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Define diffusion

The net movement of molecules/ions from a region of higher concentration to a region of lower concentration down a gradient, as a result of the random movement of particles


Describe diffusion

- The molecules/ions move down a conc gradient
- The random movement is caused by the natural kinetic energy of the molecules/ions
- As a result of diffusion, molecules/ions tend to reach an equilibrium situation, where they are evenly spread out within a given volume of space


What factors affect the rate at which a substance diffuses across a membrane?

1) The steepness of the concentration gradient (the difference in the concentration of the substance on the two sides of the surface
2) Temperature
3) The surface area across which diffusion is taking place
4) The nature of the molecules/ions


How does the steepness of the concentration gradient affect the rate of diffusion?

- The greater the difference in concentration the greater the difference in the number of molecules passing in the two directions, and hence the faster the rate the diffusion
- i.e. if there are more molecules on one side of a membrane then at any one moment, more molecules will be moving (entirely randomly) from this side than from the other


How does temperature affect the rate of diffusion?

- At high temps, molecules/ions have much more kinetic energy than at low temps
- Therefore, they move around faster and so diffusion takes place faster


How does the surface area across which diffusion is taking place affect the rate of diffusion?

- The greater the SA, the more molecules can cross it at any one moment and therefore faster diffusion can occur


How can the SA of membranes be increased?

By folding


What is the relationship between the SA:vol ratio and the size of a 3D object?

The SA:vol ratio decreases as the size of any 3D object increases
- the larger the cell, the smaller its SA in relation to its volume


What do cells rely on diffusion for and what does this result in?

Internal transport of molecules
- this results in a limit on the size of cells, because once inside a cell, the time it takes a molecule to reach a certain destination by diffusion increase rapidly with distance travelled (rate falls in proportion to distance squared)


Why are cells small (roughly 50μm) in relation to diffusion?

Diffusion is effective over very short distances therefore an aerobic cell would quickly run out of oxygen and die if it was too large


How does the size of the molecules/ions affect the rate of diffusion?

- Large molecules require more energy to get them moving than small ones do, so large molecules tend to diffuse more slowly than small molecules


How does the polarity of the molecules affect the rate of diffusion?

- Non-polar molecules e.g. glycerol, diffuse much more easily through cell membranes than polar ones, because they are soluble in the non-polar phospholipid tails
- The respiratory gases are uncharged and non-polar (+small) so can cross through the bilayer directly between the phospholipid molecules


Why can water molecules diffuse rapidly?

Despite being very polar can diffuse rapidly across the phospholipid bilayer because they are small enough


Define facilitated diffusion

The diffusion of a substance through transport proteins in a cell membrane; the proteins provide hydrophilic areas that allow the molecules or ions to pass through the membrane which would otherwise be less permeable to them


What are the two types of protein involved in facilitated diffusion?

Channel proteins and carrier proteins


What are channel proteins?

Water-filled, hydrophilic pores with a fixed shape
- they allow ions to diffuse through the membrane
- most are 'gated' which means that part of the protein molecule on the inside surface of the membrane can move to open/close the pore
- this allows control of ion exchange
- e.g. the gated proteins in nerve cell membranes


What are carrier proteins?

Proteins that can flip between two shapes
- as a result, the binding site is alternately open to one side, and then the other
- the molecules move down the concentration gradient (bc facilitated diffusion)


How can the rate of diffusion be affected by channel/carrier proteins?

The rate is affected by how many channel/carrier proteins there are in the membrane, and in the case of channel proteins, on whether they are open or not


Define osmosis

The net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane, as a result of their random motion


Solution =



What is a partially permeable membrane (PPM) ?

A membrane that only allows certain molecules through e.g. membranes in living cells


Why can only water molecules pass through the PPM during osmosis?

Because the solute molecules are too big


What happens during osmosis?

- the number of solute molecules on each side stays the same but the number of water molecules changes
- therefore, the volume of the more concentrated solution increases and the volume of the more dilute solution decreases (either side of the PPM)


Define water potential

The tendency of water to move out of a solution


What two factors does water potential depend on?

1) how much water the solution contains in relation to solutes (i.e. concentration)
2) how much pressure is being applied to it


What happens to the water potential during osmosis?

Water moves from a high WP to a low WP, down a WP gradient
- this will happen until the WP is the same throughout the system, when equilibrium has been reached


What happens when you increase the pressure on a solution?

It increases the tendency of water to move out of it (i.e. its water potential), until it is higher than the WP in the solution on the other side of the PPM


What is the water potential of pure water at atmospheric pressure and what does this mean?

0, therefore a solution must have a negative water potential


What is the solute potential?

The contribution of the concentration of a solution to water potential
- i.e. the extent to which the solute molecules decrease the water potential of the solution


How does solute potential affect water potential?

The more solute there is, the lower the tendency for water to move out of the solution
- solute potential is also 0 for pure water and negative for solutions
- therefore, adding more solute to a solution decreases its water potential

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