MYP Biology 1.3 Diffusion Notes

How Does Diffusion Actually Move Substances Without Energy?

1. Diffusion is a passive process, it doesn't require energy input from the cell (no ATP needed).
2. It works because of the concentration gradient:
   1. More particles on one side = more random movement on that side.
   2. More movement = more particles moving outward.
   3. Fewer particles on the other side = fewer particles moving back.
   4. Net movement is therefore from high to low concentration.
3. This will continue until equilibrium is reached (equal concentration everywhere).
4. Once equilibrium is reached, the concentration gradient will no longer exist.

Why Is Diffusion Fast in Some Places and Slow in Others?

Several factors affect how fast diffusion occurs:

Concentration gradient

1. Steeper gradient = faster diffusion:
   1. Large difference in concentration = more net movement.
   2. Small difference in concentration = less net movement.
2. As diffusion proceeds, the gradient decreases:
   1. The difference gets smaller over time.
   2. Diffusion slows down as it approaches equilibrium.

Temperature

1. Higher temperature = faster diffusion:
   1. Particles have more kinetic energy.
   2. They move faster and collide more frequently.
   3. More collisions = faster spreading.
2. Lower temperature = slower diffusion:
   1. Particles have less kinetic energy.
   2. They move slower with fewer collisions.

Surface Area

1. Larger surface area = faster diffusion:
   1. More space for particles to cross.
   2. More particles can move simultaneously.
2. Cells and organisms increase surface area for diffusion:
   1. Lungs have millions of tiny air sacs (alveoli) - massive surface area.
   2. Small intestine has millions of tiny projections (villi) - increases surface area for nutrient absorption.
   3. Root hair cells have long projections - more surface area for water absorption.

Distance

1. Shorter distance = faster diffusion:
   1. Particles reach their destination quicker.
   2. Diffusion is only efficient over short distances (typically less than 1mm).
2. Longer distance = much slower diffusion:
   1. Diffusion time increases exponentially with distance.
   2. This is why large organisms need transport systems (blood, xylem, phloem) - diffusion alone is too slow.

Size of Molecules

1. Smaller molecules = faster diffusion:
   1. Less mass means easier to move.
   2. Can slip between other particles more easily.
2. Larger molecules = slower diffusion:
   1. More mass means more inertia.
   2. More likely to collide and bounce back.