Transport across cell membranes

Question 1

Movements across cell membrane occurs how?

Answer 1

• simple diffusion
• facilitated diffusion
• osmosis
• active transport
• co-transport

Question 2

What is diffusion?

Answer 2

• Net movement of particles down conc gradient

- random movement, particles possess KE & passive process

Question 3

Properties of molecules that can pass through phospholipid bilayer by simple diffusion?

Answer 3

• lipid soluble
• small structures
• non charged (e.g. no ions)
• non polar

Question 4

Fick’s law

Answer 4

Rate of diffusion propertional to Conc grad X SA / DD

Question 5

What’s microvilli?

Answer 5

tiny hair projections that increase SA and therefore efficient exchange

Question 6

What’s facilitated diffusion?

Answer 6

• transport of substance across biological membrane down conc gradient by transport protein ( carrier or channel)

Question 7

What kind of process is facilitated diffusion?

Answer 7

passive

Question 8

Why are transport proteins needed?

Answer 8

• ions & polar molecules diffuse slowly

- transport proteins speed things up

Question 9

Describe how carrier proteins work?

Answer 9

• have binding sites present where molecules to be transported bind
• proteins undergo CONFORMATIONAL CHANGE
• eventually, opening up on other side of cell membrane, solute released to other side of membrane

Question 10

Describe how channel proteins work?

Answer 10

• interact weakly with material to be transported
• provides a hydrophilic passageway
• if open, specific solutes can freely be transported & pass through them

Question 11

Factors affecting facilitated diffusion?

Answer 11

• conc gradient

- Number of transport proteins! (more in use, faster rate of diffusion)

Question 12

Explain why the phospholipid bilayer is described as the ‘fluid mosaic’ model

Answer 12

FLUID: individual phospholipid molecules can move relative to each other. Gives membrane a flexible structure that is constantly changing shape

MOSAIC: proteins embedded in phospholipid bilayer vary in shape, size & pattern the same way stones & tiles do in a mosaic

Question 13

What is osmosis?

Answer 13

• movement of water molecules down water potential gradient through selectively permeable membrane

Question 14

Water potential measured in what?

Answer 14

kPa

Question 15

Pure water has a water potential of what?

Answer 15

0

Question 16

What is water potential?

Answer 16

• tendency of water to diffuse to another area by osmosis

Question 17

What’s a hypertonic solution?

Answer 17

• higher conc of solutes outside of cell

Question 18

What’s an isotonic solution?

Answer 18

• equal conc of solutes

Question 19

What’s a hypotonic solution?

Answer 19

• lower conc of solutes outside cell

Question 20

Rate of osmosis depends on what?

Answer 20

• water potential gradient
• thickness of exchange surface
• SA of exchange surface

Question 21

What is active transport?

Answer 21

• movement of molecules or ions in & out of cell against conc gradient using ATP & carrier proteins

Question 22

compare facilitated diffusion & active transport

Answer 22

1 active transport requires ATP but FD is passive
2 active transport moves up conc gradient, FD moves down
3 FD uses carrier & channel proteins but active transport only uses carrier proteins

Question 23

Describe the process of direct active process

Answer 23

1 molecules bind to receptor sites on protein, on inside cell ATP binds to protein, causing it to split into ADP and a phosphate molecule
2 as a result, protein changes shape & opens to other side of membrane
3 molecules released to other side of membrane
4 phosphate molecule released from protein which causes protein to revert back to original shape
5 phosphate molecule recombines with ADP to form ATP during respiration

Question 24

What’s co-transport?

Answer 24

• type of transport across cell membrane that requires transport proteins embedded into membrane of cell

Question 25

Describe co-transport process

Answer 25

1 Na+/K+ pump uses active tramsport to move K+ ions into epithelial cells & Na+ (sodium ions) out of cell
2 establishes conc gradient between epithelial cell & ileum 
3 sodium ions move into epithelial cell by facilitated diffusion (down conc gradient)
4 glucose (or AA) is co-transported along w/ sodium into cell
5 glucose (or AA) pass into blood plasma by facilitated diffusiom

Question 26

Describe the phospholipid bilayer

Answer 26

two layers of phospholipids with hydrophillic heads & hydrophobic tails

Question 27

Structure of plasma membrane

Answer 27

• fluid mosaic model

- made from phospholipids, cholesterol (provides rigid structure), proteins and carbohydrates

Question 28

Many different substances enter and leave a cell by crossing its cell surface membrane. Describe how substances can cross a cell surface membrane [5 Marks]

Answer 28

• simple/facilitated diffusion down conc gradient
• small/non-polar/lipid soluble molcules pass via phospholipid bilayer
• water moves by osmosis
• active transport against conc gradient
• active site and FD involves carrier proteins
• active transport requires ATP
• ref. to Na+/glucose co-transport