transport across membranes

Question 1

describe fluid mosaic model of membranes

Answer 1

fluid: phospholipid bilayer in which individual phospholipids can move = flexible shape

mosaic: extrinsic and intrinsic proteins of different sizes and shapes are embedded

Question 2

role of cholesterol in membranes

Answer 2

steroid molecule in some plasma membranes; connects phospholipids and reduces fluidity to make bilayer more stable

Question 3

role of glycolipids in membranes

Answer 3

cell signalling and cell recognition

Question 4

explain the functions of extrinsic proteins in membranes

Answer 4

• binding sites/receptors e.g. for hormones
• antigens (glycoproteins)
• bind cells together
• involved in cell signalling

Question 5

explain the function of intrinsic proteins

Answer 5

• electron carriers (respiration/photosynthesis)
• channel proteins (facilitated diffusion)
• carrier proteins (facilitated diffusion/active transport)

Question 6

explain the function of membranes within cells

Answer 6

• provide internal transport system
• selectively permeable to regulate passage of molecules into/out of organelles
• provide reaction surface
• isolate organelles from cytoplasm for specific metabolic reactions

Question 7

explain the functions of the cell-surface membrane

Answer 7

• isolate cytoplasm from extracellular environment
• selectively permeable to regulate transport of substances
• involved in cell signalling/cell recognition

Question 8

name and explain 3 factors that affect membrane permeability

Answer 8

• temperature: high temperature denatures membrane proteins / phospholipid molecules have more kinetic energy and move further apart
• pH: changes tertiary structure of membrane proteins
• use of a solvent: may dissolve membrane

Question 9

outline how colorimetry could be used to investigate membrane permeability

Answer 9

1. use plant tissue with soluble pigment in vacuole. tonoplast and cell-surface membrane disrupted = increased permeability = pigment diffuses into solution
2. select colorimeter filter with complimentary colour
3. use distilled water to set colorimeter to 0. measure absorbance/ % transmission value of solution
4. high absorbance/low transmission = more pigment in solution

Question 10

define osmosis

Answer 10

water diffuses across semi-permeable membranes from an area of higher water potential to an area of lower water potential until dynamic equilibrium is established

Question 11

what is water potential

Answer 11

• pressure created by water molecules measured in kPa
• water potential of pure water at 25 degrees celsius and 100 kPa = 0
• more solute = more negative water potential

Question 12

osmosis INTO cell affect on plant v animal cells

Answer 12

plant = protoplast swells , cell turgid
animal = lysis

Question 13

osmosis OUT of cell affect plant v animal cells

Answer 13

plant = protoplast shrinks, cell flaccid
animal = crenation

Question 14

define simple diffusion

Answer 14

passive process
net movement of small, lipid-soluble molecules directly through the bilayer from an area of high concentration to an area of lower concentration

Question 15

define facilitated diffusion

Answer 15

passive process
specific channel or carrier proteins with complementary binding sites transport large and/or polar molecules/ions down concentration gradient

Question 16

how do channel proteins work

Answer 16

hydrophilic channels bind to specific ions = one side of the protein closes and other opens

Question 17

how do carrier proteins work

Answer 17

binds to complementary molecule = conformational change which releases molecules on other side of membrane

in facilitated diffusion = passive process

in active transport = requires energy from ATP hydrolysis

Question 18

5 factors that affect rate of diffusion

Answer 18

temperature
diffusion distance
surface area
size of molecule
difference in concentration (how steep concentration gradient is)

Question 19

what is Fick’s law

Answer 19

(surface area x difference in concentration) / diffusion distance

Question 20

how are cells adapted to maximise the rate of transport across their membranes

Answer 20

• many carrier/channel proteins
• folded membrane increases surface area

Question 21

define active transport

Answer 21

active process
ATP hydrolysis releases phosphate group that binds to carrier protein, causing it to change shape

specific carrier protein transports molecules/ions from area of low concentration to area of higher concentration

Question 22

compare and contrast active transport and facilitated diffusion

Answer 22

• both may involve carrier proteins
• active transport requires energy from ATP
• facilitated diffusion may also involve channel proteins

Question 23

define co-transport

Answer 23

movement of a substance against its concentration gradient is coupled with the movement of another substance down its concentration/electrochemical gradient

substances bind to complementary intrinsic protein (symport or antiport)

Question 24

what direction does a symport intrinsic protein move substance in

Answer 24

same direction

Question 25

what direction does an antiport intrinsic protein move in

Answer 25

opposite direction

Question 26

explain how co-transport is involved in the absorption of glucose/amino acids in the small intestine

Answer 26

1) Na+ actively transported out of epithelial cells and into bloodstream
2) Na+ concentration lower in epithelial cells than lumen of gut
3) transport of glucose/amino acids from lumen to epithelial cells is ‘coupled’ to facilitated diffusion of Na+ down electrochemical gradient