Roles of Transmembrane Proteins Unit 1 Cells and

Roles of Transmembrane Proteins Unit 1 Cells and Proteins Advanced Higher Biology Miss Aitken

Movement of Ions and Molecules Across Membranes • Ions/Polar molecules must go through a protein channel in order to pass through the cell membrane • There are specific transmembrane proteins which act as sodium channels etc. • Some transmembrane proteins control the concentration gradient across a membrane by controlling numbers of ions on either side.

Channel Proteins • When ions or molecules pass through channel proteins, no change in shape is required. • This is facilitated diffusion; as the protein has made it easy for the ion or molecule to move across the membrane passively. • Types of channel protein are specific to one ion or molecule

Channel Proteins • Aquaporin is a good example of a channel protein. • It forms a channel to let polar water molecules pass through the pore. Hydrophobic R groups are found around the surface of the protein, but a narrow channel in the middle has hydrophilic R groups to help water molecules pass through.

Channel Proteins • Some channel proteins can be open or closed to allow/disallow diffusion of an ion. These are called gated channels – there are 2 types. • Ligand-gated: A signal molecule binds to the channel protein and changes its shape/opens or closes gate. • Voltage-gated: A change in ion concentration across the cell membrane causes a change in the channel protein and it opens or closes.

Transporter Proteins • Also involved in facilitated diffusion • Also specific to one type of ion/molecule • Main difference: Transporter proteins bind to ions or molecules and this causes a conformational change in the protein. • Protein actually passes ions or molecules across the membrane rather than providing a route through. • Example: GLUT 4 glucose transporter

Coupled Transport • The movement of one molecule ‘downhill’ along it’s concentration gradient powers the movement of another ‘uphill’ against it’s concentration gradient. • Example: glucose symport process

Specialisation • Different tissues in a multicellular organism have different types of channel and transporter proteins in their membrane to move different materials across their cell membranes. • e. g: glucose symporters are only found in the cells lining the small intestine and kidney nephron.

Signal Transduction • Some transmembrane proteins do not carry ions or molecules across the membrane. • Instead, they act as a receptor where a hydrophilic signal molecule can bind outside of the cell. • The binding of a molecule to a membrane receptor causes a conformational change in the protein, and since part of it is sitting in the cytoplasm, this often triggers a signaltransduction pathway and causes a response.

Signal Transduction • Hydrophobic signal molecules work by moving across the cell membrane into the cell, where they then bind to a receptor protein inside the cell and activate processes. (More later)