Selective permeability Cell Membrane Selective permeability

Diffusion: passive movement of particles from a region of high concentration to a region of low concentration.

The movement of particles is caused by the kinetic energy possessed by the particle. The direction of movement is random. Observing groups of particles it emerges that they move from regions of high concentration to regions of low concentration. Effective for moving molecules small distances (micrometers), but not over long distances

Osmosis is the passive movement of water molecules from regions of lower solute concentration to a region of higher solute concentration Only occurs if solutes (substances dissolved in water) are present.

water moves through plasma membrane pores called aquaporins. The water molecules have kinetic energy like other molecules. The water molecules move randomly and will, if they come into contact with the membrane, pass straight through. The tendency is for water to pass from lower solute to higher solute concentrations.

Simple diffusion across membranes involved particles passing between the phospholipids in the membrane. Can only happen if membrane is permeable to the particles. Only occurs if there is a concentration gradient.

Type Permeability Reason Ions Not Easy Membrane is hydrophobic, ions are +/- charged Polar Low rates Partial +/- charges Nonpolar, small Ie O2 and CO2 High rates No charge

Facilitated Diffusion Larger molecules (red) move passively through the membrane via channel proteins These proteins (grey) have large globular structures and complex 3d-shapes The shapes provide a channel through the middle of the protein, the 'pore' The channel 'shields' the diffusing molecule from the non-charged/ hydrophobic/ non-polar regions of the membrane.

Molecules are moved against the concentration gradient from a region of their low concentration to a region of their high concentration. Active mean that the membrane protein 'pump' requires energy (ATP) to function The source of energy is ATP is produced in cell respiration Transported molecules enter the carrier protein in the membrane. The energy causes a shape change in the protein that allows it to move the molecule to the other side of the membrane.

The sodium-potassium pump creates electro-chemical gradients across the cell membrane of all cells. Cells are -ve charged on the inside relative to the outside. This pump is modified in the nerve cell to create some of the electrochemical phenomena seen in nerve cells.

Cells will manufacture molecules for secretion outside of the cell Cells will manufacture molecules for secretion outside of the cell. Some of these secretion molecules are complex combinations of proteins, carbohydrates and lipids. The base protein is coded for by a gene whose expression begins the process.

Membranes are fluid.. Therefore they can change shape and move For example the making of vesicles…

Transport by a Vesicle 1. Protein is already synthesised and present in the rER. 2. The protein is moved through the rER and modified. 3. A spherical vesicle is formed from the end of the rER with the  protein inside. 4. The vesicle migrates to the golgi apparatus. 5. Vesicle and golgi membranes fuse. The protein is released into the lumen of the golgi apparatus.

6. The golgi modifies the protein further by adding lipid or polysaccharides to the protein. 7. A new vesicle is formed from golgi membrane which then breaks away. The vesicles migrates to the plasma membrane. 8. The vesicle migrates to the plasma membrane fuses and secretes content its contents out of the cell. A process called exocytosis.

a) Exocytosis: vesicle membrane fuses with the plasma membrane. b) Endocytosis: a vesicle is formed by the infolding of the plasma membrane