SURFACE
TENSION
What’s going on
at the surface
of a liquid?
What’s going on
at the surface
of a liquid?
Let’s take
a 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.
But how do intermolecular forces
influence the surface?
= intermolecular
attractions
Under the surface,
intermolecular attractions pull on
individual molecules in all directions
= intermolecular
attractions
= intermolecular
attractions
= intermolecular
attractions
At the surface,
pull on the molecules is laterally and downward;
there is negligible intermolecular attractions
above 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 that
surface particles are pulled down until
counter-balanced by the compression
resistance of the liquid:
Surface molecules are compressed
more tightly together,
forming a sort of skin on the surface,
with less distance between them
compared to the molecules below.
Surface molecules also form a
much smoother surface than
one would expect from randomly
moving molecules.
This explains the characteristic rounded
shape that liquids form when dropping
through the air: The molecules are all
being pulled toward the center.
This explains the characteristic rounded
shape that liquids form when dropping
through the air: The molecules are all
being pulled toward the center.
Water in particular
has a very high
surface tension.
What property does
water have that would
give it such a strong
surface 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
Proteins
Lipid layer 2
Membrane structure
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
hydrophilic
hydrophobic
hydrophilic
• membrane proteins inserted, also amphipathic
Membrane Proteins
Membrane proteins:
Integral: inserted in membrane
- transmembrane – span
membrane
Peripheral: next to membrane
- inside or outside
Membrane structure
• Two transmembrane proteins: different structure
Bacteriorhodopsin: proton pump
Bacterial pore protein
Membrane Proteins
Movement of molecules
Simple Diffusion: most basic
force to move molecules
• Disperse until concentration equal in all areas
Movement of molecules
Cell membranes only allow some molecules across w/out help:
• Small, non-polar molecules OK
ex. steroids, O2, CO2
• 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?
Passive Transport - Simple Diffusion
• NO ENERGY required
• DOWN concentration gradient
• molecules
equally distribute
across available
area by type
- non-polar molecules
(steroids, O2, CO2)
Passive Transport – Facilitated Diffusion
• 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 - Osmosis
• osmosis – movement
of water across cell
membrane
• water crosses cell
membranes via
special channels
called aquaporins
• moves into/out of cell until
solute concentration is balanced
Passive Transport - Osmosis
In each situation below, does water have net
movement, and which direction:
fewer solutes in
solution, than in cell
equal solutes in
solution as in cell
more solutes in
solution, than in cell
Passive Transport - Osmosis
• tonicity – # solutes in solution in relation to cell
- hypotonic – fewer
solutes in solution
- isotonic – equal
solutes in solution
animal cell
- hypertonic – more
solutes in solution
plant cell
Passive Transport - Osmosis
Paramecium example
• regulate water balance
• pond water hypotonic
• water into contractile
vacuole
– water
expelled
Passive Transport - Osmosis
Scenario: in movie theater, watching a long movie.
You are: drinking water
What happens to your
blood?
You are: eating popcorn
What happens to your
blood?
Active Transport
• ENERGY IS required
• UP/AGAINST
concentration gradient
• Ex. Na-K ion pump
- Na+ ions: inside to out
- K+ ions: outside to in
• transport proteins
a. ion pumps
(uniporters)
b. symporter/antiporter
c. coupled transport
• antiporter: two molecules move
opposite directions (UP gradient)
Active Transport - uniporter
• Ex. proton (H+) pump
• ATP used pump H+ ions out
• uniporter:
ONE molecule
UP gradient
• against concentration and charge gradients
*gradients – used by cell for energy potential
Active Transport – coupled transport
• Ex. Active glucose transporter
• coupled transport: one molecule
UP gradient & other DOWN
gradient (opposite directions)
• Na+ diffusion used for
glucose active transport
• Na+ moving DOWN
concentration gradient
• Glucose moving UP
concentration gradient
Bulk Transport
• ENERGY IS required
• Several or large molecules
• Molecules moved IN
- endocytosis
• phagocytosis
– “food” in
• pinocytosis
– water in
Bulk Transport
• receptor-mediated endocytosis
– proteins bind molecules, vesicles inside
• Molecules
moved OUT
- exocytosis
Self-Check
Type of
transport
Energy
required?
Movement
direction?
Simple diffusion
no
Down conc. gradient
Osmosis
Facilitated
diffusion
Active transport
Bulk transport

Examples:
O2, CO2, nonpolar molecules