Lab Notes and Procedures-04 Biology 1004
North Arkansas College
Topic: Membranes and Membrane Transport
- The membranes of all living organisms:
- Protects internal (and external) cell environment - maintains homeostasis.
- Maintains structural integrity of the cell.
- Regulates the movement of nutrients into and out of the cell.
- The cell membrane is a bimolecular leaflet arranged in a fluid mosaic configuration. There are 2 layers of phospholipids with proteins imbedded within.
- Phospholipids in water form a bilayer which separates both sides. These phospholipids have a head (polar) and a tail (non-polar). The head is polar and hydrophilic. The tail is non-polar and hydrophobic.
- The phospholipid tails and the proteins "wiggle" back and forth. This allows materials to pass through this membrane.
- Transport of substances through the membrane: This is a semi-permeable membrane (can partially regulate the size of materials that pass through it).
Membrane Transport
- Passive Transport - No energy is required. There are 2 basic types of passive transport.
Simple Diffusion - Solute (substance to be suspended)
Moves from an area of higher concentration to area of lower concentration. Osmosis is a type of simple diffusion in which solvent (usually water) moves from area of higher concentration to area of lower concentration.
Facilitated Diffusion - Requires no energy; but requires carrier proteins (usually permeases).
- Active Transport - Characterized by requirement for energy (ATP) and carrier proteins.
Tonicity
Tonicity is the response of cells to external environment. There are three types of tonicity.
- Isotonic - Solute concentration is the same inside and outside of the cell. There is no change to cells. We usually use 0.9% NaCl and 5% glucose (isotonic concentration of human blood).
- Hypertonic - Solute concentration is greater outside the cell. Because water moves to highest concentration, water moves outside of cell and causes cell to shrink/collapse. This process is called plasmolysis/crenation.
- Hypotonic - Solute concentration becomes greater on the inside of the cell. Water flows in and the cell swells/bursts. This process is called osmotic lysis.
