4.2 - Diffusion

Question 1

what are the 3 important concepts to understand about particle movement

Answer 1

• all particles are constantly in motion due to kinetic energy they posses
• this motion is random, no set pattern for movement
• particles are constantly bouncing off one another as well as off other objects

Question 2

what is the definition of diffusion

Answer 2

the net movement of molecules or ions from one region where they are more highly concentrated to one where their concentration is lower until evenly distributed

Question 3

what molecules can diffuse through the CSM

Answer 3

• small, non-polar molecules
• such as oxygen and carbon dioxide

Question 4

what type of process is facilitated diffusion

Answer 4

passive

Question 5

describe facilitated diffusion

Answer 5

• passive process
• relies on the kinetic energy of diffusing molecules
• occurs down a concentration gradient but occurs only at certain points in the CSM where there are special protein molecules
• happens via 2 mechanisms: protein channels and carrier proteins

Question 6

describe protein channels

Answer 6

• water-filled hydrophilic channels spread across the membrane
• allow specific water-soluble ions to pass through
• the channels are selective, each opens in the presence of a specific ion (if not present = closed)
• therefore it has control over entry/exit of ions
• ions bind with the protein causing it to change shape in a way that closes it to one side of the membrane and opens it up to the other side

Question 7

describe carrier proteins

Answer 7

• when a molecule such as glucose that is specific to the protein present, it binds with the protein
• this causes it to change shape in such a way that the molecule is released to the inside of the membrane
• No external energy is needed for this
• the molecules diffuse using only their kinetic energy

Question 8

what 6 factors determine the rate of diffusion

Answer 8

• the concentration gradient
• the surface area
• Temperature
• the length of the diffusion path
• the type of molecule or ion diffusing
• the number of proteins

Question 9

how does the concentration gradient affect the rate of diffusion

Answer 9

steeper the gradient = the faster the rate of diffusion

Question 10

how does the surface area affect the rate of diffusion

Answer 10

• greater the surface area = the faster the rate of diffusion
• because: more molecules or ions can pass at any one time

Question 11

how does the Temp affect the rate of diffusion

Answer 11

-higher temp = molecules have more kinetic energy
- molecules move faster = diffusion takes place faster
- NB: if too high = proteins denature

Question 12

how does the length of the diffusion path affect the rate of diffusion

Answer 12

• thinner exchange surfaces increase speed of diffusion

Question 13

how does the type of molecule/ion diffusing affect the rate of diffusion

Answer 13

• large molecules: need more energy to move = so diffuse slower
• Non-polar molecules: diffuse easier than polar molecules because they are soluble in the non-polar phospholipid tails

Question 14

Which direction does diffusion happen

Answer 14

• the net movement will be from high —> low (until particles are evenly distributed

Question 15

What is the concentration gradient

Answer 15

• the path from an area of higher concentration to an area of lower concentration
• for diffusion: particles diffuse DOWN the concentration gradient

Question 16

Give examples of molecules that can easily diffuse through membranes + why

Answer 16

• carbon and oxygen
  —> small: so passes through the spaces between the phospholipids
  —> non-polar: makes them soluble in lipids, so they can dissolve in the hydrophobic bilayer

Question 17

What is it known as when molecules diffuse directly through a cell membrane

Answer 17

Simple diffusion

Question 18

What types of molecules would need to diffuse via facilitated diffusion (via carrier proteins and protein channels)

Answer 18

• larger molecules (e.g. amino acids/glucose) - they diffuse extremely slowly through phospholipid bilayer because they’re so big —> therefore need facilitated diffusion to speed it up
• charge molecules (e.g. ions and polar molecules) - diffuse very slowly because they’re water soluble and the centre of the bilayer is hydrophobic —> therefore need facilitated diffusion to speed it up

Question 19

Is facilitated diffusion a passive process

Answer 19

Yes

Question 20

Which molecules do carrier proteins help move during facilitated diffusion, how is it done

Answer 20

large molecules
1) a large molecule attaches to a carrier protein in the membrane
2) then, the protein changes shape
3) this releases the molecule on the opposite side of the membrane

Question 21

What molecules do channel proteins help move during facilitated diffusion, how is this done

Answer 21

• channel proteins form pores in the membrane for charged molecules
• they can diffuse through these pores (down a concentration gradient)

Question 22

Are carrier proteins/protein channels specific

Answer 22

• yes
• different carrier proteins facilitate the diffusion of different molecules
• different channel proteins facilitate the diffusion of different charged particles

Question 23

What 3 factors does the rate of simple diffusion depend on

Answer 23

• the concentration gradient
• the thickness of the exchange surface
• the surface area

Question 24

How does the concentration gradient affect the rate of simple diffusion

Answer 24

• the higher the gradient = the faster
  because:
• As diffusion takes place, the difference in concentration between the 2 sides of the membrane decreases until it reaches an equilibrium
• this means that diffusion slows down over time

Question 25

How does the thickness of the exchange surface affect the rate of simple diffusion

Answer 25

• the thinner the exchange surface = the shorter the distance the particles need to travel = the faster the rate of diffusion

Question 26

How does the surface area affect the rate of simple diffusion

Answer 26

• the larger the surface area (e.g. of the CSM) = the faster the rate of diffusion
• larger SA means that more particles can be exchanged in the same amount of time — increasing the rate of diffusion

Question 27

Give an example of one adaptation for improving the rate of simple diffusion in the small inetstine

Answer 27

• some cells (e.g. epithelial cells in small intestine) have microvilli
• these are projection formed by the CSM folding up on itself
• the microvilli give the cell a larger surface area

Question 28

What 2 factors does the rate of facilitated diffusion rely on

Answer 28

• the concentration gradient
• the number of channel or carrier proteins

Question 29

How does the concentration gradient affect the rate of facilitated diffusion

Answer 29

• the higher the concentration gradient = the faster the rate of facilitated diffusion
• As equilibrium is reached, the rate of facilitated diffusion will level off

Question 30

How does the number of channel or carrier proteins affect the rate of facilitated diffusion

Answer 30

• once all proteins in a membrane are in use, facilitated diffusion can’t happen any faster
• even if you increase the concentration gradient
• therefore: the greater the number of channel or carrier proteins in the cell membrane = the faster the rate of facilitated diffusion

Question 31

Give an example of one adaptation for improving the rate of facilitated diffusion in kidney cells

Answer 31

• some kidney cells are adapted to have lots of aquaporins
• aquaporins are special channel proteins that allow the facilitated diffusion of water through cell membranes
• the aquaporins allow the cells to reabsorb a lot of water that would otherwise be excreted by the body