Biological membranes

Osmosis • Net diffusion of water. • Water moves down its concentration gradient across a selectively permeable membrane /

Tonicity • Describes the movement of water between two solutions separated by a semipermeable membrane /

Isotonic solution: two solutions (e.g., inside and outside of a cell) have equal solute concentration. No net movement of water molecules /

Hypertonic solution: a higher concentration of solutes exists in that solution as compared to the other. Water moves into it from the other. /

Hypotonic solution: a lower concentration of solutes exists in that solution. Water will leave the solution towards the other /

Filtration - particles are driven through membrane by physical pressure *requires force. Filtration of water and small solutes through gaps in capillary walls. Allows removal of waste from capillaries in kidneys. /

Carrier-Mediated Transport - Carrier proteins on membranes carry solutes into or out of cell (or organelle) Carrier has slot in it that allows specific molecule to fit in snug /

Properties of Carrier-Mediated Transport - Specificity– Carrier proteins are specific for particular solutes /

Saturation– As solute concentration rises, the rate of transport rises, but only to a point. /

Transport maximum (Tm) — transport rate at which all carriers are occupied. /

Three kinds of carriers? Uniport—carries one type of solute • Example: Proton pump– Symport - carries two or more solutes simultaneously in same direction (cotransport) • Example: sodium-glucose transporters Antiport - Carries two or more solutes in opposite directions (countertransport) /

Three mechanisms of carrier-mediated transport: • Facilitated diffusion (Passive) • Primary active transport • Secondary (indirect) active transport /

Facilitated Diffusion - needs a carrier* moves solute down its concentration gradient • Does not consume ATP • Solute attaches to binding site on carrier, carrier changes conformation, then releases solute on other side of membrane /

Primary Active Transport - carrier moves solute through a membrane against concentration gradient (low to high) • The carrier protein uses ATP for energy. Ex; proton pump (uniport) /

Another form of Primary Active Transport Sodium? - Potassium Pump (antiport): uses ATP while expelling three sodium ions out of and importing two potassium ions into a cell *MOST IMPORTANT IN THE BODY /

Secondary (Indirect) Active Transport − “Coupled transport” − Transport driven by the energy stored in the concentration gradient of another molecule (Na+) − Indirect use of ATP − Passive diffusion of Na+ /

Types of Secondary (Indirect) Active Transport - Co-transport by symporters - Counter-transport by antiporters /

Co-transport: substance is transported in the same direction as the “driver” ion (Na + ) /

Counter-transport (Antiport): substance is transported in the opposite direction as the “driver” ion (Na + ) /

Vesicular Transport - • Moves large particles, fluid droplets, or numerous molecules at once through the membrane in vesicles (bubble-like enclosures of membrane) • Requires ATP /

Endocytosis - vesicular processes that bring material into cell /

Pinocytosis - “cell drinking,” taking in droplets of ECF containing molecules useful to the cell. /

Receptor-mediated endocytosis—particles bind to specific receptors on plasma membrane /

Exocytosis – Secreting material – Replacement of plasma membrane removed by endocytosis /

Transcytosis — transport of material across the cell by capturing it on one side and releasing it on the other– Receptor-mediated endocytosis moves it into the cell and exocytosis moves it out the other side /