The Plasma Membrane
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The plasma membrane is a thin (~5 nm thick), semi-fluid
barrier that surrounds every cell.
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It separates the cell's interior from its external
environment, controls molecular trafficking, stores electrical charge (acts as
a capacitor with a negative interior and positive exterior), and can
self-repair through spontaneous rearrangement of its lipid bilayer.
Fluid Mosaic Model (Singer & Nicolson, 1972)
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The currently accepted model describes the membrane as
a fluid mosaic — a phospholipid bilayer in which proteins are embedded or
attached, and all components can move laterally. Key features:
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Phospholipid bilayer with hydrophilic heads facing
outward and hydrophobic tails facing inward.
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Integral proteins span the bilayer; peripheral proteins
attach to one surface; lipid-anchored proteins connect via covalent lipid
groups.
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Carbohydrates on the outer surface form glycoproteins
and glycolipids → involved in cell recognition and communication.
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Lipids and proteins are free to move laterally (diffuse
~2 μm/sec) but rarely flip-flop across leaflets.
Figure 1.1 — The fluid mosaic model of
the plasma membrane.
Selective Permeability
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The pure lipid bilayer is freely permeable to small
non-polar molecules (O2, CO2, N2, diethylurea) and slightly permeable to small
uncharged polar molecules (water, urea, glycerol). It is essentially
impermeable to ions (Na+, K+, Cl-) and large polar molecules (glucose, amino
acids) — these require specific transport proteins.
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Order of permeability through a pure bilayer: O2 >
Diethylurea > Urea > Water > Glucose > Na+
Facts to remember
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The membrane acts as a capacitor: ΔV ≈ −60 mV across ~7
nm → Electric field E ≈ 8.6 × 10⁶ V/m (enormous).
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Lipid molecules exchange places with neighbours ~10⁷
times/sec (lateral diffusion), but flip-flop is rare without enzyme help.
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Self-sealing is driven by the amphipathic character of
lipids and the hydrophobic effect — any tear is energetically unfavorable.