| Date | May 2013 | Marks available | 2 | Reference code | 13M.2.SL.TZ1.2 |
| Level | Standard level | Paper | Paper 2 | Time zone | Time zone 1 |
| Command term | Describe | Question number | 2 | Adapted from | N/A |
Question
The diagram shows how vesicles are used to transport materials in a cell.
State the name of organelle A.
State the process occurring at B.
Describe how the structure of the membrane allows the formation of vesicles.
Explain active transport across membranes.
Markscheme
Golgi apparatus/complex/body
Reject Golgi vesicle and Golgi unqualified.
endocytosis/phagocytosis/pinocytosis
Reject exocytosis.
a. fluidity of membrane allows change of shape/invagination/formation of vesicles;
b. phospholipids can move / phospholipid bilayer makes membrane fluid/flexible;
c. weak bonding between phospholipid tails;
d. bends/kinks in the phospholipid tails prevent close packing;
e. cholesterol affects membrane fluidity;
a. moves substances up/against a concentration gradient / from lower to higher concentration;
b. protein/pump (in membrane) that moves material; (reject channels)
c. ATP is used; (reject energy alone)
d. example/labeled diagram showing mechanism;
Examiners report
Most candidates correctly identified the organelle as Golgi apparatus; otherwise, it was usually mistakenly labelled as rough ER.
Instead of answering “endocytosis”, candidates often stated “exocytosis” and lost the mark.
This follow-up question involved application of knowledge about membrane structure. Sadly, candidates had trouble linking fluidity in membranes to vesicle formation. Though the phospholipid bylayer was sometimes mentioned it was not seen as giving fluidity/flexibility. Weak bonding between the phospholipid tails was rarely included. A few candidates did mention the presence of cholesterol in membranes but not much on their role in membrane fluidity. The idea that bends/kinks in the phospholipid tails prevents close packing, thereby contributing to flexibility, was never given. Some candidates confused fluidity with permeability.
Many candidates gained partial or full marks on their explanations of active transport across membranes. Movement up/against a concentration gradient was often mentioned, along with the necessity of ATP. Energy, by itself, was rejected. There was confusion over protein pumps/carrier proteins and channel proteins. The latter were unacceptable since they are used in passive transport to enable solutes to diffuse down concentration gradients.
