Facilitated Diffusion

• Molecules are assisted across the membrane by carrier proteins and ion channel proteins; molecules that are too big, insoluble or charged.

• No added energy.

• Moves down the gradient.

Carrier Proteins

• Assist particles that are lipid insoluble or too large to pass through the openings in the bilayer.

• Ex. Glucose

• One direction.

• http://www.aber.ac.uk/gwydd-cym/graffeg/biolgell/cludiant/proteincludo.gif

Ion Channel Proteins

• Small specific pathways for ions (charged particles) to pass

• Ex. Na+, K+, Ca2+, Cl-

• Two directions.

• http://www.aber.ac.uk/gwydd-cym/graffeg/biolgell/cludiant/sianel.gif

Side Note…

• It is important to realize that all three types of passive transport can happen at the same time. We study them independent of one another but they all occur at the same time to try to reach equilibrium.

Active Transport

• The cell’s movement of material across the cell membrane using added energy (ATP – the energy molecule of the cell).

• Material moves from lower concentration to higher concentration (up the concentration gradient)

Proton Pump

Sodium Potassium Ion Pump

• Proteins transport Na+ and K+ ions up their concentration gradient by using energy from ATP. (3 sodium out, 2 potassium in)

• http://student.ccbcmd.edu/~gkaiser/biotutorials/eustruct/images/sppump.gif• http://www.brookscole.com/chemistry_d/templates/student_resources/shared_resources/ani

mations/ion_pump/ionpump.html

• Occurs across

the cell membrane

in nerve cells.• Allows muscles

to contraction

Endocytosis and Exocytosis

• Process used to transport molecules in or out of a cell that are too large to fit through the lipid bilayer or proteins.

• Can also transport large

quantities at one time.

Endocytosis

• Plasma membrane surrounds the material and engulfs it into the cell.

• The membrane forms a vesicle that pinches off inside the cell.

• Vesicles fuse with lysosomes so that their contents can be digested.

Two Types…• Phagocytosis – large solid particles or

whole cells.

http://www.stolaf.edu/people/giannini/flashanimat/cellstructures/cell.swf

• Pinocytosis – liquids.

Exocytosis

• Reverse of endocytosis to release material to the environment.

http://www.aber.ac.uk/gwydd-cym/graffeg/biolgell/cludiant/ecsocytosis.gif

Endo/Exo Animations• Endo/exocytosis• http://www.sinauer.com/cooper/4e/micro/13/13-02_Phagocytosis(NL-

Large).jpg• http://student.ccbcmd.edu/~gkaiser/biotutorials/eustruct/images/phagocyt.gif• http://www.endocytosis.com/endocytosis.gif• phagocytosis vs. pinocytosis• http://upload.wikimedia.org/wikipedia/commons/thumb/1/1a/

Endocytosis_types.svg/672px-Endocytosis_types.svg.png• http://www.emc.maricopa.edu/faculty/farabee/BIOBK/exocyt.gif• endocytosis and exocytosis combined• http://www.bact.wisc.edu/Microtextbook/images/book_4/chapter_2/2-60.gif

Review – Different Types of Transport

• http://www.teachersdomain.org/resources/tdc02/sci/life/cell/membraneweb/assets/tdc02_int_membraneweb/tdc02_int_membraneweb_swf.html