Biology- Diffusion (AS)

Passive transport

No energy needed from an additional source.

Active transport

Requires metabolic activity

Diffusion

Net movement of particles from a region of higher concentration, to a region of lower concentration, down a concentration gradient.
It's a passive process that continues until there's a concentration equilibrium between the 2 regions.

How does diffusion occur?

Due to particles having kinetic energy, so they're constantly moving and colliding. Over short distances, diffusion is fast but over long distances, it's slow.

How fast is the rate of diffusion in a large cell?

It's slow, so reactions wouldn't get the substrates they need fast enough or ATP wouldn't be supplied fast enough.

Factors affecting rate of diffusion

-temperature
-concentration difference
-surface area
-thickness of membrane

How does temperature affect the rate of diffusion?

The higher the temperature, the higher the rate of diffusion, due to the particles having more kinetic energy and moving faster.

How does concentration difference affect the rate of diffusion?

Greater the concentration difference, the faster the rate of diffusion between 2 regions. This is due to the net movement of particles from the higher concentration to the lower concentration being larger.

How does surface area affect the rate of diffusion?

Larger the surface area, the the higher the rate of diffusion.

How does the thickness of membranes affect the rate of diffusion?

The thinner the membrane, the higher the rate of diffusion.

Simple diffusion

Diffusion without the presence of a membrane/barrier.

How can rate of diffusion be calculated?

Distance travelled/time OR volume filled/time

What do agar blocks contain?

Indicator, phenolphthalein which turns pink in the presence of an alkali, e.g. Sodium hydroxide.

Diffusion across membranes

-non-polar molecules can diffuse freely down a concentration gradient, e.g. O2.
-hydrophobic interior of the phospholipid bilayer repels polar substances such as H2O, so they can only diffuse through at a slow rate.

How are membranes partially permeable?

Small polar molecules pass through at a faster rate than large polar molecules.

Facilitated diffusion

Passive process.
Involves carrier proteins which change shape when a specific molecule binds to it and molecules diffuse down a concentration gradient.
Also involves channel proteins that are selectively permeable for specific molecules/ions.

Channel proteins

Form pores in the membrane for polar molecules/ions to diffuse through down a concentration gradient. They're specific to certain polar molecules/ions.

Carrier proteins

Transport large molecules in/out of a cell down a concentration gradient. They're specific to certain ions/polar molecules. The large molecule binds to the carrier protein and it changes shape, causing the molecule to be released on the other side of the

What effects the rate of facilitated diffusion?

-temperature
-thickness of membrane
-surface area
-concentration gradient
-number of channel and carrier proteins

How can agar cubes be used to investigate the effect of surface area on diffusion?

-cut agar cubes into various sizes and calculate their S.A:V ratio.
-time how long it takes for the cubes to go colourless in the same concentration of HCl.
-cube with the largest S.A:V ratio will turn colourless the fastest.

How can agar cubes be used to investigate the effect of concentration gradient on diffusion?

-prepare test using different concentrations of HCl and put the same size cube of agar jelly into each test tube and time how long it takes for them to turn colourless.
-the higher the concentration of HCl, the faster the rate of diffusion.

How can agar cubes be used to investigate the effect of temperature on diffusion?

-prepare several boiling tubes with the same concentration of hydrochloric acid and put tubes into water baths of varying temperatures.
-place equal-sized cubes of agar jelly into each boiling tube and time how long it takes for each cube to turn colourle

Why might the results of the agar cube experiment not be precise?

-deciding on colour change is subjective
-hard to say exactly when colour change occurred
-difficult to cut blocks to exactly the same size every time

How do you calculate the volume of a cube?

If cube size is 4cm, do 4^3, as volume is measured in cm^3.
Length
width
height

How do you calculate surface area of a cube?

If cube size is 5cm, do 6
5
5, as S.A is measured in cm^2.
Or, if the cube is 2cm
4cm
4cm, do 2
4
4=32 and add 4
2
2=32, to in total =64cm^2