SURFACETENSION
What’s going onat the surfaceof a liquid?
What’s going onat the surfaceof a liquid?
Let’s takea look!
Particles that make up a liquid are in constant random motion; they are randomly arranged.
You might expect the particles at the surface,at the micro level, to form a random surface,as shown below.
You might expect the particles at the surface,at the micro level, to form a random surface,as shown below.
= intermolecularattractions
But how do intermolecular forcesinfluence the surface?
Under the surface, intermolecular attractions pull onindividual molecules in all directions
= intermolecularattractions
= intermolecularattractions
= intermolecularattractions
At the surface, pull on the molecules is laterally and downward;there is negligible intermolecular attractionsabove the molecules (from the medium above, such as air).SO, the net force on surface molecules is downward.
The result of this downward force is thatsurface particles are pulled down untilcounter-balanced by the compressionresistance of the liquid:
Surface molecules are compressedmore tightly together,forming a sort of skin on the surface,with less distance between themcompared to the molecules below.
Surface molecules also form a much smoother surface thanone would expect from randomlymoving molecules.
This explains the characteristic roundedshape that liquids form when droppingthrough the air: The molecules are all being pulled toward the center.
This explains the characteristic roundedshape that liquids form when droppingthrough the air: The molecules are all being pulled toward the center.
Water in particularhas a very highsurface tension. What property doeswater have that wouldgive it such a strongsurface tension?
I. Membrane Structure
II. Permeability
III. Transport Across Membranes
A. Passive
B. Facilitated
C. Active
D. Bulk
Membrane structure
1915, knew membrane made of lipids and proteins
• Reasoned that membrane = bilayer
Where to place proteins?
Lipid layer 1
Lipid layer 2
Proteins
Membrane structure
• freeze fracture
• proteins intact, one layer or other
• two layers look different
Membrane structure
Experiment to determine membrane fluidity:
• marked membrane proteins mixed in hybrid cell
Membrane structure
Membrane fluidity
• phospholipid f.a. “tails”: saturation affects fluidity
• cholesterol buffers temperature changes
Membrane structure
“fluid mosaic model” – 1970s
• fluid – phospholipids move around
• mosaic – proteins embedded in membrane
Membrane structure
• cell membrane – amphipathic - hydrophilic & hydrophobic
• membrane proteins inserted, also amphipathic
Membrane structure
hydrophilic
hydrophilic
hydrophobic
Membrane Proteins
Membrane proteins:
- transmembrane – span membrane
Integral: inserted in membrane
Peripheral: next to membrane- inside or outside
• Two transmembrane proteins: different structure
Bacteriorhodopsin: proton pump
Membrane structure
Bacterial pore protein
Membrane Proteins
Movement of molecules
Simple Diffusion: most basic force to move molecules
• Disperse until concentration equal in all areas
• Small, non-polar molecules OK
ex. steroids, O2, CO2
Movement of molecules
Cell membranes only allow some molecules across w/out help:
• No charged, polar, or large molecules
ex. sugars, ions, water*
Transport Across Membranes
Types of transport:
A. Passive transport
- Simple diffusion
- Facilitated diffusion
- Osmosis
B. Active transport
C. Bulk transport
• Energy Required?
• Directionality?
• DOWN concentration gradient
• molecules equally distribute across available area by type
Passive Transport - Simple Diffusion
- non-polar molecules (steroids, O2, CO2)
• NO ENERGY required
• DOWN concentration gradient
• molecules equally distribute but cross membrane with the help of a channel (a) or carrier (b) protein.
Passive Transport – Facilitated Diffusion
• NO ENERGY required
• osmosis – movement of water across cell membrane
• water crosses cell membranes via special channels called aquaporins
Passive Transport - Osmosis
• moves into/out of cell until solute concentration is balanced
Passive Transport - Osmosis
equal solutes in solution as in cell
more solutes in solution, than in cell
fewer solutes in solution, than in cell
In each situation below, does water have net movement, and which direction:
• tonicity – # solutes in solution in relation to cell
- isotonic – equal solutes in solution
- hypertonic – more solutes in solution
animal cell
plant cell
- hypotonic – fewer solutes in solution
Passive Transport - Osmosis
Paramecium example
• regulate water balance
• water into contractile vacuole
– water expelled
• pond water hypotonic
Passive Transport - Osmosis
Scenario: in movie theater, watching a long movie.
You are: drinking water
You are: eating popcorn
What happens to your blood?
What happens to your blood?
Passive Transport - Osmosis
• transport proteins
a. ion pumps (uniporters)
• Ex. Na-K ion pump
- Na+ ions: inside to out
b. symporter/antiporter
- K+ ions: outside to in
Active Transport
• UP/AGAINST concentration gradient
• ENERGY IS required
• antiporter: two molecules move opposite directions (UP gradient)
c. coupled transport
• ATP used pump H+ ions out
*gradients – used by cell for energy potential
• against concentration and charge gradients
Active Transport - uniporter
• Ex. proton (H+) pump
• uniporter: ONE molecule UP gradient
Active Transport – coupled transport
• Ex. Active glucose transporter
• Na+ diffusion used for glucose active transport
• Na+ moving DOWN concentration gradient
• Glucose moving UP concentration gradient
• coupled transport: one molecule UP gradient & other DOWN gradient (opposite directions)
• phagocytosis – “food” in
• pinocytosis – water in
• Molecules moved IN - endocytosis
Bulk Transport• ENERGY IS required
• Several or large molecules
Bulk Transport
• receptor-mediated endocytosis
– proteins bind molecules, vesicles inside
• Molecules moved OUT - exocytosis
Self-Check
Type of transport
Energy required?
Movement direction?
Examples:
Simple diffusion no Down conc. gradient O2, CO2, non-polar molecules
Osmosis
Facilitated diffusion
Active transport
Bulk transport