1. 2.4: MEMBRANES

3. IB Question: Draw a labelled diagram showing the fluid-mosaic model of a biological membrane. [5]

4. IB Question: Draw a labelled diagram showing the fluid-mosaic model of a biological membrane. [5]
Award [1] for each feature clearly drawn and correctly labelled.
(phospho)lipid bilayer;
hydrophobic tails and hydrophilic heads;
integral protein;
peripheral protein;
cholesterol; (any shape different from lipid bilayer representation)
glycoprotein;
indication of size; [5 max]

5. 2.4.1: Membrane Structure

6. “water loving” Non-polar tail “water fearing”
2.2.4: Explain how the hydrophobic and hydrophilic properties of phospholipids help to maintain the structure of membranes.
Phospholipids: Fundamental unit of membranes
Phospholipid structure.
Polar head
“water loving”
Non-polar tail
“water fearing”

7. Head made from choline, phosphate and glycerate
Tail made from two fatty acids (one saturated and one unsaturated)
fatty acid tails

8. micelle

9. Arrangement of phospholipids in membranes
Phospholipids form a bilayer, a phospholipid bilayer
heads face outside the membrane (attracted to water) hydrophilic
tails face inside the membrane (repelled by water) hydrophobic
water
water

10. phospholipids held together by hydrophobic interactions;
phospholipids layers are stabilized by interaction of hydrophilic heads and surrounding water
liposome

11. phospholipids allow for membrane fluidity / flexibility which is important in endocytosis and exocytosis.
endocytosis
exocytosis

13. phospholipids can move about / move horizontally / “flip flop” to increase fluidity;

14. 2.2.4: Explain how the hydrophobic and hydrophilic properties of phospholipids help to maintain the structure of membranes. M2003
IB question.
Explain how the structure and properties of phospholipids help to maintain the structure of cell membranes. [9]
Actual IB mark scheme
phospholipid structure
hydrophobic tail / hydrophilic head;
head made from glycerol and phosphate;
tail made from two fatty acids;
saturated / unsaturated fatty acid (in tail);
arrangement in membrane
phospholipids form a bilayer;
heads face outside the membrane / tails face inside the membrane / hydrophobic
interior / hydrophilic exterior of membrane;
A suitable annotated diagram may incorporate all or many of the above points.
Award [5 max] for a suitable diagram that is labeled correctly.
phospholipids held together by hydrophobic interactions;
phospholipids layers are stabilized by interaction of hydrophilic heads and surrounding water;
phospholipids allow for membrane fluidity / flexibility;
fluidity / flexibility helps membranes to be (functionally) stable;
phospholipids with short fatty acids / unsaturated fatty acids are more fluid;
fluidity is important in breaking and remaking membranes (e.g. endocytosis /
exocytosis);
phospholipids can move about / move horizontally / “flip flop” to increase fluidity;
hydrophilic / hydrophobic layers restrict entry / exit of substances; [9 max]

15. IB question.
Explain how the structure and properties of phospholipids help to maintain the structure of cell membranes. [9]
phospholipid structure
hydrophobic tail / hydrophilic head;
head made from glycerol and phosphate;
tail made from two fatty acids;
saturated / unsaturated fatty acid (in tail);
arrangement in membrane
phospholipids form a bilayer;
heads face outside the membrane / tails face inside the membrane / hydrophobic
interior / hydrophilic exterior of membrane;
A suitable annotated diagram may incorporate all or many of the above points.
Award [5 max] for a suitable diagram that is labeled correctly.
phospholipids held together by hydrophobic interactions;
phospholipids layers are stabilized by interaction of hydrophilic heads and surrounding water;
phospholipids allow for membrane fluidity / flexibility;
fluidity / flexibility helps membranes to be (functionally) stable;
phospholipids with short fatty acids / unsaturated fatty acids are more fluid;
fluidity is important in breaking and remaking membranes (e.g. endocytosis /
exocytosis);
phospholipids can move about / move horizontally / “flip flop” to increase fluidity;
hydrophilic / hydrophobic layers restrict entry / exit of substances; [9 max]

16. IB Question: Explain how the hydrophilic and hydrophobic properties of phospholipids help to maintain the structure of cell membranes. [6]

17. IB Question: Explain how the hydrophilic and hydrophobic properties of phospholipids help to maintain the structure of cell membranes. [6]
membranes are surrounded by water;
hydrophilic molecules are attracted to water;
hydrophobic molecules are attracted to one another/repel water;
phospholipids are amphipathic/have a hydrophobic tail and a hydrophilic head;
tails are positioned away from water / heads are positioned towards water;
phospholipids have a hydrocarbon tail and a phosphate head;
phospholipid bilayer/membranes self-assemble in water;
protein association with membrane is determined by hydrophobic interactions;
phospholipid bilayer is hydrophilic on the outside and hydrophobic on the inside; [6 max]