Fluid mosaic membranes

Every cell membrane is built around a phospholipid bilayer: two layers of phospholipid molecules arranged tail-to-tail. Each phospholipid molecule has two regions with opposite affinities for water:

• a hydrophilic head containing the phosphate group, which is polar and attracted to water;
• two hydrophobic fatty-acid tails, which are non-polar and repelled by water.

A molecule with both polar and non-polar regions is called amphipathic, and this is the property that drives bilayer formation.

When phospholipids are placed in water they spontaneously arrange themselves so that the hydrophilic heads face outwards into the aqueous environment on both sides of the bilayer (cytoplasm and extracellular fluid), while the hydrophobic tails face inwards, away from water. This is the lowest-energy configuration — it minimises the unfavourable interactions between water and the non-polar tails (the hydrophobic effect). No energy input is required.

The resulting bilayer is about 7-10 nm thick and behaves as a two-dimensional fluid: molecules can move sideways within their layer, and small breaks in the membrane reseal spontaneously. The bilayer is selectively permeable: small non-polar molecules ($\text{O}_2$, $\text{CO}_2$, steroids) diffuse through easily; polar / charged molecules (ions, glucose) cannot cross and require transport proteins.

Membrane proteins are responsible for most of the membrane's functions, and they are classified by their position in the bilayer.

Integral (intrinsic) proteins are embedded within the bilayer. The portions of the protein in contact with the fatty-acid tails are made mostly of hydrophobic amino acids. Many integral proteins span the bilayer completely (transmembrane proteins) and act as transport routes or signal receptors. Their main types are:

• Channel proteins — form hydrophilic pores through which specific ions or small polar molecules diffuse passively down their gradient. Many are gated (open in response to voltage, ligand or stretch).
• Carrier proteins — bind a specific molecule (e.g. glucose, amino acid), change shape, and release it on the other side. Used in facilitated diffusion (no ATP) and in active transport (ATP required).
• Receptor proteins — bind extracellular signalling molecules (hormones, neurotransmitters, drugs) and trigger a response inside the cell. Often glycoproteins.
• Enzymes — catalyse reactions at the membrane surface (e.g. ATP synthase in mitochondria).

Peripheral (extrinsic) proteins are attached only to the surface of the bilayer (most often on the cytoplasmic side) by binding to the polar heads of phospholipids or to integral proteins. They include structural proteins that link the membrane to the cytoskeleton (e.g. spectrin on the inside of red blood cells, which gives the cell its biconcave shape).

This diversity of protein function is what turns the membrane from a simple barrier into the cell's active interface with its environment.

Cholesterol is a small steroid lipid found in animal cell-surface membranes (typically about 20-25% of lipid molecules). Its hydroxyl group hydrogen-bonds with the phosphate heads, while its flat, rigid steroid ring projects into the hydrophobic core, sitting between the fatty-acid tails of neighbouring phospholipids.

Cholesterol's main role is to regulate membrane fluidity across a range of temperatures:

• At high temperatures, cholesterol restricts the movement of the fatty-acid tails. This makes the membrane less fluid than it would otherwise be and prevents it becoming too leaky.
• At low temperatures, cholesterol prevents the fatty-acid tails packing tightly together. This maintains fluidity and prevents the membrane from solidifying / crystallising.

In effect, cholesterol is a buffer against temperature change — it keeps fluidity within a workable range whether the cell is warmer or cooler than usual.

In addition to regulating fluidity, cholesterol:

• Reduces the permeability of the bilayer to small polar molecules (water, ions) — making the barrier function more effective.
• Provides mechanical strength, especially in cells without a cell wall (e.g. red blood cells).
• Organises membrane micro-domains known as lipid rafts, which concentrate certain signalling proteins.

Plant cell membranes lack cholesterol but use related plant sterols (e.g. stigmasterol) that play similar roles.

Many membrane components have short carbohydrate chains (oligosaccharides) attached. These projections always face outwards from the outer surface of the cell — never on the cytoplasmic side. They include:

• Glycoproteins — membrane proteins with carbohydrate chains attached. Most receptor proteins are glycoproteins.
• Glycolipids — phospholipids with a carbohydrate chain attached.

These carbohydrate markers have several important functions:

1. Cell-to-cell recognition. Cells of the same organism display the same surface markers and recognise each other when they meet. Foreign cells (bacteria, virus-infected cells, transplanted tissue) display different markers and are recognised as 'non-self'. The ABO blood group antigens are glycolipid markers on red blood cells; the MHC (HLA in humans) glycoproteins are the markers used by the immune system to distinguish self from non-self.
2. Receptor sites for hormones, neurotransmitters and drugs. A signalling molecule binds the glycoprotein and triggers a response inside the cell (e.g. insulin binds insulin-receptor glycoproteins; adrenaline binds adrenergic receptors).
3. Cell adhesion. Carbohydrate-carbohydrate and carbohydrate-protein interactions help cells stick together to form tissues.
4. Antigens. The immune system uses surface carbohydrates to identify pathogens; antibodies bind specifically to these antigens.

The total carbohydrate coat on the outside of the cell is sometimes called the glycocalyx.

Before 1972, biologists viewed the membrane as a rigid sandwich of phospholipids between two layers of protein (the Davson-Danielli model). New data — including freeze-fracture electron microscopy that showed proteins inside the bilayer rather than coating it — overturned this picture.

The Singer-Nicolson fluid mosaic model (1972) describes the cell-surface membrane as follows:

• Fluid. The bilayer behaves as a two-dimensional liquid. Individual phospholipid molecules can diffuse laterally within their leaflet, rotate around their long axis, and flex their tails. Proteins also drift sideways within the bilayer (though more slowly than the lipids). Small breaks self-seal because the components rearrange to exclude water.
• Mosaic. Proteins of many different shapes, sizes and functions are scattered through the bilayer, like coloured tiles set into a sea of phospholipid. Integral proteins are embedded in (or span) the bilayer; peripheral proteins sit on the surfaces. The arrangement is asymmetric — the two leaflets of the bilayer have different lipid and protein composition, and carbohydrate chains are only on the outer face.

Other features of the model:

• Cholesterol sits between phospholipids in animal membranes, modulating fluidity.
• Glycoproteins and glycolipids project outwards for cell recognition.
• Components are held together by hydrophobic interactions rather than covalent bonds — this is what allows the membrane to behave as a fluid.

The fluid mosaic model is the currently accepted picture of every biological membrane, including the cell-surface membrane and the membranes of organelles.

Because the bilayer is held together by hydrophobic and other non-covalent interactions, it is sensitive to temperature.

Low temperatures. At low T, the phospholipid tails have less kinetic energy and pack more tightly. The membrane becomes less fluid (more 'gel-like') and transport proteins work more slowly. If the membrane were to solidify completely, transport across it would stop — but this is normally prevented by cholesterol and by membrane composition (e.g. cold-adapted organisms have more unsaturated fatty acids in their phospholipids).

High temperatures. Increasing T raises the kinetic energy of all the molecules in the membrane. Two effects on permeability appear:

1. The phospholipids vibrate more strongly, the bilayer expands, and gaps appear between phospholipids. Small polar molecules can leak through these gaps.
2. Above ~50-60 °C, the proteins denature — hydrogen and ionic bonds in their tertiary structure break and the proteins lose their shape. Channel and carrier proteins no longer function as selective transporters; some membrane proteins aggregate and form non-specific leaks.

The combined result is a steep rise in membrane permeability at high temperatures, often visible as the leakage of coloured intracellular contents.

The beetroot practical. Beetroot vacuoles contain a water-soluble red pigment called betalain. In intact cells, betalain is contained by the tonoplast and the cell-surface membrane. When standardised beetroot discs are exposed to a range of temperatures, the amount of pigment that leaks out into the surrounding water can be measured with a colorimeter (use a blue-green filter ~470-530 nm, complementary to red). The graph of absorbance vs temperature typically shows little change up to ~40 °C, then a sharp rise as gaps form and proteins denature.

Controlled variables: disc size and number, volume of water, time in water bath, age of beetroot. Repeats: at least 3 per temperature, then take the mean.

Membrane permeability is also affected by other factors that are commonly investigated in the laboratory.

Organic solvents (e.g. ethanol, methanol, acetone) dissolve membrane lipids. As ethanol concentration in the surrounding water increases, more of the phospholipid bilayer is disrupted and permeability rises sharply above a threshold (~30-40% ethanol). This is the basis of a related beetroot practical that mirrors the temperature one.

pH extremes denature membrane proteins. Hydrogen and ionic bonds in the tertiary structure of channel and carrier proteins are pH-sensitive; large changes in pH disrupt these bonds, the active site / pore loses its shape and selective transport fails. Pigment leakage rises at both very low and very high pH compared with the cell's normal range.

Detergents are amphipathic molecules that compete with phospholipids in the bilayer. At sufficient concentration they dissolve the membrane completely — a property used in biochemistry to extract membrane proteins.

In all cases the mechanism is the same: disruption of the bilayer or denaturation of proteins. You can apply the same logic of 'why does pigment leak?' to a practical involving any of these factors.