Cell Membrane Transport - Transport Across A Membrane - How Do Things Move Across A Cell Membrane

The video introduces membrane transport, explaining how substances move into and out of cells. The plasma membrane, primarily composed of phospholipid molecules with polar hydrophilic heads and nonpolar hydrophobic tails, regulates this movement. Proteins are scattered throughout the membrane, which forms a bilayer with heads facing outwards and tails inwards. The inner part is the cytosol, and the outer fluid is interstitial fluid.

Passive transport does not require cellular energy. Diffusion is the movement of a substance from a high concentration to a low concentration, aiming for equilibrium, down a concentration gradient. Cellular diffusion across the plasma membrane from high to low concentration depends on the substance's concentration in interstitial fluid and cytosol. This can be simple or facilitated diffusion.

Simple diffusion occurs when small, nonpolar solutes, like oxygen, carbon dioxide, and small fatty acids, can pass directly through the nonpolar interior of the phospholipid bilayer without assistance, moving down their concentration gradient.

Facilitated diffusion applies to small, charged, or polar solutes that cannot pass directly through the membrane. They are assisted by plasma membrane proteins: channel mediated diffusion uses water-filled protein channels (leak or gated) for ions, while carrier mediated diffusion involves carrier proteins that change shape to move polar molecules like glucose and amino acids.

Osmosis is the passive movement of water through a selectively permeable membrane due to a difference in water concentration. Water can move between phospholipid molecules or through aquaporins (integral protein channels). It moves from an area of higher water concentration to lower water concentration to achieve equilibrium, often influenced by solute concentration.

Active processes require cellular energy (ATP) for membrane transport. These include active transport (moving solutes against their concentration gradient) and vesicular transport (moving large substances via vesicles). Primary active transport uses ion pumps, like the sodium-potassium pump, to move ions across the membrane against their gradient by directly using ATP.

Secondary active transport uses the energy from the movement of one substance down its concentration gradient to move a second substance against its own concentration gradient. This can be symport (both substances move in the same direction) or antiport (substances move in opposite directions), exemplified by sodium ions moving down their gradient to power glucose or H+ movement.

Vesicular transport involves moving larger substances like proteins or polysaccharides using membrane-bound sacs called vesicles. In exocytosis, vesicles formed by the Golgi apparatus transport materials to the cell membrane, fuse with it, and release their contents outside the cell. The vesicle membrane then becomes part of the plasma membrane.

Endocytosis is when the plasma membrane folds inward to trap a substance, forming a vesicle. There are three types: phagocytosis ('cell eating') engulfs large particles which are then broken down by lysosomes; pinocytosis ('cell drinking') engulfs droplets of interstitial fluid with dissolved solutes; and receptor-mediated endocytosis uses receptors on the membrane to bind specific molecules, which are then enclosed in a vesicle. Both endocytosis and exocytosis require energy.