membranes

Question 1

Suggest why the model of the membrane is described as a fluid mosaic.

Answer 1

Molecules within the membrane able to move;

mixture of phospholipid and protein / arrangement of protein;

Question 2

Describe the structure of a phospholipid molecule and explain how phospholipids are arranged in a plasma membrane.

Answer 2

1 Phosholipid consists of glycerol;
2 (To which are joined) two fatty acids;
3 And phosphate;
4 By condensation/elimination of water molecules;
5 Arranged as bilayer in membrane;
6 Head/phosphate hydrophilic/polar and tail/fatty acid
hydrophobic/non-polar;
7 Heads outside and tails attracted to each other/inside;

Question 3

Describe the structure of a cell membrane. (5)

Answer 3

Double layer of phospholipid molecules;
Detail of arrangement of phospholipids;
Intrinsic proteins/protein molecules passing right through;
Some with channels/pores;
Extrinsic proteins/proteins only in one layer/on surface;
Molecules can move in membrane/dynamic/membrane contains
cholesterol;
Glycocalyx/carbohydrates attached to lipids/proteins;

Question 4

Describe the part played by cell surface membranes in regulating the movement of substances into and out of cells. (6)

Answer 4

Non-polar/lipid soluble molecules move through phospholipid layer/bilayer;
Small molecules/water/gases move through phospholipid layer/bilayer;
Ions/water soluble substances move through channels in proteins;
Some proteins are gated;
Reference to diffusion;
Carriers identified as proteins;
Carriers associated with facilitated diffusion;
Carriers associated with active transport/transport with ATP/pumps;

Question 5

Explain how three features of a plasma membrane adapt it for its functions.

Answer 5

1. Phospholipid bilayer (as a barrier);
2. Forms a barrier to water soluble / charged substances / allows non-polar substances to pass
   OR
   Maintains a different environment on each side / compartmentalisation;
3. Bilayer is fluid;
4. Can bend to take up different shapes for phagocytosis / form vesicles / self-repair;
5. Channel proteins (through the bilayer)/intrinsic protein;
6. Let water soluble/charged substances through / facilitated diffusion;
7. Carrier proteins (through the bilayer);
8. Allow facilitated diffusion / active transport;
   9 surface proteins / extrinsic proteins, glycoproteins / glycolipids;
   10 cell recognition / act as antigens / receptors;
   11cholesterol;
   12 regulate fluidity / increases stability;

Question 6

Describe how the distribution of cell membranes in a prokaryotic cell such as a bacterium differs from that in a cell from a plant leaf. (4)

Answer 6

Absence of nuclear envelope/membrane;
Membrane bounded organelles;
Such as mitochondria/chloroplast/vacuole/lysosome;
and membrane systems/endoplasmic reticulum/Golgi;
Mesosomes in prokaryotes;