W11_Cellular process

MPharm Programme: Cell Science - Cellular Process 1 & 2

• The lecture covers different transport processes in and out of cells (passive, active, and vesicle transport).
• Transport mechanisms across cell membranes are explained.
• Examples of transport processes are given, including in the nervous system and neurotransmission.
• Body fluid pools (intracellular and extracellular) are also detailed. The intracellular fluid makes up 2/3rds and the extracellular fluid makes up 1/3rd of the total body fluids.
• Extracellular fluid includes interstitial fluid, plasma, lymphatic fluid, and cerebrospinal fluid.
• Terminology related to solutions, solvents, solutes, and concentration gradients are defined.
• The process of molecules moving across cell membranes (specifically the plasma membrane) through concentration and electrical gradients is discussed.
• A concentration gradient is a difference in the concentration of a chemical between one place to another (e.g. inside vs outside the cell).
• An electrical gradient is a difference in electrical charges between two regions.
• The combined influence of concentration and electrical gradients on the movement of a particular ion is called an electrochemical gradient.
• Different types of transport processes are covered including passive transport (simple diffusion, facilitated diffusion, osmosis), active transport, endocytosis (receptor-mediated endocytosis, phagocytosis, fluid-endocytosis (pinocytosis)), and exocytosis.
• Passive transport does not require energy.
• Simple diffusion involves the movement of substances from an area of higher concentration to one of lower concentration.
• Facilitated diffusion uses carrier proteins to transport substances across the cell membrane from high to low concentration.
• Osmosis is the movement of water across a semi-permeable membrane from an area of higher water concentration to one of lower water concentration.
• Factors influencing diffusion include the steepness of the concentration gradient, temperature, mass of the diffusing substance, surface area, and diffusion distance.
• Examples of substances that undergo simple diffusion include gases (like CO₂, O₂, N₂), small uncharged polar molecules (like ethanol), and some small uncharged polar molecules (like water and urea). Large uncharged polar molecules (like glucose, fructose) and ions (like K⁺, Na⁺, Cl⁻, Mg²⁺, Ca²⁺, HCO₃⁻, HPO₄²⁻) are impermeable to simple diffusion.
• Facilitated diffusion requires a carrier in the membrane, but not ATP.
• Ion channels are integral transmembrane proteins that allow the passage of inorganic ions across the cell membrane.
• Facilitated diffusion (through carriers) includes protein transporters that move substances down their concentration gradient. These can be saturated, meaning they reach the maximum transport capacity. Glucose uses this mechanism.
• Osmolarity is the total solute concentration of a solution.
• One osmol of a substance is equal to one mole of solute particles. A 1 molar solution of glucose is 1 osmol. A 1 molar solution of sodium chloride (NaCl) contains 2 osmols due to the dissociation into two ions.
• Tonicity refers to the ability of a solution to cause changes in the shape or tone of cells by altering their internal water volume.
• Active transport uses energy (ATP) to move molecules/substances against the concentration gradient.
• This process includes primary active transport, using ATP directly to move substances, and secondary active transport, using the electrochemical gradient formed from primary active transport to drive the transport of other substances.
• Examples of primary active transport include the Na+/K+-ATPase pump, Ca²⁺-ATPase pump, H⁺-ATPase pump, and H⁺/K+-ATPase pump.
• Secondary active transport includes co-transporters/symporters (moving two substances in the same direction), and counter-transporters/antiporters (moving two substances in opposite directions).
• Vesicular transport types are endocytosis (including pinocytosis, receptor-mediated endocytosis, and phagocytosis) and exocytosis.
• In endocytosis, substances are enclosed in vesicles that bud from the plasma membrane, moving into the cell.
• Exocytosis releases substances contained in vesicles that fuse with the plasma membrane, moving out of the cell.
• There are three main types of endocytosis: receptor-mediated endocytosis, pinocytosis, and phagocytosis.
• receptor-mediated endocytosis is selective process that uses receptors on the cell surface to bind to specific molecules.
• Pinocytosis is the uptake of liquid by the cell.
• Phagocytosis is the uptake of solid particles or cells by the cell.
• Exocytosis is a process that moves substances out of the cell. It's used by nerve cells to release neurotransmitters to communicate with other cells.
• Transcytosis is a process where substances cross cells by undergoing endocytosis on one side of the cell and exocytosis on the other side.
• There are more detailed summary of the processes in the last section of the slides. These are summarized with descriptions.