Signalling Across Cell Membranes (A-level Biology)

Signalling Across Cell Membranes

We have previously established that the cell surface membrane is essential for cells to communicate with each other and their environment. Glycoproteins and glycolipids form receptors on the cell surface membrane, which are able to receive signals, such as hormones, from neighbouring cells or from the environment. This is then relayed to the inside of the cell to initiate specific cellular responses related to these messages.

Cell Signalling via ligands and receptors

• Communication across cells usually occur via chemical signals. These chemical signals, also known as ligands, are excreted by the sending cell into the extracellular space.
• The ligand will bind to a target cell which has a receptor that recognises the ligand. The ligand is intended for a target cell, which is able to receive this signal as it has a receptor that the ligand can specifically recognise and bind to.
• The binding of the ligand to the receptor causes an intracellular reaction. When the ligand binds to its corresponding receptor, it triggers a cascade of reactions in the cell that ultimately leads to a change within the cell. This resulting intracellular reaction can be small, such as activation of a gene, or large, such as inducing cell division.
• There can be multiple ligands being received by a cell at once. Most cells have multiple receptors, therefore, they can be receiving multiple ligands at once, each of which can trigger different intracellular reactions.

Types of ligands and receptors

Types of Ligands

• Some ligands are polar molecules. They only bind to the extracellular receptors on the cell surface membrane as they cannot cross the phospholipid bilayer. Examples include peptide hormones such as insulin.
• Some ligands are hydrophobic molecules. They can pass through the cell membrane and bind to intracellular receptors. Examples include steroid hormones such as testosterone and oestrogen.

Types of Receptors

• Some receptors sit extracellularly. Some receptors such as glycoproteins and glycolipids are anchored into the cell membrane. Their ligand-binding region (also called “domain”) remains extracellularly so that the ligand does not need to pass through the plasma membrane in order to bind to the receptor. Some extracellular receptors are coupled with another signalling protein that sits intracellularly. When a ligand binds to the extracellular domain of the receptor, it causes a conformational change in the intracellular protein the receptor is bound to, and this protein can then activate other signalling molecules within the cell to trigger further intracellular reactions.

• Some receptors sit intracellularly. Intracellular receptors are usually present in the cytoplasm or nucleus. When their ligand enters the cell and binds to them, it causes the intracellular receptor to change shape and form a hormone-receptor complex. This allows the complex to enter the nucleus (if it was not there previously) and cause a change in gene activity.