02.09 Phospholipids

Question 1

Describe the structure of a phospholipid molecule. (6 marks)

Mark Scheme:

1. A phospholipid consists of a glycerol backbone. (1 mark)
2. Two fatty acid tails are attached to the glycerol, forming the hydrophobic portion. (1 mark)
3. The third hydroxyl group of glycerol is bound to a phosphate group, forming the hydrophilic head. (1 mark)
4. The hydrophobic tails are non-polar hydrocarbon chains, repelling water. (1 mark)
5. The hydrophilic head, containing the polar phosphate group, attracts water. (1 mark)
6. This dual solubility gives phospholipids unique properties in water and their role in forming membranes. (1 mark)

Question 2

Explain how the structure of phospholipids relates to their behaviour in water. (6 marks)

Mark Scheme:

1. Phospholipids have a hydrophilic head and two hydrophobic tails. (1 mark)
2. In water, the hydrophilic heads are attracted to water molecules. (1 mark)
3. The hydrophobic tails avoid water, clustering together to minimize contact. (1 mark)
4. This leads to the formation of a phospholipid bilayer, with heads facing outward and tails inward. (1 mark)
5. The bilayer arrangement provides a semi-permeable barrier that separates the aqueous environments inside and outside the cell. (1 mark)
6. This structure is fundamental to the selective permeability and function of cell membranes. (1 mark)

Question 3

What is the significance of the dual solubility of phospholipids for cell membrane structure? (5 marks)

Mark Scheme:

1. The dual solubility of phospholipids allows them to form bilayers in aqueous environments. (1 mark)
2. The hydrophilic heads face outward, interacting with water in the extracellular and intracellular environments. (1 mark)
3. The hydrophobic tails face inward, forming a barrier to water-soluble substances. (1 mark)
4. This arrangement creates a semi-permeable membrane, allowing selective transport of molecules. (1 mark)
5. The bilayer structure also provides flexibility and the ability to self-repair, crucial for cell stability and function. (1 mark)

Question 4

Explain the role of phospholipids in the selective permeability of cell membranes. (6 marks)

Mark Scheme:

1. The hydrophobic tails in the phospholipid bilayer form a barrier to polar and water-soluble molecules. (1 mark)
2. This prevents ions and large hydrophilic molecules from freely crossing the membrane. (1 mark)
3. Small non-polar molecules, such as oxygen and carbon dioxide, can diffuse through the bilayer. (1 mark)
4. The hydrophilic heads on the outer surfaces interact with the aqueous environment, stabilizing the membrane. (1 mark)
5. Embedded membrane proteins assist in the transport of substances that cannot diffuse through the hydrophobic core. (1 mark)
6. This selective permeability maintains cellular homeostasis by regulating the internal environment. (1 mark)

Question 5

Compare the structure of phospholipids and triglycerides. (6 marks)

Mark Scheme:

Question 6

How do phospholipids contribute to membrane fluidity and flexibility? (5 marks)

Mark Scheme:

1. Phospholipids in the bilayer can move laterally, allowing flexibility and dynamic membrane structure. (1 mark)
2. The unsaturated fatty acid tails (if present) have kinks that prevent tight packing, enhancing fluidity. (1 mark)
3. Membrane fluidity allows proteins and other components to move within the bilayer, aiding in cell signaling and transport. (1 mark)
4. The bilayer can self-repair when disrupted, as phospholipids spontaneously rearrange to seal gaps. (1 mark)
5. This flexibility is essential for processes like endocytosis, exocytosis, and cell division. (1 mark)

Question 7

Why are phospholipids described as amphipathic molecules, and how is this property biologically important? (6 marks)

Mark Scheme:

1. Phospholipids are amphipathic because they have a hydrophilic head and hydrophobic tails. (1 mark)
2. The hydrophilic head contains a phosphate group that interacts with water. (1 mark)
3. The hydrophobic tails are non-polar and repel water. (1 mark)
4. In aqueous environments, this property allows phospholipids to form bilayers or micelles. (1 mark)
5. The bilayer arrangement is crucial for creating cell membranes, which act as selective barriers. (1 mark)
6. Amphipathic properties also facilitate membrane flexibility and dynamic interactions with proteins and other molecules. (1 mark)

Question 8

Explain the biological importance of phospholipid bilayers in cells. (5 marks)

Mark Scheme:

1. Phospholipid bilayers form the basic structure of cell membranes, providing a semi-permeable barrier. (1 mark)
2. This barrier regulates the movement of substances, maintaining homeostasis. (1 mark)
3. The hydrophilic heads interact with water, stabilizing the membrane in aqueous environments. (1 mark)
4. The hydrophobic core prevents unregulated movement of polar and large molecules, protecting cellular integrity. (1 mark)
5. The bilayer supports membrane proteins and other components involved in transport, signaling, and cell recognition. (1 mark)

Question 9

How does the structure of phospholipids enable self-repair of cell membranes? (5 marks)

Mark Scheme:

1. The amphipathic nature of phospholipids allows them to spontaneously rearrange in aqueous environments. (1 mark)
2. When the bilayer is disrupted, the hydrophobic tails avoid water, driving them back into alignment. (1 mark)
3. The hydrophilic heads orient toward water, resealing the bilayer. (1 mark)
4. This self-repair mechanism maintains the structural integrity of the membrane. (1 mark)
5. It is crucial for cell survival during mechanical damage or dynamic processes like endocytosis and exocytosis. (1 mark)

Question 10

Discuss the roles of phospholipids in cellular membranes and membrane dynamics. (6 marks)

Mark Scheme:

• Phospholipids form the structural framework of cell membranes, creating a bilayer. (1 mark)
• The bilayer provides selective permeability, regulating the entry and exit of substances. (1 mark)
• Phospholipids contribute to fluidity, allowing proteins and other components to move within the membrane. (1 mark)
• Their amphipathic nature supports the embedding of membrane proteins and cholesterol. (1 mark)
• They facilitate endocytosis and exocytosis by enabling membrane flexibility. (1 mark)
• Phospholipids are involved in signal transduction, as some are precursors for signaling molecules like phosphatidylinositol. (1 mark)