Unit 3 Dif f us ion of m olec ules
Learning objectives â—ŽFactors affecting the rate of diffusion â—ŽArrangement and movement of molecules
Diffusion â—ŽThe exchange of substances between cells and the environment occurs in ways that require metabolic energy (active transport) and in ways that do not (passive transport) â—ŽDiffusion is an example of passive transport
Diffusion It is necessary to understand that: â—ŽA ll particles are constantly in motion due to the kinetic energy that they possess â—ŽT his motion is random, with no set pattern â—ŽP articles are constantly bouncing off one another as well as off other objects
Diffusion Diffusion is therefore defined as:
The net movement of molecules or ions from a region of higher concentration to a region of lower concentration
Rate of diffusion ◎Conc. gradient - the greater the
difference in the conc. of molecules, the faster the rate of diffusion ◎Surface area - the larger the area of an exchange surface, the faster the rate of diffusion
◎Thickness of an exchange surface the thinner an exchange surface, the faster the rate of diffusion
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Diffusion is proportional to: Surface area x difference in concentration ______________________________ Length of diffusion path (Diffusion is also affected by the nature of the plasma membrane (composition), and the size and nature of the diffusing molecule e.g. small molecules diffuse faster than large ones)
Diffusion
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â—Ž If 10 particles occupying the left -hand side of a closed vessel are in random motion, they will collide with each other and the sides of the vessel. â—Ž Some particles from the left -hand side move to the right, but initially there are no available particles to move in the opposite direction, so the movement is in one direction only. â—Ž There is a large concentration gradient and diffusion is rapid.
Diffusion
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◎ After a short time the particles have spread themselves move evenly. ◎ Particles can now move from right to left as well as left to right. ◎ However, with a higher concentration of particles (7) on the left than on the right (3), there is a greater probability of a particle moving to the right than in the reverse direction. ◎ There is a smaller concentration gradient and diffusion is slower.
Diffusion ◎ Some time later, the particles will be evenly distributed throughout the vessel and the concentrations will be equal on each side. ◎ The system is in an equilibrium. ◎ The particles remain in random motion. ◎ With equal concentrations on each side, the probability of a particle moving from left to right is equal to the probability of one moving in the opposite direction. ◎ There is no concentration gradient and no diffusion.
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Diffusion
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◎ At a later stage, the particles remain evenly distributed and will continue to be so. ◎ Although the number of particles on each side remains the same, individual particles are continuously changing position. ◎ This situation is called dynamic equilibrium.
Gas
Liquid
Solid
Assumes the shape and volume of its container
Assumes the shape of the part of the container which it occupies
Retains a fixed volume and shape
Compressible
Not easily compressible
Not easily compressible
Flows easily
Flows easily
Does not flow easily
Particles can move past one another Lots of free space between the particles
Particles can move past one another
Particles can move/slide past one another
Little free space between particles
Particles can move/slide past one another
Rigid - particles locked into place
Little free space between particles
Rigid - particles cannot move/slide past one another
Particles in a: ◎ gas are well separated with no regular arrangement. ◎ liquid are close together with no regular arrangement. ◎ solid are tightly packed, usually in a regular pattern. P articles in a: ◎ gas vibrate and move freely at high speeds. ◎ liquid vibrate, move about, and slide past each other. ◎ solid vibrate (jiggle) but generally do not move from place to place.