Understanding Cell Membrane Structure and Transport

The fluid mosaic model represents the complex organization of cell membranes, where various proteins are embedded within a phospholipid bilayer. The membrane's flexibility comes from fluid phospholipids that can move relative to each other, while maintaining essential cellular functions.

Definition: The fluid mosaic model describes how phospholipids, proteins, and other molecules arrange themselves in the cell membrane, creating a flexible but stable barrier.

The membrane contains several key components that work together. Intrinsic proteins span the entire membrane while extrinsic proteins are partially embedded. Channel proteins create selective pathways for water-soluble molecules like glucose and ions, while carrier proteins undergo conformational changes to transport specific molecules across the membrane. The glycocalyx, a carbohydrate-rich layer, surrounds the entire membrane structure.

Transport across membranes occurs through multiple mechanisms. Diffusion represents the passive movement of molecules from high to low concentration, affected by factors like concentration gradient, surface area, and temperature. The rate of diffusion can be calculated using the formula: Rate = (Surface Area × Concentration Difference) / Diffusion Distance.

Highlight: Understanding membrane transport is crucial for WJEC Biology A Level studies, particularly in Unit 1 where cell structure and function are key topics.

Protein Translation and Post-Translational Modifications

The translation process, as outlined in WJEC A level Biology knowledge organisers, begins when processed mRNA leaves the nucleus and attaches to ribosomes. Transfer RNA (tRNA) molecules, carrying specific amino acids, dock at the ribosome using their anticodons to match the mRNA codons. This precise matching ensures the correct amino acid sequence in the growing polypeptide chain.

Vocabulary: Polysome - A structure where multiple ribosomes attach to a single mRNA strand, allowing for efficient protein production through parallel translation.

Post-translational modifications play a crucial role in protein maturation, as detailed in WJEC Biology past Papers. After synthesis, polypeptides are transported to the Golgi body, where they undergo complex folding processes to achieve their secondary, tertiary, and quaternary structures. This folding is essential for proper protein function and cellular localization.

The One Gene One Polypeptide hypothesis demonstrates the relationship between genetic information and protein production. This concept, fundamental to modern molecular biology, explains how genetic information flows from DNA to functional proteins through the processes of transcription and translation, forming the basis of cellular function and organism development.