Chapter 3 Lecture Outline

Biology

Chapter 3 Lecture Outline

The Plasma Membrane

1.  Function – barrier to external environment

a.  Question: How do materials the cell requires vs materials cell needs to dispose of cross this barrier?

2.  Composition and structure – “The fluid-mosaic model’

a.  Cell membrane is a phospholipid bilayer with proteins embedded throughout in an asymmetric manner

b.  Fluid – phopholipid comprise core of membrane

i. Lipids and proteins able to float around membrane

c.  Mosaic – asymmetrical distribution of the proteins and various components throughout membrane surface

3.  Components of Membrane

a.  Phospholipid bilayer

i. Spontaneous

ii.  Hydrophobic vs hydrophilic interactions

iii.  FA tails – saturated versus unsaturated (regulate fluidity)

b.  Proteins embedded

i. Often called glycoproteins – carbs attached to protein

1.  Integral or transmembrane proteins – pass through entire span of membrane

2.  Peripheral – attached to one side or other

c.  Cholesterol

i. Minor component

ii.  Helps regulate fluidity

d.  Glycolipids

i. Some Plipids have sugars attached

e.  Glycolipids and glycoproteins function with receiving messages from outside cell and in cell:cell recognition (immune system)

4.  Membrane is selectively permeable

a.  Only certain chemicals are able to FREELY pass thru the lipid membrane itself!!

b.  Permeable:

i. Small hydrophobic molecules

ii.  Small uncharged polar

c.  Non-permeable:

i. Larger uncharged polar molecules

ii.  Ions

d.  Anything that can not FREELY pass through lipid membrane requires as specialized protein embedded within cell membrane to use as a passageway – Transport protein!!

Diffusion and Osmosis

5.  Diffusion

a.  Movement of molecules from an area of higher concentration to lower concentration

b.  Molecules FOLLOW concentration gradient.

i. What is a concentration gradient?

c.  Notes about diffusion: Random

i. No energy required!!!

1.  Increase entropy due to increases (more disorder)

2.  Going DOWN ramp easier than going up!!

b.  Molecules will continue to move until equillbrium is reached!

d.  What does chemical equilibrium mean?

i. Molecule still move but forward movement matched with reversed!

ii.  In diffusion, concentrations will equalize, but still have movement – just no NET movement!

e.  Rate of Diffusion Dependent On:

f.  In a cell, some materials will pass FREELY, simply by diffusion over the cell membrane and will follow concentration gradients!

i. Ex: O2/CO2 exchange

6.  Osmosis

a.  Diffusion of water across a membrane!!!

b.  Water moves from area of high to low concentration just like everything else, however, often easier to think in terms of solute concentrations!

c.  Water moves from area of lower to higher solute concentration à SALT SUCKS

d.  Water always follows the solute!!

e.  Many substances are not permeable to membrane to often water moves to compensate!

i. Hypotonic

ii.  Isotonic

iii.  Hypertonic

f.  Movement of water in animal vs plant cells

i. Animal cells – shrink or burst

ii.  Plant cells – cell wall prevents them from rupturing

a.  Turgor pressure – outward pressure of cell membrane on cell wall keeps plants upright, also prevents additional water from entering

7.  Rates of diffusion and osmosis across membrane depend on:

a.  Permeability of membrane to substance

b.  Difference in concentration gradient

c.  Surface area of membrane: volume

8.  How do substances cross the plasma membrane?

a.  Passive transport

i. Simple diffusion

ii.  Facilitated diffusion

1.  Protein passageway allows for material to pass

2.  No energy required

3.  Simply follows concentration gradient

4.  Diffusion stops once equilibrium reached

iii.  Active transport

1.  Use of a protein pump to move molecules in directed fashion

2.  Allows cell to create or move substances against concentration gradient

3.  ENERGY requiring!!

4.  Energy for active transport can come from:

a.  Direct ATP hydrolysis

i. Ex: Na-K pump

b.  Coupling movement of molecule with another

i. Ex: glucose-Na transporter

c.  Redox Reactions (electron transport systems)

i. Ex: H+ Pump

5.  What happens is a molecule cannot pass freely thru membrane?

a.  Possibility of developing a concentration gradient between inside and outside cell

b.  What might be useful about this?

i. Source of potential energy a cell could utilize later

1.  Ex:

a.  Nerve impulses

b.  ATP synthesis

c.  Active transport of molecules

6.  Examples of maintain concentration gradients:

a.  Na-K in nerve cells

b.  pH regulation

c.  taking nutrients into cell

7.  Special cases of active transport

a.  Endocytosis

b.  Exocytosis