5.3 : Diffusion Flashcards
What is passive movement?
Utilises energy from the natural motion of particles, rather than energy from another source.
Define diffusion
The net movement of particles from a region of higher concentration to a region of lower concentration.
It is a passive process and will continue until a concentration equilibrium is reached between the two areas.
Why does diffusion happen?
The particles in a gas or liquid have kinetic energy.
Their movement is random and unequal distribution of particles will eventually become an equal distribution.
How does distance affect diffusion ?
Particles move at high speeds and are constantly colliding, which slows down their overall movement.
This means that over short distances diffusion is fast.
As diffusion distance increases, the rate of diffusion shows down because more collisions have taken place.
How does temperature affect diffusion?
The higher the temperature the higher the rate of diffusion. This is because the particles have now kinetic energy and move at higher speeds.
How does a difference in concentration affect diffusion?
The greater the difference in concentration between two regions the faster the rate of diffusion because the overall movement from the higher concentration to lower concentration will be larger.
What is simple diffusion?
Diffusion in the absence of a barrier or membrane.
How would you investigate rate of diffusion and surface area?
Agar blocks should contain the indicator phenolphthalein which turns pink in the presence of an alkali.
Agar blocks of different sizes immersed in a solution of sodium hydroxide for ten minutes.
The blocks were removed and the distance the sodium hydroxide had diffused was measured with a ruler.
What can and can’t pass through a phospholipid bilayer ? Why is it called partially permeable?
Non polar molecules eg oxygen , diffuse through freely down a concentration gradient.
Hydrophobic interior of the membrane repels substances with a positive or negative charge ( ions ), so they cannot easily pass through.
Polar molecules such as water, with partial positive and negative charges can diffuse through membranes, but only at a very slow rate.
Small polar molecules pass through more easily than larger ones.
Therefore membranes are partially permeable.
What affects the rate at which molecules or ions diffuse across membranes?
Surface area - the larger the area of an exchange surface, the higher the rate of diffusion
Thickness of membrane - the thinner the exchange surface, the higher the rate of diffusion
Define facilitated diffusion
Diffusion across a membrane through protein channels.
The movement of molecules is down a concentration gradient and does not require external energy.
Why are some membrane selectively permeable?
Membranes with protein channels are selectively permeable as most protein channels are specific to one molecule or ion
What’s factors affect the rate of facilitated diffusion?
Temperature
Concentration gradient
Membrane surface area and thickness
Number of channel proteins present - the more protein channels, the higher the rates of diffusion overall
How would you investigate factors affecting diffusion using a model cell?
A model cell can be stimulated by tying one end up of a section of tubing, filling with a solution and then tying the other end.
This is then placed into another solution. The solutions could contain different sizes, or concentrations, of solute molecules.
The changes in concentration of solute molecules can be measured over time. Rates of diffusion across the tubing can then be calculated.
Why is dialysis tubing used in a model cell?
It is partially permeable, with pores a similar size to those on a partially permeable membrane.
This means that small molecules like water can pass through it, but larger molecules like starch cannot fit through the pores.
The tubing is therefore a barrier to large molecules.
