4 - Transport across Cell Membranes

Question 1

functions of the plasma membrane

Answer 1

control the movement of substances in and out of cells and organelles
separate organelles from cytoplasm so that the specific metabolic reactions can occur within them
allow communication between cells and membrane-bound organelles
provide an internal transport system (eg. ER)
isolate enzymes thst might damage the cell
provide surfaces on which reaction can occur

Question 2

fluid mosaic model

Answer 2

phospholipid bilayer allows the membrane to be fluid and flexible, allows proteins to move within it
proteins do not cover whole bilayer but are spread out in mosaic pattern
classified as a model because it is based on experimental and chemical evidence

Question 3

phospholipids (in the bilayer)

Answer 3

form bilayer with hydrophobic tails facing inwards and hydrophilic heads facing outwards
lipid-soluble material moves through the membrane
prevents water-soluble substances entering or leaving
makes the membrane flexible and self-sealing

Question 4

cholesterol (in the plasma membrane)

Answer 4

adds strength to bilayer
very hydrophobic so prevents loss of water and dissolved ions from the cell
reduces lateral movement of membrane

Question 5

glycolipids (in the plasma membrane)

Answer 5

carbohydrate covalently bonded with a lipid
carbohydrate extends from membrane and acts as a cell surface receptor
act as recognition sites
helps cells to attach to one another to form tissues

Question 6

glycoproteins (in the plasma membrane)

Answer 6

carbohydrate bonded to protein

act as cell surface receptors for hormones and neurotransmitters

Question 7

diffusion

Answer 7

the net movement of molecules/ions from an area of their high concentration to an area of their low concentration

Question 8

diffusion through a membrane

Answer 8

the net movement of molecules/ions across a partially permeable membrane, from an area of their high concentration to an area of their low concentration until equilibrium is reached

Question 9

diffusion of molecules/ions across the plasma membrane

Answer 9

small molecules can diffuse across (diffuse most quickly if non-polar)
large polar molecules and ions use facilitated diffusion

Question 10

factors affecting rate of diffusion

Answer 10

steepness of concentration gradient
temperature
surface area (mor eparticles can cross membrane at any one moment)
type of molecules

Question 11

bulk transport

Answer 11

vesicles, vacuoles, and cell membranes have a common membrane
allows vesicles and vacuoles to fuse with cell membrane in order to release substances from the cell to outside (and vice versa)

Question 12

facilitated diffusion

Answer 12

assisted diffusion for large molecules and ions along a concentration gradient 
uses protein channels and carrier proteins
passive process (no ATP required)

Question 13

protein channels

Answer 13

form hydrophobic, water-filled channels across which specific water soluble IONS can diffuse

Question 14

carrier proteins

Answer 14

protein embedded in the bilayer

changes shape as molecule binds

Question 15

osmosis

Answer 15

diffusion of water from a region of higher water potential to a region of lower water potential across a partially permeable membrane

Question 16

high water potential =

Answer 16

high concentration of water molecules / low concentration of solute molecules

Question 17

water potential of pure water

Answer 17

0 kPa (kiloPascals)

Question 18

hypertonic

Answer 18

more concentrated in terms of solute than surroundings (lower water potential)

Question 19

hypotonic

Answer 19

less concentrated in terms of solute than surroundings (higher water potential)

Question 20

isotonic

Answer 20

equal concentration to surroundings

Question 21

active transport

Answer 21

an energy (ATP) requiring process that allows for the transport of substances against a concentration gradient

[ATP hydrolase hydrolyses ATP into ADP and Pi, releases large amount of energy, energy changes shape of carrier protein]

Question 22

co-transport of glucose or amino acids

Answer 22

1. sodium ions are actively transported out of epitehelial cells, by the Na-K pump, into the blood
2. maintains a much higher concentration of sodium ions in the lumen than inside the cell
3. sodium ions therefore diffuse along the concentration gradient into the cell
4. they diffuse through carrier proteins and in the process carry glucose/amino acids into the cell
5. glucose/amino acids pass out the cell by facilitated diffusion (and then active transport to prevent diffusion stopping when equilibrium is reached)