▶️ Answer/Explanation
(a)
The technique is freeze fracture (or freeze etching).
It involves freezing and cracking the cell membrane to view inside layers.
(b)
Using the scale bar (0.1 μm) and measuring it as approximately 1.5 cm (15 mm):
\[
\text{Magnification} = \frac{15 \, \text{mm}}{0.1 \, \mu\text{m}} = \frac{15{,}000 \, \mu\text{m}}{0.1 \, \mu\text{m}} = 150{,}000 \times
\]
(c)
The molecules in Region X are proteins (or integral/transmembrane proteins).
(d)
This image disproves the Davson-Danielli model because it shows proteins inside the membrane, not just on the surface.
The old model showed a protein layer coating both sides of the membrane.
(e)
- To prevent osmosis.
- Wrong osmolarity causes cells to swell, burst, or shrink.
- Hypotonic solution → water enters → cell swells.
- Hypertonic solution → water leaves → cell shrinks.
- This can distort cells and affect results.
Markscheme:
(a) freeze fracture/etching
OR
involves freezing and fracturing cells;
(b) 140000 to 150000 (X);
(c) protein/named membrane protein;
(d) the phospholipid bilayer was not coated by/sandwiched between two protein layers
OR
(transmembrane/integral) proteins are found within the phospholipid bilayer;
(e)
a. to prevent osmosis;
b. cells placed in the incorrect osmolarity might swell/burst/shrink;
c. hypotonic solution would cause water to enter cells/tissues;
d. hypertonic solutions would cause water to leave cells/tissues;
e. water loss would hinder (metabolic) reactions in cell cytoplasm
OR
distort appearance of the cells (for the investigation);
▶️ Answer/Explanation
(a)
Annotation should show:
Hydrophilic (water-loving) head – labeled at the circular parts of phospholipids (facing outward to water).
Hydrophobic (water-fearing) tails – labeled at the wavy lines (facing inward, away from water).
Phospholipids are amphipathic because they have both a hydrophilic region (head) and a hydrophobic region (tails).
(b)
- Cholesterol reduces membrane fluidity and adds stability, especially at high temperatures.
- It also prevents the membrane from becoming too rigid at low temperatures.
- It helps maintain membrane flexibility and integrity.
(c)
- Mitochondria and chloroplasts have their own circular DNA, like prokaryotes.
- They have double membranes and replicate independently by binary fission.
- These features suggest they were once free-living prokaryotes engulfed by a larger cell.
Markscheme:
a
a. line to circle labelled phosphate (head) and (tail) labelled fatty acid/hydrocarbon/lipid (tail);
b. label hydrophilic/polar/attracted to water/ and hydrophobic/non polar/not attracted to water;
b reduces fluidity of membrane / reduces permeability of membrane (to some molecules);
c
a. mitochondria/chloroplasts have their own DNA;
b. mitochondria can self-replicate/undergo a process like binary fission;
c. mitochondria/chloroplasts have double membranes;
d. mitochondria/chloroplasts have (70s) ribosomes;
e. mitochondria/chloroplasts are sensitive to antibiotics;
f. similar in size to bacteria