Diffusion

Solutions• Solvent – liquid

• Solute - solid that is dissolved

• Solution – liquid with dissolved substances

• Soluble – can dissolve

Substances may be soluble in

water, eg ions, oxygen, carbon dioxide

oils, mainly small non-polar molecules

Diffusion

net movement of particles from a region of high concentration to a

region of low concentration.

In other words: Molecules in a gas or liquid will tend to spread out as much as possible

Cells need nutrients and gases for photosynthesis, respiration and other metabolic processes

Cells excrete waste products which could become toxic if they accumulate

Why do cells need transport

Transport mechanisms

Passive transport (doesn’t require energy) Diffusion Facilitated diffusion Osmosis Filtration

Active transport (uses energy from ATP)Active transportBulk transport

} Movement down a concentration gradient

1. DiffusionThe following will increase diffusion:

1. Steep concentration gradient

2. Larger surface area

3. Thin permeable membrane

4. Small, non-polar molecules

5. Higher temperatures

Convolutions increase SA:V

Volume = 12

Surface area = 14

SA:V = 1.17

Volume = 12

Surface area = 26

SA:V = 1.86

Volume = 12

Surface area = 14

SA:V = 1.17

Volume = 1

Surface area = 4

SA:V = 4

Small organisms have a larger SA:V ratio

Limitations to cell size

Volume = 1cm3

SA = 6cm2

SA:V = 6

Volume = 8cm3

SA = 24cm2

SA:V = 3

Nutrients can diffuse to all

parts of cell fast enough to

maintain lifeLower SA:V

Takes too long for nutrients to

diffuse to all parts of cell?

O2 – Non-polar

CO2 – Polar but very small

H2O – Polar but very small

Diffuse rapidly

Transport mechanisms

Passive transport (doesn’t require energy) Diffusion Facilitated diffusion Osmosis Filtration

Active transport (uses energy from ATP)Active transportBulk transport

} Movement down a concentration gradient

2. Facilitated diffusion

Membrane impermeable to molecules such as Glucose, sodium ions & chloride ions.

• Carrier proteins can “carry solute across”

OR• Ion channels act as hydrophilic pores

• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html

Facilitated transport

Chemicals diffusing through the air

Remaining slides are extras

Kinetic movement

Molecules have kinetic energy, so they move constantly and randomly.

Molecules move down a concentration gradient

HIGH LOW CONCENTRATION.

Small molecules diffuse faster

Higher temperatures more energy faster diffusion

Equilibrium - still moving, but no net movement

AS Biology, Cell membranes and Transport

22

Diffusion of Bromine

AS Biology, Cell membranes and Transport

23

Diffusion of Bromine

Diffusion

http://www.youtube.com/watch?v=H7QsDs8ZRMI

(5min)

Concentration gradients

higher concentration

lower concentration

Molecules diffuse DOWN

a concentration

gradient

O2

O2

O2

O2

O2

O2

O2

O2

O2

O2

O2

O2

O2

O2

O2

High oxygen concentration

O2

O2

O2Lower oxygen concentration

The bigger the difference, the steeper the gradient

Molecules diffuse more easily across thin barriers

O2

O2

O2

O2

O2

O2

O2

O2

O2O2

Membrane thickness