Membranes and transport, at a glance

A mobile phospholipid bilayer forms the barrier; proteins, carbohydrate-bearing molecules and cholesterol give it selective and regulatory functions.

• Fluid means lateral mobility; mosaic refers to the varied components.
• Cholesterol regulates fluidity rather than having one identical effect at all temperatures.

Keep in mind: In the cell surface membrane, the carbohydrate chains of glycoproteins and glycolipids project from the extracellular surface.

Membranes control exchange, separate reactions into compartments and provide surfaces on which organised processes occur.

• Surface membranes control exchange and communication.
• Internal membranes compartmentalise reactions and provide organised surfaces.

Keep in mind: They create compartments and reaction surfaces, allowing different processes and gradients to coexist.

Passive transport uses an existing gradient. A membrane protein may be needed without the transport becoming active.

• Equilibrium means no net movement, not no molecular motion.
• Osmosis follows a water-potential gradient.

Keep in mind: Facilitated diffusion uses proteins but is passive. Active transport requires an energy source to drive movement against a gradient.

Active transport couples an energy source to selective movement against a gradient, enabling cells to maintain unequal distributions.

• An ATP-driven pump changes conformation and is reused.
• A fall in active transport does not automatically stop passive transport.

Keep in mind: Active describes energy-coupled movement against a gradient, not its speed.

Endocytosis forms inward vesicles from the surface membrane. Exocytosis fuses internal vesicles with the surface membrane to release contents.

• Endocytosis forms inward vesicles; exocytosis uses fusion.
• Bulk transport needs energy but is not defined by an uphill solute gradient.

Keep in mind: Membranes fuse and reorganise. The cell retains a continuous boundary as contents are released.