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RELEVANT LEARNING OUTCOME:
(a) Explain the fluid mosaic model and the roles of the constituent biomolecules (including phospholipids, proteins, glycolipids, glycoproteins and cholesterol) in cell membranes.
💡Fluid Mosaic Model
💡Fluid Mosaic Model
After numerous research done in the early 20th century, the cell membrane was discovered to
be fluid, where the individual components have lateral mobility due to weak intermolecular bonds
have a mosaic appearance, due to the random arrangements of proteins on and among the phosopholipids.
The fluid mosaic model of the cell membrane is often illustrated as follows (a phospholipid bilayer with proteins, cholesterol and carbohydrates associated with it):
📂 Recall
📂 Recall
Before delving further into the structure of the membrane, let's check your understanding about biomolecules.
What are the intramolecular bonds that help maintain the structure of proteins, and which part of the protein is involved in forming these bonds?
Hydrogen bonds, ionic bonds, disulphide bonds, and hydrophobic interactions, formed between the side chains/R-groups of the amino acid residues of the protein.
2. What do the terms 'hydrophobic' and 'hydrophilic' mean?
Hydrophobic = water-hating; Hydrophilic = water-loving
See 'Introduction and properties of Lipids' for explanation.
3. With respect to its interactions with water, what is the nature of cholesterol molecules?
Cholesterol is amphipathic, having both hydrophilic and hydrophobic parts.
See 'Introduction and properties of Lipids' for explanation.
💡Features of the fluid mosaic model
💡Features of the fluid mosaic model
👁️🗨️ Watch the video to learn about the different constituent biomolecules of the phospholipid bilayer and their respective roles.
(Note: In this video, the mosaic nature of the model is due to the presence of different types of molecules. However, for the A-Levels, we will attribute the mosaic pattern to the random arrangement of proteins.)
🖊️Check Your Understanding: Learning Goals Checkpoint
🖊️Check Your Understanding: Learning Goals Checkpoint
How does an integral protein bind to the cell membrane?
By having R-groups/side chain of different properties on the exterior of the protein:
(i) next to the phosphate head: polar/charged R-groups, to form electrostatic interaction/ionic bond/hydrogen bonds with the phosphate head
(ii) next to the fatty acid tails: non-polar R-groups, to form hydrophobic interactions with the tails
2. How does peripheral protein maintain its position in the cell membrane?
By having polar/charged R-groups on the surface in contact with the cell membrane, to form electrostatic interaction/ionic bond/hydrogen bonds with the phosphate head of the phospholipids.
🖊️Check Your Understanding: Lecture Notes Checkpoint
🖊️Check Your Understanding: Lecture Notes Checkpoint
Consider a plant that grows in a temperate region. If you were to compare the compositions of the cell membrane in summer and in winter, how would you expect them to differ?
In summer vs in winter:
Lower proportion vs higher proportion of unsaturated fatty acid chains.
Longer vs shorter fatty acid tails.
Higher vs lower proportion of cholesterol.
💡Membrane fluidity
💡Membrane fluidity
The fluidity of the membrane is critical for the survival and function of a cell, as it allows for
interactions between membrane proteins
growth and self-healing of the cell membrane
cell elongation and cell division
formation of vesicles during endocytosis and secretion of materials during exocytosis
🖊️Check Your Understanding: Conceptual Check
🖊️Check Your Understanding: Conceptual Check
Explain how fluidity of the membrane can be maintained. [4]
1. Presence of unsaturated fatty acid have kinks at sites of C=C double bonds;
2. This prevents fatty acid chains from packing closely together causing the membrane to be more fluid.
3. Shorter fatty acid tails have weaker intermolecular attraction between them.
4. This weaker attraction results in less tightly packed phospholipid molecules, allowing for greater mobility and increased fluidity.
5. Membranes with a lower cholesterol content tend to be more fluid.
6. Cholesterol generally condenses the phospholipid bilayer by filling in the gaps and reduce mobility of phospholipid bilayer, thereby decreasing fluidity of the membrane.
7. Cholesterol regulates membrane fluidity at low and high temperatures.
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