The plasma membrane and transport

Cells come in two kinds. In prokaryotic cells (bacteria), the DNA lies loose in the cytoplasm, with no organised nucleus, as if its parents were not strict. In eukaryotic cells, the DNA is kept inside a proper nucleus, and there are many membrane-bound organelles. Both kinds have a plasma membrane, cytoplasm, ribosomes and DNA.

The plasma membrane is like the wall of a house: it surrounds everything inside the cell and decides what comes in and goes out. Gorter and Grendel (1925), studying red blood cells, showed it is made of a double layer (bilayer) of lipids. The accepted picture is Singer and Nicolson's fluid mosaic model (1972): a lipid bilayer with proteins scattered in it like the pieces of a mosaic. Phospholipids have water-loving heads facing outwards, towards the watery surroundings, and water-fearing tails facing inwards. The bilayer is semi-fluid, so lipids and proteins can drift sideways.

Membrane proteins are of two kinds. Peripheral proteins sit on the surface, often for signalling; some act like watchmen, telling the cell's own molecules from foreign ones. Integral proteins are partly or fully buried in the membrane; the commonest are transmembrane proteins that cross it completely.

How substances cross:

• Passive transport: moving down the concentration gradient (from more to less), without energy: diffusion, and osmosis for water.
• Facilitated transport: still downhill and without energy, but helped by proteins: carriers (like the glucose transporter) or channels (like aquaporins for water, and ion channels).
• Active transport: moving against the gradient (from less to more), which needs energy. The Na⁺-K⁺ pump, driven by ATP, is an example.

Some carriers move two substances at once (coupled transport): in the same direction it is a symport (like glucose taken in together with Na⁺); in opposite directions, an antiport. A carrier moving just one substance is a uniport.