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.
      2. They travel from areas of high concentration (e.g., blood plasma) to areas of low concentration (e.g., inside the cell).
   2. Interaction with the Bilayer:
      1. Once a steroid approaches the membrane, its hydrophobic nature allows it to dissolve into the lipid bilayer.
      2. Unlike polar molecules, it does not encounter resistance from the hydrophobic fatty acid tails.
   3. Crossing the Membrane:
      1. The steroid molecule traverses the bilayer by diffusing through the non-polar core.
      2. Its small size and non-polarity ensure a rapid and efficient process.
   4. Release into the Cytoplasm:
      1. After crossing the membrane, the steroid enters the cytoplasm, where it can bind to intracellular receptors and exert its biological effects.

Applications and Implications

1. The ability of steroids to pass through the phospholipid bilayer has significant biological and medical implications:
   1. Hormone Signaling:
      1. Steroids like oestradiol and testosterone act as hormones, regulating processes such as reproductive development, metabolism, and stress response.
      2. Their ability to enter cells directly allows them to interact with intracellular receptors and influence gene expression.
   2. Drug Delivery:
      1. The hydrophobic nature of steroids inspires the design of lipid-based drug delivery systems.
      2. For example, synthetic steroids used in medicine, such as prednisone, can easily cross cell membranes to target specific tissues.
   3. Endocrine Disruption:
      1. Environmental pollutants, such as synthetic steroids or steroid-like compounds, can mimic natural hormones and disrupt endocrine signalling.
      2. Their ability to pass through membranes makes them particularly potent.

Identifying Steroids in Molecular Diagrams

1. Being able to recognise steroids in molecular diagrams is a key skill. Look for:
   1. The four-ring structure: three six-carbon rings and one five-carbon ring.
   2. Functional groups like hydroxyl (-OH) or ketone (=O) attached to the rings.
   3. The absence of long hydrocarbon chains, which distinguishes steroids from other lipids like triglycerides.