B1.1.13 Ability of non-polar steroids to pass through the phospholipid bilayer Notes

1. Imagine you're a cell in the human body, surrounded by a protective barrier, the phospholipid bilayer of your plasma membrane.
2. This barrier is essential for regulating what enters and exits your domain.
3. However, not all molecules are treated equally. Water-soluble molecules, for instance, often need assistance to cross.
4. In contrast, non-polar steroids glide through effortlessly.

Structure of Steroids and Their Chemical Properties

1. Steroids are a class of lipids defined by their unique molecular structure: four fused carbon rings,
2. consisting of three cyclohexane rings (A, B, and C) and one cyclopentane ring (D).
3. This consistent structural framework makes steroids easily recognizable in molecular diagrams.

Examples of Steroids: Oestradiol and Testosterone

1. Two prominent examples of steroids are estradiol and testosterone, the primary sex hormones in females and males, respectively.
2. Despite their shared structural backbone, slight differences in their functional groups account for their distinct biological roles. For example:

Hydrophobic Nature of Steroids

1. The majority of a steroid molecule consists of hydrocarbon rings, making it predominantly non-polar and hydrophobic.
2. This means steroids do not dissolve readily in water but are highly soluble in non-polar environments, such as the interior of the phospholipid bilayer.
3. While functional groups like hydroxyl (-OH) can introduce slight polarity, the overall molecule remains non-polar.

Structure and Properties of the Phospholipid Bilayer

1. The phospholipid bilayer forms the backbone of all cellular membranes. It is composed of amphipathic phospholipid molecules, each with:
   1. A hydrophilic phosphate head that interacts with water.
   2. Two hydrophobic fatty acid tails that avoid water and interact with each other.
2. When these molecules arrange themselves in water, they form a bilayer.
   1. The hydrophilic heads face outward toward the aqueous environment, while the hydrophobic tails face inward, creating a non-polar interior.
   2. This unique structure creates a selectively permeable barrier.
3. Polar or charged molecules, like glucose or ions, require transport proteins to cross the membrane.
4. However, non-polar molecules, such as steroids, can diffuse directly through the hydrophobic core of the bilayer.

How Non-Polar Steroids Pass Through the Phospholipid Bilayer

1. The ability of non-polar steroids to cross the phospholipid bilayer lies in their chemical compatibility with the hydrophobic interior of the membrane. Let’s break this process down step-by-step:
   1. Diffusion Driven by Concentration Gradient:
      1. Steroids move across the membrane via simple diffusion, a passive process that does not require energy.