Methods Used To Transport Molecules Through Cell Membranes

Question 1

What are the 5 methods to move substances in or out of cells?

Answer 1

• diffusion
• facilitated diffusion
• osmosis
• active transport
• endocytosis and exocytosis

Question 2

Describe how diffusion works

Answer 2

• net movement of molecules from high to low conc across a partially permeable membrane
• passive process so doesnt require energy
• stops when molecules have reached equilibrium
• involves non - polar, small and lipid soluble molecules e.g oxygen and CO2

Question 3

What is the equation for ficks law ( rate of diffusion )?

Answer 3

(Surface area x conc gradient) / diffusion distance

Question 4

What are the 4 factors affecting rate of diffusion?

Answer 4

• surface area
• conc gradient
• temperature
• diffusion distance

Question 5

How does temp affect rate of diffusion?

Answer 5

• increased kinetic energt
• faster rate of diffusion

Question 6

How does surface area affect rate of diffusion?

Answer 6

• larger S.A = more space for molecules to pass thru, so faster ROD
• allows more proteins to be present

Question 7

How does conc gradient affect rate of diffusion?

Answer 7

• as conc diff inc, rate of diffusion inc

Question 8

How does diffusion distance affect rate of diffusion?

Answer 8

• the shorter the diffusion difference (thinner membrane)
• the faster molecules will travel thru it

Question 9

Describe how facilitated diffusion works

Answer 9

• diffusion used when proteins help some molecules to pass thru phospholipid bilayer
• hydrophilic substances cant pass thru hydrophobic tails of bilayer
• as fatty acid tails are non - polar so repel polar molecules
• so to enter/exit cell, water soluble molecules need to move thru membrane via channel or carrier proteins
• these are a specific shape so only transport specific molecules complementary to binding site

ALL PROTEINS HAVE BINDING SITES, only enzymes have active sites

Question 10

What do all proteins have in terms of sites?

Answer 10

ALL PROTEINS HAVE BINDING SITES, only enzymes have active sites

Question 11

Explain why facilitated diffusion would level off/plateau at a lower rate?

Answer 11

• carrier/channel proteins binding sites are full or saturated
• causing it to plateau at a lower rate as there are fewer specific carrier proteins for this molecule in membrane
• number of proteins becomes limiting factor

Question 12

Describe how osmosis works

Answer 12

• net movement of water molecules from a low to high concentration of water across a partially permeable membrane
• passive process, doesnt require energy

Question 13

What affect would a lower solute concentration than tissue/organelle have on cells, in terms of osmosis?

Answer 13

Animal cells - swelling + lysis (bursting)
Plant cells - swell, inc mass, (cellulose wall prevents lysis)
- Water moves in by osmosis

Question 14

What affect would a higher solute concentration than tissue/organelle have on cells, in terms of osmosis?

Answer 14

Animal cells - shrivelling
Plant cells - membrane pulls away from cell wall
- water moves out by osmosis, mass is lost

Question 15

What affect would a isotonic (equal) solute concentration than tissue/organelle have on cells, in terms of osmosis?

Answer 15

• no net movement of water in and out of cells, no osmosis
• solute conc inside and outside of cell is at an equilibrium
• no conc gradient = no gain in mass

Question 16

Describe how active transport works

Answer 16

• used to transport molecules across membrane against their conc gradient
• from low to high conc
• requires specifically shaped carrier proteins with a complementary binding site that only complementary molecules bind to
• requires energy supplied by ATP which is produced during respiration, it is an ACTIVE process

Question 17

What are two other forms of active transport?

Answer 17

Exocytosis and endocytosis

Question 18

Describe what happens during exocytosis

Answer 18

• uses golgi vesicles to move large quantities of molecules such as enzymes/nts
• from inside to outside of the cell
• ATP is required to move the vesicles
• vesicles fuse with membrane

Question 19

Describe what happens in endocytosis

Answer 19

• cell surface membrane is pulled inwards to create a vesicle
• any molecules within this area are enclosed within vesicle
• the movement of vesicle into the cell also requires the breakdown of ATP into ADP and Pi