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