Keywords: IB Biology Topic B2.1 Membranes, Fluid Mosaic Model, Phospholipid bilayer, Simple vs Facilitated diffusion, Active transport, Osmosis, Endocytosis and Exocytosis.
Welcome to Topic B2.1: Membranes and Membrane Transport. In the new IB Biology curriculum, the cell membrane is no longer just a 'bag' holding the cell together; it is a dynamic, functional interface. The IBO now places heavy emphasis on the amphipathic nature of phospholipids and the selective permeability that maintains the internal environment. To master this topic, you must move beyond just drawing the 'Fluid Mosaic Model' and start thinking about the physics of how molecules actually cross this oily barrier.
A favorite conceptual question is why phospholipids spontaneously form bilayers. It’s all about the dual nature of the molecule: the 'head' loves water, and the 'tail' hates it.
Take a look at the question below:
The Approach: The term amphipathic is a keyword you must know. Because the environment inside and outside the cell is water-based, the hydrophobic tails hide in the middle while the hydrophilic heads face out. This is a low-energy, stable state that requires no ATP to maintain!
The IB loves to test which molecules can slip through the 'gaps' and which need a 'doorway' (protein channel).
Take a look at the question below:
The Trap: Size matters, but charge matters more. Even though a Sodium ion is small, its charge makes it "hated" by the hydrophobic fatty acid tails in the center of the membrane. Oxygen and CO2 are non-polar and can slip right through (Simple Diffusion), but ions and large polar molecules (like glucose) must use a channel protein (Facilitated Diffusion).
Whenever you see the phrase 'against the concentration gradient,' your brain should immediately scream 'Active Transport' and 'ATP'.
Take a look at the question below:
The Bio-Logic: Think of the membrane as a hill. Diffusion is like rolling a ball down the hill (passive). Active transport is like pushing the ball up the hill. To do that work, you need a fuel source: ATP. This is how cells maintain high concentrations of certain ions (like K+ inside neurons) compared to the outside.
Membrane transport is the foundation for everything from how your brain sends signals to how your kidneys filter blood. If you can categorize a molecule by its polarity and its concentration gradient, the transport method becomes obvious. Keep practicing those 'direction' questions!
Click the black box to reveal the answers!