Transport Across Cell Membranes- Diffusion and Osmosis

Question 1

Describe the fluid mosaic model of membrane structure

Answer 1

• Molecules free to move laterally in phospholipid bilayer
• Contain many components- Phospholipids, Proteins Glycoproteins and Glycolipids

:Laterally- Left right up and down

Question 2

Describe the arrangement of the components of a cell membrane (5)

Answer 2

• Phospholipids form a bilayer- Fatty acid tails face inwards, phosphate heads face outwards
• Embedded Proteins
  -Intrinsic/Integral proteins span bilayer e.g. Channe; and carrier proteins
  -Extrinsic/peripheral proteins on surface of membrane
• Glycolipids (lipids with polysaccharides) found on exterior surface
• Glycoproteins(protein with polysaccharide chains attached) found on the exterior surface
• Cholesterol (sometimes present) bonds to phospholipid hydrophobic fatty acid tails

Question 3

Explain the arrangement of phospholipids in a cell membrane (3)

Answer 3

*Bilayer, with water present on each side
* Hydrophobic fatty acid tails repelled from water so point away from water/to interior
* Hydrophilic phosphate heads attracted to water so point to water

Question 4

Explain the role of cholesterol in cell membranes

Answer 4

• Restricts movement of other molecules making up the membrane
• So it decreases fluidity (and permeability)/ increases rigidity

Question 5

Suggest how cell membranes are adapted for other functions

Glycoproteins/lipids and phospholipid

Answer 5

• Phospholipid bilayer is fluid–>Membrane can bend for vesicle formation/Phagocytosis
• Glycoproteins/glycolipids act as receptors/antigens–> involved in cell signalling/ recognition

Question 6

Describe how movement across membranes occcurs by simple diffusion (4)

Answer 6

• Lipid soluble(non polar) or very small substances e.g. O2, steroid hormones
• Move from an area of higher conc to an area of lower conc, down a conc gradient
• Across phospholipid bilayer
• Passive- doesnt require energy from ATP / respiration (only kinetic energy of substances

Question 7

Explain the limitations imposed by the nature of the phospholipid bilayer

Answer 7

• Restricts movement of water soluble (polar) & larger substances e.g. Na+/glucose
• Due to hydrophobic fatty acid tails in interior of bilayer

Question 8

Describe how movement across membranes occur by facilitated diffusion (4)

Answer 8

• Water soluble (polar/ Slightly larger substances
• Move down a conc gradient
• Through specific channel/carrier proteins
• Passive- doesnt require energy from ATP/respiration (only kinetic energy of substances)

Question 9

Explain the role of carrier and channel proteins in facilitated diffusion (3)

Answer 9

• Shape/charge of protein determines which substances move
• Channel proteins facilitate diffusion of water-soluble substances
  -Hydrophilic pore filled with water
  -May be gated (can open and close)
• Carrier proteins facilitate diffusion of slightly larger substances
  -Complementary substance attaches to binding site
  -Protein changes shape to transport substance

Question 10

Describe how movement across membranes occurs by osmosis (4)

Answer 10

• Water moves
• From an area of high to low water potential/down a water potential gradient
• Through a partially permeable membrane
• Passive- doesnt require energy from ATP/respiration (only kinetic energy of substances)

Question 11

How do the molecules move in simple diffusion

Answer 11

The molecules need to have kinetic energy

Question 12

How do molecules diffuse across a membrane in simpel diffusion

Answer 12

They must be lipid soluble(polar) and small

Question 13

Define facilitated diffusion

Answer 13

Doesnt require ATP but differs from SD as proteins are used to transport molecules.
The movement of ions and polar molecules, which cannot simply diffuse, can be transported across membranes by facilitated diffusion using protein channels and carrier proteins

Question 14

How does facilitated diffusion work

Answer 14

Protein channels form tubes filled with water and this enables water-soluble ions to pass through teh mmebrane
This is still selective as the channel proteins only opne in the presence of certain ions when the bind to the protein
Carrier proteins will bond with a molcule such as glucose which causes a change in the shape of the protein. This shape change enables the molecule to be released to the other side of the membrane

Question 15

What is water potential

Answer 15

The pressure created by water molecules and it is measured in kPa represented with the trident symbol
The more negative the water potential is the more solute must be dissolved in it

Question 16

Define isotonic hypotonic and hypertonic

Answer 16

Iso- Same water potential in solution and within the cell
Hypo-When the water potential of a solution is more positive(closer to 0) than the cell
Hyper- When the water potential of a solution is more negative than the cell

Question 17

What happens if an animal cell and a plant cell were in a hypotonic solution

Answer 17

Water will enter the cell by osmosis causing the animal cell to burst but for a plant cell it will become turgid

Question 18

What happens if an animal cell and a plantcell were placed in a hypertonic solution

Answer 18

Water will leave the cell by osmosis causing them to shrivel

Question 19

WHAT CAN PASS THROUGH BY FACILITATED DIFFUSION?

Answer 19

Larger molecules like glucose or amino acids.
Charged/polar/hydrophilic molecules
Water soluble molecules
Molecules that can’t readily pass through the cell membrane via simple diffusion.

Question 20

WHAT CAN GO THROUGH MEMBRANE BY SIMPLE DIFFUSION AND WHY?

Answer 20

Gases - Oxygen and carbon dioxide diffuse into and out of the cell for respiration.
Hydrophobic molecules - don’t repel from the hydrophobic fatty acid tails
Non-polar/uncharged - don’t repel from the hydrophobic fatty acid tails
Water - small molecule
Lipid-soluble (capable of dissolving in lipids) - dissolve in phospholipid bilayer to move through it
Small molecules - fit between phospholipids

Question 21

FACTORS AFFECTING RATE OF FACILITATED DIFFUSION

Answer 21

Concentration gradient
Greater = faster
Up to a point if all proteins are in use. As equilibrium reaches, rate plateaus.

Number of protein channels/carriers
More = faster
Once all in use, the rate of diffusion cannot increase even if other factors increase.

Question 22

FACTORS AFFECTING RATE OF OSMOSIS

Answer 22

Water potential gradient
Higher gradient = faster
As osmosis takes place, the difference in water potential on either side of the membrane decreases, so the rate of osmosis levels off over time.

Thickness of exchange surface
Thinner = faster
Shorter distance for the water molecules to travel

Surface area of exchange surface
Larger = faster
More area for the water molecules to travel through in a set amount of time.