A membrane is held together primarily by

Options

A. hydrophobic attractions
B. hydrophilic attractions
C. covalent bonds
D. ionic bonds

Answer

### Step-by-Step Solution:

1. **Understanding Membrane Structure**:
- Biological membranes are primarily composed of a phospholipid bilayer. Each phospholipid molecule has a hydrophilic (water-attracting) "head" and two hydrophobic (water-repelling) "tails".

2. **Role of Hydrophobic Attractions**:
- The hydrophobic tails of the phospholipids face inward, away from water, while the hydrophilic heads face outward, towards the water. This arrangement is crucial for the stability of the membrane.

3. **Segregation of Polar and Non-Polar Substances**:
- The hydrophobic attractions lead to the segregation of polar (hydrophilic) and non-polar (hydrophobic) substances. This segregation minimizes the contact between the polar and non-polar regions, which stabilizes the membrane structure.

4. **Maximizing Hydrogen Bonding**:
- The arrangement of the phospholipids maximizes hydrogen bonding between water molecules outside the membrane, further stabilizing the membrane structure.

5. **Conclusion**:
- The primary force holding the membrane together is the hydrophobic attractions between the hydrophobic tails of the phospholipids. This is the correct answer to the question.

### Final Answer:
A membrane is held together primarily by **hydrophobic attractions**.

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