IB DP Biology Membranes and membrane transport Study Notes
IB DP Biology Membranes and membrane transport Study Notes
IB DP Biology Membranes and membrane transport Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on IB Biology syllabus with guiding questions of
- How do molecules of lipid and protein assemble into biological membranes?
- What determines whether a substance can pass through a biological membrane?
Standard level and higher level: 4 hours
Additional higher level: 1 hour
B2.1.1—Lipid bilayers as the basis of cell membranes
- The Foundation of Life: Cell membranes are not just barriers; they are dynamic structures that enable life. The plasma membrane defines the cell’s boundaries, while internal membranes create specialized compartments within eukaryotic cells.
- A Universal Structure: The basic structure of all biological membranes is remarkably consistent: a fluid bilayer of phospholipids. This unified design emphasizes the fundamental importance of this structure for cellular life.
- A Dynamic Barrier: Despite being incredibly thin, the lipid bilayer acts as a selective barrier, controlling the passage of substances into and out of the cell. This selective permeability is essential for maintaining cellular homeostasis.
- A Fluid Mosaic: The term “fluid mosaic” perfectly captures the nature of the membrane. It’s a mosaic of lipids, proteins, and carbohydrates, all moving and interacting within a fluid environment. This dynamic nature is crucial for membrane function.
In essence, the lipid bilayer is not just a passive barrier; it’s a dynamic and intricate structure that plays a vital role in cellular life, enabling communication, transport, and maintaining cellular homeostasis.
B2.1.2- Lipid bilayers as barriers
The Hydrophobic Core: A Selective Barrier
- Amphipathic Nature: Phospholipids are amphipathic, meaning they have both hydrophilic (water-loving) and hydrophobic (water-hating) regions. The hydrophilic phosphate heads interact with the aqueous environment, while the hydrophobic hydrocarbon tails cluster together, forming the core of the membrane.
- Barrier to Hydrophilic Molecules: This hydrophobic core creates a barrier for hydrophilic molecules like ions and polar molecules (e.g., glucose). These molecules are more attracted to the aqueous environment on either side of the membrane than to the hydrophobic interior.
- Size Matters: The size of the molecule also influences membrane permeability. Smaller molecules like water can pass through more easily than larger molecules like proteins.
In essence, the lipid bilayer acts as a selective barrier due to its hydrophobic core. This core restricts the passage of hydrophilic molecules, creating a controlled environment within the cell.
Note: While the hydrophobic core repels hydrophilic molecules, it doesn’t actively repel them. The molecules are simply more attracted to the aqueous environment.