Transport across membrane

Question 1

Some substances can cross the cell-surface membrane of a cell by simple diffusion through
the phospholipid bilayer. Describe other ways by which substances cross this membrane.

Answer 1

By osmosis (no mark)

1. From a high water potential to a low water potential / down a water potential gradient;
2. Through aquaporins / water channels;

By facilitated diffusion (for large, polar, water-soluble molecules)

3. through channel or carrier protein;
4. Down concentration gradient;

By active transport (no mark)

5. through Carrier proteins
6. Against a concentration gradient;
7. Using ATP / energy (from respiration);

By phagocytosis / endocytosis (no mark)
8. Engulfing by cell surface membrane to form vesicle / vacuole;

By exocytosis
9. Fusion of vesicles (from the Golgi) with cell surface membrane and releasing the contents;

Simple diffusion
10. small, non-polar, lipid soluble molecules
can diffuse directly through phospholipid
bilayer
11. down concentration gradient

Question 2

Describe the processes involved in the absorption of the products of starch
digestion.

Answer 2

Sodium ions are actively transported out
of epithelial cell by sodium-potassium
pump;

Into blood;

So the sodium ion concentration in the cell
is lower than in the lumen of the gut;

So, sodium ions enter the cell by facilitated
diffusion (from the gut)

And glucose is absorbed with Na+ against
its concentration gradient through a
symport protein;

Glucose moves from the epithelial cell into
the blood by facilitated diffusion;

Question 3

Describe the processes involved in the absorption of the products of protein
digestion.

Answer 3

Amino acids moves in with sodium (into epithelial
cell);

Via (carrier/channel) protein/symport;

Sodium removed (from epithelial cell) by active
transport/sodium-potassium pump;

Into blood;

Maintaining low concentration of sodium (in
epithelial cell) / maintaining sodium concentration
gradient (between lumen and epithelial cell);

Amino acid moves into blood;

By (facilitated) diffusion;

Question 4

Describe how substances move across cell-surface

membranes by facilitated diffusion. [3]

Answer 4

Carrier/channel protein;

(Protein) specific/complementary to substance;

Substance moves down concentration gradient;

Question 5

Contrast the processes of facilitated diffusion

and active transport. [3]

Answer 5

Facilitated diffusion involves channel or carrier proteins whereas
active transport only involves carrier proteins;

Facilitated diffusion does not use ATP / is passive whereas active
transport uses ATP;

Facilitated diffusion takes place down a concentration gradient
whereas active transport can occur against a concentration
gradient;

Question 6

Compare and contrast the processes by
which water and inorganic ions enter cells.
[3]

Answer 6

Comparison: both move down concentration gradient;

Comparison: both move through (protein) channels in membrane;

Contrast: ions can move against a concentration gradient by active
transport;

Question 7

Oxygen and chloride ions can diffuse across cell-surface membranes.
The diffusion of chloride ions involves a membrane protein. The
diffusion of oxygen does not involve a membrane protein.
Explain why the diffusion of chloride ions involves a membrane protein
and the diffusion of oxygen does not.

Answer 7

Chloride ions water soluble/charged/polar;

Cannot cross (lipid) bilayer (of membrane);

Chloride ions transported by facilitated diffusion OR diffusion
involving channel/carrier protein;

Oxygen not charged/non-polar;

(Oxygen) soluble in/can diffuse across (lipid) bilayer;