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Biology A-Level > Cell Transport (Chapter 4) > Flashcards

Cell Transport (Chapter 4) Flashcards

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1
Q

Define diffusion

A

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

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2
Q

Describe diffusion

A
  • 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
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3
Q

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

A

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

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4
Q

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

A
  • 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
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5
Q

How does temperature affect the rate of diffusion?

A
  • 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
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6
Q

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

A
  • The greater the SA, the more molecules can cross it at any one moment and therefore faster diffusion can occur
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7
Q

How can the SA of membranes be increased?

A

By folding

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8
Q

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

A

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

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9
Q

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

A

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)

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10
Q

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

A

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

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11
Q

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

A
  • Large molecules require more energy to get them moving than small ones do, so large molecules tend to diffuse more slowly than small molecules
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12
Q

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

A
  • 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
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13
Q

Why can water molecules diffuse rapidly?

A

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

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14
Q

Define facilitated diffusion

A

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

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15
Q

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

A

Channel proteins and carrier proteins

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16
Q

What are channel proteins?

A

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
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17
Q

What are carrier proteins?

A

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)
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18
Q

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

A

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

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19
Q

Define osmosis

A

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

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20
Q

Solution =

A

solute+solvent

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21
Q

What is a partially permeable membrane (PPM) ?

A

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

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22
Q

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

A

Because the solute molecules are too big

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23
Q

What happens during osmosis?

A
  • 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)
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24
Q

Define water potential

A

The tendency of water to move out of a solution

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25
Q

What two factors does water potential depend on?

A

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

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26
Q

What happens to the water potential during osmosis?

A

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

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27
Q

What happens when you increase the pressure on a solution?

A

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

28
Q

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

A

0, therefore a solution must have a negative water potential

29
Q

What is the solute potential?

A

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

30
Q

How does solute potential affect water potential?

A

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
31
Q

What is pressure potential?

A

The contribution of pressure to the water potential of a solution

32
Q

What happens to an animal cell in a hypotonic solution (i.e. of high WP/low conc) ?

A

The cell swells and bursts (haemolysis) as water moves into the cell from the solution from a high water potential to a low water potential

33
Q

What happens to an animal cell in a hypertonic solution (i.e. of low WP/high conc) ?

A

The cell shrinks (crenation) as water moves out of the cell into the solution from a high water potential to a low water potential

34
Q

Whats the difference between plant cells and animal cells in terms of their vulnerability to osmosis?

A

Plant cells have very strong and rigid cell walls which prevents the cells from bursting

35
Q

Describe what happens and why to a plant cell in a solution of high water potential e.g pure water?

A

1) Water enters the cell through the the PPM by osmosis from an area of high WP to low WP
2) The volume of the cell increases, just like in the animal cell, but in the plant cell, the cell wall pushes back against the expanding protoplast, and pressure starts to build up rapidly
3) This is the pressure potential and it increases the WP of the cell until the WP inside the cell equals the WP outside the cell and equilibrium is reached
4) When a plant cell is fully inflated with water, it is described as fully turgid

36
Q

Why does it take very little water to achieve full turgidity (?) in a plant cell?

A

Because the cell wall is so inelastic

37
Q

What is the protoplast?

A

The living part of the plant cell

38
Q

Describe what happens and why to a plant cell in a solution of low water potential e.g. concentrated solution?

A

1) Water leaves the cell by osmosis from an area of high WP to an area of low WP
2) As it does so, the protoplast gradually shrinks until it is exerting no pressure at all on the cell wall - at this point pressure potential = 0
3) Both the solute and water molecules of the external solution can pass freely through the cell wall and therefore stay in contact with the shrinking protoplast
4) As the protoplast continues to shrink, it begins to pull away from the cell wall
5) This process is called plasmolysis and the cell has been plasmolysed
6) Eventually an equilibrium is reached when the WP of the cell has decreased until it equals that of the external solution

39
Q

What is it called when plasmolysis is about to occur?

A

Incipient plasmolysis

40
Q

Define active transport

A

The movement of molecules/ions through transport proteins across a cell membrane, against their concentration gradient, using energy from ATP
(i.e. low conc to high conc)

41
Q

How is active transport achieved?

A

By carrier proteins, each one specific to a particular type of molecule/ion

42
Q

What is the difference between active transport and facilitated diffusion?

A

Active transport requires energy because movement occurs up a concentration gradient

43
Q

How is the energy supplied and used in active transport?

A
  • the energy is supplied by the molecule ATP which is produced during respiration inside the cell
  • the energy is used to make the carrier protein change its shape, transferring molecules/ions across the membrane in the process
44
Q

What is an example of a carrier protein used for active transport?

A

The sodium-potassium pump

45
Q

Where is the Na-K pump found?

A

In the CSM of all animal cells

46
Q

What is the role of the Na-K pump?

A

To pump 3 Na+ ions out of the cell at the same time as allowing 2 K+ ions into the cell for each ATP molecule used

47
Q

What is the result of the Na-K pump?

A

The ions are both positively charged, so the net result is that the inside of the cell becomes more negative than the outside i.e. a potential difference is created across the membrane

48
Q

How does the Na-K pump get energy?

A

It has a receptor for ATP on its inner surface which acts as an ATPase enzyme, bringing about the hydrolysis of ATP

49
Q

Active transport can occur either into or out of the cell, where is it important?

A

Reabsorption of kidneys and sugar to phloem

50
Q

What is the difference between bulk transport and the other transport mechanisms?

A

It is for the transport of large quantities of materials into and out of cells compared to individual molecules/ions

51
Q

What kinds of things are transported across membranes using bulk transport?

A

Large molecules e.g. proteins/polysaccharides
Parts of cells
Whole cells

52
Q

How are bulk transport and active transport similar?

A

They both require energy (bulk transport is a form of active transport)

53
Q

Define endocytosis

A

The bulk movement of liquids (pinocytosis) or solids (phagocytosis) into a cell, by the infolding of the CSM to form vesicles containing the substance
- it is an active process, requiring ATP

54
Q

What happens during endocytosis?

A

Material is engulfed by the CSM to form a small sac, or ‘endocytic vacuole’

55
Q

What is phagocytosis?

A

The bulk uptake of solid material - ‘cell eating’

56
Q

What are the vacuoles called formed in phagocytosis?

A

Phagocytic vacuoles

57
Q

What are cells specialising in phagocytosis called?

A

Phagocytes e.g. WBCs that engulf bacteria

58
Q

What is pinocytosis?

A

The bulk uptake of liquid - ‘cell drinking’

59
Q

Why is pinocytosis sometimes called micropinocytosis?

A

Because the vesicles formed are often extremely small

60
Q

Define exocytosis?

A

The bulk movement of liquids or solids out of a cell, by the fusion of vesicles containing the substance with the cell surface membrane
- active process, requiring ATP

61
Q

Describe exocytosis

A

The reverse of endocytosis and the process by which materials are removed from cells

62
Q

Give two examples of when exocytosis occurs

A

1) the secretion of digestive enzymes from the cells of the pancreas by the Golgi body
2) transport of cell wall building materials to the outside of the CSM in plant cells

63
Q

What is the opposite of an active process?

A

A passive process

64
Q

What can transport proteins become in terms of the rate of transport?

A

A limiting factor because they can be filled up and there is a limit to how many molecules can be in a protein at one time

65
Q

How can concentration become a limiting factor for the rate of diffusion?

A

Because the molecules/ions are passing through the membrane/channels at their maximum rate

66
Q

What is the name for the solution when it has the same concentration as the cell placed in it?

A

Isotonic

67
Q

How else can water potential be reduced?

A

If the water molecules are bound to the solute and are therefore not ‘free’

Biology A-Level (44 decks)

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