TRANSPORT

TRANSPORT

1. Transport routes

   • Paracellular diffusion: between the cells
   • Transcellular diffusion: through the cell (IN/OUT)
   • Transport mechanisms: =Passive diffusion =Facilitated diffusion =Active transport =Vesicular transport =Cell to cell
     • Passive diffusion is the movement of ions and other atomic or molecular substances across the cell membrane without need of energy input (transcellular)
     • Active transport is the movement of ions or molecules across a cell membrane into a region of higher concentration, assisted by enzymes and requires energy.
       • Requires ATP, to pump a molecule against its concentration gradient, it enable a cell to maintain its internal environment. Electrogenic pump generates voltage across the membrane e.g. Sodium potassium pump. In primary active transport, the protein hydrolises ATP to power the transport directly whereas, in secondary active transport, ATP hydrolysis is done indirectly to power the transportation.
       • Exocytosis and Endocytosis: Active transport to move macromolecules, fusion of vesicles. Endocytosis - Pinocytosis, Phagocytosis and receptor mediated endocytosis.
     • Facilitated diffusion: in addition to basic diffusion, areas of high concentration to areas of low concentration, facilitated diffusion selectively allows substances to cross the membrane.
     • Movement
       • Symport: two substances, move in the same direction
       • Antiport: Two substances, move in opposite direction
     • Cell to cell: Gap junctions are specialised connections that form a narrow pore between adjacent cells. these pores permit small moleules and ions to move from one cell to another. Cardiac tissue has exstensive gap junctions, to allow the rapid movement of ions between cells thi helps the tissue to beat in rhythm.
       • Vesicular transport: Active movement of substances across the membrane via a vesicle

2. MEMBRANE CHANNELS

   • MEMBRANE CHANNEL PROTEINS: Channel proteins provide corridors/channels for molecules to travel through the cell membrane. Aquaporin allows for the quick diffusion of water. ion channels which can be gated such as sodium channels open on action potential.
   • MEMBRANE CARRIER PROTEINS: Carrier proteins undergo a subtle change in shape which transloactes a solute across the membrane. They are not opened simultaneously to either side of the membrane. GLUT1 is a carrier protein found in almost all animal cell membranes that transport glucose across the bilayer.

3. GLUCOSE TRANSPORT:

   • Two types of transport: transporter facilitated passive diffusion (GLUT) Co transporter mediated transport by sodium/glucose transporters (SGLT)
     • GLUT plays a key role in glucose diffusion across the tissue barriers. SGLT plays a key role in glucose absorption from intestinal tract and renal tubules.
       • Co-Transport: The cell membrane uses an electrochemical gradient as a means of energy, called co-transport. The system works by transporting two molecules or ions at a time and using the gradient of ones solutes concentration to force the other molecule or ion against its gradient. A single ATp powered pump can drive transport of several other solutes. A substance that has been pumped across a membrane can do work as it moves back.

4. RECEPTORS: a molecule structure within a cell or on the surface characterised by selective binding of a specific molecule and specific physiologic effect that accompanies the binding.

   • Autocrine: a substance secreted by a cell and acting on the surface receptors of the same cell. Paracrine: relating to or denoting a hormone which has effect only in the vicinty of the gland secreting it. Endocrine: glands which secrete hormones or other products directly into the blood.

5. CELL SIGNALLING: the release, by one cell, of substances that transmit information to other cells

   • LIGAND: a molecule that binds specifically and reversibly to another molecule to form a complex.
     • SIGNAL TRANSDUCTION: series of changes that convert the extracellular chemical signal to a specific intracellular response.