AS Biology 9700: Membrane Structure. Phospholipids, Proteins and Glycoproteins

A student is studying a section of a cell-surface membrane. It contains: a phospholipid bilayer; a protein that spans the full width of the bilayer and forms a water-filled channel; a short, branching carbohydrate chain attached to a protein on the outer surface of the membrane; and a second protein that is attached only to the inner (cytoplasmic) surface of the membrane and does not cross the bilayer.

(a) Describe the arrangement of the phospholipid molecules within the bilayer, and explain how the structure of a phospholipid produces this arrangement. [2]

(b) Distinguish between an intrinsic (integral) membrane protein and an extrinsic (peripheral) membrane protein, referring to the channel protein and to the protein attached only to the inner surface in your answer. [2]

(c) The carbohydrate chain attached to the protein on the outer surface forms a glycoprotein. Explain two roles that glycoproteins carry out at the cell surface. [2]

Part (a): Arrangement of the phospholipid bilayer

Each phospholipid molecule has a hydrophilic phosphate head and two hydrophobic fatty acid tails, making it amphipathic. In water, phospholipids automatically arrange into a bilayer: the phosphate heads on both surfaces of the membrane face outward into the watery environments (the tissue fluid outside the cell and the cytoplasm inside), while the fatty acid tails point inward, away from water, and pack together in the centre of the membrane. This arrangement minimises contact between the hydrophobic tails and water, which is why it forms spontaneously and is energetically stable.

Part (b): Intrinsic versus extrinsic proteins

The channel protein spans the entire width of the bilayer, so it is an intrinsic (integral) protein. Its hydrophobic surface regions sit alongside the fatty acid tails of the phospholipids, anchoring it firmly within the membrane, while a hydrophilic channel runs through its centre, providing a route for polar substances to cross the hydrophobic core.

The second protein, attached only to the inner surface, is an extrinsic (peripheral) protein. It does not enter the hydrophobic core of the bilayer at all. Instead, it is held in place by weaker interactions with the phosphate heads of the phospholipids or with an intrinsic protein, on one face of the membrane only. Because it is not embedded within the bilayer, an extrinsic protein is generally easier to detach from the membrane than an intrinsic one.

Part (c): Roles of glycoproteins

Cell signalling (acting as a receptor). The carbohydrate portion of a glycoprotein, together with its protein component, forms a shape that a specific signalling molecule (such as a hormone) can bind to. This binding changes the shape of the glycoprotein and triggers a response inside the target cell, allowing cells to communicate with one another.
Cell-to-cell recognition. The carbohydrate chains that project from glycoproteins on the outer surface act like an identity tag specific to each cell type. This lets the immune system distinguish the body’s own cells from pathogens or abnormal (e.g. cancerous) cells, and lets cells of the same tissue type recognise and adhere to one another.

Final answers

(a) Phospholipids form a bilayer with hydrophilic heads facing the water on both sides and hydrophobic tails facing inward, because each phospholipid is amphipathic.
(b) Intrinsic proteins (e.g. the channel protein) span the bilayer, embedded by hydrophobic regions; extrinsic proteins (e.g. the inner-surface protein) are attached to one face only and do not span the bilayer.
(c) Glycoproteins act as receptors for signalling molecules (cell signalling) and as identity markers for cell-to-cell recognition.